Study On Cooling Ageing Process Of 7A85 Aluminum Alloy

The non-isothermal linear cooling treatments of 7A85 aluminum alloy have been systematicly investigated in this paper. Optimized ageing processes for die forgings and reasonable precess-window of non-isothermal ageing for 7A85 alloy are obtained via considering the performance match of hardness and electrical conductivity. Through characterization of microstructures under different ageing processes, the relationship between process parameters and microstructure are disclosed.The results show that: under condition of linear heating process the hardness increase with the increase of ageing time, afer reach the peak value 179HB at 185℃hardness begin to decrease. Utilizing the linear cooling treatments, the high hardness and conductivity are obtained simultaneously.There is a broad process-window in non-isothermal ageing treatment and the ageing time in high temperature during non-isothermal ageing process is the key factor which intensively affects the microstructure and mechanical properties of 7A85. The appropriate ageing times corresponding to different high temperature are generalized in the non-isothermal ageing time and temperature diagram.Combintional analysis using TEM, HRTEM and DSC show that the main precipitation sequence under non-isothermal cooling treatment is consistent with the classical precipitation process, that is as follows: Small G.P. zones→η' phase→ηphase. The main strengthen precipitate of the 7A85 alloy is plate-likeη' phase, precipiting along the {111}Al.The tensile test results show that 7A85 aluminum alloy exhibits a strength of 554 MPa, yield strength of 531 MPa, elongation of 13.5% and conductivity of 39 %IACS which are close to typical ageing treatment after 190℃direct cooling ageing process, while just takeing only 4.5h. This result shows direc cooling ageing has a technological interest for industrial production.