Electrochemical machining of gamma titanium aluminide intermetallics

Characteristics of low density, high stiffness, good creep resistance and high strength at a wide range of temperatures make titanium aluminide a potentially important material in respect of weight savings in high performance components operating at high temperatures. Some work has already been carried out to examine properties of the machining of this alloy using mechanical stock-removal techniques such as, turning. Such methods are found to have limitations in terms of surface integrity defects and the formation of surface hardened layer. In this paper the electrochemical machining (ECM) characteristics of titanium aluminide are examined. Conditions under which reproducible ECM is viable for this material have been established and parameterised in terms of machining parameters generated from chronoamperometric analyses for both chloride and perchlorate electrolyte systems. Stable dissolution has been shown to be limited by surface passivation, in case of the chloride system, and choking in case of the perchlorate system. In case of the perchlorate surface there is evidence of an increased amount of cross-colony attack when compared to surfaces machined using chloride. Chronoamperometric analysis has indicated that this form of attack can be correlated to differences in the valency (2.4 for the perchlorate system and 3.0 for the chloride system) and the overpotential (3.8 V for the perchlorate system and 4.3 V for the chloride system). These differences have been related to measured differences in the surface dissolution characteristics of the microstructural phases of this material. Surface hardness of the ECM TiAl has been shown to have decreased by 46% with respect to conventionally machined surfaces.