Analysis of tool wear and surface integrity in turning wrought and additively manufactured Ni_50.8Ti_49.2 shape memory alloys

Nitinol (NiTi), a nickel-titanium alloy, is used in biomedical applications owing to its superelasticity (SE) property, which allows the material to revert to its original shape after significant deformation when the applied load is removed. The process chain to manufacture NiTi components involves either plastic deformation or additive manufacturing processes, both followed by post-processing by machining to achieve the part's final shape. However, machining NiTi poses challenges due to its unique stress-strain properties, and it becomes even more critical when considering the unique characteristics of its microstructure. Within this context, the present work aims to evaluate the role of microstructural features of NiTi alloy on tool wear when machined after plastic deformation and additive manufacturing. In this regard, wrought and laser power bed fusion (LPBF) cylinders were machined at different cutting speeds. Tool wear was quantitatively and qualitatively analyzed, supported with cutting force measurements. The machined surfaces were characterized in terms of austenite finish temperature, microstructural alteration, and surface roughness. LPBF NiTi alloy exhibited a coarser and more inhomogeneous microstructure compared to wrought NiTi alloy, resulting in faster tool wear. While the wrought material largely retained its superelasticity after machining, the additive-manufactured samples experienced a reduction in superelasticity. However, this reduction was mitigated as the tool wear increased, due to the higher thermal loads generated during the cutting process.