Tool Wear and Workpiece Surface Integrity during Ni-Based Superalloy Machining with Multi-layer CVD Alumina Coated Carbides

Ni-based superalloys are used in the most hostile engineering environments, including the hottest stages of aeroengines. The exceptional high-temperature performance of these alloys presents significant challenges during machining; substantial heat and high stresses are generated that impact the integrity of both the tool and the workpiece. Alumina coatings are known for their excellent chemical stability and low thermal conductivity. Therefore, they are considered well-suited for cutting hard-to-machine alloys.

In the following work, an in-depth study of alumina degradation during a series of turning experiments on wrought and powder metallurgy (PM) alloys. During axial turning experiments involving a wrought alloy (IN718), high levels of coating pull-out and delamination were observed. In contrast, when turning a PM superalloy (RR1000), a lower propensity for adhesive wear and alumina grain pull-out was observed; however, a faster wear rate and a higher concentration of Hf-rich reaction products formed across the contact zone, indicating chemical wear. It is hypothesised that thermally driven wear phenomena are more significant when turning RR1000.

Both fine and coarse grained RR1000 were machined, with the latter causing faster overall wear and significant notching during tool engagement. A higher wear rate was also detected at the higher cutting speed tested. Larger fluctuations in the machining force response were measured when turning the less homogeneous CG material. The deformation characteristics of the CG material were also observed to be more diffuse and more inhomogeneous, which was accompanied by irregular shear band formation in the produced chips.

Additional dry orthogonal experiments were performed on as-HIP RR1000. Significant adhesion and Hf-rich reaction product formation were again detected, supporting previous observations regarding the significant chemical affinity between alumina and RR1000. The results presented demonstrate the limitations of state-of-the-art textured alumina coatings in terms of Ni-based superalloy turning, highlighting the need for further research in this area.