Twin Wire Arc Spray
Process: Electric arc spraying is a thermal spraying process that uses an electric arc between two consumable electrodes of the surfacing materials as the heat source. A compressed gas atomizes and propels the molten material to the work-piece. The principle of this process is shown by figure. The two consumable electrode wires are fed by a wire feeder to bring them together at an angle of approximately 30 degrees and to maintain an arc between them. A compressed air jet is located behind and directly in line with the intersecting wires. The wires melt in the arc and the jet of air atomizes the melted metal and propels the fine molten particles to the work-piece. The power source for producing the arc is a direct-current constant-voltage welding machine. The wire feeder is similar to that used for gas metal arc welding except that it feeds two wires.
The gun can be hand held or mounted in a holding and movement mechanism. The part or the gun is moved with respect to the other to provide a coating surface on the part.
The high temperature of the arc melts the electrode wire faster and deposits particles having higher heat content and greater fluidity than the flame spraying process. The deposition rates are from 3 to 5 times greater and the bond strength is greater 5000-7000 PSI. There is coalescence in addition to the mechanical bond. The deposit is denser and coating strength is greater than when using flame spraying.
Dry compressed air is used for atomizing and propelling the molten metal. A pressure of 90-100 psi is used. Almost any metal that can be drawn into a wire can be sprayed. Following are metals that are arc sprayed: Ferrous and Non-Ferrous, aluminum, zinc, titanium, brass, bronze, copper, molybdenum, Inconel, monel, nickel, stainless steel, carbon steel, tin.
Features:
- Over-current protection
- Air driven pull system
- Air cooled or solid cables
- Easy to maintain onsite service
- Uses wires from 1.6-2.0mm dia. without changing feed rollers
- Significantly lower operating cost.
- Stable low and high deposition rates.
- Reduced substrate heating.
- Higher density coating through higher velocity.
- Significantly improved bond strengths.
- Thicker coating and more easily finished coatings than can be obtained by other metalizing method.
- Choice of coating textures
- Optional Arc beam spray concentrator
Application:
- Engineering coatings
- Reclaiming parts
- Bearing and seal surfaces
- Print rollers
- Anti-spark coatings
- Wear resistance coatings : Boiler tube, screen tube, burner wall etc..
- Large anti-corrosion applications for offshore industries components, construction, Railway Bridge, pipeline, spool, structures etc.
Comparison of three thermal spray processes:
| CRITERIA | TWAS | HVOF |
| Type of feedstock | Wire | Powder |
| Choice of material | Restricted to electrically conductive wires: metals, metal alloys, metallic composites | Metallic alloys, carbides and composites; Limitation for refractory materials |
| Versatility in the choice of material | Low | Medium |
| Reliability | Medium | High |
| Heat transfer to the substrate | Low | Very High |
| Process Cost | Low | Medium |
| Deposition Rate | 1 – 50 kg/hr | 1 – 10 kg/hr |
| Adhesion Strength | Low (~3000 PSI) | High (~10000 PSI) |
| Coating Porosity | 10% – 15% | 0.5% – 5% |
| Maximum Temperature | 3000ºC – 5500ºC | 2600ºC – 3100ºC |
| Particle Temperature | Low | Medium |
| Particle Velocity | 80 – 150 m/s | 550 – 1000 m/s |
| Particle Diameter | 30-50 μm | Powder (10-100 μm) |
| Surface Roughness | 2 – 10 μm | 1 – 4 μm |
| Onsite facility | Mostly applicable & Cost effectives | Applicable with suitable requirement |
| Use in Power Plant | BTC TPP (Thermal Power Plant) | Turbine Parts (Hydro Power) |