The basic principle in the choice of materials and forming process, we should take into account whether the material performance meets the requirements under this working condition, and whether the forming process is easy when the parts are made of this material, at the same time, we should also consider whether the production and use of materials or machine parts are economical or not.

The principle of serviceability the principle of serviceability means that the material chosen must be adaptable to service conditions and meet satisfactory service requirements. To meet the use requirements is a necessary condition for material selection, is the first consideration in the selection of materials. The use of materials requires its chemical composition, structure, mechanical properties, physical properties and chemical properties and other internal quality requirements. In order to meet the requirements of materials, the load of parts, the environment and the performance of materials are considered in the selection of materials. The load condition of a part mainly refers to the size of the load and the stress state. The use of the environment refers to the material in the environment, such as the medium, working temperature and friction, etc. . The service performance of the material requires the service life of the material, the various broad allowable stresses and broad allowable deformations of the material. Only by taking the above three aspects into full consideration, can the material meet the performance requirements.

In general, once the material has been selected, its processing technology can be generally determined. At the same time, the processing technology changes the properties of the material, and the shape and structure of the parts, the production batch and the production conditions also have a significant impact on the material processing technology. The principle of technological property means that when selecting materials, we should consider the technological property of the materials, give priority to the materials with good technological property, and reduce the difficulty and cost of the materials. Each molding process has its own characteristics and advantages and disadvantages, the parts of the same material, when using different molding process manufacturing, its difficulty and cost is different, the required material processing performance is different. For example, when the shape of the parts is complex and the size is large, it is often difficult to achieve forging molding. If the parts are cast or welded, the materials must have good casting or welding properties, the structure should also be suitable for casting or welding requirements. In addition, the elongation of the material and the effect of deformation strengthening on the mechanical properties of the material should be considered when manufacturing the key and pin by cold drawing process.

The principle of economy must consider the economy of materials while meeting the requirements of material use and technology. The principle of economy means that when selecting materials, materials with high performance-to-price ratio should be selected. The performance of a material is its serviceability. The performance of the material can generally be represented by the time of use and the degree of safety. The price of materials is mainly determined by cost. The cost of materials includes the production cost and the use cost. Generally, material cost is determined by raw material cost, material utilization ratio, material forming cost, processing cost, installation and commissioning cost, maintenance cost, management cost and other factors. Material and molding process selection steps, methods and according to the material and molding process selection steps are as follows: First, according to the use of working conditions and use requirements for material selection, and then according to the selected material, at the same time, combining 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, select the appropriate forming process.

1. Steps and methods for selecting materials and forming processes

The service conditions of the parts are analyzed, and the load, stress State, temperature, corrosion and wear of the parts are found out. Most parts are used at room temperature in the atmosphere, the main requirements of material mechanical properties. Parts used under other conditions require materials to have certain special physical and chemical properties. If used at high temperature, it is required that the parts have certain high temperature strength and oxidation resistance, chemical equipment is required to have high corrosion resistance, some instrument parts require materials with electromagnetic properties, etc. . The welding structure used in severe cold area should be attached with the requirements of low temperature toughness and atmospheric corrosion resistance when used in wet area.

 (1) by analyzing or testing, and combining the results of failure analysis of the same kind of materials, the broad allowable stress indexes, such as allowable strength, allowable strain, allowable deformation and service time, are determined. 

(2) to find out the main and secondary generalized allowable stress index, and take the important index as the main basis of material selection. 

(3) according to the main performance indicators, select several materials that meet the requirements.

 (4) choose the material and its forming process according to the technological property of the material, the complexity of the parts, the production batch of the parts, the existing production conditions and the technical conditions.

 (5) to select the most suitable material by optimization method, considering material cost, forming process, material property, reliability of use, etc. .

 (6) material selection should be tested and put into production when necessary, and then verified or adjusted.

The above is only the general rule of material selection process, and its workload and time-consuming are quite large. For important parts and new materials, it is necessary to carry out a large number of basic tests and batch production process to ensure the safety of materials. For less important, small batch parts, usually refer to the same working conditions of the same kind of material experience to select materials, determine the material grades and specifications, arrange molding process. If the part is normal damage, the original material and forming process can be chosen; if the part damage is abnormal early damage, the cause of failure should be found out and corresponding measures should be taken. If it is a material or its production process, you can consider the use of new materials or new molding process.

