What is a casting
What is a casting?
Castings are metal molded objects obtained by various casting methods, that is, objects with certain shape, size and performance are obtained by pouring, injection, suction or other casting methods into the prepared mold, cooling, grinding and other subsequent processing methods.
Castings have a long history of application. In ancient times, people used castings as and some living utensils. In modern times, castings are mainly used as blanks of machine parts, and some precision castings can also be directly used as machine parts. Castings account for a large proportion in mechanical products. For example, in tractors, the weight of castings accounts for about 50 ~ 70% of the weight of the whole machine, 40 ~ 70% in agricultural machinery, and 70 ~ 90% in machine tools and internal combustion engines. Among all kinds of castings, mechanical castings have the most varieties, the most complex shapes and the largest amount, accounting for about 60% of the total output of castings. The second is the ingot mould for metallurgy, the pipeline for engineering, and some tools in life.
Castings are also closely related to daily life. For example, the frequently used door handles, door locks, radiators, plumbing pipes, iron pans, gas stove frames, irons, etc. are all castings.
Classification of castings
There are many classification methods for castings: according to the different metal materials used, they are divided into steel castings, iron castings, copper castings, aluminum castings, magnesium castings, zinc castings, titanium castings, etc. Each type of casting can be further divided into different types according to its chemical composition or metallographic structure. For example, iron castings can be divided into gray iron castings, nodular iron castings, vermicular iron castings, malleable iron castings, alloy iron castings, etc; According to different mold forming methods, castings can be divided into ordinary sand castings, metal castings, die castings, centrifugal castings, continuous castings, investment castings, ceramic castings, electroslag remelting castings, bimetallic castings, etc. Among them, ordinary sand castings are the most widely used, accounting for about 80% of the total casting output. Aluminum, magnesium, zinc and other non-ferrous metal castings are mostly die castings.
What is steel casting?
Steel castings refer to parts made of cast steel, which have similar properties to cast iron, but better strength than cast iron. Steel castings are prone to air hole defects and inaccurate angle positioning in the casting process, which may lead to casing fracture in long-term use.
Characteristics of steel castings
One of the advantages of steel castings is the flexibility of design. Designers have the greatest freedom to design and choose the shape and size of castings, especially for parts with complex shape and hollow section. Steel castings can be manufactured by the unique process of core assembly. Its forming and shape change are very easy. The transformation from drawing to finished product is very fast, which is conducive to rapid quotation response and shortening the delivery time. The perfect design of shape and quality, the minimum stress concentration factor and the strongest overall structure all reflect the flexibility and process advantages of steel casting design:
- 1) Steel castings have strong adaptability and variability in metallurgical manufacturing. Different chemical composition and microstructure control can be selected to meet the requirements of various projects; Mechanical properties and service properties can be selected in a large range through different heat treatment processes, and have good welding and processing properties.
- 2) The isotropy of steel casting materials and the strong overall structure of steel castings improve the engineering reliability. Coupled with the advantages of reduced weight design and short delivery time, it has competitive advantages in price and economy.
- 3) The weight of steel castings can vary in a wide range. Those with small weight can be investment precision castings with only tens of grams, while the weight of large steel castings can reach several tons, tens of tons or even hundreds of tons.
- (1) Uneven tissue. After the liquid metal is injected into the mold, the first layer of liquid metal in contact with the mold wall will soon solidify into finer grains because the temperature decreases the fastest. With the increase of the distance from the mold wall, the influence of the mold wall decreases gradually, and the crystals grow into columnar crystals parallel to each other along the direction perpendicular to the mold wall. In the center of the casting, the heat dissipation has no significant directionality, and can grow freely in all directions until they contact each other, so an equiaxed crystal region is formed. It can be seen that the structure in the casting is uneven. Generally speaking, the grain is relatively coarse.
- (2) The tissue is not dense. The crystallization of liquid metal is carried out in the form of branch growth, and the liquid metal between branches finally solidifies, but it is difficult to fill all the branches with metal liquid, resulting in the general non compactness of castings. In addition, the liquid metal injected into the mold can also form porosity or even shrinkage cavity if its volume shrinks and is not sufficiently supplemented during cooling and solidification. Graphite in iron castings often appears in flake, spherical or other shapes, which can also be regarded as a kind of non dense structure.
- (3) Rough surface. Generally speaking, the surface is rough, which can not be compared with the machined surface, and the shape is complex.
Application of steel castings
Due to the characteristics of steel castings, almost all industrial departments need steel castings, which are widely used in ships and vehicles, construction machinery, engineering machinery, power station equipment, mining machinery and metallurgical equipment, aviation and aerospace equipment, oil wells and chemical equipment. As for the application of steel castings in various industrial sectors, there may be great differences due to different specific conditions in various countries.
There are many kinds of steel castings. The use of steel castings in several major industrial sectors is briefly described.
1. Power station equipment
Power plant equipment is a high-tech product, and its main parts operate continuously for a long time under high load. Many parts of thermal power plant and nuclear power plant equipment also need to withstand the corrosion of high temperature and high pressure steam, so there are strict requirements for the reliability of parts. Steel castings can meet these requirements to the greatest extent and are widely used in power station equipment.
2. Railway locomotive and rolling stock
Railway transportation is closely related to the safety of people’s lives and property. It is very important to ensure safety. Some key components of locomotive and vehicle, such as wheel, side frame, bolster, coupler, etc., are traditional steel castings.
The turnout used for railway switch is a component that bears strong impact and friction. The working conditions are very bad and the shape is very complex.
3. Construction, construction machinery and other vehicles
The working conditions of construction machinery and engineering machinery are very poor. Most parts bear high load or need to withstand impact wear. A large part of them are steel castings, such as driving wheel, bearing wheel, rocker arm, track shoe, etc. in the action system.
Steel castings are rarely used in general automobiles, but many steel castings are also used in the operation part of special off-road vehicles and heavy trucks.
Production of steel castings
(1) Melting of cast steel. Cast steel must be melted in electric furnace, mainly electric arc furnace and induction furnace. According to the different lining materials and slag systems used, it can be divided into acid furnace and alkaline furnace. Carbon steel and low alloy steel can be melted in any kind of furnace, but high alloy steel can only be melted in alkaline furnace.
(2) Casting process. Cast steel has high melting point, poor fluidity, and liquid steel is easy to oxidize and inhale. At the same time, its volume shrinkage is 2 ~ 3 times that of gray cast iron. Therefore, the casting performance of cast steel is poor, and it is easy to produce defects such as insufficient pouring, porosity, shrinkage, hot crack, sand sticking, deformation and so on. In order to prevent the above defects, corresponding measures must be taken in the process.
Molding sand for steel castings shall have high fire resistance and sand adhesion resistance, as well as high strength, air permeability and yield. Raw sand usually adopts silica sand with large and uniform particles; In order to prevent sand sticking, the cavity surface is mostly coated with coatings with higher fire resistance; When producing large parts, it is mostly used in sand mold or sodium silicate sand, which is faster than casting mold. In order to improve mold strength and yield, various additives are often added to molding sand.
In the design of gating system and riser. As cast carbon steel tends to solidify layer by layer and has large shrinkage, the principle of rigid sequential solidification is adopted to set the gating system and riser to prevent shrinkage cavity and porosity. Generally speaking, steel castings should be provided with risers. Cold iron is also widely used. In addition, the bottom pouring system with simple shape and large cross-sectional area should be used as far as possible to make the molten steel fill the mold quickly and stably.
(3) Heat treatment. The heat treatment of cast steel is usually annealing or normalizing. Annealing is mainly used for steel castings with w (c) ≥ 0.35% or with particularly complex structure. This kind of castings have poor plasticity, high casting stress and easy cracking. Normalizing is mainly used for steel castings with w (c) ≤ 0.35%. This kind of steel has low carbon content, good plasticity and is not easy to crack when cooling.
Common defects of steel castings
Although the defects produced in the casting process of steel castings are similar to those produced by ingot casting, they are still process defects. The common process defects include pores, inclusions, shrinkage cavities, looseness and cracks.
- (1) Pores (bubbles): pores (bubbles) are cavities formed due to excessive gas content in liquid metal, wet model and poor air permeability. The pores in castings can be divided into single dispersed pores and dense pores.
- (2) Inclusions: inclusions are divided into non-metallic inclusions and metallic inclusions. Nonmetallic inclusions are the products formed by the chemical reaction between metal and gas during smelting or the inclusions formed by the mixing of refractory and molding sand into liquid steel during pouring. Metal inclusions are inclusions formed when dissimilar metals occasionally fall into liquid steel and fail to dissolve.
- (3) Shrinkage cavity: shrinkage cavity is a defect formed because the volume shrinkage of liquid metal cannot be supplemented during cooling and solidification. Shrinkage cavities are mostly located near the pouring riser and at the position with the largest section or at the sudden change of section.
- (4) Looseness: due to poor melting, improper mold shape and other reasons, fine grain boundary cracks or fine voids are generated in the middle of the steel casting wall thickness, resulting in loose structure. The bonding between these grains is quite weak (cloud shadow is formed on the radiographic negative film).
- (5) Crack: crack refers to the defect formed by excessive low melting point impurities and excessive internal stress (thermal stress and structural stress) during the cooling process of liquid steel. At the sudden change of casting section size, the stress concentration is serious and cracks are easy to appear.
