What is Spray Welding?
Spray welding is a term used to classify several welding procedures in the form of thermal spraying. It is an industrial activity that involves atomizing and spraying a powder or wire onto a metal surface at a high velocity with compressed gas.
Spray welding includes the use of industrial plasma, flame, detonation guns, arc sprays, and high-velocity oxy-fuel. Due to the significant heat produced during spray welding, procedures and regulations must be followed carefully and consistently to prevent harm to individuals and the environment.
How does Spray Welding Work?
Thermal spray is a general term that represents multiple coating processes. The entire welding involves the use of 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.
Process and operations
The spray process is an umbrella name for multiple processes. All involve using a coating material in the form of a wire, rod, or powder that is melted by one of the following sources of energy.
The molten powder, wire, or rod is accelerated and propelled towards the substrate by gas or an atomization jet. The particles build up and coat the material.
- Flame and Arc Welding (also called TSA, TSZ, TWAS): uses fuel such as acetylene or an electric arc to create required heat.
- Plasma Transferred Arc Welding (PTA): uses ionized gas and powder to coat materials
- HVOF (high-velocity oxyfuel): uses pressurized gas combined with powder.
- Detonation Gun Spraying: mix of gas, oxygen, and powder is ignited in a gun barrel
- Cold Spray: Deformable particles are introduced to a supersonic preheated gas stream. Only plastic materials can be used.
The coating can be applied at different thicknesses. It is used to repair worn components and machine parts or to improve performance and promote longer component life. Components frequently last 50% to 75% longer when treated.
Different Types of Spray Welding Techniques
1. 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
2. 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 weldings, 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
3. High-Velocity Oxyfuel (HVOF)
The HVOF process combines gas (hydrogen, oxygen, propylene, air, kerosene), which is injected using high pressure into the torch’s combustion chamber.
Gas achieves supersonic speeds while at the same time powder is injected into the flame. The process provides dense thermal spray coatings with less than 1% porosity.
The result has high bond strength and fine as-sprayed surface finishes. Oxide levels are also low. The process is used for spraying wear-resistant carbides and alloys (wear or corrosion resistant) such as Iconel, Triballoy, and Hastelloy. The spraying distance is 380 – 400mm.
The process has high levels of adhesion and low porosity (less than 1%). It supports thicker coatings and has a higher amount of retained carbides when compared to plasma or flame spraying).
It is relatively noisy (greater than 130 dB) with a low deposition rate (35% – 50%). The equipment also tends to be higher in price.
- 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
4. Plasma Spraying Process (PTA)
The plasma spray process was developed to spray ceramics, although plastics and metals can be treated. The process can be automated and requires fewer steps than other spray welding processes.
The plasma spray welding process has the greatest amount of versatility. Here, gas is used (hydrogen, helium, nitrogen, argon) with an electric arc ionizing the gas.
The process operates at over 10,000C, which is hotter than the melting point for metals. The powder is injected into the flame, melted, and moved to the material being sprayed.
The advantages of the plasma transferred arc welding process include that it is easy to apply. It has bigger size of cermet particles. It has higher wear resistance, low or no porosity, thick coatings, and low heating of the substrate when compared to GTAW.
Disadvantages include high oxidation of sprayed material, and it is impossible to obtain thin coatings of 1mm or thinner.
- 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
5. 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
6. Cold Spray Process
The cold spray process uses deformable particles that are introduced to a supersonic preheated gas stream. The stream is directed onto the substrate. The coating is deposited by an impaction process.
There is no heating of particles (the gas is heated to achieve a higher sonic flow speed). Only plastic materials can be used.
Unlike other methods, the particles are not heated at all; instead, the gas is the one that gets burned. This action achieves a swifter flow speed. The process uses low temperatures of 100-500°C. This is done to the gas stream, which exits through the nozzle.
- 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 when spraying.
Advantages of Spray Welding
- Smooth Weld Bead
- High Penetration (used on metal 3/16″ or greater)
- High Weld Deposit rates
- Minimal Spatter
- Reduced cost: spray is used to strengthen a lower cost material
- Low heat input: coatings do not penetrate the base material
- Versatile: Most metals, plastics, and ceramics can be sprayed
- Works within a broad thickness range: .001 to .1 inches, can be more than 1 inch thick
- Fast processing speed: spray goes on from 3 to 60 lb/hour (depends on the process being used)
Disadvantages Of Spray Welding
- Requires welder training
- Gas Cost can be greater due to higher argon levels (> 85%)
- Recommended for flat position and horizontal fillets only
- High Heat can cause welder discomfort
- Undercut can be caused, especially on the top edge of welds
- Bonding of coating is mechanical, not metallurgical
- Line of sight process
- Poor resistance of coatings to pinpoint loading