Process–microstructure–corrosion relationship in TiO 2 -reinforced magnesium composite coatings processed by fast multiple rotation rolling

This study investigated the effect of fast multiple rotation rolling (FMRR) rotational speed on the microstructure, mechanical properties, and in vitro corrosion behaviour of TiO₂-reinforced AZ91 magnesium composite coatings produced by friction surfacing. The coatings were subjected to FMRR at 1000, 1300, and 1600 rpm under identical processing conditions. Increasing the rotational speed increased the peak interface temperature from 338 to 411°C, significantly influencing microstructural evolution. The optimum condition was obtained at 1300 rpm, where the average grain size decreased from 4.5 ± 0.3–1.6 ± 0.2 μm, TiO₂ particle size from 4.8 ± 0.6–2.5 ± 0.2 μm, and β-Mg₁₇Al₁₂ precipitate size from 0.74 ± 0.04–0.21 ± 0.02 μm. Consequently, the nano-hardness increased from 7.86 ± 0.61–9.73 ± 0.48 GPa. Electrochemical testing in modified simulated body fluid revealed that processing at 1300 rpm provided the highest corrosion resistance, reducing the corrosion current density from 58.36 ± 1.21–31.45 ± 1.05 μA cm−2 and increasing the charge-transfer resistance from 392.6 ± 6.4–745.8 ± 7.2 Ω·cm2. These improvements were attributed to the synergistic effects of grain refinement, homogeneous TiO₂ dispersion, and refined β-Mg₁₇Al₁₂ precipitates.