Advancing electric vehicle drivetrain reliability through magnetorheological finishing of 42CrMo4 motor shaft

High-speed electric vehicle (EV) motor shafts require superior surface integrity to ensure efficiency, durability, and noise–vibration–harshness (NVH) performance. Conventional finishing methods such as grinding and superfinishing provide micrometric smoothness but often fail to deliver the nanometric precision demanded by modern EV drivetrains. This study explores magnetorheological (MR) nano-finishing as a transformative technique for enhancing the performance of 42CrMo4 alloy steel shafts commonly used in EV motors. Experimental investigations were carried out on heat-treated shafts (52–56HRC) using a custom MR finishing setup under systematically varied process parameters. Statistical analysis (analysis of variance) identified the most significant factors influencing surface roughness and subsurface properties. Results revealed a reduction in surface roughness from 0.18–0.22 to 0.013–0.020 µm, representing ∼90% improvement. Residual stress analysis showed a shift from tensile (+150 MPa) to compressive (−160 to −230 MPa) states, alongside a 6% to 8% increase in near-surface hardness. Tribological tests confirmed ∼30% lower friction and ∼35% to 42% reduced wear, while fatigue life improved by ∼39%. NVH evaluations further demonstrated a reduction of ∼5 dB(A) in noise and ∼20% lower vibration amplitudes. The findings establish MR nano-finishing as a scalable and sustainable process that simultaneously improves surface quality, subsurface integrity, and functional performance. Its adoption could significantly advance EV drivetrain reliability, efficiency, and passenger comfort.