Thermal Spray Welding (Metalizing)

The thermal spray process is an umbrella term with regard to multiple processes. Thermal spraying techniques are coating processes wherein melted or heated materials are sprayed onto a surface. The coating precursor called "feedstock" is heated by electrical plasma or arc or chemical means of combustion flame. The molten powder, wire or rod is accelerated and propelled towards the substrate by way of gas or an atomisation jet. The particles build-up and coat the material.

Thermal spraying can provide thick coatings, spanning a large area at high deposition rate in comparison to other coating processes including electroplating, physical and chemical vapour deposition. Coating materials intended for thermal spraying comprise of metals, alloys, ceramics, plastics and composites. They are fed in powder or wire form, heated to a molten or semi-molten state and accelerated in the direction of substrates in the form of micrometer-size particles. Combustion or electrical arc discharge is typically made use of for the source of energy for thermal spraying. Resulting coatings are created from the accumulation of countless sprayed particles. The surface might not heat up substantially, making it possible for the coating of flammable substances.

Coating quality is typically evaluated by measuring its porosity, oxide content, macro and micro-hardness, bond strength and surface roughness. Typically, the coating quality improves along with increasing particle velocities. It can be employed to repair worn components and machine parts, or even improve performance and enhance longer component life. Components generally last 50% to 75% longer whenever treated.

Defining Thermal Spray

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Becoming familiar with thermal spray commences with comprehension of the essentials of solids. Solids may be considered as having three parts:

• The central body supplies the basic form or shape and the strength and rigidity.
• The surface is related to colour, texture, patina, and sheen.
• The barrier generates a layer between the outer surface and the main body.

Oftentimes, there's no distinction involving the three areas because they are typically of the same material. On the other hand, it is often essential to fabricate a component for which the body requires a coating or barrier with properties not present in the body material.

This is exactly where thermal spray becomes valuable.Thermal spray can provide a barrier against chemicals, extreme temperatures, abrasion, erosion, light, radiation, radar (for example with stealth aircraft), and stress associated with differential thermal expansion involving the surface and the body.

Furthermore, thermal spray is employed for repair, rebuild, and clearance control and also to alter the friction factor of the surface.

Thermal spray is a essential component of advanced fabricated products including prostheses and gas turbines. For the purpose of prostheses, thermal spray is utilized in order to apply the hydroxyapatite ceramic coating for bone bonding. With regard to gas turbines, thermal spray provides the thermal barrier coating designed for high-temperature operations and additionally for clearance control, both vital for the high performance of these systems.

The term thermal spray represents a family of processes which include a wide range of thermodynamic properties. In all scenarios, material is heated and sprayed on the part being coated, the substrate. A combustion process, whether an electric arc or a plasma process, supplies the heat. Material may be introduced in powder form or in the form of wire. In cases where a wire is used, it can be solid or it can be filled or cored with a powder.

Thermal spray comes with its own limitations. An essential consideration is that it must be a line-of-sight operation. As opposed to plating, this cannot coat deep bores, slots, or hidden areas.

Advantages & Disadvantages Of Spray Welding

Advantages:

• Smooth Weld Bead
• High Penetration (applied on metal 3/16″ or greater)
• High Weld Deposit rates
• Minimal Spatter
• Reduced cost: spray is utilized to strengthen a lesser cost material
• Low heat input: coatings never penetrate the base material
• Versatile: Most metals, plastics and ceramics can be sprayed
• Is effective within a wide thickness range: 0.001 to 0.1 inch, can be greater than 1 inch thick
• Fast processing speed: spray goes on from 3 to 60 lb/hour (varies according to the process being employed)

Disadvantages:

• Necessitates welder training
• Gas Cost can be higher owing to higher argon levels ( > 85%)
• Recommended for flat position and horizontal fillets only
• High Heat can result in welder discomfort
• Undercut can be caused, specifically on the top edge of welds
• Bonding of coating is mechanical, not metallurgical
• Line of sight process
• Poor resistance associated with coatings to pinpoint loading

