An investigation on dissimilar welding between AISI420 martensitic Stainless Steel and AISI Austenitic Stainless Steel 304

Today, resistance spot welding has become one of the most widely used welding methods in many industries. Dissimilar resistance spot welding of austenitic stainless steel and martensitic stainless-steel alloys has drawn the attention of researchers and industries due to the suitable strength, sufficient elongation and high corrosion resistance during dynamic and static loading. In the current research, the relationship between microstructure and fracture type, as well as mechanical properties and the effect of parameters of welding current and time on resistance spot welding of dissimilar AISI 420 martensitic stainless steel and AISI 304 austenitic stainless steel have been investigated. The resistance spot welding process was performed using various current limits including 6 kA, 7 kA, and 8 kA, and a time cycle of 0.2 s and 0.1 s with a constant electrode force of 2.5 bar. Optical microscopy and scanning electron microscope have been used to investigate the micro and macrostructural changes of the welding nugget. After analyzing the failure surface, it was found that the samples jointed under 7 kA current, 0.2 s welding time, and 2.5 bar electrode force had the maximum force tolerance. It was observed that the sample welded with a current of 7 kA and a welding time of 0.2s has a pullout failure (PF) mode. In this case, the fracture force was 8500 N and the amount of displacement leading to failure was 7 mm. This incident was attributed to the increase of heat input in the weld nugget, which increased the size of the weld nugget with the increase of heat input and caused the fracture toughness of the weld nugget to increase. In the other welded sample with a current of 7 kA and welding time of 0.1 s, the size of the welding nugget was reduced due to a lower heat input, and the amount of displacement leading to failure was less than the welded sample with a welding time of 0.2 s. The optimum size of the welding Nugget was obtained in the welded sample with a current of 7 kA and a welding time of 0.2 s, which led to an increase in the fracture displacement during the tensile-shear test. After analyzing the fracture pattern on the weld surface and fractography of this sample, it was observed that the crack grew from the interface of the heat-affected zone and the base metal and finally led to PF mode. With the increase in welding time and welding current, due to the increase in heat input in the welding nugget, it led to an increase in the amount of exposure of melt. The increase of exposure melt caused the failure to change from the pullout mode to the interfacial (IF) mode, which had a lower fracture toughness than the pullout mode. In welding with the current of 8 kA, the amount of exposure melt increases. After analyzing the fracture surface, it was found that the crack grew from inside the base metal and the fracture type was interfacial.