Testing length-scale considerations in mechanical characterization of WC-Co hardmetal produced via binder jetting 3D printing

Abstract

The extreme versatility of additive manufacturing (AM) as processing technology results in “AMed pieces” with intrinsic characteristics linked to the shaping route followed, which are also key for defining mechanical integrity. The latter requires validation by measuring the mechanical properties, at both macroscopic (global) and microscopic (local) levels; and thus, consideration of specific testing length-scale aspects. This work aims to study the correlation between microstructure and mechanical properties for a WC-12%wt.Co hardmetal grade produced via binder jetting 3D printing (BJT) and subsequent sintering. In doing so, macro- and micro- Vickers hardness as well as scratch tests, using different loads and indenter tips, are conducted. It is found that studied samples processed by means of BJT exhibit a microstructure consisting of a relatively wide carbide size distribution, including a significant volume fraction (higher than 15%) of carbides larger than 3 μm. This is a direct consequence of the relatively high sintering temperature needed for getting full dense specimens, when manufactured following this AM route. Meanwhile, mechanical properties are found to be isotropic, with hardness and scratch resistance values falling within ranges of those expected for hardmetals with similar binder content and mean carbide grain size. Very interesting, length-scale effects on testing are observed in terms of dispersion of measured hardness value as applied load decreases. These findings, together with similar ones linked to length-scale influence on scratch response, point out that effective selection of mechanical testing parameters become critical for studying and understanding phenomena such as elastic/plastic and deformation/fracture transitions in AMed hardmetals.