Spray welding includes multiple welding processes in thermal spraying. Spray welding is an industrial process where a powder or wire is atomized at high speed with compressed gases and sprayed onto a metal surface.
Spray welding applies industrial plasma, flame, detonation guns, arc spray, and high velocity oxyfuel. With spatter welding, the significant heat generated obliges procedures and protocol to be followed correctly and uniformly to avoid injury to mankind and the environment.
What is Spray Welding?
In spray or droplet welding, the weld wire does not short with the material (it does not contact). The plasma arc is so big that drops about the same diameter as the wire used are melted and transferred to the workpiece.
To achieve spray transfer you have to have the voltage up quite high as the spray is only possible with very high currents (for the size of wire).
| Wire Size UK | Wire Size US | Welding Current |
| 0.6mm | 0.0236″ | 135A |
| 0.8mm | 0.0315″ | 150A |
To achieve this, increase your voltage setting and tune in your wire until you hear the traditional “crackle.” Now back off the wire until you can no longer hear the crackle. The plasma is so high that every droplet is transferred to the weld pool, in a smooth stream.
If a voltage is set too low to transfer the droplets, the smooth transfer is not possible and instead becomes very erratic and has a “poppy” sound as the droplets fall to the material to be welded solely by gravity.
In general spray arcs create less spatter and higher run speeds if the material thickness to be welded accommodates it.
Related: What is Welding?
How does Spray Welding Work?
Thermal spray refers to a general term for a number of coating processes. The creaetic process fusing material or coating (for example, rod, powder or wire) using an energy source to melt the coating material.
Thermal spraying can simply be described as an industrial coating process that has a heat source, and a coating material which is melted into droplets, and is then projected at a very high speed. The spraying is directed at the substrate with an atomizing jet or with gas to propel it.
Thermal spraying is a very flexible process and is well documented for its effectiveness. It can offer a good alternative for numerous surface treatments, such as heat or nitride treatments, chrome and nickel plating, anodizing, and many others.
The thickness of the coating can be selected according to individual preferences. The coating can restore your worn components, and base components of machinery. It can enhance your components performance and longevity. If performed correctly, it can be potentially more than70% longer!
Different Types of Spray Welding Techniques
1. Spray Arc Welding
Spray arc welding is one of the processes of transferring molten material in the form of many small droplets smaller than the diameter of the filler wire. The arc stays stable and splatter-free because there are no short circuits.
For the spray arc to work properly, both the current and voltage values need to exceed certain limits. The workpiece receives more heat than with short-arc welding and spray arc welding is only suitable for materials with a thickness of 5 mm or more.
Due to the high heat input the weld pool is also large and hence welding needs to occur in a horizontal position. It should also be noted that CO2 cannot be used as the shielding gas or a pure spray arc cannot be achieved.
The shielding gas must be pure argon, possibly with a little CO2 (maximum 25%) or O2. Spray arc welding is used mainly for MIG welding of aluminum and stainless steel where Argon is the shielding gas.
It is possible to do spray arc welding with a thin filler wire at lower currents than with a thick filler wire.
The arc voltage should also be set just enough voltage to sustain a short-free arc. The filler wire is normally connected to the positive pole.
Advantages
Spray arc welding is a very efficient process. Key benefits of this process include:
- High deposition rate,
- Good fusion and penetration,
- good appearance of the weld,
- Ability to use larger diameter electrode wires and
- Presence of very little spatter.
Limitations
The limitations of spray-arc welding are:
- It is only used for material 1/8 inch (3mm) thick and thicker (handheld) and
- It is limited to flat and horizontal fillet weld positions
- Good fit-up is always required as there is no open root capability.
2. Flame Spraying Process
Flame spraying, or oxy/acetylene combustion spraying, is an age-old thermal spraying method developed some 100 years ago. The process uses the principle of a welding torch with high-velocity airstream to propel molten particles onto the substrate.
The coating material can be utilized in either powder form or wire form. Post-application of flame spray coatings are typically melted to increase adhesion and density of the coating.
Advantages
- High rates of deposition
- Low surface heating
- Versatile
- The process is simple and user-friendly
Disadvantages
- Relatively low adhesion
- Increased heating efficiency
- Not compatible with metals with melting points that exceed 2,800°C
3. High-velocity oxyfuel (HVOF)
The HVOF (high-velocity oxy-fuel) process utilizes a combustion reaction between oxygen and select combustible gases (propane, propylene, hydrogen). Although the HVOF system observes the principles of combustion, the design of the spray gun differs from the conventional oxy-fuel spray gun.
The design differences in the HVOF gun lead to increased flame temperatures and increased velocities. The increased flame and velocity provide a more thoroughly melted powder and increased kinetic energy to “flatten” the melted particles of the coating material. The end result is superior bond strength and coating density when using the HVOF process.
The HVOF method is most widely used for the deposition of hard metals and metal alloys with a high melting temperature, such as tungsten carbide, chromium carbide, etc.
Advantages
- Highly supports a thick coating
- Low porosity levels
- High adhesion levels
- More retained carbides as compared to flame spraying or plasma
Disadvantages
- 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 (non-transferred arc), employs inert gases that are continually fed past an electrode causing the gases to be induced into the “plasma” state. When those gases exit the nozzle of the gun apparatus and return to their normal state, significant levels of heat are released.
