Insights into grain refinement of Al-Mn-Fe-Si alloy via in situ reaction during laser directed energy deposition

In the present work, we studied the grain refinement by adding in situ reactants, pure titanium (Ti) or a combination of Ti and boron (B), and investigated the governing mechanism in Al-Mn-Fe-Si 3104 alloy made by laser directed energy deposition (DED) additive manufacturing (AM). The transition from large columnar grains to refined equiaxed grains was achieved by Ti addition or Ti+B additions and in situ reaction during laser AM. The sole addition of Ti was identified as the best grain refiner in this study. The increase in Ti addition from 0.5 wt% to 1 wt% resulted in a transition from partial to complete equiaxed grain structure. With the same amount of Ti, B addition would diminish grain refinement, due to the consumption of Ti to form TiB2. The direct addition of TiB2 particles was proven to be ineffective to grain refinement. Substrate heating further improved grain refinement by reducing the cooling rate and temperature gradient. The study demonstrated the higher nucleation potency of Al3Ti than in situ formed TiB2 during laser DED AM. The formation of Al3Ti plays the most critical role in grain nucleation.