Thermal spray is a general term that represents multiple coating processes. The entire welding involves the use of a coating material, for instance, a rod, powder, or wire, which is melted by various sources of energy.
In simple terms, it can be defined as an industrial coating process consisting of a heat source and a coating material melted into droplets that are sprayed at a high velocity. The spraying is propelled towards a substrate by an atomization jet or gas.
Thermal spraying is quite a versatile process and is known to be highly efficient. It can be a good alternative for several surface treatments, which include heat or nitride treatment processes, chrome and nickel plating, anodizing, among other methods.
The coating thickness differs based on individual preferences. The coating repairs worn-out components and basic machine parts. It can also be applied to improve the performance and durability of the element. This can last up to 70% longer if well treated.
The following are different processes of thermal spray welding.
Arc Spray Welding
This process is also known as TSA, TSZ, or TWAS. The method uses DC power through a gun head to energize positive and negative wires. From the head, the wires arc against each other. As a result, they create the necessary heat required to form a molten metal.
For atomization, the molten metal and compressed air is introduced directly into the arc, splashing the droplets towards the material you are working on. Once the drops fall into the material, they interlock above each other, creating the bond or the weld.
For this to come out as expected, the following should be taken into consideration:
- Amperage above or at transition level, which is achieved by short circuiting, and arc spray
- Wires should form a funnel at the end of the electrode wire when in spray arc mode
- Transition being the change point of the weld pool needs to be accurate and precise
If correctly done, the droplets quickly form and are sprayed ideally on the surface of the weld puddle.
The three means of equipment used in this process are a power source of up to 650 amp, zinc or aluminum, negative, and positive power lead hookup.
- Ensure that the surface to be sprayed is preheated. This excludes copper, aluminum, manganese alloys, and titanium because when heated, they form an oxide of the metal. The best you can do is underheat them for efficiency.
- Energize the two wires as negative and positive, respectively.
- The wires should meet at the head of the gun to form an arc.
- Use dry and compressed air to atomize the material.
- To prevent an increase in porosity, spray perpendicular towards the surface.
- For your safety, you need to spray the surface at a distance of 100 or 200mm.
- Low surface heating
- Simplicity and high rates of deposition
- Denser and thick coating
- Serves best nonmetallic substrates
- High porosity
- Low heating efficiency
Flame Spraying Process
This is a spraying process that uses heat combusted from oxygen with fuel gas to come up with a clean-looking, quality surface. This happens when a coating material is propelled onto a substrate.
This method of welding is an excellent option for surfaces that can barely withhold extreme stress.
The process uses various gases as fuel, which include propane, propylene, and acetylene.
- A stream of gas is created from a chemical reaction between oxygen and the combustion of fuel.
- The material to be sprayed is then subjected to a flame to heat it up.
- The use of compressed air atomizes the molted particles before they are forwarded to a substrate.
- In case powder sprays are used, they are softened by the flame prior to coating by the gasses as they accelerate towards the nozzle.
Just like all other spray welding, this process is among the environment-friendly methods and is less demanding. The metals have higher porosity, lower bond strength, and higher oxide levels.
Combined fuel and oxygen generate a flame which will be used to melt down the mixture. This method of welding is popular for all low-intensity operations.
- High rates of deposition
- Low surface heating
- The process is simple and user-friendly
- Relatively low adhesion
- Increased heating efficiency
- Not compatible with metals with melting points that exceeds 2,800°C
High-Velocity Oxyfuel (HVOF)
This thermal spray welding works by combining gases like oxygen, hydrogen, air, propylene, and kerosene. The mixture is injected by high pressure in the combustion chamber. This passage earns the gas a high speed, and the powder is injected directly into the flame. One advantage of this process is the production of less than a single percentage of porosity during coating.
The method is known to be conservative as far as the environment is concerned. This process offers a variety of materials with less impact on the environment as compared to other spray processes like conventional planting.
As earlier stated, this welding tactic is environmentally friendly. Therefore, there are various readily available materials that can be used in this process effectively. They include ceramics, plastics, alloys, composites, and certain metals.
- Combine the gasses to use. To produce a gas stream, you need to mix, followed by the ignition of fuel and oxygen. This should be done in a combustion chamber that accelerates high-pressure gas through a nozzle.
- Heat the material to be covered by the coating.
- Using gas steam accelerates the HVOF coating on the surface of the component and improves efficiency in order to attain the best properties.
- Introduce powder to the stream. Here, it is heated then accelerated towards the surface of a component.
When correctly done, the result will consist of overlapping thin platelets.
This process is commonly used to spray alloys and carbides that are known to be wear resistance like Triballoy, Hastelloy, and Inconel. The spraying distance of this process should be 380 to 400mm. In addition to providing thin layers, the coating adheres perfectly and is strong enough to keep the connection intact.
