Welding - using the new generation of high-power fibre-delivered lasers
TWI Bulletin, January - February 2007
Which is the most appropriate laser for your applications? The choice has risen enormously.
Geert Verhaeghe graduated in Belgium as a Mechanical Engineer in 1993, and European Welding Engineer in 1994. He worked for Arcelor's R&D centre in Ghent (Belgium) before joining TWI in 1996. Since then, he has worked in both the Arc Welding and Laser Welding Group, managing a variety of arc and laser-related projects.
Paul Hilton is Technology Manager, Lasers at TWI, where he has a responsibility for the strategic development of laser materials processing. He is a past president of the UK's Association of Industrial Laser Users and a board member of the European Laser Institute.
Only a few years ago, anyone wanting to use a high-power, fibre-delivered laser would have needed to consider its reliability, the capital outlay and the running costs, in addition to its performance for materials processing. There was no choice in the type of available laser since, at that time, the only CW fibre-delivered laser source was the lamp-pumped Nd:YAG rod laser. As Geert Verhaeghe and Paul Hilton report this situation has changed in recent years, with diode-pumped Nd:YAG rod lasers, Yb-fibre lasers and Yb:YAG thin-disc lasers all becoming commercially available at output powers of up to at least six kilowatts.
Today's laser user not only has to choose the laser source, but also needs to consider what laser beam quality or brightness is required for their application. In contrast with the original lamp-pumped CW Nd:YAG rod lasers, which could only achieve a beam parameter product (BPP) of about 20-25mm.mrad at 4kW, some of the newer technology lasers are available at the same power with a BPP as low as 2mm.mrad. The general welding performance of lasers has been the subject of many publications, with some also comparing the welding performance of the different sources.
A common problem, however, is that such comparisons are often between data obtained at different (sometimes lengthy) periods of time, on samples of slightly different thickness or composition, using slightly different processing arrangements, including spot size or even laser power, which makes it difficult to draw concise conclusions. For this reason, a controlled series of experiments was carried out by TWI, on both aluminium and steel, using a range of CW fibre-delivered laser sources and beam focusing systems to determine the available depth of penetration as a function of welding speed for a constant laser power of 4kW.
In the study, all experiments were performed on the same materials, the laser power measured using the same power meter, and the optical systems for each laser were chosen to produce, as well as a smallest spot diameter, a spot diameter as close as possible to 0.4mm. Four different lasers, one Nd:YAG rod laser, one Yb:YAG disc laser and two Yb-fibre lasers, with BPPs between 23 and 4mm.mrad, and seven combinations of delivery fibre, collimating lens and focusing lens, producing spot diameters ranging from 0.61 to 0.14mm in diameter, were used. All welding was performed with the beam waist positioned on the workpiece surface, notwithstanding the fact that the optical systems used produced a range of Rayleigh lengths from 1 to 10mm. Melt run trials were carried out on 5mm and 10mm thickness 5083-O aluminium alloy and S275 grade C-Mn steel. The samples, were machined to give a tapered profile, such that in a single pass at a constant speed, this taper produced both partial and full penetration, giving an estimate of the conditions for full penetration. To eliminate heat-sink differences, the same clamping arrangement was used for all welding trials. For each of the BPP/spot size combinations, the depth of penetration data obtained was plotted against welding speed, as the starting point for the analysis. Although the results discussed in this article are all related to the welds in aluminium, similar trends were observed for the melt-runs made in C-Mn steel. For all the work reported here on aluminium the welds were shielded using argon gas delivered by a simple side jet.
The curves in Fig.1 show the depth of penetration against the welding speed for the two extremes of BBP/spot size used in the experiments. The advantage of a smaller spot/smaller BPP in terms of depth of penetration is obvious, and more pronounced as the welding speed is increased. For example, at a welding speed of 5m/min, the 4mm.mrad source (Yb-fibre) focused in a 0.14mm spot offers a 60% increase in depth of penetration over that available from the 23mm.mrad source (Nd:YAG rod) focused in a 0.61mm spot. Or, for a depth of penetration of 5mm, the fibre laser provides a 250% increase in welding speed, over that available using the Nd:YAG laser.
Fig.1. Depth of penetration as a function of welding speed in aluminium at the two extremes of spot size/BPP used in the trials
In contrast, Fig.2 shows the set of performance curves obtained using the four available laser sources all focused to a spot size close to 0.4mm in diameter. In this case the performance curves fit very much one on top of another, with,for each curve, little difference between the relative performance at low and high speeds. The lowest performance in terms of penetration and welding speed is for the lamp-pumped Nd:YAG rod laser, with the lowest BPP of 23mm.mrad,followed by that of the 18mm.mrad Yb-fibre laser and that of the 4mm.mrad Yb-fibre laser. Unexpectedly, the 7mm.mrad Yb:YAG disc laser outperformed the 4mm.mrad Yb-fibre laser, in these welding trials. A possible reason for this is given later.
