AI-Zn-Mg-Cu-0.1wt%Sc 알루미늄 합금 압출재의 퇴화처리 및 재시효 처리
Retrogression and Reaging of Extruded Al-Zn-Mg-Gu+0.1wt%Sc Al Alloy
알루미늄합금 퇴화처리 재시효처리 금속재료공학 금속학;
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7000 series Al alloys based on Al-Zn-Mg-Cu element generally have excellent specific strength which makes them possible to apply in transportation vehicle and aircraft as a important structural material. Various 7000 series aluminum alloy with improved properties have been developed continuously until now. Addition of transition element is one of the effective methods to improve the properties of aluminum alloy. It was reported in Russia that mechanical property, corrosion resistance, fatigue strength, recrystallization temperature of aluminum alloy can be improved by Sc addition. On the other hand, improvement of stress corrosion cracking resistance of 7000 series alloy has been continuously investigated by many researchers to date and it has long been recognized by Cina et al. that RRA (Retrogression and Reaging) treatment can effectively improve the stress corrosion cracking resistance without sacrificing yield strength. And because Al_(3)Sc formed due to Sc additon has high temperature stability (melting temperature of Al_(3)Sc is 1320±7℃), it is reasonably considering that Al_(3)Sc may also have some effects in elongating the retrogression time of RRA treatment. In this study, a continuous cast Al-Zn-Mg-Cu+0.1Sc ingot was homoginezed at 380℃ for 1hour and the ingot was subject to hot extrusion process. The extrusion was carried out at 350℃, with extrusion ratio of 25:1, extrusion pressure of 190∼220MPa and extrusion speed of 0.7mm/sec. A round bar of Φ30mm was fabricated. The following conditions of a T6 treatment were chosen: supersaturation at 480℃ for 2 hours and ageing at 120℃, 140℃, 160℃. After this initial treatment, the material was subjected to the retrogression process at 180℃, 200℃, 220℃ for various time and finally, re-ageing according T6 temper to find the optimum regression time. Variation of mechanical property during RRA treatment was investigated and the stress corrosion cracking resistance was evaluated by electric resistivity measurement and Ring-test. The peak hardness of Al-Zn-Mg-Cu+0.1wt.%Sc after T6 treatment occurred when ageing at 120℃for 24 hours and this peak hardness was reoccurred after RRA treatment when the retrogression treatment was carried out at 200℃ for 20min. which was significantly elongated compared with that of Al7075 alloy. The UTS of Al-Zn-Mg-Cu+0.1wt.%Sc alloy after T6 treatment (ageing at 120℃ for 24 hours) was 690MPa while the UTS after RRA treatment (retregression 200℃ for 20min.) was 678 MPa. The Y.S after T6 treatment and RRA treament was 660MPa and 653MPa respectively. Considering there are generally about sacrificing of 10% to 15% Y.S in T73 treatment, it is clear that Sc addition was effective in elongating the retrogression time which is the most important factor in RRA treatment. The stress corrosion cracking test which was performed in atmosphere and in 3.5% NaCl solution showed that the degradation of strength after RRA treatment was only 16% while, degradation of stregnth after T6 treatment was 33%. RRA treatment greatly improved the stress corrosion cracking resistance of the Al-Zn-Mg-Cu+0.1wt.%Sc alloy.
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