Effect of compression bonding temperature on the strength-ductility balance of AA7075 alloy

Abstract

In this study, the effect of compression bonding temperature on the microstructure and strength-ductility balance of AA7075 alloy was investigated. The results showed that dynamic recovery (DRV) was the dominant restoration mechanism in the compressed samples at 25, 100, 200, and 300 °C decreasing the dislocation density of elongated grains, while dynamic recrystallization (DRX) was predominant in the compressed sample at 400 °C forming fine equiaxed grains. By increasing the compression temperature, the bonding quality is improved. Several MgZn2 peaks could be observed in the diffraction pattern of the processed sample at 400 °C, confirming the occurrence of overaging. By increasing the compression temperature from 25 °C to 300 °C, the texture parameter of {200} (as recrystallization texture) was continuously reduced from 2.13 to 1.11, while the texture parameter of {220} (as deformation texture) was continuously enhanced from 2.09 to 4.91 due to the higher height reduction in the larger compression temperature. By further increasing the compression temperature to 400 °C, the texture parameter of {200} is significantly increased from 1.11 to 3.21, while the texture parameter of {220} is remarkably decreased from 4.91 to 0.83 owing to the occurrence of DRX. By increasing the temperature from 25 °C to 400 °C, the hardness of both AA7075 and pure aluminum layers was decreased due to the intensifying recovery and recrystallization. After the compression bonding, no evidence of the Portevin Le Chatelier was found and the stress-strain curves exhibited normal behavior. With increasing the temperature from 25 °C to 400 °C, yield and ultimate tensile strength were decreased from 672.7 MPa and 689.8 MPa to 187.4 MPa and 374.9 MPa, respectively, while the total elongation and energy absorption were increased from 6.2% to 40.6 J/cm3 to 33.7% and 108.1 J/cm3, respectively. At the early stage of plastic deformation, the strain hardening rate of the compressed sample at 400 °C was significantly lower than that of other compressed samples. However, when the ε reaches 0.09, the strain hardening rate of the sample became larger than that of other processed samples due to the recovery of the strain hardening rate (RSHR) phenomenon at stage III in the compressed sample at 400 °C. The fracture features of all samples were characterized by dimples and cleavage facets, indicating the occurrence of a mixed ductile/brittle mode of fracture. However, by increasing the compression temperature, the dimpled portion of the fracture surface was increased. The dimples were coarsened when the compression temperature increased owing to the softening of samples as well as coarsening of MgZn2 precipitates at higher temperatures.