Diamond machining of additively manufactured Ti6Al4V: Novel chip formation and friction-induced welding mechanism

The ultra-precision machining of additively manufactured (AM) Ti-6Al-4V presents complex interactions between the tool, material microstructure and surface integrity. This study reveals a previously unreported solid-state friction-induced welding phenomenon occurring during the machining of AM Ti-6Al-4V, which leads to redeposited debris and localised protrusions on the machined surface. Recent studies on AM of Ti6Al4V have explored deformation and failure mechanisms during laser beam powder bed fusion fabrication; however, these have primarily focused on quasi-static compression and mechanical property optimisation. The present work extends these mechanistic insights to the dynamic cutting environment, showing that interfacial friction and severe plastic deformation can trigger solid-state bonding at the tool–chip interface without any thermal phase transformation. The findings offer a new mechanistic explanation for debris accumulation and surface artefacts in precision machining of AM titanium alloys, providing pathways for improved process control and surface quality. This finding explicitly contrasts with earlier reports that attributed such deposits to carbides formed from diamond tool wear or to titanium precipitation. Our study also uncovered a novel continuous chip formation mechanism, dominated by a plastic mode of material removal interspersed with intermittent fracture sites, differing fundamentally from the adiabatic shear-induced saw-tooth chips reported for cast Ti6Al4V. These mechanism-level insights align with recent advances in machining difficult-to-cut and anisotropic materials but extend them by establishing direct evidence of debris welding in AM Ti6Al4V. This work reframes the challenges of precision surface generation in diamond machining and highlights the need for alternative strategies to suppress friction-induced welding for optical-grade finishes.