Influence of filler wire composition and heat input on microstructure and strength of CMT brazed DP steel joints

In this study, three types of Cu-based fillers with varying Al content were used for joining high strength DP780 steels in lap position using cold metal transfer (CMT) brazing for a range of heat input 64–266 J mm⁻¹. Results showed that formation of liquation cracking and porosity occured in case of pure Cu whereas presence of intermetallic layer at the L/S interface in case of alloyed wires prevented the diffusion of Cu at the grain boundaries of base metal. Majorly bead shape and amount of dispersed intermetallic phases in the deposited bead contributed to the joint strength and geometrical notch at the edge of upper sheet influenced the failure mode of the joints. Fe in the filler resulted in higher amount of dispersive intermetallic phases in the bead in case of CuAl10Fe filler. CuAl10Fe filler exhibited higher strength in comparison to CuAl8 irrespective of bead shape at each heat input due to increased amount of dispersive phases which strengthened the joints. Orowan's model also predicted higher strength for CuAl10Fe joints due to higher amount of dispersive phases and showed good agreement with the experimental results. Based on the observations, it is recommended to alter filler composition, torch position and coating composition etc to strengthen the brazed joints by increasing the dispersive phase fraction. (paper)