Dipl.-Ing. Christian Schwalenberg (Schweißtechnische Lehr- und Versuchsanstalt Halle GmbH)
Use of the GMA laser beam hybrid orbital process for the orbital welding of pipelines
The efficiency in pipeline construction has continually increased over the years, starting with the development of special vertical-down weld electrodes right through to automated GMA processes with adapted seam preparations. Ultimately, however, only small increases in melting deposition rates can be expected, because the melting pools get out of control very quickly at higher wire feeding rates, due to low welding speeds and the influence of gravity.
The GMA laser hybrid welding process is a very promising alternative. The excellent gap bridge ability of the GMA process combined with the high penetration depth of the laser beam open up completely new possibilities when the two processes are merged, as well as synergetic effects which have a positive influence on the welding process. SLV Halle GmbH has been carrying out studies with partners from the pipeline construction industry for several years now to boost the efficiency of the GMA laser hybrid process for pipeline construction. The aim is to prove the basic suitability of this method for pipeline construction, since both welding depth and welding speed can be increased if the GMA process is supported by a laser beam welding process. The filler material added by the GMA process allows to influence the melting pool metallurgically. Through their interaction, the two methods have a mutually stabilising effect and both GMA arc and laser beam are vital for attempts to increase efficiency in pipeline construction while retaining high quality standards.
The knowledge gained in preliminary tests led to the decision to carry out welding of the pipe joints using the hybrid welding process not in one work cycle, but rather using two vertical-down welds. A further torch was attached for fast changing from the first to the second vertical-down weld. Thanks to the integration of the arc process following hybrid welding, closed seam profiles were able to be produced, and hardening of the heat effected zones in the root area was reduced.
The typical conditions in the vertical-down welding position and the continuous influence of gravity on the melting pools mean that a torch arrangement in the direction of welding cannot be considered, since the arc forces further enhance the influence of gravity and the melting pool is even more likely to advance. The best results were achieved with a torch adapted in advance and optics arranged neutrally to the joint.
The results presented reveal the potential of the hybrid process for high laser powers and brilliant beam qualities. Besides the proof of general constructionsite suitability for the process, extensive knowlege was gained about the out-of-position hybrid welding and the equipment design. Finally, it leads to the design of a specialised prototype. This is available to potential partners both for technological studies and for further field tests.