Lianghua Lin

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.

Effects of Severe Cold Rolling on Exfoliation Corrosion Behavior of Al-Zn-Mg-Cu-Cr Alloy

The exfoliation corrosion (EXCO) behavior of Al-Zn-Mg-Cu-Cr alloy after severe cold rolling was investigated by optical microscope, transmission electron microscope, and electrochemical technique. The results show that the EXCO resistance decreased with increasing cold rolling reduction because of the grain boundaries which were decorated with the continuously distributed particles. After the solution treatment, the samples with different reductions retained the fibrous grains, and the elongated grains accelerated the growth of the corrosion cracks according to crack propagation analysis. Furthermore, the increase of deformation enhanced the degree of recrystallization, while the number of corrosion cells increased greatly when in the electrolyte, which tended to reduce the resistance to EXCO.

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.

Effects of Pre-Strain on Exfoliation Corrosion Behavior in Al-Cu-Mg Alloy

Mechanical properties and exfoliation corrosion behavior in Al-Cu-Mg alloy with various pre-strain percents were characterized in this study by means of hardness measurement, optical microscope, transmission electron microscope (TEM), x-ray diffraction (XRD), and electrochemical technique. The hardness of naturally aged alloy was significantly enhanced with increasing the pre-strain percents. The immersion tests and polarization measurements revealed that the pre-straining processing before natural aging reduced the resistance of the alloy to exfoliation corrosion (EXCO), which was mainly attributed to the increase of dislocations density and grain aspect ratio. The decrease of residual tensile stress may only play a minor role in the EXCO resistance.

Effect of Overaging on Fatigue Crack Propagation and Stress Corrosion Cracking Behaviors of an Al-Zn-Mg-Cu Alloy Thick Plate

The fatigue property as well as stress corrosion cracking (SCC) resistance of an Al-Zn-Mg-Cu alloy thick plate in peak-aged and overaged tempers (T7351 and T7651) is systematically investigated by fatigue crack propagation (FCP) test and slow strain rate test (SSRT). Microstructural characterization is examined by transmission electron microscopy and scanning electron microscopy. Results reveal that the T7351 alloy has lower strength but higher electrical conductivity as compared to T7651 alloy. The FCP resistance of T7351 alloy is superior to that of the T7651 alloy due to the coarser precipitates in the highly overaged alloy in which the strain localization is reduced by promoting homogeneous slip. In addition, the SSRT test suggests a higher SCC resistance in T7351 alloy. The enhanced SCC resistance is found to depend on grain boundary precipitate characteristics and crack propagation resistance of the alloys.