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Information Sheets
Laser Cladding
The aim of most cladding operations is to overlay one metal with another to form a sound interfacial bond without diluting the clad material. The result should be a material with the required surface properties, usually of wear, corrosion or appearance but with the structural properties of the base material, usually toughness or cost. There are many competing processes for doing this; from welding routes such as TIG, MIG, spray and plasma processes to forging methods such as D Gun and other high velocity particle processes and diffusion bonding. The particular advantages offered by the laser are its ability to create a highly localised melt zone thus allowing the formation of strong weld bonds, with low dilution without having to heat the whole article.
Laser cladding can be done by melting preplaced powder, blowing powder or feeding wire into the laser generated melt pool. It can also be done by the decomposition of thermally sensitive vapours blown onto a laser defined hot spot, as in Laser Chemical Vapour Decomposition (LCVD). Laser sputtering in a vacuum is also practiced for the cladding of high melting point materials such as platinum. Laser enhanced electro plating or cementation processes are being considered for laying down conducting tracks for electronic circuits.
The most common method for laser cladding is the blown powder route in which a metal powder is blown, preferably coaxially into the laser beam. While in the beam path the powder is heated and some possibly melted but it eventually strikes the laser generated hot spot on the substrate, where it melts and sticks. The clad can be very precisely placed by the laser melt zone; a feature that has been used in the jewellery trade. Within this laser generated melt pool there is considerable stirring due to out of balance surface tension forces (Maragoni forces); this has been used to form alloys in-situ for the rapid survey of metallurgical systems for unusual properties, such as corrosion or hardness.
One variation of the process is that of "particle injection" in which two materials of different melding points such as stellite and TiC are fed together and under certain operating conditions the high melting material will only partially melt leaving a track similar to a Macadam road.
The process behaves like a "metal pencil" in that a metal track is deposited where ever the laser beam directs it. Thicknesses from a few microns up to 3mm can be deposited in a single pass. Multiple passes will lead to the build up of the track. This has been exploited to create 3D castings without the need for a mould, directly from a computer design, which has been digitally sliced to allow construction layer by layer (see technical information on rapid prototyping). Structures that are impossible to make by normal foundry routes can thus be made this way, for example conformal cooling channels in an extrusion mould tool. It has also created a new process in the form of "reverse machining". This is an additive process instead of the usual subtractive machining operations, such as milling or turning. An industry based on the repair and refurbishment of high value metal objects – such as turbine blades and rotors - has been created based on laser cladding.
Laser cladding by powder blowing has a further flexibility in that the powder can be changed while the cladding proceeds; graded cladding can be simply created or a "casting" made that has a variable composition.
The precision, bond strength and low dilution of laser cladding are its main strengths. More can be read about this and other laser processes in W.M.Steen "Laser Material Processes" 3rd Edition Springer 2003.