Comparison of mechanical properties and microstructure of weld nugget between weld-bonded and spot-welded dual-phase steel

Weld bonding, which is a combination of resistance spot welding and adhesive bonding, is finding application in vehicle structures that involve advanced high-strength steels. The strength of weld-bonded specimens is attributed to the strength of the weld nugget and adhesive strength. The existence of an insulating epoxy adhesive layer causes a rise in contact resistance and current density during the welding stage, and thus enhances the heat input. The aim of the present study is to explore the mechanical properties and microstructure of the weld nugget in weld-bonded dual-phase steel by means of comparison with a spot-welded nugget. Tensile-shear tests, weld lobe determination, microstructural characterization, and microhardness tests of weld-bonded and spot-welded specimens were conducted. The results of tensile-shear tests show that the weld nuggets of weld-bonded specimens have a higher tensile-shear force and energy absorption, and exhibit button-pullout fracture more easily at lower welding current. The weld lobe of weld-bonded dual-phase steel is too narrow. The results of microstructural characterization and microhardness tests indicate that, compared with resistance spot welding specimens, weld-bonded specimens have a larger nugget size at lower current; finer martensite and lower hardness in the heat-affected zone; and slightly more ferrite and lower hardness in the fusion zone. The comparative results are useful for optimizing the processing parameters and improving the weld quality of weld bonding.