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Further studies on the welding performance of high brightness lasers - Industrial Member Report 959/2010

 

TWI Industrial Member Report Summary 959/2010

by P A Hilton and J Blackburn

Background

In 2009 TWI published results of work (Hilton and Verhaeghe) to establish the welding performance of a range of continuous wave (cw) fibre and disc lasers. New fibre and disc lasers were compared with earlier Nd:YAG rod lasers and also with electron beams in terms of weld penetration as a function of welding speed. The work also introduced the concept of laser beam brightness. Defined as the power density in the spot per solid angle in the cone of the focused beam. Thus a large stand-off distance in combination with a small spot size implies a high beam brightness. The best performance, in terms of melt run penetration in steel, was found for the laser system with the highest brightness and not from the laser beam with the highest power density.

When the results from the high brightness laser were compared those from a conventional Nd:YAG rod laser, for example at a processing speed of 2m/min and a laser power of 4kW at least a two times increase in penetration could be seen, whilst at a penetration of 4mm, a five times increase in process speed was available. The optimum performance enhancement could only be achieved when a jet of argon was directed at the interaction point of the laser beam and the steel (termed side-jet and directed opposite to the travel direction) and an additional gas jet (termed cross-jet and directed perpendicular to the travel direction) was used to disturb the air flow in the region between the beam focusing lens and the workpiece. When using the highest brightness laser beam, if the gas system was switched off, then penetration was reduced and the process became more unstable. However, from the melt runs taken in this optimum condition, it would appear that a 4kW laser beam would be capable of autogenous butt welding 10mm steel at a speed of about 1m/min.

It was suggested that the effect of the power density of a very narrow laser beam passing through the atmosphere between its focusing lens and a workpiece, could be sufficient to produce an effect of thermal blooming on this gas column, changing its refractive index. This could significantly aberrate the laser beam. If the cross-jets applied to the high brightness laser beam were reducing or eliminating such a thermal blooming effect, this might enable the best performance from the beam. Another possibility suggested, was that under some circumstances, the beam might induce the generation of ionised plasma, in or above the welding keyhole and this could affect both weld penetration and stability.

One of the lasers used in the earlier work was a fibre laser with a beam parameter product (BPP) of 1.6mm.mradians. BPP is the product of the laser beam diameter and divergence angle at the waist. This laser was used in conjunction with a lens of 500mm focal length to produce the beam with the highest calculated brightness. The welding work using this laser was undertaken at IWS in Dresden. Based on the results obtained and outstanding questions summarised above, a further visit to IWS was arranged to perform an additional series of experiments using the same laser source. This report details the work performed on melt runs and the results obtained, as well as reporting on a series of butt welds made using the same laser, in 10mm thickness steel plate.

Objectives

  • Determine the effects of side and cross gas jets when welding with a high brightness fibre laser using two different focal lengths for the beam focusing lens.
  • Better understand the mechanisms which influence welding performance.
  • Investigate the capability to produce butt welds using a 4kW high brightness fibre laser beam.

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