What is a aluminum ingot
What is a aluminum ingot?
Aluminum ingot is an industrial raw material produced by electrolysis of alumina cryolite.
Classification of aluminum ingots
Aluminum ingots can be divided into remelting aluminum ingots, high-purity aluminum ingots and aluminum alloy ingots according to their composition; According to the shape and size, it can be divided into bar ingot, round ingot, plate ingot, T-shaped ingot, etc.
Several common aluminum ingots
- Aluminum ingot for remelting — 15kg, 20kg (≤ 99.80% Al);
- T-shaped aluminum ingot — 500kg, 1000kg (≤ 99.80% Al);
- High purity aluminum ingot — l0kg, 15kg (99.90% ~ 99.999% Al);
- Aluminum alloy ingot — 10kg, 15kg (Al — Si, Al — Cu, Al — Mg);
- Plate ingot — 500 ~ 1000kg (for plate making);
- Round ingot — 30 ~ 60kg (for wire drawing).
Production process of aluminum ingot
The production of aluminum ingot is composed of bauxite mining, alumina production, aluminum electrolysis and other production links.
Bauxite is mined first, then aluminum ore powder is produced through water washing, grinding and other processes, and then alumina is obtained through four complex processes such as roasting. There are three main types of bauxite for alumina production: gibbsite, diaspore and diaspore.
Among the proven global reserves of bauxite, 92% are weathered laterite bauxite, belonging to gibbsite type. These bauxites are characterized by low silicon, high iron and high aluminum silicon ratio, which are concentrated in western Africa, Oceania and central and South America. The remaining 8% is sedimentary bauxite, belonging to diaspore and diaspore type, with medium and low grade, mainly distributed in Greece, the former Yugoslavia and Hungary.
Due to the different characteristics of the three bauxites, alumina production enterprises have adopted different production processes, mainly Bayer process, alkali lime sintering process and Bayer sintering combined process. Usually, high-grade bauxite is produced by Bayer process, and low-grade bauxite is produced by combined process or sintering process. Because of its simple process and low energy consumption, Bayer process has become the most important method used in alumina production, and its output accounts for about 95% of the total global alumina production.
Aluminum ingot alumina electrolyzes aluminum metal under the action of strong current. This process is called electrolytic aluminum.
Process description of aluminum ingot production
The aluminum ingot casting process adopts liquid aluminum injection into the mold. After cooling into the billet and taking it out, the injection process is the key step for the quality of the product. The casting process is also a physical process in which liquid aluminum crystallizes into solid aluminum. The process flow of casting aluminum ingot is roughly as follows:
- Aluminum discharging – slag raking – weighing – batching – furnace loading – refining – casting – aluminum ingot for remelting – finished product inspection – finished product weighing – warehousing;
- Aluminum discharging – slag raking – weighing – batching – furnace loading – refining – casting – alloy ingot – casting alloy ingot – finished product inspection – finished product weighing – warehousing;
The common casting methods are divided into continuous casting and vertical semi continuous casting:
Continuous casting can be divided into mixing furnace casting and external casting. Continuous casting machines are used. Mixing furnace casting refers to the casting of molten aluminum in the mixing furnace after it is loaded into the mixing furnace. It is mainly used to produce aluminum ingots and casting alloys for remelting. External casting is casting directly from the ladle to the casting machine, which is mainly used when the casting equipment can not meet the production, or the incoming material quality is too poor to be directly put into the furnace. Since there is no external heating source, it is required to lift the package at a certain temperature, generally 690 ~ 740 ℃ in summer and 700 ~ 760 ℃ in winter, so as to ensure a better appearance of aluminum ingot.
For casting in the mixing furnace, ingredients shall be added first, then poured into the mixing furnace, stirred evenly, and then flux shall be added for refining. The casting alloy ingot must be clarified for more than 30min. After clarification, it can be cast by scraping the slag. During casting, the furnace hole of the mixing furnace is aligned with the second and third molds of the casting machine, so as to ensure a certain mobility when the liquid flow changes and the mold is changed. The furnace hole and the casting machine are connected by a runner, and a shorter runner is better, which can reduce the oxidation of aluminum and avoid vortex and splash. When the casting machine is stopped for more than 48h, the mold shall be preheated for 4H before restarting. The molten aluminum flows into the mold through the runner, and the oxide film on the surface of the molten aluminum is removed with a shovel, which is called slag raking. After the first mock exam is completed, the runner is moved down to a mold, and the casting machine is advancing continuously. The mold advances in turn, and the aluminum liquid cools gradually. When it reaches the middle of the casting machine, the aluminum liquid has solidified into aluminum ingot, and the melting number is marked by the printer. When the aluminum ingot reaches the top end of the casting, it has been completely solidified into aluminum ingot. At this time, the mold is turned over, the aluminum ingot is demoulded and falls on the automatic ingot receiving trolley. The finished aluminum ingot is automatically stacked and bundled by the stacker. The casting machine is cooled by water spray, but water supply can only be carried out after the casting machine is started and turned for a full circle. About 8-10t water is consumed per ton of molten aluminum, and air blowing is required for surface cooling in summer. The ingot belongs to flat mold casting. The solidification direction of liquid aluminum is from bottom to top, and the upper part is finally solidified in the middle, leaving a groove shaped shrinkage. The solidification time and conditions of each part of aluminum ingot are different, so its chemical composition will be different, but it meets the standard as a whole.
