20 Hz synchrotron X-ray diffraction analysis in laser-pulsed WC-Co hard metal reveals oscillatory stresses and reversible composite plastification

Abstract

The lifetime of WC-Co inserts used in cutting processes, such as milling, is limited by millisecond temperature and mechanical pulses, which occur as a result of interrupted tool-workpiece contact, thermal fatigue and wear. In the current work, synchrotron X-ray diffraction (XRD) was used in conjunction with a pulsed laser heating set-up to characterise the time-dependent development of stresses and microstructure in locally irradiated WC-Co inserts coated by chemical vapour deposition with 6.5 and 3.5 μm thick TiCN and α-Al2O3 films, respectively. Diffraction data from the WC phase were used to evaluate the time and temperature-dependent evolution of in-plane stresses, thermal strains and integral breadths of WC diffraction peaks in experiments with a single and five successive laser shocks applied within 2.2 and 20 s, respectively, using a laser spot diameter of ~5.8 mm and an X-ray beam size of 1 × 1 mm2. The laser heating induces the formation of compressive stresses in the inserts' substrates. Above a temperature of ~750 °C, at the onset of WC-Co composite plastification, compressive stresses relax and then vanish in WC at the maximal applied temperature of ~1300 °C, followed by the build-up of tensile stresses. The applied cyclic heating up and cooling down led to the repetitive formation of compressive and tensile stresses, with temperature dependencies oscillating with the number of applied laser pulses. The observed relatively high tensile stress level of ~1100 MPa in WC was a consequence of the stabilising function of the coating, which hindered the initiation of surface hot cracks and stress relaxation. The stress evolution was coupled with changes in XRD peak broadening, which however strongly depended on the particular hkl reflections and showed oscillatory behaviour within a single temperature cycle. In summary, the unique diffraction set-up revealed stress levels and provides insight into the WC-Co composite plastification mechanism governing the stress build-up and relaxation in locally thermo-shocked WC-Co inserts at millisecond time resolution.