1、 concept of cold heading
Cold heading (extrusion) is a main processing technology in the forming process of fasteners. Cold heading (extrusion) belongs to the category of metal pressure processing. In production, under normal temperature, the metal is forced to form in a predetermined die by applying external force. This method is usually called cold heading.
In metallurgically, forging of materials heated to recrystallization temperature (steel is about 700 ℃) is called hot forging.
For steel forgings, forging with recrystallization temperature below and higher than normal temperature is called temperature forging.
2、 Advantages of cold heading
High utilization rate of steel: cold heading (extrusion) is a kind of less and no cutting processing method, such as hexagon head bolt of processing rod and hexagon screw of cylinder head. Cutting processing method is adopted. The utilization rate of steel is only 25% - 35%, while the utilization rate of cold heading (extrusion) method can reach 85% - 95%, which is only some process consumption of material head, material tail and hexagon head cutting.
High productivity: compared with general cutting, the cold heading (extrusion) forming efficiency is more than dozens times higher.
Good mechanical properties: the strength of the parts processed by cold heading (extrusion) method is much better than that of cutting because the metal fiber is not cut off.
Suitable for automatic production: Fasteners (including some special parts) suitable for cold heading (extrusion) method are basically symmetrical parts, suitable for high-speed automatic cold heading machine production and the main method of mass production.
In a word, cold heading (extrusion) method is a kind of processing method with high comprehensive economic benefits, which is widely used in fastener industry, and also an advanced processing method widely used and developed at home and abroad.
3、 Cold forging process
Generally speaking, cold forging is to obtain the final shape of parts by the combination of various processes. Figure 2 is an example of cold forging. After cutting off the blank, the shaft rod is extruded, the cup cylinder is backward extruded, the cup barrel is extruded, upsetting, punching, and the pipe is extruded.
The process is divided into multiple processes to avoid excessive pressure during one forming. Because the lower the cost of the process, the key to process design is to reduce the forming pressure and reduce the number of processes.
4、 Main processing methods of cold forging
Upsetting
Fig. 3a is a free upsetting of the outer surface without the restriction of the die. The machining pressure increases with friction constraint, when the height of billet is higher than diameter d0 (h/d0 > 1.0), C = 1.2, but C will rise to about 2.5 when the blank becomes thin.
When the reduction rate increases, cracks will occur in the oblique and longitudinal directions of the peripheral plane, as shown in Figure 4. The crack occurrence depends on the ductility of the material. Therefore, when the reduction rate increases, the size of the constraint coefficient of various upsetting forming is given in Figure 3.
In free upsetting, when the initial height of the blank is more than 2 times the diameter, as shown in Figure 5, the bending of the blank is caused by the instability of the material, thus forming a folding defect. In order to prevent the instability of the material, the die shaped in Figure 6 is usually used for preparation.
Semi closed forging
As shown in Figure 7, semi closed forging is a method to promote material filling by generating the flying edge to increase the pressure in the mold cavity. When the flying edge is compressed, the constraint coefficient C will increase to 6.0-9.0, and the thickness of the flying edge shall be controlled above the necessary thickness as much as possible. Figure 8 is an example of cold forging using semi closed forging.
Extrusion of shaft rod
Shaft rod extrusion is a processing method to reduce the diameter of material, usually called positive extrusion. The extrusion of shaft rod can be divided into the internal constraint extrusion of the die in which the blank is put into the die for extrusion as shown in Fig. 9, and the free extrusion shown in Fig. 13b. Free extrusion is applied to forming with small machining degree.
As shown in Figure 10, internal cracking is easy to occur. The flow of material in the last stage of shaft rod extrusion is in unsteady state, as shown in Figure 11, which is easy to produce center cavity or crack.
Cup cylinder extrusion
The extrusion of cup is the most common method in cold forging, which is to squeeze the punch into the material to form the cylinder parts with bottom while the outer diameter of blank is constrained by the die.
Usually, the extrusion punch is pushed into the material, and the material flow direction is opposite to the direction of the punch movement, so it is called back extrusion, but there is also a positive extrusion method of the punch not moving to form the cylinder parts through the extrusion material. Figure 12 shows,
5、 Cold forging method of compound drive
As shown in Figure 2, cold forging generally requires multiple processes, which mainly takes into account the excessive pressure of the die when forming with one process. High die surface pressure not only causes strength damage, but also causes the elastic deformation of the die, which leads to the decrease of the precision of the forging products.
The recent cold forging process of gear requires high forming pressure to fill the tooth profile if the usual forging method is used. In order to forge gears with as few working procedures as possible, block forging or split forging with compound motion function is needed.
Block forging
Figure 15 is the block forging principle and bevel gear forging. The blank is put into the cavity formed by the upper and lower die, and the material is compressed and deformed by the upper and lower punch.
The contact area between material and punch is almost unchanged, and the material is squeezed to radius direction, which can greatly form force compared with the compressed flying edge in semi closed forging.
In addition to the movement and the die closing force of the upper and lower punch, the special design of the die base device is required to use the method. By means of closed forging, the CV joint such as bevel gear and constant velocity joint has been successfully produced.
Split forging
The principle of split forging is that there is space for material flow in both the main and the reverse directions of material flow, thus reducing forging pressure.
When reverse extrusion is shown in Fig. 16a, a method of extrusion exit is also designed in front of it is called the method of abandoning shaft. Fig. 16b is to flow the material to the tooth shape of the outside, while a hole is designed inside the material to make the material flow to the inside at the same time, which is called hole setting method.
The method is applied to gear forging by using the compound action die. According to the characteristics of material flow, the method is called split forging. In recent years, the precision forging of helical gears has been successfully developed by using this method and by using the compound movement of the die.
