Strengthening Mechanism Of Heavily Cold Deformed7050Aluminium Alloy During Ageing Treatment

Thermal-mechanical treatment applied on7050aluminum has been studied in thepast few decades, an improved overall property was achieved. However, thestrengthening mechanism during the process was still unclear. Within this study, wefocused on strengthening mechanism of7050aluminum during a simplified thermal-mechanical treatment, which was solution treatment followed by80%deformation coldrolling and subsequent ageing process. The strengthening effect of both dislocation andprecipitation was expected to be kept simultaneously, therefore a lower ageingtemperature was chosen. Mechanical properties and microstructure evolution during theprocess were studied with the test methods of room temperature tensile test, OM, TEM,DSC, and XRD.It was proved that the strength of7050aluminum was dramaticlly improved aftercold rolling with the deformation of80%on account of the work hardening effect andearly stage precipitation. TEM results showed small precipitation particles formationduring cold rolling process. Yield strength exhibited a rapid drop with the first hour ofageing, even under the ageing temperature of60°C, which is the half of T6peak ageingtemperature. After6hours of ageing, the strength was increased gradually. The yieldstrength of7050alloy after30h ageing treatment under100°C was successfully increasedto approximatelly660MPa. Temporary strength increase during2h to4h ageing under80°C proved the interaction of dislocation and clusters.Microstructure analysis showed that the main precipitation occurred during lowtemperature ageing was G.P. zones and small clusters with the size of1nm, which is thecause of rapid strengthening behavior happened during the low temperature ageing. Smallη’ precipitates was observed in the later stage of ageing, and these particles should be inthe transformation from cluster to η’ precipitates. Recovery of dislocation occurred withinthe whole ageing process, especilly the first6hours. Recovery rate of dislocationdecreased in the later stage of ageing. It was also proved that modified Williamson-Hallplot was a effective way to measure the dislocation density of metals.Contrast tests of5A06alloy showed the work hardening effect provided a220MPaincrease of thermal-mechanical treated7050alloy yield strength compared with thesolution treated alloy. Small clusters formed during the cold rolling made a contribute of100MPa to the yield strength. Strength decrease caused by dislocation recovery after30hours of low temperature ageing was appromixately40~50MPa. Ageing strengtheningof7050alloy compensated the strength loss and elevated the strength slightly.