Investigation of a novel TIG-spot welding vis-à-vis resistance spot welding of dual-phase 590 (DP 590) steel: Processing-microstructure-mechanical properties correlation

Resistance spot welding (RSW) is the most widely used joining technique for thin sheets in automotive industries. However, this process requires access from both sides of the sheet metals, which is unavailable in many intricate applications. To address this gap, a novel one-sided tungsten inert gas spot welding (TIG-spot welding) is investigated for the joining of 1 mm thick dual phase (DP 590) steel. To establish the processing window for TIG-spot welding, weld current and arc time were varied between 100 and 180 A and 0.2–1.2 s respectively, considering one variable at a time approach. For RSW, to obtain a comparable nugget diameter with TIG-spot welds, the welding current was varied between 5 and 6.8 kA at a constant welding time of 0.2 s and an electrode force of 2.5 kN. For the investigation of the mechanical properties of the joints, tensile-shear, Coach-Peel, and Cross-tension tests were conducted. It was revealed that joints fabricated using both RSW and TIG-spot welding having a nugget diameter greater than 4 mm possessed load-bearing capacity greater than the threshold requirement. Similar to the RSW process, with an increase in welding current, nugget diameter increases, resulting in a significant rise in the load-bearing capacity of the joints. Further, a desirable pull-out mode of failure with fully ductile fracture could be achieved with the TIG-spot welding technique. Microstructure-hardness correlation is established for both RSW and TIG-spot welded joints. Softening of the sub-critical heat-affected zone in the TIG-spot welding process was associated with extensive tempering of martensite, which was not the case in the RSW process. Microstructural differences resulting in a relatively lower hardness in different zones of welding explain the slightly lower load-bearing capacity for the TIG-spot welds compared to RSW. However, the joint strength is adequate as per the standard and hence can meet the application requirements.