There have been many changes in laser technology over the past 30 years. With each advancement comes new challenges and opportunities. The CO2 laser with 10-micron wavelength was king for many of those years because it was versatile, from cutting thin to thick plates and welding fast while maintaining high quality welds. A little over 10 years ago, high brightness 1-micron wavelength fiber and disk lasers came into the market with great promise. Everyone thought the higher absorption in steel (from ~5% up to now ~40%) (Figure 1) was the holy grail for faster, better and greater flexibility in laser processing, with a 200% increase in energy efficiency.
Although the lasers were very energy efficient and could even cut 1/8” steel much faster, we started to see challenges come to light. Cutting thick plates was a challenge because the new wavelength was absorbed much better, which meant you had less energy getting to the bottom of the thick plate. These challenges have since been conquered with new cutting optics, steeper angled focusing optics and cutting nozzle design.
One of the tools TRUMPF developed and patented was BL (BrightLine) using a 2in1, or dual core, delivery fiber (Figure 2). This concept, along with some features in our system design, allows us to put the laser energy in the center core when cutting thin plate steel. When cutting thicker plate, it can activate a switch in the laser to direct the light into the outer core, increasing our spot size for the thicker plate without having to change the optical set up. This gives a quick and easy way to switch from one job to another without changing the dynamics of the system's motion control. Nowadays we can cut foils to 1.0” thick plate steel with disk lasers with ease.
This was the first hurdle the laser industry took on, as about 80% or more of the lasers sold are used for cutting applications, not welding. In recent years, however, laser welding has grown significantly. Many of the parts being welded today are small and thin gauge materials where traditional MIG & TIG processes will not work. As with cutting, the 1-micron vs 10-micron wavelength provided new challenges. The major benefit of higher absorption was dramatically less heat put into the part, reducing the HAZ (heat affected zone), which allows us to weld faster and use thinner materials (Figure 3).
However, due to the higher absorption in the weld, pool/keyhole formation was much more violent, causing significant spatter (Figures 4 and 5). High spatter results in weaker welds with some material being expelled, moving parts and gears can bind up or get damaged from the debris, and with electronics, it can cause electrical shorts. Maintenance costs increase as more frequent cleaning of tooling/fixturing is required, along with lens protection cover glasses, which can be damaged quickly.
Figure 5 shows the amount of spatter comparison between standard and BLW. We have adjusted spot sizes and found optimal feed rates to reduce the spatter, but they tend to limit processing time. Additionally, many have introduced mechanical shielding methods to protect parts and make cleaning easier. TRUMPF has now introduced a novel new concept, expanding on its Bright-Line cutting mechanism to now offer BLW (BrightLine Weld). BL lets you put laser power in one core or the other; in BLW we split the laser power into both cores at the same time, allowing you to vary power 10 - 90% between the two cores for optimized results (Figure 6). One of the unique features of the TRUMPF TruDisk is the free space optic configuration before launching into the process fiber; we utilize the entire power of the laser and reduce splicing of multiple fiber modules into the core. This enables us, in some cases, to use a 2 or 4 kW BLW laser instead of a 7 or 8 kW fiber spliced design to achieve the same performance.
The first application looked at was automotive powertrain welding. The application has controlled pressed fit joints, consistent material quality, and requirements of fast welding, low distortion from heat input, and little to no spatter formation as it goes into a transmission box with many moving gears. BrightLine Weld technology allows a very flexible range of parameters for optimizing energy efficiency or machine productivity. If BrightLine Weld is used to optimize energy efficiency, as shown in Figure 7 on the left, the part can be welded at common feed rates, such as, v = 5 m/min with a laser power of P = 2 kW and a penetration depth of about 3 mm. The spatter formation is low, so no exhaust is required, reducing costs. On the other hand, it is possible to improve the productivity by increasing the feed rate combined with using higher laser power. This case is shown on the right side of Figure 7. The same high-quality weld seam with the low spatter behavior and equal penetration depth can be achieved with three times the weld speed at v = 16 m/min and P = 5 kW.
