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

This study investigated the laser-beam-weld geometry and tensile properties under various butt-joint gap conditions as a function of the use of undermatching filler for 2.3-mm thick ultra-high strength steels (UHSSs) with a yield strength of 610–850 MPa. An undermatching filler wire with 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 applied to UHSSs was proven to overcome the underfill issues without affecting the tensile properties while maintaining the low-temperature transformation microstructure of the weld metal (WM). Regardless of the use of filler, the underfill-depth acceptance criterion for 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 new optimisation criteria can be used as guidelines for butt joints of laser beam welding in UHSSs.