Influence of wire feed rate to speed ratio on arc stability and characteristics of cold metal transfer weld–brazed dissimilar joints

To explore the influence of wire feed rate (WFR) to speed ratio on arc stability, interfacial microstructure and mechanical properties, cold metal transfer (CMT) weld-brazed lap joints of aluminium alloy (AA5052) to dual-phase (DP780) steel were formed at a constant optimized heat input of 80 J/mm. Cyclic representation of recorded voltage and current showed that joining process was comparatively less stable at high WFR to speed combinations. At high WFR and higher speed combinations, greater spatial variations in intermetallic compound (IMC) layer thickness due to spilling of τ₅-Fe₂Al_7.4Si phase and higher porosity in the bead reduced the resistance to failure of the joints especially in the case of AlSi12 filler. Addition of Si successfully increased the wettability of the joints and also reduced the average thickness of the IMC layer at the interface. Formation of hard and brittle orthorhombic η-Fe₂Al₅ and monoclinic θ-FeAl₃ compounds was observed for pure Al filler whereas less brittle ternary Fe-Al-Si intermetallic phases formed with Si-enriched fillers. Three modes of failure, in the Al heat-affected zone (HAZ) (due to softening), bead (due to porosity) and interface (due to poor wettability/increased brittleness), were observed during shear tensile testing. The joints formed using AlSi5 filler at lower joining speed and low WFR combinations, i.e. high WFR/speed ratio, showed higher and consistent mechanical properties.