How to choose the right metal materials to make parts?

Basic principle

In the selection of metal materials and forming process, it is necessary to consider whether the material properties meet the requirements under this working condition, whether the forming process is easy when the material is used to make parts, and whether the production and use of materials or parts are economical, i.e. from three aspects of applicability, process and economy.

Applicability principle

The applicability principle refers to that the selected materials must be able to adapt to the working conditions and meet the requirements of satisfactory use. It is the necessary condition of material selection to meet the use requirements, and it is the first problem to be considered in material selection.
The application requirements of materials are embodied in the requirements of internal quality such as chemical composition, structure, mechanical properties, physical properties and chemical properties. In order to meet the use requirements of materials, the selection of materials is mainly considered from three aspects: the load of parts, the use environment of materials and the use performance requirements of materials. The load condition of parts mainly refers to the load size and stress state. The use environment of material refers to the environment in which the material is located, such as medium, working temperature, friction, etc. The service performance requirements of materials refer to the service life of materials, various generalized allowable stresses and deformation of materials. Only when the above three aspects are fully considered, can the material meet the requirements of service performance.

Technological principle

Generally, once the material is selected, the processing technology can be generally determined. At the same time, the processing technology changes the performance of materials, and the shape and structure of parts, production batch and production conditions also have a significant impact on the material processing technology.
The principle of processability refers to that when selecting materials, the processability of materials should be considered, and the materials with good processability should be selected first, so as to reduce the manufacturing difficulty and cost of materials.
All kinds of forming processes have their own characteristics, advantages and disadvantages. When the parts of the same material are manufactured by different forming processes, the difficulty and cost are different, and the required material process performance is also different. For example, when the shape of parts is complex and the size is large, forging is often difficult to achieve. If casting or welding is used, the materials must have good casting or welding properties, and the structure must also meet the requirements of casting or welding. For example, when making keys and pins by cold drawing process, the elongation of the material should be considered, and the effect of deformation strengthening on the mechanical properties of the material should be considered.

Economic principle

While meeting the requirements of material use and technology, the economy of material use must also be considered. The principle of economy refers to the selection of materials with high performance price ratio. The performance of material refers to its use performance. The service performance of materials can be generally represented by service time and safety degree. The price of materials is mainly determined by the cost. The cost of materials includes production cost and use cost. Generally, material cost is determined by raw material cost, utilization rate of raw material, material forming cost, processing cost, installation and commissioning cost, maintenance cost, management cost and other factors.

Steps, methods and basis of material and forming process selection

The selection steps of materials and forming process are as follows: first, select materials according to the use conditions and requirements, and then select the appropriate forming process according to the selected materials, combined with the cost of materials, the forming process of materials, the complexity of parts, the production batch of parts, the existing production conditions and technical conditions.

Steps and methods of selecting materials and forming process

Analyze the service conditions of the parts, find out the specific load, stress state, temperature, corrosion and wear during the use of the parts.
Most parts are used in normal temperature and atmosphere, which mainly requires the mechanical properties of materials. For parts used under other conditions, materials must have some special physical and chemical properties. For example, when used under high temperature conditions, parts materials are required to have certain high temperature strength and oxidation resistance; chemical equipment requires materials to have high corrosion resistance; some instrument parts require materials to have electromagnetic properties, etc. For welded structures used in severe cold areas, requirements for low temperature toughness shall be attached; for those used in humid areas, requirements for atmospheric corrosion resistance shall be attached.

  • (1) Through analysis or test, combined with the results of failure analysis of similar materials, the generalized allowable stress indexes, such as allowable strength, allowable strain, allowable deformation and service time, are determined.
  • (2) Find out the primary and secondary generalized allowable stress indexes, and take the important indexes as the main basis of material selection.
  • (3) According to the main performance index, select several materials that meet the requirements.
  • (4) According to the forming process of materials, the complexity of parts, the production batch of parts, the existing production conditions and technical conditions, select materials and forming process.
  • (5) Considering the cost of materials, forming process, performance and reliability of materials, the most suitable materials are selected by optimization method.
  • (6) If necessary, the materials shall be tested and put into production, and then verified or adjusted.

