Porous tungsten is used to manufacture dispenser cathodes due to it refractory properties. Surface porosity is critical to functional performance of dispenser cathodes because it allows for an impregnated ceramic compound to migrate to the emitting surface, lowering its work function. Likewise, surface roughness is important because it is necessary to ensure uniform wetting of the molten impregnate during high temperature service. Current industry practice to achieve surface roughness and surface porosity requirements involves the use of a plastic infiltrant during machining. After machining, the infiltrant is baked and the cathode pellet is impregnated. In this context, cryogenic machining is investigated as a substitutionary process for the current plastic infiltration process. Along with significant reductions in cycle time and resource use, surface quality of cryogenically machined un-infiltrated (as-sintered) porous tungsten has been shown to significantly outperform dry machining. The present study is focused on examining the relationship between machining parameters and cooling condition on the as-machined surface integrity of porous tungsten. The effects of cryogenic pre-cooling, rake angle, cutting speed, depth of cut and feed are all taken into consideration with respect to machining-induced surface morphology. Cermet and Polycrystalline diamond (PCD) cutting tools are used to develop high performance cryogenic machining of porous tungsten. Dry and pre-heated machining were investigated as a means to allow for ductile mode machining, yet severe tool-wear and undesirable smearing limited the feasibility of these approaches. By using modified PCD cutting tools, high speed machining of porous tungsten at cutting speeds up to 400 m/min is achieved for the first time. Beyond a critical speed, brittle fracture and built-up edge are eliminated as the result of a brittle to ductile transition. A model of critical chip thickness (hc) effects based on cutting force, temperature and surface roughness data is developed and used to study the deformation mechanisms of porous tungsten under different machining conditions. It is found that when hmax = hc, ductile mode machining of otherwise highly brittle porous tungsten is possible. The value of hc is approximately the same as the average ligament size of the 80% density porous tungsten workpiece.

The thesis deals with plasma overlay welding of cobalt alloys. In the theoretical part of the thesis, suitable cobalt alloys for plasma overlay welding were described, out of which cobalt alloy Stellite 21, was chosen for overlay welding for particular part. Subsequently, the plasma overlay welding machine PPC 250 PTM and the torch PHP 250 S, used in the practical part, were described. In the practical part were debug overlay welding parameters and suggest the suitable overlay welding procedure for particular part.

Powder metallurgy (PM) steel is produced by near net shape manufacturing, which is used to fabricate alloy steels for many purposes. Designing new powder metal steels that can form a significant fraction of martensite relies on hardenability calculations developed for wrought steels. These proven tools are built upon assumptions for wrought steels that do not hold true for PM steels. One assumption is that the alloying elements are homogenized throughout the material. In admixed powder blends that are industrially sintered this is not the case. Using prealloyed powder is a solution to this issue, yet it places restrictions on alloy design and compressibility. There are tools available to computationally optimize diffusion problems, yet the complexity during the sintering of PM steel is such that a robust model has yet been produced. It is intuitive that with smaller particles of Fe sintering time can be reduced. A direct experimental investigation linking Fe-powders’ sizes and hardenability on Fe-C-Cr-Mn-Mo-Ni PM steel was subject to microstructure analysis and mechanical properties (Jominy test) for comparative analysis. Another assumption that is made for wrought steel is a consistent density of 7.87g/cm3. This is not the case for PM steel as the press and sinter method produces pores, decreasing the density. This directly affects the thermal conductivity and phase transformation of the steel. In an effort to understand how these differences affect Grossmann’s predictions of hardenability, a direct experimental investigation linking the density to hardenability was launched on prealloyed FL-4605 and FL-4605+2%Cu. Specifically the Jominy test was completed on a range of densities, as well as compared to software predictions. The chemical variations in admixed and sintered PM steel produce a unique system where one TTT diagram cannot predict the entire final microstructure. PM steel such as this is observed in industry, and can be created through incorporating larger Fe-particles such that less alloying constituents have a chance to fully alloy these regions. Since the large particles will not have the chance to be alloyed, they will not have the ability to form martensite. Since the regions between large particles will be alloyed, martensite will form, creating a hard matrix surrounding softer particles. This structure is characteristic of a metal matrix composite (MMC), and therefore should be treated as such. There are methods of MMC design that involve numerical methods of predicting strength and toughness. These methods, along with experimental data (tensile and Charpy testing) of Fe-C-Cr-Mn-Mo-Ni PM steels with ranging volume fractions of pearlitic inclusions were compared. / Thesis / Master of Applied Science (MASc)