Machining behavior and phase transformation in WEDM of Ti₅₀Ni₄₉Co₁ shape memory alloy

The study reported in this research article focuses on the machining behavior and phase transformation of Ti₅₀Ni₄₉Co₁ shape memory alloy during wire-electrical-discharge-machining (WEDM). Experiments were performed to determine the combined effect of the pulse-on time (Ton) and servo voltage (SV) under high-energy discharge conditions, using a Taguchi L₂₅ orthogonal array. The results show that the pulse-on time significantly increases discharge energy, which reached a peak MRR of 8.43 mm³/min with Ton = 125 µs and SV = 20 V (Run order 21). However, it also increased the surface roughness (Ra) from 2.85 μm to 5.01 μm, producing deeper craters and hardened recast layers as well as re-solidified debris because of rapid melting and solidification. Major morphological changes were observed at high discharge energy, as confirmed by 3D surface profilometry. The servo voltage influenced the stability of the spark: higher voltages reduced discharge intensity, resulting in lower material removal rate but improved surface finish. XRD analysis revealed the formation of secondary phases, containing NiTi₂, TiO₂, and CuZn, was produced during machining, resulting from oxidation and transfer of the electrode material. DSC examination showed an increase in transformation peaks, a decrease in transformation enthalpy, and an insignificant change in transformation temperatures, indicating that the martensitic transformation in the recast layer was partially suppressed by compositional alterations and oxide formation. Nevertheless, no loss of shape-memory property was observed in the bulk material, although slight variations in transformation behavior were observed. Overall, the results support the importance of properly optimizing pulse-on time and servo voltage to achieve a balance among good machining performance, consistent surface quality, and good functional performance, particularly in precision biomedical, aerospace, and smart actuator applications.