Laser welding is usually done without filler metals so parts need to have good fit with a gap that's less than 15% of the thickness of the thinnest component. Parts should be relatively clean since 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, penetration-mode and keyhole welds. Conduction-mode welds are rather shallow penetration and wide welds, similar to a GTAW or TIG weld shape.
Penetration-mode welds have a weld penetration that is equal to, or slightly deeper than, the weld width. When using penetration-mode laser welding the heat input is reduced due to the lower melt volume, thus creating deep penetration low heat input welding from even low average power lasers.
Laser Welding Benefits
Laser welding offers numerous advantages over tradition welding techniques such as resistance spot and arc welding.
- 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;
- Versatile (the same tool can be used for laser cutting and drilling).
Fiber lasers are an effective way to weld very small parts, commonly used in the engineering, medical and electronics industries. JK Lasers' research has demonstrated how the excellent beam quality, low power (100 - 200W) continuous wave (CW) fiber lasers with modulation consistently achieve high quality spot welds on thin stainless steel foils (20μm - 150μm).
During the trials, a single mode fiber laser with Gaussian beam profile produced spatter-free spot welds between 76μm - 175μm diameter. A flat top beam profile, available as an optional feature, achieved spatter-free welds between 150μm and 260μm in diameter.
Our higher powered fiber lasers are highly capable of welding thicker materials. For example, the 2kW fiber laser (JK2000FL) can weld 8mm low carbon steel and 8mm 316 stainless steel. This heavy duty welding is more suited to applications in the automotive and aerospaces sectors.
Nd:YAG Pulsed Lasers
Nd:YAG Pulsed 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 Nd:YAG laser so versatile. Even a lower average power pulsed Nd:YAG laser can produce large spot welds or deep spot and seam welds as the interaction with the material is defined by the pulse parameters.
A pulsed Nd: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 Nd:YAG laser is usually about 2kW minimum to as high as 10kW.
In general, pulsed Nd:YAG lasers are used for spot welding applications, in the 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. They can weld up to 3mm penetration. Peak power of around 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 "shaped" to optimize the weld quality and deal with dissimilar metals. 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
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 enables tacking and seam welding as well as power ramping.
Super modulation can improve weld 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 that scatters the beam and this soot disperses quickly when the laser energy is reduced. Super modulation takes advantage of this by delivering energy quickly before the soot level reaches a scattering concentration.
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 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 that simplifies welding system design and creates a much more consistent and robust laser welding process. Fiber optics have standard lengths of 5m - 50m and standard focusing end-effectors called Focus Heads. These take the laser beam from the fiber and produce a focused spot on the workpiece. Focus heads can be simple straight units or they can have a 90 degree turn in them. CCTV viewing is a common option. Other options include multi-spot prisms, ring-focus optics, welding nozzles and air knives.
Time-Share and Energy-Share Multiplexing
Fiber optic delivery allows the use of Time-share and Energy-share multiplexing of the laser beam to multiple workstations or to different areas within the same workstation. Time-sharing can direct the laser beam into six different fibers with a switching time as low as 50msec. The laser parameters can be changed for each different workstation or process.
Time-sharing is common where the laser processing time is low compared to fixturing or index time and where 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 two fibers, or equal share on three or four fibers. Systems can also be set up for non-equal shares depending on the process requirements.
Energy-sharing enables simultaneous laser welding without distortion by having the welding forces equal and opposite when positioned symmetrically on a part. It can also improve cycle times when simultaneous welds are possible. Time-sharing between energy-share banks is also an option.
Let Us Help You Choose
JK Lasers has specialist applications centres where we can develop a laser welding solution to suit you. We can help determine the appropriate laser source, beam delivery and weld schedule. For can also help with alloy selection, weld joint design, fixture development, custom optical systems, controls integration and the production of test pieces.