Benefits of Thermal Spraying

• Comprehensive variety of coating materials: metals, alloys, ceramics, cermets, carbides, polymers and plastics
• Thick coatings can be applied at high deposition rates
• Thermal spray coatings are mechanically bonded to the substrate - could spray coating materials that happen to be metallurgically incompatible with the substrate
• Could spray coating materials which has a higher melting point compared to the substrate
• The majority of parts can be sprayed using little or no preheat or postheat treatment, and additionally component distortion is minimal
• Parts can be rebuilt rapidly and at low cost, and typically at a fraction of the price of a replacement
• Using a premium material for the thermal spray coating, the lifetime of new components can be extended
• Thermal spray coatings could possibly be applied both manually and mechanised.

Thermal spray coatings are extensively employed in the manufacturing of gas turbines, diesel engines, bearings, journals, pumps, compressors and oil field equipment, in addition to coating medical implants.

Thermal spraying is specially an alternative solution to arc welded coatings, eventhough it is in addition made use of as an alternative to additional surfacing processes, including electroplating, physical and chemical vapour deposition and ion implantation for engineering applications.

Benefits of thermal spraying as compared to other coating processes tend to be numerous, and include:

• Versatility with regards to the coating material, which can be metal, cermet, ceramic and polymer, in the form of powder, rod or wire. There is also a comprehensive selection of coating materials in order to satisfy the requirements associated with a wide variety of applications, specifically protection from wear and corrosion damage.
• Coatings of metal, cermet, ceramic and plastic can be applied to any sort of substrate that won't degrade from the heat of the impinging particles or gas jets.
• The coating is formed by means of minimal heating of the substrate and the coating doesn't need to fuse together with the substrate to form a bond. Substrate temperature rarely exceed 250°C. Consequently, coatings can be applied to components using little or no pre- or post-heat treatment and component distortion is minimal. The coatings can also be applied to thermally sensitive substrates which includes low melting point metals and plastics or composites.
• Thick coatings, characteristically close to 10mm, can be applied and frequently at high deposition rates. Consequently thermal spraying may also be used for component reclamation and spray forming. Parts can be rebuilt quickly and at a tiny proportion of the replacement price.
• Thermal spraying has the capacity to form barrier and functional coatings on a wide range of substrates.
• More recently, thermal spraying has become recognised as being a essential process for the synthesis of specialised coatings and materials. It provides the cabability to create free standing structures for net-shaped manufacture of high performance ceramics, composites and functional graded materials. It is additionally employed for the rapid-solidification synthesis of specialised materials. Thermal spraying is currently being taken into consideration for the synthesis of advanced functional surfaces which includes catalytic coatings, dielectrics, ferrites, bioactive materials and solid oxide fuel cells.

Thermal Spraying Processes

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Thermal spraying processes fall into two categories:

• lower energy
• higher energy

Lower Energy Processes

The lower energy processes, also called metallising processes, are arc and flame spraying. These processes are widely used for reclamation of worn or damaged components, and for depositing coatings of metals including aluminium and zinc alloys to protect steel structures from corrosion.

Coatings prepared using lower energy processes are quite porous and adhesion is lower as compared to that accomplished while using the higher energy techniques. The pores are frequently impregnated using a sealant or lubricant to enhance coating performance. Sealants are widely used in applications in which the surface is required to be resistant to corrosive environments.

With the lower energy processes of flame and arc spraying, adhesion to the substrate is considered largely mechanical and is dependent on the substrate surface being extremely clean and suitably rough. Roughening is accomplished by grit blasting and, sometimes, by machining.

Higher Energy Processes

The higher energy processes of plasma spraying, high velocity oxyfuel spraying and detonation spraying have been developed to produce coatings with much lower porosity and oxide levels, combined with greater adhesion to the substrate. This is accomplished to a certain extent since spray particles possess higher impact velocities, specifically with HVOF and D-gun coatings.

