Role of grain refinement in modifying intergranular liquid film properties and mitigating hot tearing in Al–9Si–3Cu casting alloys

Al–9Si–3Cu casting alloys are widely used in the manufacturing of automotive engine cylinder heads, but with the requirements of thin-walled castings, the probability of hot tearing defects increases significantly. This study focuses on the effect of grain refinement on the hot tearing of alloys. Three different methods of adding Sr, Al–5Ti–B and Sc are used to refine the Al–9Si–3Cu grains to a similar level, and their sensitivity to hot tearing is evaluated. The effects of three refiners on the stress‒strain characteristics of Al–9Si–3Cu intergranular liquid films between two single-crystal grains were studied using a custom-made device, and the mechanism by which the refiners affect the hot tearing of the liquid alloy films was analyzed. The results show that grain refinement can promote the internal shrinkage feeding behavior of the alloy, thereby reducing the hot tearing sensitivity of the alloy. The addition of Sr also delays the formation of the eutectic silicon phase and reduces the effect of the eutectic silicon phase blocking the feeding channel, thus contributing the most to the reduction in hot tearing sensitivity. Compared with the other two grain refinement methods, the addition of Sc generates second phases that block the feeding channel, resulting in a higher hot tearing sensitivity.