In the context of fabrication techniques and particularly metalworking, cladding means that some kind of material is deposited on another material – for example, a metallic layer on some other metal. There are various cladding techniques, some of which do not involve a laser. Laser-aided cladding processes use the absorption of laser light, supplied in the form of a laser beam, as the required heat source, e.g. instead of a gas flame or electric arc.
The main application of laser cladding is to make the surface of machine tools more resistant against abrasion, corrosion or temperature shocks. Such challenges are met in many industry, for example when machine parts are exposed to intense forces, hot aggressive exhaust gases, to sea water or to abrasive substances. Laser-applied claddings then often provide substantial advantages, particularly in terms of lifetime or possibilities for repair.
Laser cladding may be applied either in the original fabrication process of a part or later on in the context of refurbishing or repair. Examples for repaired objects are turbine blades, gear wheels and cutting tools. Claddings can be applied to large areas or alternatively only to isolated critical areas.
In contrast to other methods such as laser alloying, cladding processes involve only moderate heating and no melting of the base surface, so that there is no substantial intermixing of the materials at the interface – only the formation of a sufficiently strong metallurgical bond. As the material becomes fully solid already at a relatively high temperature, substantial mechanical stress can be “frozen” into the microstructure in case of rapid cooling, but may be reduced by annealing processes for slower cooling.
Laser cladding is also called buildup welding In contrast to other methods of laser welding, the purpose is not to join different solid parts, but to form a cladding from a material which is applied in different form, e.g. as a powder or a wire. Usually, the thickness of the cladding layer is small compared with the thickness of the base part.
Since some kind of layer is added, laser cladding belongs to the methods of laser additive manufacturing.
Laser Cladding Processes
A typical process involves a slow movement of a laser processing head over the surface while the cladding material is supplied to the region of the beam focus. Absorption of the laser light causes that material to melt, but when the laser beam has moved away, it quickly solidifies, forming a solid joint with the base material.
The cladding material is usually supplied either in the form of a hot (resistance heated) or cold wire or as a stream of powder, transported by a process gas. Sometimes, that material feed is fully integrated into the laser processing head (coaxial feed), while in other cases it is connected to the head from the side (lateral feed).
In some cases, a process gas is applied in addition to the cladding material. It can protect the involved metals (particularly the cladding metal) against oxidation and thus improve the quality of the results.
While a single cladding process of the described type is sufficient in many cases, thicker claddings can be built up layer by layer in subsequent steps. In some cases, additional laser milling is employed for achieving a better quality of the final top surface.
Process parameters like laser power, spot size and shape, speed of movement and the amount of supplied cladding material need to be optimized for best results with a given combination of materials. Automated laser cladding machines can automatically control such parameters, possibly also adapting parameters for variable speeds of movement. Additional facilities for process monitoring may also be used in automated processes.
Frequently used cladding materials are iron alloys of nickel, cobalt or titanium, to be deposited on iron or steel, or sometimes on other base metals such as aluminum or titanium alloys. The cladding alloys usually have a substantially lower melting point than the base metal, so that they can be melted without melting that base.
Besides the material composition, the microscopic details of the fabricated cladding can have an important impact on the obtained metallurgical properties of the cladding, including its resistance e.g. to abrasion or to stress by temperature changes. Such properties can in turn depend on the process details.
Continuous-wave operation of the laser is normally suitable, and the required optical intensity is moderate, e.g. compared with what ones needs for laser cutting. With sufficient laser power, the laser spot size can be relatively large in the interest of a high processing speed.
An elliptical beam focus with the longer direction perpendicular to the direction of movement may be used, particularly for high-power operations.
A similar method concerning the results is laser plating, but here the thickness of the deposited layer is usually much smaller, and the process details can also be quite different.
There are also techniques of laser coating; the differences to laser cladding concern the wider choice of materials and the typically far smaller coating thickness.
Another method for laser-aided deposition of materials is pulsed laser deposition. Here, the laser beam does not directly interact with the workpiece. Instead, a pulsed ultraviolet beam hits some target material, from which it sputters material to the substrate to be coated. The whole process is done in a vacuum chamber, possibly with some low-pressure process gas. A wide range of target materials is used, including oxides, nitrites, carbides, ceramics, metals and even polymers.
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