The Influence of Cold Drawing on Mechanical Properties of Metallic Materials

Abstract​

Cold drawing, as a widely used metal forming process, plays a crucial role in modifying the mechanical properties of metallic materials. This paper aims to comprehensively explore the effects of cold drawing on the mechanical properties of metallic materials. It begins by introducing the basic principles and process characteristics of cold drawing, then focuses on analyzing the changes in strength, plasticity, hardness, toughness, and other mechanical properties of metallic materials after cold drawing. Additionally, the microstructural mechanisms underlying these property changes are discussed, and the practical applications and considerations of cold-drawn metallic materials in various industries are addressed. Through this research, a deeper understanding of the relationship between cold drawing and the mechanical properties of metallic materials can be achieved, providing theoretical guidance for the rational application of cold drawing technology in industrial production.​

1. Introduction​

Metallic materials are indispensable in modern industrial production, and their mechanical properties directly determine their application scope and service life. To meet the diverse performance requirements in different engineering fields, various metal processing technologies have been developed, among which cold drawing occupies an important position. Cold drawing is a plastic forming process in which a metal blank is pulled through a die at room temperature to reduce its cross-sectional area and change its shape. Compared with hot working processes such as forging and rolling, cold drawing offers unique advantages. It can achieve high dimensional accuracy and good surface finish of the products, and more importantly, it can significantly improve the mechanical properties of the materials through work hardening.​

In recent years, with the rapid development of industries such as automotive, aerospace, machinery manufacturing, and construction, the demand for metallic materials with excellent mechanical properties has been increasing. Cold-drawn metallic materials, due to their superior performance, have been widely used in these fields. For example, cold-drawn steel bars are commonly used in construction structures to enhance the strength and stability of the buildings; cold-drawn seamless steel tubes are applied in the automotive and aerospace industries for their high pressure resistance and good toughness. Therefore, studying the impact of cold drawing on the mechanical properties of metallic materials is of great practical significance for optimizing the cold drawing process, improving product quality, and expanding the application of cold-drawn materials.​

This paper will systematically review the research progress on the effect of cold drawing on the mechanical properties of metallic materials. It will first elaborate on the basic principles and process parameters of cold drawing, then analyze the changes in various mechanical properties in detail, explore the microstructural reasons behind these changes, and finally summarize the applications and existing problems of cold-drawn materials, as well as prospects for future research.​

2. Basic Principles and Process Characteristics of Cold Drawing​

2.1 Basic Principles​

Cold drawing is a process that utilizes the plastic deformation of metals. When a metal blank (such as a wire, rod, or tube) is subjected to a pulling force, it passes through a die with a certain shape and size. Under the action of the die's constraint and the external pulling force, the metal undergoes plastic deformation, the cross-sectional area is reduced, and the length is increased, thereby obtaining the desired shape and size of the product. During the cold drawing process, since the deformation is carried out at room temperature, there is no obvious recovery and recrystallization of the metal, and the work hardening effect is significant.​

The plastic deformation of metals in cold drawing is mainly achieved through the movement of dislocations. Under the action of external forces, dislocations in the metal crystal lattice move, multiply, and interact with each other. As the deformation increases, the number of dislocations increases, and their movement is hindered, leading to an increase in the strength and hardness of the metal, which is the work hardening phenomenon.​

2.2 Process Characteristics​

Cold drawing has several distinct process characteristics:​

• High dimensional accuracy: The die used in cold drawing has high precision, and the size of the cold-drawn product can be accurately controlled, with a tolerance range generally within a few micrometers to tens of micrometers. This makes cold-drawn products suitable for applications with strict dimensional requirements.​

• Good surface quality: During cold drawing, the metal surface is in contact with the die, and the die's smooth surface can improve the surface finish of the product. Cold-drawn products usually have a bright and smooth surface, reducing the need for subsequent surface treatment.​

• Significant work hardening: As mentioned earlier, cold drawing at room temperature results in obvious work hardening, which can significantly improve the strength and hardness of the metal. However, this also leads to a decrease in plasticity and toughness, which is a trade-off in the cold drawing process.​

• Flexible process: Cold drawing can be used to produce a variety of cross-sectional shapes, such as round, square, hexagonal, and special-shaped, to meet different design requirements. In addition, multiple passes of cold drawing can be performed to achieve a larger deformation.​

