Classification of metal materials
Metal materials refer to materials composed primarily of metallic elements and possessing metallic properties. They possess excellent electrical and thermal conductivity, ductility, and mechanical properties. They are one of the most widely used materials in modern industrial production, and are used in everything from daily necessities to aerospace equipment. Metal materials can be classified in a variety of ways, including by chemical composition, structure, performance characteristics, and application. These classifications facilitate a better understanding and application of metal materials, meeting the needs of diverse applications.
Based on their chemical composition, metal materials can be divided into two main categories: pure metals and alloys. Pure metals are composed of a single metallic element, such as copper, aluminum, iron, gold, and silver. Pure metals are found in low concentrations in nature, mostly in the form of alloys or compounds. Pure metals possess unique physical and chemical properties. For example, copper has excellent electrical and thermal conductivity and is commonly used in wire and cable; aluminum, with its low density and corrosion resistance, is widely used in aerospace and construction; and gold and silver, with their excellent chemical stability and ductility, are commonly used in jewelry and electronics. However, the mechanical properties of pure metals often struggle to meet the requirements of complex working conditions. For example, pure iron has low strength and hardness, limiting its application. Alloys are materials with metallic properties, such as steel, aluminum alloys, and copper alloys, composed of two or more metallic elements (or metal and nonmetal elements). Alloying can significantly improve the mechanical properties, corrosion resistance, and wear resistance of pure metals, expanding their applications. For example, adding carbon to pure iron forms steel, which has much greater strength and hardness than pure iron, making it a crucial metal material in industrial production.
Metal materials can be divided into ferrous and non-ferrous metals based on their structural structure. Ferrous metals primarily refer to iron, chromium, manganese, and their alloys. Iron-based alloys (steel and cast iron) are the most widely used, accounting for over 90% of total metal production. Steel is an iron-carbon alloy with a carbon content of less than 2.11%. Depending on the carbon content, it can be divided into low-carbon steel (carbon content <0.25%), medium-carbon steel (carbon content 0.25%-0.6%), and high-carbon steel (carbon content >0.6%). Low-carbon steel has good plasticity but low strength, making it suitable for stamping. Medium-carbon steel has moderate strength and toughness , making it suitable for manufacturing parts such as shafts and gears. High-carbon steel has high hardness and good wear resistance, making it suitable for making tools, springs, and other parts. Cast iron is an iron-carbon alloy with a carbon content greater than 2.11%. It boasts low cost and excellent casting properties. It can be categorized into gray cast iron, ductile iron, and malleable cast iron. Gray cast iron offers excellent wear resistance and shock absorption, and is often used in machine tool beds. Ductile iron, with mechanical properties similar to those of steel, can be used to manufacture load-bearing parts such as crankshafts and gears. Non-ferrous metals refer to all metals and their alloys other than ferrous metals, including aluminum and its alloys, copper and its alloys, titanium and its alloys, and magnesium and its alloys. Non-ferrous metals possess numerous excellent properties, such as the low density and high strength of aluminum alloys; the excellent electrical conductivity and corrosion resistance of copper alloys; and the high strength and excellent high and low-temperature resistance of titanium alloys, making them irreplaceable in the aerospace, automotive, and electronics industries.
Metal materials can be divided into structural and functional materials based on their performance characteristics and applications. Structural materials are metal materials primarily used for mechanical properties, focusing on strength, hardness, toughness, and fatigue strength. They are primarily used to bear or transmit loads, such as steel used in construction, steel used in machinery manufacturing, and aluminum alloys used in aerospace. The selection of structural materials depends on factors such as the stresses on the component and the operating environment. For example, steel used in bridges requires high strength and toughness to withstand vehicle loads and environmental loads; steel used in machine tool spindles requires high fatigue strength and wear resistance to ensure long-term stable operation. Functional materials are metal materials primarily used for physical or chemical properties, focusing on electrical conductivity, thermal conductivity, magnetism, and corrosion resistance. They are primarily used to achieve specific functions, such as copper for electrical applications, aluminum for thermal conductivity, iron-nickel alloys for magnetic properties, and stainless steel for corrosion resistance. Functional materials are widely used in the electronics, energy, and information technology sectors. For example, highly conductive copper is used in motor windings, highly thermally conductive aluminum is used in heat sinks, soft magnetic materials are used in transformer cores, and stainless steel is used in chemical equipment.
Based on the production process, metal materials can be divided into cast metal materials, deformed metal materials, and powder metallurgy materials. Cast metal materials are made through the casting process (pour molten metal into a mold and cool and solidify), such as cast iron, cast aluminum alloys, and cast copper alloys. Cast materials have good formability and can be used to make complex-shaped parts such as engine blocks and machine tool beds, but their mechanical properties are generally lower than deformed materials. Deformed metal materials are made through plastic deformation processes (such as rolling, forging, and stamping), such as steel plates, steel pipes, forged steel parts, and aluminum alloy profiles. Deformation densifies the material’s internal structure, improving its mechanical properties and significantly increasing its strength and toughness. They are suitable for parts that bear heavy loads, such as automotive drive shafts and aircraft landing gear. Powder metallurgy materials are made by pressing and sintering metal powders, such as powder metallurgy gears and cemented carbide. Powder metallurgy materials can produce materials with specialized properties (such as porous materials and refractory metals) and have high material utilization, making them suitable for mass production of complex-shaped parts, but with higher production costs. Metal materials produced by different production processes have their own characteristics. In practical applications, they must be reasonably selected based on the shape of the parts, performance requirements and production batch.
The classification of metal materials is the basis for understanding and applying metal materials. Different classification methods reflect the characteristics and application scope of metal materials from different perspectives. With the development of materials science, new metal materials continue to emerge, such as high-temperature alloys, shape memory alloys, hydrogen storage alloys, etc. These materials have unique properties and have expanded the application fields of metal materials. In practical applications, it is necessary to select appropriate metal materials based on specific usage requirements, taking into account the chemical composition, organizational structure, performance characteristics and production process of the materials, so as to achieve a balance between product performance, cost and reliability. At the same time, the classification of metal materials is not absolute. A material may belong to multiple categories at the same time. For example, stainless steel is both a ferrous metal and a structural material and a corrosion-resistant functional material. Understanding this intersectionality will help us to have a more comprehensive understanding of the characteristics and applications of metal materials.
