Effect of GMAW-brazing and Cold Metal Transfer Welding Techniques on the corrosion behaviour of Aluminium-Steel lap joints

Dissimilar metal welds of steel and aluminium find major application in automotive sector as it makes the vehicles fuel efficient. But a major concern in the Al-steel joints is the microstructural in-homogeneities in the weld zone due to the low solid solubility issues between steel and Aluminium alloy and large differences in their thermal and electrochemical properties. The durability of such welds in service environment is still an issue to be resolved. Hence the main aim of this study is to compare the corrosion behaviour of H32 5052 Aluminium-galvanized mild steel joints welded by Gas metal arc welding-brazing (GMAW-brazing) and Cold metal transfer (CMT) techniques as a function of welding parameters viz., weld speed and wire feed rate. Lap joints were fabricated using Al-5%Si filler wire, followed by the microstructural characterization using Scanning electron microscopy, hardness measurement by nano-indentor and phase analysis by X-ray diffraction technique. Corrosion testing was done on the joints following ASTM G 67-04. The results establish the effect of welding parameters on the microstructure and the corrosion behaviour of the welds. A thick Al-Fe-Si intermetallic compound (IMC) layer was formed at the weld bead-mild steel interface with Zinc entrapping in the foot region of both welds. The composition of this layer was confirmed by EDAX and XRD results. In both the cases, it had shown a severe impact on the intergranular corrosion behaviour of the welds. The results have also shown an increase in the thickness of the intermetallic layer with increase in the heat input, enhancing the galvanic and intergranular corrosion rate.