Flame spraying can be seen as a similar concept to paint spraying. Once the system is connected up, the sprayer operates a valve to start the gases flowing and lights the gas stream. A trigger is then used to start/stop the wire feed into the flame and the coating deposited in a similar way to spray painting. These systems are used to reclaim surfaces by applying a similar material or give the surface different properties by coating it with a different material. Most metals can have an aluminium coating applied by the flame spray process. This can be for aesthetic, anti-corrosion, conductivity or many other reasons. Zinc can also be applied to most substrates using the flame spray process. This is often to provide galvanic protection of the substrate, but may be for a number of other reasons. The gas fuel and oxygen are mixed and ignited to produce a flame. The material, either a wire or powder is fed into the flame.
With flame spraying, you are using the heat that is generated from the combustion of a mixture of oxygen and a fuel gas, commonly oxy/propane or oxy/acetylene. This gas mixture is ignited to produce a flame that heats the consumable, either a wire or powder. The molten material is atomised and sprayed to build up a coating layer. Propane gas is most commonly used for spraying low melting point materials such as zinc, aluminium and their alloys at high throughput rates. Propane can also be used to spray bronzes, coppers, Babbitt, nickel, tin/zinc and some steels, although optimum throughput rates may not be achieved with these materials. The Metallisation MK73 system uses propane gas.
Acetylene gas is most commonly used for spraying higher melting point materials such as varying grades of steel, nichrome and molybdenum. When spraying with acetylene, parameters can be easily changed that will give different coating properties. For example, molybdenum coatings can be applied as either a soft, strongly adherent bond coating or as a harder top coating, just by changing the ratio’s of gas to oxygen. Similarly, some decorative coatings of copper and bronzes can have their final colour appearance influenced by the spray parameters. As with propane gas, acetylene gas systems can also spray the lower melting point materials of zinc, aluminium and their alloys, but again, not necessarily at their optimum conditions. The Metallisation MK61 system uses acetylene gas.
As a general principle, the throughput rate of the spray system is linked to the wire diameter, for example, a 1.6mm wire will spray considerably slower than a 4.76mm wire.
Anti-corrosion coatings are applied to generally protect steelwork from a corrosive atmosphere with the most commonly sprayed materials being zinc, aluminium and their alloys. Common application examples include the spraying of steel bridges, in-situ pipework in petrochemical refineries, street furniture and vehicle chassis’.
Engineering coatings cover a much wider range of applications where the coating could, for example, provide a wear protection surface, a thermal barrier coating or an electrically conductive path. Common applications include spraying steels to build up worn or mis-machined areas on a variety of components, conductive heater elements on carbon fibre wing edges and hard molybdenum coatings on selector forks.
Flame spray systems are commonly manually operated but it is possible to semi-automate or fully-automate the process if required. Capital costs of flame spraying are typically lower than arc spraying but the running costs are typically higher. The amount of material that can be sprayed by the flame spray process is limited by the size of wire and the material being sprayed.