Principles and methods of selecting metal materials

Basic principle

When choosing materials and manufacturing processes, it is critical to evaluate whether the performance of the material meets the needs of the working conditions, whether the manufacturing process is feasible with this material, and whether the production and use of the material or parts are economical. This assessment must be conducted from three perspectives: suitability, feasibility and cost-effectiveness.

Applicability principle

The principle of suitability requires that the materials chosen are capable of withstanding the working conditions and meet the requirements for satisfactory use. Ensuring that materials meet usage requirements is a crucial step in the materials selection process.

The usage requirements of materials are reflected in their internal quality specifications, such as chemical composition, structure, mechanical properties, physical properties and chemical properties.

When selecting materials, it is important to take into account the loading conditions of the parts, the use environment of the materials, and the performance requirements of the materials.

The load conditions of the parts refer to the size and stress state of the load. The use environment of materials refers to the environment in which materials are used, including the medium, working temperature and friction. The service performance requirements of materials refer to their service life and various generalized allowable stresses, strains, etc.

Only by carefully considering these three aspects can materials meet performance requirements.

Technological principle

Once the materials have been selected, the processing technology can usually be determined. However, it is important to keep in mind that the processing process can change the properties of the materials. In addition, factors such as shape, structure, batch size and production conditions of parts also play a significant role in determining the material processing technology.

The feasibility principle requires considering the processability of materials when selecting them, and materials with good processability should be preferred to minimize manufacturing difficulty and cost. Each manufacturing process has its own characteristics, advantages and disadvantages.

When parts made from the same material are manufactured using different processes, the difficulty and cost can vary, as can the performance required in processing the material. For example, forging may not be viable for parts with complex shapes and large sizes. In such cases, casting or welding can be used, but the material must have good casting or welding performance and the structure must meet the requirements for casting or welding.

In another example, in the manufacture of keys and pins by cold drawing, the stretching of the materials and the impact of reinforcing deformation on their mechanical properties must be considered.

Economic principle

In addition to meeting the requirements for material use and processing, it is also important to consider the cost-effectiveness of materials.

The cost-benefit principle refers to the selection of materials that offer a high performance/price ratio. The performance of materials refers to their use performance, which can generally be represented by their service life and safety level. The price of materials is mainly determined by their cost, which includes both the production cost and the usage cost.

The cost of materials is influenced by several factors, including the cost of raw materials, utilization rate of raw materials, material forming cost, processing cost, installation and commissioning cost, maintenance cost, management cost and others.

Steps, methods and basis for selection of materials and forming processes

The steps for selecting materials and manufacturing processes are as follows:

• Choose materials based on usage conditions and requirements.
• Based on the selected materials, choose an appropriate manufacturing process, taking into account factors such as the cost of materials, processing properties of materials, complexity of parts, batch size of parts, existing production conditions and technical requirements .

1. Steps and methods of material selection and their forming processes

To evaluate the service conditions of the parts, the specific load, stress state, temperature, corrosion and wear conditions that the parts will suffer during use must be determined.

For parts used under normal temperature conditions, the main requirement is that the materials have adequate mechanical properties. However, for parts used in different conditions, the materials must have specific physical and chemical properties.

If parts are used at high temperatures, the materials must have high temperature resistance and oxidation resistance. Parts used in chemical equipment must have high resistance to corrosion. Some instrument parts require materials with electromagnetic properties. For welding structures used in extremely cold areas, low temperature resistance requirements must be considered.

When used in humid areas, atmospheric corrosion resistance requirements must be included. The following are the general steps for material selection:

• Through analysis or testing, together with the failure analysis results of similar materials, determine various generalized indicators of allowable stress, such as allowable strength, allowable strain, allowable deformation and service time.
• Identify the main and secondary generalized allowable stress indicators and use the most important indicators as the primary basis for material selection.
• Based on key performance indicators, select several materials that meet the requirements.
• Choose the materials and their forming process based on the materials forming process, the complexity of the parts, the production batch of the parts, the existing production conditions and the technical conditions.
• Consider factors such as material cost, forming technology, material performance and usage reliability to select the most suitable material using an optimization method.
• If necessary, test materials and put them into production for verification or adjustment.

Please note that these are just general guidelines for material selection and the process can be time-consuming and complex.

For important parts and new materials, a significant number of basic tests and trial production processes are required to ensure material safety during selection. For small and less important batch parts, materials are typically selected based on the experience of using similar materials under the same working conditions, and the brand and specification of materials are determined, followed by organizing the forming process.

If parts are damaged normally, the original materials and forming process can be used. If the damage is due to abnormal premature damage, the cause of the failure must be determined and appropriate measures must be taken. If it is a result of the material or its production process, new materials or a new molding process may be considered.

