Resistance spot welding systems based on 1 kHz medium-frequency inverters (MFDC) have recently gained increasing importance in industry. The uninterrupted energy input into the weld nugget using direct current (DC) enables the programming of welding parameters in the smallest controllable time unit of around 1 ms. This even allows for the welding of multi-material joints such as steel with aluminum by controlling the necessary welding heat via advanced multi-pulse welding programs.
As part of this study, advanced multi-pulse welding programs have been developed to successfully produce joints between commercial dual phase (DP) steel and a precipitation hardenable 6000 series aluminum alloy, with sheet thicknesses of the welded materials being about 1-2 mm. It is worth noting that the electrodes used for the MFDC spot welding device are standard electrodes with only minor modifications and that there was no need for any additional transition material or interlayer between the steel and the aluminum sheet.
The effect of welding parameters on the microstructure, particularly with regard to the typical intermetallic compound layer formed between aluminum and steel will be reported on. Interestingly, the interfacial region developed a profound bulging of the steel into the aluminum during the welding process. Light microscopy and FIB / SEM target preparation results will be presented. Furthermore it will be discussed the implications of microstructure on strength. Tensile shear tests have been performed to test this. The influence of subsequent natural aging of the 6000 series aluminum alloy on strength will also be outlined. Physical effects that lead to the unique shape and structure of the fused and heat affected zones within the multi-material joint will be illuminated as well.