Complete columnar-to-equiaxed transition and significant grain refinement in an aluminium alloy by adding Nb particles through laser powder bed fusion

3 wt% Nb particles were added to Al7075 alloy powder and processed by laser powder bed fusion (L-PBF) with the aim of improving the processability of the alloy and promoting grain refinement. It was found that with the addition of Nb, the grain structure changed from large columnar grains into ultrafine equiaxed grains and the hot tearing was completely avoided. Electron microscopy study reveals that the L-PBF-processed Al7075-Nb sample contains a high density of Al3Nb and MgZn2 precipitates. The intra-granular Al3Nb particles were found to show coherent orientation relationships with the matrix and should have acted as potent heterogeneous nucleation inoculants. A high population of Al3Nb particles were also distributed along grain boundaries which may have impeded grain growth during L-PBF. The ultrafine MgZn2 precipitates which have a lower melting point than the matrix is believed to have formed during thermal cycling of L-PBF. The Al7075-Nb samples demonstrate high yield strength and decent ductility. Transmission electron microscopy study reveals that during deformation, the relatively larger Al3Nb particles were looped around by dislocations whereas the relatively smaller MgZn2 particles have been cut through by dislocations. Serrated flow was observed during plastic deformation which is believed to be associated with the interaction between dislocations and solute elements such as Mg.