Selecting the source

Today’s users of fiber-delivered laser beams for welding must choose among Nd:YAG, fiber, and disc technology

Paul Hilton and Geert Verhaeghe

Only a few years ago, anyone wishing to utilize the benefits of fiber-optic beam delivery from a high-power laser source would need to consider the capital outlay, the running costs, the reliability, and the capability of the laser to perform the process required. What they didn’t need to consider was the type of laser source to be used, as the only CW fiber-delivered laser source available was the lamp-pumped Nd:YAG rod laser. Today’s users must now be concerned with the type of technology used in generating the laser beam, due to recent advances made in the generation of CW infrared laser radiation with wavelengths suitable for transmission down an optical fiber. Diode-pumped Nd:YAG rod lasers, Yb-fiber lasers, and Yb:YAG thin-disc lasers are all now commercially available at powers up to at least 6kW.

Laser users are now faced with the additional questions of laser beam quality and brightness, as the original lamp-pumped CW Nd:YAG rod lasers could only achieve a beam parameter product (BPP) of about 20-25mm.mrad at 4kW, whereas some of the advanced laser technology mentioned above can operate at 4kW with a BPP as low as 2mm.mrad. This article reports on the welding process capability of a range of CW fiber-delivered laser sources and beam focusing systems, in a controlled series of experiments on aluminum and steel, to determine depth of penetration as a function of welding speed for a constant laser power of 4kW.

In our work, all the experiments were performed on the same materials, all laser powers were measured with the same power meter, and the optical systems were chosen to produce, as well as a ‘smallest’ spot diameter, a spot diameter as close as possible to 0.4mm, for each of the different laser sources used. Four different lasers, an Nd:YAG rod laser, a Yb:YAG disc laser, and two Yb fiber lasers, with BPPs between 23 and 4mm.mrad were used. With these four lasers, seven combinations of delivery fiber, collimating lens, and focusing lens produced beam waists in the range from 0.61 to 0.14mm in diameter. In addition, four beam focusing systems were configured to produce a beam waist very close to 0.4mm in diameter. In the work reported here, all welding trials were made with the beam waist positioned on the surface of the workpiece, however the optical systems used produced a range of Rayleigh lengths from 1 to 10mm.

Prometec and Primes laser beam analyzers were used to measure the beam caustic in the region of the beam focus. In one series of measurements, both analyzers were used for the same laser with the same processing optics, revealing only small differences in measured values, within ±3 percent. In all cases the laser power was adjusted so that the welding trials were carried out with a laser power of 4kW at the workpiece, although some of the lasers used for the trials could operate well above this power. An air knife was used with each beam focusing system to reduce the risk of smoke, fume, and/or spatter damaging the cover slide and focusing optic. The aluminum weld pool was shielded using 8 and 5 liter/min of argon applied to the top and bottom of the weld, respectively. No gas shielding was used for the steel welds.

Melt run trials were carried out on 5mm and 10mm thickness S275 grade C-Mn steel and 5083-O aluminum alloy. The samples were machined to give a tapered profile, so that in a single pass at a constant speed, both a partially penetrating weld profile and an estimate of the conditions for full penetration could be found. The samples were clamped with the tapered machined side facing downwards. To eliminate differences in heat-sink, the same clamping arrangement was used for all four lasers. For each of the laser/spot size combinations, the data obtained was used to construct graphs of penetration against welding speed, as a starting point in the analysis of the results. Although the results discussed in this article are related to the welds in aluminum, similar trends were observed for the melt-runs made in C-Mn steel.


