Thermal Spray Forming

While the thermal spray technology described above originated primarily as a coating or surfacing process, with materials being deposited as thin (<250 ^m), nonload-bearing coatings for dimensional restoration, wear, corrosion, and thermal protection, it has also evolved, during the last 50 years, into a process capable of spray forming free-standing materials onto mandrels at thicknesses of >100 mm. Early applications included zirconia A-oxygen sensors and bulk ceramic materials for crucibles. Historically "difficult-to-form" materials such as the refractory metals tungsten, hafnium, molybdenum, and tantalum; superalloys; and intermetallics have now all been deposited with varying degrees of success, with the ultimate mechanical properties of the sprayed deposit being the main limitation. Process technology advances, such as HVOF, which can produce sprayed deposits with lower tensile stresses and degrees of thermal degradation than other processes and with increased deposit density, controlled-atmosphere plasma spray, and improved powder formulations and morphologies have all contributed to the successful development of thermal spray forming of monolithic and composite materials.

Thermally sprayed coating deposits are usually <1000 / m thick, more typically —400 /'m. Thermal spray forming, however, can yield deposit thicknesses in excess of 25 mm. Free-standing shapes can be produced by spraying onto sacrificial mandrels, which are mechanically or chemically removed after spraying.

The incremental nature of thermal spray processes, using particulate-based feedstocks, and with deposited layers typically —15 to 25 /,!m thick, enables graded and laminated structural materials to be formed. Also, because of the high processing temperatures and high localized forming energies (high particle kinetic energies at impact), traditionally difficult-to-form materials can be processed into near-net-shape components. Virtually all common and refractory metals, many intermetallics, ceramics, and combinations of these have been spray formed as "composite" materials. The properties (apart from the ductility of metals) of these deposited materials, after postdeposition heat treatment, have been reported to be close to, if not exceeding, those of cast or wrought materials. The ductility of metals is limited by contamination sources occurring either within the feedstock powders used, or in the spray process itself, due to oxidation of the material during heating under atmospheric conditions. Nickel-base superalloys and other heat-resistant alloys have all been thermally spray formed during the last 20 years, either as preforms or as a component repair technique. Plasma spray in low pressure inert/controlled atmosphere (VPS) chambers has been found to be most suitable for forming these alloys; however, recent investigations using HVOF have shown that this too may be a viable alternative to plasma spraying in controlled atmospheres, at least for some component repair applications. Despite the technical capability of thermal spray, however, economics and process/coating reliability concerns related to material structure, uniformity, and material properties still limit the use of thermal spray forming.

The current major advantages of thermal spray forming techniques are in the forming of refractory metals, both in graded or layered structures and in composites. Some of the more innovative spray forming application developments are described below, together with some insights into emerging technologies that may ultimately lead to expanded implementation of thermal spray forming as a materials processing technique.

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