TWI Industrial Member Report Summary 1013/2012
By A Woloszyn
Ytterbium (Yb) fibre and disc lasers have attracted a lot of interest from laser users in recent years, because of the advantages of electrical opposed to optical power efficiency (between 25 and 30%), maximum output power (up to 16kW for disc and 50kW for fibre lasers) and beam quality (<1mm.mrad). These lasers are now generically referred to as 'high brightness' lasers. The improved beam quality and beam brightness of these lasers is of particular interest, allowing smaller spot sizes or longer stand-off distances to be used when welding. These offer both improved welding performance and better protection for the optics in the process head from spatter, fume and debris. The availability of these solid state lasers, at ever increasing output powers, allows thick-section welding at high speeds in a single pass, previously only possible with in-vacuum electron beam techniques (Verhaeghe and Dance, 2005).
The current welding performance study aimed to apply the process knowledge gained during earlier preliminary investigations to an industrial / commercially available (IPG YLS-5000) 5kW fibre laser system, where it was found that a 'cross-jet' of air flowing between the workpiece and the focusing lens could greatly influence weld penetration, with an associated range of standard process optics. The work scope also aimed to quantify the interaction of a range of key variables in combination with maximum laser power output. These included:
- Focused spot size (0.2, 0.38, 0.47 and 0.50mm).
- Focus distance (160, 300, 400 and 500mm).
- Cross-jet shielding (Applied or not-applied).
- Mode of relative movement between laser beam and workpiece (workpiece or process head moves).
- Changes and variability in penetration were documented across a full range of variable combinations.
- Determine the maximum performance, in terms of welding speed, that can be achieved in 1-10mm thickness C-Mn steel using commercially available 5kW high-brightness fibre-delivered laser beams.
- Quantify the influence of the cross-jet shielding for maximising depth of penetration.
- Determine the optimum focusing and collimating lens combination for maximising weld penetration.