The resistance welding process is divided into pre-pressing, heating, upsetting, maintenance, and rest procedures. The first three procedures are involved in the formation of the resistance welding joint, and the latter two are necessary auxiliary procedures in operation. In equal pressure resistance welding, it is difficult to distinguish between upset and maintenance.
1. Preloading The purpose of preloading is to establish good and uniformly distributed physical contact points. For this reason, the connection surface of the weldment and the surface where the current is introduced should be cleaned well, and the error of the parallelism of the connection surface should be as small as possible. To ensure that the initial contact points are as evenly distributed as possible.
2. The purpose of heating is the main stage of resistance welding. Under the combined effect of mechanical force and resistance heat, the contact point is rapidly heated and deformed, which causes the contact area to increase, and finally extends to the entire joint surface, so that the contact resistance tends to zero. Weldment resistance increases with temperature. Under the effect of heat conduction, the temperature of the end surface gradually becomes uniform, and a certain temperature distribution is formed along the depth of the end of the weldment. The temperature distribution curve during resistance welding. At the same time, under the action of pressure, the weldment gradually develops plastic deformation and shortens.
During heating, the following points should be noted: 1. The maximum temperature on the joint surface of the two weldments should not exceed the melting point of the material, which is generally 80% to 90% of the material melting point (calculated according to the Celsius temperature). The temperature distribution everywhere on the end face should be made uniform by heat conduction.
2. Reduce and prevent oxidation of the bonding surface during heating. If necessary, a protective atmosphere (such as a single or mixed gas of Ar, N2, H2, and CO gas) can be introduced; the groove shown in Fig. 19a can also be used to isolate the end face from the atmosphere during the initial stage of heating.
3. Upset forging When the temperature of the end face of the weldment reaches uniform and the temperature distribution along the depth of the weldment is appropriate, the plastic deformation speed will be significantly accelerated, and the upset forging phase is entered, at which time the current should be cut off. Impurities such as oxides on the end face should be completely eliminated during upset, so that the subsequent pure metal will cause the metal interface to disappear under certain plastic deformation, forming a common crystal grain, thereby forming a joint. When equal pressure resistance welding is used, the upset force is the same as the heating force, so the distinction between the two stages is not clear. When variable pressure resistance welding is used, the upset force is greater than the heating force. In addition to the thorough elimination of impurities such as oxides, upset forging should also achieve sufficient plastic deformation. In addition to the appropriate temperature distribution and pressure, the end faces of the rotating body symmetrical weldment are processed as shown in Figure 19c, Figure 1-9d, and Figure 19f, which is conducive to the removal of oxides.
4. Maintenance The purpose of maintenance is to cool the weldment under pressure to avoid defects caused by shrinkage stress.
5, rest for resetting the device.