WIRE AND POWDER COMBUSTION

Combustion thermal spray includes coatings produced using powder or wire as a feedstock.

Wire Combustion Process
The wire combustion process uses a wire — either solid or cored — and continuously drives the wire into a combustion flame — usually oxygen/acetylene. The wire may be driven into the flame using either an electric motor or an air turbine which transmits power to drive rollers through a geared transmission. Compressed air is used to propel the molten droplets onto the surface to be coated. The coating is built up layer by layer until the desired coating thickness has been achieved. Coating feedstock is limited to materials that can be drawn into wire form. A very limited number of cored wires are also available, but development of new materials for combustion wire spraying stopped long ago.

Characteristics
Today's combustion wire applications are frequently driven by aging specifications. Combustion wire coatings tend to be less well-bonded and more porous than coatings produced with other processes. Bond coats are often used — special layers applied directly on the workpiece to improve the bond. The coatings are quite economical and can be sprayed quite thick producing very serviceable coatings that last for many years. Even with bond coats, combustion wire coatings are still less well bonded than coatings applied with other thermal spray processes. The arc spray process has overtaken many of the applications that were once performed using combustion wire.

Materials
Combustion wire coatings are normally applied using oxygen/acetylene gasses, but for lower melting point materials, Mapp and other relatively lower BTU gasses may be used. The coatings available for combustion wire include virtually any material that can be drawn into a wire.

Commonly used combustion wire materials:

Steel Stainless Steel Copper Aluminum Bronze

Zinc Brass Molybdenum Babbitt

Silver Gold Platinum

Characteristics
The combustion powder process produces coatings that are well bonded, fairly dense and serviceable. Coatings can be applied quite thick because of the relatively low stress involved in the production of these coatings.

Materials
Coating materials available for the combustion powder process include self-fluxing alloys, most of the pure metals, many alloys, carbides, ceramics and a class of coatings called self-bonding Self-bonding materials exhibit high bond strengths and excellent machinability or grindability. Properly applied, they can be machined to a feather edge

Spray and Fuse
Spray and fuse is terminology used to describe coatings applied with the combustion powder process that are subsequently heated to about 2000 F. and melted to the substrate material.  These coating are typically nickel or cobalt based and form hard carbide and boride phases during processing - thus are considered hard facing applications.  Spray and fuse as a process has several advantages over conventional weld overlay hard facing techniques.  The component being sprayed is heated all at once during the fusing operation instead of being heated in a localized area as happens in welding - therefore distortion of the component is minimized.  Since the coating thickness can be controlled to a much better extent than is possible in weld overlay, it is possible to use a minimum of material, and since that material is fairly smooth after fusing, machining/grinding time can be minimized.  Coatings applied using the spray and fuse process are 100% dense and show a diffusion layer with the substrate.  They are specified and used where there is high potential for corrosion or corrosion combined with severe wear. Therefore they are often used in such machine elements as pump sleeves and valves. They may also be used in impact applications such as hammers.