The micro-mechanistic understanding of deformation of titanium alloy Timetal® 54M during machining

The next generation of aircraft platforms will employ increased tonnages of aerostructural titanium alloys.  Pressure to meet the delivery targets of many of these aircraft programmes will require higher productivity, which from a machining standpoint equates to higher surface speeds. Unfortunately, the very properties that make titanium a popular engineering alloy also lead to difficulties during machining: low thermal conductivity and high cutting forces due to retention of mechanical strength at high temperatures lead to excessive tool wear and prohibitive manufacturing costs. Therefore to allow for this progression in the methods employed during the machining of titanium alloys, a greater awareness of the mechanisms which are involved needs to be developed. Assessment of surface condition and optimisation of machining parameters are often traditionally limited to surface topography and for critical applications would include relatively low magnification light microscopy. In this paper high resolution electron backscatter techniques were used to identify microstructural subsurface damage in the form of intense slip bands and mechanical twins, induced during a series of high-speed turning trials carried out on the titanium alloy Timetal® 54M.