Performance and wear mechanisms of uncoated cemented carbide cutting tools in Ti6Al4V machining

Characterisation of worn WC tool using STEM‐EDS aided by principal component analysis

“…While PC4 is present over most parts of the spectrum image due to the presence of carbon contaminants on the specimen surfaces (particularly in the bottom half due to an earlier, aborted SI acquisition), its greatest impact can be seen to the right in the image: the weighting is positive in the interior of the tool, while it is negative along the interaction layer seen as a bright band in the topmost HAADF image. This is consistent with the presence of metallic W (or C‐deficient WC) forming in the interaction region 9,13,30 …”

“…A nanobeam electron diffraction pattern recorded from the interaction layer further corroborates the presence of metallic W; as shown in Figures 5(C), (D) this region yields strong diffraction at spacings that cannot be ascribed to the nearby phases in the tool or alloy. The formation of ‐W has also been reported previously from X‐ray 30 and electron diffraction 9,13 characterisations of similarly worn tools. Based on the simulations presented in Figure 2(A), it should have been possible to distinguish W 2 C as a separate component from WC and W under the current conditions if it had been present in as little as 2% of the pixels in the spectrum image.…”

“…High‐pressure directed cooling was used with a 8 % oil–water emulsion at 90 bar pressure. The cutting tool reached a maximum temperature of approximately 870°C based on IR‐CCD images during operation 9,25 . The turning operation was stopped after 216 seconds to limit the flank wear to 300 μm.…”

“…While mechanical mechanisms (e.g. abrasion and adhesion) play a role, the wear processes are increasingly being connected to interdiffusion and chemical reactions between the tool and the workpiece, 2,5–8 between components in the tool, 9 and between the tool and the surrounding atmosphere 10 . The diffusional mechanisms have been studied particularly in the context of crater formation 5–7,11 – considered the most important wear type in titanium machining – but they will occur in all areas of contact between tool and workpiece, for example, also the flank region.…”

“…For scanning electron microscopy (SEM), there are several reports of its application on analysing the wear zone either directly on the surface 2,8,10 or in cross section after cleaving and polishing worn tools 6,13 . The use of scanning transmission electron microscopy (STEM) is more recent, but the higher spatial resolution this technique offers can be crucial; the interaction layer and adjacent material in the workpiece and tool have been shown to contain variations on the nanometre scale after both emulsion‐cooled 9,12 and cryogenic machining 14 . Interpretation of both SEM‐ and STEM‐EDS spectrum images can however be complex due to, for example, physically overlapping phases and inclusions in the sample, and overlapping X‐ray peaks in the spectra.…”