3. Effects of Cold Drawing on the Mechanical Properties of Metallic Materials​

3.1 Effect on Strength​

Strength is one of the most important mechanical properties of metallic materials, which refers to the ability of the material to resist plastic deformation and fracture under external forces. Cold drawing has a significant enhancing effect on the strength of metallic materials, mainly reflected in the increase in tensile strength, yield strength, and ultimate strength.​

When the metal undergoes cold drawing deformation, the number of dislocations in the crystal lattice increases rapidly. Dislocations interact with each other, such as tangling and pinning, which makes it more difficult for dislocations to move. As a result, a higher external force is required to cause further deformation, leading to an increase in strength. For example, cold-drawn steel wires have much higher tensile strength than hot-rolled steel wires. Studies have shown that with the increase of cold drawing deformation (i.e., the reduction in cross-sectional area), the tensile strength and yield strength of steel increase linearly or non-linearly. When the deformation reaches a certain extent, the rate of strength increase slows down due to the saturation of dislocations.​

Different metallic materials show different degrees of strength improvement after cold drawing. For example, low-carbon steel has a more obvious increase in strength after cold drawing because its initial strength is relatively low and it has good plastic deformation ability, allowing for more dislocations to be generated. High-alloy steels, on the other hand, due to the presence of alloying elements that can pin dislocations, the increase in strength after cold drawing is relatively small.​

3.2 Effect on Plasticity​

Plasticity is the ability of a material to undergo permanent deformation without fracture under external forces, usually measured by elongation and reduction of area. Unlike the effect on strength, cold drawing generally leads to a significant decrease in the plasticity of metallic materials.​

During cold drawing, as the deformation increases, the dislocations in the metal accumulate and entangle, making it difficult for the crystal lattice to slide and deform further. At the same time, the grains are elongated along the deformation direction, forming a fibrous structure. This fibrous structure reduces the ability of the material to undergo uniform plastic deformation, resulting in a decrease in elongation and reduction of area. For instance, after cold drawing, the elongation of copper wires can decrease from over 50% to less than 10%, depending on the degree of deformation.​

The decrease in plasticity is a major limitation of cold drawing. In practical applications, if the material needs to have certain plasticity for subsequent processing (such as bending, forming), it is necessary to control the degree of cold drawing deformation or perform intermediate annealing to restore some plasticity. Intermediate annealing is a heat treatment process that is carried out between multiple passes of cold drawing, which can eliminate work hardening, reduce dislocations, and recrystallize the grains, thereby restoring the plasticity of the material.​

3.3 Effect on Hardness​

Hardness is the ability of a material to resist indentation or scratching by another harder object. Cold drawing can significantly increase the hardness of metallic materials, which is consistent with the change in strength. The increase in hardness is also due to work hardening caused by dislocation accumulation and grain refinement.​

When measuring the hardness of cold-drawn materials, it is found that the hardness value increases with the increase of cold drawing deformation. For example, the Brinell hardness of aluminum alloys can increase by 30% to 50% after a certain degree of cold drawing. The distribution of hardness in the cold-drawn material is generally uniform, but in some cases, due to uneven deformation, there may be a slight difference in hardness between the surface and the core. The surface layer, which is in direct contact with the die, may have a higher deformation degree and thus a higher hardness.​

The increase in hardness expands the application range of cold-drawn materials. For example, cold-drawn bearing steel has high hardness, which can improve the wear resistance of the bearing and extend its service life.​

3.4 Effect on Toughness​

Toughness is the ability of a material to absorb energy during fracture, which is a comprehensive performance index reflecting strength and plasticity. Cold drawing has a complex effect on the toughness of metallic materials, generally leading to a decrease in toughness, but the degree of decrease is related to factors such as material type, deformation degree, and microstructure.​

The decrease in toughness is mainly due to the decrease in plasticity. Since toughness is related to the ability of the material to undergo plastic deformation before fracture, a decrease in plasticity leads to a reduction in the energy absorbed during fracture, resulting in lower toughness. For brittle materials, cold drawing may cause a more significant decrease in toughness, making them more prone to brittle fracture. For example, cold-drawn high-carbon steel, if the deformation is too large, may have low toughness and be easy to break under impact loads.​

