Effect of the Heat-Affected Zone on the Mechanical Properties and Microstructure of TIG-Welded Cast Iron/Aluminum Alloy Joints

Abstract

Dissimilar-metal welding between Gray Cast Iron (ASTM A48M) and Aluminum Alloy 6063 presents a persistent engineering challenge, particularly in the heat-affected zone (HAZ) where rapid thermal cycling drives microstructural changes that govern the mechanical performance of the entire joint. To address this, TIG-welded specimens (200 mm × 200 mm × 30 mm) were fabricated at 240 V and 90 A and subjected to three post-weld cooling conditions natural air cooling, oil quenching, and water quenching to systematically examine how cooling strategy shapes both microstructure and mechanical behaviour. Vickers hardness, Charpy impact, and tensile tests were carried out across the weldment, HAZ, and base metal regions, complemented by optical metallography and scanning electron microscopy (SEM) to characterise the underlying microstructural changes. The findings show clearly that oil quenching drives weldment hardness to its peak in both cast iron (310.7 HV) and aluminum alloy (146.8 HV), while water quenching produces the highest HAZ hardness in cast iron (292.0 HV). Where toughness and tensile strength are concerned, however, natural cooling consistently outperforms both quenching methods across all zones and both materials. At the microstructural level, cooling rate was found to directly govern graphite morphology in cast iron and precipitate distribution in the aluminum alloy two mechanisms that together explain the full range of mechanical property gradients observed. Taken together, these results offer concrete, quantitative guidance for selecting post-weld cooling strategies in dissimilar-metal TIG welding, with direct relevance to automotive and structural engineering applications.