Increasing the feed rate leads to a higher cooling rate of the molten material. The tests were done on an axial seam between a shaft and a disk. The shaft is made of 20MnCr5, the disk with a thickness of 5 mm is made of 16MnCr5. Further investigation of the cross sections of the weld seams show a slim V-shape weld. The weld with BrightLine Weld is narrow compared to state of the art. Similarly, the HAZ is narrower. The cross-sectional areas are 1.08 mm2 for the seam area (comparison state-of-the-art 1.70 mm2) and for the HAZ a total of 0.79 mm2 for both sides (comparison 1.76 mm2). The average welding depth is 3.6 mm.
The structural changes in the heat-affected zone are similar for the BrightLine Weld welding process on both sides of the seam. With the fine-grained material, rather than the coarser-grained steel, only the former pearlite grains are converted to martensite. The former ferrite grains are not converted or only near the grain boundaries. The higher welding speed of the BrightLine Weld means that there is less time for carbon diffusion and less conversion of the ferrite.
The hardness curves in Figure 8 show the hardened zones for the BrightLine Weld process are narrower compared to the state-of-the-art weld without beam shaping. This reflects the described widths and areas of the welds and heat-affected zones. Regarding the size of the hardening, both methods are approximately equal. Hardness of about 450 to 470 HV0.1 are achieved in the welds. In the heat affected zones, the measured hardness depends on whether the hardness impressions affect ferritic or martensitic grains. Hardening occurs in martensitic grains up to 650 HV0.1. On average, in the BrightLine Weld process, HAZ appears to contain a lower proportion of highly hardened sites, which is related to the higher proportion of residual ferrite grains
As you can see in this application of a tight, pressed-fit butt weld, the BLW technology has reduced spatter by 90 – 95% while still giving the same or better performance hardness, HAZ and cracking. Additionally, if productivity is not a major concern, you can buy a less expensive, lower-power laser. If productivity is the main driver, a similar power of a single beam laser can increase feed rate up to 300%.
Once the technology proved out on this application, we turned to other welding materials and joint configurations. Stainless steel saw dramatic reduction of spatter formation of 90 – 95%, but speed increase was only about 100% vs 300% on mild steel. This is important since many stainless welding applications are for cosmetic weld seam, so a combination of BLW with a shield gas can solve the cosmetic issues. Other weld joint designs have also been evaluated and BLW has shown the same reduction of spatter, but changes in the process include increasing spot sizes or oscillation of the beam. This is required as the BLW is fairly narrow when using 1:1 imaging ratio optics and weld strength on overlap welding. With fillet welding, the weld strength is measured by the weld interface zone of the two pieces of material instead of the weld depth on a butt weld.
A very active market now is electric cars with battery and motors, along with electronics, general electrical and power storage industries, that have a heavy use of copper and aluminum. The BLW dramatically improves the weld quality and speed in these materials along with reducing spatter. In these industries you need to maintain high quality electrical conductivity and minimal impedance while minimizing brittleness in the weld zone and making the weld look good cosmetically. Figure 9 shows an example of welding aluminum with a single core fiber vs a BLW dual core fiber, and Figure 10 shows welds in copper. Testing in these materials is underway.
In summary, beam shaping using BLW in laser welding by means of coaxially superposed beams was successfully applied in gear wheel welding at high speeds. In this case, the welds were conducted with three-times the weld speed compared to the state-of-the-art. Endurance testing of high-speed welds shows that typical endurance strength for laser welds is ensured.
In welding of materials with low viscosity in the liquid phase, such as copper, beam shaping BrightLine Weld has shown further positive effects. Significant spatter prevention is achieved for deep penetration key hole welding. Welding of 6000 series aluminum suffers from hot crack phenomena in lap weld conditions with small flange sizes. By applying beam shaping, well controlled partial penetration welds free from spatter were achieved. This offers new possibilities for welding tasks in automotive hang-on parts due to high feed rates that are possible by applying linear welding.