AMS taks to Trumpf Laser and System Technology on the underlying power of lasers used for body-in-white production, the benefits of these systems – and potential drawbacks

There are four basic techniques that can be used to achieve a weld in the automotive body-in-white (BIW) process; spot, resistance, arc and laser, with a range of alternatives available within each methodology. Of these, laser was the most recently introduced, yet despite the familiarity of the phrase ‘laser welding’, the science behind the joining process is largely unfamiliar to those outside the body shop.

Dr. Martin Bea, Industry Manager Automotive (Scanner Applications) at Trumpf Laser outlines what makes a laser, the development process leading up to today’s diodepumped lasers, and the new joining types being developed to make best use of the technology “Industrial high-power lasers use specific laser-active materials that can be pumped using electrical energy, the pumping action ‘exciting’ photons within the material. Mirrors aligned around this laser-active material arranges the photons, which when amplified creates a collimated monochromatic laser beam.” Bea explains this was the root of the first lamp-pumped rod lasers in the 1970s, the laser-active material being a cylindrical yttrium-aluminium garnet (YAG) crystal rod that featured a special neodymium doping. As neodymium has the periodic symbol Nd, these machines were referred to as Nd:YAG lasers. Technicians used arc lamps to illuminate the crystal rod, with those photons delivered in the correct wavelength resonating through the rod to create a laser beam.

“Neodymium atoms are very selective about the (light) wavelength they absorb to get excited,” says Bea. “The spectrum of light coming from an arc lamp is extremely broad, so the efficiency level was quite poor. Altogether, the extracted laser light represented an overall energy return of around 3%.”

To improve the efficiency of these early lasers, Trumpf technicians started to use a new doping material, ytterbium, creating Yb:YAG lasers, as well as diode lasers for optical pumping instead of arc lamps. In conjunction with this, the company also altered the internal crystal format. “Extracting (heat) wastage out of the original crystal rod was very inefficient,” explains Bea, “so we took the rod and shortened it to a very thin disk. This was the origin of a completely new type of high-power laser, the so-called (diode-pumped) disk laser. The increased surface area made it easy to extract the excess heat from the laser-active material.” The new doping, with pumped diode lasers, returned a much-improved conversion rate, which could be further enhanced using the extremely thin Yb:YAG disks that perfectly dissipate any heat build up. Heat is essentially the killer of laser efficiency and power. Bea: “Without effective cooling of the laser material, the heat builds to saturation point and no further laser light can be produced.”

Beside removing heat, another way to improve efficiency is to directly employ laser diodes for laser material processing. Within the TruDiode high-power solid state laser series, Trumpf combines the laser output of many low-powered diode lasers into one common, collimated laser beam, coupled into a transport fibre still within the laser device. In this way, laser output power levels of up to 4kW can be generated at overall efficiencies of up to 40%. But, aside from their limited output power, a drawback of such diode lasers is the limited quality of the created beam, the reduced focusability resulting in larger spot diameters and wider welds with less penetration. This restricts the number of possible applications in automotive manufacturing.

Compared to TruDisk lasers, TruDiode lasers are the ideal choice where laser material processing can afford a relatively large focus spot dimensions on the work piece at medium laser power levels, in operations such as heat conduction welding, brazing, powder remelting and other surface modification processes. Within the TruDisk lasers, the light of the pumping diodes gets further redefined within the disk-type amplifiers to improve beam quality (focusability). Combining several disks in a series allows for accordingly higher laser output power levels, without any deterioration of the generated laser beam’s quality.

Fit for purpose
Why would a BIW specialist elect to use laser welding over another methodology? “The big advantage of laser welding over traditional arc welding is that the (laser) focus spot is much smaller than the area heated up by an arc welding gun,” says Bea. “Lasers can generate narrower and deeper seams, more comparable with an electron beam than with electric arc welding. This has the practical effect of reducing material shrinkage at the welding seam and resulting distortion of the welded panels.”

