Effect of Deep Cryogenic Treatment on Precipitation Hardening of Aluminum 2024 and 7075

Sub-zero treatment is an efficient method for increasing the efficiency of metal alloys. In addition, aging is a common method for increasing the ratio of the strength to weight of aluminum alloys 2024 and 7075 for using in a passenger aircraft. In this research, the effect of sub-zero treatment at temperature of -196° C (deep cryogenic treatment) and time of 4 hours on the hardness of aluminum alloys 2024 and 7075 are studied. For this purpose, scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) have been applied to study the microstructure. Also, tensile and hardness tests have been applied to study mechanical properties. The results show that the formation of precipitation around the particles (especially for iron particles) is facilitated by performing sub-zero treatment. Because the difference in the coefficient of iron contraction with the aluminum matrix at sub-zero treatment causes elements (such as copper in 2024 and magnesium in alloy 7075) are easier to absorb the areas around iron particles at aging temperature of 100 ° C. The new precipitates increased the yield strength of aluminum 2024 and 7075 compared to the control specimen, 32 and 20 MPa, respectively, and the tensile strength of aluminum 2024 and 7075 increased 26 and 21 MPa, respectively. While in both specimens, the change of hardness was not noticeable.