Cladding (metalworking)

Cladding is the bonding together of dissimilar metals. It is different from fusion welding or gluing as a method to fasten the metals together. Cladding is often achieved by extruding two metals through a die as well as pressing or rolling sheets together under high pressure.

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The United States Mint uses cladding to manufacture coins from different metals. This allows a cheaper metal to be used as a filler. For example, dimes and quarters struck since have cores made from pure copper, with a clad layer consisting of 75% copper and 25% nickel added during production. Half dollars struck from to for circulation and in for collectors also incorporated cladding, albeit in the case of those coins, the core was a mixture of 20.9% silver and 79.1% copper, and its clad layer was 80% silver and 20% copper. Half dollars struck since are produced identically to the dimes and quarters.

Laser cladding is an additive manufacturing approach for metal coatings or precise piece restorations by using high power multi-mode optical fiber laser.[1]

Roll bonding

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In roll bonding, two or more layers of different metals are thoroughly cleaned and passed through a pair of rollers under sufficient pressure to bond the layers. The pressure is high enough to deform the metals and reduce the combined thickness of the clad material. Heat may be applied, especially when metals are not ductile enough. As an example of application, bonding of the sheets can be controlled by painting a pattern on one sheet; only the bare metal surfaces bond, and the un-bonded portion can be inflated if the sheet is heated and the coating vaporizes. This is used to make heat exchangers for refrigeration equipment.[2]

Explosive welding

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In explosive welding, the pressure to bond the two layers is provided by detonation of a sheet of chemical explosive. No heat-affected zone is produced in the bond between metals. The explosion propagates across the sheet, which tends to expel impurities and oxides from between the sheets. Pieces up to 4 x 16 metres can be manufactured. The process is useful for cladding metal sheets with a corrosion-resistant layer.[2]

Laser cladding

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Laser cladding[3][4] is a method of depositing material by which a powdered or wire feedstock material is melted and consolidated by use of a laser in order to coat part of a substrate or fabricate a near-net shape part (additive manufacturing technology).

It is often used to improve mechanical properties or increase corrosion resistance, repair worn out parts,[5][6] and fabricate metal matrix composites.[7] Surface material may be laser cladded directly onto a highly stressed component, i.e. to make a self-lubricating surface. However, such a modification requires further industrialization of the cladding process to adapt it for efficient mass production. Further research on the detailed effects from surface topography, material composition of the laser cladded material and the composition of the additive package in the lubricants on the tribological properties and performance are preferably studied with tribometric testing.

Process

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A laser is used to melt metallic powder dropped on a substrate to be coated. The melted metal forms a pool on the substrate; moving the substrate allows the melt pool to solidify in a track of solid metal. Some processes involve moving the laser and powder nozzle assembly over a stationary substrate to produce solidified tracks. The motion of the substrate is guided by a CAM system which interpolates solid objects into a set of tracks, thus producing the desired part at the end of the trajectory.

Automatic laser cladding machines are the subject of ongoing research and development. Many of the process parameters must be manually set, such as laser power, laser focal point, substrate velocity, powder injection rate, etc., and thus require the attention of a specialized technician to ensure proper results. By use of sensors to monitor the deposited track height and width, metallurgical properties, and temperature, constant observation from a technician is no longer required to produce a final product. Further research has been directed to forward processing where system parameters are developed around specific metallurgical properties for user defined applications (such as microstructure, internal stresses, dilution zone gradients, and clad contact angle).

Advantages

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• Best technique for coating any shape => increase lifetime of wearing parts.
• Particular dispositions for repairing parts (ideal if the mould of the part no longer exist or too much time is needed for a new fabrication).
• Most suitable technique for graded material application.
• Well adapted for near-net-shape manufacturing.
• Low dilution between track and substrate (unlike other welding processes and strong metallurgical bond.
• Low deformation of the substrate and small heat affected zone (HAZ).
• High cooling rate => fine microstructure.
• A lot of material flexibility (metal, ceramic, even polymer).
• Built part is free of crack and porosity.
• Compact technology.

See also

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References

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• Cladding at Wikimedia Commons

Laser cladding, also known as laser metal deposition, is a technique for adding one material to the surface of another. Laser cladding involves the feeding of a stream of metallic powder or wire into a melt pool that is generated by a laser beam as it scans across the target surface, depositing a coating of the chosen material.

Laser cladding technology allows materials to be deposited accurately, selectively and with minimal heat input into the underlying substrate.

The laser cladding process allows for property improvements for the surface of a part, including better wear resistance, as well as allowing for the repair of damaged or worn surfaces. Creating this mechanical bond between the base material and the layer is one of the most precise welding processes available.

Laser cladding can be performed using either a wire (including hot or cold wire) or powder feedstock. The laser develops a molten pool on the surface of the workpiece into which the wire or powder is simultaneously added. Despite the high power of the laser as a heat source, the exposure time is short which means that solidification and cooling times are fast.

The result is a metallurgically bonded layer which is tougher than can be achieved with thermal spray and less dangerous to health than the process of hard chromium plating.

