by Paul Hilton
The Nd:YAG laser is one of the most versatile laser sources used in materials processing. The relative robustness and compactness of the laser and the possibility for the 1.06 micron light it produces to be transmitted to the workpiece via silica optical fibres, are two features which contribute to its success. Nd:YAG lasers were first commercialised operating mainly in pulsed mode, where the high peak powers which can be generated were found useful in applications such as drilling, cutting and marking. These pulsed lasers can also be utilised for welding a range of materials. More recently, high power (up to 10kW), continuous wave (CW) Nd:YAG lasers have become available. The Nd:YAG crystals in these lasers can be pumped either using white light flashlamps or, more efficiently, using laser diodes. The latter methods are used to produce high quality beams, which can be focused to smaller spots (and therefore produce higher power densities) than the flashlamp pumped lasers. Because of the possibility of using fibre optic beam delivery, these lasers are often used in conjunction with articulated arm robots, in order to work on components of complex shape.
Because of the wide range of applied power and power densities available from Nd:YAG lasers, different welding methods are possible. If the laser is in pulsed mode, and if the surface temperature is below the boiling point, heat transport is predominantly by conduction and a conduction limited weld is produced. If the applied power is higher (for a given speed), boiling begins in the weld pool and a deep penetration weld can be formed. After the pulse, the material flows back into the cavity and solidifies. Both these methods can be used to produce spot welds. A seam weld is produced by a sequence of overlapping deep penetration 'spot' welds or by the formation of a continuous molten weld pool. For the former, once the energy input is sufficient to ensure that the weld does not solidify between pulses, the 'keyhole' type weld normally associated with CO 2 laser welding can be formed. Pulsed laser welding is normally used at thicknesses below about 3mm. Higher power 4-10kW CW Nd:YAG lasers are capable of keyhole type welding in materials from 0.8mm (car body steel) to 15mm (ship steel) thickness.
Nd:YAG laser welding is used commercially on a wide range of C-Mn steels, coated steels, stainless steels, aluminium alloys, titanium and molybdenum. The low heat input welding offered by Nd:YAG lasers is utilised in the electronics, packaging, domestic goods and automotive sectors, and significant interest has been shown more recently, particularly for the high power CW lasers, in the shipbuilding, oil and gas, aerospace and yellow goods sectors. Important R&D issues involve development of high power lasers of better beam quality, use of distributed energy in the beam focus, weld quality maintenance for both thick and thin sections and weld classification.
The principal risks involved in Nd:YAG laser welding are: optical (the beam can burn the skin or damage the retina if focused by the eye), electrical, and fume generation. A current application issue is safe use of Nd:YAG lasers in anything other than a fully opaque (to the Nd:YAG laser wavelength) enclosure, such as might be found in a shipyard for example. (twi.org)
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