Metal Material Selection: Basic Principles and Methods

Basic principle

When selecting materials and forming processes, it is necessary to consider whether the material performance meets the requirements under this working condition, whether the forming process is easy when using this material to manufacture parts, and whether the production and use of materials or parts are economical, that is, from the three aspects of applicability, processability and economy.

Metal Material Selection: Basic Principles and Methods 1

Applicability principle

The applicability principle means that the selected materials must be able to adapt to the working conditions and meet the requirements of satisfactory use.

Meeting the use requirements is a necessary condition for material selection, which is the first thing to consider when selecting materials. 

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

In order to meet the use requirements of materials, when selecting materials, the load conditions of parts, the use environment of materials and the use performance requirements of materials are mainly considered.

The load condition of parts mainly refers to the load size and stress state.

The use environment of materials refers to the environment in which the materials are located, such as medium, working temperature and friction.

The service performance requirements of materials refer to the service life, various generalized allowable stresses, generalized allowable deformation, etc. of materials.

Only when the above three aspects are fully considered, can the materials meet the performance requirements.

Technological principle

Generally, once materials are selected, their processing technology can be generally determined.

At the same time, the processing process changes the properties of materials;

The shape, structure, production batch and production conditions of parts also have a significant impact on the material processing technology.

The principle of processability refers to that the processability of materials shall be considered when selecting materials, and materials with good processability shall be preferred to reduce the manufacturing difficulty and cost of materials.

Each molding process has its own characteristics, advantages and disadvantages.

When parts of the same material are manufactured with different molding processes, their difficulty and cost are different, and the required material process performance is also different.

For example, when the shape of a part is complex and the size is large, forging is often difficult to achieve.

If casting or welding is used, the material must have good casting performance or welding performance, and the structure must also meet the requirements of casting or welding.

For another example, when manufacturing keys and pins by cold drawing process, the elongation of materials and the influence of deformation strengthening on mechanical properties of materials should be considered.

Metal Material Selection: Basic Principles and Methods 2

Economic principle

While meeting the requirements of material use and process, the use economy of materials must also be considered.

The principle of economy refers to the selection of materials with high performance price ratio.

The performance of materials refers to their use performance.

The service performance of materials can generally be represented by service time and safety degree.

Material price is mainly determined by cost.

The cost of materials includes production cost and use cost.

Generally, material cost is determined by raw material cost, raw material utilization rate, 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, existing production conditions and technical conditions.

1. Steps and methods for selecting materials and their forming processes

Analyze the service conditions of the parts, and find out the specific load, stress state, temperature, corrosion and wear conditions of the parts in the use process.

Most parts are used in normal temperature atmosphere, which mainly requires mechanical properties of materials.

Parts used under other conditions require materials to have some special physical and chemical properties.

If it is used under high temperature, the part materials shall have certain high temperature strength and oxidation resistance;

Chemical equipment requires materials with high corrosion resistance;

Some instrument parts require materials with electromagnetic properties.

For welding structures used in severe cold areas, requirements for low temperature toughness shall be attached;

Requirements for atmospheric corrosion resistance shall be attached when it is used in humid areas.

(1) Through analysis or test, combined with the results of failure analysis of similar materials, determine various generalized allowable stress indicators that allow materials to be used, such as allowable strength, allowable strain, allowable deformation and service time.

(2) Find out the main and secondary generalized allowable stress indicators, and take the important indicators as the main basis for material selection.

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

(4) The materials and their forming process are selected according to the forming process of materials, the complexity of parts, the production batch of parts, the existing production conditions and technical conditions.

(5) Considering the material cost, forming technology, material performance, reliability of use, etc., the most suitable material is selected by optimization method.

(6) If necessary, the materials shall be tested and put into production before verification or adjustment.

The above is just the general rule of material selection steps, and its workload and time consumption are quite large.

For important parts and new materials, when selecting materials, a large number of basic tests and batch trial production processes are required to ensure the safety of materials.

For less important and small batch parts, the materials are usually selected by referring to the use experience of similar materials under the same working conditions, the brand and specification of materials are determined, and the forming process is arranged.

If the parts are damaged normally, the original materials and forming process can be used;

If the damage of parts belongs to abnormal early damage, find out the cause of failure and take corresponding measures.

