Tailoring Ti-6Al-4V during additive manufacturing with in situ sulphur microadditions to enhance machinability

Improving the machinability of titanium alloys remains a major barrier in aerospace manufacturing, particularly for additively manufactured Ti-6Al-4V components that exhibit heterogeneous microstructures and high cutting resistance. This study introduces a new alloy modification machinability enhancement strategy by incorporating trace sulphur (<300ppm) via MoS₂ during wire-based directed energy deposition (W-DED). The modified alloy meets the composition requirements of Ti-6Al-4V ELI (ASTM Grade 23) titanium and retains its structural integrity while developing sulphur-rich particles at the grain boundary that act as low-strength shear paths and facilitate plastic flow during cutting. Machinability improvements include an approximate 8% reduction in average cutting forces (F r ) and a th r eefold decrease in tool flank wear compared to the unmodified alloy. Additionally, there was a noticeable change in chip morphology, shifting from distinctly serrated to more continuous. Machining trials show lower cutting forces, delayed crater and flank wear, and a shift in chip morphology from distinctly serrated to more continuous. This behaviour parallels the classical free-cutting effect of sulphur in steels yet is demonstrated here for the first time in an additively manufactured titanium alloy system. This work establishes in-process sulphur micro-addition as an effective pathway for tailoring the machining response of additively manufactured Ti-6Al-4V, offering a practical route for producing difficult-to-cut aerospace components with improved manufacturing efficiency. • Trace sulphur added to Ti-6Al-4V via W-DED without affecting mechanical properties • Sulphur precipitates at prior-β grain boundaries, forming particles that ease machining • Cutting forces drop ~8%, with crater and flank wear greatly reduced • Chip morphology shifts to continuous, aided by higher shear angle and less segmentation • Up-milling roughness improves from 3.16 µm to 0.71 µm due to stable chip formation