Colloquia

Samenvatting

Due to the electrification of the automotive industry cars need to be lighter to achieve the highest possible range on a single battery charge. Therefore, more and more car manufacturers are turning to aluminum castings to achieve high-volume, complex parts with excellent performance at a lower mass. Aluminum has only good mechanical properties when alloyed. To further increase the mechanical properties, the microstructure can be altered, for example, by grain refinement and/or modification of eutectic Si. Recently, more research has been conducted on the addition of rare earth elements to increase the performance of grain refinement and/or modification of the eutectic Si. Er and Eu are potent grain refiners and modifiers, respectively. However, it is known that the castability of an alloy can differ greatly when grain refiners and/or modifiers are introduced to the melt. In gravity die casting, castability is often limited by fluidity. Therefore, this study focuses on quantifying the effect of the addition of Er and Eu on the fluidity of a common foundry alloy A356.

To this end, two improved fluidity test molds have been designed, to minimise the variability in the input parameters. These molds have been produced and fluidity experiments have been conducted on four compositions; the base alloy, 0.1wt% Er, 0.2wt% Eu, and  0.1wt% Er + 0.2wt% Eu addition. Lastly, the microstructure of the samples has been investigated to see the influence of these additions to the base alloy and explain the results obtained by the fluidity tests.

The results show no significant in- or decrease in fluidity after the addition of Er and/or Eu to the A356 base alloy. From the results of the Reduced Pressure Test, it was found that the melt cleaning treatment was not successful resulting in extremely bad melt quality during the experiments. After examination of the microstructure of the castings, it was found that the eutectic Si of the base alloy was already modified before any addition, which could not be explained by the knowledge currently present in the field. The eutectic could be modified even further when Eu was added to the melt, the effects on fluidity stayed minimal. Surprisingly, Er was not able to grain refine the microstructure this was contributed to the nucleation of Er on the oxide bifilms which settled to the bottom of the crucible, disabling the ErAl3 as heterogeneous nucleation sites. From the results obtained with the scanning electron microscope it was concluded that Er and Eu formed intermetallics and that only in the Er + Eu composition the morphology and size of the intermetallics might influence the fluidity.