by Chris Pilcher
Keyhole welding processes offer higher power densities and faster welding speeds than conduction welding technologies
The majority of joining techniques use conduction welding and typically add excess heat to a part, creating metallurgical damage to the substrate. Other welding techniques use high power densities on small areas 0.008 in. (0.2 mm), such as Electron Beam and Laser Beam Welding (EBW and LBW).
Laser beam welding, the focus of this article, is a more economical choice because it doesn’t operate under a vacuum, as does Electron Beam welding.
How it works
Laser welding relies on an incredible amount of power and power density focused on a small spot. Power densities are typically more than 1 x 106 Watts/cm2. The laser vaporizes the material before it’s conducted into the substrate, forming the keyhole. The laser moves the keyhole along the part, melting material in front of the keyhole while molten material flows around the sides of it, solidifying the keyhole. The result is a small weld with little distortion.
The light in the laser beam is different than normal incandescent light because it is monochromatic, collimated (meaning it travels in a straight line and doesn’t disperse) and is coherent with all photons travelling in the same direction.
The laser beam focus is accomplished with an optical head. The laser resonator delivers the beam using a process fiber or mirrors. The beam gets collimated, travels through a focusing lens, and converges on the part.
The fiber laser
Fiber laser technology emerged in the late 1990s and has been making inroads into the industry because it is considered more energy efficient than YAG lasers, and offers electrical efficiency of more than 30 per cent. Today, there are fiber lasers ranging in power up to 50 kW and with prices down almost a third of what they were five years ago, it’s making the technology more accessible to manufacturers.
The fiber laser can be doped with other materials such as Erbium, which will produce light in the 1.5 micron wavelength. Changing the doping material to Thulium will produce a wavelength of light that is 2 microns. Why do this? Each wavelength of light will absorb into each material at a different rate. Two microns, for example, has an advantage for welding clear plastics. On the other hand, 1 micron will absorb into metal better than CO2. It is important to understand that not one wavelength of light will take over the market.
Designing lasers for welding
Welding geometries must be designed for lasers. Trying to use a joint geometry typically used in processes like GMAW have a maximum gap between the components being joined as a result of the addition of filler wire. Although lasers can use filler wire, they typically do not. Subsequently, lasers rely on little to no gap between the parts. Typical joint geometries found in laser welding include butt, overlap, T and Fillet joints. SMT
Chris Pilcher is regional sales manager-Canada, for IPG Photonics Corp.