Characterization of WC-Co tools used for machining of Alloy 718 under the influence of high-pressure coolant supply

Licentiate thesis, 2017

The use of high-pressure jets for supply of coolant to the cutting zone has the potential to significantly increase the productivity and process stability when machining difficult-to-cut materials such as Ni-based superalloys. Apart from better chip-breakability, the jets enable the coolant to reach closer to the cutting edge and hence enhance its capability to dissipate heat. This work deals with characterization of conventional and surface-modified uncoated WC-Co tools used for machining of a Ni-based superalloy (Alloy 718) with assistance of high-pressure coolant supply. Specifically, scanning electron microscopy (SEM) together with energy-dispersive X-ray spectroscopy (EDX) were used to study the flank wear lands and areas impinged by the coolant of conventional and surface-modified tools. As compared to conventional tools, the surface-modified tools (with arrays of square pyramidal shaped dimples on rake and flank faces) enabled to reduce flank wear. This was linked to the larger surface area provided by the modifications which can enhance the tool-coolant interaction. In that way, the heat dissipation during cutting is improved and the tool is subjected to less thermal softening and wear. At the tool surfaces, the Co-binder was removed by erosion during the coolant-impact when machining. The erosion occurred on areas of both rake and flank surface which are impinged by the high-pressure coolant jets and are not in contact with the workpiece during cutting. The erosion damage can be used to determine the access of coolant to the cutting edge. Such observations are not possible during the machining since the cutting zone is visually inaccessible. In addition, the existence of an erosion-free zone adjacent to the flank wear land was observed. This indicated that the coolant jets did not reach this location. Instead, the high temperature occurring at the tool surface during machining led to vaporization of the coolant and the formed vapor-barrier prevented the coolant to reach closer to the cutting zone. After the conducted machining tests, the worn flank faces were covered by workpiece material. Removal of the adhered material allowed to characterize the underlying tool microstructure. However, fragments of workpiece precipitates were still found on the worn tool surface. Because of longer contact times and larger thermal loads in the tertiary shearing zone in connection with increasing flank wear, the morphology of the fragments changed to a more smeared-out appearance.