Influence of Heat Treatments on Microstructure and Hardness of a High-Strength Al-Zn-Mg-Cu-Zr Alloy Processed by Laser Powder Bed Fusion

Processing 7075 alloy by laser powder bed fusion is complex due to hot cracking. Zr addition is one of the solutions to reduce this susceptibility to cracking and permit the elaboration of crack-free parts. This study explored the microstructural and hardness evolutions of an Al-Zn-Mg-Cu-Zr alloy with respect to different heat treatment routes. In particular, natural and artificial agings are considered from as-built and solution heat-treated tempers. Microstructural characterization was principally carried out by optical, scanning, and transmission electron microscopy. In the as-built state, the melt pools exhibited a bimodal grain structure consisting of very fine grains at their boundaries and coarser elongated grains in the center. The grain boundaries were composed of η-Mg(Al, Zn, Cu)2 eutectic. The associated hardness is quite significant (140 HV0.1). Natural and artificial aging from this state did not show any significant microstructural and hardness evolutions. In contrast, solution heat treatments modified the microstructure by dissolving eutectics and Al3Zr precipitation without significant grain growth. Natural and artificial aging produced interesting hardening: a maximum of 220 HV0.1 was achieved for the T6 temper. The resulting value completely exceeded the hardness of the AW-7075-T6 alloy. This excellent value was attributed to η′ and Al3Zr precipitation without significant grain growth.