RETROGRESSION AND RE-AGING OF ALUMINUM ALLOYS (AA 7075) CONTAINING NICKEL

In this study the effects of nickel additions in improving the mechanical properties and microstructure for high-strength aluminum alloys (AA7075) produced by semi-direct chill casting were investigated. Aluminum alloys were homogenized at different temperatures, aged at 120 degrees C for 24 h (T6), and retrogressed at 180 degrees C for 30 min and then re-aged at 120 degrees C for 24 h (RRA). The results of the microstructural analysis showed that adding nickel to aluminum alloy led to form nickel-rich dispersoid particles, such as Al7Cu4Ni, Al4Ni3, Al75Ni10Fe15, Al3Ni2, and Al50Mg48Ni7. These provided particles strengthening of dispersion and fine-grain that led to prevent the recrystallization besides restricted the grain growth. The mechanical properties of the alloys were improved by the strengthened dispersoid particles and precipitation of the matrix base alloy. The highest ultimate tensile strength and Vickers hardness of aluminum alloy containing nickel after the retrogression and re-aging treatment were about 400 MPa and 225 HV, respectively. Microstructure characterization of the alloys were carried out using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and X-ray diffraction (XRD).