Cold forging is a general term that encompasses various plastic processing techniques such as cold die forging, cold extrusion, and cold heading.
Cold forging is a metal forming process that takes place at temperatures below the recrystallization temperature of the material. It is also known as forging at a temperature below the recovery temperature. The term “cold forging” is typically used to describe the production of unheated blanks.
The materials most commonly used in cold forging include aluminum and its alloys, copper and its alloys, low carbon steel, medium carbon steel, and low alloy structural steel. These materials have low deformation resistance and good plasticity at room temperature.
One of the benefits of cold forging is the high quality surface finish and dimensional accuracy it provides. This can often replace some cutting processes, resulting in cost savings and improved efficiency. Additionally, cold forging can strengthen the metal and increase the strength of the part.
Definition
Cold forging, also referred to as cold volume forming, is a manufacturing process that involves shaping metal into a desired form. It is similar to the cold stamping process.
The cold forging process is composed of three key elements: materials, molds, and equipment. While the materials used in the stamping process are primarily plates, the materials used in cold forging are usually discs or wires.
Basic concept
Cold forging refers to a range of metal forming techniques performed at temperatures below the recrystallization temperature of the metal. According to metallographic theory, the recrystallization temperature of different metal materials varies. It is generally estimated to be between 0.3 to 0.5 times the melting temperature of the metal.
The melting temperature of various metal materials can be seen in the following table:
| Metal | Min recrystallization temperature /℃ | Metal | Min recrystallization temperature /℃ |
| Iron (Fe) and copper | 360~450 | Tin (Sn) | 0 |
| Copper (Cu) | 200~270 | Lead (Pb) | 0 |
| Aluminum (Al) | 100~150 | Tungsten (W) | 1200 |
From the information in the table, the lowest recrystallization temperature of both metallic and non-metallic iron can be determined. Lead and tin, on the other hand, cannot be considered cold forging even if they are formed at room temperature or normal temperature. Instead, they would be considered hot forging. However, forming iron, copper, and aluminum at normal temperature can be referred to as cold forging.
Over time, the shapes of cold forged parts have become increasingly complex, evolving from simple shapes like stepped shafts, screws, nuts, and conduits to parts with intricate designs. One example is the spline shaft, which is created through the following process: positive pressing of the rod portion, intermediate portion of the upset, and squeeze spline. The main process for creating a spline sleeve involves anti-squeezing the cup-shaped part, forming the bottom part into a ring-shaped part, and squeezing the sleeve.
Cold extrusion technology has also been successfully used in the production of cylindrical gears. In addition to ferrous metals, cold extrusion applications of copper alloys, magnesium alloys, and aluminum alloy materials are becoming increasingly common.
Related reading: Ferrous vs Non-ferrous Metals
Process introduction
Cold precision forging is a near net-forming process that results in parts with high strength, precision, and good surface quality. Currently, the total weight of cold forgings used in a typical foreign car is between 40 to 45 kg, with tooth-shaped parts weighing more than 10 kg. Cold-forged gears can weigh over 1 kg and have tooth profile accuracy up to 7th level.
Innovation in the continuous forging process has driven the development of cold extrusion technology. Since the 1980s, precision forging experts have started applying the split forging theory to the cold forging of spur gears and helical gears.
The main principle of split forging is to create a split or channel in the blank or mold that the material can flow through during the forging process. This enables the production of high-precision gears with smaller and quicker scale production, without the need for cutting.
For extrusions with a length-to-diameter ratio of 5, such as piston pins, cold extrusion can be used for one-shot forming through axial splitting of the residual mass, with good punch stability. Radial residual blocks can also be used for cold extrusion of flat-type spur gear forming.
Occlusion forging is a one-way or opposite-direction extrusion of metal in a closed die with one or two punches, resulting in a near net shape precision forging without flash.
For car precision parts like planetary and semi-axle gears, star sleeves, and cross bearings, using the cutting method results in low material utilization rates (less than 40% on average), high labor and time costs, and high production costs. However, by using occlusion forging technology, these net forgings can be produced with most cutting processing eliminated, reducing costs significantly.
