The electro-discharge machining characteristics of TiNi shape memory alloys

The electro-discharge machining (EDM) characteristics of TiNi shape memory alloys (SMAs) have been investigated in this study. Experimental results sh ow that the material removal rate of TiNi SMAs in the EDM process significa ntly relates to the electro-discharge energy mode, involving the pulse curr ent I-P and pulse duration tau (P). It also has a reverse relationship to t he product of the melting temperature and thermal conductivity of TiNi SMAs . In addition, a longer pulse duration tau (P) and a lower pulse current I- P should be selected to have a precise EDM machining of TiNi SMAs. Many ele ctro-discharge craters and re-cast materials are observed on the EDM surfac e of TiNi SMAs. The re-cast layer consists of the oxides TiO2, TiNiO3 and t he deposition particles of the consumed Cu electrode and dissolved dielectr ic medium. The thickness of the re-cast layer initially increases, reaches a critical value, and then decreases with increasing pulse duration tau (P) . The specimen's hardness near the outer surface can reach 750 Hv for EDM T iNi 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 b ending strain due to the depression of the re-cast layer. All the Ti49Ni51, Ti50Ni50 and Ti50Ni40Cu10 SMAs exhibit similar EDM characteristics althoug h they have different crystal structures and mechanical properties at room temperature. (C) 2001 Kluwer Academic Publishers.