If it is a matter of material or its production process, new materials or new molding process can be considered.

Metal Material Selection: Basic Principles and Methods 3

2. Material selection basis

(1) Load conditions

Engineering materials are subject to various forces during use, including tensile stress, compressive stress, shear stress, cutting stress, torque, impact force, etc.

When materials work under load, their mechanical property requirements and failure forms are closely related to load conditions.

In engineering practice, any machinery and structure must be able to work safely and reliably while fulfilling the movement 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 being stressed.

Another example is that when a jack lifts a load, its screw must be kept in a balanced state in a straight line without sudden bending.

For engineering components, only when they meet the requirements of strength, stiffness and stability, can they work safely and reliably.

In fact, there are specific conditions for the use of these three aspects of materials in material mechanics.

When analyzing the stress conditions of materials or selecting materials according to the stress conditions, in addition to considering the mechanical properties of materials, the relevant knowledge of material mechanics must also be used for scientific material selection.

Table 1 Stress, Failure Forms and Required Mechanical Properties of Several Common Parts

Spare parts

Working conditions

Common failure forms

Main mechanical property requirements

Stress category

Load properties

Other forms

Ordinary fastening bolt

Tensile stress and shear stress

Static load

Excessive deformation and fracture

Yield strength Shear strength

Transmission shaft

Bending stress Torsional stress

Cyclic shock

Friction and vibration at journal

Fatigue failure, excessive deformation and wear at journal

Comprehensive mechanical properties

Transmission gear

Compressive stress and bending stress

Cyclic shock

Strong friction, vibration

Wear, pitting peeling, tooth breakage

Surface: hardness, bending fatigue strength, contact fatigue resistance; Center: yield strength, toughness

Spring

Torsional stress Bending stress

Cyclic shock

Vibration

Loss of elasticity, fatigue fracture

Elastic limit, yield ratio, fatigue strength

Oil pump plunger pair

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

Hardness, sufficient strength and toughness

Die-casting die

Complex stress

Cyclic shock

High temperature, friction, liquid metal corrosion

Thermal fatigue, brittle fracture, wear

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

Rolling bearing

Compressive stress

Cyclic shock

Strong friction

Fatigue fracture, wear, pitting peeling

Contact fatigue resistance, hardness and wear resistance

Crankshaft

Bending stress Torsional stress

Cyclic shock

Journal friction

Brittle fracture, fatigue fracture, erosion and wear

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

Connecting rod

Tensile stress and compressive stress

Cyclic shock

Brittle fracture

Compressive fatigue strength, impact fatigue resistance

(2) Service temperature of materials

Most materials are used at room temperature, of course, there are also materials used at high or low temperatures.

Due to different service temperatures, the properties of materials required are also very different.

With the decrease of temperature, the toughness and plasticity of steel materials decrease continuously.

When the temperature decreases to a certain extent, its toughness and plasticity will decrease significantly.

This temperature is called ductile brittle transition temperature.

When used below the ductile brittle transition temperature, materials are prone to brittle fracture under low stress, thus causing harm.

Therefore, when selecting the steel used at low temperature, the material with ductile brittle transition temperature lower than the working condition should be selected.

Alloying of various low temperature steels aims at reducing carbon content and improving low temperature toughness of materials.

With the increase of temperature, the properties of steel materials will undergo a series of changes, mainly including the decrease of strength and hardness, the increase and then decrease of plasticity and toughness, and the oxidation or corrosion of steel at high temperature.

This has an impact on the performance of the material, and even makes the material invalid.

For example, the service temperature of carbon steel and cast iron should not exceed 480 ℃, while that of alloy steel should not exceed 1150 ℃.

(3) 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 in unit time;

It can also be expressed by the corrosion depth of metal materials in unit time.

The corrosion resistance rating standards of 6 categories and 10 grades are commonly used in industry, ranging from Class I complete corrosion resistance to Class VI non corrosion resistance, as shown in Table 2.

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

Corrosion resistance classification

Corrosion resistance classification

Corrosion rate, mm/d

I

Complete corrosion resistance

1

<0.001

Very corrosion resistant

2

3

0.001~0.005

0.005~0.01

III

Corrosion resistance

4

5

0.01~0.05

0.05~0.1

IV

Corrosion resistance

6

7

0.1~0.5

0.5~1.0

V

Poor corrosion resistance

8

9

1.0~5.0

5.0~10.0

VI

Non corrosion resistance

10

>10.0

Most engineering materials work in atmospheric environment, and atmospheric corrosion is a common problem.

