Weld Processing and Mechanical Responses of 1-GPa TRIP Steel Resistance Spot Welds

It is essential to assess the weldability of newly developed steels, which are candidates for manufacturing vehicle autobodies.This work aims to study the weld processing and weld mechanical performance of a 1-GPa grade, transformation-induced plasticity steel.It also tried to disclose the relationships between the thermophysical properties of the alloy and its response to the resistance spot welding process.The rich chemistry and complex thermomechanical processing of the alloy result in changes to its thermophysical properties.It was found that the steel had higher electrical resistivity and lower heat conductivity in contrast to the conventional low carbon steels, resulting in more weld penetration depth.The special thermophysical properties of the alloy also allowed for nugget formation at lower welding currents.Due to the enhancement of the metallurgical bond induced by the weld penetration, the weld samples exhibited more load bearing and energy absorbing capacities in the tensile-shear tests in contrast to the conventional and high-strength steels (HSS).The relationships between the sheet intrinsic properties, metallurgical bond characteristics, and mechanical responses of the welds are disclosed here.In studying the mechanism of the nugget formation and growth, it was seen at high heat input conditions, the rates of the nugget growth and penetration were almost unchanged, an indication of the balance between the heat generation and heat transfer by the water-cooled electrodes.The results indicate a higher nugget penetration depth was obtained in the resistance spot welds of 1-GPa grade, transformation-induced plasticity steel in contrast to those of conventional low-carbon steels.This was a key factor in limiting the nugget size because the higher nugget penetration depth reduced the distance between the water-cooled electrodes and molten nugget, and thus enhanced the efficiency of the heat transfer by the water-cooled electrodes.The welds showed high load and energy bearing capacities; however, the size of the weld button in the tensile experiments was less than the corresponding nugget size.It was seen that both cross-tension and tensile-shear strength of the welds increased with the nugget size, but their ratio (ductility ratio) decreased at high welding currents.This can be an indication of the existence of the weld discontinuities, inhomogeneities, and stress concentration factors in the welds obtained at high-heat input conditions, which need to be assessed with more detail.