Microstructure and Wear Properties of Al-Fe Surface Composites: Impact of Four Friction Stir Processing Passes

The number of passes during friction stir processing (FSP) can influence the microstructure and wear properties of thin metallic films. In this study, thin Aluminum-Iron (Al-Fe) films are deposited on a low carbon steel substrate via a plasma spray process with an initial coating thickness of approximately 100 μm. The effect of FSP pass numbers on the microstructure evolution at the stir zones (SZs) and heat-affected zones (HAZs) is investigated using optical microscopy and electron backscatter diffraction (EBSD) techniques. The study also investigates surface wear behavior by conducting a reciprocating wear test (1000 m sliding distance under 15 N load) and utilizing scanning electron microscopy (SEM) to examine the topography of worn surfaces, revealing that the wear track depth decreased from approximately 160 μm in the As-sprayed coating to 152, 120, 18, and 20 μm after one to four FSP passes, respectively. The films were analyzed using X-ray diffraction (XRD), which revealed the formation of Fe-Al intermetallic compounds in all of the surface composites. Applying the second pass of FSP led to the formation of the Widmanstätten ferrite plates from recrystallized austenite grains, both at the stir zones (SZs) and the heat-affected zones (HAZs). The grains underwent thickening and elongation as a result of an increased diffusivity of carbon and higher heat inputs applied to the surface, which occurred with an increase in pass numbers. Applying additional FSP passes led to a more anisotropic microstructure, which was attributed to the accumulation of a higher number of dislocations. Conducting multiple FSP passes (three or four) has improved the wear resistance and reduced the running-in distance in friction coefficient measurements, leading to a reduction in total wear rate from 1.6 × × 10 −2 μm m −1 in the As-sprayed condition to 0.15 and 0.17 × × 10 −2 μm m −1 after three and four passes, respectively. Correspondingly, the Vickers microhardness of the stir zone increased from 198 HV in the As-sprayed coating to 405 HV after four passes, while friction coefficients remained in the range of 0.33‐0.42. An analysis of the wear track revealed a shift from adhesive wear to abrasive wear as the number of FSP passes increased.