Cold forging is a general term for plastic processing such as cold die forging, cold extrusion, and cold heading.
Cold forging is a forming process below the recrystallization temperature of the material, and is forging at a temperature below the recovery temperature.
Forging in the production of unheated blanks is called cold forging.
Cold forging materials are mostly aluminum and some alloys, copper and some alloys, low carbon steel, medium carbon steel and low alloy structural steel with less deformation resistance and good plasticity at room temperature.
Cold forgings have good surface quality and high dimensional accuracy, which can replace some cutting processes.
Cold forging can strengthen the metal and increase the strength of the part.
Cold forging, also known as cold volume forming, is a manufacturing process and a processing method.
It is basically the same as the cold stamping process.
The cold forging process is also composed of three elements: materials, molds and equipment.
Only the materials in the stamping process are mainly plates, and the materials in the cold forging process are mainly discs or wires.
Cold forging refers to various volume forming performed below the recrystallization temperature of the metal.
From the theory of metallography, the recrystallization temperature of various metal materials is different; the recrystallization temperature = 0.3~0.5 melting temperature.
T melting temperature is shown 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 figures in the table, the lowest recrystallization temperature of iron metal and nonmetal is known.
Even at room temperature or normal temperature, the forming of lead and tin cannot be called cold forging, but hot forging.
However, the forming of iron, copper and aluminum at normal temperature can be called cold forging.
The shape of cold forged parts is becoming more and more complex, from the initial stepped shafts, screws, nuts and conduits to parts with complex shapes.
The typical process of the spline shaft is: positively pressing the rod portion – the intermediate portion of the upset – the squeeze spline;
The main process of the spline sleeve is: anti-squeezing the cup-shaped part – the bottom part is made into a ring-shaped part – the sleeve is being squeezed.
The cold extrusion technology of cylindrical gears has also been successfully used in production.
In addition to ferrous metals, cold extrusion applications of copper alloys, magnesium alloys and aluminum alloy materials are becoming more widespread.
Related reading: Ferrous vs Non-ferrous Metals
Cold precision forging is a (near) net forming process.
The parts formed by this method have high strength and precision, and the surface quality is good.
At present, the total amount of cold forgings used in a foreign ordinary car is 40~45kg, and the total amount of tooth-shaped parts is more than 10kg.
The weight of the cold forged gear can reach more than 1kg and the tooth profile accuracy can reach 7th level.
Continuous process innovation has driven the development of cold extrusion technology.
Since the 1980s, precision forging experts at home and abroad have begun to apply split forging theory to cold forging of spur gears and helical gears.
The main principle of split forging is to create a split or split channel for the material in the forming portion of the blank or mold.
During the forging process, while the material fills the cavity, part of the material flows to the split or split channel.
The application of the split forging technology has enabled the production of high-precision gears to be small and without cutting to quickly reach the industrial scale.
For extrusions with a length to diameter ratio of 5, such as piston pins, cold extrusion can be used for one-shot forming by axially splitting the axial residual mass, and the stability of the punch is good.
For flat-type spur gear forming, cold extrusion of the product can also be achieved with radial residual blocks.
The occlusion forging is a one-way or opposite-direction extrusion of the metal in one or two punches in the closed die to obtain a near net shape precision forging without flash.
For some car precision parts such as planetary and semi-axle gears, star sleeves, cross bearings, etc., if the cutting method is used, not only the material utilization rate is very low (less than 40% on average), but also the labor and time cost are high, and the production cost is extremely high.
Foreign countries use occlusion forging technology to produce these net forgings, eliminating most of the cutting processing, and the cost is greatly reduced.