The above is only the general rule of material selection steps, and its workload and time are quite large. For important parts and new materials, a large number of basic tests and batch trial production processes are needed to ensure the safety of materials. For the parts that are not very important and small in batch, the materials are usually selected according to the experience of the same kind of materials under the same working condition, the brand and specification of the materials are determined, and the forming process is arranged. If the parts are damaged normally, the original materials and forming process can be selected; if the parts are damaged abnormally in the early stage, the cause of failure shall be found out and corresponding measures shall be taken. If it is a matter of material or its production process, new material or new forming process can be considered.

Basis of material selection

Load condition

Engineering materials are affected by various forces in the process of use, including tensile stress, compressive stress, shear stress, shear stress, torque, impact force, etc. When materials work under load, their mechanical properties and failure modes are closely related to the load.
In engineering practice, any machinery and structure must be able to work safely and reliably while fulfilling the motion requirements. For example, to ensure the normal operation of the machine tool spindle, the spindle is neither allowed to break, nor to produce excessive deformation after stress. Another example is that when Jack lifts a heavy object, its screw must maintain a straight-line form of balance, and it is not allowed to bend suddenly. For engineering components, only when they meet the requirements of strength, rigidity and stability, can they work safely and reliably. In fact, in the mechanics of materials, there are specific conditions for the use of these three aspects. In the analysis of material stress or material selection according to the stress, in addition to considering the mechanical properties of materials, it is also necessary to use the relevant knowledge of material mechanics for scientific material selection.

Table.1 Stress conditions, failure forms and required mechanical properties of several common parts

Spare parts

Working conditions

Common failure modes

Main mechanical performance requirements

Types of stress

Load properties

Other

General fastening bolts

Tensile stress

Shear stress

Dead load

Excessive deformation and fracture

Yield strength

Shear strength

Transmission shaft

Bending stress

Torsional stress

Cyclic impact

Friction and vibration at the Journal

Fatigue damage, excessive deformation, wear at the neck

Comprehensive mechanical properties

Transmission gear

Compressive stress

Bending stress

Cyclic impact

Strong friction and vibration

Wear, pitting, tooth breakage

Surface: hardness and bending fatigue strength, contact fatigue resistance; heart part: yield strength and toughness.

Spring

Torsional stress

Bending stress

Cyclic impact

Vibration

Loss of elasticity and fatigue

Elastic limit, yield ratio and fatigue strength

Piston pair of oil pump

Compressive stress

Cyclic impact

Friction, oil corrosion

abrasion

Hardness and compressive strength

Cold working die

Complex stress

Cyclic impact

Strong friction

Wear and brittle fracture

Hardness, strength and toughness

Die casting die

Complex stress

Cyclic impact

High temperature, friction, corrosion of molten metal

Gelau, brittle fracture and wear

High temperature strength, thermal fatigue resistance, toughness and red hardness

Rolling bearing

Compressive stress

Cyclic impact

Strong friction

Fatigue fracture, wear and pitting

Resistance, hardness and wear resistance of contact fatigue

crankshaft

Bending stress

Torsional stress

Cyclic impact

Journal friction

Brittle fracture, fatigue fracture, bitten corrosion and abrasion

Fatigue strength, hardness, impact fatigue resistance and comprehensive mechanical properties

connecting rod

Tensile stress

Compressive stress

Cyclic impact

Brittle fracture

Compressive fatigue strength and impact fatigue resistance

Service temperature of materials

Most materials are used at room temperature, of course, there are materials used at high or low temperature. Due to the use of different temperatures, the properties of materials are also very different.
With the decrease of temperature, the toughness and plasticity of steel materials are decreasing. When the temperature is reduced to a certain extent, the toughness and plasticity of the alloy decrease significantly, which is called the ductile brittle transition temperature. When the temperature is lower than the ductile brittle transition temperature, the material is prone to low stress brittle fracture, resulting in damage. Therefore, when choosing the steel used at low temperature, the material with the ductile brittle transition temperature lower than the working condition should be selected. The purpose of alloying of various low temperature steels is to reduce carbon content and improve the low temperature toughness of materials.
With the increase of temperature, a series of changes will take place in the properties of iron and steel materials, mainly including the decrease of strength and hardness, the increase of plasticity and toughness first and then the decrease, and the high-temperature oxidation or high-temperature corrosion of iron and steel. All of these affect the properties of materials and even make them fail. For example, the service temperature of carbon steel and cast iron should not exceed 480 ℃, while that of alloy steel should not exceed 1150 ℃.

