Flame spray
The flame spraying method is a type of thermal spray coating process, whereby, the material to be sprayed is used in two ways, either in powder or a wire form.
Concept
First, the material to be sprayed melts in flames of oxyacetylene (natural flame) then it is sprayed onto the surface of the base metal by blowing it through a stream of compressed air.
So that it spreads around, interlocks with projections, is contained in the pits, and cools and overlaps to form a coherent structure, and joins with each other. Spraying material is usually applied in layers less than 0.25 mm thick.
Equipment used in spraying equipment
The equipment used in flame spraying system can be divided into two parts based on their processes, such as
- Wire spray system and
- Powder spray system.
Wire flame spraying equipment
A wire flame spraying process includes the following equipment and supplies.
- Wire-type spray gun
The gun resembles an oxy-fuel gas torch with a hole for the wire through the center of its tip. The gun also has the arrangement to drive a pair of rolls to feed the wire into the flame. An air cap surrounds the tip of the gun and directs an explosion of compressed air to pick up the molten metal and project it completely.
- Wire reel and straightener
- Fuel (acetylene) gas cylinder and regulator.
- Flow meters for both compressed air and acetylene cylinders.
- Hose
- Flame arrestors.
Roll of Compressed Air in a wire flame spraying process
- The compressed air forms an air envelope around the outside of the flame and the liquid metal, it atomizes the molten wire metal that has been melted by the oxyacetylene flame.
- Compressed air cools the substrate, thus maintaining its temperature below 205 degrees centigrade.
Compressed air accelerates the molten metal particles on its path to the base metal and thus deposits them there.
Typical data for Wire flame spraying | Pressure of | Zinc | 25kg/hr |
| Weight of a manually operated gun | 1.35 to 2.75 kg | Lead | 45kg/hr |
| Weight of a machine-mounted gun | up to 4.50 kg | Acetylene | 0.7kh/cm² |
| Wire electrode size | up to 4.75 mm | Oxygen | 1.12kg.cm² |
| Deposition rate for steel | 8 kg/hr | Air | 2.45kg/cm² |
Advantages of Wire type flame spraying system
- A wire-type metal spraying system is probably the most commonly used metalizing process employed today. Wire metalizing is a fast and low-cost method of applying a wide variety of metal coatings, such as carbon steel, stainless steel, brass, and bronze, over a grit-blasted or nickel aluminize-undercoated substrate. Such coatings are often used for shaft buildup and general restoration of worn or mismatched parts.
Powder flame spraying equipment
A powder flame spray system includes the following equipment and supplies:
- Powder spray gun
- Cylinder, pressure regulator, and flow meter.
- Dry compressed air supply with pressure control and flow meter.
- Powder container.
- Hosepipe for oxygen, acetylene, and air
Concept
The metal powder is carried by compressed air from the powder container to the spray gun where it feeds through the air from the powder container orifice into the nozzle and from there into the oxyacetylene flame. As it enters the flame, the powder is melted and the resulting molten globules are blown onto the work surface.
Advantages and limitations of Powder flame spraying
- This process has the potential to be applied to a large variety of metal, ceramic, and cermet blending materials.
- Compared with wire flame spraying equipment, powder spraying equipment is simpler and lighter in weight.
One problem experienced in the powder spray process is the difficulty of maintaining a steady feed of powder. Also, the deposition capacity is low.
See More– Electric arc spraying
