Using particle-loaded inks to selectively change the material properties in binder-jetted WC–Co parts

Abstract

Functionally graded hardmetals offer a superior combination of fracture toughness and wear resistance and alleviate the compromise that conventional hardmetals must face between these two material properties. In tooling applications, they exhibit superior cutting performance compared to the conventionally employed hardmetals. Traditionally, graded tools are enabled by a surface treatment with nitrogen or carbon. This enables a tungsten carbide–cobalt (WC–Co) tool with a surface layer containing less cobalt (Co) than the core of the part. With this Co gradation, the wear properties can be improved due to the harder surface. If different loads act at different points in a tool and the geometric complexity of a tool increases, the requirements for a targeted positioning of the Co gradients become higher. A simple surface treatment does not allow a flexible distribution of the desired gradients. Binder jetting provides a solution to produce location-independent graded structures. In this work, the process was evaluated for the generation of Co gradients in WC–Co structures. Inks loaded with graphite particle contents of 2.5, 5, 7.5, and 10 m% were used to create differences in the carbon content in the green part. This induced a Co migration during the liquid phase sintering process. The sintering parameters were varied in time and temperature and studied regarding their effect on the microstructure and the porosity, the formation of the Co gradient, and the hardness. At low sintering times and temperatures, all carbon differences led to a Co gradient in the sintering part with a deviation of up to 10 m%. With an increase in sintering time and temperature, the Co distribution began to even out, and a deviation was observed only in the parts created with the inks containing 7.5 and 10 m% of particles. Hardness values of up to 1300 HV10 were obtained, which is comparable to conventionally produced WC–Co parts. The excess of carbon resulted in porosity in the parts leading to a reduced density. However, with inks containing 10 m% particles, densities of 94.26% were achieved at the longest sintering times and the highest temperature.