Puyou Ying

Multistage-aging process effect on formation of GP zones and mechanical properties in Al–Zn–Mg–Cu alloy

Abstract The tensile properties and fatigue behavior of an Al–Zn–Mg–Cu alloy were investigated by performing tensile tests and fatigue crack propagation (FCP) tests. The tensile results show that lower aging temperature modified retrogression and re-aging (RRA) process enhances the elongation, but reduces the strength of the alloy, as compared to conventional RRA process which employs peak aging temperature. Both ductility and strength, however, are increased by employing a natural aging prior to re-aging based on the former modified RRA process. Fatigue test results show that both routes reduce FCP rate. Especially, the lower re-aging temperature modified RRA process obtains the lowest FCP rate. Natural aging treatment could enhance the nucleation rate of GP zones. A large amount of GP zones could be cut by dislocations, which is responsible for the highest tensile strength and elongation, as well as lower FCP rate.

Improved Stress Corrosion Cracking Resistance and Strength of a Two-Step Aged Al-Zn-Mg-Cu Alloy Using Taguchi Method

Multi-step heat treatment effectively enhances the stress corrosion cracking (SCC) resistance but usually degrades the mechanical properties of Al-Zn-Mg-Cu alloys. With the aim to enhance SCC resistance as well as strength of Al-Zn-Mg-Cu alloys, we have optimized the process parameters during two-step aging of Al-6.1Zn-2.8Mg-1.9Cu alloy by Taguchi’s L9 orthogonal array. In this work, analysis of variance (ANOVA) was performed to find out the significant heat treatment parameters. The slow strain rate testing combined with scanning electron microscope and transmission electron microscope was employed to study the SCC behaviors of Al-Zn-Mg-Cu alloy. Results showed that the contour map produced by ANOVA offered a reliable reference for selection of optimum heat treatment parameters. By using this method, a desired combination of mechanical performances and SCC resistance was obtained.

Texture Effect on Fatigue Crack Propagation Behavior in Annealed Sheets of an Al-Cu-Mg Alloy

The effects of texture, including Brass, Goss + Cube and Cube, on the fatigue crack growth behavior in annealed Al-Cu–Mg alloy sheets were characterized in this investigation. Results showed that the Goss + Cube sheet possessed the greatest fatigue threshold value and the lowest fatigue crack propagation (FCP) rate, and the Brass sheet presented the smallest threshold value and the greatest FCP rate among the three textured sheets. SEM results showed more slip bands and more tortuous crack path formed in Goss + Cube sheet during cyclic loading, and EBSD results showed that numerous dramatic crack deflections occurred when fatigue crack propagated across Goss-grain boundary in Goss + Cube sheet, indicating that the grain orientations including Goss and Cube had positive effect on enhancing the FCP resistance. On the one hand, Goss and Cube grains have more high Schmid factor slip systems than Brass grain, promoting persistent slip bands formation and externally applied energy consumption, consequently giving rise to a high resistance to FCP. On the other hand, Goss grains could induce crack deflection with large twist angles, resulting in more energy consumption during the cracking process and thereby enhance the FCP resistance.