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Eingeladener Vortrag
- nein (14)
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.
Short Term Metallurgy and Hot Cracking During Laser Beam Welding of Austenitic Stainless Steels
(2011)
Industrial application of high alloyed austenitic stainless steel laser welding has grown significantly in the recent time due to the continuous improvement of compact and high power density lasers systems. The application of such processes meanwhile ranges from pipeline or railway car body manufacturing to the production of household wares. The largest advantages of the laser application to welding production are much higher welding speeds, reduction or complete exclusion of welding consumables, easy design of the weld joints, decrease of thermal distortions and thus, costs saving. In contrast to arc welding, laser beam welding might particularly be associated with metallurgical defects, like the formation of hot cracks. Such phenomena are related to an order of magnitude higher temperature gradients and cooling rates in the solidification zone, providing rapid solidification kinetics which may cause significant segregation of alloying elements such as Ni and Cr and respective undercooling of the solute at the solidification front. In specific metastable austenitic stainless steels alloys in vicinity of the so called eutectic rim of the Fe-Cr-Ni constitutional diagram, such effects might entail a change of solidification mode from primary ferrite to austenite, providing an increased risk of solidification cracking. Previous studies has shown that the primary solidification mode change during laser beam welding of Cr-Ni austenitic stainless steels such alloys could be effectively influenced by nitrogen absorption as well as by the laser plasma type and also proved the occurrence of metastable primary ferritic solidification. In the present contribution, such results are compared to recent investigations of laser welding newer austenitic Fe-Cr-Mn-Ni steel grades by identification of respective hot cracking critical welding parameter intervals and strain rates in the Controlled Thermal Weldability (CTW) Test.
Schweißeigenspannungen - im Besonderen infolge von Strahlschweißprozessen - annähernd der werkstoffspezifischen Streckgrenze können durch eine mögliche Spannungsrelaxation oder verstärkte Spannungsrisskorrosion lebensdauerreduzierende Konsequenzen mit sich ziehen. Aus diesem Grund wurden viele Verfahren entwickelt, welche diese Eigenspannungen reduzieren können. Jedoch sind diese Verfahren kostenintensiv oder nur für breite Schweißnähte und einfache Bauteilgeometrien anwendbar. Die in dieser Arbeit dargestellte Methode nutzt den Schweißstrahl für den Schweißprozess sowie in defokussierter Form für eine thermische Nachbehandlung des Bauteils. Hierfür läuft der defokussierte Strahl durch hochfrequente Strahlablenkung beidseitig der Schweißnaht mit einer bestimmten zeitlichen Distanz dem Schmelzbad nach und wird zum Aufheizen dieser Materialbereiche genutzt. Mit dieser Prozedur ist es möglich die Spannungen in schmalen Schweißnähten mit großen Spannungsgradienten im Vergleich zu anderen spannungsreduzierenden Verfahren ohne Kontaktflächen oder zusätzlichem Equipment effizient zu reduzieren. Abhängig von der Bauteilgeometrie und der Strahlleistung können unterschiedliche Prozessparameter für dieses Verfahren angewandt werden. Es werden die Mechanismen der Eigenspannungsreduzierung in diesem Prozess und deren Abhängigkeit von den Prozessparametern anhand von Ergebnissen aus FEM-Simulationen diskutiert. Zudem wird in einer Reihe von experimentellen Ergebnissen die Anwendbarkeit von bereits für lineare Schweißnähte ermittelte Verfahrensparameter auf axiale und radiale Rundnähte nachgewiesen. Hierbei zeigen die besten experimentellen Ergebnisse für Elektronenstrahl- sowie für Laserstrahlschweißnähte Spannungsreduktionen größer als 90 %.
Experimental investigation of the laser-plume interaction during high power fiber laser welding
(2011)
The effect of the well-known plasma absorption and refraction in CO2-laser metal welding plumes is in case of high power solid state laser welding negligibly small. By contrast, the diffraction effects of shorter wavelength laser radiation are considerable. According
to the results of preliminary studies, the fine condensed metal particles in the welding plume can lead to essential worsening of the laser beam quality.
This work is devoted to the investigation of the lasermatter interaction during up to 20 kW ytterbium fiber laser welding of thick mild steel plates. The plume attenuation of a probe 1.3 µm wavelength diode laser beam as well as of continuous radiation in 250-600 nm
wavelength range was measured during welding with and without Ar shielding gas supply. The measured results allow it to calculate average size and concentration of fine condensed metal particles in
different plume areas using the multi-wavelength method and the Mie scattering theory. The plume temperature, which determines the condensation conditions, was measured by means of Fe I atom
spectral line emission registration.
The obtained results can be also of interest for remote metal treatment with high-power fiber or disc lasers.
The results of an in-situ plume-laser interaction measurement during welding of mild steel with a 5 kW ytterbium fiber laser are reported. A measurement of the attenuation of probe laser beam passing through the plume has allowed to estimate the plume characteristics like the size of the extinction area and the spatial distribution of the extinction coefficient. The power loss of the fiber laser radiation propagating through the whole plume length was calculated. Together with a measured temporal characteristics of extinction the result indicates a significant decreasing of the laser radiation stability, which can lead to the formation of the macroscopic welding defects.
Influence of the main laser welding parameters on process stability and resulting weld quality were investigated. Effects of weld edge preparation on the weld appearance and quality were considered. The obtained welded joints were subjected to radiographic tests for detection of internal imperfections. Metallurgical characterisation of the samples regarding the resulting phase composition and hardness was conducted. Tensile and potentiodynamic tests were performed to evaluate the mechanical and corrosion properties. The results provide an insight into the advantages and limitations of the laser welding process for joining high-manganese alloyed stainless steels. Conditions for the production of defect-free and corrosion-resistant welds having good mechanical properties could be determined.
Different types of lasers are used nowadays for a number of technological Solutions, Modem high power solid-state lasers with high brightness of Radiation like Fiber or disc lasers find ever more applications in welding technology. Apart from the replacement of the CC>2-iasers traditionaHy used fcr these purposes, they provide new technological possibilities of metal treatment, such as using industrial robots and remote welding with high power laser radiation. The possibility of flexible optical fiber delivery of the radiation to any point gives a great technological advantage and permits welding processes to be realized with complicated weld seam geomelries and different positions using a minimum of additional fixtures. The short wavelength of the high power solid state lasers (1.07 ’m) causes another advantage. The coefflcient of inverse 170
Bremsstrahlung absorption from free electrons in plasma for such radiation is about 100 times lower than for the COi-iaser radiation. Owing to this fact, using the solid state lasers allows it to avoid a negative influence of the welding plume.
However, a short wavelength can lead to another kind of laser Radiation attenuation, which is connected with the formation of a cloud consisting of small metal particles that absorb and scatter laser radiation. This cloud arises due to the condensation of hot metal vapor by flying out from the deep Penetration channel and broadening in relatively cold atmosphere over the metal Surface.
Some theoretical and experimental approaches were made in advance in Order to estimate the influence of the Condensed particles on laser radiation and welding quality.
In this paper a systematic study including the determination of the Cloud size and its dynamical characteristics is presented. The results were obtained within the scope of the first stage of the metal vapor condensation process Investigation during welding with high power fiber lasers.
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 ISO 22 9 3 N L 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.