Performance and wear mechanisms of different PcBN tools when machining superalloy AD730

Nickel-based superalloys are known to be difficult to machine. These alloys are generally machined with cemented carbide tools under low productivity process conditions. Superhard polycrystalline cubic boron nitride (PcBN) tooling offers the possibility of increased production rates, however, a thorough understanding of the material performance and degradation is required before a transition from carbide tools occurs. This study investigated the performance and wear mechanism of three different PcBN tools with low (50 vol. %), medium (65 vol. %), and high (90 vol. %) cBN content, when turning AD730, a new Ni-Co-based superalloy. Due to the novelty of the material, little research has been done on its machinability with carbide and even less with PcBN tooling. The results showed the high-cBN grade was the best performer in terms of machining time. However, an undesirable wear morphology is generated, leading to excessively high cutting forces. Both the medium-cBN and low-cBN showed promise with stable rake and flank wear, but with notching as a drawback. Degradation analysis of medium-cBN grade revealed diffusional dissolution of cBN and formation of a reaction layer atop the cBN grains. The reaction layer, which acts as a diffusional barrier, consisted of three sub-layers: (1) Al2O3, (2) (Ti, Nb, Zr)N, and (3) (Ti, Cr, Nb, Zr)N. The low-cBN grade also showed a reaction layer; however, the smaller cBN grain size of this grade did not allow the layer to sufficiently stabilize.