Ingenieurwissenschaften und zugeordnete Tätigkeiten
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The geometry of the melt pool in laser beam welding plays a major role to understand the dynamics of the melt and its solidification behavior. In this study, a butt configuration of 15 mm thick structural steel and transparent quartz glass was used to observe the weld pool geometry by means of high-speed camera and an infrared camera recording. The observations show that the dimensions of the weld pool vary depending on the depth. The areas close to the weld pool surface take a teardrop-shape. A bulge-region and its temporal evolution were observed approximately in the middle of the depth of the weld pool. Additionally, a 3D transient thermal-fluid numerical simulation was performed to obtain the weld pool shape and to understand the formation mechanism of the observed bulging effect. The model takes into account the local temperature field, the effects of phase transition, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature. The numerical results showed good accordance and were furthermore used to improve the understanding of the experimentally observed bulging effect.
The hybrid laser arc welding (HLAW) process provides many advantages such as improved gap bridgeability, deep penetration and misalignment of edges, that is why the process is used increasingly in industrial applications e.g. shipbuilding, power plant industry and line-pipe manufacturing. The obvious encountered problem for single pass welding in flat position is the gravity drop-out at low welding velocities. With the usage of an electromagnetic weld pool support system, which is based on generating Lorentz forces within the weld pool, wide seams followed by reduced welding velocities could be achieved in this study leading to the realization of a gap bridgeability up to 1 mm, misalignment of edges up to 2 mm and a single pass weld up to 28 mm thickness with a 20-kW fibre laser. These developments expand the boundaries of the HLAW process for different industrial applications. As a result, less accurate preparation of the edges would be sufficient, which saves time for manufacturing.
Recently developed fibre lasers provide multi-kilowatt beam power with high quality at impressive energy efficiency. Combined with gas metal arc welding (GMAW) equipment these lasers can be used in a hybrid process to weld thick-walled constructions single-pass, that are currently welded using multi-pass techniques. The main benefits are a reduction of heat induced distortions, due to the low heat input, as well as savings in filler material and process time. Probable applications can be found in power generation, ship building and pipeline constructions. An orbital (girth) laser-hybrid process using a 20 kW fibre laser and a GMAW torch is currently examined at the BAM, Berlin. The aim of this research is to obtain a stable and crack free girth welding process and to demonstrate its application in pipeline construction. The experiments are carried out on 16 mm thick plates as well pipe rings with 36" (914 mm) pipe diameter of X65. Particular welding parameters, such as welding speed, GMAW power, arc length are varied and their influence on the appearance of the weld in the different welding positions is analyzed. Even though issues remain that demand further research it could already be shown that the rings can be welded using a girth hybrid process that is divided into two half girth processes in downward direction.
The weldability of materials is still for many years a highly contentious issue, particularly regarding the causes of the hot crack formation. Because of the process-related temperature and emissions, direct measurement for the arising strain in the close vicinity of the welding process is challenged. therefore, the externally loaded hot cracking testes remain for decades the only way to determine the critical straining conditions for solidification cracking. In this study, a novel optical two-dimensional in situ observation technique has been developed to analyse the strain evaluation during the welding process in the moment of crack formation. Additionally, the Controlled Tensile Weldability test (CTW test) was used to generate the hot crack under different global straining conditions. To record the welding process and the moment of the solidification crack initiation a CMOS camera was used which inserted coaxially into the optical path of the welding laser. As illumination source a diode laser with wave length 808 nm was employed to illuminate the welding region. An interference filter was placed on the camera lens, allowing only the illumination wavelength to pass through and reflecting all other wavelengths, so that the melt pool and the re-solidifying metal could be visualized in a single image. in order to obtain good temporal resolution, the frame rate of the camera was set to 1100 frame per second in. The contrast in images obtained using this unique setup allows to apply the optical flow technique based on Lucas-Kanade (LK) algorithm to follow the pixels in each image sequence and then to calculate the displacement field. The strain was calculated based on the estimated displacement. Using this technique, the local strains and strain rates under different global straining condition has been determined and analysed. The results shown Dependency between the external strain rate and the critical local strain and strain rate has been observed. That is to say, the critical local strain and strain rate are increased with an increase of the strain rate. Moreover, the described procedure of the optical measurement allows to determine the real martial dependent values of critical strain and strain rate characterizing transition to the hot cracking during laser welding processes.The experiments as well as the measurement has been performed on the stainless steel 316L (1.4404)
This paper represents the results for proposed optical flow method based on the Lucas-Kanade (LK) algorithm applied to two different problems. The following observations can be made:
- The estimated strain and displacement for conducted tensile test are generally very close to those measured with conventional DIC-technique.
- The LK technique allows measurement of strain or displacement without special selection of a region of interest.
Using a novel optical measurement technique together with the optical flow algorithm, a twodimensional deformation analysis during welding was conducted. This technique is the first to provide a measurement of the full strain field locally in the immediate vicinity of the solidification front. Additionally, the described procedure of the optical measurement allows the real material-dependent values of critical strain characterizing the transition to hot cracking during laser welding processes to be determined.