The common defects of aluminum ingot for remelting are:
- ① Porosity. It is mainly because the casting temperature is too high, there is more gas in the aluminum liquid, there are many pores (pinholes) on the surface of aluminum ingot, the surface is dark, and thermal cracks occur in serious cases.
- ② Slag inclusion. The main reasons are as follows: first, the slag is not clean, resulting in slag inclusion on the surface; Second, the temperature of molten aluminum is too low, resulting in internal slag inclusion.
- ③ Ripple and flash. It is mainly caused by poor operation, too large aluminum ingot, or unstable operation of casting machine.
- ④ Crack. The cold crack is mainly caused by the low casting temperature, resulting in the non dense crystallization of aluminum ingot, resulting in porosity and even cracks. The hot crack is caused by the high casting temperature.
- ⑤ Component segregation. It is mainly caused by uneven stirring when casting alloy.
Vertical semi continuous casting
Vertical semi continuous casting is mainly used for the production of aluminum ingots, plate ingots and various deformed alloys for processing profiles. The molten aluminum is poured into the mixing furnace after batching. Due to the special requirements of wires, the intermediate hinge Al-B needs to be added before casting to remove titanium and vanadium (wire ingot) from the molten aluminum; Al Ti — B alloy (ti5% B1%) shall be added to the ingot for refining. Refine the surface structure. Add 2 # refining agent to the high magnesium alloy, the dosage is 5%, stir evenly, stand for 30min, remove the scum, and then cast. Before casting, raise the casting machine chassis and blow out the water on the chassis with compressed air. Then raise the chassis into the mold, apply a layer of lubricating oil to the inner wall of the mold, put some cooling water into the water jacket, and place the dry and preheated distribution plate, automatic adjusting plug and chute so that each port of the distribution plate is located in the center of the mold. At the beginning of casting, press the automatic regulating plug by hand, block the flow nozzle, cut open the furnace hole of the mixing furnace, and let the aluminum liquid flow into the distribution plate through the chute. When the aluminum liquid reaches 2/5 in the distribution plate, release the automatic regulating plug to make the aluminum liquid flow into the crystallizer, and the aluminum liquid will be cooled on the chassis. When the aluminum liquid reaches 30mm high in the mold, the chassis can be lowered and cooling water can be sent. The automatic adjusting plug controls the aluminum liquid to flow into the mold evenly and keeps the aluminum liquid height in the mold unchanged. The scum and oxide film on the surface of molten aluminum shall be removed in time. When the length of aluminum ingot is about 6m, block the furnace hole, remove the distribution plate, stop water supply after all the aluminum liquid solidifies, remove the water jacket, take out the cast aluminum ingot with a monorail crane, cut it off on the sawing machine according to the required size, and then prepare for the next casting.
During casting, the temperature of molten aluminum in the mixing furnace is maintained at 690 ~ 7l0 ℃, the temperature of molten aluminum in the distribution plate is maintained at 685-690 ℃, the casting speed is 190 ~ 21omm/min, and the cooling water pressure is 0.147 ~ 0.196mpa. The casting speed is proportional to the ingot with square section:
In the formula:
- V is the casting speed, mm/min or M/h;
- D is the side length of ingot section, mm or m;
- K is a constant value, m2/h, generally 1.2 ~ 1.5.
Vertical semi continuous casting is a sequential crystallization method. After the molten aluminum enters the casting hole, it begins to crystallize on the chassis and the inner wall of the mold. Due to the different cooling conditions at the center and edge, the crystallization forms the form of low in the middle and high around. The chassis descends at a constant speed. At the same time, liquid aluminum is continuously injected into the upper part, so that there is a semi solidification zone between solid aluminum and liquid aluminum. Due to the contraction of liquid aluminum during condensation and a layer of lubricating oil on the inner wall of the crystallizer, with the decline of the chassis, the solidified aluminum exits the crystallizer, and there is a circle of cooling water holes at the lower part of the crystallizer. The cooling water can be sprayed onto the surface of the stripped aluminum ingot for secondary cooling, Until the whole ingot is cast.
Sequential crystallization can establish satisfactory solidification conditions, which is beneficial to the particle size, mechanical properties and conductivity of crystallization. There is no difference in mechanical properties in the height direction of the comparative ingot, the segregation is small, the cooling speed is fast, and a very fine crystalline structure can be obtained.
The surface of aluminum wire ingot shall be flat and smooth without slag inclusion, crack, air hole, etc., the length of surface crack shall not be greater than 1.5mm, the depth of slag and edge wrinkles and cracks on the surface shall not exceed 2mm, the section shall not have crack, air hole and slag inclusion, and the slag inclusion less than LMM shall not be more than 5.
