Among thermal projection techniques, flame spray is the most versatile and cost efficient. It allows the projection a large variety of chemical formulas as long as it comes in wire or powder form.
The technique uses a chemical reaction between oxygen and a fuel gas (acetylene, hydrogen) to provide a heat source. This thermal source creates the flame. If the feeding material is a wire, the flame melts it before spraying it with compressed air.
Flame spraying with powder is simple: particles are injected in the torch, where they become in a plastic state, and are then sprayed with compressed air on the part.
Nonetheless, this technique is to be used only with materials that have a low fusion temperature because of the relatively low heat production of the gun. Projection speeds of flame spray are slower than other equipment, this results in coating that are more porous and less adhesive to substrates.
This process consists in creating an arc between two metallic wires used as electrodes. A compressed air jet oriented towards the point of initiation of the arc pulverizes the melted metal on the substrate.
One of the advantages of this technique is that two different kinds of wire can be used simultaneously to achieve a very specific alloy. Coatings obtained with this technique are more dense and provide a better adherence than those from flame spraying due to a greater velocity of the particles.
High Velocity Oxy-Fuel
HVOF is a thermal projection process in which a supersonic flame supplies the needed energy to bring the particles to a semi melted state. The particles are propelled by the combustion of a gas (propane, propylene, hydrogen, acetylene, natural gas) or liquid (kerosene) and oxygen. The gases burned in a combustion chamber are accelerated to a velocity greater than the speed of sound (Mach 3) at the nozzle exit. Temperatures and accelerations attained by the flame allow the projection of the particles on the substrate such that the coating has a greater density compared to other techniques.
It results in an excellent adherence and low porosity of the coating. The relatively low temperature (3000℃) is advantageous for certain materials, such as carbides which experience less decarburization and metallic powders which are les oxidized. The coating thus obtained as excellent mechanical properties: high density, strong binding, great wear resistance and corrosion resistance.
The high velocity air fuel process, commonly known as HVAF, is a thermal spraying technique used for the deposition of high performance coatings of cemented carbides and metallic powders. We use it to protect parts against abrasion, erosion and corrosion.
It is a hot spraying technique that is colder than HVOF but hotter than cold spray. The HVAF injects powder axially in an air-fuel jet at a mean temperature of 1900 to 2500℃. It operates with a combustible gas and compressed air (from a compressor at 400 CFM / 125 PSI). Consequently, this process can apply efficiently carbide based materials. Tungsten carbide is a material of choice applied by HVAF to provide wear resistance.
HVAF prevents overheating of powder material which can be seen in the hot jets of conventional thermal spraying techniques (such as Plasma or HVOF). Thereby, particles are mainly solid when they hit the substrate, slowing down oxygen diffusion. This process enables metal application with almost no oxidation, as in cold spray. All common powder projection materials can be applied with HVAF except for ceramics.
HVAF can apply metals, carbides and other alloys of metallic powders at a thickness between 5 and 53 microns. Deposition efficiency is between 50% to 75% for carbide powders, and from 60% to 85% for metallic powders.
The plasma technology is made of an anode and a cathode both water cooled. An electric arc is initiated with high frequency and maintained with a low tension electric current.
Gas molecules (argon or nitrogen with hydrogen or helium) are dissociated and ionize to form an electric arc. As gases go through the anode, they expand. This expansion blocks the anode, forcing gases to exit towards the cathode.
Accelerated and fused, particles are projected on the substrate to be coated with a high kinetic energy, achieving optimal adherence.
Plasma is a process for materials with a high fusion point. It provides a very high quality coating.
|Characteristics||Powder Flame SPRAY||WIRE FLAME SPRAY||WIRE ARC SPRAY||H.V.O.F||H.V.A.F||Plasma Spray|
|Heat source||Flame||Flame||Electric arc||Flame||Flame||Plasma|
|Source temperature (deg.°C)||3000||3000||6000||3000||2500||12000|
|Particle Carrier||Gas Flame||Compresse d’air||Compresse d’air||Gas|
|Particle speed (m/s)||40||150||300||700||700||200|
|Filler Material State||Powder||Wire||Cored Wire||Powder||Powder||Powder|
|Filler Material Category||Metals|
A technique other than thermal projection is often used at Metallitech:
PTA (Plasma Transferred Arc)
We use this technique to melt a metallic coating on a substrate to increase its resistance to wear and/or corrosion.
During this process, powdered metal is introduced in a fusion bath generated by the high temperature plasma arc (up to 5000 ℃). This coating hardened by PTA is a real welding process with a high binding to the substrate.
In this process, two separate electric currents are used to establish first a non transferred arc (Pilot Arc) between the tungsten electrode (-) and the anode injector nozzle (+). Then an electric arc is transferred between the tungsten electrode (-) and the substrate (+).
The pilot arc is equipped with a high frequency device. The plasma gas around the cathode is ionized at the electrode tip.
When the transferred arc is ignited, the substrate becomes part of the electrical circuit and the plasma arc is concentrated through the aperture of the torch to the substrate. The powder is injected in the plasma to be projected in the fusion bath. It is possible to dope some existing alloys to meet specific needs.
The thickness of the deposit varies from 0.6 to 6.0 mm and its width from 3 to 10 mm when using one pass. With multiple passes, thickness of the deposit can reach 20 mm with a width of 30 mm.
One of the most important characteristic of the PTA process is dilution control. PTA produces a dilution as low as 5% compared to the usual 20-25% obtained with the MGAW (MIG) and GTAW (TIG) processes. It is therefore possible to maintain the noble properties of deposits even with only one run.