result of the expanding gases from the torch. Other times, compressed gases are added to the process for atomization. In either case, the droplets are reduced to a smaller size and accelerated, thus atomized. Table 1 features a comparison of the common thermal spray processes. Where Thermal Spray is Useful Thermal spray is used to change the surface properties of components, including dimensional control, wear resistance, corrosion resistance, electrical properties, thermal barrier, and traction/antiskid. One Common Application Because the part being sprayed is normally kept below 300°F, thermal spray is extremely useful where keeping heat to a minimum is required. An example of a common application for thermal spray is repairing a damaged shaft. Smaller shafts can distort with applying heat sufficient enough to melt the base material. In this case, the repaired area does not require a metallurgical bond, but it requires a bond strong enough to withstand shear forces from postcoat machining. In addition to dimensional restoration, a material can be selected that improves the wear resistance. The basic thermal spray process for this type of repair includes machine undersize, surface cleaning, mask, surface texturing, thermal spray, clean, and finish machine. Surface Preparation When a shaft is damaged due to a spun bearing or a worn bearing seal area, the damaged material must be removed. This is typically done by machining the damaged area undersize deep enough to create a uniform diameter and clean up any damaged areas. All oil and grease on the part must be removed before spraying. Some common methods include degreasing with solvents, alkaline water-based washes, and baking in an oven. Extra care should be taken with castings to ensure there is no trapped grease or oil in the casting porosity. JULY 2016 / WELDING JOURNAL 57 Fig. 1 — Thermal spray processes melt, atomize, and spray molten droplets. Table 1 — Thermal Spray System Comparisons Combustion Powder • Melting energy is from fuel gas/oxygen. • Material source is powder, metals, alloys, and some ceramics. • Atomization is from compressed air, plus expanding combustion gases. • Low purchase cost and low operating cost • Useful entry-level technology Combustion Wire • Melting energy is from fuel gas/oxygen. • Material source is wire, metals, and alloys. • Atomization is from compressed air, plus expanding combustion gases. • Low purchase cost and low operating cost • Useful entry-level technology Twin Wire Electric Arc • Melting energy is electric arc, typically a DC welding power supply. • Material source is wire. • Atomization is from compressed air. • Medium purchase cost and low operating cost • Highest spray rates for thermal spray Plasma Spray • Melting energy is from ionized plasma gas. • Material source is powder, metals, alloys, carbides, and ceramic. • Atomization is from inert gases used in the process. • High purchase cost and high operating cost • Best for ceramics High-Velocity Oxygen Fuel • Melting energy is from fuel gas/oxygen and particle velocity. • Material source is powder, metals, alloys, and carbides. • Atomization is from compressed air, plus expanding combustion gases. • Medium purchase cost and high operating cost • Best for carbides Cold Spray • Melting energy is from particle velocity. • Material source is soft metal powders. • Atomization is from compressed gases. • High purchase cost and high operating cost • This is a relatively new and developing technology.
Welding Journal | July 2016
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