Coupled Effects of Lubrication on Tool/Chip Interfacial Friction and Primary Shear Deformation in Cutting of Ti-6Al4V

Abstract

The mechanics of machining Ti-6Al4V are governed by an intrinsic coupling between tool–chip interfacial friction and primary shear deformation. Conventional cooling and lubrication strategies such as flood cooling, Minimum Quantity Lubrication (MQL), and cryogenic-MQL effectively reduce average friction and improve tool life, yet their continuous delivery and cooling action dynamically alter interfacial conditions, masking the inherent link between frictional behavior and shear deformation. In this study, a pre-coated lubrication film (PCFL) was employed as a controlled boundary condition to isolate and examine this coupling during orthogonal cutting of Ti-6Al4V. Synchronized force measurements and high-speed Particle Image Velocimetry (PIV) were employed to characterize chip–tool contact and primary deformation-zone (PDZ) behavior under dry and lubricated conditions for uncut thicknesses (h) of 10, 50, and 100 µm. The cutting force (Ft) remained nearly constant for both conditions, while the feed force (Ff) decreased by about 28 % under lubrication, indicating reduced rake-face adhesion. The corresponding reduction in friction angle (β) reorients the equilibrium resultant force closer to the rake-face, steepening the shear plane and increasing the shear angle (ϕ) in accordance with Merchant’s minimum energy principle, which predicts that the shear plane reorients to minimize total energy under varying frictional conditions. PIV results further confirmed a shorter, more stable PDZ, demonstrating that interfacial friction directly governs shear-zone geometry and deformation stability in titanium machining.