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4. Description of processes within the scope of this guide
4.1.1 A torch burning acetylene, propane, propylene or hydrogen with oxygen is used - see Figure 1. The spray consumable is in the form of a wire, cord or a powder. Wire or cord consumables are fed into the centre of the flame where the material is melted and propelled by the atomising gas towards the component. In powder spraying, the consumable is supplied to the gun in powdered form, and heated to the plastic or molten form in the gas flame. In some cases an additional gas jet is used to accelerate the powder particles.
Figure 1(a) Wire Flame Spray
Figure 1(b) Powder flame spray
Figure 1: Flame spray process
schematics (pictures courtesy Metallisation Ltd)
4.1.2 The process requires a flame spraying gun, gas delivery and storage. It is a process that can easily be transported to site, and is generally operated manually. However, the process can be programmed.
4.2 Arc Spraying
4.2.1 In the arc spray process, a DC electric arc is struck between two consumable wires, which are fed continuously into the gun. The molten material is atomised and propelled towards the component by a compressed air (or in very specialised applications, inert gas) jet, see Figure 2.
Figure 2: Arc Spray (Picture courtesy Metallisation Ltd)
4.2.2 The process requires an arc gun, power supply (usually constant voltage) with twin wire feed unit, and a compressed air or inert gas supply. It is readily transported to site applications, for example to spray bridges and offshore structures. It is also suited to mechanised application, where the gun and/or the component can be manipulated by a robot or turntable.
4.3.1 In plasma spraying, a DC electric arc is struck between two non-consumable electrodes, while a flow of inert gas is fed through the arc. This generates a stream of ionised gas which is heated and expanded and then accelerated by the torch nozzle so that it emerges as a high velocity plasma jet. Plasma gases are commonly argon, hydrogen, helium or nitrogen, or mixtures of these. Powder particles are fed in an inert gas into this plasma, where they are heated and propelled to the component - see Figure 3.
4.3.2 There are two distinct variants - vacuum or low pressure plasma spraying, which is conducted inside an evacuated chamber, and air plasma spraying which is carried out at normal pressures. Provision may be made to cool the component during spraying, using cryogenic gases. The gun itself requires water cooling.
Figure 3: Plasma Spray (Picture courtesy Metallisation Ltd)
4.3.3 The equipment needed includes a power supply (usually constant current), a plasma gun, powder feed unit, process control console, gas delivery system and a heat exchanger. Air plasma spray would generally be operated within an enclosed booth with fume and dust extraction to protect the operator from the noise, fume, dust and ultraviolet light. Mechanised, automated or robotic manipulation may be required to apply the coating, The reduced pressure variants would require a purpose built chamber and vacuum pumps.
4.4 High Velocity Oxy-fuel Spraying, HVOF
4.4.1 In this process, the gun has been designed to burn the oxy-fuel mixture in a combustion chamber. High gas flow rates and high pressures within the system lead to the generation of supersonic flames emerging from the gun - up to 2000 m s-1. Fuels include gases such as acetylene, hydrogen, propane, propylene and liquid fuels such as kerosene. The consumable is fed as a powder entrained in an inert gas into the gun, where it is heated and propelled by the flame towards the component, see Figure 4. The component may be cooled if required.
Figure 4: High Velocity Oxy-Fuel Spraying, HVOF (A Liquid Fuelled Example) (Picture courtesy Metallisation Ltd)
4.4.2 The equipment needed includes the spray gun and its cooling circuit and heat exchanger, gas flow control unit, gas delivery system, powder feed unit, and mechanised or robotic manipulation of the gun and/or component. The process is very noisy, and an enclosure with fume and dust extraction will normally be required.