자동차용 7075 알루미늄 합금의 소부경화성 향상

Abstract

Aluminum alloys have attracted much attention for the automotive industry due to their significant weight reduction for last decades. Up to this time, A5000 and A6000 series aluminum alloys have been usually used in these fields. Recently, Al-Zn-Mg-Cu aluminum alloys(A7000 series aluminum alloys) have been investigated for additional weight reduction of auto-mobile structure. It is well known that the Al-Zn-Mg-Cu alloys could usually obtain high mechanical properties by age-hardening. The alloy sheets for auto-mobile structure are transported and press-formed then undergo paint bake treatment. This paint curing process show a similar effect as artificial aging from which the strength can be increased. In this study paint-bake response of twin roll cast 7075 aluminum alloy sheet has been investigated. By the conventional paint-bake treatment(commonly aged at 180 ℃ for 30 min) the yield strength of the sheet was not changed, which was caused by the compromise of the formation of new precipitates and coarsening of η′-phase. To improve the paint-bake hardenability, pre-aging treatment was additionally applied after the solutionization where followed by paint-bake treatment. The samples pre-aged at 120 ℃ for 5 ~ 60 min showed the remarkable increase of hardness. In terms of the values of formability and yield strength, the optimum pre-aging condition should be decided the temperature of 120 ℃ for 30 min after that the hardness significantly rose by +124 ㎫ and finally the yield strength was 432 ㎫. The effect of pre-aging treatment on the precipitation sequence has been analyzed by using the transmission electron microscopy(TEM) observation.