Overcoming underfill defect with undermatching filler and establishing new acceptance criteria for underfill depth in laser beam welding of ultra-high strength steels

Abstract

Abstract This study investigated the laser-beam-weld geometry and tensile properties under various butt-joint gap conditions for 2.3-mm thick ultra-high strength steels (UHSSs) as function of the use of undermatching filler. An undermatching filler wire having a yield strength of 430 MPa was used to overcome the underfill. Without the filler, the tensile properties were acceptable under a gap of up to 0.2 mm, which is less than 10% of the base metal (BM) thickness. With the filler, the gap tolerance increased to 0.5 mm, and the undermatching filler was proven to overcome the underfill issue. The underfill-depth acceptance criterion for realising good tensile properties was defined as 0.39 mm (= 17% of UHSSs’ thickness), which was significantly higher than the currently defined American Welding Society specification (7% of UHSSs’ thickness). These optimisation criteria can be used as guidelines for the butt joints of laser beam welding in UHSSs.