Microstructure and fretting wear behaviour of gradient-structured Cu sheets prepared by plastic flow machining

Plastic flow machining (PFM) is a novel severe plastic deformation (SPD) approach that produces gradient-structured surface sheets by a one-step forming method. This paper investigates the influence of constraint tool angles on the microstructure and mechanical properties of gradient-microstructured Cu sheets. The tribological behaviour and wear mechanisms of the sheets were studied in the fretting tests and compared with those of coarse-grained microstructural copper. The results indicated that significant grain sizes and hardness gradients were still formed along the sheet thickness direction by changing the constraint tool angle. As the constraint tool angle was increased, the thickness of the ultrafine grain layer (UGL), maximum microhardness, and residual compressive stress decreased. The fretting wear resistance of the sheet was enhanced, and the volume loss was significantly lower than that of the coarse-grained (CG) copper. The gradient-microstructured surface sheets prepared at the constraint tool angle of 90° exhibited lower coefficients of friction (CoF) and superior anti-wear performance, which were attributed to the gradient-structured surface that reduced adhesion, abrasion, and delamination wear, as well as to the prevention of crack initiation and development.