To sum up, the remarkable feature of process defects in steel castings is complex shape; The service defects of steel castings are mainly fatigue cracks, including mechanical fatigue cracks and thermal fatigue cracks.
Inspection of steel castings
1. Poor ultrasonic penetration
The complex interface of coarse grain and uneven structure enhances the scattering of ultrasonic wave, and the energy attenuation is large, so that the detectable thickness is smaller than that of forgings.
2. There are many interference clutter
When the sound wave scatters on the interface of non-uniform, non dense structure and coarse grain, the scattered signal intensity is large and is received by the probe; The rough casting surface will form clutter when it reflects the sound wave; These will be displayed as messy forest echo (also known as grass echo) on the oscillograph screen, which may drown the defect echo and hinder the identification of defect echo.
3. Poor surface coupling conditions
The surface of steel castings is rough, which is not conducive to acoustic coupling. The surface hardness is large and grinding is difficult.
4. It is difficult to quantify defects
Due to the large attenuation of sound wave and complex defect shape of steel castings, the quantitative evaluation of defects based on artificial defects has large error, and it is more difficult to quantify defects by calculation method.
The above is the difficulty of casting inspection, which limits the casting inspection to a certain extent. On the other hand, due to the low quality requirements of castings, the size and quantity of single defects are allowed to be large, and the regularity of casting defects is strong, so the casting inspection still has a certain value.
- 1) For small and medium-sized castings (especially investment castings), they have small volume, light weight and less processing capacity. They can be magnetized in at least two roughly vertical directions on a fixed magnetic particle flaw detector. It is better to use DC or pulsating DC current and test it by wet continuous method. Direct energization method, rod threading method, magnetization method and coil method can be used.
- 2) For large and heavy castings, magnetization shall be carried out locally or in zones in at least two roughly vertical directions. It is better to use the portable or mobile magnetic particle flaw detector with DC or half wave rectification, and use the contact method or yoke method, dry method continuous method or wet method continuous method to detect the parts or zones of castings. The detection shall generally be carried out in two mutually perpendicular directions.
- 3) In order to prevent the casting in contact with the electrode from burning out, it is recommended to take the following measures: do not turn on the current when the contact is not in full contact with the casting surface, and remove the contact only when the current has been disconnected. And use sufficiently clean and suitable contacts. For machined smooth surfaces, the yoke method should be used.
- 4) Due to the influence of casting stress, some cracks (cold cracks) of steel castings will delay cracking, so they should not be tested immediately after casting, but should be tested again after 1 ~ 2 days.
- 5) If the casting defect exceeds the acceptance standard and is rejected, and excavation (shovel) and repair welding are allowed, pay attention to controlling the generation of delayed cracks in the repair welding area.
- 6) The inspection shall be carried out with the naked eye, and a magnifying glass of no more than 3 times can be used only in the inspection of quality grades 001 and 01.
What is iron castings?
Articles cast with molten iron are collectively referred to as iron castings. Due to the influence of many factors, there are often defects such as pores, pinholes, slag inclusions, cracks, pits and so on.
Quality of iron castings
Including appearance quality, internal quality and use quality. Appearance quality refers to surface roughness, surface defects, dimensional deviation, shape deviation and weight deviation of castings; It mainly refers to the and metallographic structure of the casting, as well as the holes, cracks, inclusions and segregation existing in the casting; Service quality refers to the working durability of iron castings under different conditions, including wear resistance, corrosion resistance, chilling and heat resistance, fatigue, shock absorption and other properties, as well as machinability, weldability and other process properties.
Due to many factors, iron castings often have defects such as pores, pinholes, slag inclusions, cracks, pits and so on. The commonly used repair equipment is argon arc welding machine, resistance welding machine, cold welding machine, etc. Casting defects with low quality and appearance requirements can be repaired by argon arc welding machines with high calorific value and high speed.
The quality of iron castings has a great influence on the performance of mechanical products. For example, the wear resistance and dimensional stability of machine tool castings directly affect the accuracy and service life of machine tools; The size, accuracy and surface roughness of impeller, shell and inner cavity of hydraulic parts of various pumps directly affect the working efficiency of pumps and hydraulic systems, energy consumption and the development of cavitation; The strength and heat shock resistance of cast copper parts such as cylinder block, cylinder head, cylinder liner, piston ring and exhaust pipe of internal combustion engine directly affect the working life of the engine.
Factors affecting quality
The first is the design process of iron castings. During the design, in addition to determining the geometry and size of iron castings according to the working conditions and metal material properties, the rationality of the design must also be considered from the perspective of casting alloy and casting process characteristics, that is, obvious size effect and solidification, shrinkage, stress and other problems, so as to avoid or reduce the occurrence of defects such as composition segregation, deformation and cracking of iron castings.
Second, there should be a reasonable casting process. That is, according to the structure, weight and size of iron castings, casting alloy characteristics and production conditions, select appropriate parting surface, modeling and core making methods, and reasonably set up casting reinforcement, cold iron, riser and gating system. To ensure high-quality iron castings.
The third is the quality of raw materials for casting. The quality of metal charge, refractory, fuel, flux, modifier, casting sand, molding sand binder, coating and other materials is not up to standard, which will cause defects such as porosity, pinhole, slag inclusion and sand sticking, affect the appearance quality and internal quality of iron castings, and scrap iron castings in serious cases. The fourth is process operation. It is necessary to formulate reasonable process operation procedures, improve the technical level of workers and correctly implement the process procedures.
In manufacturing, the quality of castings should be controlled and inspected. First of all, we should formulate process codes and technical conditions for the control and inspection of raw materials, auxiliary materials and each specific product. Each process shall be controlled and inspected in strict accordance with the process code and technical conditions. Finally, the quality of finished castings is inspected. Reasonable testing methods and appropriate testing personnel shall be equipped. Generally, for the appearance quality of professional iron castings, comparative samples can be used to judge the surface roughness of castings; Minor cracks on the surface can be inspected by coloring method and magnetic particle method.
How to calculate the price of iron castings?
Confirm whether we can produce the casting.
We need to check whether we can use sand casting process to produce rough castings. Some may use wet sand, some may use resin sand, or shell casting, or automatic molding process. Wet sand is cheaper and shell casting is the most expensive. Relevant factors include dimensional tolerance, surface quality, structural complexity, annual demand, weight and size, etc. These factors will affect the cost of blank castings. Of course, if these processes can not meet the requirements, we will not be able to produce.
Confirm whether we can meet the processing requirements.
For normally processed works, most iron casting plants with processing capacity will be able to process in-house. Otherwise, they will have to entrust other professional processing workshops for processing. Because the cooperation between foundry and processing workshop may cause more difficulties and delay delivery, this is not a good choice.
Since the quotation needs to take into account the relevant processing equipment, working time and price, it is difficult to calculate the processing cost. In addition, the annual demand is also an important factor affecting the processing cost.
Consider other costs.
Other costs include packaging, surface treatment, painting, inland and sea freight, and, of course, exchange rates. These costs will be related to the specific needs of the customer.
After talking so much, you may not know how to calculate the casting cost. In fact, it is not simply to make an accurate quotation or price evaluation. There are many factors affecting the price.
However, we can still provide the approximate price range as a reference.
For rough castings without machining work.
For the processing cost, it is simple processing or rough processing, and the price range is 1 yuan/kg to 5 yuan/kg. For complex or high-precision machining, the price range is 5 yuan/kg to 10 yuan/kg.
Manifestations of defects in iron castings
The surface area carbon and wrinkle skin of iron castings are two forms of surface defects of iron castings. During the casting process, the solid and liquid phase products formed by gasification and decomposition of foam plastic models accumulate on the surface of the castings to form orange shaped carbon defects, and the surface wrinkling defects of the casting surface due to the poor temperature of the liquid metal filling. According to the most serious wrinkle on the casting surface, 100mm × The severity of wrinkled skin in an area of 60mm is divided into 5 levels. Level 1: very slight wrinkle (to fire); Grade 2: slight wrinkle; Grade 3: moderate wrinkle; Grade 4: severe wrinkle; Grade 5: severe wrinkle.
What is aluminum casting?
Cast aluminum is a process in which molten aluminum is poured into the mold and cooled to form aluminum parts with the required shape. The castings obtained by casting aluminum are called aluminum castings.
During the casting process, aluminum castings are prone to internal porosity, shrinkage cavity, air hole and other defects. After machining, the surface dense layer parts of these castings with defects are removed, so that the internal structural defects are exposed. For automotive cast aluminum parts with sealing requirements, such as cylinder block, cylinder head, intake manifold, brake valve body, etc., during the pressure seal test, the existence of defective micropores will lead to the leakage of sealing medium and a large number of waste products, and these defects are often found only after pressure test after machining, resulting in serious waste of man hours, raw materials and energy. In order to solve the problem of high scrap rate of automotive aluminum castings and save the castings that may be scrapped due to the above defects, certain treatment measures should be taken in production. The most common technology is infiltration treatment, that is, plugging. The so-called “infiltration” is a process technology that infiltrates the infiltration agent into the micropores of aluminum castings under certain conditions, connects the filler infiltrated into the pores with the inner wall of the pores of the castings after curing, blocks the micropores, and enables the parts to meet the conditions of pressurization, anti-seepage and anti leakage.
