Microstructure and mechanical properties of laser DED produced crack-free Al 7075 alloy: Effect of process parameters and heat treatment

Abstract

Laser directed energy deposition (DED) is an attractive additive manufacturing (AM) technique to fabricate high-strength Al 7075 alloy components for various applications. However, its poor weldability and easy occurrence of cracks during the highly non-equilibrium process have limited the adoption of this material for AM. In this research, various DED process conditions were employed to fabricate crack-free Al 7075 samples, and double aging combined with solution treatment was performed on the as-built materials thanks to the precipitation hardening nature of Al 7075. The influences of DED process parameters and heat treatment on microstructure and mechanical properties were investigated. It was found that the setting of laser power at 280 W and scanning speed at 500 mm/min led to crack-free material deposition with the highest density among all conditions investigated. Compared with the as-built Al 7075, the average micro-hardness of the alloy increased from 91.73 to 144.91 HV, and the ultimate tensile strength increased from 233.18 to 342.16 MP after heat treatment. Meanwhile, it was discovered that mass Cu elements are segregated at grain boundaries (GBs) for the as-built material. The existence of Al2Cu phase at GBs is believed to effectively mitigate the hot cracking, and the coherent strengthening mechanism of Cu segregation to the Al GBs is discussed. In addition, it was shown that Mg and Zn contents increased at the GBs after heat treatment, which tend to form precipitated phases and lead to the improved mechanical properties for DED produced Al 7075.