Study on Quench Sensitivity and Premature Precipitation Behavior of the Cryogenic Deformed 6061 Aluminum Alloy

The temperature–time-property (TTP) curve of the cryogenic deformed 6061 aluminum alloy was fitted in this study, and the sensitive temperature range was determined through interrupted quenching experiments. The precipitation behavior during isothermal treatment of the cryogenic deformed samples was observed and analyzed by the differential scanning calorimetry (DSC), optical microscope (OM), and transmission electron microscope (TEM), and the premature precipitation mechanism during isothermal treatment was discussed in detail. The results revealed that the critical time for a 5% drop in hardness at the nose temperature (400 °C) was 25.2 s. Furthermore, the sensitive temperature range was determined to be from 360 to 420 °C. At 400 ℃, the supersaturated solution transformation rate reached its maximum, and coarse β equilibrium phases were rapidly precipitated in the grain. However, when the temperature was outside the sensitive range, the transformation rate decreased, and several β" and β′ phases were observed, indicates a lower degree of premature precipitation of the alloy. The high sensitivity at nose temperature can be attributed to the relatively high nucleation and growth rates of phases. Based on the experimental results, the cooling rate at the sensitive temperature range should be increased, while the cooling rate from the solution temperature to 420 °C should be properly decreased during the quenching process to obtain relatively high mechanical properties and low residual stresses.