| Summary: | 碩士 === 國立中央大學 === 機械工程研究所 === 87 === High-strength 7000 series aluminum alloys have excellent weight to strength ratio, but suffer from severe stress corrosion cracking(SCC) as material is peak aging(T6) treated. The SCC susceptibility of material can be improved by using overaging(T73) heat treatment but decrease 10∼15% strength at the same time. The purpose of this study is to investigate the effect of Zn/Mg ratio and Fe+Si content on mechanical properties and SCC resistance of 7050 aluminum alloy. By means of differential scanning caloarimeter (DSC), transmission electron microscope (TEM), scanning electron microscope (SEM), optical microscope (OM), tensile test and slow strain rate testing (SSRT), the variations of microstructure, mechanical properties and SCC resistance due to the change of Zn/Mg ratio and Fe, Si content can be clarified. Each alloy was given four different heat treatments : T6, T73, RRA and Step-quench. It is our final goal to obtain a suitable combination of heat treatment and alloy composition in order to produce the 7050 aluminum alloy with both high strength and SCC resistance.
The results show that increasing Zn/Mg ratio, the strength of T6, RRA, Step-Quench tempered alloys have only minor change, but the strength of T73 tempered alloy descends remarkably when Zn/Mg ratio is higher than 2.80. The SCC resistance is better for high Zn/Mg ratio and step-quench tempered alloys. To increase content of Fe, Si impurity elements reduce the strength, elongation and SCC resistance of all four different tempered alloys.
As the results above, the high strength and SCC resistance of 7050 aluminum alloy can be obtained using step-quench heat treatment. Combining with alloy composition of high Zn/Mg ratio and alloy purity can improve SCC resistance more.
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