Controlled powdered coating material is inserted into the plasma “flame” and propelled on to the substrate.
Ceramic coatings are most frequently applied using plasma spray due to their high melting temperatures. (Often > 3500 oF). There are several varieties of ceramic coatings that can be applied using plasma spray.
Advantages
- 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
Disadvantages
- High oxidation on the sprayed material
- Difficult to get a thin layer of 1mm or less
5. Detonation Spraying
Detonation spraying generally refers to the cooling process of a substrate that undergoes a thermal spray with the substrate at supersonic spraying speeds to alter surface characteristics for improvements to service life of a component.
The actual term detonation spraying (spraying) was made known in 1955 by H.B. Sargent, R.M. Poorman, and H. Lamprey, and is applied to a component by a special unit called a detonation gun (D-gun).
It should be stated that before any spraying occurs to the component part, a considerable amount of surface preparation will have to be conducted to remove all contamination such as oils, greases and foreign debris, and the surface structure of the components must be roughened in order to form an extremely well bonded detonation spray coating.
Detonation spraying is the method which incorporates the highest velocity (shockwave of≈3500m/s that propels the coating materials) and temperature (≈4000 °C), process-wise of all the thermal spraying coating methods.
Hence, detonation spraying can produce low porous (below 1%) and low oxygen content (0.1-0.5%) protective coatings used to protect against corrosion, abrasion and adhesion – especially at low loading applications.
This process allows pathways for applying very hard and dense surface coatings which is very critical for wear resistant coatings; as detonation spraying is normally used in applications for protective coatings in aircraft engines, plug and ring gauges, cutting edges (skiving knives), tubular drills, rotor and stator blades, guide rails or any metal material subject to excessive wear.
In most applications, the materials injected onto a component are powders of metals, metal alloys, and cermets; and their oxides (aluminum, copper, iron, etc.) which are sprayed coats onto surfaces.
Detonation spraying is an industrial method of process which can present hazards, if preformed inappropriately or in an unsafe way.
Thus, following a few health and safety procedures for this thermal spraying method in usage for an industrial application, will help prevent the inherent unsafe elements that can occur during the application of the detonation spraying process.
6. Cold Spray Process
Cold spraying (CS) is a coating deposition process that utilizes solid powders 1 to 50 micrometers in diameter that are quickly accelerated with a supersonic gas jet, which can result in velocities of up to approximately 1200 m/s.
When the particles impinge on a substrate, they undergo plastic deformation due to the kinetic impact of the particles and adhere to the surface of the substrate.
A spray nozzle is manually or semi-manually scanned across the substrate to create uniform thickness. Cold sprays can also be used to deposit metals, polymers, ceramics, composites, and nanocrystalline powders
The kinetic energy of the particles is a result of the expansion of the gas and is converted to energy for plastic deformation during the bonding of the particles employing CS. Unlike thermal sprays (e.g., plasma, arc, flame, or high-velocity oxygen fuel (HVOF)) as the powders themselves are not melted during the spraying process.
Advantages of Spray Welding
- Effective and very penetrating for metal thicknesses over 3/16.
- Spray welding is smooth welding.
- Spray welding has high weld deposit rates making for improved productivity.
- Produces very little duff.
- Economical: it is less expensive material to melt and then spray weld and strengthen.
- Spray welding is versatile – can spray most metals, ceramics, and some plastics.
- Spray at various thicknesses.
- Speed of process – spray times of 3 to 60 lb/hour.
Disadvantages Of Spray Welding
- Needs specific training for welders to spray.
- Gas costs are costly (>85%) due to the high argon concentration.
- Heat is high and can become uncomfortable for the welders.
- The coating is mechanically bonded, not metallurgically.
- Only flat position and horizontal fillets are recommended.
- The risk of undercutting occurs especially at the weld edge.
- Coatings have a low resistance to pinpoint loading.
FAQs
What are the disadvantages of spray welding?
Although spray welding has many advantages, it also has some disadvantages. One of the main disadvantages is that it can be expensive compared to other welding methods. Additionally, the process can produce high levels of heat and noise, which can be dangerous if proper safety precautions are not taken.
What gas is used for spray welding?
Argon-heavy gases are ideal for welding in spray transfer. In contrast, helium-heavy gases are suitable for short circuits in stainless steel.
What is the difference between MIG and spray arc welding?
Spray transfer is a hotter weld, sounds different, and is best used only on thick metal for horizontal welds. Weldingtipsandtricks Description: Mig welding using short circuit transfer can handle sheet metal thickness all the way up to the beefy thick metal if done right.
What is the easiest welding for DIY?
MIG welding is the most beginner-friendly type of welding. It is a faster process that is easy to learn and forgiving of mistakes. TIG welding is a slower process that requires more time to master and produces more precise welds on a variety of materials types.
Is spray welding strong?
Moreover, spray welding produces exceptionally strong metallurgical bonds, ensuring the durability and longevity of the welds. Additionally, the low heat input characteristic of spray welding helps in reducing substrate distortion, preserving the integrity and precision of the components being welded.
Can you spray weld aluminum?
When MIG welding aluminum, the desired mode is spray transfer. This process is a very smooth transfer of molten metal droplets from the end of the electrode to the molten pool. The droplets crossing the arc are smaller in diameter than the electrode.