Surface preparation by cleaning and grit blasting is, on the other hand, still extremely important. The range of coating types which can be deposited by higher energy techniques is wider, and increases the range of applications to incorporate protective coatings meant for severe wear, high temperature oxidation and gaseous corrosion.

Types of Thermal Spraying

Several different types of thermal spraying can be distinguished:

1. Plasma Spraying: The plasma spray process (non-transferred arc), makes use of inert gases fed past an electrode inducing the “plasma” state of the gases. As soon as the gases leave the nozzle of the gun apparatus and return to their normal state, a tremendous amount of heat will be released. A powdered coating material is injected into the plasma “flame” and propelled onto the substrate.
2. Detonation Thermal Spraying: The detonation gun comprises of a long water-cooled barrel along with inlet valves designed for gases and powder. Oxygen and fuel (acetylene most frequently used) are fed into the barrel together with a charge of powder. A spark is utilized that will ignite the gas mixture, and then the resulting detonation heats and accelerates the powder to supersonic velocity through the barrel. A pulse of nitrogen is utilized to help purge the barrel subsequent to each and every detonation. This procedure is repeated many times a second. The high kinetic energy of the hot powder particles upon impact along with the substrate leads to a accumulation of a very dense and strong coating.
3. Electric Wire Arc Spraying: Electric wire arc thermal spraying uses the identical principles as employed in wire arc welding systems. The coating material that is in wire form, is electrically charged, and thereafter contacted producing an arc. The molten droplets of the metal wire are subsequently sprayed upon the substrate utilizing a high velocity air stream in order to atomise and propel the material. Arc spray coatings are extremely economical and therefore are typically employed to apply metals including pure aluminium, zinc, copper, and metal alloys which include stainless steel. Arc spray additionally enables modifications to accomplish varied coating texture.
4. Flame Spraying: Flame spray, also referred to as oxy/acetylene combustion spray is the original thermal spray technique. It makes use of the essential principles associated with a welding torch with the addition of a high velocity air stream to propel molten particles onto the substrate. The coating material can be either a wire or powder form. Typically flame spray coatings are fused subsequent to being applied to enhance bond strengths and coating density.
5. High Velocity Oxy-fuel Coating Spraying (HVOF): The High Velocity Oxy-fuel Coating Spraying or HVOF process combusts oxygen and another of select group of ignitable gases which include propane, propylene, or hydrogen. Even though the HVOF method makes use of the fundamental principle of combustion, the spray gun is designed in a different way ın comparison to the conventional oxy-fuel spray gun. The HVOF gun dissimilarities generate higher flame temperatures and higher velocities. The end result is a more thoroughly melted powder and more kinetic energy available to “flatten” the molten particles of coating material. The HVOF technique generates superior bond strength and coating density. The HVOF process is generally employed to apply high melting temperature metals and metal alloys.
6. High Velocity Air Fuel (HVAF): High Velocity Air Fuel or HVAF process is a newer technique wherein air is utilized as the oxidiser. It happens to be comparable to HVOF nevertheless distinct for the reason that the process is not self-sustaining in the gun; a separate ignition source is required to maintain the combustion. As a consequence of the reduced combustion temperature, coatings have a relatively reduced oxide content along with enhanced performance. Typical HVAF coating materials include, but are not restricted to; tungsten carbide, chrome carbide, stainless steel, Hastelloy, and Inconel. As a consequence of ductile characteristics, HVAF coatings can help resist cavitation damage.
7. Warm Spraying: Warm spraying can be described as innovative modification of high velocity oxy-fuel spraying, wherein the temperature of combustion gas is reduced by way of blending nitrogen together with the combustion gas, consequently bringing the process closer to the cold spraying process. The resulting gas possesses substantial water vapour, unreacted hydrocarbons along with oxygen, and therefore is dirtier compared to cold spraying. Nevertheless, the coating efficiency is actually higher. Alternatively, lower temperatures of warm spraying reduce melting and chemical reactions of the feed powder, in comparison to HVOF. These advantages are specifically a consideration for such coating materials as Ti, plastics, and metallic glasses, which rapidly oxidise or deteriorate at high temperatures.
8. Cold Spraying: In cold spraying, deformable particles are introduced into a supersonic preheated gas stream. The coating is deposited by means of an impaction process. There is no heating of particles, merely the gas is actually heated to attain an increased sonic flow speed. Only plastic materials can be used with this process. The deposition efficiency is usually very low for alloy powders, and the window of process parameters and suitable powder sizes is narrow. To accelerate powders to higher velocity, finer powders are utilized. It is possible to accelerate powder particles to much higher velocity utilizing a processing gas possessing high speed of sound.
9. Spray and Fuse: Spray and fuse makes use of high heat to enhance the bond between the thermal spray coating and the substrate of the part. It generates a metallurgical bond between the coating and the surface. Consequently rather then depending upon friction for coating adhesion, it melds the surface and coating material into one material. This process typically necessitates spraying a powdered material onto the component subsequently following with an acetylene torch. The torch melts the coating material along with the top layer of the component material; fusing them together. As a consequence of high heat of spray and fuse, some heat distortion might manifest, and additionally attention must be taken to ascertain whether a component constitutes a good candidate. These high temperatures are comparable to those made use of in welding. This metallurgical bond generates an extremely wear and abrasion resistant coating. Spray and fuse delivers the main advantages of hardface welding along with the ease of thermal spray.

