Although fiber lasers are well-established as a successful welding method in several automotive production applications, there are still some specific uses in which they deliver less than ideal results. Often, these involve welding “difficult” materials, such as aluminum or galvanized steel. One reason these materials can be difficult is that they frequently contain constituents having a relatively low boiling point compared to the main alloy metal. The presence of these more volatile materials can make it difficult to control the precise dynamics of the welding process, leading to poor or inconsistent results. The key to overcoming these difficulties is to alter the intensity distribution of the incident laser power to tailor it for the specific requirements of the material.

Beam Shaping for Enhanced Control

Traditionally, fiber lasers have utilized a single, round core delivery fiber which outputs either a single mode or multi-mode circular spot. Today there are several approaches that deliver more complex intensity distributions at the workpiece. These include utilizing delivery fibers that have either a non-round core (square or other shape), fibers which utilize multiple cores, and so-called “beam wobble” methods that rapidly move a traditional, round beam on the work surface.

Extensive development work at Coherent has proven that one solution for high speed, reduced spatter, deep welding of metals is to use a beam profile consisting of a central spot, surrounded by another concentric ring of laser light. Achieving this unusual configuration in the focused fiber laser spot is accomplished by using a delivery fiber that augments the conventional circular core by surrounding it with another, annular cross-section fiber core.

In Coherent’s embodiment of this technology, the power in the center and the ring can be independently adjusted on demand over a range of 1% to 100% of the nominal maximum output (which can be from 2.5 kW to 10 kW). The center and ring beams can even be independently modulated, at repetition rates of up to 5 kHz. The ability to ramp up or modulate the ring and center independently enhances the ability to control the weld geometry and melt pool dynamics.

While there is virtually an unlimited number of possible combinations in terms of the power ratio of the inner to the outer beam, these can be broadly grouped into the configurations shown in the drawing. These basic patterns can then be varied to deliver a wide range of processing characteristics to optimally service a diverse set of applications.

Zero Gap Welding of Galvanized Steel

Figure 1. Simplified schematic of the Coherent Adjustable Ring Mode delivery fiber and the five basic power patterns possible in the focused laser spot.

Galvanized steel is used widely in automobile “hang-on” parts, as well as in auto bodies. Zero-gap lap welding of this material has presented a challenge for lasers because the more volatile zinc evaporates first when the laser energy is applied to the material. This creates gas pressure which can blow out the molten steel, resulting in an inconsistent weld seam, as well as spatter that needs to be subsequently cleaned. This problem can be mitigated by either dimpling the material, or adding spacers between the metal sheets, so that there is sufficient space (~ 0.1 – 0.5 mm) for the vaporized zinc to vent in a controlled manner to the side, rather than the top of the weld keyhole during the process. But this approach has obvious drawbacks – added steps increase manufacturing complexity and/or cost.

The adjustable ring mode laser enables galvanized steel welding without the need for a gap between the parts. For optimized results, much of the laser power is distributed away from the beam center and into the edges. Doing this both pre- and post- heats the material surrounding the weld keyhole, thus increasing its elasticity. This reduces the pressure at the center of the keyhole and allows the zinc gas to vent out easily through the center without producing spatter, even when the parts are clamped together with zero gap. Plus, this arrangement stabilizes the keyhole and keeps it open; eliminating keyhole fluctuation is also key to reducing spatter.

It’s also worth noting that this power distribution is symmetric, so the orientation of the beam doesn’t have to be changed to follow the direction of the weld seam, which might vary substantially on a contoured or shaped part. This greatly simplifies its implementation and is totally compatible with a scanner head.

Powertrain Component Welding

Figure 2. Weld cross section showing four 0.77 mm thick sheets of galvanized steel welded together with zero gap. No spatter, “blowouts” or pores are seen in the weld.

Welding of automobile powertrain components is one area in which CO2 lasers are still utilized, because fiber laser welding typically produces some spatter. Such contamination is particularly unacceptable when welding moving parts, such as powertrain gears or bearing surfaces. Furthermore, spatter is often accompanied by weld porosity (since the spattered material may leave a void or an undercut), which can affect weld quality, strength, and consistency.

