Effect of holding time on microstructure and mechanical properties of resistance spot welds between low carbon steel and advanced high strength steel

The effect of holding time on microstructure, hardness and mechanical properties of unequal thickness weld joints between low carbon steel and advanced high strength steel is investigated in the present work. The microstructures in the regions of weld nugget, heat affect zone and base metal are obviously different. The hardness of fusion zone both at low carbon steel and advanced high strength steel sides shows a slight variation and it is much higher than that of base metal. The peak load and failure mode of the weld joints are carried out via tensile shear test and finite element analysis. The test result demonstrates that the peak load and failure mode depend on the resistance force generated via the failure zone. For a given fusion size, the increase of holding time raises the hardness of fusion zone, and affects the peak load and failure mode of weld joints via enhancing the interfacial resistance capability. The increase of holding time reduces the welding efficiency, thus, a suggestion based on the test result is proposed that the holding time should be less than 15 cycles to ensure the manufacturing efficiency. A nonlinear finite element analysis is applied to simulate the tensile test procedure. The simulation demonstrates the necking phenomenon and failure process at interfacial or base metal around the weld nugget. (C) 2016 Elsevier B.V. All rights reserved.