Sinter-based additive manufacturing of hardmetals: Review

Hardmetals, commonly referred to as cemented tungsten carbide (WC) or simply tungsten carbide, is one of the oldest liquid phase sintered, two phase composite, powder metallurgical (PM) product. In fact, this year marks the 100th year since the first hardmetal patent was issued. The classic two-phase composite hardmetal structure typically consists of hard ceramic-like WC particles that are embedded in a softer matrix binder phase primarily consisting of cobalt (Co), sometimes in conjunction with other metals such as nickel and iron, that takes into solution some tungsten and carbon. Various shaping methods have been used to fabricate hardmetals into useful products. Traditional machining to shape hardmetal components was extremely difficult due to the high hardness of these materials (significantly harder than any steels used for cutting). Press and sinter was the process of choice for making relatively simple shapes from hardmetals and is still extensively used. To accommodate some shape complexity, die pressed or cold isostatically pressed “green” parts were machined followed by subsequent sintering to shape hardmetal components. Extrusion using organic polymers was another technique that was adapted to make hardmetal rods and bars. Using the powder injection molding (PIM) process to form more complex shapes from hardmetals was a natural evolution. However, the tooling cost and the typically low volume of parts required for many hardmetal applications was a major barrier. At the turn of the century, there was a rapid growth of the additive manufacturing (AM) sector that is capable of fabricating extremely complex shaped components without the use of any tooling. However, the early AM processes that gained commercial viability were founded on the melt-based techniques, which was generally not conducive to processing of hardmetals. Since 2015, a number of sinter-based AM technologies started to emerge and these technologies were found to be better suited for the processing of hardmetals into complex shapes in an economic manner. This paper will review the developments in the processing of hardmetals using some of the different sinter-based AM techniques.