Gas metal arc welding with undermatched filler wire for hot-press-formed steel of 2.0 GPa strength: Influence of filler wire strength and bead geometry

Commercial welding filler wires have less strength than hot-press-forming (HPF) steels. As the 2.0 GPa-HPF steel sheets have been released, their lap welding characteristics were investigated using gas metal arc welding in this study. The base metal was 1.1 mm-thick 2.0 GPa-HPF steel sheets, and three filler metal wires considered in this study (W540, W920, and W980) had tensile strengths of 540, 920, and 980 MPa, respectively. Gas metal arc welding was performed under a controlled short-circuit mode, and the wire feed speed (WFS) was selected as a process parameter. Tensile-shear test and microscopy were performed to evaluate the joint strength and metallurgical characteristics. The joint strength increased when WFS increased. When the WFS was 6 m/min or higher and high strength filler wires were applied to it, a heat affected zone (HAZ) fracture was observed in the tensile-shear test, with a tensile strength of approximately 1150 MPa. The fracture location was the boundary of the sub-critical HAZ (comprising tempered martensite) and intercritical HAZ (comprising polygonal ferrite and martensite). The weld metal (WM) hardness for W540 welds was 270 HV, and that for W920 and W980 was 414-419 HV, while the joint strength for the WM fracture was proportional to the throat thickness. For low WFS (when the heat input per unit length and welding current were low), high strength filler metals enhanced the joint strength, while high welding currents and deep penetration welding modes were recommended for W540. This study provided the filler wire and bead geometry design for the lap welds of 2.0 GPa HPF steel sheets.