Effect of Creep Aging on Mechanical Properties of Under-Aged 7075 Aluminum Alloy

The 7075 alloy is widely used in the manufacture of aerospace components, such as aircraft wings and fuselage plates, owing to its high strength and light weight. Moreover, it is well-suited for manufacturing these massive aviation components using creep aging forming (CAF) technology. In this present study, the effect of creep aging on the mechanical properties of under-aged 7075 alloy was systematically studied in detail by means of a uniaxial creep tensile test and a stress-free artificial aging test. EBSD, SEM, and TEM observations were used to characterize the evolution of dislocations and precipitates with creep aging time. A quantitative analysis was performed on the relationship between mechanical properties and microstructure evolution. The results show that creep aging greatly improves the plasticity of the under-aged 7075 aluminum alloy while maintaining its high strength. The mechanical properties of the alloy are sensitive to creep stress. The sample aged for 6 h under 260 MPa and 426 K has the maximum yield strength, reaching 537.9 MPa. In comparison to the artificial aging sample, the dimple distribution of the creep aging sample is denser and the grain is more inclined toward a high Schmid factor orientation, which is 15% higher than the artificial aging sample. TEM results show that the primary phase in the crystal is.' phase. The size of the precipitated phase in the crystal grows with increasing creep aging time from 3.04 nm for 2 h to 4.27 nm for 6 h and the volume fraction increases from 0.22% to 0.46%. The size of the grain boundary precipitates increases and the transition from continuous to discontinuous occurs. The EBSD results show that no significant change in the recrystallization and subgrain ratio occurred in any of the samples, and the average grain size remains approximately 80 mu m. The distribution of geometrically necessary dislocations (GND) decreases first and subsequently increases with the extension of creep aging time. The contribution of grain boundary strengthening to the yield strength contribution model is shown to be essentially constant at about 17 MPa, and the coupling effect of dislocation and precipitation strengthening is the primary reason for the increase in strength.