Aluminum castings have the advantages of low cost, good processability, remelting and regeneration, saving resources and energy, so the application and development of this material will last for a long time; The advanced technology of desulphurization and filtration of molten iron is widely used; Manufacturing technology of thin-walled high-strength iron castings; Manufacturing technology of Cast Iron Composites; Surface or local strengthening technology of iron castings; Complete set technology of isothermal foreign fired nodular cast iron; Special processes and equipment such as metal mold casting, metal mold sand coated casting and continuous casting are adopted. Aluminum casting light alloy will be more widely used in aviation, aerospace, automobile, machinery and other industries because of its series of excellent characteristics such as low density, high specific strength and corrosion resistance. Especially in the automobile industry, in order to reduce fuel consumption and improve energy utilization, aluminum It is a long-term development trend for nickel alloy castings to replace steel and iron castings. It focuses on pollution-free, efficient and easy to operate refining technology, modification technology, grain refinement technology and rapid detection technology in front of furnace. In order to further improve material properties and maximize material potential, high-quality aluminum alloy materials, especially aluminum matrix composites, can be developed to meet the performance requirements of different working conditions; Strengthen the research of cluster alloy melting process, and continue the development and research of alloy die casting and squeeze casting process and related technologies; Improve the development and research of ferroalloy smelting equipment and related technologies and processes.
What is die casting?
Die casting is a kind of pressure casting parts. It uses the pressure casting mechanical die casting machine equipped with the casting mold to pour the heated liquid metals such as copper, zinc, aluminum or aluminum alloy into the feed port of the die casting machine. After die casting by the die casting machine, copper, zinc, aluminum parts or aluminum alloy parts with the shape and size limited by the mold are cast. Such parts are usually called die castings. Die castings have different names in different places, such as die-casting parts, pressure castings, die-casting parts, die-casting aluminum, die-casting zinc parts, die-casting copper parts, copper die castings, zinc die castings, aluminum die castings, aluminum die castings, aluminum die alloy castings, aluminum alloy die-casting parts, etc.
Characteristics of die castings
Because the metal copper, zinc, aluminum and aluminum alloy have good fluidity and plasticity, and the casting process is cast in the pressure die casting machine, the aluminum die casting can make a variety of complex shapes, as well as high precision and finish, thus greatly reducing the machining amount of the casting and the casting allowance of metal copper, zinc, aluminum or aluminum alloy, It not only saves electricity and metal materials, but also greatly saves labor costs; Copper, zinc, aluminum and aluminum alloys have excellent thermal conductivity, small specific gravity and high machinability; Therefore, die castings are widely used in automobile manufacturing, internal combustion engine production, motorcycle manufacturing, motor manufacturing, oil pump manufacturing, transmission machinery manufacturing, precision instruments, landscaping, power construction, architectural decoration and other industries.
Application of die casting
Die castings can be manufactured into die-casting auto parts, die-casting auto engine pipe fittings, die-casting air conditioning parts, die-casting gasoline engine cylinder head, die-casting valve rocker arm, die-casting valve support, die-casting power parts, die-casting motor end cover, die-casting shell, die-casting pump shell, die-casting building parts, die-casting decoration parts, die-casting guardrail parts, die-casting wheels, etc, With the continuous improvement of the development level of domestic manufacturing equipment industry, the equipment level of die-casting machine has also been significantly improved, the types of parts that can be manufactured are also expanding, and the accuracy and complexity of die-casting parts have also been greatly improved. I believe that die-casting parts will better serve our production and life in the near future.
Defects of die castings
Other name: stripe.
Features: the surface of the casting presents a smooth pattern of local subsidence which is consistent with the flow direction of liquid metal and can be felt by hand. This defect has no development direction and can be removed by polishing method.
Causes: 1. Marks left by two metal flows filling the cavity out of sync. 2. Low mold temperature, such as zinc alloy mold temperature lower than 150 ℃ and aluminum alloy mold temperature lower than 180 ℃, are prone to such defects. 3. Filling speed is too high. 4. Excessive amount of paint.
Troubleshooting: 1. Adjust the sectional area or position of the inner gate. 2. Adjust the mold temperature and increase the overflow tank. 3. The filling speed is properly adjusted to change the flow pattern of liquid metal filling cavity. 4. The coating shall be thin and uniform.
Cold water barrier
Other names: cold joint (butt joint), water grain.
Features: the gap caused by the butt joint but non fusion of metal flows with low temperature presents an irregular linear shape, including penetrating and non penetrating. It has a development trend under the action of external force.
Causes: 1. The pouring temperature of molten metal is low or the mold temperature is low. 2. The alloy composition does not meet the standard and the fluidity is poor. 3. The liquid metal is filled separately, and the fusion is poor. 4. The gate is unreasonable and the process is too long. 5. Low filling speed or poor exhaust. 6. The specific pressure is low.
Troubleshooting measures: 1. Appropriately increase the pouring temperature and mold temperature. 2. Change the alloy composition and improve the fluidity. 3. Improve the gating system, increase the inner gate speed and improve the filling conditions. 4. Improve the overflow conditions and increase the overflow flow. 5. Increase the injection speed and improve the exhaust conditions. 6. Increase the specific pressure.
Other names: tensile force, pull mark, die sticking mark.
Features: along the demoulding direction, due to metal adhesion, the mold manufacturing slope is too small, resulting in the scratch trace on the casting surface, which becomes the scratch surface in serious cases.
Causes: 1. The casting inclination of core and wall is too small or inverted. 2. There are pressure scars on the core and wall. 3. Alloy adhesion die. 4. Casting ejection deflection, or core axis deflection. 5. The wall surface is rough. 6. Paint is often not sprayed. 7. The iron content in aluminum alloy is less than 0.6%.
Troubleshooting: 1. Correct the mold to ensure the manufacturing slope. 2. Polish the indentation. 3. Reasonably design the gating system to avoid metal flow against the core and wall, and appropriately reduce the filling speed. 4. Correct the die structure. 5. Polish the surface. 6. The coating dosage is thin and uniform, and the coating shall not be missed. 7. Appropriately increase the iron content to 0.6 ~ 0.8%.
Other names: concavity, depression, suffocation and edge collapse.
Features: the concave part on the smooth surface of the casting, and its surface is in a natural cooling state.
Causes: 1. The structural design of the casting is unreasonable, and there are local thick parts, resulting in hot spots. 2. The alloy has large shrinkage. 3. The sectional area of the inner gate is too small. 4. Lower than. 5. Mold temperature is too high.
Elimination measures: 1. Improve the casting structure to make the wall thickness slightly uniform. The joints with large thickness difference should gradually ease the transition and eliminate hot spots. 2. Select the alloy with small shrinkage. 3. Properly set the gating system and appropriately increase the cross-sectional area of the inner gate. 4. Increase the injection force. 5. Properly adjust the heat balance conditions of the die, adopt temperature control device and cooling, etc.
Other name: bubbling.
Features: bubbles formed by gas accumulation and bulging under the skin of the casting.
Causes: 1. The mold temperature is too high. 2. The filling speed is too high and the metal flow is involved in too much gas. 3. The coating has large gas output, excessive dosage and is not burned out before pouring, so that the volatile gas is wrapped on the surface of the casting. 4. The exhaust is not smooth. 5. Premature mold opening. 6. Alloy melting temperature is too high.
Troubleshooting: 1. Cool the mold to the working temperature. 2. Reduce the injection speed and avoid vortex aeration. 3. The coating with low gas output shall be selected, the dosage shall be thin and uniform, and the mold shall be closed after burning out. 4. Clean and add overflow trough and exhaust duct. 5. Adjust the mold retention time. 6. Trim the smelting process.
Other names: air hole and air eye.
Features: holes with regular shape and smooth surface formed by the gas involved in the die casting.
Causes: mainly caused by wrapped gas: 1. Improper gate position selection and diversion shape lead to front impact and vortex when liquid metal enters the cavity. 2. Poor runner shape design. 3. The pressure chamber is not full enough. 4. The inner gate speed is too high, resulting in turbulence. 5. Poor exhaust. 6. The mold cavity is too deep. 7. There is too much paint and it is not burnt out before filling. 8. The charge is not clean and the refining is poor. 9. Machining allowance is too large.
Elimination measures: 1. Select the gate position and diversion shape conducive to the elimination of gas in the mold cavity to prevent the metal liquid from sealing the overflow system on the parting surface first. 2. The nozzle cross-sectional area of the sprue shall be as large as possible than that of the inner gate. 3. Improve the fullness of the pressure chamber, select a smaller pressure chamber as far as possible and adopt quantitative pouring. 4. Under the condition of good molding, the inner gate thickness is increased to reduce the filling speed. 5. The overflow groove and exhaust passage shall be set at the last filling part of the cavity, and the overflow groove and exhaust passage shall be prevented from being closed by liquid metal. 6. An exhaust plug is set at the deep cavity to increase exhaust in the form of splicing. 7. The amount of coating is thin and uniform. It is filled after burning out, and the coating with low gas generation is adopted. 8. The furnace charge must be treated clean and dry, and the smelting process must be strictly followed. 9. Adjust the conversion point of injection speed, slow injection speed and fast injection speed. 10. Lower pouring temperature and increase specific pressure.
Other names: shrink eye and empty.
