The effects of depth of cut and pre-cooling on surface porosity in cryogenic machining of porous tungsten

Cryogenic machining of porous tungsten is an effective method to achieve as-machined porous surfaces. Dispenser cathodes, which are high performance electron emitting devices, rely on the (surface) porosity of porous tungsten for their functionality. Conventional (dry, flood, MQL) machining results in unwanted smearing of surface pores and requires the use of a plastic infiltrant to stabilize pores during machining. Previous studies have shown the ability of PCD tools to achieve controlled micro-fracture which occurs only at low cutting speeds (vc~10-20 m/min) along with cryogenic cooling. This study investigates the influence of cryogenic pre-cooling time and depth of cut on the attainable surface morphology of porous tungsten. Negative rake cermet tool inserts were used to demonstrate the ability of non-diamond tools to achieve similar results. Three cryogenic pre-cooling times (60, 120 and 180s) are compared to establish their relative effectiveness in enabling the infiltrant-free cryogenic machining of porous tungsten with the objective of achieving maximum surface porosity. It is found that increased pre-cooling time increases the asmachined surface porosity and significantly alters the deformation mechanism during machining. Lastly, a qualitative relationship between the chips generated during the cryogenic machining and as-machined surface porosity is laid out.