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Laser Plasma Hybrid Welding of Austenitic Stainless Steels - Phenomena of Process Instability
(2007)
Laser plasma hybrid welding has been proved to be a very stable hybrid welding process and
welds of high quality can be produced, especially if high surface quality and low spattering is
demanded such as in welding fabrication of high alloyed austenitic stainless steels.
In particular cases, even though welds display high outer quality, X-ray examinations
revealed weld defects which may range from low porosity to blowhole-like cavities. The
phenomena and the main influencing parameters such as arc current, welding speed and
focal point position are discussed. Parameter fields will be suggested for welding plates of
different austenitic stainless steel grades with thicknesses ranging from 3 to 8 mm. The
results are based on welding experiments carried out using a 4.4 kW diode pumped Nd:YAGlaser.
Thus, also the influence of the feeding fibre diameter has been investigated and it was found
that the resulting beam shape has a major effect on the welding performance.
The results of a collaborative research project on laser beam weldability of carbon steels of
high sheet thickness are presented. That includes single and multiple pass welding of 16 mm
and 20 mm thick plates, as well as the investigation of acceptable tolerances i.e. gap
bridgeability and edge misalignment. For the welding experiments fibre lasers with 8 kW,
20 kW laser power and different MAG-techniques were used in various applications.
With the 20 kW fibre laser 16 mm plates could be welded with a single pass, 20 mm required
a seam preparation or alternatively preheating of the material. For multi pass welding with
8 kW laser power a joint preparation with a single V-butt joint with a broad root face (Ygroove)
was applied. The root pass was always welded with a hybrid process, the filler
passes with a hybrid process as well as a MAG process which produced the best results.
With an availability of the modern high power fibre lasers the application range of laser and laser hybrid welding can be significantly increased. For the implementation of this new technology in the economically extremely interesting application of pipeline construction it is necessary to fulfil a series of systematic research studies. As it has been shown in the past the fibre laser hybrid wielding of pipeline Steel X65 up to 32 mm thickness is possible with appropriate quality. For the use of laser-hybrid welding in pipe laying insensibility towards tolerances and the possibility of girth welding are very important factors in addition to the weld seam quality. The aim of a Research project carried out in the Federal Institute for Materials Research and Testing (BAM) was the development of a welding procedure using high power fibre laser in combination with GMAWProcess which could be applied for welding of pipelines. For tests segments of linepipes with an outer diameter of 914 mm and a wall thickness of 16 mm have been welded using specially designed equipment for girth welding of pipes. First promising results for the use of laser-hybrid welding of thick materials could be demonstrated. The gap-bridgeability and misalignement was studied.
New brilliant high-power lasers such as disc laser and fibre laser open up new fields of applications for laser-hybrid welding, e.g. power generation, shipbuilding and pipeline construction. For the use of laser-hybrid welding in pipe laying, insensibility towards tolerances and the possibility of orbital welding are very important factors in addition to the weld seam quality.
Within the scope of a basic research project, first promising results for laser-hybrid welding of thick materials could be demonstrated. As base material, the typical pipeline steel API 5L X65 was selected. With the help of a 20 kW fibre laser in combination with an arc-welding process it was possible to produce high-quality welds in plates of up to 20 mm thickness in a single pass and of up to 32 mm in three to five passes, both welded in position PA. Various joint preparations were examined to weld 20 mm thick plates in one pass. Besides different welding positions, gap bridgeability and misalignment were studied. Results for microhardness and Charpy toughness are presented to proof the applicability of laser-hybrid welding for joining in pipe laying.
Single-pass solid-state laser welding of plates in the thickness range of 10 to 20 mm became
possible with the invention of the fibre laser. This new technique provides excellent beam
quality at powers as high as 20 kW or more, and has proved applicable in several industrial
applications. By replacing conventional methods with the fibre laser, it is possible to avoid
multiple-pass welding that requires time-consuming bevelling. The high energy density of the
fibre laser beam also reduces the heat input and consequently the distortion. However, the
rapid solidification and cooling associated with laser welding can cause imbalance of the
microstructure of duplex stainless steel weldments, where excessively high ferrite contents
may reduce the corrosion resistance and the ductility of the material. The solution is normally
to add nickel-based filler wire and to increase the heat input. By using a hybrid welding
process where the laser beam and the gas metal arc (GMA) process act in a common
process zone, filler metal can be added to the molten pool at higher heat input and at the
same time, higher welding speed and deeper penetration can be achieved. In this work,
13.5 mm thick 2205 (EN 1.4462, UNS S31803) was fibre laser-GMA hybrid welded in a
single-pass using 14 kW of laser power and 2209 (ISO 22 9 3 L N) as filler wire for the GMA
process. The resulting welds were free from defects, with smooth surfaces and full
penetration. The investigation examines the weld metal microstructure and the effect on
corrosion resistance and mechanical properties. The option to add nickel foil when hybrid
welding was also investigated as comparison and the effect on austenite formation was
evaluated.