FIGURE 1. Penetration (Al) vs. speed at two spot size/BPP used in the trials.
Click here to enlarge image

Figure 1 shows the penetration versus speed curves for the two extremes of BPP/spot size used in these experiments. The potential advantages of the smaller spot/better BPP are very clear from this figure. For example, at a welding speed of 5 m/min, the 4mm.mrad source (fiber) offers a 70 percent increase in penetration over that available from the 23mm.mrad source (rod Nd:YAG). Similarly, for a penetration of 4 mm, the 4mm.mrad source provides a 250 percent increase in welding speed, over that available using the 23mm.mrad source. In addition, it is apparent from this figure, that as the welding speed increases, the increase in performance offered by the better beam quality laser gets larger.


FIGURE 2. Penetration (Al) vs. speed for the four laser sources used, each configured to provide a spot size close to 0.4mm in diameter.
Click here to enlarge image

In contrast, Figure 2 shows the set of curves obtained using the four available laser sources, but with the focusing optics arranged to produce, in all cases, a spot size close to 0.4mm in diameter. What can be seen from this figure is that these performance curves fit very much one on top of another, with, for each curve, little difference seen between the performance at low and high speeds. As to be expected, the poorest performance, in terms of penetration and speed, can be seen from the rod Nd:YAG laser, with a BPP of 23mm.mrad. The next curve is from one of the fiber lasers, with a BPP of 18mm.mrad, and the curve above this is for the second fiber laser, with a BPP of 4mm.mrad. In this set of trials the highest performance, in terms of penetration and speed, was recorded from the disc laser, which had a BPP of 7mm.mrad. It might be expected that these two latter curves should be positioned the other way round, with the 4mm.mrad laser outperforming the 7mm.mrad laser. This apparent discrepancy in the results will be discussed again later.


FIGURE 3. Penetration gain vs. welding speed when changing spot size.
Click here to enlarge image

In Figure 1, the observed gain in performance by increasing the beam quality or decreasing the spot size becomes larger as the welding speed increases. Figure 3 shows the relative increase in penetration as a function of welding speed, when changing from a 0.4mm spot to a 0.14mm spot for the 4mm.mrad fiber laser, and for the 23mm.mrad Nd:YAG rod laser, when changing from a spot size of 0.6mm to 0.44mm. For the fiber laser, the graph shows that the advantages to be gained in terms of penetration from moving to the smaller spot are limited to less than 10 percent for welding speeds lower than 7.5m/min, but increase sharply, and apparently linearly, above this speed. This indicates that the mechanism that determines the depth of penetration is changing at a speed of about 7m/min. When welding with near-infrared light, it is usual to consider an energetic plume of excited vapor as emerging from the keyhole, as opposed to an ionized vapor or plasma, seen when welding with CO2 lasers. It is possible that the lack of increase in performance seen in this work, below speeds of 7m/min, is in fact related to the formation of plasma above the keyhole at the high power densities available with the fiber laser. However, the data for the Nd:YAG rod laser show a similar trend, and it has been shown previously that plasma is not present when welding with the Nd:YAG laser, at the power densities used in this experiment.


FIGURE 4. Penetration vs. 1/spot size.
Click here to enlarge image

A significant speed dependence can also be observed in Figure 4, which shows the depth of penetration obtained for each of the seven combinations of BPP and spot size used in this work, plotted against the inverse of the spot size, for three different welding speeds of 1, 5, and 15m/min. Up to a value of 3 mm-1, that is, corresponding to spot sizes between 0.3 and 0.61 mm used in the trials here, the data points show an approximate linear behavior, with different slopes, corresponding to the different welding speeds, as might be expected. However, above l/spot size values of 3 mm-1, changes in the slope of the data become obvious. At the slowest welding speed of 1m/min, no additional gain in penetration whatsoever can be seen, for any spot size below 0.3mm in diameter. This behavior is similar for the welding speeds of 5 and 15m/min, however, at these speeds some small increase in depth of penetration is still observed above the value of 3 mm-1, although the inflection points in the data are still clear. This would indicate that, unless welding at high speeds, essentially over 7m/min, there would seem to be little reason to use a focused spot size of less than 0.3mm in diameter.

