Abstract
Machining-induced white layers and severely deformed layers are undesirable surface integrity features which can be formed when machining high-strength aerospace alloys. An orthogonal milling process has been designed and performed to assess the impact of cutting speeds, tool wear, cutting edge radius and climb vs conventional milling on white layer formation and plastic strain distribution. The plastic deformation in the machined surface associated with the formation of white layers in Ti-6Al-4V has been quantified using micro-grids of different length scales printed using the electron beam lithography technique. It was found that white layers formed via the severe plastic deformation mechanism, at equivalent plastic strain values in excess of 1.2 and in regions of the cutting arc with the instantaneous chip thickness of less than the cutting-edge radius and ploughing and rubbing being the dominant mechanisms. The results indicated that the magnitude of the measured strains and the depth of plastically deformed material was greater at lower cutting speeds, during climb milling and when machining with a larger cutting edge radius and tool flank wear land.
Highlights
Machining-induced white layers can be formed when a difficult-tocut material, such as aerospace superalloys or hardened steels, is machined at high cutting speeds (Chou and Evans, 1999) or using a severely worn tools (Brown et al, 2019) or alternatively tools with a large radius of curvature (Denkena et al, 2015). Griffiths (1987), identified that these white layers can form via a thermally driven phase transformation mechanism or a severe plastic deformation (SPD) mechanism due to mechanical work, which can involve dynamic recrystallisation
High speeds and deformation rates have been associated with white layer formation, these white layers are often formed via phase transformation, and it is well-known that this mechanism is favourable at higher cutting speeds (Hosseini et al, 2015)
At lower speeds, the SPD mechanism is dominant and the formation of white layers at only the lower cutting speed used in this study proves that SPD rather than phase transformation was the active mechanism
Summary
Machining-induced white layers can be formed when a difficult-tocut material, such as aerospace superalloys or hardened steels, is machined at high cutting speeds (Chou and Evans, 1999) or using a severely worn tools (Brown et al, 2019) or alternatively tools with a large radius of curvature (Denkena et al, 2015). Griffiths (1987), identified that these white layers can form via a thermally driven phase transformation mechanism or a severe plastic deformation (SPD) mechanism due to mechanical work, which can involve dynamic recrystallisation. Irrespective of the material among the common aerospace engine alloys, the white layer region typically possesses a highly refined nanoscale grain structure, increased hardness relative to the bulk material and extreme residual stresses, as shown by Brown et al (2018) for Ti-6Al-4V, Bushlya et al in IN-718 (Bushlya et al, 2011) and hardened steel (Fang-yuan et al, 2017). Whilst these properties may appear beneficial in certain circumstances, Herbert et al (2014) have shown that the presence of a white layer can lead to a significant reduction in the fatigue life of components, compared to surfaces without a white layer but similar levels of residual stress. The phase transformation mechanism is associated with the high cutting speeds, whilst the SPD mechanism can dominate at lower speeds, as identified by Hosseini et al (2015). Guo et al (2010) reported that, due to the greater heat generation at higher speeds, phase transformation white layers can be associated with a more tensile residual stress state, relative to an SPD white layer in the same material
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