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 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

A comparative study of the effects of milling and abrasive water jet cutting on flexural performance of CFRP

Machining of carbon fibre reinforced polymers is part of the production process that introduces several challenges due to inherent characteristics of CFRPs such as non-homogeneity of their mechanical properties. A comparative analysis of conventional milling and abrasive water jet (AWJ) cutting is performed to quantify the effects of machining induced damage on flexural strength of woven CFRP laminates. The machined surfaces quality is characterized using optical and scanning electron microscopy methods prior to flexural mechanical testing. High-speed Digital Image Correlation technique is also used to measure deformation evolutions and determine fracture mechanisms in relation to the applied machining operation and produced machined surfaces. The effect of machining induced damage on strength of milled samples was less than expected with the AWJ processed samples having the least mechanical properties. The surface morphology analysis revealed that the entry and exit point of the water jet introduced severe surface and subsurface damage across the full thickness. The failure initiation sites were determined by strain distribution maps indicating that machining induced damage promotes failure of the tested CFRPs away from maximum compressive stress observed under the loading points

On deformation characterisation of machined surfaces and machining-induced white layers in a milled titanium alloy

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

Destructive and non-destructive testing methods for characterization and detection of machining induced white layer: A review paper

The presence of machining-induced white layer in the near-surface of critical aeroengine alloys has a detrimental effect on the lifetime of a component. Present techniques for identifying and characterizing white layer, such as optical microscopy and hardness testing, whilst effective, are destructive, costly and time-consuming. Non-destructive testing methods may, therefore, offer improvements to the process of white layer detection. This paper discusses the formation mechanisms and the defining physical properties of machining-induced white layers before offering a comprehensive review of the current state-of-the-art in both destructive and non-destructive testing methods for detecting this anomalous surface feature

The effect of cutting speed and feed rate on hole surface integrity in single-shot drilling of metallic-composite stacks

AbstractExperimental work was carried out to evaluate the influence of cutting speed (Ti/CFRP/Al: 30/120/120 and 36/144/144m/min) and feed rate (0.05, 0.08, 0.12 and 0.15mm/rev) on workpiece surface integrity following single-shot drilling of multilayer metallic-composite stacks (Ti-6Al-4V/CFRP/Al-7050) using CVD diamond coated tooling. When operating with the lower cutting speed set of 30/120/120m/min and a feed rate of 0.08mm/rev, average hole surface roughness (Ra) was ∼ 0.60, 0.87 and 0.27μm in the Ti, CFRP and Al layers respectively after 30 holes. Roughness values in the stack increased to 0.84, 1.6 and 0.43μm Ra when employing the higher cutting speed of 36/144/144m/min, with the drill lasting only 20 holes. Microhardness depth profile evaluation of the machined surfaces showed no appreciable variation in both the Ti and Al material, irrespective of cutting conditions. Matrix cracking and burn were apparent in the CFRP layer as feed rate increased due to greater wear of the drill corner