Laser welding is a very successful application for Rofin and there are a number of distinct subdivisions:
- Continuous laser welding or pulsed welding
- Conduction limited or keyhole welding
- Spot welding or seam welding
- Reflow welding or welding with addition of new material from a wire or rod
- Transmission welding (of plastics) or surface absorption
Laser welding relies upon the absorption of laser power first at the surface of the material to be welded and then beyond this by thermal conduction or ‘keyholing’ which allows a deep, narrow penetration weld and occurs at higher focused power densities in metals. At lower power densities, in the absence of the keyhole effect, the penetration which can be reached in metal tends to be limited by the rate at which thermal energy conducts away from the absorption point in all directions. Such ‘conduction limited’ welds can be desirable in terms of surface finish for some applications, but it is generally the deeper penetration of keyhole welding that is sought.
Spot welding can be very efficient if a galvo head deflects the beam, as in laser marking. With this beam delivery method, the time wasted between spot welds can be as little as a millisecond. Seam welding can be achieved using overlapping spots or with a continuous beam. The latter has the potential to be faster, but is likely to need more average power (hence cost) for some applications.
In general, laser welding uses less heat and directs it more closely to the region required than conventional means. Provided the fit-up of the two parts to be joined is good (i.e. a small gap relative to the thickness of material), no additional material need be added, such as welding rod. There are occasions however when it is desirable to add material, for example:
- When dissimilar materials cannot be welded together to yield a strong joint, metallurgy can point to a suitable intermediate material which can be added to the weld in rod form to make a crack-free joint possible
- In tool repair, worn surfaces can be built up so that tools can be refurbished. See application example below and dedicated tool repair section under Markets
- In jewellery repair, it is often necessary to add material to effect repairs such a reclawing jewel mounts or resizing rings. Laser welding with fine wire of the same material as the original item makes this possible. See dedicated jewellery section under Markets
Finally it is worth drawing attention to plastic welding, and the growing niche within this category of transmission welding. For thin plastic sheets it is possible to join by heating from the top with a CO2 laser and relying on a degree of transmission and conduction through the polymer to reach the sheet below. However, for plastics of order of 1mm or thicker this usually fails and a means of heating the plastic junction itself is needed. This is achieved by selection a solid state laser outputting in the region of 900nm, which is quite well transmitted by most undoped plastics. Once the lower plastic material is doped so that it absorbs the laser, it is possible to bond them by heating the interface directly. See application example below and more under plastics under Markets.
Mould Tool Repair by Material Addition
When plastic injection or other mould tools or inserts need modifying or repairing, it is often necessary to add material to fill cracks or build up surfaces. It is desirable to add the same metal as the tool is made from and to do so with minimal thermal impact so that surfaces or structures created may be subsequently machined back or polished without leaving a ‘witness mark’:
An intense pulsed Nd:YAG laser coupled with feed-in of fine welding wire accomplishes this task and can be facilitated by an eyesafe dedicated tool repair system comprising microscope optics and a joggable XYZ stage.
See dedicated Tool & Mould section under Markets.
Transmission Welding of Plastics
A key point to remember when selecting the laser and polymers types with a view to welding in this way is that a polymer which is transparent to the laser does not need to be transparent to the eye. Hence it can be possible to weld black plastic to black plastic by this means. Careful material and additive selection yields welds which are extremely strong and clean as the weld is internal and no glues or solvents are needed.
See dedicated Plastics section under Markets.