System selection is based on the required laser power level and focus spot size. The achievable focus geometry is based on the beam parameter product (BPP), the measurement of beam quality. A 4kW laser with a BPP of 40 – the lower end of the BPP scale – is ideal for surface treatment or brazing (lasers above 1kW are considered high power). “Even a 500W laser with excellent beam quality (BPP below 1) is sufficient to achieve welding speeds of 20m/min,” Bea points out. “But this only provides penetration of several tenths of a millimetre into the metal part. As the metal on car bodies averages about 1mm in thickness, lasers for these operations have an approximate output of 4kW and a BPP below 10. These lasers are typically used to replace resistance spot welding.”

Following on from this, he says that TruDiode lasers are suited for brazing operations such as the joining of roof and bodyside panels. The TruDisk system is designed for deep-penetration, where a very narrow weld seam is desired in combination with high speed. An example of this is the so-called remote welding of hand-on parts, such as doors, and for other subassemblies like bumpers, cross-members, seat frames and door sills.

Which laser type returns a finish suitable for painting with only minimal rework? “During deep penetration, a laser can direct the equivalent of several million watts of energy per square centimetre, which heats up the melt pool to evaporation temperature through to its centre,” says Bea. “This releases strömungen impurities into the meltpool, which settle on the surface as the metal cools, affecting the upper weld bead. So this method is preferred for covered areas, such as adding side impact protection beams. It is not for welding outer car body structures.” Bea explains that during laser brazing, such intense thermodynamic flows inside the meltpool do not occur, since the metal is not brought to the boiling point. Thus, laser brazing produces a better join for exterior work, as the joins require comparatively little rework. “Laser brazing results in a join that only needs polishing to be flat, smooth and invisible under paint,” he says.

Decision process
Dr. Martin Bea believes that cost per unit, in combination with achievable product features, is driving adoption of laser welding technology in automotive BIW production. “OEMs demand improved process efficiency, in terms of cost and reliability, but also better mechanical properties of the welded product. ”

Trumpf relies on its Technology Centers, located at its headquarters in Ditzingen, Germany and at subsidiary locations around the world, for testing data related to improved process efficiency. Results are shared with carmakers, who can then bring the technology in-house for evaluation. The laser manufacturer offers guidelines on installation and process, then a cost analysis is carried out. The material to be worked is also a key factor in the decision process. Working with aluminium poses particular problems when using lasers, as the conductivity and reflectivity of aluminium is higher than that of steel. As such, cutting and laser welding aluminium requires higherpowered lasers in the range of 4 to 6kW.” Bea adds that pure aluminium achieves the best results in laser welding, as alloy components, including magnesium, silicon and copper, have different evaporation temperatures compared to the base metal. If this is not addressed by means of choosing the right type of filler wire, he says, the risk of degradation (cracking, inclusions, porosity) during welding is very high, while joining choices are also limited. Essentially, any material that can be arc welded can be welded using lasers, but all steel grades are easier to work with than aluminium.

Joining techniques
When chemistry does not allow direct fusion of the different structures, copper flux can be used. An alternative to this is to use the small spot focus capability of a laser to heat the leading edge of the steel part. Press this white-hot edge against the aluminium and the heat flow melts the softer metal, gluing the parts together and creating an overlap joint. Bea says that given the material limitations, this is the most efficient technique to laser weld steel and aluminium blanks. Additionally, weld speed can be maintained, as only one metal layer is heated.

The cost of clamping parts is also prohibitive, though an alternative could be the company’s newly-developed K-joint, which aims to replace overlap welds with butt welds. In the case of welding seat rails onto a floor panel, which would normally require a comprehensive clamping system to precisely position and fix the rail in an overlap configuration, Bea says that a small addition to the floor panel could save both time and money. “Our idea is to apply a small tool within the deep drawing tool that lifts a part of the floor panel, creating a mechanical stop for the rail. With three of these stops, you create the perfect geometric positioning for the part. The only addition would be some simple clamps for welding in butt joint configuration.”

Over the past decade, with the switch from lamp pumped rod to diode-pumped disk lasers, energy efficiency has increased by more than by 20%. Bea says that users switching from overlap to butt-joint welding can expect to increase process efficiency by at least 50%, since both panels do not have to be welded through the complete cross section, the thickness of the thinner joining partner representing the required weld seam depth. “We recommend this type of join to all our users. Butt joints offer design-related potential to improve laser efficiency by 50% or more, as you can perform the butt weld at the same speed with a laser that requires 50% less energy compared to the overlap scenario.”