Metal spraying (also commonly called thermal spraying) is a coating process in which metals or ceramics are melted or softened and sprayed onto an object. There are four commonly used processes in thermal spraying; Flamespray, Arcspray, Plasma Spray and High Velocity Oxygen Fuel (HVOF). In this piece, we will take a back-to-basics look into the technical background and workings of the wire flamespray process.
In the Metallisation Wire Flamespray process, the raw material in the form of a single wire or cord, is fed by a driven roller system into the centre of an oxygen-gas flame where it is melted. An annular air nozzle then applies a jet of high-pressure air, which atomises and projects the molten material towards the work piece. The molten spray solidifies instantly on the component surface to form a dense, strongly adherent coating that has no drying or curing time. The driving of the wire is typically via an air motor and gearbox that forms part of the pistol.
The gas fuel used varies, depending on the wire to be sprayed and in some cases, the application. The two most common gas fuels used are Propane and Acetylene.
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.
Acetylene fuelled systems are also used to spray ceramic cords and rods. The principle is the same, but instead of wire, the spray material is in the form of a flexible cord available on a plastic reel or a solid ceramic rod.
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.
Typical applications can be broken down into two different categories – anti-corrosion and engineering coatings.
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 (see video), in-situ pipework in petro-chemical 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.
Metallisation offer a full range of wire flame spray systems. More details can be seen by following the links below or visiting our products pages:
MK61 oxy-acetylene system - typically used for engineering spraying.
MK66E-M semi-automatic flame spray system for tube mills, engineering coatings etc.
MK66E-PC fully automatic flame spray system for roboticly automated processes.
In 2010, Metallisation launched a new deflected Flamespray extension to fit its MK73 and MK61 Flamespray systems, as a direct result of customer requests for a system that could apply good quality coatings in difficult access areas. The new extension uses the same consumable spares as standard Metallisation pistols. More details can be found here and a video can be viewed here.
A more detailed article can be downloaded as a pdf and for more information on Flamespray projects, processes or equipment, please contact Stuart Milton, Sales and Marketing Manager, Metallisation on +44 1384 252 464.