Laser Welding

Laser welding is usually done without filler metals so parts need to have good fit-up with a gap less than 15% of the thickness of the thinnest component. Parts should be relatively clean since the welding is very fast with no time to burn-out contaminants. Shield gas is required for more reactive metals but many alloys can be welded in air.
Weld heat input and weld shape can be controlled with laser parameters and optics to generate conduction-mode welds, penetration-mode welds, and keyhole welds. Conduction-mode welds are rather shallow penetration and wide welds similar to a GTAW or TIG weld shape. This mode is common for small devices where the smoothness and cosmetics of the weld are important such as medical devices and tools or in electronic devices such as relay cans and batteries. Penetration-mode welds have a weld penetration that is equal to or slightly deeper than the weld width. When using penetration-mode welding the heat input is reduced due to the lower melt volume creating deep penetration low heat input welding from even low average power lasers. Keyhole-mode welding can only be performed with CW or Super Modulated CW lasers due to the requirement to maintain the keyhole during the weld cycle. Keyhole welds have high penetration to width ratios, as high as 6:1 and are the most efficient welding process. Pulsed, CW, and Super Modulated CW lasers can weld in conduction-mode and penetration-mode regimes with the CW and Super-Modulated CW lasers the only candidates for keyhole welding.

Pulsed Nd:YAG lasers create discrete pulses of controllable energy, peak power, and temporal profile, or shape, to create a weld. It is the control of these pulses that make the pulsed YAG such a versatile welding system. Even a lower average power pulsed YAG laser can produce large spot welds or deep spot and seam welds since the interaction with the material is defined by the pulse parameters with average power having little effect on the weld. A pulsed YAG laser can produce energies from a few tens of millijoules per pulse, however the average power of the laser that produces these pulses can be on the order of 100W. The peak power of a pulsed YAG laser is usually about 2kW minimum to as high as 10kW. In general, pulsed Nd:YAG lasers are used for all spot welding applications or in seam welding of temperature-sensitive components or where aluminum and copper alloys are to be joined. Their higher energy per pulse can create a large melt volume from a single pulse and spot welding penetration is function of pulse energy not mean power. The peak power of a pulsed laser will overcome the reflectivity and heat conductivity of aluminum, copper, and other similar alloys. Pulsed lasers can weld with up to 3mm penetration. Peak power of about 1kW is needed for welding ferrous alloys and high nickel alloys. For aluminum alloys peak powers of about 3kW are needed and 5kW for copper alloys. The temporal profile of the pulse can be ÒshapedÓ to optimize the weld quality and deal with dissimilar metals. JK Lasers invented the modern pulse-shaping pulsed YAG laser. Adjusting the peak power throughout the pulse will control cooling rates to reduce cracking, eliminate porosity, and improve weld esthetics.

Continuous Wave (CW) and Super Modulated CW lasers are employed where fast, low heat input, seam and stitch welds are needed. These lasers produce a continuous or high speed modulated and Super Modulated output which can produce a continuous molted pool for high speed welding and deep penetration welding. Unlike a true pulsed laser that must re-initiate the melt with each pulse and overlap the previous pulse by up to 90%, these lasers can produce a weld with very low heat input. CW and Super Modulated lasers, however, must use a higher average power to create a deeper penetration weld. To increase weld penetration the laser's power is increased or the welding speed reduced. These lasers can produce conduction-mode, penetration-mode, and even keyhole-mode welding regimes similar to electron-beam welding. Modulating the laser beam for tacking and seam welding as well as power ramping. However many of these laser models have a patent pending Super Modulation feature which can improve welding penetration or speed by as much as 40% in ferrous alloys and increase welding capability in aluminum alloys by up to 600%. Super Modulation is a modulated sine or square waveform with a peak power up to 2X the laser's mean power while still producing the laser's rated mean power. For example, a 1kW mean power laser can Super Modulate with a waveform from 100-1000Hz with a peak power of 2kW while still producing a 1kW average power output. Soot and particulate from the weld zone can scatter the laser beam and up to 40% of the laser's energy can be scattered away from the focus spot. It takes time for the soot level to reach a concentration to begin scattering the beam and this soot disperses quickly when the laser energy is reduced. Super Modulation takes advantage of this effect by putting the energy in quickly before the soot level reaches a scattering concentration and the laser output goes through a minimum as the soot level decays below the scattering threshold just in time for the next high peak power cycle from the laser. Different alloys can have a different optimum Super Modulation frequency. Using Super Modulation also reduces porosity and heat input during welding. In ferrous alloys these CW and Super Modulated lasers can weld with up to 1.5mm penetration at 500W, 3.5mm penetration at 1kW, and 8mm penetration at 2kW.

Both types of lasers usually employ fiber optic beam delivery which simplifies welding system design and creates a much more consistent and robust welding process. Fiber optics have standard lengths of 5-50m and have standard focusing end-effectors called Focus Heads that take the laser beam from the fiber and produce a focused spot on the workpiece. These focus heads can be simple straight units or can have a 90 degree turn in them. CCTV viewing is a common option with viewing through the laser lenses with the camera focus being the same as laser focus so with the addition of a cross-wire generator the system set up can be as simple as Òpoint and shootÓ. Other options are available such as multi-spot prisms, ring-focus optics, welding nozzles, air knifes, etc.

Fiber optic delivery allows the use of Time-Share and Energy-Share Multiplexing of the laser beam to multiple workstations or areas within the same workstation. Time-Sharing can direct the laser beam into up to 6 different fibers or more with a switching time as low as 50msec. The laser parameters can be changed for each different workstation or process. Timesharing is common where the laser processing time is low compared to fixturing or index time and the synchronization of workstations is not stringent. Energy-Sharing is the simultaneous delivery of laser energy down multiple fibers. This can be an equal 50:50 share with 2 fibers, or equal share on 3 or 4 fibers. Systems can also be set-up for non-equal shares depending on the process requirements. The use of energy sharing allows welding without distortion by having the welding forces be equal and opposite when positioned symmetrically and occur simultaneously on a part. Energy sharing can also improve cycle time where simultaneous welds are possible. Timesharing between Energy-share banks is also an option.

GSI Group has Applications Centers around the world to help you develop a laser welding solution for you. We can provide minimum support such as determining the appropriate laser source, beam delivery, and weld schedule. For other projects we can help with alloy selection, weld joint design, fixture development, custom optical systems, controls integration, and the production of test articles.

Designers and product engineers should familiarize themselves with the properties, advantages, and applications of lasers, as well as how to design components, applications, and systems to best utilize the advantages of laser welding. Since the advent of the industrial laser, laser welding has been chosen over conventional welding processes, such as resistance spot or arc welding, due to several primary advantages:

• Minimum heat input, resulting in minimal distortion of the component
• Consistent, repeatable welds
• Small heat affected zone (HAZ)
• Narrow weld bead with good cosmetic appearance
• High strength welds
• Easily automated
• High degree of accuracy and control
• Ability to weld dissimilar materials
• Generally no flux or filler material required
• Flexibility of beam manipulation, including fiber-optic delivery
• Ability to weld in areas difficult to reach with other techniques
• Often faster than other techniques, with greater throughput
• Versatility (the same tool can be used for laser cutting and laser drilling)