The defects of aluminum ingot mainly include:
- ① Crack. The reason is that the temperature of molten aluminum is too high and the speed is too fast, which increases the residual stress; When the silicon content in the aluminum liquid is greater than 0.8%, the aluminum silicon melt is generated, and then a certain amount of free silicon is generated, which increases the thermal cracking of the metal: or the cooling water is insufficient. When the mold surface is rough or no lubricating oil is used, cracks will also occur on the surface and corners of the ingot.
- ② Slag inclusion. The slag inclusion on the surface of aluminum ingot is caused by the fluctuation of aluminum liquid, the rupture of oxide film on the surface of aluminum liquid, and the scum on the surface entering the side of ingot. Sometimes the lubricating oil can also bring in some slag. The internal slag inclusion is caused by the low temperature and high viscosity of liquid aluminum, the slag can not float in time, or the frequent change of aluminum liquid level during casting.
- ③ Cold insulation. The formation of cold barrier is mainly caused by excessive fluctuation of liquid aluminum level in the mold, low casting temperature, too slow ingot speed or uneven vibration and decline of the casting machine.
- ④ Pores. The pores mentioned here refer to small pores with a diameter of less than 1mm. The reason is that the casting temperature is too high and the condensation is too fast, so that the gas contained in the liquid aluminum can not escape in time. After solidification, small bubbles gather and stay in the ingot to form pores.
- ⑤ Rough surface. Because the inner wall of the mold is not smooth and the lubrication effect is not good, aluminum nodules on the crystal surface will be formed in serious cases. Or segregation caused by uneven cooling due to too large iron silicon ratio.
- ⑥ Aluminum leakage and reanalysis. It is mainly the operation problem, and it also causes tumor crystals in serious cases.
Plastic forming method of aluminum ingot
Production of plate and strip
Flat roll rolling is adopted, and the basic processes are hot rolling, cold rolling, heat treatment and finishing. LY12, LC4 and other hard aluminum alloys with complex chemical composition shall be homogenized before hot rolling. The treatment temperature is generally 10 – 15 ℃ lower than the eutectic temperature of the low melting point phase of the alloy and kept for 12 – 24 hours. The cladding of duralumin alloy is that the cladding plate is placed on both sides of the billet after milling and welded by means of hot rolling. The thickness of aluminum clad layer is generally 4% of the plate thickness. Hot rolling is generally carried out above the recrystallization temperature. Hot rolling can be carried out on a single stand reversible mill or continuous rolling on multiple stands. In order to improve the yield and production efficiency, large ingot rolling is developed, and the ingot weight is more than 10 – 15 tons. In factories with an annual output of less than 100000 tons, four roll reversible hot rolling and hot coiling process are generally used, and the thickness of hot rolled strip is about 6 – 8mm. For factories with an output of more than 100000 tons, single stand or two stand, three stand and five stand continuous rolling are mostly adopted after the blank opening of four high reversible hot rolling mill, and hot finishing rolling is implemented, with a strip thickness of 2.5 – 3.5mm. Hot rolled strip is used as cold rolled blank after being rolled. In order to ensure the best plasticity of metal, hot rolling should be carried out in single-phase microstructure. The hot rolling bloom temperature of LY11, LY12 and other alloys is 400 – 455 ℃. The deformation rate of the first few passes is generally within 10%, and then increases gradually. The pass deformation rate of pure aluminum and soft aluminum alloy can reach 50%, and that of hard aluminum alloy is about 40%. The total deformation rate of hot rolling can reach more than 90%.
Cold rolling is often carried out at room temperature. Through cold rolling, thin plates and strips with accurate size, smooth and flat surface can be obtained, and work hardened plates and strips with specific mechanical properties can be obtained. Cold rolling mainly adopts the strip production process. Four high reversible mill or four high irreversible mill is used for cold rolling. At present, irreversible mill is developed for cold rolling. The rolling mill is equipped with hydraulic screwdown, hydraulic roll bending, automatic thickness control system or automatic thickness control system for measuring roll gap and shape controller. Various parameters are controlled, recorded and stored by microcomputer to obtain plate and strip with accurate size and flat shape. For example, the tolerance of 0.18 mm strip can reach ± 5 microns. Small factories also produce plates by block method. After annealing, the cold deformation rate of aluminum can reach more than 90%. Multiphase hard aluminum alloy has obvious cold work hardening and needs intermediate annealing. The cold deformation rate after intermediate annealing is 60 – 70%. Emulsion lubrication and cold rolling have been developed from emulsion to full oil lubrication in hot rolling. The multi-stage cooling system with separate control nozzle is adopted to reduce the friction between aluminum plate and roll, cool roll, control roll shape, wash aluminum powder and other impurities, so as to obtain good surface quality and shape.
After cold rolling and heat treatment, the coil is often finished on the roll straightener or on the stretch bending continuous straightener train. The flat quenched plate shall be carried out within the aging incubation period, generally within 30 – 40 minutes after quenching. The total deformation of flat calendering of quenched plate shall not exceed 2%.