Corrosion

In industry, corrosion rate is generally used to express the corrosion resistance of materials. The corrosion rate is expressed by the loss of metal material per unit area per unit time, and also by the corrosion depth of metal material per unit time. The corrosion resistance rating standards of 6 categories and 10 grades are commonly used in industry, from complete corrosion resistance of category I to non corrosion resistance of category VI, as shown in Table 2.
Table.2 Classification and rating standards for corrosion resistance of metal materials

20200227024048 48776 - How to choose the right metal materials to make parts?

Most engineering materials work in atmospheric environment, atmospheric corrosion is a common problem. The humidity, temperature, sunshine, rain water and corrosive gas content of atmosphere have great influence on the corrosion of materials. In common alloys, the corrosion rate of carbon steel in the industrial atmosphere is 10 ^ – 605 M / D, which can be used after coating the protective coating such as paint. The low alloy steel containing copper, phosphorus, nickel, chromium and other alloy components has a great improvement in atmospheric corrosion resistance, which can be directly used without painting. Aluminum, copper, lead, zinc and other alloys are very resistant to atmospheric corrosion.

Wear resistance

The factors affecting the wear resistance of the material are as follows:

  • ① Properties of the material itself: including hardness, toughness, work hardening ability, thermal conductivity, chemical stability, surface state, etc.
  • ② Friction conditions: including the characteristics of the grinding materials, the pressure, temperature, speed, lubricant characteristics, corrosion conditions, etc.

Generally speaking, the materials with high hardness are not easy to be pierced or ploughed in by abrasive objects, and the fatigue limit is generally high, so the wear resistance is high; if they are punctured or ploughed in, they will not be torn off into pieces, so the wear resistance can be improved; therefore, the hardness is the main aspect of wear resistance. In addition, the hardness of the material is also variable in use. Metals that are easy to work and harden harden in the process of friction, while metals that are easy to be softened by heat will soften in the process of friction.

Selection basis of material forming process

Generally speaking, when the material of the product is determined, the type of forming process is generally determined. For example, if the product is cast iron, casting molding shall be selected; if the product is sheet metal, stamping molding shall be selected; if the product is ABS plastic, injection molding shall be selected; if the product is ceramic, ceramic molding process shall be selected. However, the forming process also has a certain impact on the performance of the material, so the final performance requirements of the material must be considered in the selection of the forming process.

Properties of product materials

① Mechanical properties of materials.
For example, for gear parts made of steel, when their mechanical property requirements are not high, they can be cast; when their mechanical property requirements are high, they should be pressure processed.
② Performance of materials.
For example, if steel die forging is used to manufacture flywheel parts in cars and automobile engines, due to the high speed of cars and the requirement of stable driving, it is not allowed to expose the fiber of flywheel forgings in use, so as to avoid corrosion and affect its service performance, so it is not suitable to use open die forging, but closed die forging should be used. This is because the open die forging process can only forge the flywheel forging with flash. In the subsequent trimming process, the fiber structure of the forging will be cut off and exposed, while the closed die forging has no flash, which can overcome this disadvantage.
③ Technological properties of materials.
The technological properties of materials include casting properties, forging properties, welding properties, heat treatment properties and cutting properties. For example, due to the poor weldability of non-ferrous metal materials which are easy to oxidize and inhale, the argon arc welding process should be used for their connection, rather than the ordinary manual arc welding process; while polytetrafluoroethylene material, although it is also a thermoplastic, is not suitable for injection molding process due to its poor fluidity, and only the pressing sintering process should be used.
④ Special properties of materials.