Das bislang zur Vermeidung unzulässiger Wurzelüberhöhungen beim Laserstrahlschweißen von Aluminiumlegierungen bzw. austenitischem Stahl eingesetzte Verfahren der elektromagnetischen Schmelzbadstütze konnte innerhalb der vorliegenden Untersuchung erfolgreich zur Kompensation des hydrostatischen Druckes von ferromagnetischen Stählen übertragen werden. Es wurden dabei Laserstrahlschweißversuche in PA-Position an bis zu 20 mm dickem Duplexstahl 1.4462 sowie Baustahl S235JR durchgeführt. Unter konstanten Schweißparametern wurden Durchschweißungen generiert. Dem hydrostatischen Druck wurde unter Verwendung der Technologie zur elektromagnetischen Schmelzbadunterstützung durch Variation der Oszillationsfrequenz und der AC-Leistung des Magnetsystems entgegengewirkt.
Zunächst konzentrierten sich die Versuche auf den Duplexstahl 1.4462, welcher jeweils aus 50 % Ferrit und Austenit besteht. Hierbei konnte festgestellt werden, dass zur idealen Kompensation von 15 mm bei einer Frequenz von 1,7 kHz eine AC-Leistung von 1,6 kW erforderlich ist, die Schweißnähte aber bereits bei einer AC-Leistung von ca. 0,8 kW in die Bewertungsgruppe B der DIN EN ISO 13919-1:1996-09 eingeordent werden können. Zur idealen Kompensation des hydrostatischen Druckes bei 20 mm dickem Duplexstahl war eine um 20 % höhere AC-Leistung notwendig. Im Anschluss an die Versuche mit dem Duplexstahl wurden die Untersuchungen auf bis zu 20 mm dicke Proben aus Baustahl S235JR erweitert. Für 15 mm konnten die Schweißnähte bei einer Frequenz von 1,7 kHz ab einer AC-Leistung von 1,3 kW in die Bewertungsgruppe B eingeordnet werden. Zur idealen Kompensation von 20 mm dickem Baustahl war eine AC-Leistung von 1,6 kW bei einer Frequenz von 636 Hz nötig. Mit steigender AC-Leistung konnte in allen Versuchsreihen eine sukzessive Verringerung
der Wurzelüberhöhung demonstriert werden.
Electromagnetic weld pool support system is illustrated successfully for hybrid laser arc welding of 20 -30 mm ferromagnetic Steels Welding of thick-walled steels in flat position and reduced welding velocities is possible Gap and misalignment between welded plates can be tolerated by use of this Technology Opportunity to reduce the laser beam power because of reduced welding velocity respectively increase of weldable material thickness with a 20 kW- fibre laser Mechanical properties of welds are in correspondence with requirements of Standards.
The laser hybrid welding process offers many advantages regarding deep penetration, increased welding velocity and with the help of the supplied filler wire an improved bridgeability to gap and misalignment tolerances. High power laser systems with a power of approx. 30 kW are already available on the market. Nevertheless, multi-layer technology with an arc process is still used for welding of plates from a thickness from 20 mm. A potential cause is the process instability with increasing laser power. It is inevitable that gravity drop-out due to the high hydrostatic pressure at increasing wall thickness especially at welding in flat position and with a low welding speed. The surface tension decreases with increasing root width resulting from low welding velocities. To prevent such inadmissible defects of the seam a use of weld pool support is required. Usual weld pool support systems such as ceramic or powder supports require a mechanical detachment which is time-consuming. The electromagnetic weld pool support system described in this work shows an alternative weld pool support which works contactless. It is based on generating Lorentz forces in the weld pool due to oscillating magnetic field and induced eddy currents. This innovative technology offers single pass welds up to 28 mm in flat position and reduced welding velocity with a laser power of just 19 kW. It also leads to improved mechanical-technological properties of the seams because of the slow cooling rate. With usage of an electromagnetic weld pool support the limitation of the hybrid laser arc welding process in the thick sheet metal will be extend.
The present research work encloses results of experimental investigations of the interaction between welding process parameters for laser-beam and hybrid-laser-arc as well as type of the filler metal and the achievable mechanical properties of the weld joints on steel grade X8Ni9+QT640 for cryogenic service containing 9% nickel. The results obtained contribute to the development and conversion in the industrial practice a new laser beam-based welding technology for the automated manufacturing of facilities for the liquefaction, storage and the transport of natural gases (LNG facilities). The results show, that the martensitic microstructure of the laser weld metal including low amount of retained austenite not exceeding 3.5% leads to the relatively low V-notch impact energy. The remarkable heterogeneity in the chemical composition of the weld metal through the weld thickness could be recognized in the case of hybrid-laser-arc welding with ERNiCrMo-3 austenitic filler metal, what also led to insufficient impact toughness of the weld metall. The most promising results could be achieved by using 11%Ni filler wire, which is similar to the base metal and provides a homogeneous microstructure with uniform distribution of Ni through the weld seam. It is remarkable, that a correlation between Charpy impact toughness and wire feeding speed and respectively process heat input exists. The highest toughness values were 134±58 J at -196C. The both laser as well as laser-hybrid welds passed the tensile test. The failure stress of 720±3 MPa with a fracture location in the base material was achieved for all samples tested.
In the article considered the problem of hot cracks occurrence during laser welding process. The main reason of their appearance is strain. The optical method for measuring full field strain locally near the solidification front during laser welding process is proposed. The proposed method of optical measurement allows to determine the real values of the critical strain for various materials characterizing the occurrence of hot cracks in laser welding process.