During the seam welding process (intermittent) seam welding, each solder joint also goes through three stages of pre-pressing, heating by heating and cooling and crystallization. However, due to the rapid change of the relative position between the roller electrode and the weldment during seam welding, these three stages are not as obvious as those in spot welding. It can be considered that: 1) Under the direct pressing of the roller electrode, the metal being heated by electricity is in the "electric heating stage". 2) The adjacent metal that is about to enter under the roller electrode is subjected to certain preheating and partial pressure of the roller electrode, and it is in the "pre-pressing stage". 3) The adjacent metal that has just come out from under the roller electrode starts to cool on the one hand, and at the same time is subjected to the partial pressure of the roller electrode, which is in the "cooling and crystallization stage".
Like spot welding, seam welding is a welding process under the combined action of thermal mechanical (force). In comparison, the mechanical (force) effect is insufficient in the welding process (except for step seam welding), and the faster the welding speed is, the more obvious it is.
The seam weld is composed of overlapped welding points, so the shunting during welding is much more serious than that of spot welding, which makes it difficult to weld thick plates of high conductivity aluminum and magnesium alloys. The surface of the roller electrode is easy to be damaged and the surface quality of the weld is deteriorated. Therefore, the electrode dressing is a particularly noteworthy problem.
Since the cross-sectional area of the seam weld is usually more than twice the longitudinal load area of the base metal (the thinner the plate, the greater the ratio), the damage must occur in the heat affected zone of the base metal. Therefore, the joint strength of seam welded structure is seldom emphasized, and it is mainly required to have good sealing and corrosion resistance.