The continuous casting and rolling of aluminum plate and strip successfully tested in 1955 can produce thin plate and aluminum foil blank. China began to produce thin plates by this method in the early 1970s.
Aluminum foil can be divided into industrial aluminum foil and packaging aluminum foil. The chemical composition of industrial aluminum foil is relatively pure, with a thickness of 0.005 – 0.2mm. It is mainly used as capacitor, thermal insulation material and moisture proof material in electrical and electronic industry. The thickness of packaging aluminum foil is generally 0.007 – 0.1mm. There are many products such as flat foil, printing foil, colored printing foil and paper mounted aluminum foil. It is mainly used as packaging materials for food, tea, paper smoke, etc. The minimum thickness of aluminum foil produced by belt production method can reach 0.0025 mm and the width can reach 1800 mm. Aluminum foil rolling is roll gap free rolling, and the roll is always in elastic flattening state. During rolling, the thickness of foil is controlled by adjusting rolling force, rolling speed and controlling tension. During rough rolling, the thickness of foil is controlled by rolling force; During finish rolling, the thickness of foil decreases with the increase of rolling speed; The greater the tension, the smaller the thickness; In order to prevent fragmentation, the tension is usually the conditional yield strength of the foil σ 0.2 – 0.4 of 0.2. In low-speed rolling, “thick oil” or “thin oil” is often added to the lubricating oil to adjust the rolling thickness of aluminum foil. Lubricant and roll state have a very important influence on the quality of foil. Aluminum foil blank comes from cold rolled aluminum coil, which is generally pre annealed at 340 – 480 ℃, and its thickness is 0.4 – 0.7mm. During rolling, the pass deformation rate is about 50%, and the total deformation rate can reach more than 95%. The foil with finished product thickness less than 0.01 – 0.02mm shall be rolled in coils and double sheets.
Wire rod production
It is mainly produced by drawing process. Products include rivet wire, welding rod and wire. The billets are produced by extrusion, rolling or continuous casting and rolling. The extrusion method can produce billets with greater flexibility and better product performance. Rolling method and continuous casting and rolling method are suitable for billet production of single alloy variety with high production efficiency.
It is mainly used in aircraft and machine manufacturing. Forgings are divided into free forgings and die forged parts, and their blanks are cast and extruded. The largest die forging hydraulic press is 70000 tons, and the projection area of the maximum size of forgings is 4.5 m2. The critical deformation rate of aluminum alloy is about 5 – 15%. In order to avoid the formation of coarse grains, the deformation rate of die forging should generally be greater than 15%. Multidirectional free forging is often used to reduce uneven deformation.
Production of pipes, bars and profiles
The usual process for the production of pipes, bars and profiles is: first, the blank is made by hot extrusion, and then the finished product is made by rolling (or stretching), finishing and heat treatment. It can also be directly made into finished products by hot extrusion. Now it has developed from short ingot to long ingot extrusion. According to the extrusion pressure of the extruder, all kinds of profiles and pipes are extruded by shunting combined die and tongue die. The length of the product can reach more than 60m. After stretching and straightening, it is cut into the required length. The maximum extruder for aluminum production is 20000 tons. It can be extruded into a ribbed tube with a diameter of 800 mm. Building profiles are extruded products developed in the 1960s, accounting for more than 35% of the total extruded products, of which 80% are used as door and window frames. Almost all building profiles are made of aluminum magnesium silicon alloys (LD30 and LD31). After anodizing and coloring treatment, the product forms various color oxide films on the surface, has good corrosion resistance, and the doors and windows are beautiful, durable and well sealed. In addition to hot extrusion, cold extrusion, isothermal extrusion, residue free extrusion and hydrostatic extrusion have also been developed (see oxidation coloring of aluminum).
Aluminum and aluminum alloy are extruded by forward extrusion and reverse extrusion. The appropriate extrusion speed should be selected according to different alloys. In order to obtain aluminum alloy extruded products with good microstructure and properties, the extrusion coefficient of profile and bar is（λ） More than 8 – 12, for forging blank λ Greater than 5. Extrusion die has a great influence on the quality of extrusion products. Flat die is commonly used for extruding bar and profile, and conical die is commonly used for pipe. When extruding hollow profiles and pipes with complex shapes, shunt combined die and tongue die are widely used. Some use liquid nitrogen to cool the extrusion die to prolong the service life of the die and ensure the accuracy of products. One die can extrude 30 tons of aluminum. Some aluminum alloy extruded products are prone to “coarse grain ring”, that is, a layer of coarse grain area appears around the products after heat treatment. High temperature extrusion can reduce this phenomenon. After solution aging treatment, the strength of some aluminum alloy extruded products that can be strengthened by heat treatment increases and the plasticity decreases.
The pipe drawing adopts multiple rapid and disc barrel drawing. The diameter of the drum of the floating core coil stretching machine is 630 – 2900mm, and the diameter of the stretched pipe blank is 40 – 50mm. The pipe drawing force is 16 – 18 tons, the drawing speed is 24 m/min, and the pass processing rate is generally 25 – 40%. The pipe length produced by this process can reach 6000 meters.
