Joint Interface Characteristics and Phase Transformations in Laser Brazed Steel

Zn-coated steel was laser brazed in a flare V-groove configuration, and the interfacial joining microstructure and solidification pathway were investigated for different wire feed rates (WFRs) with the assistance of thermodynamic calculations. Thermodynamic calculations indicate that Si atoms diffuse toward the steel substrate and this Si-enrichment layer along the steel/bead interface further promoted the formation of an α-Fe(Si) reaction layer, which was thin (~ 1.9 to 2.9 µm) and uniform at the laser direct irradiation region, and thick at the molten-Fe-enrichment region. Multiple phases are formed in the joint with a precipitation sequence involving α-Fe/α-Fe(Si), Cu and other phases (FeSi, Cu56Si11, Mn5Si3) in the braze bead. More molten Fe was stirred into the bead for the laser brazed joint when a low WFR of 3.5 m/min is applied and an Fe-based liquid phase appeared in addition to the Cu-based liquid according to the Scheil–Gulliver calculation results. The α-Fe/α-Fe(Si) phase was first precipitated from the Fe-based liquid at 1227 °C and then precipitated from the Cu-based liquid during the solidification process. However, when applying a WFR of 5 m/min, the α-Fe/α-Fe(Si) phase was directly precipitated from the Cu-based liquid at 1051 °C. As a results of these differences in phase types and content of precipitates within the bead, the microhardness of the bead decreased with WFR, reaching average values of 143 ± 2.5 and 117 ± 2.3 HV at WFRs of 3.5 and 5 m/min, respectively.