Effect of Radial Force and Radial Torque on Hole Quality Characteristics During Micro-Drilling of Nitinol Shape Memory Alloy

Micro-drilling of Nitinol, a shape-memory alloy, presents unique challenges owing to its high strength, elasticity, and work-hardening properties. In addition, drill tip wandering caused by the radial force and radial torque during micro-drilling severely affects the hole quality. However, measuring the radial force and torque is difficult because of their low values. Therefore, the current study focuses on the measurement and analysis of radial force and radial torque and correlates them with hole quality characteristics. This study aims to examine the impact of micro-drilling parameters on hole quality when drilling Nitinol with carbide tools, to optimize conditions for better machining efficiency and precision. This is crucial given Nitinol’s widespread use in applications like medical devices requiring high precision. Additionally, hole quality affects component functionality, and this research aids in optimizing manufacturing by reducing material waste and improving yield. Micro-holes were drilled in Nitinol using the direct drilling method at varying speeds and feeds. The size effect during micro-drilling was characterized by the behavior of radial force, which fluctuated in relation to spindle speed. The “size effect” zones were recognized by the deformed layer’s pattern with respect to spindle speed. The measured oversize error exhibited a correlation with the radial force. The experimental results demonstrated a maximum oversize error of 8.5 µm at 30,000 rpm and 0.25 µm/rev and a minimum oversize error of 0.5 µm at 10,000 rpm and 0.075 µm/rev. The various zones in micro-drilling were differentiated by specific radial forces, which consequently explained the size effect phenomenon. The deformation layer thickness exhibited a maximum value of 6.113 µm at 20,000 rpm and 0.25 µm/rev and a minimum value of 3.211 µm at 10,000 rpm and 0.075 µm/rev. Scanning electron microscopy (SEM) revealed microcracks formed in the deformed layer at higher speeds. However, the ploughing and transition regions resulted in the material buildup with minimal or no cracks because of the size effect. Therefore, this study established a definite correlation between the size effect, radial force, radial torque, and hole quality characteristics.