The heat treatment to ensure the final performance of aluminum products is called finished aluminum product heat treatment, including finished product annealing, solution treatment, quenching, natural aging and artificial aging; In addition, graded aging and thermomechanical treatment processes have been developed. Aging treatment not only improves the mechanical properties of products, but also improves the stress corrosion resistance and fracture toughness of products. The transfer time from solution heating to quenching shall generally be controlled within 30 seconds. The cooling rate of quenching should not only ensure the acquisition of supersaturated solid solution, but also prevent excessive quenching stress and bending deformation of products. The coils and plates are annealed in box furnace, well furnace, vertical plate continuous annealing furnace or air cushion continuous annealing furnace with strong circulating ventilation. The new furnace adopts protective atmosphere. Quenching is generally heated in salt bath, and the new process adopts air cushion continuous quenching furnace for quenching.
Classify according to the stress and deformation mode of the workpiece during deformation
Classify according to the stress and deformation mode of the workpiece during deformation
According to the stress and deformation mode (stress and strain state) of the workpiece in the deformation process, aluminum processing can be divided into rolling, extrusion, drawing, forging, spinning, forming processing (such as cold stamping, cold deformation, deep drawing, etc.) and deep processing.
Rolling is a plastic deformation process in which the ingot is pulled into the rotating roll by friction, and its cross section is reduced, its shape is changed, its thickness becomes thinner and its length increases with the help of the pressure exerted by the roll. According to different roll rotation directions, rolling can be divided into longitudinal rolling, transverse rolling and cross rolling. In longitudinal rolling, the rotation direction of the working roll is opposite, and the longitudinal axis of the rolled piece is perpendicular to the axis of the roll. It is the most commonly used method in flat roll rolling of aluminum alloy plate, strip and foil; During cross rolling, the rotation direction of the working roll is the same, and the longitudinal axis of the rolled piece is parallel to the roll axis, which is rarely used in the rolling of aluminum alloy plate and strip; During cross rolling, the rotation direction of the working roll is the same, and the longitudinal axis of the rolled piece forms a certain inclination angle with the roll axis. Double roll or multi roll cross rolling is often used in the production of aluminum alloy pipes and some special-shaped products. According to different roll systems, aluminum alloy rolling can be divided into two roll (one pair) system rolling, multi roll system rolling and special roll system (such as planetary rolling, V-shaped rolling, etc.): according to different roll shapes, aluminum alloy rolling can be divided into flat roll rolling and pass roll rolling. According to different product varieties, aluminum alloy rolling can be divided into plate, strip and foil rolling, bar, flat bar and special-shaped profile rolling, pipe and hollow profile rolling, etc.
Extrusion is a processing method in which the ingot is loaded into the extrusion barrel, the metal is pressed by the extrusion shaft to extrude it from the die hole of a given shape and size, resulting in plastic deformation to obtain the required extruded products. Extrusion can be divided into forward extrusion, reverse extrusion and combined extrusion. In forward extrusion, the motion direction of extrusion shaft is consistent with the flow direction of extruded metal, while in reverse extrusion, the motion direction of extrusion shaft is opposite to the flow direction of extruded metal. According to the heating temperature of ingot, extrusion can be divided into hot extrusion and cold extrusion. During hot extrusion, the billet is heated above the recrystallization temperature for extrusion, and cold extrusion is extrusion at room temperature.
Drawing is a processing method in which the drawing machine (or drawing machine) pulls the aluminum blank (wire blank or pipe blank) from the die l of a given shape and size through the clamp to produce plastic deformation and obtain the required pipe, rod, shape and wire rod. According to the variety and shape of products produced, drawing can be divided into wire drawing, pipe drawing, bar drawing and profile drawing. Pipe stretching can be divided into empty stretching, core head stretching and swimming core head stretching. The main elements of drawing processing are drawing machine, drawing die and drawing drum. According to the drawing die matching, drawing can be divided into single-mode drawing and multi-mode drawing.
Forging is a processing method in which forging hammer or press (mechanical or hydraulic) exerts pressure on aluminum ingot or forging blank through hammer or indenter to cause plastic deformation of metal. Aluminum alloy forging includes free forging and die forging. Free forging is to place the workpiece between flat anvil (or die anvil) for forging; Die forging is to put the workpiece in the die with given size and shape, and then apply pressure to the workpiece for forging deformation to obtain the required die forging.
Other plastic forming methods of aluminum
People have also developed a variety of new aluminum processing methods, mainly including:
- (1) pressure casting forming method, such as low, medium and high pressure forming, extrusion forming, etc.
- (2) semi solid forming method, such as semi-solid rolling, semi-solid extrusion, semi-solid drawing, liquid die forging, etc.
- (3) continuous forming method, such as continuous casting and extrusion, high-speed continuous casting and rolling, conform continuous extrusion, etc.
- (4) compound forming method, such as lamination rolling method, multi blank extrusion method, etc.
- (5) deformation heat treatment method, etc.