2. Material selection basis

(1) Loading conditions

Engineering materials are exposed to various forces during operation, such as tensile stress, compressive stress, shear stress, shear stress, torque and impact force, among others.

The mechanical properties and failure modes of materials are closely linked to the loading conditions to which they are subjected.

In engineering, it is crucial that machines and structures function safely and reliably while meeting their movement requirements.

For example, the spindle of a machine tool must be able to operate normally without breaking or deforming excessively under stress. Another example is that when a jack lifts a load, the screw must remain straight and balanced without suddenly bending.

The safe and reliable operation of engineering components depends on meeting strength, rigidity and stability requirements.

There are specific conditions for each of these aspects of materials in material mechanics that must be considered when analyzing stress conditions or selecting materials.

When selecting materials based on stress conditions, it is important to consider not only the mechanical properties of the materials, but also relevant knowledge of the mechanics of the materials to make a scientifically informed choice.

Table 1 Stress, failure modes and required mechanical properties of several common parts

Spare parts	Work conditions			Common failure forms	Main mechanical property requirements
	Stress Category	Load properties	Other ways
Common fixing screw	Tensile stress and shear stress	static charge		Excessive deformation and fracture	Yield resistance Shear resistance
Drive shaft	Bending stress Torsional stress	Cyclic shock	Friction and vibration in the trunnion	Failure due to fatigue, excessive deformation and wear on the trunnion	Comprehensive mechanical properties
Transmission gear	Compressive stress and bending stress	Cyclic shock	Strong friction, vibration	Wear, peeling, tooth breakage	Surface: hardness, flexural fatigue resistance, contact fatigue resistance; Center: yield strength, toughness
Spring	Torsional stress Bending stress	Cyclic shock	Vibration	Loss of elasticity, fatigue fracture	Elastic limit, yield rate, fatigue resistance
Pair of oil pump plungers	Compressive stress	Cyclic shock	Friction, oil corrosion	abrasion	Hardness and compressive strength
Cold working die	Complex stress	Cyclic shock	Strong friction	Wear and brittle fracture	Sufficient hardness, strength and toughness
Casting Mold	Complex stress	Cyclic shock	High temperature, friction, liquid metal corrosion	Thermal fatigue, brittle fracture, wear	High temperature resistance, thermal fatigue resistance, toughness and red hardness
Bearing	Compressive stress	Cyclic shock	Strong friction	Fatigue fracture, wear, peeling	Contact fatigue resistance, hardness and wear resistance
Crankshaft	Bending stress Torsional stress	Cyclic shock	Diary Friction	Brittle fracture, fatigue fracture, erosion and wear	Fatigue resistance, hardness, impact fatigue resistance and comprehensive mechanical properties
connecting rod	Tensile stress and compressive stress	Cyclic shock		Brittle fracture	Compressive fatigue resistance, impact fatigue resistance

(2) Service temperature of materials

Most materials are typically used at room temperature, however, there are also materials that are used at high or low temperatures.

Due to these varying service temperatures, the required material properties also vary greatly.

As the temperature decreases, the toughness and plasticity of steel materials decrease continuously. At a certain point, there is a significant decrease in toughness and plasticity, known as the ductile-brittle transition temperature.

When used below the ductile-brittle transition temperature, materials are susceptible to brittle fracture under low stress, which can result in damage. Therefore, when selecting steel for use at low temperatures, materials with a ductile-brittle transition temperature lower than working conditions should be chosen.

Alloying various low-temperature steels aims to reduce carbon content and improve their low-temperature toughness.

As the temperature increases, the properties of steel materials undergo several changes, including a decrease in strength and hardness, an increase and then a decrease in plasticity and toughness, and oxidation or corrosion at high temperatures.

These changes impact the performance of the material and may render it unusable. For example, the service temperature for carbon steel and cast iron should not exceed 480 ℃, while the service temperature for alloy steel should not exceed 1150 ℃.

(3) Corrosion

In industry, corrosion rate is commonly used to express the corrosion resistance of materials.

Corrosion rate is measured as the loss of metallic material per unit area over a specific period of time, or as the depth of corrosion in the metallic material over time.

The industry typically uses a 6-category, 10-grade corrosion resistance classification system, ranging from Class I with complete corrosion resistance to Class VI without corrosion resistance, as shown in Table 2.