Thermal Spraying Gases

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Depending on the version of thermal spaying process employed, usage of gas is made for one or more of the subsequent functions:

• carrying the powder consumable material
• atomising molten particles from the wire consumable
• creation of the energy source, flame or plasma

The carrier gas employed to feed the powder consumable is primarily an inert gas for instance argon or nitrogen.

For the arc spray process, compressed air is usually made use of as an atomising gas because of its low cost. By utilizing an inert gas like nitrogen or argon as the atomising gas, the quantity of oxide in the coating could be reduced.

In the HVOF and flame spraying processes, combustion results from the reaction between oxygen and fuel. Fuels made use of are generally in the gas form, eventhough certain liquid fuels including kerosene are also used in combination with the HVOF process. A listing of the primary fuel gases used in the combustion process, in conjunction with their maximum flame temperatures, is specified in the table.

Together with pure gases, mixtures of gases are widely-used for the combustion process; instances of these kind of mixtures are -

Plasma spraying depends on the generation of a jet of high temperature plasma. Gases ionised during the process are mainly:

• argon
• mixture of argon and helium
• mixture of argon and hydrogen
• mixture of nitrogen and hydrogen

Selection of gas is influenced primarily through the capability to melt the sprayed particles and obtain a narrow spray cone.

Arc Spray Welding

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Wire arc spray can be described as form of thermal spraying whereby two consumable metal thermal spray wires, typically of the same composition as the coating feedstock, are fed independently into the spray gun at matched, controlled speeds. These thermal spray wires are subsequently charged and an arc is generated between them. The heat created by the arc melts the incoming thermal spray wire, which is subsequently entrained in an air jet from the gun. This entrained molten feedstock is subsequently deposited upon a substrate by making usage of compressed air. This technique is frequently employed for metallic, heavy coatings.

The electric arc spray process works by using DC power to energise negative and positive thermal spray wires which are fed via a gun head. The thermal spray wires arc against each other within the head, creating the heat essential for the generation of molten metal. Air which is compressed is introduced within the arc, atomising the molten metal from the thermal spray wires thereafter moving the droplets to the material being worked on. The droplets interlock on top of each other to generate the weld or bond.

