Quantitative characterization of machining-induced white layers in Ti–6Al–4V

Machining-induced white layers can affect the functional performance of engineered components, due to the resulting mechanical and microstructural properties. Destructive inspection methods such as cross-sectional microscopy are typically used to identify white layers, however, these methods are inherently costly and time-consuming. It is, therefore, desirable to detect this anomalous surface feature using non-destructive methods which requires improved knowledge around the characteristics of white layers. The present paper reports on the characterization of white layers formed during machining of Ti–6Al–4V, to aid future development of a reliable non-destructive assessment method. The microstructure of the material in the white layer was found to have a basal (0002) α-hexagonal close packed texture and there was no evidence of an α→β phase transformation during white layer formation. The white layer has a highly refined grain structure with an increased nanohardness of up to 15% compared with the bulk material. It is proposed that white layers in Ti–6Al–4V are formed by continuous dynamic recrystallization driven by severe plastic deformation during machining. According to the measured micro-mechanical properties of the white layer, suitable non-destructive testing methods are suggested for the detection of this surface feature.