Adhesive layer formation and its dual role in tribological performance and surface integrity of Ti-6Al-4V: implications for the machining process

The poor machinability of Ti-6Al-4V (Ti64), characterized by adhesive and abrasive wear, low thermal conductivity, and high chemical reactivity, continues to hinder efficient manufacturing. Among these challenges, adhesive layer formation on tool flank faces remains poorly understood despite its critical influence on tool degradation and workpiece surface integrity. To address this, this study investigates the tribological behaviour of WC/Co-Ti64 pin-on-disc sliding contacts under dry and minimum quantity lubrication (MQL) conditions through both experimental and numerical approaches. Experimental results show that thick, stable, and intact adhesive layers transferred from Ti64 discs was formed on WC/Co pin surfaces under dry and low MQL flowrate conditions. These layers are associated with reduced friction coefficients and lower disc wear but simultaneously contribute to compromised surface integrity. Comparative boundary element method (BEM) simulations with 316 L stainless steel reveal that the lower elastic modulus of Ti64 adhesive layers significantly reduces nominal contact pressure and subsurface von Mises stress, lowering friction coefficients and enhancing mechanical stability of adhesive layer. However, the accompanying increase in surface roughness intensifies local stress concentrations and result in thicker work-hardened layers on Ti64 disc, which align well with BEM simulation results. Conversely, high MQL flowrate inhibited adhesive layer formation, leading to higher friction and wear but producing smoother surfaces and thinner work-hardened layer. The findings offer new mechanistic insights into complex interplay between adhesive layer, lubrication and surface topography, and present the first direct evidence of the dual role of adhesive layer: reducing friction and tool-side wear but compromising workpiece surface integrity.