High-deposition laser brazing with enhanced surface quality, bridge performance and mechanical properties

Laser brazing of Zn-coated steel in a flare V-groove configuration was conducted using high wire feed rates (WFR) at travel speeds (TS) of 2.5 m/min. With a high WFR/TS ratio of 1.6–2.2, high-quality joint surface without visible surface defects was obtained exhibiting surface roughness values from 1.6 to 2.2 μm, and the bridge width and height were improved. The fracture load and nominal tensile strength were improved with wire deposition rate, reaching maximum values of 3.94 kN in a 15-mm long weld, or a nominal equivalent tensile stress of 529 MPa at high WFR/TS ratios of 2 and 1.8 respectively, due to an enhanced bead strength resulting from decreased content of Fe mixed in the bead. This suppressed formation of Fe-rich precipitates, while refined the solidified bead microstructure. With the enhancement of the bead strength, the interfacial reaction layer/bead interface becomes a weak-bonded interface due to a mismatch in the lattice structure and mechanical properties in the braze versus base metal, with fracture initiating from the unwetted gap at the steel/bead interface. With low WFR values of 3 and 3.5 m/min, the joint easily fractured across the bead due to segregation of large Fe-rich precipitates and hot cracking within a concave joint geometry. The joint produced with a WFR of 4 m/min fractured by a combination of through-bead and interfacial fracture. This work provides an insight for improving the mechanical properties and broadening the application of laser brazing.