Resistance Nut Welding: Improving the Weldability and Joint Properties of Ultra High Strength Steels

This study covers resistance projection welding of three different M6 and M8 weld nut types to high- and ultra high strength thin sheet steels ranging up to 1 500 MPa tensile strength. Static tensile testing, torsion testing and fracture evaluation as well as metallographical examinations were used to evaluate the suitability of the selected nut types for applications in high and ultra high strength steels. Different resistance welding equipment were compared and both alternating current and direct current were evaluated. A new etchant was used for examining the fusion zone in the nut joints, which indicated that no significant melting of the interfaces had occurred. This was evident for all the weld nut types welded to the galvanized thin sheet steels. The electrical characteristic of the power source plays an important role for the weld result. A rapid current build-up during the first period (AC) of weld time increased the initial heating of the weld nut projections that improved the joint strength. Significant differences in pull-out strength were recorded in joints welded with different AC power sources. Current curve modification for MFDC welding machines is a promising way of increasing the joint strength. A welding cycle with an initial high peak current pulse followed by a current plateau improved the pull-out strength by more than 100%.