Influence Of Flame Correction On Microstructure And Properties Of Al-Zn-Mg Alloy

Al-Zn-Mg alloys are widely used because of their high specific strength, hardness, good weldability and corrosion resistance. However, it is difficult to control residual stress and deformation for aluminum alloys because of their higher linear thermal expansion coefficient and lower yield strength. Flame correction is usually used to correct deformation because of its simple operation and low cost. But the heat input must be strictly controlled during flame correction. However, the performance of the alloy should not degrade greatly, and simultaneously the deformation should be corrected completely. In actual production, the corrosion property of Al-Zn-Mg alloy weldment usually decreases after flame correction, which affects the overall performance of the component. So, it is necessary to study the process of flame correction of Al-Zn-Mg alloy to analyze the influence of flame correction on mechanical properties and corrosion properties of Al-Zn-Mg alloy.This paper aims to study the microstructure, tensile properties, intergranular corrosion resistance and exfoliation corrosion resistance of Al-4.5Zn-1.5Mg(wt%) alloy after flame correction in different temperatures and for different times and provide the reference for the actual production.The experimental results reveal that the strength and plasticity of Al-4.5Zn-1.5Mg(wt%) alloy appeared to improve in different degrees after flame correction in 300℃. After one time of flame correction in 300℃, the tensile strength of the alloy raised both inside and outside the region of flame correction which was related to the significant increase of precipitation phases. After two times of flame correction, the plasticity of the alloy enhanced inside the region of flame correction, and the tensile strength of the alloy increased outside the region of flame correction. After three times of flame correction, the plasticity of the alloy improved both inside and outside the region of flame correction and incomplete recrystallization organization appeared outside the region of flame correction. Corrosion tests show that the corrosion performance of the alloy declined after flame correction in 300℃ The degree of the decrease aggravated with the increase of times of flame correction. After flame correction in one time, two times and three times, the maximum intergranular corrosion depth inside the region of flame correction was 0.076mm,0.086mm and 0.089mm, respectively, and the level of exfoliation corrosion was EC, ED and ED, respectively. The samples without flame correction and outside the region of flame correction in one time, two times and three times did not appear intergranular corrosion. And the level of exfoliation corrosion before flame correction and after flame correction in 300℃ for one time, two times and three times was EA, EB, EB and ED, respectively.After flame correction in 350℃, the tensile strength and elongation of Al-4.5Zn-1.5Mg(wt%) alloy inside the region of flame correction degraded sharply after one time of flame correction, which was related to the decrease of precipitation phases and the grain growth. Then, the tensile strength and elongation of the alloy inside the region of flame correction improved after two and three times of flame correction in 350℃ because of the increased precipitation phases. Outside the region of flame correction, the tensile strength of the alloy increased and the elongation of the alloy decreased after one time, two times and three times of flame correction compared with the specimen before flame correction due to partial recrystallization of the alloy. Corrosion tests indicate that the corrosion performance of the alloy reduced after flame correction in 350℃. Among those, the corrosion performance deteriorated significantly after one time of flame correction in 350℃. After flame correction in one time, two times and three times, the maximum intergranular corrosion depth inside the region of flame correction was 0.105mm,0.080mm and 0.076mm, respectively, and the level of exfoliation corrosion was all ED. The samples outside the region of flame correction did not appear intergranular corrosion, and the level of exfoliation corrosion was EC, EB and EB, respectively, after one time, two times and three times of flame correction in 350℃. TEM results suggest that the sharp decline in corrosion performance after one time of flame correction in 350℃ was related to the semi-continuous distribution of the precipitation phases in grain boundary and the formation of precipitate free zone.Based on the above experiments, two times of flame correction in 300℃ is more suitable for Al-4.5Zn-1.5Mg(wt%) alloy, which has higher tensile strength and elongation and slighter declined corrosion performance compared with other parameters.