Being able to mix two or more powders and control the feed rate for both separately means that this is a flexible process that can be used to fabricate heterogeneous components or functionally graded materials. In addition, laser cladding allows the material gradient to be designed at the microstructural level due to the localised fusion and mixing in the melt pool, which means that clad materials can be tailored for functional performance in specific applications.

There are many variations of laser cladding and laser cladding technology.

The descriptions in this article will primarily focus on conventional (and mainstream) laser cladding. However, there are newer and more advanced variants of the technology, including Extreme High-speed Laser Application (EHLA). 

In the EHLA process, the powder is fed into the line of the focused laser beam above the substrate. This ensures that the deposited material is already molten before making contact with the substrate, on the substrate a very shallow melt pool is still formed, allowing the deposited material to cool and solidify in contact with the underlying material, reducing the amount of heat reaching the component below and the depth of the dilution and heat effects. This small dilution forms the capability for producing much thinner coatings (20-300µm) that achieve desired chemistry within 5-10µm. This also forms the core of the high traverse speeds achievable with EHLA, which can exceed over 100m/min.

Laser cladding offers several advantages when compared with conventional coating processes. The advantages of laser cladding, include the deliver of a higher quality coating material (Inc. high bond strength & integrity) with very little distortion and dilution, as well as enhanced surface quality. These advantages include:

• Ability to place tailored performance enhancing material exactly where required
• Can be used with a wide choice of materials, both as the substrate and the layer including bespoke alloy or metal matrix composite (MMC) design
• Little or no porosity within the deposits (>99.9% density)
• Relatively low heat input results in a narrow heat affected zone (EHLA as low as 10µm)
• Minimal distortion in the substrate reduces the need for corrective machining
• Easy automation and integration into CNC and CAD/CAM production environments
• Reduced production times
• Improved thermal control with laser power modulation
• Ability to produce functionally graded parts
• Precise deposition rates, dependant on equipment and application characteristics
• Good mechanical properties
• Suitable for repair of worn parts

While there are lots of advantages to laser cladding, there are also a few drawbacks to the technology, these include:

• Expensive set-up costs for capital equipment
• Large equipment means that it is generally not portable, though portable on-site solutions do exist.
• High build rates can lead to cracking (though with some materials this can be eliminated with additional thermal control measures such as preheat and post deposit-cooling control).

Laser cladding can be performed with a variety of metals including:

• Aluminium alloys (Al-(Mg)-Si)
• Cobalt alloys (Co, C, Cr, W)
• Copper alloys
• Nickel self-fluxing alloys (Ni-Cr-B-Si)
• Stainless steels (Fe, Cr, Ni)
• Super alloys (Ni, Co, Mo, Cr, Si)
• Titanium alloys4
• Tool steels (Fe, C, Cr, V)
• MMC including carbides (WC, TiC, CBN)
• Nano additive alloys (oxide dispersion strengthened alloys)

This wide range of materials means that laser cladding can be used for a large selection of industrial applications, including rapid manufacture, repair of parts, and surface enhancement. Materials such as tungsten carbide in a MMC, for example, offers durability making it ideal for coating applications that require superior wear resistance.

As mentioned above, laser cladding is suitable for a variety of applications across industry. These applications cover areas ranging from agriculture and aerospace to drilling, mining and power.

Some example applications include:

Cutting Tools

Laser clad materials can be used as layers to protect saw blades, counter blades, disc harrows and other cutting tools from wear and corrosion, while providing superior cutting characteristics. The lack of distortion with this process means that these tools are kept straight while different coating thicknesses can be achieved to suit requirements. These coated tools can find applications across industry, including construction and agriculture.

Drilling Tools

High performance drilling tools are used in a range of industries including oil and gas, mining, and geothermal. These tools need wear protection to withstand the stresses they are subjected to and reach their required lifetimes. Laser cladding is becoming increasingly common as a technique for applying coatings due to the materials performance this process provides. 

Heat Exchangers

Heat exchangers can suffer corrosion from the corrosive liquids and gases that they come into contact with. Laser cladding with coatings such as nickel alloys with good corrosion resistance and toughness can help avoid cracking in heat exchangers, while also offering improved wear protection even at high temperatures. 

Hydraulic Cylinders

Hydraulic cylinders, such as those used in the mining industry, require coating in order to mitigate against the corrosion caused by the local atmosphere. Chrome plating was the primary method used in the past, but this is increasingly being superseded by laser cladding, due to the superior durability it offers. Some estimates say that laser cladding can improve durability of these products by 100%.

Replacement for Hard Chromium Plating

Hard chromium plating has been facing prohibitive measures from the EU, leading the industry to try and seek alternative solutions. Laser cladding had been discounted as a solution in the past because it wasn&#;t deemed fast enough or able to deliver thin enough coatings. However, developments in the technology (specifically, extreme high-speed laser application) now allow for higher speed deposition with thinner layers in a more power efficient manner, meaning that laser cladding can provide an effective alternative to hard chromium plating for particular applications.  

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