What is Thermal Spray Welding?

Thermal spray welding is an umbrella term for a group of surface coating processes in which a feedstock material — supplied as a wire, rod, or powder — is melted or softened by a heat source and then propelled onto a prepared substrate by a compressed gas or atomization jet. The arriving particles flatten (splat), interlock, and solidify in successive layers to form a dense, adherent coating.

Unlike conventional fusion welding, thermal spray does not form a metallurgical bond with the parent material in most process variants. The bond is primarily mechanical, achieved through surface anchoring in a rough, blasted substrate. The exception is the fused spray-and-fuse variant, where a self-fluxing alloy powder is applied and then re-fused with an oxyacetylene torch or furnace to create a true metallurgical bond.

The six principal thermal spray processes are:

How Thermal Spray Works — The Core Mechanism

Regardless of process variant, all thermal spray systems share a common sequence: feedstock heating → particle acceleration → impaction → coating formation. Understanding each stage is key to controlling coating quality.

Feedstock Heating

The heat source can be a combustion flame, DC electric arc, plasma arc, or supersonic combustion chamber, depending on the process. In most processes, the feedstock is fully melted; in cold spray, only the carrier gas is heated and the particles remain below their melting point.

Particle Acceleration

Molten or semi-molten particles are entrained in a high-velocity gas stream. Particle velocity at impact ranges from approximately 50 m/s for flame spray up to 1000 m/s for cold spray. Higher impact velocity generally produces denser, stronger coatings.

Impaction and Splat Formation

When a particle strikes the substrate, it flattens into a thin disc (splat) and cools rapidly — often at rates exceeding 10⁶ °C/s. Successive splats interlock over the roughened substrate asperities to create the coating’s mechanical bond. Porosity arises when particles do not fully deform or when gases are trapped between splats.

Coating Buildup

Coating thickness is controlled by the number of passes, spray distance, and deposition rate. Thermal spray can produce coatings from 0.025 mm (25 µm) up to several millimetres in thickness, with the working range for most applications being 0.1 mm to 2.0 mm.

Arc Spray Process (TWAS / TSA)

Arc spray, also called Twin Wire Arc Spray (TWAS) or Thermal Spray Arc (TSA), is the most energy-efficient thermal spray process for metallic coatings. Two consumable wires of the coating material are fed through the spray gun and connected to opposite polarities of a DC power source. At the gun nozzle, the wires converge and create a sustained arc that melts the wire tips continuously.

Compressed air (or inert gas for oxidation-sensitive alloys) is then directed through the arc zone, atomising the molten metal into fine droplets and propelling them toward the substrate. The droplets interlock on the surface to form the coating.

Arc Spray — How the Arc Mode Works

Arc spray requires operation above the transition current level — the amperage at which the droplet transfer mode changes from globular to spray. Below this threshold, wire ends contact the substrate (short circuit transfer), producing coarse droplets and a rough, porous coating. At and above the transition level, the wire tip forms a conical point and very fine droplets are projected in a focused plume.

Arc Spray Equipment

• DC power source rated at 650 A minimum
• Positive and negative power lead connections (one per wire)
• Wire feed system with twin spool mounts (Zn, Zn/Al, Al, steel, stainless, bronze, etc.)
• Compressed air supply: dry, oil-free, at 4–6 bar
• Spray gun head with guide tubes and air cap
• Control unit for wire feed speed (ipm) and voltage

Arc Spray — Step-by-Step Process

1. Preheat the substrate surface (do not preheat aluminium, copper, titanium, or manganese alloys — oxide films form rapidly and impair adhesion; keep these at or below ambient).
2. Mount one wire to the positive terminal and one to the negative terminal of the DC supply.
3. Feed both wires simultaneously through the spray gun at equal wire-feed speeds.
4. The wires converge at the gun nozzle and strike an arc at approximately 25–35 V, generating temperatures around 6000 °C at the arc zone.
5. Compressed dry air is directed through the air cap, atomising the molten wire tips into fine droplets (100–300 µm).
6. Maintain a stand-off distance of 100–200 mm and traverse the gun perpendicularly to the surface at controlled speed for uniform layer buildup.

