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The properties of metallic materials are primarily determined by their internal structural organization, which includes the arrangement of atoms, phase composition within the solid, grain size, and metallic fiber structure. Given the wide variety of metal materials and their diverse properties, understanding the impact of metal microstructure on properties requires first gaining an understanding of the requirements for metal materials in the field of machining. In mechanical engineering, metallic materials are the most widely used structural materials, primarily due to their excellent service properties and processability. In terms of performance, the mechanical properties of metal materials play a relatively prominent role. There are many test indicators for the mechanical properties of metallic materials, such as strength, hardness, plasticity, and elasticity. These performance indicators are the main basis for selecting materials in mechanical engineering. Therefore, it is necessary to fully understand the microstructure of metallic materials, so as to know how the microstructure affects the properties of these materials. 1 The influence of metal material microstructure on its mechanical properties 1.1 The effect of grain size on the mechanical properties of metals In terms of processability, the impact of grain size on the mechanical properties of metals is primarily reflected in the metal’s plastic formability. In current industrial production, the average area or average diameter of the grains, that is, the grain size, is generally used to measure the size of the grains. The higher the grain size grade, the finer the grain size. Consequently, there are more grain boundaries per unit volume, which in turn results in a greater number of grains with different phases. This enhances the plastic deformation and compressive resistance of the metallic material. For general metal materials, the finer the grain size, the higher their strength, hardness, plasticity, and toughness may be. Therefore, controlling the grain size of castings is of great significance. In actual production, we can see many methods used to refine grains, such as reducing the pouring temperature of the melt, selecting casting mold materials with high thermal conductivity, adding nucleators, and using mechanical vibration. 1.2 The influence of the isomorphous transformation properties of iron on the microstructure and mechanical properties of steel materials: It is well known in mechanical engineering and modern industry that the isomorphous transformation properties of iron have a significant impact on the microstructure and mechanical properties of steel materials, which are among the most widely used alloys. These alloys are essentially based on iron and carbon as their main components, but due to variations in the amounts of these elements – such as different carbon contents in the alloys – their microstructure, properties, and areas of application vary. Under normal conditions, there are five basic microstructures in iron-carbon alloys; among them, the solid solutions are ferrite and austenite. The interstitial solid solution formed in α-Fe is ferrite; however, the size of the interstices in the lattice affects properties such as plasticity, toughness, strength, and hardness. Therefore, since α-Fe has a body-centered cubic lattice structure and relatively small interstitial spaces, the solubility of carbon in α-Fe is quite low. As a result, the properties of such interstitial solid solutions at room temperature are similar to those of pure iron. This means that they exhibit good plasticity and toughness, along with relatively low strength and hardness. However, austenite is an interstitial solid solution formed when carbon dissolves in γ-Fe. Based on the properties of r-Fe itself, it exists at high temperatures and has a face-centered cubic crystal structure. Such a crystal structure features relatively large interstitial spaces; as a result, austenite has a greater capacity to absorb carbon compared to ferrite. Hence, its strength and hardness are higher than those of ferrite, and it also possesses good plasticity and toughness, especially excellent formability. Iron exhibits allotropic transformation, which is why heat treatment can alter the properties of steel materials. For example, in the case of ductile iron, heat treatment can be used to alter its matrix structure, thereby changing its mechanical properties. Generally, annealing can be used to obtain a ferrite structure, thereby improving its plasticity and toughness to some extent and relieving stresses. Normalizing yields a pearlite matrix, which enhances strength and wear resistance. Quenching and tempering produces a tempered sorbite matrix, thus improving the material’s overall mechanical properties. 2 Influence of the microstructure of metallic materials on their processability 2.1 Processability of metallic materials The processability of metallic materials refers to their ability to adapt to various processing methods. Generally, these include: castability, forgeability, weldability, machinability, and heat treatability, etc. 2.2 Casting properties of metal materials The degree of segregation in castings is an important indicator for assessing the metallurgical quality of metal materials. After a metal solidifies, the uneven distribution of its chemical components within it is known as segregation. Research has shown that when segregation is relatively severe, it may lead to significant differences in the mechanical properties of various parts of the casting; in other words, it has a certain impact on the uniformity of the casting’s properties. Segregation is difficult to completely avoid, and it inevitably reduces the quality of castings; its impact is particularly significant on large-scale castings. Thus, it can be seen that whether the structure of a cooled and solidified metal material is uniform has a great influence on the castability of castings. 2.3 Forgeability of metallic materials. Taking forgeability as an example, it refers to the ease with which high-quality forgings can be obtained through forging processes. Generally, it is evaluated comprehensively using two criteria: plasticity and resistance to deformation. Generally, the better the plasticity, the lower the resistance to deformation, and thus the better the forging properties of the metal. Generally, the forging properties of pure metals are better than those of ordinary alloys. In iron-carbon alloys, the lower the carbon content, the better the forging properties; in alloy steels, the more types and amounts of alloying elements present, the worse the forging properties. Both single-phase solid solutions and multiphase mixtures are types of alloy structures, each exhibiting its own characteristics regarding plasticity and deformability. It can be seen that different microstructural compositions of metal materials have varying effects on their forging properties. 2.4 Effect of welding on metal microstructure The surface defects caused by welding in metal materials can be summarized as incomplete fusion between the deposited metal and the base material, resulting in certain gaps, such as weld beads, undercuts, and lack of penetration. We can employ certain macroscopic methods to improve some of the gaps, such as using mechanical processing to enhance the surface quality of the welded areas. However, the effect of some gaps on the metal’s microstructure cannot be eliminated. For example, in electrolyte solutions (taking an aqueous NaCl solution as an example), crevice corrosion often occurs in the crevices, which reduces the performance of the metal material. Although crevice corrosion requires certain conditions to occur—such as the size of the crevice, the retention state of the medium, and the characteristics of the corrosion itself—surface defects caused by welding undoubtedly provide an ideal environment for the formation of crevice corrosion. Therefore, welding-induced surface defects inevitably have an impact on the metal structure. Such effects also have different consequences in various corrosive environments. For instance, in welded joints of austenitic stainless steel, the ferrite present in the originally molten region becomes the phase that is preferentially corroded in hydrochloric acid solution; this leads to the easy formation of network-like cracks. Meanwhile, chromium carbides that precipitate along grain boundaries in the heat-affected zone cause a significant deterioration in the intergranular stress and corrosion resistance of the steel in corrosive environments such as aqueous NaCl solutions. In summary, the impact of welding on metal materials is very complex, and the metal’s microstructure also affects the quality of welding. In practice, it is necessary to minimize the impact of welding on metal materials, so as to control welding defects and preserve the original properties of the metal, thereby preventing damage to the relevant structures. 3 Conclusion In summary, the metal microstructure has a certain influence on its mechanical properties as well as its processability. To make better use of metal materials, it is necessary to fully understand their structural characteristics and gain a thorough understanding of their impact on properties. This can, to a certain extent, reduce the adverse effects of various defects on the performance of metal materials and improve their processability. A thorough understanding of the effect of the microstructure of metallic materials on their properties can, to some extent, promote the development of new materials and technologies, as well as advance modern materials science research.