Effect of heat treatment on microstructural evolution and properties of cemented carbides (WC-17Co) reinforced with 3% volume hexagonal-boron nitride (h-BN) and processed by selective laser sintering (SLS)

Abstract

In the manufacturing and mining industries, cemented carbides (WC-Co) are desired due to their extraordinary properties such as high strength, good wear resistance and high fracture toughness. Moreover, due to design limitations, such as intricate shapes of parts manufactured using conventional powder metallurgy, metal-3D printing is being sought after for better processing and fabrication of parts with complex shapes and geometries. In this study, a cemented carbide (WC-Co) reinforced with 3% Hexagonal-Boron Nitride (h-BN) was processed using selective laser sintering and heat treated to understand the effect of the elemental composition and post-processing heat treatment on the microstructural evolution and mechanical integrity of processed parts. Generally, the as-printed WC-Co-hBN sample had distinct polyangular WC carbides with sizes between 8 μm to 18 μm, W-C-Co matrix and large areas of fused eta phases (40% area fraction). X-ray diffraction analysis revealed that beside the major hexagonal WC phases, the h-BN breaks down to form minor phases such as CoWB, Co5.47N together with W3Co3C, W3Co3N, W9Co3C4 phases. Post-processing heat treatment at 400 °C resulted in the evolution of well-defined WC carbides and eta phases including reduced W-C-Co matrix when compared with the as-printed samples. X-ray diffraction analysis showed the disappearance of the W3Co3N and W3Co3C4 phases coupled coalescence and coarsening of the WC phases and the minor W9Co3C4, CoWB and Co5.47N phases. However, post-processing at 600 °C resulted in the breakdown of the coarsened WC carbides (~10 μm to 12 μm), evolution of more eta phases (63% area fraction) and more W3Co3C phases when compared with the as-printed sample. Nevertheless, post-processing at 800 °C resulted in extensive depletion of the W-C-Co matrix and the growth of irregular WC carbides and eta phases. At 1000 °C, the relative sizes of the WC carbides were smaller but well-defined and closely packed including large volume fractions of the eta phase when compared with the samples at 800 °C. Also, the h-BN phase appeared, for the first time, in this sample with high characteristic peaks. An inverse correlation was observed with the heat treatment on the residual stresses. Thus, the heat treatment aided in relieving the residual stresses in the heat-treated samples. The as-printed sample recorded the highest average hardness value of 4200 HV which is approximately 200% higher than the SLS printed WC-17Co. The as-printed sample had the highest fracture toughness (7.5 MPa√m) whiles the 600 °C had the lowest fracture toughness. Additionally, the wear properties of the W-C-Co-hBN samples were significantly higher with the 1000 °C having the best wear properties. In general, the h-BN reinforcement in this study demonstrated an improvement and stability in the mechanical properties of fabricated parts even up to 1000 °C.