The wire electro-discharge machining characteristics of TiNi shape memory alloys

The wire electro-discharge machining (WEDM) characteristics of TiTi shape m emory alloys (SMAs) have been investigated in this study. The maximum feedi ng rate without breakage of wire electrode increases with increasing pulse duration tau (p) for the TiNi SMAs. The wire electrode will easily break of f if the pulse duration tau (p) raises up to 0.7-0.8 mus due to the excessi ve discharge energy produced in the over-long pulse duration. The maximum f eeding rate of wire electrode has a reverse relationship to the product of the melting temperature (lambda) and thermal conductivity (theta) of machin ed materials. Besides, longer pulse duration tau (p) will have rougher WEDM surfaces of TiNi SMAs. Many electrodischarge craters and re-cast materials are observed in the WEDM surface of TiNi SMAs. The re-cast layer consists of the oxides TiO2, TiNiO3 and the deposition particles of consumed brass e lectrode. The thickness of re-cast layer decreases with increasing pulse du ration tau (p). The specimen's hardness near the outer surface can reach 73 0 Hv for TiNi SMAs. This hardening effect arises from the formation of the oxides TiO2, TiNiO3 and the deposition particles of consumed electrode in t he re-cast layer. The WEDM TiNi SMAs still exhibit a nearly perfect shape r ecovery at a normal bending strain, but a slightly reduced shape recovery a t a higher bending strain due to the depression of the re-cast layer. All t he Ti49Ni51, Ti50Ni50 and Ti50Ni40Cu10 SMAs exhibit similar WEDM characteri stics although they have different crystal structures and mechanical proper ties at room temperature.