The humidity, temperature, sunshine, rainwater and corrosive gas content of the atmosphere have a great impact on the corrosion of materials.

In common alloys, the corrosion rate of carbon steel in industrial atmosphere is 10 ^ – 605m/d, which can be used after painting and other protective layers are often applied.

Low alloy steel containing copper, phosphorus, nickel, chromium and other alloy components has greatly improved atmospheric corrosion resistance, and can be used directly without painting.

Aluminum, copper, lead, zinc and other alloys have good atmospheric corrosion resistance.

(4) Wear resistance

The factors affecting the wear resistance of materials 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 phase abrasive materials, the pressure, temperature, speed during friction, the characteristics of lubricants, corrosion conditions, etc.

Generally speaking, materials with high hardness are not easy to be penetrated or ploughed by grinding objects, and the fatigue limit is generally high, so the wear resistance is high;

At the same time, it has high toughness.

Even if it is punctured or ploughed, it will not be torn off in pieces, which can improve the wear resistance;

Therefore, hardness is the main aspect of wear resistance.

In addition, the hardness of materials is also variable during use.

The metal that is easy to work hardening becomes hard during friction, while the metal that is easy to be softened by heat will be softened during friction.

Metal Material Selection: Basic Principles and Methods 4

3. Basis for selection of material forming process

Generally speaking, 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;

If the product is sheet metal, the sheet metal shall be selected for stamping;

If the product is ABS plastic, injection molding should be selected;

If the products are ceramic parts, the corresponding ceramic forming process shall be selected.

However, the forming process also has a certain impact on the performance of materials, so the final performance requirements of materials must also be considered when selecting the forming process.

(1) Performance of product materials

① Mechanical properties of materials.

For example, gear parts made of steel can be cast when their mechanical properties are not required;

When the mechanical properties are required to be high, pressure processing shall be used.

② Service 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 requirements for smooth driving, it is not allowed to expose fibers in flywheel forgings in use to avoid corrosion and affect their performance.

Therefore, open die forging is not suitable, but closed die forging should be used.

This is because the open die forging process can only forge flywheel forgings with flash, and the fiber structure of the forgings will be cut off and exposed in the subsequent trimming process;

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, since the weldability of nonferrous metal materials that are easy to oxidize and breathe is poor, the argon arc welding process should be used for their connection, rather than the ordinary manual arc welding process;

Although PTFE is also a thermoplastic material, due to its poor fluidity, it is not suitable to use the injection molding process, but only the pressing and sintering molding process.

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 the acid resistant pump are made of stainless steel, they can only be cast;

If plastic is selected for manufacturing, injection molding can be used;

If it is required to be both heat-resistant and corrosion resistant, then it should be made of ceramics, and the grouting molding process should be selected accordingly.

(2) Production batch of parts

For mass production products, the forming process with high precision and productivity can be selected.

Although the manufacturing cost of these molding process equipment is relatively 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 forming processes shall be selected;

For mass production of non-ferrous alloy castings, metal mold casting, die casting, low pressure casting and other molding processes should be selected;

For mass production of MC nylon parts, injection molding process should be selected.

When producing these products in small batches, the forming processes with low precision and productivity can be selected, such as manual molding, free forging, manual welding, and the forming processes combined with cutting.

(3) Shape complexity and accuracy requirements of parts

For metal parts with complex shapes, especially those with complex inner cavities, casting process can be selected, such as box, pump body, cylinder block, valve body, shell, bed, etc;

For engineering plastic parts with complex shapes, injection molding process is often used;

For ceramic parts with complex shapes, injection molding or injection molding process is often used;

For metal parts with simple shape, pressure processing or welding forming process can be selected;

For engineering plastic parts with simple shape, blow molding, extrusion molding or molding process can be selected;

The ceramic parts with simple shape are mostly molded.

If the product is a casting, ordinary sand casting can be used if the size requirement is not high;

For those requiring high dimensional accuracy, investment casting, evaporative pattern casting, pressure casting and low pressure casting can be selected according to different casting materials and batches.

