Numerical simulation of solidification within the shot sleeve of high-pressure die casting and the prediction of secondary dendrite arm spacing for externally solidified crystals

Abstract

The formation of externally solidified crystals (ESCs) within the shot sleeve of cold chamber high-pressure die casting (HPDC) is undesirable for mechanical properties, as it is often associated with defects (oxides and shrinkage porosities). Understanding the solidification conditions within the shot sleeve is critical to reduce the formation of ESCs. Using finite element simulations, the effect of interfacial heat transfer coefficient $\left(h\right)$, fill volume ($f_v)$, and piston displacement has been simulated to systematically investigate the development of thermal gradient and solid fraction. More emphasis is provided on the temperature loss and solidification occurring at distances up to 5 mm from the alloy-shot sleeve interface as a function of $h$ and $f_v$. Simulation results indicate that the onset of solidification along the wall regions (up to 2 mm) occurs within 0.5 s after complete filling for all $h$ $\ge$ 5 kW/m²K. Under standard HPDC operating conditions and for a given alloy composition, increasing the $f_v$ from 20 to 40 % can significantly decrease the total solid fraction (from 20 % to 10 %) along the sleeve wall. Based on the cooling rate predicted from numerical simulations and correlating the solidification conditions with unsteady (un-constrained) directional solidification studies for Al-7Si alloys, both high-growth and slow-growth regimes are identified. Using the relationships between growth rate and cooling rate with secondary dendritic arm spacing (SDAS), the locations for the growth of large size ESCs and cold flakes with finer SDAS are predicted as a function of $h$ and distance from the alloy-shot sleeve interface. Potential mechanisms for the origin, growth of large size ESCs and migration during piston displacement have been discussed.