What Is Spray Welding?- Process, And Techniques

What is Spray Welding?

Spray welding refers to several welding processes in the form of thermal spraying. It is an industrial activity in which a powder or wire is atomized at high speed with compressed gas and sprayed onto a metal surface.

Spray welding involves the use of industrial plasma, flame, detonation guns, arc spray, and high-velocity oxyfuel. Due to the significant heat generated by spatter welding, procedures and regulations must be followed carefully and consistently to avoid harm to people and the environment.

Related: What is Welding?

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, 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.

Related: What is Thermal Spray?

Spray welding is a term used to classify several welding procedures in the form of thermal spraying.

Different Types of Spray Welding Techniques

1. Spray Arc Welding

Spray arc welding is one of the processes used to transfer metal from the electrode or wire to the weld. Tiny droplets of molten metal travel through the arc and onto the base metal or joint being worked.

Spray Transfer is ideal for use on thicker metals for butt or fillet welds. It is not suitable for positioned welding work because the flying metal droplets and gravity do not create an ideal situation for the welder or the welding work.

Technically, spray arc welding is a spray of molten metal that is transferred across the arc, much like water escaping from a garden hose with a restricted orifice. Spray arc welding reduces weld spatter and creates a finer weld seam.

Process

The process involves high current and voltage levels. When the wire is brought close to the base metal, it generates a current before it touches the metal. The current heats the wire very quickly and melts it. The molten metal travels in the form of tiny droplets across the generated arc, giving it the name spray arc welding.

Spray Arc Welding can achieve high metal deposition rates when equipped with the ideal combination of shielding gas, metal, and wire gauge and contact-to-tip distance. With the perfect combination of everything, the process can generate very high transient currents. The process is also called axial spraying.

The current levels used in this process must be higher than the transient current. Only then will the metal be transferred in the form of drops, and not just melted. The process requires the use of high current levels with sufficient voltage levels to ensure a spatter-free weld.

Advantages

Spray arc welding is a very efficient process. Some of the significant benefits offered by this process include:

  • High metal deposition rates
  • Good metal fusion and penetration
  • Excellent weld bead appearance
  • Ability to use larger diameter electrode wires
  • Very little spatter generation

Despite the presence of such extensive benefits, the spray arc transfer method does have substantial limitations.

Limitations

The limitations of spray-arc transfer include, but are not limited to:

  • It is only suitable for use on thick materials (about 1/8 in. (3 mm) and thicker)
  • It is limited to flat and horizontal fillet weld positions
  • It does not have open root capabilities

2. Flame Spraying Process

Flame spraying, also known as oxy/acetylene combustion spraying, is the original thermal spraying technique developed about 100 years ago. It uses the basic principles of a welding torch with the addition of a high-velocity airflow to propel molten particles onto the substrate.

The coating material can be in either wire or powder form. Flame spray coatings are often melted after application to improve adhesion and coating density.

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 exceeds 2,800°C

3. High-Velocity Oxyfuel (HVOF)

The HVOF (High-Velocity Oxy-Fuel) process combusts oxygen and a select group of flammable gases including propane, propylene, or hydrogen. Although the HVOF system uses the basic principle of combustion, the spray gun is designed differently than the standard oxy-fuel spray gun.

The HVOF gun differences produce higher flame temperatures and higher velocities. The result is a more thoroughly melted powder and more kinetic energy available to “flatten” the melted particles of coating material. The HVOF process produces superior bond strength and coating density.

The HVOF process is most commonly used to deposit high melting temperature metals and metal alloys such as tungsten carbide, chromium carbide.

Advantages

  • Highly supports 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), uses inert gases fed past an electrode inducing the “plasma” state of the gases. When the gases exit the nozzle of the gun apparatus and return to their normal state, a tremendous amount of heat is released.

A powdered coating material is injected into the plasma “flame” and propelled onto the substrate.

Ceramic Coatings are most often applied using plasma spray due to their high melting temperatures. (Often > 3500 F). Several types of ceramic coatings 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 is one of the many forms of thermal spraying techniques that are used to apply a protective coating at supersonic velocities to a material in order to change its surface characteristics. This is primarily to improve the durability of a component.

It was first invented in 1955 by H.B. Sargent, R.M. Poorman, and H. Lamprey and is applied to a component using a specifically designed detonation gun (D-gun). The component being sprayed must be prepared correctly by removing all surface oils, greases, debris, and roughing up the surface in order to achieve a strongly bonded detonation spray coating.

This process involves the highest velocities (≈3500 m/s shockwave that propels the coating materials) and temperatures (≈4000 °C) of coating materials compared to all other forms of thermal spraying techniques.

This means detonation spraying is able to apply low porous (below 1%) and low oxygen content (between 0.1-0.5%) protective coatings that protect against corrosion, abrasion, and adhesion under low load.

This process allows the application of very hard and dense surface coatings which are useful as wear-resistant coatings. For this reason, detonation spraying is commonly used for protective coatings in aircraft engines, plug and ring gauges, cutting edges (skiving knives), tubular drills, rotor and stator blades, guide rails, or any other metallic material that is subject to high wear and tear.

Commonly the materials that are sprayed onto components during detonation spraying are powders of metals, metal alloys, and cermet’s; as well as their oxides (aluminum, copper, iron, etc.).

Detonation spraying is an industrial process that can be dangerous if not performed correctly and in a safe environment. As such there are many safety precautions that must be adhered to when using this thermal spraying technique.

6. Cold Spray Process

Cold spraying (CS) is a coating deposition method. Solid powders (1 to 50 micrometers in diameter) are accelerated in a supersonic gas jet to velocities up to ca. 1200 m/s. During the impact with the substrate, particles undergo plastic deformation and adhere to the surface.

To achieve a uniform thickness the spraying nozzle is scanned along the substrate. Metals, polymers, ceramics, composite materials, and nanocrystalline powders can be deposited using cold spraying.

The kinetic energy of the particles, supplied by the expansion of the gas, is converted to plastic deformation energy during bonding. Unlike thermal spraying techniques, e.g., plasma spraying, arc spraying, flame spraying, or high-velocity oxygen fuel (HVOF), the powders are not melted during the spraying process.

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

FAQs.

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.

What is Spray Arc Welding?

Spray arc welding is one of the processes used to transfer metal from the electrode or wire to the weld. Minute molten droplets of metal travel via the arc and onto the base metal or the joint being worked on. Spray Transfer is ideal for use on thicker metals for butt or fillet joints.

How do I set up a spray arc?

For higher production speeds use spray transfer. Greater than 80% argon mix set the voltage at 23-4 volts to begin. Set the amperage with about 300-400 inches of wire feed speed. Again increase/decrease wire feed speed until 150 amps are reached.

How thick can you spray weld?

Thermal spraying can provide thick coatings (approx. thickness range is 20 microns to several mm, depending on the process and feedstock), over a large area at a high deposition rate as compared to other coating processes such as electroplating, physical and chemical vapor deposition.

Is spray welding strong?

High-Velocity Oxyfuel Spray Welding. 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.