Among the aluminum processing materials, calendered materials (plate, strip, strip and foil) and extruded materials (pipe, rod, shape and wire) are the most widely used and have the largest output. According to statistics, the annual output of these two kinds of materials account for about 58% and 39% of the total annual output (average) of aluminum in the world respectively. The rest of aluminum processing materials, such as forging products, account for only a few% of the total output of aluminum.
Classify according to the temperature characteristics of the workpiece during processing
Classification and characteristics of aluminum processing methods
There are many aluminum plastic forming methods, which are usually classified according to the temperature characteristics of the workpiece during processing and the stress and deformation mode (stress-strain state) of the workpiece during deformation.
According to the temperature characteristics of the workpiece in the processing process, aluminum processing can be divided into hot processing, cold processing and warm processing.
Hot working refers to the plastic forming process of aluminum ingot above the recrystallization temperature. During hot processing, the billet has high plasticity and low deformation resistance. Products with large deformation can be produced with equipment with small capacity. In order to ensure the structure and performance of the product, the heating temperature, deformation temperature and deformation speed, deformation degree, final deformation temperature and cooling speed after deformation of the workpiece shall be strictly controlled. The common hot working methods of aluminum alloy include hot extrusion, hot rolling, hot forging, hot upsetting, liquid die forging, semi-solid forming, continuous casting and rolling, continuous casting and rolling, continuous casting and extrusion, etc.
Cold working refers to the plastic forming process completed below the temperature without recovery and recrystallization. The essence of cold working is the process combination of cold working and intermediate annealing. Cold processing can reach the final products with smooth surface, accurate size, good microstructure and performance and can meet different performance requirements: the most common cold processing methods include cold extrusion, cold top forging, pipe cold rolling, cold drawing, plate, strip and foil cold rolling, cold stamping, cold bending, spinning, etc.
Warm working is a plastic forming process between cold and hot working. The main purpose of warm processing is to reduce the deformation resistance of metal and improve the plastic properties (processability): the most common warm processing methods are warm extrusion, warm rolling, warm upsetting, etc.
Processing technology of aluminum
Aluminum is the most widely used and widely used metal material in non-ferrous metals, and its application range is still expanding. There are many kinds of aluminum products produced with aluminum materials. According to statistics, there are more than 700000 kinds. There are different needs from construction and decoration industry to transportation industry, aerospace and other industries. Today, I’d like to introduce the processing technology of aluminum products and how to avoid processing deformation.
The advantages and characteristics of aluminum are as follows:
- 1. Low density. The density of aluminum is about 2.7g/cm3. Its density is only 1/3 that of iron or copper.
- 2. High plasticity. Aluminum has good ductility and can be made into various articles by pressure processing means such as extrusion and stretching.
- 3. Corrosion resistance. Aluminum is a highly negatively charged metal. Under natural conditions or anodic oxidation, a protective oxide film will be formed on the surface, which has much better corrosion resistance than steel.
- 4. Easy to strengthen. The strength of pure aluminum is not high, but it can be improved by anodizing.
- 5. Easy surface treatment. Surface treatment can further improve or change the surface properties of aluminum. Aluminum anodizing process is quite mature and stable. It has been widely used in the processing of aluminum products.
- 6. Good conductivity and easy recovery.
Processing technology of aluminum products
Punching of aluminum products
1. Cold punching
Use aluminum particles. One time forming with extruder and die is suitable for cylindrical products or product shapes that are difficult to achieve by drawing process, such as elliptical, square and rectangular products.
The tonnage of the machine used is related to the cross-sectional area of the product. The gap between the upper die punch and the lower die tungsten steel is the wall thickness of the product, and the vertical gap from the upper die punch and the lower die tungsten steel to the lower dead center is the top thickness of the product.
Advantages: the die opening cycle is short, and the development cost is relatively low compared with the drawing die.
Disadvantages: the production process is long, the product size fluctuates greatly in the manufacturing process, and the labor cost is high.
The material used is aluminum sheet. The continuous die machine and die are used for multiple deformation to meet the requirements of shape, which is suitable for non cylindrical products (aluminum products with bending)（ As shown in Fig. 5 machine, FIG. 6 mold and Fig. 7 product)
Advantages: more complex and multiple deformation products have stable dimensional control in the production process, and the product surface is smooth.
Disadvantages: high mold cost, relatively long development cycle, high requirements for machine selection and precision.
Surface treatment of aluminum products
1. Sand blasting (shot blasting)
The process of cleaning and roughening metal surface by using the impact of high-speed sand flow.
The surface treatment of aluminum parts by this method can make the surface of the workpiece obtain certain cleanliness and different roughness, and improve the mechanical properties of the workpiece surface. Therefore, it improves the fatigue resistance of the workpiece, increases the adhesion between it and the coating, prolongs the durability of the coating film, and is also conducive to the leveling and decoration of the coating. We often see this process in various products of apple.
Using mechanical, chemical or electrochemical action to reduce the surface roughness of the workpiece, so as to obtain a bright and flat surface. Polishing process is mainly divided into: mechanical polishing, chemical polishing and electrolytic polishing. Aluminum parts can approach the stainless steel mirror effect after mechanical polishing + electrolytic polishing. This process gives people the feeling of high-grade simplicity, fashion and future.
