New insights into variable thermomechanical loads due to chip formation size effects in machining of Ti-6Al4V alloy

Using state-of-the-art in-situ characterization via high-speed optical microscopy and full-field digital image correlation analysis with nanometer resolution, Particle Image Velocimetry (PIV) as well as high bandwidth time-correlated process force analysis, this paper provides new quantitative insights into the complex dynamic variability present in the machining of Ti-6Al4V alloy, particularly at low uncut chip thicknesses where microstructural effects are most influential. Dynamically varying loads were analyzed across a wide range of uncut chip thicknesses (from 6 µm to 150 µm) and typical industrial cutting speeds for Ti-6Al4V alloy (30–120 m/min). The results reveal three distinct regimes of uncut chip thickness: from microstructural size effects at low thicknesses to quasi-steady state chip formation at intermediate uncut chip thickness to continuum-scale adiabatic shear banding at higher uncut chip thicknesses. Each regime is characterized by significantly different dominant mechanisms of chip formation, leading to variable sub-surface elastic and plastic loading with varying frequency/time and characteristic length scales. To minimize variability in thermomechanical loading and improve surface integrity in the cutting of Ti-6Al4V alloy at speeds between 30 and 120 m/min, our findings indicate an optimal uncut chip thickness range of 20–60 µm. These findings advance machining practices for Ti-6Al4V alloy, contributing to a new process optimization paradigm based on minimizing process and material-specific load variability to maximize as-machined component quality across multiple length scales.