Nitinol Hollow Manufacturing – Role of Cutting Parameters on Dimensional Accuracy and Microstructural Optimization

Nitinol hollows are intermediate mill products that are subsequently processed into tubes for critical medical applications. To ensure functional integrity and geometric precision, seamless tubes are essential in these demanding uses. Industry practice commonly employs gun drilling to produce hollows from solid Nitinol bars. These processes generate significant friction and pressure, which may cause localized heating and plastic deformation, particularly at the drilled surface. Consequently, gun drilling produces a complex interplay of thermal and mechanical effects that alter the local microstructure. Understanding these effects is vital for optimizing process conditions and achieving reliable material performance in Nitinol hollows and the fine tubes or components derived from them.

This study investigates the evolution of microstructure and geometry during gun drilling and honing. Three hollows with identical outer diameters but varying wall thicknesses were examined to assess strain hardening, grain refinement, and geometric quality. Microstructural features—including grain morphology, microcleanliness, and microhardness—were evaluated across the wall thickness with an emphasis on the layer near internal surface. Dimensional measurements of internal diameter, outer diameter, and wall thickness were conducted to determine concentricity, while scanning electron microscopy (SEM) was performed to determine local characteristics of internal surface and the effect of gun drilling speed and subsequent honing.