When the penetration data for the seven combinations of BPP/spot size are plotted against the brightness of the focused beams in use, another interesting trend can be seen, as shown in Figure 5. Here brightness is defined as the available power density per solid angle in the cone of the focusing beam, in W/mm2.steradian. As can be seen, the depth of penetration increases with an increasing laser beam brightness up to around 33x105 W/mm2.steradian, which appears to be the optimum brightness for maximizing the depth of penetration when welding aluminum, regardless of travel speed. Beyond this brightness, the depth of penetration appears to reduce, although we note that this behavior is based only on one set of data points.


FIGURE 5. Penetration vs. brightness for three different welding speeds. The dotted line approximately represents the ‘optimum’ beam brightness. The sets of points highlighted by the vertical boxes represent data points from the 7mm.mrad disc laser and the 4mm.mrad fiber laser, at spot sizes of 0.4mm.
Click here to enlarge image

This trend is also seen for welding steel, although the ‘optimum’ brightness for steel appears to be slightly speed dependent. This figure can also be used to explain the apparently anomalous behavior seen in Figure 2 and described earlier, in that the performance of the 7mm.mrad disc laser appeared better than the 4mm.mrad fiber laser. The curve in Figure 2 for the 4mm.mrad fiber laser was established using the same data points as can be seen on the extreme right hand side of Figure 5, (at a brightness of about 55x105 W/mm2.steradian), whereas the curve in Figure 2 for the disc laser was assembled from the set of data points lying at a brightness of about 18x105 W/mm2.steradian. Both data sets correspond to a spot size close to 0.4mm in diameter. When these two sets of data are seen in Figure 5, it becomes clear that the points for the disc laser are systematically higher with respect to penetration, than those for the fiber laser. It is believed this is because the disc laser, and its particular beam delivery system, sits closer to the ‘optimum’ brightness than the fiber laser, even though its BPP is less than the fiber laser. The figure also shows, that with the ‘optimum’ beam delivery system, either the fiber laser or the disc laser would be capable of producing a set of penetration versus speed data, which should sit above any of the curves currently presented in Figure 2.

The results shown in Figure 4 indicate that when choosing an optimum welding system with the capability to process over a range of welding speeds and material thicknesses, there would appear to be no real benefit in using a focused spot smaller than 0.3mm in diameter. Combining this figure with the ‘optimum’ brightness figure of 33x105 W/mm2.steradian indicates that this should be achievable using a lens with a focal length of around 350mm, for a focusing system with an aperture of 50mm, for instance. If the numerical aperture of the beam delivery fiber is of the order of 0.2, then, in order to achieve the 0.3mm spot size with a 175mm focal length collimating lens, a delivery fiber with a diameter of about 0.15mm would be necessary. In order to use such a fiber, the beam parameter product of the laser required would then have to be between 5 and 7mm.mrad.

Conclusions

The consistency of the results from this carefully controlled series of trials has shown that the type or wavelength of the lasers used does not impact significantly on the resulting welding performance in terms of penetration and speed for any given source. What is important is the combination of beam parameter product available and the focused spot size used. It would also appear that these two parameters are linked (in practical terms by the design of the beam delivery and focusing system) to the brightness of the resulting laser beam. Maximum performance in terms of penetration and speed is available at an ‘optimum’ brightness, which is less than the brightness available from several of the laser/focusing optic combinations used in this work. Much more work is needed to explain the physics of some of the results presented here and to obtain the best welding performance from the new range of high-beam-quality fiber and disc laser sources that are now commercially available.

Acknowledgements

Thanks to Anthony Elliott, Paul Fenwick, and Harvey Whitmore, from TWI, and to TRUMPF and IPG for their assistance. The U.K.’s Yorkshire and Humber Regional Development Agency provided support.

Paul Hilton (paul.hilton@twi.co.uk) and Geert Verhaeghe (geert.verhaeghe@twi.co.uk) are with TWI Ltd, Great Abington, Cambridge, UK.

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