Thermal Spray Wire
Wire is the preferred form of feed material for many coating operations. The reasons include lower material costs and ease of handling. Also, wire operations are often characterized by high deposition rates and high deposit efficiencies. The two common applications for thermal spray wire is the combustion wire process and the wire arc spray process. The arc spraying process by far uses more total pounds of wire per year when compared to combustion wire.
Applications are limited to coating materials that can be formed into a malleable wire. The manufacturing process of wire is what limits what materials you can make in thermal spray wire. The initial process could include forging or hot rolling to an intermediate size. The final processing requires cold drawing using hardened dies. It may also include heat treating operations to remove cold work from the drawing process. If the material is too hard or too brittle to go through a cold drawing process, it will not be available as a thermal spray wire.
Wires are categorized by “Base” material, the metal that is predominant in the wire. Wires for both electric arc and combustion spray include bases of aluminum, cobalt, copper, iron, molybdenum, nickel, tin and zinc. Titanium wire is available for electric-arc spraying but not combustion arc spraying since titanium has a higher melting point (over 3000o F) then is available with combustion spraying. Lead wire is available for combustion spraying but not electric-arc spraying due to the poor electrical conductivity of lead (caution needs to be used when spraying lead. Lead was used in the manufacture of hats and is the basis for the expression “mad as a hatter”).
A key factor is the wire size. In other industries, wire sizes are typically expressed as “gauge”. However, there are a number of conflicting definitions of “gauge” (American Wire Gauge [AWG], Standard Wire Gauge [SWG] (British), Birmingham Wire Gauge [BWG], US Standard for Stainless Steel, etc.). AWG is used for non-ferrous wire and SWG is used for ferrous wire. Since it is essential to have a close match between the thermal spray gun set-up and the wire size, it is recommended that wire size be specified in mm or inches as opposed to gauge.
The following table has the common sizes used in Thermal Spray:
| Gauge | Inches | Millimeters |
|---|---|---|
| 8 | .125 | 3.2 |
| 11 | .090 | 2.3 |
| 12 | .079 | 2 |
| 14 | .062 | 1.6 |
| 15 | .057 | 1.4 |
Wire feed is critical for any wire based thermal spray operation. A single wire feed is used for flame spray systems while twin wires are typically used for electric-arc spray systems The use of generic wire manufactured for other industries can cause a lot of grief. Standard industrial wire may have surface scaling which can lead to wire feed jams, excessive wear of feed components and lead to inclusions in the final coating. Also, other industries may not be concerned with providing wire rolls free of kinks, weld joints or other features that could lead to feed problems. Some wires have lubrication added to the surface during the final spooling process. A “level winding” process is used during the spooling process, which is a method of coiling the wire on the spool so it is wound one level layer at a time.
In setting up a wire feed system, adequate room is needed to allow for free wire feeding without binding. This is especially important for wire guns that use a “pull” drive only (some systems have both a push and pull drive that are synchronized). Including a wire-straightener system in the feed is also beneficial for a smooth, trouble-free thermal spray operation. When wire is Level wound, it comes off the spool without getting tangled.
In addition to solid wire, there is a family of hollow wires filled with powder. These are referred to as “cored” wires in that they have a hollow core filled with powder. This takes advantage of the ease of use of a wire system with the ability to add materials, such as a powder, that is not easily made into a solid wire. One example is a wire that we add Tungsten Carbide and Titanium Dioxide (P-Met 297), resulting in a material that has over 30% carbide in the coating after it is sprayed. A wire with that much carbide would be too hard to complete the final wire drawing process and until relatively recently, was not available as a wire.