Typical Arc Spray Parameters

Flame Spraying Process

Flame spraying is the oldest and most straightforward thermal spray process. An oxyacetylene, oxypropane, or oxypropylene flame melts a feedstock wire, rod, or powder, which is then propelled onto the substrate by a compressed gas stream. Ignition takes place outside the torch nozzle, and the combustion front is open to the atmosphere.

Because of its portability, low equipment cost, and versatility for on-site repairs, flame spraying remains widely used for corrosion protection (zinc, aluminium), anti-friction coatings (bronze, babbitt), and dimensional restoration of worn shafts and bearing housings.

Flame Spray Fuels

• Acetylene — highest flame temperature (~3150 °C), used for metals with higher melting points
• Propane — lower cost, slightly cooler flame; suitable for zinc, aluminium, most bronzes
• Propylene — intermediate temperature and cost; good deposition rate for steel wire

Flame Spray — Wire vs. Powder

Wire flame spray feeds a continuous metallic wire through a rotating-collet torch head. The wire melts at the flame tip and is atomised by a surrounding air or nitrogen annulus. Wire flame spray achieves deposition rates of 3–12 kg/h and is the preferred route for large-area corrosion coatings.

Powder flame spray injects a powder feedstock (metals, alloys, ceramics, cermets) into the flame via a hopper and carrier gas. Powder processes unlock a wider material range but produce higher porosity (10–20%) and lower bond strength compared to wire processes.

Flame Spray Advantages & Limitations

High-Velocity Oxyfuel (HVOF)

HVOF is the process of choice for dense, high-strength wear coatings. A fuel gas (hydrogen, propylene, propane) or liquid fuel (kerosene) is combined with high-pressure oxygen and ignited in a confined combustion chamber. The expanding gases exit through a convergent-divergent nozzle and accelerate to supersonic velocities — typically 1500 to 2000 m/s at the nozzle exit. Powder feedstock is injected axially or radially into the jet, reaching particle velocities of 400–700 m/s at impact.

Because particle temperatures are lower than in plasma spray (combustion temperature ~3000 °C versus plasma >10 000 °C) yet impact velocity is much higher, HVOF produces coatings with very low porosity (<1%), high bond strength (70–100 MPa), and excellent carbide retention. It is particularly suited to tungsten carbide–cobalt (WC-Co), chromium carbide–nickel chromium (Cr₃C₂-NiCr), and high-performance nickel superalloys (Inconel, Hastelloy, Triballoy).

HVOF Equipment Components

• High-pressure oxygen and fuel gas supply (or liquid fuel pump for kerosene systems)
• Combustion chamber and convergent-divergent nozzle (Laval nozzle)
• Powder feeder with carrier gas
• Cooling water supply for the gun body (HVOF guns run hot)
• Acoustic enclosure (mandatory — noise >130 dB)
• Robotic or CNC traversing system for consistent stand-off and traverse speed

HVOF Process Characteristics

Common HVOF Coating Materials

• WC-Co / WC-CoCr — hardface wear coatings for pump shafts, roll surfaces, landing gear
• Cr₃C₂-NiCr — high-temperature oxidation and wear resistance up to 900 °C
• Inconel 625 / 718 — corrosion-resistant overlays in oil & gas
• Hastelloy C-276 — chemical corrosion resistance in aggressive media
• Triballoy T-400 / T-800 — galling resistance in valve and bearing applications

Plasma Transferred Arc (PTA) / Plasma Spraying

Plasma spraying uses a non-transferred or transferred DC arc to ionise a gas (typically argon, with additions of hydrogen, helium, or nitrogen) into a plasma state. The plasma flame reaches temperatures exceeding 10 000 °C — far above the melting point of any known engineering material — making it the only thermal spray process capable of depositing refractory ceramics, ultra-high-temperature compounds, and a broad range of functional coatings.

In Plasma Transferred Arc (PTA) welding — a related but distinct process — the arc is transferred to the substrate through the powder cloud, producing a metallurgical fusion bond rather than a mechanical bond. PTA is therefore closer to fusion welding than to pure coating, and is widely used for hardfacing applications in the oil & gas, mining, and agricultural equipment industries.

