Microstructure, aging behavior, and friction and wear properties of ultrafine-grained 7050 aluminum alloy produced by cryogenic temperature extrusion machining

Cryogenic temperature extrusion machining (CT-EM), as a novel cryogenic temperature severe plastic deformation (CT-SPD) process, is an effective method to produce ultrafine-grained (UFG) materials. In this paper, the microstructure, microhardness, friction and wear properties of the UFG chips prepared by CT-EM and room temperature extrusion machining (RT-EM) were analyzed. The results show that CT-EM has a better grain refinement effect. The grain size of the CT-EM samples is smaller, the dislocation density is larger, and the microhardness is higher. After aging treatment, a large number of small secondary precipitates (eta '/eta) appear in the microstructure of the samples, which further improves the microhardness. This is the result of a synergistic effect of fine grain, dislocation, and precipitation strengthening. Cryogenic treatment can increase the precipitation kinetics of the secondary phase and shorten the time for the samples to reach peak hardness. The CT-EM samples have a lower friction coefficient and wear mass. With the increase of the machining velocity, the wear mechanism of the samples changes from abrasion wear to oxidation, adhesion, and fatigue wear.