7055铝合金的非等温双级时效行为

The Non-Isothermal Double Ageing Behaviour of 7055 Aluminum Alloy

采用硬度测试、电导率测试、室温拉伸实验、剥落腐蚀实验、DSC分析以及TEM观察,研究了非等温双级时效对7055铝合金组织及性能的影响。结果表明:在第二级时效的连续升温阶段,晶内由GP区、η′和α-Al三相共存状态逐渐演变为η′、η和α-Al三相共存态。在第二级时效的连续降温阶段,晶内二次析出GP区和η′相,合金硬度再次升高。晶界η相在时效过程中不断粗化并呈断续分布,合金电导率持续增加。非等温双级时效的第二级升温速率和最高时效温度(Tp)决定了合金的性能。在相同性能水平下,快速升温对应相对更高的Tp。以电导率22 MS/m为标准,1℃/min升温速率对应的Tp为215℃,而3℃/min升温速率下Tp则为225℃。经过105℃、24 h预时效并包含升降温的非等温二级时效,7055铝合金的抗拉强度和剥落腐蚀等级可达610MPa和EB级别,表现出比T6和T73态更加优异的综合性能,且取消等温保温阶段实现了热处理工艺的短流程操作。

Thick plates of 7055 aluminum alloy are widely used as structural components,especially in the aerospace industry,due to their high strength,low density,excellent hot workability,and high stresscorrosion resistance,which are dependent on the type of thermal treatment the alloy is subjected to.Because of the heating and cooling stages in such components,non-isothermal ageing has attracted a lot of research interests.Replacing isothermal ageing with non-isothermal ageing is needed for higher efficiency and practicability.Herein,a novel isothermal-ageing technique based on double ageing is developed.Hardness test,electrical conductivity test,room-temperature tensile test,exfoliation corrosion test,DSC,and TEM analyses were employed to study the influence of non-isothermal double ageing on microstructure and properties of the 7055 aluminum alloy.The results showed that in the heating stage of the second ageing treatment,inner grains of the microstructure evolved from a three-phase coexistence state containing the GP zone,η′phase,andα-Al to that containingη′phase,ηphase,andα-Al.On the other hand,in the continuous cooling stage of the second ageing,GP zone andη′phase re-precipitated,resulting in improved hardness.Theηphase on the grain boundary became coarse and discontinuously distributed,which resulted in a progressive improvement of the electrical conductivity.The heating rate and highest ageing temperature(T_p)of the second ageing stage determined the final properties.With a standard electrical conductivity of 22 MS/m,1℃/min heating rate corresponds to the T_p of 215℃,while Tpof225℃is needed when heating by 3℃/min.After pre-aged by 105℃,24 h and non-isothermal ageing including heating and cooling stages,the strength and exfoliation corrosion resistance of approximately610 MPa and EB level were achieved,respectively.The alloy showed a better comprehensive performance than the T6 and T73 state ones.Additionally,the non-isothermal ageing removing the heat preservation stage realized the short process preparation.