Joakim Wahlsten, Erik Tolf

Abstract

Resistance spot welding of three-sheet joints has been performed for a number of years in the industry but it is still a demanding task. A three-sheet joint with a thin outer member of ultra low carbon thin sheet steel often requires extended weld times for the nugget to grow into the thin sheet. The current ranges are also in many cases limited. It is of great interest to be able to improve the weldability of these joints.

The investigated methods in this project have been use of process tapes and an alternative weld primer. The use of stainless process tape proved to be the best of the investigated methods for improved weldability. The alternative coating, Gardoprotect®, as well as Cu process tape did influence the approved current range. However, the penetration of the thin sheet was still minor, compared to the result from stainless tape.

Another unwanted phenomenon in resistance spot welding is shunting. Measurements of shunt currents through a preceding weld spot resulted in the following graph. The same linearity could however not be observed for boron steel.