Features: irregular shape and rough surface holes caused by insufficient internal compensation during condensation of die casting.
Causes: 1. The alloy pouring temperature is too high. 2. The wall thickness of the casting structure is uneven, resulting in hot spots. 3. The specific pressure is too low. 4. The overflow tank has insufficient capacity and the overflow outlet is too thin. 5. The filling degree of the pressure chamber is too small, and the residual material (material cake) is too thin, so the final feeding cannot play a role. 6. The inner gate is small. 7. The local temperature of the die is too high.
Troubleshooting measures: 1. Comply with alloy melting specifications. The overheating time of alloy liquid is too long and the pouring temperature is reduced. 2. Improve the casting structure, eliminate metal accumulation, uniform wall thickness and slow transition. 3. Properly increase the specific pressure. 4. Increase the capacity of overflow tank and thicken the overflow port. 5. To improve the fullness of the pressure chamber, quantitative pouring is adopted. 6. Properly improve the gating system to facilitate good pressure transmission.
Features: the smooth stripes on the surface of the casting can be seen by the naked eye, but can not be felt by hand. The color is different from the veins of the base metal, which can be removed by wiping with 0# abrasive cloth.
Causes: 1. The filling speed is too fast. 2. Too much paint. 3. The mold temperature is low.
Troubleshooting: 1. Reduce the injection speed as much as possible. 2. The coating dosage is thin and uniform. 3. Raise the mold temperature.
Features: the alloy matrix on the casting is damaged or broken to form a filament like gap. There are two kinds of penetrating and non penetrating, with a development trend.
Cracks can be divided into cold cracks and hot cracks. The main difference between them is that the metal at the crack of cold crack casting is not oxidized, and the metal at the crack of hot crack casting is oxidized.
Causes: 1. The casting structure is unreasonable, the shrinkage is hindered, and the casting fillet is too small. 2. The core pulling and ejection device deflects and bears uneven force during operation. 3. The mold temperature is low. 4. Mold opening and core pulling time is too late. 5. Improper alloy selection or excessive harmful impurities will reduce the plasticity of the alloy. Zinc alloy: lead, tin, cadmium and iron are on the high side. Aluminum alloy: zinc, copper and iron are on the high side. Copper alloy: zinc and silicon are on the high side. Magnesium alloy: aluminum, silicon and iron are on the high side.
Elimination measures: 1. Improve the casting structure, reduce the wall thickness difference and increase the casting fillet. 2. Correct the die structure. 3. Increase the working temperature of the die. 4. Shorten the mold opening and core pulling time. 5. Strictly control harmful impurities, adjust alloy composition, comply with alloy melting specifications or re select alloy brand.
Other names: insufficient pouring, unclear outline and incomplete corners.
Features: the mold cavity is not filled with liquid metal, and there are parts with incomplete filling on the casting.
1. Poor alloy flow: (1) high gas content and serious oxidation of liquid metal, resulting in decreased fluidity.（2) Alloy pouring temperature and mold temperature are too low.（3) The inner gate speed is too low.（4) Insufficient nitrogen pressure in the accumulator.（5) The plenum is low.（6) Improper design such as too thin casting wall or great thickness difference.
2. Poor gating system: (1) improper selection of gate position, diversion mode and number of inner gate strands.（2) The sectional area of the inner gate is too small.
3. Poor exhaust conditions: (1) poor exhaust.（2) The paint is too much and has not been dried out.（3) The mold temperature is too high, the gas pressure in the mold cavity is high, and it is not easy to discharge.
Elimination measures: 1. Improve the fluidity of the alloy: (1) adopt the correct melting process to eliminate gas and non-metallic inclusions.（2) Properly increase the alloy pouring temperature and die temperature.（3) Increase the injection speed.（4) Supplement nitrogen to increase the effective pressure.（5) Quantitative pouring is adopted.（6) Improve the casting structure and properly adjust the wall thickness. 2. Improve the gating system: (1) correctly select the gate position and diversion mode, which is beneficial to the use of multi strand inner gate for non-good shape castings and large castings.（2) Increase the cross-sectional area of the inner gate or increase the injection speed. 3. Improve the exhaust conditions: (1) add overflow groove and exhaust duct, and vent plug can be set at the deep concave cavity.（2) The coating shall be thin and uniform, and the mold shall be closed after blowing dry and burning out.（3) Reduce the mold temperature to the working temperature.
Other names: push rod mark, insert or movable block splicing mark.
Features: convex and concave marks left on the casting surface due to the collision of mold cavity and the splicing of push rod, insert block, movable block and other parts.
Causes: 1. The push rod is not adjusted evenly or the end is worn. 2. Poor fit of the die cavity, sliding block splicing part and its movable part. 3. The push rod area is too small.
Troubleshooting: 1. Adjust the push rod to the correct position. 2. Fasten inserts or other moving parts to eliminate unnecessary concave convex parts. 3. Increase the area or number of push rods.
Other names: Net trace, net pattern, crack and burr.
Features: reticulated raised marks and metal thorns on the casting surface caused by thermal fatigue on the surface of the die cavity.
Causes: 1. For the traces caused by the surface crack of the mold cavity, the heat conduction near the inner gate area is the most concentrated, the friction resistance is the largest, the erosion of molten metal is the strongest, the cold and hot alternation is the most intense, and it is most likely to produce thermal crack and form crack. 2. Improper die material or incorrect heat treatment process. 3. The temperature difference between cold and hot dies varies greatly. 4. The pouring temperature of alloy liquid is too high and the preheating of die is not enough. 5. The surface roughness Ra of die cavity is too large. 6. The metal flow rate is too high and the front scouring wall.
Troubleshooting measures: 1. Correctly select mold materials and reasonable heat treatment process. 2. The die must be preheated to the working temperature range before die casting. 3. Reduce the alloy pouring temperature as much as possible. 4. Improve the surface quality of die cavity and reduce Ra value. 5. Insert shall be annealed regularly to eliminate stress. 6. The gating system shall be designed correctly, and the injection speed shall be as small as possible under the condition of good molding.
Other names: oil spots, black spots.
Features: spots on the surface of castings different from the base metal are generally formed by coating carbides.
Causes: 1. Impure coating or excessive dosage. 2. The paint contains too much graphite.
Elimination measures: 1. The coating shall be thin and uniform without accumulation, and shall be blown away with compressed air. 2. Reduce the graphite content in the coating or select graphite free water-based coating.
Features: a small pitted distribution area formed on the surface of the casting under the condition of approximate under pressure due to the low mold temperature or alloy liquid temperature during mold filling.
Causes: 1. During filling, the metal is dispersed into dense droplets and hits the wall at high speed. 2. The thickness of inner gate is too small.
Troubleshooting measures: 1. Correctly design the gating system to avoid splashing of liquid metal, improve the exhaust conditions, avoid excessive gas in the liquid flow, reduce the speed of the inner gate and increase the mold temperature. 2. Adjust the inner gate thickness properly.
Other name: drape joint.
Feature: sheet metal appearing on the edge of the casting.
Causes: 1. The clamping force of the machine is not adjusted well before injection. 2. The die and sliding block are damaged, and the locking element is invalid. 3. Die inserts and sliding blocks are worn. 4. Deformation caused by insufficient die strength. 5. The sundries on the parting surface are not cleaned up. 6. The calculation of the projected area is incorrect and exceeds the clamping force. 7. The injection velocity is too high and the pressure shock peak is too high.
Troubleshooting: 1. Check the clamping force or pressurization, and adjust the injection pressurization mechanism to reduce the peak value of injection pressurization. 2. Check the damage of the die slide and repair it to ensure that the locking element works. 3. Check for wear and repair. 4. Correctly calculate the die strength. 5. Remove sundries on the parting surface. 6. Correctly calculate and adjust the clamping force. 7. Adjust the injection speed properly.
Other name: septum.
Features: there are obvious metal layers on the casting.
Causes: 1. The rigidity of the mold is not enough, and the formwork shakes during the filling process of liquid metal. 2. The press chamber punch is not well matched with the press chamber, and the forward speed is not stable in the press firing. 3. Improper gating system design.
Troubleshooting measures: 1. Strengthen the die stiffness and fasten the die parts. 2. Adjust the injection punch and pressure chamber to ensure good fit. 3. Reasonably design the inner gate.
Features: the casting surface presents loose and loose macro structure.
Causes: 1. The mold temperature is too low. 2. Alloy pouring temperature is too low. 3. Smaller than pressure. 4. Too much paint.
Troubleshooting: 1. Increase the mold temperature to the working temperature. 2. Properly increase the alloy pouring temperature. 3. Increase the specific pressure. 4. The coating is thin and uniform.
Wrong side and wrong buckle
Other name: staggered joint.
Features: one part of the casting is staggered with the other part on the parting surface, resulting in relative displacement (called wrong thread for thread).
Causes: 1. Displacement of die insert. 2. Die guides worn. 3. Insert manufacturing error of two half dies.
Troubleshooting: 1. Adjust the insert and tighten it. 2. Replace the guide post and guide sleeve. 3. Trim to eliminate errors.
Other names: twist and warpage.
Features: the overall deformation of the casting whose geometry is inconsistent with the design requirements.
Causes: 1. Poor structural design of castings, resulting in uneven shrinkage. 2. Premature mold opening and insufficient rigidity of castings. 3. Casting slope is too small. 4. Improper operation of taking and placing castings. 5. The push rod position is not arranged properly.
