Effects of bioceramic particles in dielectric of powder-mixed electrical discharge machining on machining and surface characteristics of titanium alloys

In powder-mixed electrical discharge machining (PMEDM), a suspension of powder particles is added to the dielectric to achieve a high surface quality and to improve the material removal rate (MRR). However, for biomedical applications, the machined surface must be nontoxic and also contain bioactive species; these constraints limit the choice of powder material. This study focuses on machining orthopedic-implant materials based on titanium and titanium-tantalum alloys using bioactive hydroxyapatite (HA)-powder suspension as the dielectric. The influence of the powder particles and the work-piece composition on the machining performance was investigated. Employing a suspension dielectric with 5 g/L HA caused a smoother surface (Ra 2.1 μm) with a thinner recast layer (∼9 μm) as compared to using water to machine titanium which has surface roughness of Ra 2.4 μm with a recast layer close to 10 μm. In addition, the MRR of titanium machined in the HA-powder-suspension dielectric (6.4 × 10−4 g min−1) was greatly lower than that in water (28.6 × 10−4 g min−1). However, increasing the HA concentration led to a gradual increase in the MRR, electrode wear rate (EWR), surface roughness, and recast-layer thickness. The MRR, EWR, and the surface roughness of titanium and titanium-tantalum alloys under PMEDM exhibited an inverse relation with respect to the melting temperature and thermal conductivity of the alloy. Furthermore, calcium and phosphorus were incorporated into the recast layer, and their amounts decreased when the discharge current was increased.