Complex machining processes require tools made of cemented carbides, which have a superior stiffness, strength and dimensional stability when compared to iron alloys. The Laser-Powder Bed Fusion process (L-PBF) offers the possibility to introduce internal cooling structures in an optimized flow, shape and diameter due to the layer-by-layer technology. This makes it possible to increase the tool life and working with higher cutting speeds, which leads to a significant reduction in production costs.
The type of powder that is used shows a decisive influence on the component quality. Important powder characteristics for the manufacturing of non-porous components with an appropriate microstructure are a suitable chemical composition, a spherical powder shape to ensure a good flowability and the powder layers must exhibit a high powder density. Apart from a spherical granule shape, commercially available WC-Co granules show a low apparent density, based on their high inner porosity and an unsuitable chemical composition. This results in a high volumetric porosity, phases that lead to embrittlement and an inhomogeneous WC grain size.
On a laboratory scale, powder synthesis with 88 wt% WC and 12 wt% Co was carried out with the aim of developing more suitable powder grades for the L-PBF process. The synthesis process comprised a mechanical alloying and a subsequent spray drying step for granulation (d99 <30 μm). Aim of the investigation was to reduce the internal porosity and to vary the chemical composition using additives. These additives are, for example, grain growth inhibitors, such as VC, Cr3C2, as well as carbon black and TiC, in order to reduce WC grain growth during the L-PBF process and to avoid the formation of the brittle η-phase at C-contents < 6.13 wt%. Suitable milling parameters for the preparation of a homogeneous suspension, which contains a solvent, WC and Co particles, as well as additives, have been defined. The granule size and the reduction of the internal porosity was adjusted by means of feed rate, spray gas flow, solid particle concentration and binder amount variation.
The powder batches were characterized by electron and light microscopy in terms of surface characteristics, morphology, inner porosity, homogeneity and size distribution. With the manufactured powder types, multilayered model specimens were produced by L-PBF process and the resulting microstructure was evaluated.