However, for some materials with good toughness, appropriate cold drawing deformation may have a less significant impact on toughness, or even a slight increase in some cases. This is related to the refinement of grains during cold drawing. Grain refinement can improve both strength and toughness, which is the so-called "grain refinement strengthening" effect. If the grain refinement effect is more significant than the negative impact of work hardening on plasticity, the toughness may not decrease or even increase slightly.​

3.5 Effect on Fatigue Properties​

Fatigue properties refer to the ability of a material to resist fracture under cyclic loading. Cold drawing has a certain influence on the fatigue properties of metallic materials, and the effect can be either positive or negative, depending on various factors.​

On the one hand, cold drawing increases the strength of the material, which can improve the fatigue limit to a certain extent. The higher strength allows the material to withstand higher cyclic loads without fatigue failure. On the other hand, cold drawing may introduce residual stresses and microcracks in the material. Residual stresses, especially tensile residual stresses, can reduce the fatigue life of the material because they superimpose with the external cyclic stresses, accelerating the initiation and propagation of fatigue cracks. Microcracks generated during cold drawing can also act as fatigue crack sources, reducing the fatigue resistance.​

The overall effect of cold drawing on fatigue properties depends on the balance between these two factors. For example, for cold-drawn steel wires used in springs, appropriate cold drawing deformation can increase the strength and improve the fatigue limit, but excessive deformation may introduce more residual stresses and microcracks, leading to a decrease in fatigue life. Therefore, in practical applications, it is necessary to optimize the cold drawing process to obtain better fatigue properties.​

4. Microstructural Mechanisms Underlying the Changes in Mechanical Properties​

The changes in the mechanical properties of metallic materials after cold drawing are closely related to their microstructural evolution. Understanding these microstructural mechanisms is crucial for explaining the property changes and optimizing the cold drawing process.​

4.1 Dislocation Behavior​

As mentioned earlier, dislocations play a key role in the plastic deformation of metals during cold drawing. In the initial state, the number of dislocations in the metal is relatively small. With the progress of cold drawing, under the action of external forces, dislocations start to move and multiply. The movement of dislocations is hindered by grain boundaries, second-phase particles, and other dislocations, leading to dislocation tangling and the formation of dislocation cells. As the deformation increases, the density of dislocations continues to increase, and the dislocation cells are refined. This high density of dislocations and their interactions make it difficult for the material to deform further, resulting in an increase in strength and hardness, and a decrease in plasticity.​

4.2 Grain Deformation and Texture Formation​

During cold drawing, the grains of the metal are no longer equiaxed but are elongated along the drawing direction, forming a fibrous structure. The degree of grain elongation increases with the increase of deformation. This fibrous structure makes the mechanical properties of the material exhibit anisotropy, that is, the properties along the drawing direction (longitudinal direction) are different from those perpendicular to the drawing direction (transverse direction). For example, the tensile strength and elongation in the longitudinal direction are higher than those in the transverse direction.​

At the same time, cold drawing can also induce the formation of texture in the material. Texture refers to the preferred orientation of grains. Due to the directional deformation during cold drawing, the grains tend to rotate to a certain orientation, resulting in a specific texture. The formation of texture further enhances the anisotropy of the material's mechanical properties. For example, cold-drawn copper wires have a strong <111> texture, which makes their electrical conductivity and mechanical properties in the longitudinal direction better than those in the transverse direction.​

4.3 Precipitation of Second Phases​

In some alloy systems, cold drawing can promote the precipitation of second phases. The second-phase particles can pin dislocations, thereby enhancing the strength of the material through the "Orowan mechanism". However, the precipitation of second phases is also affected by factors such as the type of alloy, cold drawing deformation, and subsequent heat treatment. For example, in aluminum-magnesium alloys, cold drawing can increase the supersaturation of solute atoms, promoting the precipitation of Mg2Al3 particles during aging treatment, which significantly improves the strength of the alloy.​

5. Practical Applications of Cold-Drawn Metallic Materials​

Due to their excellent mechanical properties and process characteristics, cold-drawn metallic materials are widely used in various industries.​

