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Simulation of an inductive weld pool support for deep penetration laser beam welding of metal parts
(2012)
three-dimensional laminar steady state numerical model was used to investigate the influence of an altemating current (ac) magnetic field during single pass high power laser beam keyhole welding of 20 mm thick aluminum. The three-dimensional heat transfer, fluid dynamics and electromagnetic field equations were solved with the commercial finite element package COMSOL Multiphysics. Dominant physical effects of the process were taken into account: Thermo-capillary (Marangoni) convection at the upper and lower weld pool surfaces, natural convection due to the gravity influence and the latent heat of solid-liquid phase transition. Simulations were conducted for several magnetic field strengths and it was found that the gravity drop-out associated with welding of thick plates due to the hydrostatie pressure can be prevented by the application of an ac magnetic field below the weld specimen of around 70 mT (rms) at an oscillation frequency of 450 Hz. The inductive support System allows for single-pass laser beam welding of thick aluminum plates. The flow pattem in the molten zone and the temperature distributions are significantly changed by the application of the electromagnetic forces in the weld pool.
Full penetration 15 kW Yb fibre laser butt welding of thick AlMg3 (AW 5754) plates was performed in PA position. A contactless inductive electromagnetic weld pool support system was used to prevent gravity dropout of the melt. The welding speed needed to achieve 20 mm penetration was ~0·5 m min-1. An ac power supply of ~244 W at 460 Hz was necessary to completely suppress gravity dropout of the melt and eliminate sagging of the weld pool root side surface. The oscillating magnetic field can suppress the Marangoni convection in the lower part of the weld pool. The system was also successfully used in the full penetration welding of 30 mm thick AlMg3 plates.
A three-dimensional laminar steady-state numerical model was developed to investigate the influence of an alternating current (ac) magnetic field during high-power full-penetration laser welding on the weld pool dynamics and weld cross section of a 20 mm thick aluminium plate in flat position. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were solved iteratively with the commercial finite element software COMSOL Multiphysics using temperature-dependent material properties up to evaporation temperature. Thermocapillary convection at the weld pool surfaces, natural convection and latent heat of solid–liquid phase transition were taken into account in this model. Solidification was modelled by the Carman–Kozeny equation for porous media morphology. The ac magnet was mounted on the root side of the weld specimen. The magnetic field was aligned perpendicular to the welding direction. The flow pattern in the melt and thus also the temperature distribution were significantly changed by the application of oscillating magnetic fields. It was shown that the application of an ac magnetic field to laser beam welding allows for a prevention of the gravity drop-out. The simulation results are in good qualitative agreement with the experimental observations.
Numerical simulation of electromagnetic melt control systems in high power laser beam welding
(2013)
The availability of laser sources with a power of
20 kW upwards prepared the ground for laser beam
welding of up to 20 mm thick metal parts. Challenges
are the prevention of gravity-driven melt drop-out and
the control of the dynamics mainly due to the
Marangoni flow.
Coupled numerical turbulent fluid flow, thermal and
electromagnetic simulations and experimental
validation with aluminum AlMg3 and stainless steel
AISI 304 were done for alternating and steady
magnetic fields perpendicular to the process direction.
The first can prevent melt sagging in full-penetration
welding by Lorentz forces in the melt induced by an
AC magnet located below the weld specimen
counteracting gravitational forces. The latter controls
the Marangoni flow by Lorentz braking forces in the
melt by the so-called Hartmann effect.
The simulations show that the drop-out of aluminum
and stainless steel can be avoided for 20 mm thick fullpenetration
welds with moderate magnetic flux
densities of 70 mT and 95 mT at oscillation
frequencies of 450 Hz and 3 kHz, respectively. The
experiments are in good agreement but show
somewhat larger values for steel, whose weakly
ferromagnetic properties are a possible reason. The
investigations with steady magnetic fields reveal the
possibility to mitigate the dynamics significantly
beginning with around 500 mT at laser penetration
depths of approximately 20 mm.