2. Basis for selection

(1) engineering materials under load are subjected to various forces, such as tensile stress, compressive stress, shear stress, shear stress, Torque, impact force, etc. . Material working under load, its mechanical performance requirements and failure forms are closely related to the load situation. In engineering practice, any machinery and structure must be able to work safely and reliably while meeting the requirements of motion. For example, to ensure the normal work of the machine tool spindle, then the spindle is not allowed to break, nor allow force to produce excessive deformation. Also such as Jack Lifting weight, the screw must maintain a straight-line form of balance, and do not allow sudden bending. For engineering components, only when the requirements of strength, stiffness and stability are met, can they work safely and reliably. In fact, in the mechanics of materials, these three aspects of material requirements have specific conditions of use. In the analysis of material stress, or according to the stress of the material selection, in addition to considering the Mechanical Properties of materials, it is necessary to apply the relevant knowledge of material mechanics to scientific material selection.

(2) temperature of material most materials are used at room temperature, of course, there are materials used at high or low temperature. Due to the different use temperature, the performance of materials is also very different. With the decrease of temperature, the toughness and plasticity of iron and steel materials decrease. When the temperature decreases to a certain extent, its toughness and plasticity decrease significantly. This temperature is called ductile-brittle transition temperature. When used below the ductile-brittle transition temperature, the material is prone to brittle fracture under low stress, which results in damage. Therefore, the choice of low-temperature use of steel, should choose the ductile-brittle transition temperature is lower than the use of materials. All kinds of low temperature steels are alloyed to reduce carbon content and improve low temperature toughness. With the increase of temperature, a series of changes will occur in the properties of steel, mainly strength, hardness, plasticity, toughness first increased and then decreased, steel oxidation or high temperature corrosion. All of these will affect the performance of the material, and even make the material invalid. For example, the general use of carbon steel and cast iron temperature should not exceed 480 °C, Alloy Steel use temperature should not exceed 1150 °C.

(3) corrosion conditions in industry, corrosion rate is generally used to indicate the corrosion resistance of materials. Corrosion rate is expressed by the amount of metal material loss per unit area per unit time, or by the depth of metal material corrosion per unit time. Industry commonly used class 610 corrosion resistance rating standards, from Class I complete corrosion resistance to Class VI non-corrosion, see table 2.

Most engineering materials work in atmospheric environment, and atmospheric corrosion is a common problem. Atmospheric humidity, temperature, Sunshine, rainwater and corrosive gas content have great influence on material corrosion. In common alloys, the corrosion rate of carbon steel in the industrial atmosphere is 10 ^-605m/d, and it is often applied with protective coating such as paint. Low Alloy steel containing copper, phosphorus, nickel, chromium and other alloy components, its atmospheric corrosion resistance has been greatly improved, generally can not paint direct use. Aluminum, copper, lead, zinc and other alloys resistant to atmospheric corrosion very good.

(4) the factors that influence the wear resistance of materials are as follows: 1 the properties of materials themselves, including hardness, toughness, work hardening ability, thermal conductivity, chemical stability, surface state, etc. . Friction conditions: including the characteristics of phase wear material, friction pressure, temperature, speed, lubricant characteristics, corrosion conditions, etc. . Generally speaking, materials with high hardness are not easy to be pierced or ploughed into by a grinding object, and the fatigue limit is generally higher, so they have higher wear resistance, can improve wear resistance; therefore, hardness is the main aspect of wear resistance. In addition, the hardness of the material is variable during use. Metals that are easy to work and harden become hard in the process of friction, while metals that are easy to heat and soften in the process of friction.