• Requires Amperage at or above Transition Level (short circuit then globular then spray arc)
• Transition – The point at which the weld pool changes. For instance, in the event the voltage is too low for the IPM then the wire will strike the parent material, as a result of raising the voltage to the transition level the arc length is going to be increased to the short circuit transition level.
• In spray arc mode, the wire forms a point (funnel) at the electrode wire end
• Very small droplets are formed and sprayed on the weld puddle

The electric wire arc spraying and welding process is as follows:

1. Preheat surface to be sprayed. Never pre-heat aluminium, copper, titanium, manganese alloys owing to oxide film formation. Best to under heat.
2. One thermal spray wire is energised positive and the other negative
3. The thermal spray wires are fed via a spray welding system
4. Both thermal spray wires meet at the gun head and generate an arc
5. Dry compressed air atomises the material and propels it
6. Spraying torch is required to be verticle with respect to the surface otherwise porosity increases. Avoid sharp edges and narrows holes.

Typical coatings are generally:

• Iron-based alloys
• Nickel-based alloys
• Copper and Copper alloys
• Aluminium, Zinc, Al/Zn alloys
• Babbitt alloys

Significant characteristics of the wire arc spray welding:

• Excellent portability for on-site coatings
• Does not necessitate any sort of process water or gasses with the exception of compressed air
• High spray rates

Flame Spray Welding

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This is a important process meant for applying comparatively inexpensive coatings which usually possess high levels of oxides and porosity together with the option of achieving a rough surface finish. The process depends on the chemical reaction involving oxygen and fuel of combustion to generate a heat source. This particular heat source generates a gas stream having a temperature in excess of 3,000°C with correctly balanced conditions between oxygen and acetylene.

The process equipment continuously feeds the spray material in thermal spray wire or powder form into a fuel gas-oxygen flame where it is melted through the heat of that combustion. Compressed air surrounds the flame and atomises the molten tip of the wire or the powder. This approach accelerates the spray of molten particles towards the prepared workpiece surface. Combustion thermal wire spray serves as a frequent preference for machine element repair and corrosion coatings. The alternatives in flame spraying is of either hard (higher melt temperature) or soft (lower melt temperature) wires.

The process makes use of one among a number of welding gasses for fuel:

• Acetylene
• Propane
• Propylene

The feedstock material, the thermal spray wire or powder, being sprayed is fed into the flame and compressed air is subsequently utilized to atomise the molten metal and accelerate the particles upon the substrate. Equipment requirements are minimal therefore can be applied off or on-site. It is additionally inexpensive. Ignition happens outside the torch. The spraying distance is 100 to 200 mm.

Amongst others, the process is commonly employed for applying bond coat materials or materials for corrosion resistance applications.

Advantages of flame spray comprise of comparatively low surface heating (350 to 450°C), high deposition rates (60-95%), flexibility and simplicity of the process.

The disadvantages of flame welding comprise of low adhesion, high porosity, low heating efficiency along with being not possible to spray materials with a melting point over 2800°C.

Typical coatings for flame thermal wire spraying applications are:

• Iron-based alloys
• Nickel-based alloys
• Molybdenum
• Copper & Copper alloys
• Aluminium, Zinc, Al/Zn alloys

The crucial characteristics of flame spraying are:

• Portable for on-site coatings
• High spray rates along with low gas consumption

Plasma Spray Welding

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The plasma spraying process involves the latent heat of ionised inert gas called plasma being utilized to generate the heat source. The most prevalent gas employed to generate the plasma is argon, termed as the primary gas. In the plasma spraying process, the material being deposited (feedstock) - usually in the form of powder, occasionally in the form of liquid or suspension or wire - is introduced into the plasma jet, emanating from a plasma torch. In the jet, where the temperature is in the order of 10,000 K, the material is melted and propelled in the direction of a substrate. There, the molten droplets flatten, rapidly solidify and form a deposit.

Argon flows between the electrode and nozzle. A high frequency or high voltage alternating electric arc is struck between the nozzle and the electrode, which ionises the gas stream. By means of increasing the arc current, the arc thickens and increases the degree of ionisation. This has the consequence of increasing the power and in addition, as a result of expansion of gas, an increase in the velocity of gas stream.

