Studies on Partially Melted Zone and Pitting Corrosion Resistance of A356 Aluminium-Silicon Alloy GTA Welds

Abstract

Gas Tungsten Arc Welding (GTAW) of Pressure Die cast A356 Al-Si alloy with strontium modification was done both with and without filler. The microstructural changes in Weld metal zone (WM), Heat affected zone (HAZ) and Partially Melted Zone (PMZ) were studied. PMZ of aluminium alloy weld is an important region, as it is the weak link in the weldments. It is significantly affected by the welding parameters, filler metal and prior thermal condition. In the present work affect of welding techniques i.e. Continuous Current Gas Tungsten Arc Welding (CCGTAW) and pulsed Current Gas Tungsten Arc Welding (PCGTAW) on microstructure and pitting corrosion resistance of weld metal, Partially melted zone and Heat affected zone in the prior conditions of as cast and T6 conditions were studied. Susceptibility to liquation was found to be less in the weld made in as cast condition of pulsed current GTA welds compared to that of artificially aged condition (T6) of continuous current GTA welds. This was mainly attributed to the silicon enrichment of eutectics at the grain boundaries. Potentiodynamic polarization testing was carried out to study the pitting corrosion behavior of the welds. Pitting corrosion resistance of weld made with pulsed current GTAW of as cast alloy is better than the that of weld made with continuous current GTAW of T6 alloy. This is mainly attributed to the discontinuous eutectic formation at the grain boundary base metal and PMZ.