Fig.2. Depth of penetration as a function of welding speed in aluminium, for the four laser sources used, each configured to provide a spot close to 0.4mm in diameter
Significant welding speed dependence is also found for the depth of penetration, as a function of spot size, as shown in Fig.3. This figure shows data for each of the seven combinations of BPP/spot size used, 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 , the data shows an approximate linear behaviour with different slopes for the different welding speeds, but above this value a change in the slope of the curves is clear. The depth of penetration remains essentially constant for spot sizes below 0.3mm in diameter. These welds in aluminium were carried out with argon side-jet shielding only, with no special measures taken to minimise plasma/plume effects. It is possible that this behaviour was influenced by absorption/scattering of the incident laser beam by a plasma/plume above the keyhole. Further work in this area is required.
Fig.3. Depth of penetration vs 1/spot size for three different welding speeds
A plot of molten area (obtained from the weld cross-sections by using AutoCAD software) against the inverse of the spot size at welding speeds of 1 and 15m/min, reveals a similar trend to that observed for the depth of penetration.The molten area increases linearly as the spot size is reduced, down to a spot size of 0.3mm; below which the molten area remains approximately independent of spot size. For spot sizes larger than 0.3mm the molten area also increases with laser beam quality/brightness, as seen in the cross-sections in Fig.4, which show two welds made with a 0.4mm spot size at a welding speed of 15m/min, but with different BPPs of 23 and 4mm/mrad.
Fig.4. Cross-sections of two welds made using a spot size of 0.4mm at a welding speed of 15m/min, using a laser with a BPP of 23mm.mrad (left) and 4mm.mrad (right)
The inflection point of 3 mm -1 is thought to be the condition at which maximum melting efficiency is achieved, with the transverse cross-sectional area remaining essentially unchanged beyond this point for a given welding condition. Below3mm -1 any change in spot size or beam quality (brightness) only affects the shape of the weld, with a narrower and deeper weld obvious for a higher brightness and/or smaller spot size. The latter is demonstrated by the welds, shown in Fig.5, which were both made using the 7mm.mrad Yb:YAG disc laser at a welding speed of 15m/min, but with different spot sizes of 0.34 and 0.2mm.
Fig.5. Cross-sections of two welds made using the 7mm.mrad Yb:YAG disc laser at a welding speed of 15m/min, with a spot size of 0.34mm (left) and 0.2mm (right)
A plot of the performance data for each of the seven combinations of BPP/spot size against the brightness of the focused beams reveals another interesting trend, as shown in Fig.6. Here, brightness is defined as the available power density per solid angle in the cone of the focusing beam, in W/mm 2 .steradian. The figure shows the depth of penetration increasing with an increasing laser beam brightness, up to around 33x105 W/mm 2 .steradian, which appears to be the optimum brightness for maximising the depth of penetration when welding aluminium, independent of welding speed.
Fig.6. Depth of penetration as a function of laser beam 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 Yb:YAG disc laser (top) and the 4mm.mrad Yb-fibre laser (bottom), at spot sizes of 0.4mm
Beyond this brightness, the depth of penetration apparently reduces, again independent of welding speed, although it is noted that this behaviour is currently based only on one set of data points. This trend, which is also seen from the data on steel (although the 'optimum' brightness for steel appeared to be slightly speed dependent) provides an explanation for the results shown in Fig.2, which show the welding performance of the 7mm.mrad Yb:YAG disc laser to be better than that of the 4mm.mrad fibre laser.
The curves in Fig.2 for the 4mm.mrad Yb-fibre and the 7mm.mrad Yb:YAG disc laser were constructed using the same data points as those shown in Fig.6 at a brightness of about 55x105 and 18x105 W/mm2.steradian, respectively. Both these sets of data correspond to a spot size close to 0.4mm in diameter and from Fig.6, it can be observed that the points for the 7mm.mrad Yb:YAG disc laser are systematically higher with respect to penetration, than those for the 4mm.mrad Yb-fibre laser. It is thought that this is because in these experiments, the Yb:YAG disc laser and its particular beam delivery system, despite a lower BPP, sits closer to the 'optimum' brightness than the 4mm.mrad Yb-fibre laser. This also means, that with an 'optimum' beam delivery system,both these lasers with their respective BPPs should be capable of improving on the performance recorded in this work.
The results shown in Fig.3 indicate that, if choosing an optimum welding system with the capability to process over a range of welding speeds and material thicknesses, using the set-up and conditions used in these trials, there is little benefit in aiming to achieve a focused spot smaller than 0.3mm diameter.
Combining this with the 'optimum' brightness value of 33x105 W/mm 2 .steradian for aluminium, would mean that this is achievable using a focusing lens with a focal length of around 350mm, for a process head with an aperture of 50mm, for instance. If the numerical aperture of the beam delivery fibre 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 fibre with a diameter of about 0.15mm would be necessary. In order to use such a fibre,the beam parameter product of the laser required, would then have to be between five and 7mm.mrad.
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 in terms of welding performance for both aluminium and steel, 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 appears available at an 'optimum' brightness, which is less than the brightness available from several of the laser/focusing optic combinations used in this work. It is clear that more effort is needed to interpret some of the results presented here and to obtain the best welding performance from the new and exciting range of high beam quality fibre and disc laser sources which are now commercially available.
