Tensile Properties and Intergranular Corrosion Sensitivity of Al-Mg-Si-Cu Alloy with Different Zn Coatents

Due to the combination of low density，medium strength，high corrosion resistance，good weldability and plastic deform-ability，the Al-Mg-Si-Cu alloys had found wide applications in the field of automotive，rail transportation and aviation. However，the addition of Cu element could increase the strength of the alloys in peak-aging state but at the cost of corrosion resistance. In order to improve the strength and corrosion resistance simultaneously，the effects of Zn content on the tensile properties and intergranular corrosion（IGC）susceptibility of Al-1.0Mg-0.95Si-0.9Cu-xZn-0.6Mn-0.12Zr alloys（x=0，0.4，0.8，1.2）in the natural aging（T4）state for 14 d and peak aging（T6）state at 180 ℃ for 8 h，were investigated by Vickers hardness test，tensile test，immersion corrosion test and differential scanning calorimetry（DSC），scanning electron microscopy（SEM），transmission electron microscopy（TEM）. The results showed that the addition of low Zn contents（0~0.8%）had no significant effect on the characteristic of hardness variation and tensile properties（strength and elongation）of T4 and T6 alloys. The yield strength，ultimate tensile strength and elongation of T4 alloys were in the range of 156~163 MPa，304~308 MPa and 27.5%~28.1%，respectively，and T6 alloys had the yield strength，ultimate tensile strength and elongation in the range of 336~340 MPa，395~404 MPa and 14.4%~14.8%，respectively. Furthermore，T4 alloys almost had no IGC susceptibility，whereas serious IGC occurred in T6 alloys，which decreased with the increase of Zn content. When Zn content of the alloy was 0.8%，the maximum corrosion depth was 320 μm，which was 220 μm less than that of Zn-free alloy. When Zn content of the alloy increased to 1.2%，the precipitation of Guinier Prestion zone（GP zone）and β" phase was greatly promoted within the grains and caused significant improvement of age-hardening rate and the alloy strength. The yield strength and ultimate tensile strength of T4 alloy were 196 MPa and 363 MPa，respectively，which were increased by 30 MPa and 44 MPa compared with Zn-free alloy. And the T6 alloy had the yield strength and ultimate tensile strength of 368 MPa and 443 MPa，respectively，which were 44 MPa and 57 MPa higher than those of Zn-free alloy，respectively. At the same time，T4 and T6 alloys could still achieve high elongations of 25.3% and 12.6%. The increased IGC susceptibility of T4 and T6 alloys should be related to the compositional change of the grain boundary precipitates and precipitate free zones caused by the addition of high Zn content，which resulted in a larger gap between their corrosion potential. In addition，the tensile fractographies of the four alloys were not affected by the content of Zn，but dependent mainly on the aging state. The dimple induced transgranular fracture was observed for T4 alloys，whereas T6 alloys were characterized by the combination of intergranular fracture and dimple induced transgranular fracture. Furthermore，the microcracks were initiated only around those second phase particles such as coarse AlFeMnSi constituents and sub-micron dispersoids，which led to the dimple induced transgranular fracture for T4 alloys. By comparison，the low strength of the precipitate free zones at grain boundaries in T6 alloys was also responsible for the initiation of microcracks and led to the intergranular fracture. In general，the appropriate amount of Zn alloying in Al-Mg-Si-Cu alloy could accelerate the aging hardening rate of the alloy and significantly improve the strength of the alloys in T4 and T6 state by promoting the precipitation of aging strengthening phase，while maintaining high level of elongation and corrosion resistance，but only when Zn content was relatively high，such as 1.2%. Or IGC resistance of the alloy could be greatly improved by bridging part of the high corrosion potential caused by Cu element without sacrificing the aging hardening rate and tensile properties，such as 0.8% Zn content. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.