Surface integrity of a high speed milled gamma titanium aluminide

The paper is concerned with the surface integrity of a high speed ball end milled gamma titanium aluminide, Ti–45Al–2Nb–2Mn–0.8 vol.%, TiB2XD™ produced by Howmet, USA. Commercial interest in gamma titanium aluminide is centred mainly in the aerospace and automotive sectors. Possible component applications include, compressor blades, swirl nozzles, automotive engine valves and turbo impellers. Machining tests were carried out on a Matsuura FX5 20,000 rpm, prismatic machining centre using 6 mm diameter, coated tungsten carbide ball end tooling. Operating parameters were used that were representative of finish machining. The majority of surface roughness values recorded were <1.5 μm Ra. Workpiece surface alterations included material pullout, fracture, smear and deformation of the lamellae. Microhardness evaluation of the subsurface indicated a hardened layer to a depth of 300 μm. Initial residual stress analysis, using ‘blind hole drilling’, showed that the surface contained compressive stresses >500 MPa. The effect of high speed milling parameters, including, flank wear (new tool/worn tool), cutting speed and angle-of-workpiece, on residual stress was further analysed using a Taguchi orthogonal array. This demonstrated that flank wear and cutting speed had the most significant effect.