If the product is a forging with low dimensional accuracy requirements, it is usually formed by free forging;

For those with high precision requirements, die forging and extrusion forming are selected.

If the product is made of plastic and requires low precision, hollow blow molding is preferred;

For those with high precision requirements, injection molding is selected.

(4) Existing production conditions

The existing production conditions refer to the existing equipment capacity, technical level of personnel and outsourcing possibility of products.

For example, when producing heavy machinery products, under the condition that there is no large capacity steelmaking furnace and large tonnage lifting and transportation equipment on site, the process of casting and welding combined forming is often used, that is, first divide the large pieces into several small pieces to cast, and then weld them into large pieces.

For another example, the oil pan parts on the lathe are usually formed by stamping thin steel plates under the press, but if the site conditions are not available, other process methods should be used.

For example, there are no thin plates or large presses on site, so they have to be produced by casting process;

When there are thin plates on site, but there is no large press, it is necessary to choose an economical and feasible spinning forming process to replace stamping forming.

(5) Fully consider the possibility of using new processes, technologies and materials

With the increasing demand of the industrial market, users have increasingly strong requirements for product variety and quality updating, which makes the production nature change from mass production to multi variety and small batch production, thus expanding the application scope of new processes, new technologies 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 forming, injection molding, powder metallurgy, ceramics and other static pressure forming, composite material forming, rapid forming, etc. are adopted under possible conditions to make the parts nearly net shaped.

Thus, the product quality and economic benefits can be significantly improved.

In addition, in order to reasonably select the molding process, it is also necessary 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 blank forming processes of metal materials are shown in Table 3.

Table 3 Characteristics of various blank forming processes

 CastingForgingStamping partsWeldmentRolled stock
Molding characteristicsForming under liquid stateSolid plastic deformationSolid plastic deformationConnection under crystallization or solid stateSolid plastic deformation
Requirements for material process performanceGood liquidity and low shrinkageGood plasticity, small deformation resistanceGood plasticity, small deformation resistanceHigh strength, good plasticity, good chemical stability in liquid stateGood plasticity, small deformation resistance
Common materialsSteel materials, copper alloys, aluminum alloysMedium carbon steel, alloy structural steelMild steel, nonferrous metal sheetLow carbon steel, low alloy steel, stainless steel, aluminum alloyLow and medium carbon steel, alloy steel, aluminum alloy, steel alloy
Metal structure characteristicsCoarse grain and loose tissueThe grains are fine, dense and directionally arrangedForming new streamline organization along the stretching directionThe weld zone is of casting structure, and the fusion zone and overheat zone are coarseThe grains are fine, dense and directionally arranged
Metal structure characteristicsCoarse grain and loose tissueThe grains are fine, dense and directionally arrangedForming new streamline organization along the stretching directionThe weld zone is of casting structure, and the grains in the fusion zone and superheat zone are coarseThe grains are fine, dense and directionally arranged
Mechanical propertySlightly lower than forgingsBetter than castings of the same compositionThe strength and hardness of the deformed part are high, and the structural rigidity is goodThe mechanical properties of the joint can reach or approach the base metalBetter than castings of the same composition
Structural characteristicsUnrestricted shape, can produce parts with rather complex structureSimple shapeLight structure and slightly complex shapeSize and structure are generally unrestrictedSimple shape, less changes in horizontal dimensions
Material utilization ratehighlowhigherhigherLower
Production cyclelongShort free forging, long die forginglongShortershort
Production costsLowerhigherThe larger the batch, the lower the costhigherLower
Main scope of applicationVarious structural and mechanical partsTransmission parts, tools, molds and other partsVarious parts formed by sheetVarious metal structural parts, partially used for parts blanksStructural blanks
Application examplesFrame, bed, base, workbench, guide rail, gearbox, pump body, crankshaft, bearing seat, etcMachine tool spindle, transmission shaft, crankshaft, connecting rod, bolt, spring, die, etcAutomobile body, engine meter housing, electrical instrument housing, water tank, oil tankBoiler, pressure vessel, chemical vessel pipeline, plant structure, bridge, vehicle body, hull, etcSmooth shaft, lead screw, bolt, nut, pin, etc

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