Using a plasma produced by argon only, it requires an exceptionally large arc current to generate sufficient power to melt the majority of materials. Utilizing this level of arc current, the velocity could possibly be too high to permit materials having a high melting point to be made molten. Consequently, to increase the power to a level sufficiently enough to melt ceramic materials, it is crucial to modify the thermal and electrical properties of the gas stream. This is typically accomplished with the addition of a secondary gas to the plasma gas stream, in most cases Hydrogen. Once the appropriate gas stream has been established for the material being sprayed, the feed stock is injected into the gas stream.

Plasma spraying systems is usually categorised by way of a number of criteria.

• Plasma jet generation:
• direct current (DC plasma), in which the energy is transferred to the plasma jet using direct current, high-power electric arc
• induction plasma (RF plasma), in which the energy is transferred as a result of induction from a coil around the plasma jet, through which an alternating, radio-frequency current passes
• Plasma-forming medium:
• gas-stabilised plasma (GSP), in which the plasma forms from a gas; commonly argon, hydrogen, helium or their mixtures
• water-stabilised plasma (WSP), in which the plasma forms from water (as a result of evaporation, dissociation and ionisation) or other suitable liquid
• hybrid plasma , by means of combined gas and liquid stabilisation, normally argon and water
• Spraying environment:
• atmospheric plasma spraying (APS), carried out in ambient air
• controlled atmosphere plasma spraying (CAPS), typically carried out within a closed chamber, either filled with inert gas or evacuated
• variations of CAPS: high-pressure plasma spraying (HPPS), low-pressure plasma spraying (LPPS), the extreme case of which is vacuum plasma spraying (VPS, see below)
• underwater plasma spraying
• vacuum plasma spraying (VPS), employed for etching and surface modification to produce porous layers with high reproducibility and for cleaning and surface engineering of plastics, rubbers and natural fibres additionally for replacing CFCs for cleaning metal components.
• Feedstock type:
• solid plasma spraying process (PTA) is the thermal spray process in which the feedstock is in the form of a solid powder or a continuously feed solid thermal spray wire,
• solution precursor plasma spray (SPPS) is a thermal spray process in which a feedstock solution, a liquid or a suspension, is heated thereafter deposited onto a substrate.

The plasma spray process was developed to spray ceramics, even though plastics and metals can be treated. The process can be automated and necessitates fewer steps as compared to other spray welding processes. The plasma spray welding process delivers the greatest degree of versatility.

Advantages of plasma transferred arc welding process include that it is straightforward 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 in comparison to GTAW.

Disadvantages consist of high oxidation of sprayed material along with being impossible to obtain thin coatings of 1 mm or thinner.

Typical coatings for atmospheric plasma spray technique are:

• Ceramics
• Carbides & Cermets
• Iron, Nickel & Cobalt based alloys
• Abradables

The essential characteristics of atmospheric plasma spray technique are:

• Quickly generates thin, dense, full-coverage coatings
• Capable of applying the full range of coating materials, including metals, ceramics, and cermets
• Keeps the environment clean and safe considering that it is a chambered process along with full filtration package

High-Velocity Oxyfuel Spray Welding

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The high velocity oxygen fuel (HVOF) thermal spray process combines gas or liquid including hydrogen, oxygen, propylene, air, kerosene, which is injected by using high pressure into the torch’s combustion chamber. Gas achieves supersonic speeds while simultaneously powder is injected into the flame. The process delivers dense thermal spray coatings having less than 1% porosity. The result comes with high bond strength and fine as-sprayed surface finishes. Oxide levels are also low. The spraying distance is 380 – 400mm.

A mixture of gaseous or liquid fuel and oxygen is fed into a combustion chamber, where these are ignited and combusted continuously. The resultant hot gas at a pressure close to 1 MPa emanates through a converging–diverging nozzle and travels through a straight section. The fuels can be gases including hydrogen, methane, propane, propylene, acetylene, natural gas, and so on. or liquids including kerosene, etcetera. The jet velocity at the exit of the barrel (>1000 m/s) surpasses the speed of sound. A powder feed stock is injected into the gas stream, which accelerates the powder up to 800 m/s. The stream of hot gas and powder is aimed towards the surface to be coated. The powder partially melts in the stream, and deposits upon the substrate. The resulting coating possesses low porosity and high bond strength.

