When a seam welder's seam is formed, each welded joint must also undergo three stages: pre-compression, electrical heating, cooling and crystallization. However, these three stages are not as obvious as in spot welding, because the relative position between the rolling electrode and the weld piece changes quickly during the seam welder welding process. The following cases can be considered.
1. The electrically heated metal is in the electrical heating stage under direct pressure.
2. The adjacent metal which enters under the roller electrode by certain preheating and partial pressure is in the pre-pressure stage.
3. On the one hand, the adjacent metal just emerging from underneath the roller electrode starts to cool down. At the same time, it is in the cooling crystallization stage and is under the partial pressure of the roller electrode.
As a result, the weld area under the roller electrode and the adjacent metal material will be in different stages. For any welded joint on a weld, the process through the roller is in three stages: pre-pressure, heating, cooling and crystallization. Since the process is carried out under dynamic conditions, the pre-pressure and cooling crystallization stages have insufficient pressure effects. Seam welded joints should typically be of lower quality than spot welds and are prone to cracking and shrinkage holes.