3. Wire drawing
Metal wire drawing is the manufacturing process of repeatedly scraping aluminum plates out of lines with sandpaper. Wire drawing can be divided into straight wire drawing, random wire drawing, rotary wire drawing and thread drawing. The metal wire drawing process can clearly show each fine wire mark, so that the fine hair luster can be reflected in the metal matte. The product has a sense of fashion and science and technology.
4. High gloss cutting
The diamond knife is reinforced on the main shaft of the precision engraving machine with high-speed rotation (generally 20000 RPM) by the precision engraving machine to cut parts, resulting in local bright areas on the product surface. The brightness of cutting highlight is affected by the milling bit speed. The faster the bit speed is, the brighter the cutting highlight is. On the contrary, it is darker and easy to produce knife marks. High gloss high gloss cutting is particularly used in mobile phones, such as iPhone 5. In recent years, some high-end TV metal frames have adopted high gloss milling technology, coupled with anodizing and wire drawing technology, which makes the TV full of fashion and sharp sense of science and technology.
Anodizing refers to the electrochemical oxidation of metals or alloys. Aluminum and its alloys form an oxide film on aluminum products (anodes) under the action of external current under the corresponding electrolyte and specific process conditions. Anodizing can not only solve the defects of aluminum surface hardness and wear resistance, but also prolong the service life of aluminum and enhance the beauty. It has become an indispensable part of aluminum surface treatment. It is the most widely used and very successful process at present.
6. Two color anode
Two color anode refers to anodizing on a product and giving different colors to specific areas. Two color anodizing process is rarely used in TV industry because of its complex process and high cost; However, through the comparison between two colors, it can better reflect the high-end and unique appearance of the product.
Technological measures and operation skills to reduce aluminum processing deformation
There are many reasons for processing deformation of aluminum parts, which are related to material, part shape, production conditions and so on. It mainly includes the following aspects: deformation caused by internal stress of blank, deformation caused by cutting force and cutting heat, and deformation caused by clamping force.
Technological measures to reduce machining deformation
1. Reduce the internal stress of wool culture
Natural or artificial aging and vibration treatment can partially eliminate the internal stress of the blank. Pre processing is also an effective process method. For the blank with fat head and big ears, due to the large allowance, the deformation after processing is also large. If the surplus part of the blank is processed in advance and the margin of each part is reduced, it can not only reduce the processing deformation of the subsequent process, but also release part of the internal stress after being placed for a period of time.
2. Improve cutting ability of cutting tools
The material and geometric parameters of the tool have an important impact on the cutting force and cutting heat. The correct selection of the tool is very important to reduce the machining deformation of the part.
(1) Reasonably select tool geometric parameters.
- ① Rake angle: under the condition of maintaining the strength of the blade, the rake angle should be appropriately larger. On the one hand, it can grind a sharp edge, and on the other hand, it can reduce the cutting deformation, make the chip removal smooth, and then reduce the cutting force and cutting temperature. Do not use negative rake tools.
- ② Back angle: the size of the back angle has a direct impact on the wear of the back cutter surface and the quality of the machined surface. Cutting thickness is an important condition for selecting back angle. During rough milling, due to large feed rate, heavy cutting load and large calorific value, it is required that the tool heat dissipation conditions are good. Therefore, the back angle should be smaller. During finish milling, the cutting edge shall be sharp to reduce the friction between the flank and the machined surface and reduce elastic deformation. Therefore, the back angle shall be larger.
- ③ Helix angle: in order to make milling smooth and reduce milling force, the helix angle should be as large as possible.
- ④ Main deflection angle: properly reducing the main deflection angle can improve the heat dissipation conditions and reduce the average temperature of the processing area.
(2) Improve tool structure.
- ① Reduce the number of milling cutter teeth and increase the chip holding space. Due to the large plasticity of aluminum parts, large cutting deformation in machining and large chip holding space, it is better to have a large radius of chip holding groove bottom and a small number of milling cutter teeth.
- ② Fine grinding of cutter teeth. The roughness value of the cutting edge of the cutter tooth shall be less than RA = 0.4um. Before using a new knife, gently grind the front and back of the knife teeth with a fine oilstone to eliminate the residual burrs and slight serrations when grinding the knife teeth. In this way, not only the cutting heat can be reduced, but also the cutting deformation is relatively small.
- ③ Strictly control the tool wear standard. After tool wear, the workpiece surface roughness increases, the cutting temperature increases, and the workpiece deformation increases. Therefore, in addition to the selection of tool materials with good wear resistance, the tool wear standard should not be greater than 0.2mm, otherwise it is easy to produce chip buildup. During cutting, the temperature of the workpiece shall not exceed 100 ℃ to prevent deformation.