Controlling the dynamics in the weld pool is a highly demanding challenge in deep-penetration laser beam welding with modern high power laser systems in the multi kilowatt range. An approach to insert braking forces in the melt which is successfully used in large-scaled industrial applications like casting is the so-called Hartmann effect due to externally applied magnetic fields. Therefore, this study deals with its adaptation to a laser beam welding process of much smaller geometric and time scale. In this paper, the contactless mitigation of fluid dynamic processes in the melt by steady magnetic fields was investigated by numerical simulation for partial penetration welding of aluminium. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were solved based on temperature-dependent material properties up to evaporation temperature for two different penetration depths of the laser beam. The Marangoni convection in the surface region of the weld pool and the natural convection due to the gravitational forces were identified as main driving forces in the weld pool. Furthermore, the latent heat of solide-liquid phase transition was taken into account and the solidification was modelled by the Carman-Kozeny equation for porous medium morphology. The results show that a characteristic change of the flow pattern in the melt can be achieved by the applied steady magnetic fields depending on the ratio of magnetic induced and viscous drag. Consequently, the weld bead geometry was significantly influenced by the developing Lorentz forces. Welding experiments with a 16 kW disc laser with an applied magnetic flux density of around 500 mT support the numerical results by showing a dissipating effect on the weld pool dynamics.
A three-dimensional laminar steady
state numerical model was used to investigate
the influence of an alternating current (ac)
magnetic field during high power laser beam
keyhole welding of 20 mm thick nonferromagnetic
aluminum. COMSOL
Multiphysics was used to calculate the threedimensional
heat transfer, fluid dynamics and
electromagnetic field equations. Most
important physical effects of the process were
taken into account: Thermo-capillary
(Marangoni) convection at the upper and lower
weld pool boundaries, natural convection due
to gravity and latent heat of solid-liquid phase
transition. It is shown that the gravity drop-out
associated with welding of thick plates due to
the hydrostatic pressure can be prevented by
the application of an ac magnetic field. The
application of an oscillating magnetic field of
70 mT was investigated to allow for singlepass
laser beam welding of thick aluminum
plates. The flow pattern in the molten zone and
the temperature distributions are significantly
changed.
A three-dimensional turbulent steady state numerical model was used to investigate the influence of a stationary magnetic field during partial penetration high power laser beam keyhole welding of thick aluminum parts. COMSOL Multiphysics was used to calculate the three-dimensional heat transfer, fluid dynamics and electromagnetic field equations. Thermo-capillary (Marangoni) convection at the upper weld pool surface, natural convection due to gravity and latent heat of solid-liquid phase transition were taken into account. It shows that the application of steady magnetic fields produces a braking Lorentz force in the melt based on the Hartmann effect. The flow pattern in the weld pool and also the temperature distribution and associated weld pool geometry thus change significantly. Convective flows in the melt can effectively be suppressed and the influence of thermo-capillary flow is diminished to a thin surface layer.
Welding is one of the most critical operations for the construction of reliable metal structures in everything from ships to reactor vessels. When welds fail, often the entire structure fails, and expectations on weld quality have never been higher. Any process that uses a localized heat source, such as welding, is likely to result in some distortion. The welding process of very thick metal components is not inherently stable and is barely controllable without external forces.
Welding is one of the most critical operations for the construction of reliable metal structures in everything from ships to reactor vessels. When welds fail, the entire structure often fails—so expectations on weld quality have never been higher. Any process that uses a localized heat source, such as welding, is likely to result in some distortion. The welding process of very thick metal components is not inherently stable and is barely controllable without external forces.
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.
During laser beam welding of aluminum alloys an electromagnetic field may favour pore outgassing through the top oxide layer. High frequencies cause a small penetration depth and thus exert a stabilizing effect on the weld surface. The point at which the laser beam between the two magnetic poles hits the workpiece surface is crucial to the influence of the magnetic field on the weld surface roughness. Using analyzed parameters for different laser points of application cause a change in weld surface roughness could be observed. The weld surface roughness could be reduced by 50%. The outgassing effect in terms of a reduction of pores could be observed for all parameter sets investigated.
