Effect of a slightly concave electrode on resistance spot welding of Q&P1180 steel

Abstract The resistance spot welding (RSW) of advanced high strength steel exhibits a high tendency to fail in unfavorable interfacial fracture. To solve this problem, a novel slightly concave electrode was designed. The RSW joints of Q&P steel obtained with the conventional dome-radius electrode and the novel slightly concave electrode in terms of microstructure, micro-hardness and tensile-shear properties were investigated and compared. Numerical simulations were conducted to reveal the thermo-electric field evolution and explain reasons for the improvement of weldability with the slightly concave electrode. The results show that by aid of slightly concave electrode, the welding current range meeting the requirement of button pullout failure was doubled and upper limits on tensile-shear load reached 28.5 kN and achieved 16 % improvement. By the employment of slightly concave electrode, a ring-shaped distribution of current density formed at the initial stage due to the ring-shaped contact interface of electrode/steel, which fabricated an annular nugget. Because of the lower current density and the stronger corona bond zone after the formation of nugget, the slightly concave electrode decreased the possibility of early expulsion, resulting in larger upper limits on nugget and wider welding current range.