The special properties of materials include wear resistance, corrosion resistance, heat resistance, conductivity or insulation. For example:

  • If the impeller and shell of acid resistant pump are made of stainless steel, they can only be cast;
  • If they are made of plastic, they can be injection molded;
  • If they are required to be heat-resistant and corrosion-resistant, they should be made of ceramic, and the injection molding process should be selected accordingly.

Production batch of parts

For mass-produced products, the molding process with high precision and productivity can be selected. Although the manufacturing cost of these molding process equipment is high, this part of investment can be compensated by the reduction of material consumption of each product. For mass production of forgings, die forging, cold rolling, cold drawing, cold extrusion and other molding processes should be selected; for mass production of non-ferrous alloy castings, metal mold casting, pressure casting, and low-pressure casting and other molding processes should be selected; for mass production of MC nylon parts, injection molding process should be selected.
In the small batch production of these products, the molding technology with low precision and productivity can be selected, such as manual molding, free forging, manual welding, and their combined with cutting process.

Shape complexity and precision requirements of parts

For the metal parts with complex shape, especially for the parts with complex inner cavity shape, the casting molding process can be selected, such as box body, pump body, cylinder body, valve body, shell, bed, etc.; for the engineering plastic parts with complex shape, the injection molding process is often selected; for the ceramic parts with complex shape, the injection molding or injection molding process is often selected; for the metal parts with simple shape, the pressure can be selected processing or welding molding process; blow molding, extrusion molding or molding process can be selected for engineering plastic parts with simple shape; and molding process is often selected for ceramic parts with simple shape.
If the product is a casting, ordinary sand casting can be used if the size requirement is not high; and if the size accuracy requirement is high, investment casting, gasification casting, pressure casting and low-pressure casting can be used respectively according to the casting material and batch. If the product is a forging with low dimensional accuracy requirements, free forging is often used; if the accuracy requirements are high, die forging, extrusion molding, etc. are used. If the products are plastic parts with low precision requirements, hollow blow molding is often used; if the precision requirements are high, injection molding is used.

Existing production conditions

The existing production conditions refer to the existing equipment capacity, personnel technical level and outsourcing possibility of production products. For example, in the production of heavy machinery products, under the condition that there is no large capacity steel-making furnace and large tonnage lifting and transportation equipment on site, the casting and welding combined forming process is often selected, that is, the large parts are divided into several small pieces for casting first, and then welded into large parts.
Another example is that the oil pan parts on the lathe are usually formed by stamping with steel sheet under the press, but if the site conditions are not available, other process methods should be adopted. For example, if there is no sheet metal or large press at the site, it is necessary to adopt casting forming process; if there is sheet metal but there is no large press at the site, it is necessary to choose economical and feasible spinning forming process to replace stamping forming.

Fully consider the possibility of using new process, new technology and new materials

With the increasing demand of industrial market, users have more and more strong requirements for product variety and quality renewal, which makes the production nature from mass production to multi variety and small batch production, thus expanding the application scope of new technology, new technology and new materials.
Therefore, in order to shorten the production cycle and update the product type and quality, a large number of new processes, new technologies and new materials such as precision casting, precision forging, precision blanking, cold extrusion, liquid die forging, superplastic molding, injection molding, powder metallurgy, ceramic isostatic pressing, composite material molding and rapid molding are adopted under possible conditions, so that the parts are formed without allowance near net type, so as to significantly improve product quality and economic benefits.

In addition, in order to select the molding process reasonably, we must have a clear understanding of the characteristics of various molding processes, the scope of application and the influence of the molding process on the material properties.

Source: China Pipe Fittings 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.)

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how to choose the right metal materials to make parts - How to choose the right metal materials to make parts?
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How to choose the right metal materials to make parts?
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In the selection of metal materials and forming process, it is necessary to consider whether the material properties meet the requirements under this working condition, whether the forming process is easy when the material is used to make parts, and whether the production and use of materials or parts are economical, i.e. from three aspects of applicability, process and economy.
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