Additive Manufacturing of 7xxx Aluminium Alloys by Laser Powder Bed Fusion

Abstract

Additive manufacturing of high strength wrought grade aluminium alloys is of great industrial and academic interests, but still difficult in practice due to the tendency of crack formations in the microstructure. This work aimed at producing dense crack-free samples of AA7075-type aluminium alloys by the laser powder bed fusion (LPBF) technology, and to investigate the microstructure. It was found that LPBF of pure AA7075 powder gave micro-cracks for all tested 25 processing parameters which covered a wide range of laser power and speed. In order to avoid the cracks, different methods were tried including high preheating temperature, introducing nano-particles, and adding Si into the alloys. Introducing ZrH₂ nano-particles into AA7075 by mechanical blending could successfully produce crack-free samples using LPBF. Adding 5% Si into the AA7075 powder by mechanical blending could also give crack-free samples with high relative density. It was also found that the as-built samples with preheating at 250°C had hardness as-low-as 70HV. Solutionizing at 480°C with water quenching followed by artificial aging at 120°C can bring the hardness to 140HV and 120HV, for the AA7075 variants with ZrH₂ and with addition of Si, respectively. Detailed characterizations revealed clear influences of both preheating temperature and alloying on the microstructure. It is concluded that preheating temperature should be carefully selected for AM of high strength precipitation hardening Al alloys.