Table 2 Classification and Classification Criteria for Corrosion Resistance of Metallic Materials

Corrosion resistance rating		Corrosion resistance rating	Corrosion rate, mm/d
I	Complete corrosion resistance	1	<0.001
Ⅱ	Very resistant to corrosion	23	0.001~0.005 0.005~0.01
III	Corrosion resistance	45	0.01~0.05 0.05~0.1
4	Corrosion resistance	67	0.1~0.5 0.5~1.0
V	Poor corrosion resistance	89	1.0~5.0 5.0~10.0
SAW	No corrosion resistance	10	>10.0

Most engineering materials operate in atmospheric environments and suffer from atmospheric corrosion, which is a common problem.

Atmospheric humidity, temperature, sunlight, rainwater and the content of corrosive gases have a great impact on the corrosion of these materials.

In common alloys, carbon steel has a corrosion rate of 10^-605 m/d in industrial atmospheres, but can be used after being painted or treated with other protective layers.

Low-alloy steel containing elements such as copper, phosphorus, nickel and chromium has greatly improved resistance to atmospheric corrosion and can be used without being painted.

Materials such as aluminum, copper, lead and zinc have good resistance to atmospheric corrosion.

(4) Wear resistance

The following are the factors that affect the wear resistance of materials:

① Material properties: including hardness, toughness, ability to undergo work hardening, thermal conductivity, chemical stability, surface condition, etc.

② Friction conditions: including the characteristics of the abrasive material in friction, pressure, temperature, friction speed, lubricant properties and the presence of corrosive conditions.

In general, materials with high hardness are less susceptible to penetration or abrasion by grinding objects and have a high fatigue limit, resulting in high wear resistance. Furthermore, the high toughness ensures that even if the material is penetrated or worn away, it will not break, further improving its wear resistance.

Therefore, hardness is the main aspect of wear resistance. It is important to note that the hardness of materials may change during use. For example, metals that undergo hardening become harder during friction, while metals that can be softened by heat can soften during friction.

3. Basis for selection of material forming process

Generally, once the material of a product has been determined, the type of forming process is typically identified.

For example, if the product is made of cast iron, casting must be used; if it is made of sheet metal, stamping should be the choice; if it is made of ABS plastic, injection molding is the best option; and if they are ceramic parts, the appropriate ceramic forming process must be selected.

However, it is important to keep in mind that the forming process can also affect the performance of the material, so the final performance requirements of the material must be taken into consideration when selecting the forming process.

Performance of product materials

① Mechanical Properties of Materials

For example, steel gear parts can be cast when their mechanical properties are not critical, but when high mechanical properties are required, pressure processing must be used.

② Material service performance

For example, when manufacturing steering wheel parts for cars and automobile engines, steel die forging should be used instead of open die forging. This is because the high speed of cars and the requirement for smooth driving means that exposed fibers in the forged steering wheel parts can cause corrosion and affect performance. Closed die forging is preferable to open die forging as it eliminates burr and prevents cutting and exposing the fiber structure of forged parts.

③ Technological Properties of Materials

Technological properties include casting properties, forging properties, welding properties, heat treatment properties and cutting properties. For example, non-ferrous metallic materials with poor weldability should be connected using argon arc welding instead of manual arc welding. PTFE, as it is a thermoplastic material with low fluidity, is not suitable for injection molding and should only be molded by pressing and sintering.

④ Special Properties of Materials

Special properties include wear resistance, corrosion resistance, heat resistance, conductivity or insulation. For example, the impeller and casing of an acid-resistant pump must be made of stainless steel and cast. If plastic is used, injection molding is an option. If heat and corrosion resistance is required, ceramic must be used and shaped through the grouting process.

(2) Parts production batch

For mass production of products, the forming process with high precision and productivity must be selected to ensure accuracy and efficiency. Although the equipment required for these molding processes may have a relatively high manufacturing cost, this investment can be offset by the reduction in material consumption per product.

For mass production of forgings, recommended forming processes include forging, cold rolling, cold drawing and cold extrusion.

For mass production of non-ferrous alloy castings, metal die casting, pressure casting and low pressure casting are the recommended molding processes.

For mass production of MC nylon parts, injection molding process is the preferred choice.

For small batch production, forming processes with lower precision and productivity can be selected, such as manual molding, free forging, manual welding and processes involving cutting.

(3) Shape complexity and accuracy requirements of parts

For metal parts with complex shapes, particularly those with complex internal cavities, the casting process is often selected, such as for casing, pump body, cylinder block, valve body, casing and bed components.

Engineering plastic parts with complex shapes are typically produced using the injection molding process.

Ceramic pieces with complex shapes can be produced using the injection molding or casting process.

For metal parts with simple shapes, pressure processing or welding forming processes can be used.

Engineering plastic parts with simple shapes can be produced using blow molding, extrusion molding, or molding processes.

Ceramic pieces with simple shapes are usually molded.

