A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 1 4 0 iTSSe TSS iTSSe TSS NEXT GENERATION PLASMA SPRAY TORCH Innovations built into a new plasma spray torch address shortcomings of previous axial injection equipment. Ash Kamble Northwest Mettech Corp., Vancouver, BC, Canada The Axial III, launched in 1994, is a patented plasma spray torch with axial injection of feedstock. In comparison to the traditional radial injection, axial injection technology has proven to offer greater deposition efficiency, throughput, and cost savings on numerous applications[1]. Despite these benefits, the original Axial III M600 torch had several limitations due to its size, mass, and number of components. To address these shortcomings, a smaller, lighter, and more ergonomic version of the torchwas developed, called the Axial III Plus. This article highlights the innovations in this new torch and the potential applications it opens for the plasma spraying industry. RADIAL VS. AXIAL INJECTION Conventional plasma spraying equipment uses a radial injection design whereby feedstock is injected radially into the plasma plume. Due to the size distribution of the feedstock, particles have different momentums as they enter the plasma stream, in turn leading to different trajectories[2]. The larger and heavier particles tend to pass through the plumewith little deflection toward the substrate whereas the smaller and lighter particles may be unable to penetrate the plume and simply bounce off. The result is that only a portion of the injected particles, being of an optimumsize, are successfully carried by the plasma plume toward the substrate. The axial injection technology enables the injection of feedstock along the axis of the plasma plume. This overcomes many of the issues posed by radial injection because more particles are completely entrained by the plume, leading to a more uniform particle heating and a much less dispersed trajectory. The major benefits that stem from this are [1]: • Higher deposition efficiencies • Higher deposition rates • Better reproducibility of coatings • Denser, lower porosity coatings • More homogeneous coatings • Less overspray • Lower defect rate • More effective spraying of nanoparticles in suspension LIMITATIONS OF AXIAL INJECTION To enable axial injection, the current Axial III design uses three electrodes symmetrically positioned around a central feed passage. This configuration inadvertently increases the size of the torch, thereby limiting the size and geometry of substrates that can be coated. A larger torch also means it is heavier and thus takes more effort to handle. Furthermore, having three electrodes instead of one increases the number of components in the torch, making assembly and disassembly more difficult and time-consuming. INNOVATIONS IN THE NEW TORCH In the 15 years of service of the original Axial III M600, stakeholders in industrial and research sectors have assessed its performance and identified a list of potential upgrades for the torch. The Axial III Plus was thus developed, incorporating those upgrades. Fig. 1 shows the two torches side-by-side. Improvements to the torch include: • 90ᵒ cable connections to reduce the overall length of the torch. This not only enables the spraying of smaller internal bores, but also allows easy access to the fasteners for assembly and disassembly of the torch. • Optimized part features and dimensions to reduce the overall mass of the torch. • Larger gas jet lines to reduce back pressure. • Integrated features to reduce the total number of parts. • Multi-start threaded nozzle cap and threaded cathodes for quick and easy fastening. • Pre-aligned suspension injector assembly to eliminate the need for manual alignment. • Modified anode retainer design and tolerances for better electrode alignment. • Refined electrical cable contacts for manual assembly and disassembly, greater longevity, and more secure electrical connection. Fig. 1 — Axial III Plus (left) and Axial III M600 (right). 7 8 FEATURE
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