Effect of Mg alloying and cooling rate on the microstructure of silicon

In response to the escalating global demand for solar photovoltaic (PV) energy, there is a critical need for more cost-effective and environmentally sustainable production methods for upgrading metallurgical-grade silicon (MG-Si). Among various metallurgical approaches, acid leaching is an economical and effective method to upgrade MG-Si. However, the impact of cooling rates during solidification, a potentially significant factor for optimization of the leaching process, has been rarely investigated. In this work, the effects of magnesium alloying content and cooling rate on microstructural evolutions in MG-Si are studied. MG-Si was alloyed with two different magnesium contents (5.5 wt% and 9.0 wt%), using an induction furnace for the melting, alloying, and casting process. The cast alloys were subsequently remelted under five distinct cooling rates, specifically 3, 10, 25, 40, and 80°C/min. Microstructural analysis and grain size measurement were conducted using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and the ASTM E112 standards. It was observed that the Mg 2 Si phase was formed along the primary Si grains and with other intermetallic silicide-containing impurities embedded inside. Moreover, higher cooling rates resulted in finer primary Si grains with highly diverse crystallographic orientations, while slower rates induced coarser Si grains and a concentrated silicide phase along the grain boundaries. Importantly, the results also indicate that a higher magnesium alloying content (9.0 wt%) led to finer grain sizes. The present work establishes links between alloying content, cooling rate, and the resulting microstructure, offering valuable insights for optimizing the alloying–leaching process.