Demonstrating the roles of solute and nucleant in grain refinement of additively manufactured aluminium alloys

Grain refinement is one of the most effective approaches to improving mechanical properties, reducing anisotropy and hot-cracking of additively manufactured alloys. Recent studies have shown that the extreme solidification conditions associated with additive manufacturing (AM), including large thermal gradient and high cooling rate, result in the difference in nucleation and grain growth mechanisms between AM and conventional casting. Thus, it is necessary to re-consider the grain refinement mechanisms, particularly the role of solute during AM. The present work investigates the grain refining efficiencies of different solute additives (Si, Cu and Ni) and their integration with nucleants (LaB6 nanoparticles) in additively manufactured pure Al. It was found that, despite the fast cooling during AM and nucleant inoculation, solute addition is essential for activating heterogeneous nucleation upon solidification to achieve high grain refining efficiency. However, the role of solute in grain refinement during AM cannot be readily interpreted by the classic grain growth restriction theory (Q value). This is attributed to the large thermal gradient in the melt pools during AM solidification, which significantly limits the development of constitutional supercooling. Alternatively, the role of solute can be better understood in terms of the lag in dendrite growth induced by solute rejection during solidification. This causes the difference between the actual dendrite growth and the theoretical pull rate, generating large thermal undercooling ahead of the solidification front to elicit heterogeneous nucleation. This work sheds new light on the factors affecting grain refinement under fast cooling.