This study investigates isothermal transformation kinetics in Ti-6Al-4V alloys structures produced by vacuum plasma spray forming. As-sprayed samples were homogenized in the β phase followed by fast cooling to the two-phase temperature region, then quenching to suppress further transformation. The microstructure of heat-treated specimens was examined by optical microscopy and equilibrium phases were measured using image analysis. The kinetics of the β → α+β phase transformation are revealed by plotting the amount of α-phase obtained over a 10 to 60 s interval at isothermal temperatures of 800, 850, 900, and 950 °C. Corresponding phase transformation rates are also calculated based on Johnson-Mehl-Avrami (JMA) theory. At temperatures below 900 °C, the main phase transformation mechanism is homogeneous nucleation and growth of α-phase. At higher temperatures, phase transformations are driven by two mechanisms: the formation of α-phase in grain boundaries and α-plate nucleation and growth.

Open pore foams can be used as gas filters, catalyst supports, and heat exchangers due to their high gas permeability and heat conductivity. In this study, Ni-Cr skins were deposited on each side of a foam sheet by HVOF spraying to form a sandwich structure for use as a heat exchanger. The microstructure of the skins and the interface with the nickel foam struts were examined and the hydraulic characteristics and heat transfer properties of the sandwich structure were experimentally determined. Pressure drops across the heat exchanger were measured and found to be proportional to the square of the velocity of the coolant and a least square fit was used to solve for the permeability, K, and form coefficient, C, of the foam.

Nickel-based superalloys can be used at temperatures up to 1050 °C in air. Superalloy open cell foam sheets with skin layers plasma sprayed on both sides can be used as high temperature heat exchangers provided that the two deposited skins are dense and well adhered to the open cell foam. In this study alloy 625 skins were deposited on each side of a sheet of metal foam by APS and HVOF to form a sandwich structure. Two densities of open cell foams, 20 and 10 pores per linear inch (ppi), were used in this study as the core. The initial Ni foam was converted to an alloy composition by plasma spraying aluminum and chromium on the foam’s struts with subsequent diffusion/solutionizing heat treatments before the alloy 625 skins were deposited. The microstructure of the coatings and the interface between the struts and skins was investigated. A layer of Ni-Al alloy was formed near the surface of the struts as a result of the heat treatment. The foam struts were imbedded more deeply into the coatings deposited by HVOF than the coatings deposited by APS.

The current study has been focused on the final morphology of atmospheric plasma sprayed 8% yttria stabilized zirconia single splats. Single splats of two different sizes (-25 µm and +25/-45 µm) of ZrO 2 8Y 2 O 3 powder have been collected on polished stainless steel substrates at three different temperatures (Room, 300°C, and 600°C). The splat morphology and diameter, satellite particles, and splashing behavior were investigated using both scanning electron microscopy and image analysis software. The splat/substrate interface and splat curl up were studied from cross-sections prepared by focused ion beam milling. Results showed primarily pancake morphology and no evidence of delamination along the splat/substrate interface at 300oC substrate temperature and 100 mm spray distance. Overlapped splats showed evidence of melting (microwelding) at the splat boundaries. Splat thickness was measured to be less than 1 µm for all spray conditions. Roughness profiles of the surface of the deposited splats indicated microcracks had formed within the splats. Image analysis results exhibited a higher volume fraction of the splats relative to satellite particles at longer spray distance and higher substrate temperature. The average splat diameter increased as the substrate temperature increased.

The structure of thermally sprayed deposit consists of individual lamellae formed from melted and re-solidified particles, along with unmelted and partially melted particles, pores, microcracks, and splat boundaries. The elastic modulus of a vacuum plasma sprayed Ti-6Al-4V alloy parallel to the splat plane determined by standard uniaxial tensile testing was found to be approximately 30% lower than that of conventionally processed materials with the same level of porosity. The relationship between the elastic modulus and the microstructure was studied using an in-situ tensile testing stage in an optical microscope combined with analytical and finite element models. An idealized microstructure was used for the analytical model, which yielded an estimate of the modulus higher than that measured. The finite-element program OOF was also used to compute the elastic modulus based on micrographs of polished and etched surfaces and predicted a reduction of about 37% in the modulus.

Boron carbide has been successfully deposited on Ti alloy by vacuum plasma spraying (VPS). Mechanical properties of the deposited structure were assessed by micro-hardness and nano-hardness indentation. Chemical and phase compositions of the starting powder and the as-sprayed structure were characterized using hot gas extraction (LECO), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and Raman spectroscopy. The microstructure consisted of equiaxed boron carbide grains, microcrystalline boron carbide particles, and amorphous carbon regions at the grain boundaries. The amount of boron oxide and amorphous carbon increased during spraying. Carbon segregation to grain boundaries in the as-deposited B 4 C was observed. The measured micro-hardness was slightly higher than values previously reported (1033 ± 2009 HV). There was significant variation of nano-hardness from point to point in the material due to the existence of multiple phases, splat boundaries, and porosity in the deposited structure.

The growing need for new materials and material combinations with superior properties for severe service applications has led to the development of near net-shape forming techniques for certain materials, such as superalloys, refractory metals and highly reactive metals, including Titanium and its alloys. In this work, the vacuum plasma spraying process (VPS) was optimized to prepare dense spray-formed components from high purity plasma atomized Ti-6Al-4V powder. VPS offers a unique environment for spraying reactive materials such as Ti- 6Al-4V as a controlled inert atmosphere is used during deposition of the material. Three particle size distributions of the powder were used to investigate the influence of the starting powder characteristics (size, chemical composition) on the resulting spray-formed material. Post-deposition heat-treatments were subsequently applied to modify the deposit structure in order to improve their mechanical properties. The as-deposited and post-treated specimens were characterized for their internal microstructure and mechanical properties. Results indicate that a combination of high purity starting powder, controlled environment, and tailored deposition and post-processing conditions can be used to produce dense spray-formed Ti-6Al-4V structures with properties comparable to those of cast and wrought materials. Yield strength in the order of 800 MPa, with ultimate tensile strength close to 900 MPa and elongation near 10% were measured for spray-formed and heat-treated Ti-6Al-4V specimens. The results of this investigation on vacuum plasma spray forming of Ti-6Al-4V are presented in a series of two papers. The first one (this one) focuses on the preparation of the spray-form components, and on the resulting mechanical characteristics. The second paper is dedicated to the detailed characterization of the internal microstructure of the as-sprayed and heat-treated deposits, and the correlation with the measured mechanical properties.

Titanium alloys are extremely reactive in the molten state, therefore near-net shape and complex shape components cannot be produced by many techniques used for steel, aluminum and other less reactive metals. The vacuum plasma spray (VPS) process is a well-known method for coating and forming reactive metals and alloys such as titanium alloys. In this study the internal microstructure of individual splats, porosity, and mechanical properties of Ti-6Al-4V deposited by vacuum plasma spray forming (VPSF) were studied using SEM, XRD, mercury porosimetry, image analysis and mechanical testing. Results described in a companion paper show that while tensile and yield strength rise with increasing initial powders size, elongation still remains as low as 1%. The as-sprayed structure consists of α’ martensite with a small amount of residual β between the martensite colonies as well as pores between the splats. The fracture surface within the splats is indicative of ductile failure. The low cohesion between the splats results in damage accumulation at the boundaries and failure at small elongations. Post deposition heat treatments were conducted to improve the coating structure and were successful in improving the ductility to levels approaching that of traditionally processed material.