Plasma Spray Process Steps

1. Plasma gas (argon primary, hydrogen or helium secondary) flows through the torch between a tungsten cathode and copper nozzle anode.
2. A high-frequency spark initiates the arc; the arc ionises the plasma gas, forming a plasma jet at 6000–15 000 °C.
3. Powder feedstock is injected into the jet via a carrier gas (argon or nitrogen), where it melts within milliseconds.
4. Molten particles are accelerated to 200–400 m/s and deposited on the substrate.
5. For APS (Atmospheric Plasma Spray): coating forms in air — some oxidation is inevitable.
6. For VPS/LPPS (Vacuum/Low Pressure Plasma Spray): operation in a controlled chamber eliminates oxidation, used for MCrAlY bond coats in TBC systems.

Plasma Spray Advantages & Limitations

Detonation Gun Spraying (D-Gun)

Detonation gun spraying is a ballistic process in which a measured charge of powder, oxygen, and acetylene is loaded into a long barrel (approximately 25 mm bore, 1 m length) and then ignited. The controlled detonation — rather than a simple combustion — generates a shock wave that accelerates the powder charge to extraordinarily high velocities (750–1000 m/s), producing some of the densest thermal spray coatings achievable.

After each detonation, nitrogen purges the barrel before the next charge is loaded. The process fires 4–8 detonations per second, giving a controlled feed rate of 0.5 to 12 kg/h. Stand-off distance is kept short — 50 to 200 mm — to preserve particle velocity at impact.

D-Gun Process Parameters

D-Gun Advantages & Limitations

Cold Spray Process

Cold spray is fundamentally different from all other thermal spray processes: the feedstock is never melted. Instead, deformable powder particles (typically 1–50 µm in diameter) are introduced into a preheated supersonic gas jet — where the gas temperature may be 300–800 °C but the particles themselves remain below their melting point — and are accelerated to velocities of 500–1200 m/s through a converging-diverging (de Laval) nozzle.

When the particles strike the substrate, the kinetic energy causes extreme plastic deformation and adiabatic shear instability at the particle–substrate interface, generating enough heat to create a solid-state metallurgical-like bond. The process produces coatings with virtually zero oxidation, near-zero porosity, and compressive residual stresses — a significant advantage over thermally-deposited coatings, which tend to have tensile residual stresses and oxide inclusions.

Cold Spray Characteristics

• Only ductile metallic materials can be deposited (brittle ceramics will shatter on impact)
• Typical materials: copper, aluminium, titanium, nickel, tantalum, silver, gold, stellite
• Coating porosity: <0.5% (high-pressure systems)
• Bond strength: 30–80 MPa (substrate and particle dependent)
• No thermal distortion, no phase changes, no grain growth
• Gas: helium (high-pressure, highest velocity) or nitrogen (lower cost, lower velocity)

Substrate Preparation for Thermal Spray

Substrate preparation is arguably the most critical variable in thermal spray quality. Since the bond is mechanical in most processes, the substrate surface must be both chemically clean and physically rough to maximise coating adhesion. Inadequate preparation is the single most common cause of coating delamination in service.

Preparation Sequence

1. Inspection & machining: Restore substrate geometry if needed. For worn shafts, undercut to a uniform diameter below minimum, with a slight undercut radius at shoulders to prevent edge lifting.
2. Degreasing: Remove all oils, greases, and cutting fluids by solvent wipe (acetone, MEK), vapour degreasing, or ultrasonic cleaning in alkaline bath. Oils are the number one adhesion killer.
3. Baking (porous substrates): Porous or contaminated materials (castings, sintered parts) are baked at 315–345 °C to drive out absorbed hydrocarbons before blasting.
4. Abrasive grit blasting: Blast with angular aluminium oxide (Al₂O₃) grit — typically 16 to 30 mesh — at 4–6 bar to achieve a surface profile (Ra) of 6–12 µm. Chilled iron grit or steel shot produce rounded profiles and are not recommended for thermal spray.
5. Inspection of blasted surface: Confirm surface profile with profilometer or comparator disc. Surface should appear matte white-grey with no shiny spots (unblasted areas).
6. Spray within 2 hours: Begin thermal spraying within 2 hours of blasting. Re-contamination from handling or humidity will compromise adhesion; re-blast if delay exceeds this window.