If the product is a casting and dimensional accuracy is not a high requirement, ordinary sand casting can be used. For high dimensional accuracy, precision casting, evaporative pattern casting, pressure casting or low pressure casting can be selected based on casting material and batch size.

For low dimensional accuracy requirements in forging, free forging is commonly used. For high precision requirements, die forging or extrusion forming are selected.

If the product is plastic and requires low precision, hollow blow molding is preferred. For high precision requirements, injection molding is selected.

(4) Existing production conditions

Existing production conditions refer to the current capacity of equipment, technical expertise of personnel and the possibility of outsourcing products.

For example, when producing heavy machinery products, if there is no large-capacity steelmaking furnace or heavy lifting and transportation equipment on site, the combined process of casting and welding is often used. This involves breaking large parts into smaller parts for casting and then welding them together to form larger parts.

As another example, crankcase parts for a lathe are typically produced by stamping thin steel plates with a press. If on-site conditions are not suitable for this process, alternative methods must be used.

For example, if there are no thin sheets or large presses on site, it may be necessary to use the casting process. If thin plates are available but a large press is not available, an economical and viable spin forming process can be used as a substitute for stamping forming.

(5) Consideration of new processes, technologies and materials

With the increasing demands of the industrial market, users have increasingly high requirements for product variety and quality upgrades, leading to a shift from mass production to multiple variety and small batch production. This expands the scope of application of new processes, technologies and materials.

To shorten the production cycle and improve product types and quality, it is necessary to consider the use of new processes, technologies and materials such as precision casting, precision forging, precision cutting, cold extrusion, liquid die forging , superplastic forming, injection molding, powder metallurgy, ceramics and other static pressure forming, composite material forming and rapid forming. This will enable near-network-shaped parts and a significant improvement in product quality and economic benefits.

Furthermore, in order to make a reasonable selection of the molding process, it is important to have a clear understanding of the characteristics and scope of application of various molding processes, as well as the impact of the molding process on the material properties.

The characteristics of various metal materials molding processes are shown in Table 3.

Table 3 Characteristics of various blank forming processes

	Foundry	Forging	Stamping of parts	Welding	Laminated Stock
Molding Features	Forming under liquid state	Solid plastic deformation	Solid plastic deformation	Connection under crystallization or solid state	Solid plastic deformation
Requirements for material process performance	Good liquidity and low shrinkage	Good plasticity, small resistance to deformation	Good plasticity, small resistance to deformation	High strength, good plasticity, good chemical stability in the liquid state	Good plasticity, small resistance to deformation
Common materials	Steel materials, copper alloys, aluminum alloys	Medium carbon steel, alloy structural steel	Mild steel, non-ferrous sheet metal	Low carbon steel, low alloy steel, stainless steel, aluminum alloy	Low and medium carbon steel, alloy steel, aluminum alloy, alloy steel
Characteristics of the metal structure	Coarse grain and loose weave	The grains are fine, dense and arranged directionally	Forming a new simplified organization along the stretching direction	The welding zone is molten structure, and the fusion zone and superheat zone are coarse	The grains are fine, dense and arranged directionally
Characteristics of the metal structure	Coarse grain and loose weave	The grains are fine, dense and arranged directionally	Forming a new simplified organization along the stretching direction	The weld zone is molten in structure and the grains in the fusion zone and superheat zone are coarse	The grains are fine, dense and arranged directionally
Mechanical property	A little lower than forged ones	Better than castings of the same composition	The strength and hardness of the deformed part are high and the structural rigidity is good	The mechanical properties of the joint can reach or approach that of the base metal	Better than castings of the same composition
Structural features	Unrestricted format, can produce parts with a very complex structure	Simple form	Light structure and slightly complex shape	Size and structure are generally unrestricted	Simple shape, fewer changes to horizontal dimensions
Material utilization rate	high	low	higher	higher	Lower
Production cycle	far away	Short free forging, long forging	far away	Shorter	short
Production costs	Lower	higher	The larger the batch, the lower the cost	higher	Lower
Main scope of application	Various structural and mechanical parts	Transmission parts, tools, molds and other parts	Multiple pieces formed per sheet	Various metallic structural parts, partially used for blanks	Structural blanks
Application examples	Frame, bed, base, bench, guide rail, gear box, pump body, crankshaft, bearing seat, etc.	Machine tool spindle, drive shaft, crankshaft, connecting rod, screw, spring, die, etc.	Automobile body, engine gauge housing, electrical instrument housing, water tank, oil tank	Boiler, pressure vessel, chemical vessel piping, plant structure, bridge, vehicle body, hull, etc.	Smooth shaft, lead screw, bolt, nut, pin, etc.