J. Schoop

Increased Surface Integrity in Porous Tungsten from Cryogenic Machining with Cermet Cutting Tool

In order to eliminate the process of backfilling porous tungsten with a plastic infiltrant during machining to prevent unwanted smearing of surface pores, cryogenic machining is investigated as a viable alternative. Porous tungsten is mainly used in the manufacture of dispenser cathodes where demands for surface quality and dimensional tolerances are extremely high. For these reasons, the ability of cryogenic machining to provide increased surface integrity and tool life compared to conventional dry machining is explored. Moreover, some preliminary results of machining with various cutting edge radii and effects on surface stress state are presented. Overall, cryogenic machining does provide significant surface quality and tool wear improvements over conventional dry machining practices.

Improved Product Quality and Resource Efficiency in Porous Tungsten Machining for Dispenser Cathode Application by Elimination of the Infiltration Process

Porous tungsten is a difficult-to-machine material commonly used in the manufacture of high-performance dispenser cathodes [1]. The functional performance of such cathodes is directly linked to the surface porosity of the machined porous tungsten workpiece. Conventional machining leads to smearing of surface pores [1]. Current industry practice involves the use of a plastic infiltrant that stabilizes the pores during machining [2]. In order to increase the sustainability of the dispenser cathode manufacturing process, we propose a materials-science driven approach to machining porous tungsten. Heated and cryogenic machining are compared to determine an effective method to increase surface porosity. The ductile/brittle transition of the body-centered-cubic (BCC) refractory metal tungsten is exploited to alter the cutting mode between shear cutting (heated machining) and controlled brittle fracture (cryogenic machining). We conclude that cryogenic machining via controlled brittle fracture using a fine-grained PCD tool is the most effective approach to infiltrantfree machining of porous tungsten for dispenser cathode applications.

Observations on cutting edge radius effects in cryogenic machining of porous tungsten

Porous tungsten is a refractory metal commonly used to manufacture dispenser cathode pellets. To produce the required geometric shape of a cathode, precision machining of the emitting surface is necessary. Because dry machining leads to excessive tool wear and smearing of surface pores, a plastic infiltrant is used to both prevent smearing and lubricate the cut. In order to develop a sustainable alternative to this plastic infiltration process, infiltrant-free cryogenic machining of porous tungsten has been identified by previous studies to be capable of producing excellent levels of surface porosity [1-3]. Cryogenic cooling during machining of porous tungsten allows for controlled brittle fracture machining, leading to relatively poor surface roughness [3]. Consequently, improving the surface quality of cryogenically machined porous tungsten surfaces is a necessary condition for the successful implementation of this technology.

High performance infiltrant-free cryogenic machining of 82% density porous tungsten under computer numerical control

Cryogenic machining was used as a means to eliminate the conventional methyl-methacrylate infiltration process used in the manufacture of dispenser cathode matrices. The resulting surfaces were analyzed before and after impregnation to determine their suitability for dispenser cathodes.