Preparation Methods Summary

Thermal Spray Process Comparison Table

Industrial Applications of Thermal Spray

The breadth of thermal spray processes makes it applicable across virtually every heavy industry. Common application categories include:

• Corrosion protection: Arc-sprayed zinc, aluminium, and Zn/Al alloys on structural steel (bridges, offshore platforms, pipelines) replace galvanising and painting for extended service lives of 20–40 years.
• Wear resistance: HVOF WC-Co on pump shafts, roll surfaces, mandrels, and cutting tools, replacing hard chrome plating (RoHS-compliant).
• Thermal barrier coatings (TBC): APS or VPS yttria-stabilised zirconia (YSZ) on turbine blades and combustion chamber components, with MCrAlY bond coat, enabling turbine inlet temperatures above 1500 °C.
• Dimensional restoration: Flame or arc spray to rebuild worn bearing journals, pump housings, and cylinder bores to original dimension at a fraction of replacement cost.
• Electrical conductivity / resistivity: Copper arc spray for EMI shielding; alumina plasma spray for electrically isolating coatings on heater mandrels.
• Biomedical: Plasma-sprayed hydroxyapatite (HA) on orthopaedic implants to promote osseointegration.
• Additive repair (cold spray): Titanium and nickel alloy deposits on aerospace structural components — no HAZ, preserves fatigue life.

Advantages & Disadvantages of Thermal Spray

Equipment & Operating Requirements

General electrical and mechanical requirements that apply across thermal spray processes:

• Power source: High-voltage DC sources — typically 26 V to 37 V for arc spray; dedicated plasma and HVOF power packs for those processes. Small welding machines are incapable of sustaining the required spray arc mode.
• Shielding gas: Minimum 85% argon, remainder CO₂ where shielding is required. 92% Ar / 8% CO₂ (C8) is the most commonly specified mixture for spray-mode MIG. 100% argon or Ar/He blends are used for plasma.
• Machine duty cycle: Thermal spray is a continuous process; equipment must have an adequate duty cycle at the operating current. Exceeding duty cycle causes thermal shutdown and inconsistent coatings.
• Machine settings: Begin at the manufacturer’s suggested volts/ipm, then adjust voltage until the audio signature is half “crackle” and half “whoosh” — the characteristic sound of spray-mode transfer.
• Wire/electrode size: Typically 0.045″ (1.1 mm) or larger. Smaller wires cannot sustain the high amperages (180–440 A) required for stable spray arc operation.
• Noise control: HVOF (>130 dB) and detonation gun (>140 dB) processes require acoustic enclosures or sealed spray booths with interlocked access doors.
• Ventilation: Fume extraction is mandatory. Thermal spray generates metal fumes and fine particulate that must be captured at the source.

How to Select the Right Thermal Spray Process

Process selection depends on the required coating performance, available equipment, substrate geometry, and budget. Use the following decision logic:

• Large-area corrosion protection on structural steel or storage tanks → Arc Spray (Zn, Al, ZnAl). Fastest deposition, lowest cost, fully portable for site work.
• Wear-resistant carbide coating (<1% porosity required) → HVOF. Best carbide retention, highest bond strength for hard chrome replacement on pump shafts, rotors, and landing gear.
• Ceramic or thermal barrier coating → Plasma Spray (APS/VPS). Only process capable of depositing ceramics such as alumina, YSZ, and hydroxyapatite in production volumes.
• Ultra-high-performance wear or repair, no budget constraint → Detonation Gun. Maximum bond strength and density, but restricted to specialist facilities.
• Temperature-sensitive or oxidation-critical repair → Cold Spray. No heat input, zero oxidation; suitable for copper bus bars, titanium aerospace components, and electronics.
• Low-budget, on-site repair of worn shafts or bearing seats → Flame Spray. Self-fluxing Ni-Cr-B-Si powder spray-and-fuse provides a metallurgical bond for bearing journals and pump housings.