Workpiece surface integrity considerations when finish turning gamma titanium aluminide

Gamma titanium aluminides (γ-TiAl) are currently being evaluated by aeroengine manufacturers as possible replacements for conventional titanium alloys and nickel based superalloys in gas turbine engines. Unfortunately, even when machining using operating parameters selected to minimise workpiece damage, turned surfaces contain cracks and microstructural alterations. The paper initially reviews prior work on the surface integrity of γ-TiAl produced by various machining processes. Following on from this, surface integrity data from a full factorial experiment are presented when turning Ti–45Al–2Mn–2Nb+0.8 vol.% TiB2 employing extremely fine finishing cuts. Workpiece surfaces/subsurfaces were evaluated in relation to microstructural alterations, strain hardening/microhardness changes, and 2D surface roughness (Ra). All the machined surfaces contained cracks, cracked TiB2 particles, deformed microstructure and microhardness increases regardless of the operating parameters. However, when comparing the surfaces with those obtained in previous work using more abusive parameters, a significant reduction in surface damage was obtained.