High-power laser welding of austenitic stainless steel with electromagnetic control of weld pool
(2014)
Laser deep-penetration welding became a widely applied tool in industrial applications due to available laser power of 20 kW and more for the single-pass welding of steel plates of up to 20 mm thikness. Above a critical limit, liquid metal tends to drop out of the bead due to hydrostatic pressure. Laser welding, in contrast to electron beam welding technique, allows for an electromagnetic manipulation of fluid flow in the weld pool. AC electromagnetic system for compensation of the hydrostatic pressure by induced Lorentz forces in the melt was experimentally and numerically investigated for single-pass full-penetration welding of up to 20 mm thikness austenitic stainless steel plates of grade AISI 304. It was shown that the application of 200-234 mT magnetic fields at oscillation frequency of around 2.6 kHZ lead to a full compensation of hydrostatic forces in the melt for plate 10-20 mm thick, respectively. Coupled fluid flow, thermal and electromagnetic finite element simulations were done with different applied magnetic flux densities and oscillation frequencies calculating for the optimal magnetic field strength to avoid melt sagging in the weld pool. The simulation results point to a lower magnetic field density needed for that purpose. The reason for that can lie in the magnetic properties of the material not being totally non-ferromagnetic. 17 Ref., 1 Table, 5 Figures.
The paper describes an experimental investigation of high power laser beam welding with an electromagnetic weld pool support for up to 20 mm thick plates made of duplex steel (AISI 2205) and mild steel (S235JR). The results of the welding tests show a successful application of this technology at ferromagnetic metals. Irregular sagging was suppressed successfully. An ac-power of less than 2 kW at oscillation frequencies between 800 Hz and 1.7 kHz is necessary for a full compasation of the hydrostatic pressure. Thus, it was demonstrated that the electromagnetic weld pool support is not only limited to non-ferromagnetic metals like austenitic steels. For future studies with duplex steel, the use of filler material has to take into account with regard to the balance of the mixed austenitic and ferritic phases.
High power laser beam welding of austenitic stainless steel with electromagnetic weld pool support
(2013)
Full penetration high power bead-on-plate laser beam welding tests of up to 20 mm thick 2205 duplex steel plates were performed in PA position. A contactless inductive electromagnetic (EM) weld pool support system was used to prevent gravity drop-out of the melt. Welding experiments with 15 mm thick plates were carried out using IPG fiber laser YLR 20000 and Yb:YAG thin disk laser TruDisk 16002. The laser power needed to achieve a full penetration was found to be 10.9 and 8.56 kW for welding velocity of 1.0 and 0.5 m min−1, respectively. Reference welds without weld pool support demonstrate excessive root sag. The optimal value of the alternating current(AC) power needed to completely compensate the sagging on the root side was found to be ≈1.6 kW for both values of the welding velocity. The same EM weld pool support system was used in welding tests with 20 mm thick plates. The laser beam power (TRUMPF Yb:YAG thin disk laser TruDisk 16002) needed to reach a full penetration for 0.5 m min−1 was found to be 13.9 kW. Full penetration welding without EM weld pool support is not possible—the surface tension cannot stop the gravity drop-out of the melt. The AC power needed to completely compensate the gravity was found to be 2 kW.
Full penetration high power bead-on-plate laser beam welding tests of up to 20 mm thick 2205 duplex steel plates were performed in PA position. A contactless inductive electromagnetic (EM) weld pool support system was used to prevent gravity drop-out of the melt. Welding experiments with 15 mm thick plates were carried out using IPG fiber laser YLR 20000 and Yb:YAG thin disk laser TruDisk 16002. The laser power needed to achieve a full penetration was found to be 10.9 and 8.56kW for welding velocity of 1.0 and 0.5 m min(-1), respectively. Reference welds without weld pool support demonstrate excessive root sag. The optimal value of the alternating current (AC) power needed to completely compensate the sagging on the root side was found to be approximate to 1.6 kW for both values of the welding velocity. The same EM weld pool support system was used in welding tests with 20 mm thick plates. The laser beam power (TRUMPF Yb:YAG thin disk laser TruDisk 16002) needed to reach a full penetration for 0.5 m min(-1) was found to be 13.9 kW. Full penetration welding without EM weld pool support is not possible-the surface tension cannot stop the gravity drop-out of the melt. The AC power needed to completely compensate the gravity was found to be 2 kW. (C) 2016 Laser Institute of America