Thermal Spray — The Misunderstood Process Six thermal spray processes are detailed along with a common application Thermal spray is often a misunderstood technology. This process melts the feedstock, atomizes it, and sprays the molten material. Molten particles then hit the part being sprayed, splat, and solidify rapidly. On the flip side, welding melts the feedstock as well as the base material. In the past, “spray welding” was used to describe this process. For clarification, the industry has shifted away from using that term. Both thermal spray and welding are similar in that they melt the feedstock. The fundamental difference is thermal spray does not melt the base material whereas welding does. The sprayed material sticks because of a mechanical bond. This article not only details the difference between welding and thermal spray but also reviews a typical application where thermal spray is useful. Identifying Various Thermal Spray Processes Six common thermal spray processes are as follows: combustion powder, combustion wire, twin wire electric arc, plasma spray, high-velocity oxygen fuel (HVOF), and cold spray. All of these processes require three main components — an energy source, a material source, and an atomizing gas. Figure 1 illustrates how they work together. Three Main Components Energy Source Similar to welding, the energy in thermal spray processes can be from one of three sources — electric arc, combustion gases, and plasma arc. An additional source of energy not used in welding is the velocity of the feedstock material. In HVOF and cold spray processes, powder is accelerated to high velocities, and the final melting occurs when the particle impacts the surface being sprayed. The twin wire electric arc process and the plasma process both use large power supplies, similar to what you would use for welding. Manufacturers supply the DC power to melt the feedstock material. The combustion powder/wire and HVOF along with cold spray processes use propane, propylene, hydrogen, natural gas, or vaporized kerosene. Combined with oxygen in the correct ratio, a combustion flame is created that melts the feedstock material. In cold spray and HVOF, the powder particles are not fully melted until impact. Material Source All the materials in thermal spray are either a powder or a wire. The most common wire sizes are 1⁄16, 1⁄8, and 3⁄16 in. depending on the material, equipment selected, and end application. An additional source of material is available. Cored wire is a hollow wire filled with powder. If a material can be made into a powder or a wire, there is a thermal spray process that can be used to spray it. Thermal spray processes can build up at a rate of 0.0005 to 0.008 in. per pass depending on the material and process. The surface speed of the thermal spray torch over the part being sprayed is in the range of 150 to 300 ft/min. Thus, the time it takes to coat a part with thermal spray is often significantly faster than welding. Additionally, the surface temperature of the part being coated is kept below 300°F. Atomization In welding, the feedstock is melted from wire or powder, and the droplets are transferred to a melted base material using low-velocity techniques. In thermal spray, as the material is being melted, it is subjected to a relatively high pressure and velocity atomizing gas. Sometimes, this atomizing gas is a 56 WELDING JOURNAL / JULY 2016 BY ROBERT MCDEMUS Twin wire electric arc thermal spray is shown here on stainless steel.
Welding Journal | July 2016
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