Constitutive Behaviour and Hot Tearing during Aluminium DC Casting

The prediction of hot tears during DC casting is complicated because of the complex cooling conditions and subsequent development of thermal stresses. Models developed for this purpose require a detailed knowledge of the process conditions and hot tearing mechanisms. Therefore, a micro structural investigation of hot tearing in commercial alloys was carried out in two ways: tensile specimens were deformed at various temperatures above the solidus, which conditions led to fracture. Further, crack propagation was studied in-situ at semi-solid temperatures. In both cases, the fracture surfaces of the specimens were investigated by SEM. Microstructural investigations of these cracks show that they initiate at any weak spot such as a pore or partially liquid grain boundary and occur by a combination of fluid film separation and rupture of solid bridges along grain boundaries. This leads to brittle behaviour on the large scale although local deformation can be very ductile. Similarities between hot tears in industrial ingots and cracked specimens indicate that important aspects of hot tearing during casting can be simulated by tensile experiments at semi-solid temperatures