3. The choice of material forming process is generally based on, when the material of the product is determined, the type of its forming process is generally determined. For example, if the product is cast iron, it should be cast into shape; if the product is thin plate, it should be pressed into shape; if the product is ABS plastic, it should be injection molded; if the product is ceramic, it should be selected corresponding ceramic forming process, etc. . However, the forming process also has some influence on the properties of the material, so the final properties of the material must be considered in the selection of the forming process. (1) properties of Product Materials 1 Mechanical Properties of materials. For example, steel gear parts, when its mechanical performance requirements are not high, can be cast; and mechanical performance requirements are high, should choose pressure processing molding. 2 Performance of materials. For example, if the steel die forging is selected to manufacture the flywheel parts in car and automobile engine, because of the high rotational speed of the car, it is required to run smoothly, and it is not allowed to have fiber exposed in the flywheel forging in use, so as to avoid corrosion and affect its performance, therefore, the open die forging should not be used, but closed die forging should be used. This is because the open die forging process produces only flywheel forgings with flanges, in which the fibrous tissues of the forgings are cut off and exposed during the subsequent trimming process, while closed die forgings do not have flanges, this shortcoming can be overcome. 3 The technological properties of the material. The technological properties of the materials include casting properties, forging properties, welding properties, heat treatment properties and cutting properties. For example, non ferrous materials that are readily oxidized and aspirated have poor weldability and should be joined by Argon arc welding rather than the normal manual arc welding process; polytetrafluoroethylene materials, although they are also Thermoplastic, however, due to its poor fluidity, it is not appropriate to use injection molding process, but only press sintering molding process. 4 Special Properties of materials. Special Properties of materials include wear resistance, corrosion resistance, heat resistance, conductivity or insulation. For example, the acid-resistant pump impeller, Shell, etc. , if the choice of stainless steel manufacturing, can only use casting molding; plastic manufacturing, you can use injection molding; if the requirements are both heat-resistant and corrosion-resistant, then should choose ceramic manufacturing, and correspondingly chooses the grouting molding technology. (2) the production batch of parts for mass-produced products, precision and productivity can be selected relatively high molding process. Although the manufacturing costs of these process equipment are high, this investment can be compensated by a reduction in material consumption per product. If a large number of forgings are produced, molding processes such as die forging, cold rolling, cold drawing and cold extrusion should be adopted; if a large number of non-ferrous alloy castings are produced, molding processes such as permanent mold casting, pressure casting and low pressure casting should be adopted; Appropriate choice of injection molding process. When producing these products in small batches, the molding processes with lower precision and productivity, such as hand molding, free forging, hand welding, and their combination with cutting, can be selected. (3) the complicated shape and precision of the parts require the complicated shape of metal parts, especially the parts with complicated inner cavity shape, can choose casting molding process, such as box body, pump body, cylinder body, valve body, Shell body, bed body, etc. For engineering plastic parts with complex shapes, injection molding process is usually used; for ceramic parts with complex shapes, grouting or injection molding process is often used; for metal parts with simple shapes, pressure processing or welding molding process is used; Engineering plastic parts with simple shape can be blow-molded, extruded or moulded. Ceramic parts with simple shape can be moulded. If the product is a casting, the ordinary sand mold casting can be used if the size requirements are not high, and the high dimensional accuracy requirements are different according to the casting materials and batches, investment Casting, vaporization casting, pressure casting and low pressure casting can be used respectively. If the products are forgings, low dimensional accuracy requirements, the use of free forging, and high accuracy requirements, the use of die forging, extrusion molding. If the product is plastic parts, low precision requirements, more choose hollow blow molding, and high precision requirements, then choose injection molding. (4) the existing production conditions the existing production conditions refer to the existing equipment capacity, the technical level of personnel and the possibility of outsourcing, etc. . For example, in the production of heavy machinery products, in the absence of large-capacity steel-making furnace and large-tonnage lifting and transportation equipment, often choose casting and welding combined molding process, that is, first large pieces into a few small pieces to cast, and then weld them together to make big pieces. For example, the oil plate parts on the lathe are usually formed by pressing with thin steel plate under the press, but if the site conditions are not available, other processes should be used. For example, when there is no sheet metal on site or large press, the casting process has to be used. When there is a sheet metal on site but no large press, an economical and feasible spinning process has to be used instead of stamping. (5) to give full consideration to the possibility of utilizing new processes, technologies and materials as the demand for new products in the industrial market increases, the requirements of users for product varieties and quality updates become more and more intense, the production property is changed from mass production to multi-variety and small-batch production, so the application range of new technology and new material is enlarged. Therefore, in order to shorten the production cycle, update the product type and quality, new Technologies, new materials, new technologies, new technologies such as precision casting, precision forging, precision blanking, cold extrusion, liquid die forging, superplastic forming, injection molding, static pressure forming of Powder metallurgy, ceramic, composite forming, rapid forming, etc. , adopt no-margin molding, make the parts close to the net-shape, thus significantly improve product quality and economic efficiency. In addition, in order to choose the molding process, it is necessary to have a clear understanding of the characteristics of various molding processes, the scope of application and the impact of molding processes on material properties. The characteristics of various blank forming processes for metallic materials are shown in Table 3.

Source: Caitong