Fig. 2 — Before and after repair using thermal spray: A — The pump sleeve before repair; B — the pump sleeve after spray with chrome oxide and grind to finish. Even though we are keeping the temperature below 300°F, grease and oil will weep out during thermal spray and cause bonding issues. Surface Profile The bonding of standard thermal spray processes is a mechanical bond. The molten droplets of the material being sprayed hit the surface being coated and “splat.” The molten material quenches rapidly and attaches to the surface being sprayed as it solidifies. If it is a smooth surface, there is not much for it to attach to. If there are jagged peaks and valleys, the molten material will surround those peaks and grip it. A raised angular profile of 1 to 3 mils is optimal for the bond strength of a thermal-sprayed coating. This is referred to as a “white metal finish.” It can be achieved using 24- to 36-grit aluminum oxide or steel grit as the blast media. Other alternatives to grit blast are knurling and cutting threads. However, the bond strength is inferior to that from grit blast. Figure 2A, B shows an example of a pump shaft repair. This part had significant damage to the surface; it was machined undersized down to a uniform surface, grit blasted with 24-grit aluminum oxide, and sprayed. The application required an inert and hard surface. A chrome oxide was selected and a plasma spray process was used to spray the ceramic. The final finish was achieved using aluminum oxide grinding wheels. Conclusion Thermal spray is an important, often misunderstood technology that is used in many industries today. And even though it shares many of the same technologies used in welding, it is not a welding process. As with all special methods, it has certain niches where it is the best process to choose from. 58 WELDING JOURNAL / JULY 2016 WJ ROBERT MCDEMUS (bob@thermalspraydepot.com) is the founder of Thermal Spray Depot, Washington, Pa. For info, go to aws.org/adindex A B
Welding Journal | July 2016
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