The paper discusses the principles of cold extrusion, emphasizing the characteristics, process flow, and requirements for forming a connector aluminum alloy shell. By optimizing the part’s structure and establishing control requirements for the raw material’s crystal structure, the quality of the cold extrusion process can be enhanced. This approach not only improves the forming quality but also reduces processing allowances and overall costs.
01 Introduction
The cold extrusion process is a non-cutting method of shaping metal that utilizes the principle of plastic deformation. In this process, a certain pressure is applied to the metal within the extrusion die cavity at room temperature, allowing it to be forced through the die hole or the gap between convex and concave dies. This results in the formation of the desired part shape.
The term “cold extrusion” encompasses a range of forming processes, including cold extrusion itself, upsetting, stamping, fine punching, necking, finishing, and thinning stretching. In most applications, cold extrusion serves as the primary forming process, often supplemented by one or more auxiliary processes to produce a finished part of high quality.
Cold extrusion is an advanced method in metal plastic processing and is increasingly replacing traditional techniques such as casting, forging, drawing, and cutting. Currently, this process can be applied to metals such as lead, tin, aluminum, copper, zinc and their alloys, as well as low carbon steel, medium carbon steel, tool steel, low alloy steel, and stainless steel. Since the 1980s, the cold extrusion process has been effectively utilized in manufacturing aluminum alloy shells for circular connectors and has since become a well-established technique.
02 Principles, characteristics, and processes of cold extrusion process
2.1 Principles of cold extrusion
The press and die collaborate to apply force on the deformed metal, creating a three-dimensional compressive stress state in the primary deformation zone, which enables the deformed metal to undergo plastic flow in a predetermined manner.
The effect of the three-dimensional compressive stress is as follows.
1) Three-dimensional compressive stress can effectively prevent relative movement between crystals, significantly enhancing the plastic deformation of metals.
2) This type of stress can help make deformed metals denser and effectively repair various micro-cracks and structural defects.
3) Three-dimensional compressive stress can prevent the formation of stress concentrations, thereby reducing the harm caused by impurities within the metal.
4) Additionally, it can significantly counteract the extra tensile stress caused by uneven deformation, thereby minimizing damage from this tensile stress.
During the cold extrusion process, the deformed metal flows in a specified direction. This causes larger grains to be crushed, while the remaining grains and intergranular materials become elongated along the direction of deformation. As a result, the individual grains and grain boundaries become difficult to distinguish and appear as fibrous stripes, which is referred to as a fibrous structure. The formation of this fibrous structure increases the deformation resistance of the metal and imparts directional mechanical properties to the cold-extruded parts.
Additionally, the lattice orientation along the metal flow direction transitions from a disordered to an ordered state, enhancing the strength of the component and leading to anisotropic mechanical properties in the deformed metal. Throughout the forming process, different parts of the component experience varying degrees of deformation. This variation results in differences in work hardening, which in turn leads to distinct differences in mechanical properties and hardness distribution.
2.2 Characteristics of cold extrusion
The cold extrusion process has the following characteristics.
1) Cold extrusion is a near-net forming process that can help save raw materials.
2) This method operates at room temperature, features a short processing time for single pieces, offers high efficiency, and is easy to automate.
3) It ensures the accuracy of key dimensions and maintains the surface quality of important parts.
4) The material properties of the deformed metal are enhanced through cold work hardening and the creation of complete fiber streamlines.
2.3 Cold extrusion process flow
The primary equipment used in the cold extrusion process includes a cold extrusion-forming machine, a forming die, and a heat treatment furnace. The main processes are blank-making and forming.
(1) Blank making: The bar is shaped into the required blank by sawing, upsetting, and metal sheet stamping, and then it is annealed to prepare for the subsequent cold extrusion forming.
(2) Forming: The annealed aluminum alloy blank is positioned in the mold cavity. Under the combined action of the forming press and the mold, the aluminum alloy blank enters a yield state and flows smoothly within the designated space of the mold cavity, allowing it to take on the desired shape. However, the strength of the formed part may not reach optimal levels. If higher strength is required, additional treatments, such as solid solution heat treatment and aging (particularly for alloys that can be strengthened through heat treatment), are necessary.
When determining the forming method and the number of forming passes, it is important to consider the complexity of the part and the established benchmarks for supplementary processing. The process flow for the J599 series plug and socket shell includes the following steps: cutting → rough turning on both sides → annealing → lubrication → extrusion → quenching → turning and milling → deburring. Figure 1 illustrates the process flow for the shell with a flange, while Figure 2 depicts the process flow for the shell without a flange.
03 Typical phenomena in cold extrusion forming
(1) Work hardening is the process where the strength and hardness of a deformed metal increase while its plasticity decreases as long as the deformation occurs below the recrystallization temperature. This means that as the level of deformation rises, the metal becomes stronger and harder but less malleable. Work hardening is an effective method for strengthening various metals, such as rust-proof aluminum alloys and austenitic stainless steel.
(2) Thermal Effect: In the cold extrusion forming process, most of the energy used for deformation work is converted into heat. In areas with significant deformation, temperatures can reach between 200 and 300°C, particularly during rapid and continuous production, where the temperature increase is even more pronounced. These thermal effects significantly influence the flow of both lubricants and deformed metals.
(3) During the cold extrusion forming process, there are two main types of stress in the deformed metal: basic stress and additional stress.
04 Process requirements for cold extrusion
Given the issues present in the production process of cold extrusion for 6061 aluminum alloy connector shells, specific requirements are established regarding its structure, raw materials, and other lathe process properties.
4.1 Requirements for the width of the back-cut groove of the inner hole keyway
The width of the back-cut groove in the inner hole keyway should be at least 2.5 mm. If structural constraints limit this width, the minimum acceptable width should be greater than 2 mm. Figure 3 illustrates the comparison of the back-cut groove in the inner hole keyway of the shell before and after the improvement. Figure 4 shows the comparison of the groove before and after the improvement, specifically when limited by structural considerations.
4.2 Single-key length and shape requirements for inner hole
Incorporate a back cutter groove or chamfer into the inner hole of the shell. Figure 5 illustrates the comparison of the inner hole of the shell before and after the addition of the back cutter groove, while Figure 6 shows the comparison of the inner hole of the shell before and after the chamfer has been added.
4.3 Bottom requirements of inner hole blind groove
Chamfers or back-cuts are added to inner hole blind grooves. Figure 7 illustrates the comparison of a rectangular shell’s inner hole blind groove before and after the chamfer is added.
4.4 Requirements for the bottom of the external cylindrical key
A relief groove has been incorporated into the bottom of the external cylindrical key of the housing. The comparison before and after the addition of the relief groove is illustrated in Figure 8.
4.5 Raw material requirements
The crystal structure of the raw material significantly affects the surface quality achieved after cold extrusion. To ensure that the surface quality standards are met, it is essential to establish control requirements for the raw material’s crystal structure. Specifically, the maximum allowable dimension of the coarse crystal rings on one side of the raw material should be ≤ 1 mm.
4.6 Requirements for the depth-to-diameter ratio of the hole
The depth-to-diameter ratio of the hole is required to be ≤3.
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Post time: Dec-03-2024