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 zon ein 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 %

Reducing the cooling time in a resistance spot welding schedule offers two possible advantages. The first is decreased weld cycle time, which is of great importance in particular to car body assembly lines performing hundreds of millions of spot welds each year. Decreasing each resistance spot weld cycle time by a mere 0.1 s leads to substantial cost savings. The second advantage is the possibility to reduce the hardness in the weld metal and thereby improve fracture behaviour. Reducing the cooling time leads to reduced cooling rate of the weld metal and potentially a softer material. Two and three-sheet joint combinations of martensitic, dual phase, transformation induced plasticity (TRIP), complex phase and hardened boron steels were investigated in this study. The joints were evaluated by shear-, peel and cross-tension testing as well as metallographical examinations and hardness measurements. With knowledge of the materials weldability, and evaluation of the behaviour when welding with reduced cooling time, it was possible to develop optimised weld schedules and thereby increase productivity. It is recommended that the programming of the hold time in the power sources is modified in order to enable full flexibility in setting of the hold time /cooling time. This study showed that the cooling time could be reduced significantly without endangering the joint integrity.

In normal production of resistance spot welded galvanised structures, it is difficult to completely avoid surface breaking cracks. Known key factors to cause cracking are zinc coating, electrode wear during subsequent welding and insufficient electrode cooling. In this report, an embrittlement mechanism was investigated that could be coupled to the galvanisation method for dual phase steels. With identical bulk material and weld parameters, the first 50 spot welds were crack free with electrogalvanised coating, while only 10 out of 50 were crack free with hot dip galvanised coating. Energy dispersive X-ray spectroscopy analysis of the worn electrode surfaces used for welding of the hot dip galvanised coating revealed areas of aluminium oxide. Since aluminium oxide is a very strong isolator, the electrical resistance will increase, which in turn is suggested to increase the surface temperature of the spot weld and thereby increase the probability for liquid metal embrittlement and surface cracks.