The process is employed for spraying wear-resistant carbides and alloys (wear or corrosion resistant) which include Iconel, Triballoy and Hastelloy. The process possesses high levels of adhesion and low porosity (less than 1%). It facilitates thicker coatings and possesses a higher amount or retained carbides in comparison to plasma or flame spraying. It happens to be comparatively noisy (greater than 130 dB) with a low deposition rate (35% – 50%). The equipment additionally is frequently more expensive.

HVOF coatings can be as thick as 12 mm. It is usually employed to deposit wear and corrosion resistant coatings upon materials, which includes ceramic and metallic layers. Common powders consist of WC-Co, chromium carbide, MCrAlY, and alumina. The process has been most effective for depositing cermet materials including WC–Co, etcetera and various corrosion-resistant alloys including stainless steels, nickel-based alloys, aluminium, hydroxyapatite for medical implants, and so on.

Amongst the important benefits of this system's high velocity is the extremely high coating density and low oxide content. The low oxides are due partly to the speed of the particles spending reduced time inside the heat source and partly as a result of lower flame temperature (around 3,000 °C) of the heat source in comparison with alternative processes. In addition to delivering excellent bond strength, certain HVOF coatings can be sprayed very thick because of the exceptionally high velocities delivering coatings in compression as opposed to tension. This permits materials that include carbide being applied very quickly in excess of 6 mm.

Typical coatings for high velocity oxygen fuel thermal spray systems are:

• Nickel & Cobalt-based alloys, Stellite,
• Triballoy, Inconel,
• Iron-based alloys, AISI 316L, etc.
• Carbides & Cermets
• MCrAlY

The major characteristics of high velocity oxygen fuel thermal spray systems are:

• Produces coatings which are especially clean, hard, and dense along with fine, homogeneous structures
• Coatings are tenaciously bonded to the substrate
• Possesses low compressive stress, which in turn results in very thick coatings

Detonation Gun Thermal Spraying

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The detonation gun consists of a long water-cooled barrel with inlet valves for gases and powder. Oxygen and fuel, typically acetylene, are fed into the barrel accompanied by a charge of powder. A spark is utilized to ignite the gas mixture, and the resulting detonation heats and accelerates the powder to supersonic velocity through the barrel. A pulse of nitrogen is employed to purge the barrel after each detonation. This process is repeated many times a second. The high kinetic energy of the hot powder particles upon impact with the substrate results in a buildup of a very dense and strong coating.

The advantages of the detonation gun spraying process comprise of superior adhesion, reduced porosity (less than 1%) and a increased feed rate (up to 12 kg/h). The process boasts a higher amount of retained carbides in comparison to plasma and flame spraying. the spraying distance is 50 to 200mm.

Disadvantages include that it is hard to spray materials with low density which include Iic, high noise (great than 140 dB), is known for a need for sealed boxes and a high price.

Typical coatings for detonation gun thermal spraying systems are:

• Al₂O₃
• Cu–Al
• Cu–SiC
• Al–Al₂O₃
• Cu–Al₂O₃
• Al–SiC
• Al–Ti
• TiMo(CN)–36NiCo
• Fe–A

Cold Spray Welding

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In cold spraying or gas dynamic cold spraying, particles are accelerated to very high speeds by way of the carrier gas forced via a converging–diverging de Laval type nozzle. Upon impact, solid particles with a sufficient amount of kinetic energy deform plastically and bond mechanically to the substrate to form a coating. The critical velocity necessary to form bonding is dependent upon the material's properties, powder size and temperature. Metals, polymers, ceramics, composite materials and nanocrystalline powders can be deposited by using cold spraying. Soft metals including Cu and Al are most suitable for cold spraying, but coating of other materials (W, Ta, Ti, MCrAlY, WC–Co, etc.) by cold spraying is reported.

