LASER WELDING CONSIDERATIONS
There are two different approaches to laser welding. One is the low-power method for relatively thin materials; and the other is the "brute force" high-power approach that generally involves keyholing. In both cases, since filler material is rarely used, a tight fitup of the parts being welded is necessary. For butt and seam welds, the laser energy is applied to the junction of the materials, minimizing heat input and distortion and permitting high processing speeds. However, these butt joints must fit accurately, which often limits laser butt welding to circular parts which can be turned to close tolerances and press-fit together prior to welding.
For lap joints, the tolerances for seam alignment are somewhat looser. The width of the weld is the major consideration. The upper material forms most of the fusion zone so that a good laser-weldable material could be welded to less suitable material by putting the former material on top.
ADVANTAGES OF LASER WELDING
Many manufacturing engineers have read about the technical advantages of using lasers in place of more conventional techniques. But, what about economic justifications, which form the basis of most production purchases? The industrial laser user gets no return from publicizing the cost cutting realized with a laser welding system and might even tip off competitors. So most manufacturers tend to regard their usage of lasers as proprietary information.8 Several applications were recently listed in MAN.9 The advantages of using lasers in comparison with the most popular techniques are shown in Table 2.
EVALUATING LASER WELDING SYSTEMS
As we have already discussed, the two laser types being used for laser welding are CO2 and Nd:YAG or Nd:Glass. Both laser types can operate in either the continuous or the pulsed mode. CO2 lasers, which range in power from 50 to 15,000 watts, are more efficient in their conversion of electrical power to laser radiation than Nd:YAG lasers, which range from about 50 to 800 watts output power. However, as discussed above, the reflectivity of most metals is much higher at the CO2 wavelength than the Nd:YAG wavelength.
Recent advances in fast-axial-flow CO2 lasers provide improved beam characteristics, making these systems competitive with electron beam welding for deep-penetration applications. Fast-spiral-flow CO2 lasers now are able to produce fundamental-mode outputs in the kilowatt range, which gives higher energy densities suitable for welding thermally sensitive alloys or materials where thermal distortion is a problem.10
Slow-axial-flow lasers with enhanced pulsed capabilities offer an advantage over fast-axial-flow units for applications requiring rapid energy coupling and low heat input. In pulsed operation, the peak power in the pulse is several times greater than the continuous-wave power, although the average power is lower. This peak power overcomes surface reflectivity and minimizes thermal damage to the surrounding material.
Solid state lasers (the generic name for Nd:YAG, Nd:Glass and similar lasers), are preferred for low- to moderate- power applications. They have found extensive application in the electronic/electrical industries for spot welding and beam lead welding integrated circuits to thin film interconnecting circuits on a substrate.
One consideration that can be important in evaluating laser welding is the physical size of the equipment. Solid state laser welding systems are relatively small compared to CO2 systems, which could occupy an average room to achieve the high powers required. Still, if you need the brute power, it can be guided to the workpiece through optics or articulating arms (attached to robots, if desired).
For delicate welding operations, such as welding lamp filaments, the solid state welding systems offer the advantage of coaxial viewing optics. Closed-circuit TV viewing provides a magnification factor of better than 40X, and the exact spot of the laser beam focus can be easily seen. This feature is very useful for alignment and beam focusing as well as workpiece viewing. Since the wavelength of the Nd:YAG laser is close to the visible spectrum, standard lenses can transmit both the laser light and the image of the workpiece.
While we have provided a quick theoretical approach to determining your laser needs, the actual laser powers required are frequently determined by experimentation. There is a trade-off of weld penetration versus travel speed for any given laser output power; in general, the higher the travel speed, the lower the penetration. This will vary from material to material and with other factors such as the focus of the beam.
