Mechanisms of externally solidified crystal formation in aluminum high-pressure die casting

Abstract Aluminum high-pressure die castings (HPDC) are widely used in the automotive and other industries to achieve lightweight components with high productivity. However, the formation of externally solidified crystals (ESCs) during HPDC process can significantly reduce the mechanical performance, particularly the elongation and fatigue, of these cast parts. ESCs can be classified into two main types: Type I, consisting of large α-Al dendrites, and Type II, characterized by large crystals with fine dendrites that exhibit a clear boundary with the matrix. This study investigates the formation mechanisms of these two types of ESCs during the HPDC process. The effects of various process parameters on the formation and movement of ESCs were analyzed through high-pressure die casting trials, computer simulations, and water analog experiments. The investigation suggests that both types of ESCs start within the shot sleeve. Type I ESCs form and float within the melt, while Type II ESCs develop along the shot sleeve wall and plunger tip post-pouring. Increasing the melt temperature was found to reduce the formation of Type II ESCs. Both types of ESCs are carried into the die cavity during the filling process. Their distribution is closely related to the fast shot speed and the turbulence of the molten aluminum. Notably, a reduced fast shot speed was found to significantly decrease the number of ESCs transported into the die cavity, especially Type II ESCs. This reduction led to a considerable improvement in the mechanical performance of the cast components, particularly in terms of elongation. These findings on ESC formation as related to process parameters provide important guidance to achieve high-performance castings in industrial applications.