The cold spray process makes use of deformable particles which are introduced to a supersonic preheated gas stream. The stream is directed upon the substrate. The coating is deposited by way of an impaction process. There isn't any heating of particles (the gas is heated to attain a higher sonic flow speed).

The deposition efficiency is usually low for alloy powders, and the window of process parameters and suitable powder sizes is narrow. To accelerate powders to higher velocity, finer powders (<20 micrometers) are utilized. It's possible to accelerate powder particles to much higher velocity employing a processing gas possessing high speed of sound (helium as opposed to nitrogen). On the other hand, helium is costly and its flow rate, thereby consumption, is higher. To enhance acceleration capability, nitrogen gas is heated nearly about 900 °C. Consequently, deposition efficiency and tensile strength of deposits increase.

Typical coatings for gas dynamic cold spraying systems are:

• Polymers, ceramics, composite materials and nanocrystalline powders
• Ductile materials & alloys

Warm Spray Welding

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Warm spray welding can be considered being a new “HVOF” technology in which the kinetic energy is increased while the thermal energy is lowered. By using warm spray, it's possible to spray virtually oxide-free coatings. The coating material particles are being accelerated in a heated gas stream (600 °C), up to a particle velocity of >1200m/s. The extreme high particle velocity combined with the low particle temperature results in very dense and oxide-free coatings.

Warm spraying is a innovative modification of high velocity oxy-fuel spraying process, wherein the temperature of combustion gas is lowered as a result of mixing nitrogen with the combustion gas, thereby bringing the process closer to the cold spraying. The resulting gas is made up of considerable water vapour, unreacted hydrocarbons and oxygen, and for that reason is dirtier compared to cold spraying. On the other hand, the coating efficiency is higher.

In contrast, lower temperatures of warm spraying reduce melting and chemical reactions of the feed powder, in comparison to HVOF. These advantages are specifically important for such coating materials as Ti, plastics, and metallic glasses, which rapidly oxidise or deteriorate at high temperatures.

Uses tend to be found in the automotive industry, corrosion protection and electronics industry.

Typical coatings of warm spray systems are:

• Ductile materials & alloys, Zn, Al, Ni, Ti, Cu, Ag, NiCr, CuAl, MCrAlY’s, etc.
• High-end materials: Niobium or Tantalum

The advantages of this kind of process are as follows:

• the critical velocity required to form a coating can be significantly reduced by heating,
• the degradation of feedstock powder including oxidation can be significantly controlled compared to conventional thermal spraying where powder is molten, and
• various coating structures can be actualised from porous to dense ones by controlling the temperature and velocity of the particles.

Spray and Fuse Welding

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Spray and fuse thermal spray welding makes use of high heat to increase the bond between the thermal spray coating and the substrate of the part. In contrast to other types of thermal spray, spray and fuse generates a metallurgical bond between the coating and the surface. Consequently rather then relying on friction for coating adhesion, it melds the surface and coating material into one material. Spray and fuse comes down to the difference between adhesion and cohesion.

This process typically necessitates spraying a powdered material onto the component subsequently following with an acetylene torch. The torch melts the coating material and the top layer of the component material; fusing them together. As a result of high heat of spray and fuse, some heat distortion may occur, and care is required to be taken to ascertain if a component is a good candidate. These high temperatures are similar to those employed in welding. This metallurgical bond generates an extremely wear and abrasion resistant coating.

The spray and fuse method is probably the most frequently used technique for hard surfacing. Spray and fuse provides the main advantages of hardface welding along with the ease of thermal spray. The powders used for spray and fuse hardfacing tend to be compositions of Ni, Cr, Co, Bo, Fe, W and WC in differing blends.

The essential characteristics of spray and fuse thermal spraying system are:

• Excellent with regard to basic wear, corrosion resistance and abradable/clearance control applications
• Coatings can be machined to final dimensions and finish
• Extremely hard and metallurgical bond of coating to substrate
• Resistant to impact and chipping together with corrosion resistant