The influence of aluminium additions on titanium during machining through the application of a novel orthogonal cutting test method

Due to the high production rates of civil aircraft the demands for increased rates of production through high speed machining are ever increasing. A lack of consistency in the current practices for determining `machinability' means that a bespoke trial is needed to ascertain whether a particular combination of material, tool and machining parameters yields a sufficiently beneficial outcome. The current approach of large scale trials, while effective, is inefficient and costly to OEMs. Most significantly, with the aerospace industries desire to build engines outside the capabilities of current alloys, conventional machining trials cannot be conducted on new alloys that have yet to reach production scale due to the material quantities required in large scale trials. With regard to titanium alloys, the majority of alloys contain some level of aluminium as a major addition.
A research programme has been undertaken to investigate the influence of aluminium on the sub-surface deformation of Ti-xAl binary alloys during high speed machining processes with a view to developing a testing method for determining `machinability'. Such a novel test method would fast track the current inefficient and uneconomical approach. Microstructural analysis of the region immediately below the machined surface has been performed for high speed milled material, focusing on quantifying the depth to which deformation occurs and the dominant plastic deformation mode by backscatter electron imaging and light microscopy. The influence of aluminium was measured through the penetration of the severe plastic deformation region and twin depth penetration and showed that the behaviour was parabolic, with Ti - 4 wt.\% Al showing the maximum level of deformation. The resultant plastic deformation from high speed milling also causes the near surface material to undergo a crystallographic reorientation, demonstrating characteristics of a simple shear process. Comparisons were made with a well established simple shear test method, axisymmetric compression testing, analysing the deformation behaviour, from high speed milling, with the mechanical behaviour of axisymmetric compressions. Use of work hardening principles, traditionally applied to FCC materials, were used to identify characteristics that exhibit trends comparable to those observed during high speed milling, in contrast to conventional mechanical behaviours such as yield stress. A novel orthogonal cut test was successfully developed to generate deformation behaviour from high speed milling and flow behaviour from axisymmetric compression tests in a single test method. This suggests the potential to incorporate `machinability' testing during alloy development rather then a final test once large scale production has commenced.