5.1 Automotive Industry​

In the automotive industry, cold-drawn steel bars and tubes are widely used. For example, cold-drawn seamless steel tubes are used in the manufacturing of automotive engines, transmissions, and suspension systems, due to their high strength, good dimensional accuracy, and pressure resistance. Cold-drawn steel bars are used to produce automotive fasteners (such as bolts, nuts), which require high strength and precision. In addition, cold-drawn aluminum alloys are used in automotive body parts to reduce the weight of the vehicle and improve fuel efficiency.​

5.2 Aerospace Industry​

The aerospace industry has strict requirements on the performance of metallic materials, requiring them to have high strength, light weight, and good fatigue resistance. Cold-drawn titanium alloys and high-strength steels are commonly used in the manufacturing of aircraft structural parts, engine components, and fasteners. For example, cold-drawn titanium alloy wires are used in aircraft cables, which have high strength and corrosion resistance, ensuring the safety and reliability of the aircraft.​

5.3 Construction Industry​

In the construction industry, cold-drawn steel bars are widely used in reinforced concrete structures. Compared with hot-rolled steel bars, cold-drawn steel bars have higher strength, which can reduce the amount of steel used and save construction costs. At the same time, their good surface quality ensures a good bond with concrete, improving the overall performance of the structure. Cold-drawn steel tubes are also used in construction for scaffolding, handrails, and structural supports.​

5.4 Machinery Manufacturing Industry​

In the machinery manufacturing industry, cold-drawn materials are used to produce various mechanical parts, such as shafts, gears, and bearings. Cold-drawn shafts have high dimensional accuracy and good surface finish, reducing the machining allowance and improving production efficiency. Cold-drawn bearing steel has high hardness and wear resistance, ensuring the long service life of the bearing.​

6. Considerations in the Application of Cold-Drawn Metallic Materials​

Although cold-drawn metallic materials have many advantages, there are also some considerations in their application:​

6.1 Control of Deformation Degree​

The degree of cold drawing deformation directly affects the mechanical properties of the material. Excessive deformation can lead to excessive work hardening, resulting in low plasticity and toughness, and even cracking of the material during processing or use. Therefore, it is necessary to reasonably control the deformation degree according to the requirements of the product. For materials that require large deformation, multiple passes of cold drawing with intermediate annealing should be adopted to ensure the quality of the product.​

6.2 Residual Stresses​

Cold drawing can introduce residual stresses in the material, which can affect the dimensional stability and service performance of the product. Tensile residual stresses on the surface can reduce the fatigue life and corrosion resistance of the material. Therefore, post-processing treatments such as stress relief annealing can be performed to reduce residual stresses. Stress relief annealing is a heat treatment process carried out at a temperature lower than the recrystallization temperature, which can reduce the residual stresses without significantly reducing the strength of the material.​

6.3 Material Selection​

Different metallic materials have different responses to cold drawing. When selecting materials, it is necessary to consider their plastic deformation ability, work hardening rate, and the required mechanical properties of the product. For example, low-carbon steel and copper have good plastic deformation ability and are suitable for cold drawing to obtain high-strength products; while high-carbon steel and some high-alloy steels have poor plasticity and require more careful control of the cold drawing process.​

6.4 Environmental Factors​

The service environment of cold-drawn materials also needs to be considered. For example, in corrosive environments, cold-drawn materials with poor corrosion resistance may be prone to corrosion, which can reduce their mechanical properties and service life. Therefore, appropriate surface treatment (such as plating, painting) or the selection of corrosion-resistant materials (such as stainless steel) should be adopted according to the environmental conditions.​

7. Conclusion and Prospects​

7.1 Conclusion​

Cold drawing is an important metal forming process that has a significant impact on the mechanical properties of metallic materials. Through cold drawing, the strength, hardness, and dimensional accuracy of the materials can be improved, but the plasticity and toughness are generally reduced. These changes in mechanical properties are mainly caused by the microstructural evolution during cold drawing, such as dislocation accumulation, grain elongation, texture formation, and precipitation of second phases.​

Cold-drawn metallic materials, with their excellent performance, have been widely used in automotive, aerospace, construction, machinery manufacturing, and other industries, playing an important role in promoting the development of these industries. However, in the application process, attention should be paid to the control of deformation degree, the reduction of residual stresses, the selection of appropriate materials, and the consideration of environmental factors to ensure the safe​