Development of the Structure of Cemented Carbides during Their Processing by SLM and HIP

• Main Authors: David Bricín, Michal Ackermann, Zdeněk Jansa, Dana Kubátová, Antonín Kříž, Zbyněk Špirit, Jiří Šafka
• Format: Article
• Language: English
• Published: MDPI AG 2020-11-01
• Series: Metals
• Subjects: WC-Co; cemented carbides; SLM; HIP; porosity; grain coarsening
• Online Access: https://www.mdpi.com/2075-4701/10/11/1477

The study focuses on microstructural evolution in a WC-Co powder mixture during Selective Laser Melting (SLM) and hot isostatic pressing (HIP) processing. This powder mixture contained a 13 ± 0.6% weight fraction of Co binder and WC particles of mean size of 3.0 ± 1.9 μ<inline-formula><math display="inline"><semantics><mi mathvariant="normal">m</mi></semantics></math></inline-formula>. SLM of the mixture produced samples of various densities, depending on the volumetric energy density (VED) applied. High VED levels led to densities of up to 88%. The aspects affected by changes in VED included the pore density as well as the resulting types of phases and the size of WC phase particles. At high VED, the material began to develop cracks due to embrittlement. This had multiple causes: coarsening of <inline-formula><math display="inline"><semantics><mi>α</mi></semantics></math></inline-formula>-phase (WC), evaporation of <inline-formula><math display="inline"><semantics><mi>β</mi></semantics></math></inline-formula>-phase (Co binder), and precipitation of <inline-formula><math display="inline"><semantics><mi>η</mi></semantics></math></inline-formula>-phase. At low VED levels, pores formed, typically of nonsymmetric shapes, with sizes larger than 500 μ<inline-formula><math display="inline"><semantics><mi mathvariant="normal">m</mi></semantics></math></inline-formula>. Subsequent HIP processing led to an increased density, up to 96% of solid material. Contributions to this increased density were provided by structure transformations, namely, coarsening of <inline-formula><math display="inline"><semantics><mi>α</mi></semantics></math></inline-formula>-phase by up to 1300% when compared to the powder grain size, and formation of <inline-formula><math display="inline"><semantics><mi>η</mi></semantics></math></inline-formula>-phase. The results provided a basis for steering further research to explore to a greater depth the SLM and HIP processing of selected WC-Co powder mixtures with as yet unused ranges of process parameters.