TWI Industrial Member Report Summary 918/2009
By P A Hilton and G Verhaeghe
Background
Only five years ago, anyone wishing to utilise the benefits of optical fibre 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 did not need to be considered, at that time, was the type of laser source to be used, as the only continuous-wave (CW) fibre-delivered laser source available was the lamp-pumped Nd:YAG rod laser. Since then, considerable advances have been made in high-power, fibre-delivered solid-state laser technology, which not only resulted in improvements to existing Nd:YAG laser technology, but also produced two new types of laser, these being the disc and fibre laser. These new solid-state lasers, both operating at similar wavelengths as the lamp-pumped Nd:YAG rod laser, are available at several kilowatts of output power and deliver this power through optical fibres, sometimes as small as 50µm in diameter. For instance, diode-pumped Nd:YAG rod lasers are now commercially available at powers up to 6kW, whereas Yb:YAG disc lasers are (commercially) available up to 8kW and Yb-fibre lasers up to 30kW.
These new solid-state lasers also possess very good beam quality. Beam quality signifies how well the beam can be focused into a small spot, and is usually expressed by the factor beam parameter product (BPP), measured in mm.mrad, with a low BPP value signifying a high beam quality. From a practical point of view, when welding, a better beam quality means that, for a given size of processing head, the beam can be focused into a smaller spot size, which in turn gives a high power density.
When laser welding using a given laser power, it is generally accepted that a smaller focal spot, ie higher power density, produces a better welding performance in terms of depth of penetration or welding speed, than a large focal spot. Additionally, a higher (laser) beam quality allows a larger stand-off distance to be used, for a given focal spot diameter (and power density), which reduces the risk of damage to the processing optics during processing. A large stand-off distance in combination with a small spot size results in a high beam brightness, which is defined as the power density in the spot per solid angle in the cone of the focused beam. It has been demonstrated by various researchers, that such (highly) focused laser beams are capable of producing welds with an exceptionally high aspect ratio, ie the ratio of weld penetration over weld width, resembling aspect ratios that, to date, could only be achieved using in-vacuum electron beam welding. However, achieving such high aspect ratio welds is not an obvious result when welding with high-brightness laser systems, with work by Verhaeghe and Hilton (2005a and b), Verhaeghe and Dance (2008) suggesting a limit in laser beam brightness beyond which the depth of penetration hardly changes.
This report details work carried out at TWI to further understand the advantages in welding performance, in terms of depth of penetration and/or travel speed, offered by high-brightness solid-state laser systems. An initial study consisted of a carefully controlled experiment comparing the welding performance (in terms of depth of penetration and welding speed) of a rod pumped Nd:YAG laser, with those of a disc laser and two Yb-fibre lasers, ranging in BPP from 23 to 4mm.mrad, on aluminium and steel. In this experiment, the power was kept constant at 4kW. Following this initial work, a further set of experiments was performed on steel, again at 4kW power, to determine the welding performance, in terms of depth of penetration and welding speed, of a 1.6mm.mrad Yb-fibre laser and a 4mm.mrad disc laser, and to compare the results obtained with those produced using an in-vacuum electron beam with an estimated BPP of 1mm.mrad.
Objectives
- Improve the understanding of the effects of beam quality/brightness on the welding performance, in terms of depth of penetration and/or welding speed, when welding with fibre-delivered solid-state lasers.
- Compare the welding performance, in terms of depth of penetration and/or welding speed, of a selection of new generation high-power, high-brightness lasers.
- Compare the welding performance, in terms of depth of penetration and/or welding speed, of a selection of new generation high-power, high-brightness lasers with that of in-vacuum electron beams.