Elimination measures: 1. Improve the casting structure to make the wall thickness uniform. 2. Determine the best mold opening time and strengthen the rigidity of the casting. 3. Zoom in on the casting slope. 4. The casting shall be handled with care. 5. The casting shall be stacked in a special box, and the gate removal method shall be appropriate. 6. Some deformed castings can be eliminated by shaping.
Features: the damage caused by the impact on the casting surface.
Causes: 1. Accidental bumps during gate removal, cleaning, correction and handling.
Troubleshooting measures: 1. Be careful when cleaning the castings. The castings shall not be stacked or collided with each other. Special storage and transportation boxes shall be used.
Other names: oxidation inclusion and slag inclusion.
Features: there are fine particles or blocks with hardness higher than that of metal matrix in the casting matrix, which makes processing difficult and tool wear serious. After processing, hard points with different brightness are often displayed on the casting.
Cause: metal or non-metallic substances harder than the base metal are mixed or precipitated in the alloy, such as Al2O3 and free silicon.
1. Alumina (Al2O3): (1) aluminum alloy is not refined well（ 2) Oxide was mixed during pouring.
2. The complex compound composed of aluminum, iron, manganese and silicon is mainly formed by mnal3 at the colder part of the molten pool, then Fe precipitates with mnal3 as the core, and silicon participates in the reaction to form the compound. 3. Free silicon mixture: (1) aluminum silicon alloy has high silicon content（ 2) When Al Si alloy is poured in semi liquid state, there is free silicon.
Elimination measures: 1. Unnecessary agitation and overheating shall be reduced during smelting to keep the purity of the alloy liquid. When the aluminum alloy liquid is kept warm in the furnace for a long time, it shall be periodically refined and degassed. 2. When the aluminum alloy contains titanium, manganese, iron and other components, do not make segregation and keep it clean. Refine with dry refining agent, but pay attention to compensation when the aluminum alloy contains magnesium. 3. When the aluminum alloy contains a large amount of copper and iron, the silicon content should be reduced to less than 10.5%, and the pouring temperature should be appropriately increased to precipitate silicon first.
Features: the base metal grain of the casting is too coarse or fine, which makes the casting easy to break or smash.
Causes: 1. The alloy liquid is overheated too much or the holding time is too long. 2. Intense supercooling and fine crystallization. 3. There are too many impurities such as zinc and iron in aluminum alloy. 4. The content of copper in aluminum alloy exceeds the specified range.
Elimination measures: 1. The alloy should not be overheated to avoid long-term thermal insulation of the alloy. 2. Increase the mold temperature and reduce the pouring temperature. 3. Strictly control the chemical composition of the alloy. 4. Keep the coating layer of crucible intact.
Features: water leakage, air leakage or water seepage of die casting parts after test.
Causes: 1. Insufficient pressure. 2. The gating system design is unreasonable or the casting structure is unreasonable. 3. Improper alloy selection. 4. Poor exhaust.
Troubleshooting measures: 1. Increase the specific pressure. 2. Improve gating system and exhaust system. 3. Select good alloy. 4. Avoid machining as much as possible. 5. Castings shall be impregnated.
Features: through chemical analysis, the alloy elements of the casting do not meet the requirements or there are too many impurities.
Causes: 1. Incorrect ingredients. 2. Raw materials and returned materials can be put into use without analysis.
Elimination measures: 1. The furnace charge can be used only after chemical analysis. 2. The furnace charge shall be strictly managed, and the old and new charge shall be allocated according to a certain proportion. 3. Strictly follow the smelting process. 4. Melting tools shall be painted.
Features: the mechanical strength and elongation of casting alloy are lower than the required standards.
Causes: 1. The chemical composition of the alloy does not meet the standard. 2. There are pores, shrinkage cavities, slag inclusions, etc. in the casting. 3. Unequal treatment of samples. 4. The unreasonable casting structure limits the casting to meet the standard. 5. Improper smelting process.
Elimination measures: 1. Chemical composition and impurity content shall be strictly controlled during batching and melting. 2. Strictly follow the smelting process. 3. Samples shall be made as required, and the castings shall be subject to process test regularly in production. 4. Strictly control the alloy melting temperature and pouring temperature, and try to eliminate various factors of alloy oxide formation.
Factors affecting die casting quality
The control of die casting die surface temperature is very important for the production of high-quality die castings. Uneven or inappropriate mold temperature will also lead to unstable casting size, ejection casting deformation in the production process, resulting in defects such as thermal pressure, die sticking, surface depression, internal shrinkage cavity and hot bubble. When the mold temperature difference is large, it has different effects on the variables in the production cycle, such as filling time, cooling time and spraying time.
The die casting part is not formed and the cavity is not fully filled.
Causes: 1. The temperature of die casting die is too low; 2. Low temperature of molten metal; 3. The press pressure is too low; 4. Insufficient liquid metal; The injection speed is too high; 5. The air can’t be discharged.
Adjustment methods: 1-2. Increase the temperature of die casting die and liquid metal; 3. Replace high pressure die casting machine; 4. Add enough liquid metal to reduce the injection speed and increase the thickness of the feed inlet.
Design of die castings
The design of die castings must take into account seven aspects: the wall thickness of die castings, the casting fillet and demoulding slope of die castings, stiffeners, the casting holes on die castings and the minimum distance from the holes to the edges, the rectangular holes and grooves on die castings, the inserts in die castings, and the machining allowance of die castings .
Code for design of cast fillet
Generally, each part of the die casting shall be intersected with rounded corners (except at the parting surface), which can make the metal flow stable during filling, and the gas can be easily discharged, and the crack caused by acute angle can be avoided. For die castings requiring electroplating and coating, the fillet can be coated evenly to prevent coating accumulation at sharp corners. Generally, the fillet radius r of die castings should not be less than 1mm, and the minimum fillet radius is 0.5mm.
Code for design of inserts in die castings
Firstly, the number of inserts on die castings should not be too large; Secondly, the connection between the insert and the die casting must be firm. At the same time, it is required to slot, bulge, knurling, etc. on the insert; Thirdly, the insert must avoid sharp corners to facilitate placement and prevent stress concentration of the casting. If there is serious electrochemical corrosion between the casting and the insert, the surface of the insert needs coating protection; Finally, heat treatment should be avoided for castings with inserts to avoid volume change caused by phase transformation of two metals and loosening of inserts.
Design specification for wall thickness of die castings
Thin wall die castings have higher strength and better compactness than thick wall die castings. In view of this, the design of die castings should follow this principle: on the premise of ensuring that the castings have sufficient strength and rigidity, the wall thickness should be reduced as much as possible and the wall thickness should be kept uniform. Practice has proved that the wall thickness design of die-casting parts is generally 2.5-4mm, and the parts with wall thickness more than 6mm should not be produced by die-casting process. The influence of too thick and thin wall of die casting on casting quality: if the casting wall is too thin in the design, the metal fusion will be poor, which will directly affect the casting strength and cause difficulties in forming; When the wall is too thick or seriously uneven, shrinkage and cracks are easy to occur. On the other hand, with the increase of wall thickness, defects such as porosity and shrinkage porosity in the casting also increase, which will also reduce the strength of the casting and affect the quality of the casting.
Design specification for machining allowance of die casting parts
Generally, due to the limitations of die-casting process, when some dimensional accuracy, surface roughness or geometric tolerance of die-casting parts can not meet the requirements of product drawings, enterprises should first consider adopting finishing methods such as correction, polishing, extrusion and shaping for repair. When finishing can not completely solve these problems, Some parts of die castings should be machined. It should be noted that small machining allowance should be considered when machining, and the surface not affected by parting surface and movable forming should be taken as the blank datum plane as far as possible, so as not to affect the machining accuracy.
Code for design of demoulding slope of die castings
When designing die-casting parts, there should be a structural slope on the structure. If there is no structural slope, there must be a demoulding process slope where necessary. The direction of the slope must be consistent with the demoulding direction of the casting.
Metal materials of die castings
The main die casting alloy materials are tin, lead, zinc, aluminum, magnesium, copper, etc. Zinc and aluminum alloys are the most widely used, followed by magnesium and copper alloys. At present, zinc, aluminum and magnesium alloys are mainly used in the electronic industry.
Die cast zinc alloy
The melting state of zinc alloy has very good fluidity and low melting point, only 365 degrees. It can carry out various post-processing treatments, such as electroplating, etc. Therefore, it is widely used. It was also used in the mobile phone industry before, but it is gradually eliminated by aluminum alloy because of its large ratio and poor corrosion resistance. The bathroom industry is the most used. There are 2, 3, 4, 5 and 7 alloys commonly used, and the most widely used is 3 alloy.
Die cast aluminum alloy
Aluminum alloy has low density but high strength. It is close to or higher than high-quality steel and has good plasticity. It can be processed into various profiles. It has excellent conductivity, thermal conductivity and corrosion resistance. It is widely used in industry, and its usage is only second to that of steel. The specific weight of aluminum alloy is only about 1 / 3 of that of iron, so it is very important in all walks of life, especially in the lightweight process of aircraft, ships, automobiles and electronic instruments.
