As industry continues to develop at a rapid pace, increasingly higher demands are being placed on the selection and application of materials in mechanical design. Choosing the right materials has become a critical aspect of the design process as well as one of the industry's major challenges.
While ensuring that machinery meets the required strength and rigidity, engineers must also strive to reduce material consumption, improve energy efficiency, protect the environment, and enhance the overall economy and practicality of mechanical products. As a result, material selection has become an important area of research in mechanical engineering.
With rapid economic growth, issues related to the environment and natural resources have become increasingly prominent. After years of extensive industrial development, many of the resources commonly used in mechanical manufacturing have become more limited. Although China possesses abundant natural resources, its large population results in relatively low per capita resource availability. In addition, market competition and fluctuations in resource supply have increased the cost of raw materials used in mechanical manufacturing, further limiting their application.
For these reasons, greater attention should be given to material selection in mechanical design. Appropriate material selection can help reduce manufacturing costs, improve production efficiency, extend the service life of machinery, conserve energy, reduce emissions, and minimize environmental impact, thereby contributing to sustainable industrial development.
Among all engineering materials, metals—especially steel—are the most widely used. According to industry statistics, steel materials account for more than 90% of materials used in mechanical products.
Steel is extensively used not only because of its excellent mechanical properties, such as high strength, good plasticity, and toughness, but also because it is relatively inexpensive, readily available, and capable of meeting a wide range of engineering requirements. Among various types of steel, alloy steel is widely used in the manufacture of mechanical components due to its superior performance.
Metals other than steel are generally classified as non-ferrous metals. Among them, aluminum and its alloys are the most commonly used. They offer advantages such as low weight, excellent thermal conductivity, and good electrical conductivity. In addition, many non-ferrous metals provide good wear resistance and corrosion resistance, making them suitable for specialized engineering applications.
Polymer materials generally fall into three main categories: plastics, rubber, and synthetic fibers. They offer many advantages. For example, their raw materials are abundant and can be derived from petroleum, natural gas, and coal. They also require relatively low energy consumption during production. In addition, polymer materials have low density—typically only about one-sixth that of steel—good elasticity within an appropriate temperature range, and excellent corrosion resistance.
A typical example is polytetrafluoroethylene (PTFE), often referred to as the "King of Plastics." PTFE has outstanding corrosion resistance and excellent chemical stability. It does not become brittle at extremely low temperatures or soften in boiling water. As a result, PTFE is widely used in chemical processing equipment and refrigeration equipment.
However, polymer materials also have some disadvantages. They are generally prone to aging, many have poor flame resistance, and most have relatively low heat resistance.
Engineering ceramics include high-temperature structural ceramics based on silicon nitride and silicon carbide, as well as tool ceramics primarily made from alumina.
Ceramic materials are characterized by high hardness, excellent wear resistance, corrosion resistance, high melting points, high stiffness, and lower density than steel. They are often described as materials that are "as strong as steel, as hard as diamond, and as light as aluminum."
Today, ceramic materials are widely used in components such as seals, rolling bearings, and cutting tools. However, they also have some limitations, including high brittleness, low fracture toughness, relatively high cost, and poor machinability.
Composite materials are made by combining two or more materials with significantly different physical and mechanical properties. These constituent materials generally consist of a matrix material and a reinforcing material.
The reinforcing material improves the strength and stiffness of the composite, while the matrix material binds and supports the reinforcement, allowing the composite to achieve performance characteristics that cannot be obtained from a single material alone.
The selection of materials in mechanical design should be based on various factors, including operating conditions, manufacturing requirements, and economic considerations. The following principles should be considered when selecting suitable materials.
The selection of materials should first consider the magnitude and characteristics of the loads applied to the components. This requires a thorough understanding of the mechanical properties of different materials.
In principle, brittle materials are only suitable for components that operate under static loads. When a certain level of impact or shock loading is involved, ductile materials should generally be selected.
The properties of metallic materials can often be improved through heat treatment. Therefore, heat treatment processes should be fully utilized to enhance material performance and maximize the potential of materials. For example, tempered steels with different tempering temperatures can achieve different mechanical properties and are widely used in mechanical manufacturing.
The operating conditions of machinery are also an important factor affecting material selection.
Regarding operating temperature, two aspects should be considered. First, the coefficients of thermal expansion of two mating components should not differ significantly. Otherwise, temperature changes may generate excessive stress or cause loose fitting between components.
Second, changes in the mechanical properties of materials at different temperatures should also be considered.
During operation, some mechanical components may experience wear. In such cases, surface hardness should be increased to improve wear resistance. Therefore, suitable materials such as hardened steel, carburized steel, and nitrided steel should be selected according to the specific application requirements.
The complexity of a component's structure and the manufacturability of the selected material should also be considered.
For components with complex structures, cast blanks are generally preferred. Alternatively, components can be formed by stamping sheet materials and then assembled through welding.
For components with simpler structures, forging processes can be used to produce the required blanks.
Economic factors are also important when selecting materials. A reasonable material choice should not only meet performance requirements but also help reduce manufacturing costs.
When a low-cost material can satisfy the required operating conditions, there is no need to select a more expensive material. This is particularly important for components manufactured in large quantities.
The processing cost of materials should also be considered. For example, although cast iron may be less expensive than steel plates for manufacturing certain box-type components, when production quantities are small, welded steel plate structures may be more economical because they can eliminate the production costs associated with casting.
Improving material utilization is another important consideration. Using manufacturing methods that require little or no cutting, such as precision casting, die forging, and cold drawing, can significantly improve material efficiency.
Composite structural designs can also help reduce material costs. For example, railway wheels commonly use a wheel core made of ordinary material with a wear-resistant wheel rim that is heat-fitted onto the outside.
The use of alternative materials can help conserve limited resources. For example, aluminum bronze can replace tin bronze in bearing applications, and manganese-boron alloy steel can replace nickel alloy steel in certain applications.
The supply conditions of materials should also be considered. For small-batch production, it is preferable to reduce the number of different material types and specifications used in the same machine to simplify procurement and manufacturing.
The selection of materials in mechanical design plays a very important role in engineering applications. To achieve more environmentally friendly, efficient, and reliable designs, engineers should carefully consider both the characteristics of different materials and the principles of material selection.
By choosing suitable materials based on mechanical performance, manufacturing requirements, operating conditions, and economic factors, mechanical products can achieve better performance, longer service life, and improved overall efficiency while reducing resource consumption and environmental impact.