3. Improve the clamping method of workpiece
For thin-walled aluminum workpieces with poor rigidity, the following clamping methods can be adopted to reduce deformation:
- ① For thin-walled bushing parts, if they are clamped radially with three jaw self centering chuck or spring collet, once they are loosened after processing, the workpiece will inevitably deform. At this time, the axial end face compression method with good rigidity should be used. Locate with the inner hole of the part, make a self-made threaded mandrel, insert it into the inner hole of the part, press the end face with a cover plate, and then tighten it with a nut. When machining the outer circle, the clamping deformation can be avoided, so as to obtain satisfactory machining accuracy.
- ② When processing thin-walled sheet metal workpiece, it is best to select vacuum suction cup to obtain evenly distributed clamping force, and then process it with small cutting parameters, which can well prevent workpiece deformation.
- In addition, the packing method can also be used. In order to increase the process rigidity of thin-walled workpiece, medium can be filled in the workpiece to reduce the deformation of workpiece during clamping and cutting. For example, fill the workpiece with urea melt containing 3% ~ 6% potassium nitrate. After processing, immerse the workpiece in water or alcohol to dissolve and pour out the filler.
4. Reasonably arrange the process
In high-speed cutting, due to large machining allowance and intermittent cutting, the milling process often produces vibration, which affects the machining accuracy and surface roughness. Therefore, NC High-speed machining process can be generally divided into: rough machining – semi finishing – corner cleaning – finishing and so on. For parts with high precision requirements, it is sometimes necessary to carry out secondary and semi finishing, and then finish machining. After rough machining, the parts can be cooled naturally to eliminate the internal stress caused by rough machining and reduce deformation. The allowance left after rough machining shall be greater than the deformation, generally 1 ~ 2mm. During finish machining, the finished surface of parts shall maintain a uniform machining allowance, generally 0.2 ~ 0.5mm, so that the tool is in a stable state in the machining process, which can greatly reduce the cutting deformation, obtain good surface machining quality and ensure the accuracy of products.
Operation skills to reduce machining deformation
In addition to the above reasons, the operation method is also very important in practical operation.
- 1. For parts with large machining allowance, in order to have better heat dissipation conditions during machining and avoid heat concentration, symmetrical machining should be adopted. If a 90mm thick plate needs to be processed to 60mm, if one side is milled immediately and the other side is milled to the final size at one time, the flatness will reach 5mm; If repeated feed symmetrical processing is adopted, each side is processed to the final size twice to ensure that the flatness reaches 0.3mm.
- 2. If there are multiple cavities on the plate parts, the sequential processing method of one cavity one cavity should not be adopted during processing, which is easy to cause uneven stress and deformation of the parts. It adopts multiple processing in layers, and each layer shall be processed to all cavities at the same time as far as possible, and then the next layer shall be processed to make the parts bear the force evenly and reduce the deformation.
- 3. Cutting force and cutting heat can be reduced by changing cutting parameters. Among the three elements of cutting parameters, the back draft has a great influence on the cutting force. If the machining allowance is too large and the cutting force of one-time tool walking is too large, it will not only deform the parts, but also affect the rigidity of the machine tool spindle and reduce the durability of the tool. If we reduce the amount of back knife, the production efficiency will be greatly reduced. However, in NC machining, high-speed milling can overcome this problem. While reducing the back draft, as long as the feed is increased accordingly and the rotating speed of the machine tool is increased, the cutting force can be reduced and the machining efficiency can be guaranteed.
- 4. Pay attention to the order of cutting. Rough machining emphasizes the improvement of machining efficiency and the pursuit of cutting rate per unit time. Generally, reverse milling can be used. That is to cut off the excess material on the surface of the blank at the fastest speed and in the shortest time to basically form the geometric contour required for finishing. The finishing emphasizes high precision and high quality, and forward milling should be adopted. Because the cutting thickness of the cutter teeth gradually decreases from the maximum to zero during forward milling, the work hardening degree is greatly reduced and the deformation degree of the parts is reduced at the same time.
- 5. The deformation of thin-walled workpiece during machining due to clamping is difficult to avoid even finish machining. In order to minimize the deformation of the workpiece, the pressing part can be loosened before the finish machining is about to reach the final size, so that the workpiece can be freely restored to the original state, and then slightly pressed, subject to the ability to clamp the workpiece (completely by hand feeling), so as to obtain the ideal machining effect. In short, the action point of the clamping force is best on the bearing surface. The clamping force should act in the direction of good rigidity of the workpiece. On the premise of ensuring that the workpiece is not loose, the smaller the clamping force, the better.
- 6. When machining parts with cavity, try not to let the milling cutter directly plunge into the parts like a drill bit, resulting in insufficient chip holding space of the milling cutter and unsmooth chip removal, resulting in overheating, expansion, tool collapse, tool breakage and other adverse phenomena of the parts. First drill the cutter hole with a drill of the same size or larger than the milling cutter, and then mill with a milling cutter. Alternatively, the spiral cutting program can be produced with CAM software.
Source: Network Arrangement – China Aluminum Ingot Manufacturer – Yaang Pipe Industry Co., Limited (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.)
If you want to have more information about the article or you want to share your opinion with us, contact us at email@example.com
Please notice that you might be interested in the other technical articles we’ve published: