Understanding the effect of surface generation rates for finish milling of Ti-5Al-5Mo-5V-3Cr aero-structural components

A detailed investigation into the influence of cutting parameters on the surface integrity, subsurface microstructure, tool wear and fatigue performance for finish milling of Ti-5Al-5Mo-5V-3Cr landing gear components. Low cost, small scale testing methodologies have been developed to replicate complex machining strategies employed in aerospace manufacturing, to understand and develop an improved processing window. Machinability trials identifieded the influence of cutting speed (Vc), feed per tooth (fz) and surface generation rates (SGR) on surface roughness and subsurface deformation. The detrimental impact of fz can be offset somewhat by comparable increases in Vc. The subsequent reduction in imposed cutting forces, increased
coolant delivery and improved chip evacuation achieves moderate responses in surface integrity and subsurface deformation. Severe plastic deformation at the near
surface, exacerbated by increased cutting parameters, is accommodated through dislocation slip along basal, prismatic and pyramidal planes of primary alpha
grains. Deformation modes, which are intensified by increased tool wear. A transition from abrasive to adhesive tool wear mechanisms have been identified and shown to be accelerated by increased fz. Through the application of a bespoke four point bend test rig, low cycle fatigue testing at R=0.1, has proposed some important observations for fatigue performance of machined high strength titanium allows. Cycles
to failure were shown to be reduced for identical applied stress for increased SGR. Fatigue cracks has been identified within the severe plastic deformation region of the subsurface, as a result of machining. Initiation was produced through quasi-cleavage along dislocation slip planes of alpha grains and propagated in a mixed mode, significantly influenced by bulk beta grains. A competing mechanism between initiation influenced by machining damage and propagation retardation as a result of machining induced compressive residual stresses has been postulated.