Direct observation of hot tearing of Al-7%Si-0.35%Mg alloy

Hot tearing of an Al-7%Si-0.35%Mg alloy T-shaped specimen was analyzed by direct observation of cracking and a finite element method (FEM) stress simulation. The semi-solid state during cooling enables direct observation of corner surface cracks. Crack opening appears to begin before the solid fraction reaches 0.3. However, a liquid phase infiltrates and disturbs the crack opening process. After the onset of the eutectic reaction, crack opening resumes and is detected as hot tearing. The FEM stress analysis revealed that tensile stress concentrates at the corners of the T-shaped specimen. The tensile stress increases with time at the initial stage of solidification, peaks at around 170 s, and then decreases. Microstructural observation of cooled and quenched specimens reveals that if the inter-dendritic liquid phase is adequate and has high fluidity, then healing of the crack occurs.