Al Si Cu system is commonly used, ADC12 (A383), ADC10 (A380), etc. In addition, there are Al Si system, Al Si mg system and Al Mg system. Also known as aluminum titanium alloy, it is improved by adding titanium metal to the above materials. Adding titanium metal can significantly improve the mechanical properties of the products.
Die cast magnesium alloy
Magnesium alloy is an alloy based on magnesium and added with other elements. It is characterized by low density, high specific strength, large specific elastic modulus, good heat dissipation, good shock absorption, greater impact load bearing capacity than aluminum alloy, and good corrosion resistance to organic matter and alkali. The specific gravity of magnesium is about 2/3 of aluminum and 1/4 of iron. Therefore, it is widely used in mobile phones, communication equipment and other electronic equipment, auto parts and medical devices.
Magnesium aluminum alloy is the most widely used, followed by magnesium manganese alloy and magnesium zinc zirconium alloy. The commonly used brands are AZ91D, AM60B, AM50A, AS41B, etc.
Advantages of using resin sand casting machine bed castings:
- 1) The resin sand mold has good stiffness and high sand mold strength at the initial stage of pouring, which makes it possible to effectively eliminate shrinkage cavity and porosity defects by using the graphitization expansion in the solidification process of cast iron, so as to realize less riser and no riser casting of gray iron and nodular iron.
- 2) Polystyrene foam pattern is used in full mold casting production, and furan resin self hardening sand is used for molding. When the molten metal is poured into the mold, the foam pattern rapidly vaporized and burned away under the action of high temperature molten metal. The metal liquid replaced the position occupied by the original foamed plastics and cooled into a solid casting with the same shape.
- 3) Relatively speaking, lost foam casting has great advantages over traditional sand molds in the production of single or small batch automobile panels, machine bed and other large molds. It not only saves the expensive wood mold cost, but also is easy to operate, shortens the production cycle, improves the production efficiency, and has the advantages of high dimensional accuracy, small machining allowance and good surface quality; In addition, the machine tool processing of castings has good wear resistance, shock absorption and good process performance.
Engraving machine body casting
Pouring technology of machine tool bed casting
In the pouring process production of machine tool bed castings, the principle of high temperature discharge and low temperature pouring should be followed. Because increasing the discharge temperature of molten metal is conducive to the complete melting of inclusions and the floating of molten slag, which is convenient for slag removal and degassing, and reduces the slag inclusion and pore defects of machine tool castings; A lower pouring temperature is conducive to reduce the gas solubility and liquid shrinkage in the liquid metal and the baking of the cavity surface by the high-temperature liquid metal, so as to avoid defects such as pores, sand sticking and shrinkage. Therefore, on the premise of filling the mold cavity, try to use a lower pouring temperature. The process of injecting molten metal from the ladle into the mold is called pouring. Improper pouring operation will cause defects of machine tool castings such as insufficient pouring, cold shut, air hole, shrinkage cavity and slag inclusion, and cause personal injury.
Instructions for use of castings
The theory of casting forming metal liquid forming is often called casting, and the casting forming technology has a long history. As early as 5000 years ago, our ancestors were able to cast red copper and bronze products. Casting is the most widely used liquid metal forming process. It is a method of pouring liquid metal into the mold cavity and obtaining a blank or part of a certain shape after it is cooled and solidified.
In machinery and equipment, liquid molded parts account for a large proportion, and in machine tools, internal combustion engines, mining machinery and heavy machinery, liquid molded parts account for 70% ~ 90% of the total weight; It accounts for 50% ~ 70% of automobiles and tractors; It accounts for 40% ~ 70% of agricultural machinery. The liquid forming process can be so widely used because it has the following advantages:
- (1) It can produce blanks with complex inner cavity and shape. Such as various boxes, machine bed, cylinder block, cylinder head, etc.
- (2) Process flexibility and wide adaptability. The size of liquid molded parts is almost unlimited, its weight can range from a few grams to hundreds of tons, and its wall thickness can range from 0.5mm to 1m. In industry, all metal materials that can dissolve into liquid can be used for liquid forming. For cast iron with poor plasticity, liquid forming is the only way to produce its blank or parts.
- (3) The cost of liquid molded parts is low. Liquid forming can directly use waste parts and chips, and the equipment cost is low. At the same time, the machining allowance of liquid formed parts is small and saves metal.
However, there are many processes of metal liquid forming, and it is difficult to accurately control, which makes the casting quality unstable. Compared with forgings made of the same material, the liquid forming structure is loose and the grain is coarse
It is easy to produce shrinkage, porosity, porosity and other defects. Its mechanical properties are low. In addition, the labor intensity is high and the conditions are poor. It has excellent mechanical and physical properties. It can have a variety of comprehensive properties with different strength, hardness and toughness. It can also have one or more special properties, such as wear resistance, high temperature resistance, low temperature resistance, corrosion resistance, etc.
The weight and size range of castings are very wide. The lightest weight is only a few grams, the heaviest can reach 400 tons, the thinnest wall thickness is only 0.5 mm, the thickest can exceed 1 meter, and the length can range from a few millimeters to more than ten meters, which can meet the use requirements of different industrial departments.
Casting has been widely used in hardware and the whole mechanical and electronic industry, and its use is becoming a growing trend. Specifically used in construction, hardware, equipment, construction machinery and other large machinery, machine tools, ships, aerospace, automobiles, locomotives, electronics, computers, electrical appliances, lamps and other industries, many of which are metal objects that ordinary people contact all day but don’t understand.
Quality of castings
It mainly includes appearance quality, internal quality and use quality. Appearance quality refers to surface roughness, surface defects, dimensional deviation, shape deviation and weight deviation of castings; Internal quality mainly refers to the chemical composition, physical properties, mechanical properties, metallographic structure, holes, cracks, inclusions and segregation in the casting; Service quality refers to the working durability of castings under different conditions, including wear resistance, corrosion resistance, quench heat resistance, fatigue, shock absorption and other properties, as well as machinability, weldability and other process properties.
Casting quality has a great influence on the performance of mechanical products. For example, the wear resistance and dimensional stability of machine tool castings directly affect the accuracy and service life of machine tools; The size, accuracy and surface roughness of impeller, shell and inner cavity of hydraulic parts of various pumps directly affect the working efficiency of pumps and hydraulic systems, energy consumption and the development of cavitation; The strength and heat shock resistance of castings such as cylinder block, cylinder head, cylinder liner, piston ring and exhaust pipe of internal combustion engine directly affect the service life of the engine.
There are many factors affecting the quality of castings. The first is the design process of castings. During the design, in addition to determining the geometry and size of the casting according to the working conditions and metal material properties, the rationality of the design must also be considered from the perspective of casting alloy and casting process characteristics, that is, obvious size effect and solidification, shrinkage, stress and other problems, so as to avoid or reduce the occurrence of defects such as component segregation, deformation and cracking of the casting. Second, there should be a reasonable casting process. That is, according to the casting structure, weight and size, casting alloy characteristics and production conditions, select the appropriate parting surface, modeling and core making method, and reasonably set the casting reinforcement, cold iron, riser and gating system. To ensure high quality castings. The third is the quality of raw materials for casting. The quality of metal charge, refractory, fuel, flux, modifier, casting sand, molding sand binder, coating and other materials is not up to standard, which will cause defects such as porosity, pinhole, slag inclusion and sand sticking, affect the appearance and internal quality of the casting, and scrap the casting in serious cases. The fourth is process operation. It is necessary to formulate reasonable process operation procedures, improve the technical level of workers and correctly implement the process procedures.
In casting production, the quality of castings should be controlled and inspected. First of all, we should formulate process codes and technical conditions for the control and inspection of raw materials, auxiliary materials and each specific product. Each process shall be controlled and inspected in strict accordance with the process code and technical conditions. Finally, the quality of finished castings is inspected. Reasonable testing methods and appropriate testing personnel shall be equipped. Generally, for the appearance quality of castings, the surface roughness of castings can be judged by comparison samples; Minor cracks on the surface can be inspected by coloring method and magnetic particle method. For the internal quality of castings, audio, ultrasonic, eddy current, X-ray and γ X-ray and other methods to check and judge.
Defects of sand casting castings include cold shut, insufficient pouring, air hole, sand sticking, sand inclusion, sand hole, sand expansion, etc.
1) If the cold insulation and pouring are insufficient, the filling capacity of liquid metal is insufficient, or the filling condition is poor, the liquid metal will stop flowing before the cavity is filled, which will cause the casting to have insufficient pouring or cold insulation defects. Insufficient pouring will make the casting unable to obtain a complete shape; During cold insulation, although the casting can obtain a complete shape, the mechanical properties of the casting are seriously damaged due to incomplete fusion joints.
Prevent insufficient pouring and cold shut: improve pouring temperature and pouring speed.
2) The pore gas does not escape in time before the liquid metal crusts, and the pore defects are generated in the casting. The inner wall of pores is smooth, bright or with slight oxidation color. The effective bearing area of the casting will be reduced when there are pores in the casting, and stress concentration will be caused around the pores, which will reduce the impact resistance and fatigue resistance of the casting. Porosity also reduces the compactness of castings, resulting in the scrapping of some castings requiring hydrostatic test. In addition, porosity also has a bad effect on the corrosion resistance and heat resistance of castings.
Prevent the generation of pores: reduce the air content in the molten metal, increase the air permeability of the sand mold, and add an air outlet riser at the highest part of the mold cavity.