- Highly supports thick coating
- Low porosity levels
- High adhesion levels
- More retained carbides as compared to flame spraying or plasma
- Relatively loud with a noise level of up to 130 dB
- Low deposition rate
- Slightly expensive
Plasma Spraying Process (PTA)
This kind of spray welding was developed purposely to spray ceramics. After different trials, it was also found to be able to treat metals and plastics. It is said to be the most effortless process as compared to other welding processes because it uses fewer and more precise steps than the rest.
Like the arc spraying process, the plasma method is also versatile since it uses varieties of gasses from helium, hydrogen argon, and nitrogen. An electric arc is used to ionize the gas.
- Use a powdered alloy and a constricted arc
- The mixture is then released to the plasma torch past feeder in the company of argon gas
- In the third stage, the powder is directed into the effluent arc from the torch
- In the influent arc, the powder is melted later fused to the intended fuse metal.
From one pass, 5% to 20% substrate dilution is deposited. The deposit is said to weaken the coating hardness at a range of 5RC. A minimum of two passes is the most preferred.
The plasma spraying process operates with high degrees of over 10,000°C. This temperature is higher than the melting point of most metals. The method uses plasma torch as the principal tool used for spraying and heating of the instrument.
- Easy to apply
- Cermet particles are bigger in size
- Wear resistance
- Very low or zero porosity
- Thick coating
- Low substrate heating as compared to GTAW
- High oxidation on the sprayed material
- Difficult to get a thin layer of 1mm or less
Detonation Gun Spraying
A detonation gun is a device used in depositing ceramic coating and material varieties into a workpiece at high speed by controlling the detonation of acetylene and oxygen. Detonation gun spraying is known to provide wear resistance, microstructure, and hard coatings.
This process is highly preferable when mechanical properties and extraordinary wear is needed. The end result of this method is an outstanding strength bond and density.
Some of the typical applications required for this process include knife seals, which generate power, fan blades to be used in aviation engines, turbine brads, steel rollers, and extruders.
- A mixture of powder, gas, and oxygen is ignited inside a gun barrel.
- By the use of nitrogen, the gun barrel is purged between detonations.
- The feed rate page from 0.5 to 12 kilograms per hour.
- The spraying distance should be maintained from 50 to 200mm.
The process is considered to have the highest velocity, which gives extreme adhesive strength. The coated surface contains impressive residual stress. It is used largely in applications like abrasion coating and also used to prevent corrosion.
- High adhesion
- Low porosity of less than one percent
- High feed rate ranging from 12 kg per hour
- High carbides retention as compared to flame and plasma spraying
- Hard to use on low-density materials
- Noise level of up to 140 dB
- Requires sealed boxes
Cold Spray Process
This kind of process works by introducing deformable particles to a supersonic heated stream of gas. The flow containing deformable particles is later directed into a substrate. The coating formed is deposited through a process of impaction.
Unlike other methods, the particles are not heated at all; instead, the gas is the one that gets burned. This action achieves a more swift flow speed. The process uses low temperatures of 100-500°C. This is done to the gas stream, which exists through the nozzle.
This process mostly relies on enough energy able to cause plastic deformation towards the substrate and the particle. Metals such as aluminum, copper, stainless steel, alloys, and titanium limit a present coating.
The machine setting can profoundly affect the final weld result. The unit requires high voltages of 27-37 volts.
- Ensure you have all the chemicals then do the vapor degreasing.
- Baking of porous material follows.
- Once done, ultrasonic cleaning takes place accompanied by wet or dry abrasive blasting
- 2 hours of rest is emphasized before spraying. Dry grit blasting or macro roughening can be used.
- The deposit at the lowest temperature is then sprayed on the surface to be worked on, allowing the particles to rest on each other, giving out a uniform spray.
- Light melting of particles at low temperatures
- Slight oxidation
- Cold work microstructure causes tough hardness
- Low heat input
- Produces minimal spatter
- Versatile since it can use plastics, ceramics, and a variety of metals.
- Consumes high volumes of gas
- Hard ceramics must be sprayed by the use of ductile binders
There are so many ways through which this kind of process can be used. Some of these include protection against corrosion, solders replacement, and purity coating. For a perfect bonding, you have to use high verbosity on impact during when spraying.
Thermal spray welding processes are highly used in different industries. In a nutshell, all the coating consists of a molten powder and a wire. The liners are exposed to oxy- or plasma-type fuel combustion. The heat produced by the spraying device powers the heated mixture.
The mixture is then sprayed onto various surfaces compatible with the kind of process you are using, retaining a firm and consistent coating. As reviewed by this article, these thermal spray weldings have a broad spectrum of uses on various applications. These applications include buildings, airplane coating, and others.
The main purpose of welding through sprays is to protect the surface from extreme conditions like chemicals, temperature, and other external conditions like rain and humidity. Since there are 5 of these processes, you can use the methods you feel most comfortable with. They all provide excellent results if everything is done according to the instructions.