The electro-discharge machining characteristics of TiNi shape memory alloys

Abstract

The electro-discharge machining (EDM) characteristics of TiNi shape memory alloys (SMAs) have been investigated in this study. Experimental results show that the material removal rate of TiNi SMAs in the EDM process significantly relates to the electro-discharge energy mode, involving the pulse current I _P and pulse duration τ_P. It also has a reverse relationship to the product of the melting temperature and thermal conductivity of TiNi SMAs. In addition, a longer pulse duration τ_P and a lower pulse current I _P should be selected to have a precise EDM machining of TiNi SMAs. Many electro-discharge craters and re-cast materials are observed on the EDM surface of TiNi SMAs. The re-cast layer consists of the oxides TiO₂, TiNiO₃ and the deposition particles of the consumed Cu electrode and dissolved dielectric medium. The thickness of the re-cast layer initially increases, reaches a critical value, and then decreases with increasing pulse duration τ_P. The specimen's hardness near the outer surface can reach 750 Hv for EDM TiNi SMAs. This feature originates from the hardening effect of the re-cast layer. The EDM TiNi SMAs still exhibit a nearly perfect shape recovery at a normal bending strain, but a slightly reduced shape recovery at a higher bending strain due to the depression of the re-cast layer. All the Ti₄₉Ni₅₁, Ti₅₀Ni₅₀ and Ti₅₀Ni₄₀Cu₁₀ SMAs exhibit similar EDM characteristics although they have different crystal structures and mechanical properties at room temperature.