3) Bonded sand is a layer of sand that is difficult to remove on the surface of bonded sand castings. Sand sticking not only affects the appearance of castings, but also increases the workload of casting cleaning and cutting, and even affects the service life of the machine. For example, it is easy to be damaged when there is sand on the surface of casting teeth. If there is sand in machine parts such as pump or engine, it will affect the flow of fluid such as fuel oil, gas, lubricating oil and cooling water, and pollute and wear the whole machine.
Prevention of sand sticking: add pulverized coal into the molding sand and apply anti sand coating on the mold surface.
4) The groove and scar defects formed by sand inclusion on the surface of castings are easy to occur when wet casting thick and large flat castings.
Most of the parts with sand inclusion in the casting are in contact with the upper surface of the sand mold. The upper surface of the mold cavity is easy to arch and warp due to the radiant heat of the metal liquid. When the warped sand layer is continuously scoured by the metal liquid flow, it may break and remain in place or be brought into other parts. The larger the upper surface of the casting, the greater the volume expansion of molding sand and the greater the tendency to form sand inclusion.
5) Sand holes are holes filled with molding sand inside or on the surface of castings.
6) During sand expanding pouring, under the pressure of liquid metal, the mold wall moves and the casting expands locally. In order to prevent sand expansion, the strength of the sand mold, the stiffness of the sand box, the box pressing force or fastening force when closing the box should be increased, and the pouring temperature should be appropriately reduced to make the surface of the molten metal crust early, so as to reduce the pressure of the molten metal on the mold.
Quality inspection of castings
The inspection of castings mainly includes dimensional inspection, visual inspection of appearance and surface, chemical composition analysis and mechanical property test. For castings with important requirements or prone to problems in casting process, nondestructive testing is also required. The nondestructive testing technologies that can be used for the quality inspection of nodular iron castings include liquid penetrant testing, magnetic particle testing, eddy current testing Radiographic testing, ultrasonic testing and vibration testing, etc.
Detection of surface and near surface defects of castings
1) Liquid penetrant testing
Liquid penetrant testing is used to inspect various opening defects on the surface of castings, such as surface cracks, surface pinholes and other defects that are difficult to find by naked eyes. The commonly used penetrant testing is dye testing, which is to wet or spray the colored (generally red) liquid (penetrant) with high permeability on the casting surface, infiltrate the penetrant into the opening defects, quickly wipe off the surface penetrant layer, and then spray the easy to dry display agent (also known as developer) on the casting surface, After the penetrant remaining in the opening defect is sucked out, the display agent is dyed, so as to reflect the shape, size and distribution of the defect. It should be pointed out that the accuracy of penetrant testing decreases with the increase of the surface roughness of the tested material, that is, the brighter the surface, the better the detection effect. The surface polished by the grinder has the highest detection accuracy, and even intergranular cracks can be detected. In addition to dye detection, fluorescent penetrant detection is also a common liquid penetrant detection method. It needs to be equipped with ultraviolet lamp for irradiation observation, and the detection sensitivity is higher than that of dye detection.
2) Eddy current testing
Eddy current testing is applicable to the inspection of defects less than 6 ~ 7mm deep below the surface. Eddy current testing is divided into two types: placement coil method and through coil method. When the test piece is placed near the coil with alternating current, the alternating magnetic field entering the test piece can induce eddy current (eddy current) flowing in the direction perpendicular to the excitation magnetic field in the test piece. The eddy current will produce a magnetic field opposite to the direction of the excitation magnetic field, partially reducing the original magnetic field in the coil, resulting in the change of coil impedance. If there are defects on the casting surface, the electrical characteristics of eddy current will be distorted to detect the existence of defects. The main disadvantage of eddy current testing is that it can not directly display the size and shape of the detected defects. Generally, it can only determine the surface position and depth of the defects. In addition, its detection sensitivity to small opening defects on the workpiece surface is not as sensitive as penetrant testing.
3) Magnetic particle testing
Magnetic particle testing is suitable for testing surface defects and defects several millimeters deep below the surface. It requires DC (or AC) magnetization equipment and magnetic particle (or magnetic levitation liquid) for testing operation. Magnetization equipment is used to generate magnetic field on the inner and outer surfaces of castings, and magnetic particle or magnetic suspension fluid is used to show defects. When a magnetic field is generated within a certain range of the casting, the defects in the magnetized area will produce a magnetic leakage field. When the magnetic particle or suspension is sprinkled, the magnetic particle is absorbed, so that the defects can be displayed. The defects displayed in this way are basically the defects that cross cut the magnetic line of force, but the long strip defects parallel to the magnetic line of force cannot be displayed. Therefore, the magnetization direction needs to be constantly changed during operation to ensure that all defects in the unknown direction can be detected.
Inspection of internal defects of castings
For internal defects, the commonly used nondestructive testing methods are radiographic testing and ultrasonic testing. Among them, the effect of radiographic testing is the best. It can get an intuitive image reflecting the type, shape, size and distribution of internal defects. However, for large castings with large thickness, ultrasonic testing is very effective. It can accurately measure the location, equivalent size and distribution of internal defects.
1) Radiographic testing (micro focus Xray)
X-ray testing, generally X-ray or γ As the ray source, the ray generating equipment and other auxiliary facilities are required. When the workpiece is irradiated in the ray field, the radiation intensity of the ray will be affected by the internal defects of the casting. The radiation intensity emitted through the casting varies locally with the size and nature of the defect, forming a radiographic image of the defect, which is imaged and recorded by radiographic film, or detected and observed in real time by fluorescent screen, or detected by radiation counter. Among them, the method of radiographic film imaging recording is the most commonly used method, that is, commonly known as radiographic detection. The defect image reflected by radiography is intuitive, and the defect shape, size, quantity, plane position and distribution range can be presented. Only the defect depth can not be reflected generally, so it can be determined only by taking special measures and calculation. The application of radiographic computer tomography in the international casting network can not be popularized because of its expensive equipment and high cost, but this new technology represents the future development direction of high-definition radiographic testing technology. In addition, the micro focus X-ray system using an approximate point source can actually eliminate the fuzzy edges generated by large focus devices and make the image contour clear. The use of digital image system can improve the signal-to-noise ratio of the image and further improve the image definition.
2) Ultrasonic testing
Ultrasonic testing can also be used to check internal defects. It uses the sound beam with high-frequency sound energy to reflect when it touches the internal surface or defects. The reflected sound energy is a function of the directivity and nature of the inner surface or defect and the acoustic impedance of this reflector. Therefore, the sound energy reflected by various defects or inner surfaces can be used to detect the location, wall thickness or depth of defects under the surface. As a widely used nondestructive testing method, ultrasonic testing has the following main advantages: high detection sensitivity and can detect small cracks; It has large penetration ability and can detect thick section castings. Its main limitations are: it is difficult to interpret the reflection waveform of disconnected defects with complex contour size and poor directivity; Undesirable internal structures, such as grain size, microstructure, porosity, inclusion content or fine dispersed precipitates, also hinder waveform interpretation; In addition, it is necessary to refer to the standard test block during testing.
How to repair casting defects?
The most fundamental focus of the method to solve the shrinkage defects of castings is “heat balance”. The method is:
- (1) Rapid solidification is carried out at the thick and hot joints formed by the machine tool casting structure, which artificially causes the basic balance of the temperature field of the machine tool casting. Adopt internal and external cold iron, and locally adopt zircon sand with large heat storage capacity, chromite ore or special coating.
- (2) Reasonable process design. The internal sprue is located at the relative Pu wall of the machine tool casting, which is more and scattered for several hours. The metal liquid that first enters the thick wall solidifies first and then solidifies at the thin wall, so that all parts basically achieve balanced solidification. For machine tool castings with uniform wall thickness, multiple ingates and air vents are used. There are many internal sprues, which are dispersed and evenly distributed to balance the overall heat. The air outlet holes are thin and many, which not only makes the exhaust unobstructed, but also plays the role of heat dissipation.
- (3) Change the position of the ingate.
- (4) The selection of modeling materials with large heat storage is very important for the production of anti-wear products with EPC! Chromite sand replaces other sand with small heat storage such as quartz sand, which will achieve good results, and micro earthquake is better after pouring.
- (5) Low temperature fast firing, open gating system. Make the molten metal fill the mold quickly, stably and evenly. This depends on the circumstances.
- (6) The machine tool mold strength of nodular iron is high, the surface hardness is ≥ 90, and the rigidity of sand box is large, which is beneficial to eliminate shrinkage porosity.
- (7) When the riser is required, the first hot riser shall be moved and leave the hot joint. If the riser is placed on the hot joint, the riser size will be increased to form “hot heating”. If it is not done well, not only the shrinkage is difficult to remove, but also centralized shrinkage will occur, which reduces the process yield.
- (8) Both tilting and alloying of the mold benefit. Eliminating shrinkage defects of machine tool castings is a complex understanding and implementation process. Based on the basic principle of “heat balance”, make a scientific analysis on the shell casting, formulate a reasonable process scheme, select appropriate molding materials, tooling and correct operation and standardization. Then any shrinkage defects of machine tool castings can be solved.
