Effect of Heat Treatment on Microstructure and mechanical hardness of aluminum alloy AA7075

The mechanical properties of high strength aluminum alloy AA7075 were greatly affected by heat treatment. This paper was intended to clarify the effect of heat treatment on the microstructure and the mechanical hardness of commercial alloy AA7075-T651. Heat treatment was performed at a variation temperature of 300, 400, 500, and 600°C for 1 h in a vacuum furnace followed by a water quench. The surface observation was done by using optical and electron microscopy and the mechanical hardness was measured by the Vickers Hardness test. The XRD result on as-received alloy revealed the precipitates in the alloy was MgZn₂, Al₇Cu₂Fe, and Al₂CuMg. The average hardness of the as-received alloy was 136 HV. The hardness became significantly lower (78.5 HV) after heat treatment at 400°C while heat treatment at others temperature gave hardness in the range 116-122 HV. Heat treatment induced grain growth and dissolution of metastable precipitates. Reduction in the number of MgZn₂ and Fe-rich phase decreased the hardness significantly. Heat treatment above 400°C promoted segregation of fresh precipitates which enhanced the metal hardness again. The metallic grain size increased with heat treatment temperature. At 600°C, most of the precipitates were segregated along the equiaxed grain boundaries.