While completely optimizing powertrain component welding with fiber lasers involves multiple process factors, including laser power and gas delivery nozzles, the single biggest improvement comes from changing the focused beam profile away from a simple Gaussian or multimode circular spot to a power distribution which isn’t as highly peaked in the center. This minimizes turbulence in the melt pool, which then reduces spatter.

Figure 3. Fiber laser and CO2 laser welding of powertrain gears show essentially identical results, that is, a deep penetration weld with good aspect ratio and no spatter. Both welds were done at the same feed rate of 3 m/min.

The photo in Figure 3 shows that a properly configured fiber laser beam can deliver the same weld quality as a CO2 laser. In this case, there is a narrow weld seam (width < 1 mm), which is useful when welding parts with high dimensional accuracy (e.g. zero gap). This delivers minimal heat affected zone and distortion. The weld root width remains >0.3 mm, similar to a CO2 laser weld. However some powertrain welding applications have to bridge gaps, due to part inaccuracies. By smart adjustment of the optical setup, gaps of up to 0.3 mm can be bridged (without filler) by using a very calm weld with very little spatter and porosity.

In addition to reducing spatter, the fiber laser welding parameters used in this case also enabled higher speed processing. In fact, an automotive supplier who had utilized this technique stated that their throughput had increased by 20% over their previous fiber laser process.

Automotive Hang-on Parts Welding

In the manufacture of automobiles, the frame and various other body components (side panels, doors, hood, etc.) are first fabricated separately. Then these separate components, are “hung-on” to the frame to complete the assembly. Fabrication of these so-called “hang-on” parts presents several challenges to the manufacturer. For example, hang-on parts often have curved or unusual geometries, which can make them difficult to weld. In addition, hang-on parts form most of the exterior, visible (to the driver) surface of the automobile. Thus, weld and assembly cosmetics are very critical.

Hang-on parts frequently incorporate aluminum, which is one of the previously mentioned “difficult” materials for laser welding. This is because aluminum has a tendency to “hot crack” due to the loss of its alloying elements during welding. This creates the need to add material into the melt pool, typically in the form of filler wire. Additionally, spatter is sometimes problematic for hang-on parts since it introduces contamination that can get trapped within an assembly (such as a door). For example, manufacturers report problems with spattered material subsequently migrating into and blocking door draining holes.

Figure 4. These two fillet weld cross sections show the dramatic effect of changes in weld results with just minor adjustments in focused spot intensity distribution. In the first sample (A), the weld does not penetrate well into the bottom piece. A slight change in the intensity profile, which increased the pre-and post-heating effect, produced a weld (B) with excellent penetration, at the same feed rate of 5 m/min.

Minimizing or eliminating spatter and cracking again requires utilizing a laser intensity distribution that spreads the applied heat more evenly than traditional lasers. With a power distribution that ramps part heating up and down more gently, the cracking that tends to occur due to rapid part cooling is minimized. Spatter is reduced and the need to use filler wire is eliminated. Additionally, the pre-heating function of the ring beam increases the absorption of the aluminum at the 1 μm fiber laser wavelength (at lower temperatures, aluminum is highly reflective at this wavelength). This pre-heating allows higher and more stable coupling of the power from center beam into the material.

The results, and effectiveness of subtle changes in laser power distribution, are shown in the photo. By adjusting the beam intensity profile, the depth of weld can be controlled while keeping the weld width constant.

Conclusion

Altering the traditional intensity distribution of fiber lasers, which is usually strongly peaked in the center, has proven to be a useful approach for improving weld quality in a number of different applications. There are several ways to accomplish this end, and the Adjustable Ring Mode fiber laser presented here is just one of them. But no matter what the precise technology employed, process development – that is exploring the parameter space of the process to determine what consistently delivers the best results – is key to practical success. Thus, working with a laser or laser system vendor that has existing process knowledge and experience, and the ability to perform this type of development work, is often as important to success as the specific laser technology utilized.

This article was written by Jarno Kangastupa, Business Director & Product Line Manager of Custom Fiber Lasers, Coherent, (Canta Clara, CA). For more information, visit here .

Photonics & Imaging Technology Magazine

This article first appeared in the March, 2020 issue of Photonics & Imaging Technology Magazine.

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