Due to the influence of many factors, defects such as pores, pinholes, slag inclusions, cracks and pits often appear. The commonly used repair equipment is argon arc welding machine, resistance welding machine, cold welding machine, etc. Casting defects with low quality and appearance requirements can be repaired by argon arc welding machines with high calorific value and high speed. However, in the field of defect repair of precision castings, due to the great influence of argon welding heat, secondary defects such as casting deformation, hardness reduction, sand hole, local annealing, cracking, pinhole, wear, scratch, undercut, or insufficient bonding force and internal stress damage will be caused during repair. The cold welding machine just overcomes the above shortcomings. Its advantages are mainly that the heat affected area is small, the casting does not need preheating and cold welding repair at room temperature, so it has no deformation, undercut and residual stress, does not produce local annealing and does not change the metal structure state of the casting. Therefore, the cold welding machine is suitable for repairing surface defects of precision castings. The scope of cold welding repair is Φ 1.5- Φ. In the process of repeated melting and accumulation of 1.2mm weld repair points, the repair efficiency is the only factor restricting its wide application in the process of large-area defect repair. For large defects, the combined application of traditional welding repair process and casting defect repair machine is recommended. But sometimes we don’t have many defects, so it’s unnecessary to invest a large cost. We can repair them with some repair agents, which is convenient and simple. For example, for iron materials, we can use JS902 to repair them. If we can’t use them up, we can use them later, which can save costs for our manufacturers, Let our foundry invest more money in improving the quality of products and let users create more wealth.
Casting quality inspection results
Casting quality inspection results are usually divided into three categories: qualified products, repaired products and scrap products.
- 1) Qualified products refer to castings whose appearance quality and internal quality meet relevant standards or technical conditions for delivery acceptance;
- 2) Repaired products refer to castings whose appearance quality and internal quality do not fully meet the standards and acceptance conditions, but are allowed to be repaired. After repair, they can meet the requirements of standards and technical conditions for delivery acceptance of castings;
- 3) Scrap refers to castings whose appearance quality and internal quality are unqualified and are not allowed to be repaired, or castings still fail to meet the requirements of standards and casting delivery acceptance technical conditions after repair. Waste products are divided into internal waste and external waste. Internal waste refers to the waste castings found in the foundry or foundry workshop; External waste refers to the waste found after delivery of castings, which causes much greater economic loss than internal waste.
Factors affecting solidification mode of castings
There are many solidification methods for castings. In the process of solidification, the section of casting is generally divided into three areas: 1 – solid phase area. 2 – solidification zone. 3 – liquid phase area. The width of the solidification zone has a great influence on the solidification zone, and the solidification mode is divided according to this. First, intermediate solidification: the solidification of most alloys is between layer by layer solidification and paste solidification. Second, layer by layer solidification: pure metals and eutectic alloys have no solidification zone in the solidification process. The liquid and solid phases in the section are clearly separated by a boundary. With the decrease of temperature, the solid phase layer increases and the liquid phase layer decreases, reaching the center. Third, paste solidification: the alloy crystallization temperature range is very wide. During a certain period of solidification, there is no solid layer on the casting surface, and the solidification zone runs through the whole section, paste first and then solidify. Relevant experts said that the factors affecting the solidification mode of castings are summarized as follows: first, the temperature gradient of castings. When the crystallization temperature range of the alloy is certain, the width of the solidification zone depends on the temperature gradient of the inner and outer layers of the casting. The smaller the temperature gradient, the wider the solidification zone（ Large internal and external temperature difference, fast cooling and narrow solidification zone). Second, the crystallization temperature range of the alloy. Small range: the solidification zone is narrow, and the more inclined it is to solidify layer by layer. Such as sand casting, low carbon steel solidification layer by layer, high carbon steel paste solidification.
Casting defect repair agent is a two-component, mastic like, room temperature curing polymer resin adhesive
, polymetallic composite cold welding repair material with metal and alloy as reinforcing filler. It has high bonding strength with metal, and can basically preserve the same color. It has the characteristics of wear resistance, corrosion resistance and aging resistance. The cured material has high strength and no shrinkage, and can be machined. It has excellent properties such as wear resistance, oil resistance, water resistance and various chemical corrosion resistance. At the same time, it can withstand high temperature of 120 ℃.
Application of casting defect repair agent
Casting defect repair agent is a high-performance polymeric metal material compounded by a variety of alloy materials and modified toughened heat-resistant resin. It is suitable for the repair of various metal castings and the repair and bonding of pores, sand holes, pits, cracks, wear and corrosion of various castings with defects greater than 2mm. It is generally used to repair various casting defects with less strict color requirements, has high strength, and can be machined together with the substrate.
How to test the hardness value of castings
The properties of castings directly affect the processing quality, and the hardness value is an important index to determine the processing of castings.
- 1) Brinell hardness: it is mainly used to measure the hardness of castings, forgings, non-ferrous metal parts, hot rolled billets and annealed parts, and the measurement range is ≯ HB450.
- 2) Rockwell hardness: HRA is mainly used for high hardness test pieces to measure the hardness of materials and surfaces with hardness higher than hrc67, such as cemented carbide, nitrided steel, etc. the measurement range HRA is > 70. HRC is mainly used for hardness measurement of steel parts (such as carbon steel, tool steel, alloy steel, etc.) after quenching or tempering. The measurement range is HRC20 ~ 67.
- 3) Vickers hardness: it is used to measure the hardness of thin parts and steel plates, and also to measure the hardness of case hardened parts such as carburizing, cyaniding and nitriding.
- 1) Before work, check whether the furnace body, furnace door, trailer, steel wire rope pulley, blower and other equipment are normal, and whether there are unsafe factors such as collapse, fracture and cracking. If any, it shall be handled properly before starting the furnace. When annealing in electric furnace, the oxide scale and metal on or near the resistance wire shall be carefully cleaned.
- 2) When burning a fire, carefully check the explosives in the coal. In the process of air stop, the coal seam shall not be thickened to prevent gas explosion. The coal cinder shall be disposed of at any time and shall not be overstocked.
- 3) Before loading and unloading, check whether there are obstacles on the track groove and whether the hook, chain and other tools are reliable.
- 4) When loading and unloading large castings or thin-wall workpieces, they must be leveled and placed stably. Proper clearance shall be reserved between adjacent, and the loading height shall not exceed the specified value.
- 5) Before discharging the furnace, clean the refractory bricks and refractory mud on the furnace door, and check whether the pulling mechanism is easy to use.
- 6) During discharging and handling, irrelevant personnel shall not be close to both sides of the flat car to avoid personal injury caused by the collapse of the sand box or workpiece. It is forbidden to rest or stack articles around the furnace body.
- 7) When the working temperature is higher than 400 ℃, it is forbidden to lift with chain or steel wire rope (or take thermal insulation measures) to avoid fracture after annealing. When using crane to lift work objects, the safety operation regulations for crane and hooker shall be observed.
- 8) When using a gas stove, the gas ignition sequence should be observed: first, use compressed air to blow off the remaining gas in the furnace, close the compressed air valve, ignite the igniter bar, slightly open the gas nozzle valve, and adjust the compressed gas and gas flow rate after burning the nozzle, so as to fully burn it. In case of cease-fire, the order shall be reversed. It is strictly prohibited to reverse the order.
- 9) When firing in an oil stove, the ignition sequence is: first close the air door, ignite the igniter bar, open the throttle slightly, and adjust the air valve and throttle to make full combustion, and prohibit the “wind pressure oil”.
- 10) When using pulverized coal fuel boiler, it is forbidden to burn the boiler when the blower is faulty or the pipeline is cracked.
- 11) When repairing the electric furnace, the main switch must be cut off, the warning sign of “no switching on for maintenance” shall be hung, or someone shall be arranged for supervision.
Development trend of castings in China
China is a large foundry producer, and there is a serious overcapacity of castings in China. However, China’s serious overcapacity of castings refers to China’s overcapacity of low-end castings. China’s medium and high-end casting production is difficult to meet the needs of the market. Therefore, China’s casting industry needs to transform the low-end casting capacity into high-end casting capacity, so as to enhance the progress of China’s casting industry.
In recent years, China’s casting industry, especially stainless steel hardware castings, has been favored by the broad and increasing demand at home and abroad. Industry experts summed up two reasons: first, with the continuous improvement of China’s economic status and the continuous development of China’s casting industry, more and more hardware casting products are exported to other countries. The increase of overseas demand directly drives the increase of product output of China’s casting manufacturing enterprises. Second, high quality wins the world. The improvement of domestic hardware casting product quality is also an important reason for its increased demand. China’s hardware casting industry is booming.
Energy conservation and emission reduction will become the focus of China’s casting capacity policy: in order to cope with global warming and reduce the consumption of fossil energy, the state attaches great importance to the improvement of energy efficiency in the industrial field and the reduction of greenhouse gas emissions. The process of energy efficiency improvement is also the process of transformation or withdrawal of backward production capacity with high energy consumption and high emission. China’s casting industry as a whole is an industry with high energy consumption and high pollution. The state is also paying high attention to energy conservation and emission reduction in the field of casting. According to experts’ prediction, in order to do a good job in energy conservation and emission reduction of the casting industry in the future, the state will formulate a policy of combining coercion and incentive, and its policy measures may include: control and standards, fiscal policies, agreements and objectives, etc. In short, energy conservation and emission reduction will become the focus of China’s casting capacity policy.
Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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