When the seam welding process (intermittent) seam welding of the welding head of the roller welder is carried out, each welding point must pass through three periods of pre-pressing, electrified heating cooling and crystallization. However, due to the sensitive change of the relative orientation between the roller electrode and the weldment during seam welding, the three periods are not as distinct as those during spot welding.
1) Under the direct pressing of the roller electrode, the metal being heated by electricity is in the "heating period of electricity".
2) The adjacent metal that is about to enter under the roller electrode is subjected to certain preheating and some pressure from the roller electrode, and it is in the "pre-pressing period".
3) The adjacent metal that has just come out from under the roller electrode is initially cooled on the one hand, and at the same time is subjected to some pressure from the roller electrode, which is in the "cooling and crystallization period".
Therefore, the roller welder under the roller electrode and the metal material at both ends adjacent to it will be in different periods respectively at the same time. For any solder joint on the weld, the process of passing under the roller is also the process of going through the three periods of "pre-pressing - electrified heating - cooling and crystallization". Since this process is carried out dynamically, the pressure effect during the preloading and cooling crystallization periods is not sufficient. The quality of seam welded joints should generally be worse than that of spot welding, and defects such as cracks and shrinkage holes are prone to occur.
2. Three working methods of roller welder
According to the mechanical-electrical cooperation method of the rotation of the roller electrode (movement of the weldment) and the passage of the welding current (energization), roller welder can be divided into the following three types.
Continuous seam welding
The roller electrode rotates continuously, the weldment moves at a constant speed, and the welding current passes continuously, forming a welding spot every half-circle. Welding speed can reach 10～20m/min Due to the poor surface quality of the weld, the actual use is limited
Intermittent seam welding
The weldment moves continuously and at the same speed, and the welding current passes through intermittently, and each "on-off" constitutes a welding spot. According to the thickness of the plate, the welding speed can reach 0.5 ~ 4.3m/min. It is widely used and mainly produces gas, water, and oil sealing welds of ferrous metals.
Step seam welding
The weldment moves intermittently, and the welding current passes through when the weldment stops, each "on-movement" constitutes a welding spot, and forging pressure can be applied. The joint composition is very similar to spot welding. The welding speed is low, generally only 0.2 ~ 0.6m/min. It is only used to make high-sealing welds such as aluminum alloys and magnesium alloys.