Engineered Surfaces for Exceptional Performance
Engineered Surfaces for Exceptional Performance

Article Index

Heat Source

Gas fuel* and oxygen flame (*commonly propane or acetylene).

Material

Wire or powder (depending on gun).

Transfer

Compressed air (wire flame).

Process

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. For wire flame spray, the material is melted and the compressed air, passing through a spray nozzle atomises the molten metal and sprays it onto the work piece. The larger the wire diameter, the higher the spray rate. 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. For powder flame spray, the powder particles (metal or ceramic) are softened in the flame and the speed of the flame gases through the nozzle sprays the softened powder onto the work piece. 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.

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/powder feed into the flame and the coating is 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. 

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. 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.