Grain refiner fade in aluminium alloys

Grain refinement is usually an integral part of melt treatment undertaken in order to consistently produce castings with premium mechanical properties. However, the successful grain refinement of foundry alloys compared to wrought alloys has proven difficult due to a higher alloy content and long holding times encountered in the foundry. Often a melt is held for long periods and the grain refinement efficiency decreases, or fades, with time. Depending on the severity of fade, significant variations in the grain size can occur, which in turn can affect the mechanical properties, porosity distribution and consistency of the casting. Settling of the grain refining particles is believed to make a significant contribution to the mechanism of fade. However, the rate of fade is much faster than predicted by Stokes law and this suggests that other factors such as particle agglomeration, formation of oxide layers and melt turbulence also contribute to the loss of grain refinement during casting. Dedicated laboratory trials have been conducted to determine the contributions of the separate mechanisms to the overall fade observed.