Ultrafine grained Al 7075 alloy fabricated by cryogenic temperature large strain extrusion machining combined with aging treatment

Abstract

In this study, cryogenic temperature large strain extrusion machining (CT-LSEM) as a severe plastic deformation (SPD) method is applied to fabricate chips with ultra-fine grained (UFG) structure. The chip morphology, microstructure evolution, aging behavior, as well as micro-hardness characteristics of a solution treated (ST) Al 7075 alloy subjected to CT-LSEM and room temperature (RT) LSEM are investigated. It is observed that the processing temperature has a great influence on the chip morphology, i.e., the chips produced by CT-LSEM have better surface quality than that by RT-LSEM. The SPD process of LSEM, especially under CT, can accelerate the aging kinetics of the alloy. Much more deformation energy stored in the CT-LSEM sample could promote the dynamic recrystallization. A higher dislocation density exists in the CT-LSEM samples due to effective suppression of dynamic recovery. The optimized hardness for all samples is obtained at an aging temperature of 120 °C. The hardness of CT and RT LSEM samples after peak aging (PA) have increased from 102 Hv to 201 Hv and 192 Hv respectively. A large number of fine and closely spaced ηʹ and η precipitates are present in the LSEM samples after PA treatment, accounting for the further enhancement of hardness. The ST Al 7075 samples subjected to LSEM are severely deformed. The mean grain/sub-grain size of PA samples has hardly changed as compared with the initial LSEM samples. It is feasible to realize simultaneous hardening including grain refinement, dislocation and precipitation in the Al 7075 alloy via application of CT-LSEM and subsequent aging.