Effect of Microstructural Characteristics on the Fretting Wear Behavior of 2A70 Aluminum Alloy with Different Grain Size Grades

Fretting wear behavior of 2A70 forged aluminum alloy with different grain size grades was investigated. Specimens (Grades 3.5 and 5) were tested against 42CrMoA steel at 115 °C under 40 and 240 μm amplitudes and 90–150 N loads. EBSD characterized the microstructural features, while SEM and laser confocal microscopy analyzed the wear surfaces. Grade 5 exhibited fine equiaxed grains, higher high-angle grain boundary (HAGB) density, lower second-phase content, and weaker texture. Grade 3.5 showed mixed grains, lower boundary density, and a higher unindexed fraction possibly originating from unindexed Al9FeNi particles as well as local micro-defects. At 40 μm, the microstructural effect was minor. At 240 μm, Grade 5 showed stable friction, lower wear depth, finer debris, and a continuous compacted layer. Conversely, Grade 3.5 showed fluctuating friction, higher and non-monotonic wear depth, coarse debris, and discontinuous compacted layers with microcracks. The anomalous maximum wear depth of Grade 3.5 at 90 N may be attributed to severe third-body abrasion before a protective layer forms. The superior wear resistance of Grade 5 is the result of the combined effect of multiple microstructural parameters, primarily driven by dense HAGBs, microstructural uniformity, and weak texture.