Improving the processability and grain structures of additively manufactured Al-Fe-Cu-xZr alloy: Experiment and high-fidelity simulation

Abstract

Additive manufacturing (AM) offers significant advantages over conventional manufacturing techniques in processing Al alloys, however, high-strength Al alloys produced by AM commonly encounter a few challenges including poor processability and undesirable microstructures with coarse columnar grains and even micro-cracks, leading to poor mechanical properties of the resultant Al alloys. Here, we demonstrate that these issues can be largely resolved by micro-alloying of Zr element in the Al-based alloys. In this study, a series of novel Al-1Fe-0.6Cu-xZr alloys with different content of Zr (x=0.3, 0.6, 0.8, 1.3 at%) are designed and then subjected to laser powder bed fusion (LPBF). It is found that, with increasing Zr content, the processability of Al alloys is significantly improved, as evidenced by the gradual reduction of melt splashing and defects (including pores, balling, necking and discontinuity) during LPBF, which eventually leads to decrease of surface roughness and improvement of processability. High-fidelity simulation reveals that the processability is closely related to the melt velocity gradient (MVG) in melt pools. Increasing Zr content results in a more uniform distribution of MVG and decreased Marangoni convection, which favors the stability of melt pool and good processability. On the other hand, Zr content also plays an important role in microstructure. Although the four LPBF Al alloys all exhibit notably heterogeneous structures with alternate distribution of coarse grain zones (CGZs) and fine grain zones (FGZs) in the whole samples, with increasing Zr content, the coarse columnar grain structure in CGZs is suppressed and transforms to the fine equiaxial grain structure. The occurrence of columnar-to-equiaxed transition (CET) is attributed to the decreased ratio between temperature gradient (G) and solidification rate (R) at the solid-liquid interface with increasing Zr content. In addition, the increase in Zr content also leads to a gradual refinement of the grain size in both CGZs and FGZs due to the increased cooling rate. Beneficial from the good processability and fully refined equiaxial gain structure, nearly full-dense and high-strength Al alloys with 0.8 and 1.3 at% Zr are successfully fabricated, which show a good combination of high strength and ductility. The findings in this work provide a guild for design of novel Al alloys with good processability and advanced mechanical properties for additive manufacturing.