Study On Corrosion Resistance Of Nonisothermal Aged7050Aluminum Alloy

A new type of heat treatment aging process called non-isothermal aging isresently applied on the age-hardening7xxx aluminum alloys. But the relationshipbetween the corrosion behavior and the microstructure of the non-isothermalaged7xxx aluminum has not been studied by now.This thesis has studied the age-hardening behavior and the microstructure characteristics of the non-isothermalaged7050aluminum. The exfoliation corrosion behavior and the stress corrosioncrack behavior have also been studied. We have tried to build the correspondingrelations between the corrosion resistance and the microstructure of non-isothermal aged7050aluminum.The characteristics of the microstructure （MPt、GBP and PFZ） of7050aluminum alloy during the specific non-isothermal aging process（（40,190）*V20+（190,100）/V20） have been analysed by TEM. At first, the grainboundary was a continuous chain with few small-sized and scattered GBP aroundthe grain boundary. Numerous and fine MPts were dispersedly distributed. As theageing processing, the GBP became discontinuous.The alloys tended to be Peakaged with larger and added metastable phase η’. GBP became larger-sized andPFZ was obvious with the width of50nm. At the end of the ageing process,numerous fine metastable phase η’ emerged around the previous thick phase η’, itwas secondary age-hardening. The degree of discontinuity of GBP was obviouslydecreased, it turned to be semi-continuous status. The hardness of the7050aluminum alloys accordingly increased slightly.Three different methods were used to study the exfoliation corrosionbehavior of non-isothermal aged7050aluminum alloy. EXCO test is visualevaluation. Electric resistance method and metallographic method were alsoapplied to measure the degree of exfoliation corrosion quantificationally. Theprocess was: pitting→wide blisters→cracks→continuous sheets lift. With theaging proceeded, the exfoliation resistance was improved between the T6peak-aging and T74aging.The stress corrosion crack behavior of non-isothermal aged7050aluminum alloy was studied by DCB spicemens through consistant displacement method.The alloy’s resistance to stress corrosion crack was evaluated by K_ISCCand（da/dt）Ⅱ. The SCC was observed mainly to be intergranular cracking through themicro-morphology, accompanied by a small number of transgranular cracking.This thesis focuses on the alloy composition, the distribution of precipitates, themicrostructure characteristics and the corrosion resistance of7050alloy. Thecontinuous Chain shape structure with small precipitates discretely distributedaround the grain boundaries is not beneficial to the corrosion resistance. Asnumerous fine metastable phase η’ separated out around the previous thick phaseη, GBP was semi-continuous, then the7050alloy got the best corrosion-resistantperformance.