Filtern
Dokumenttyp
- Zeitschriftenartikel (13)
Sprache
- Englisch (13)
Referierte Publikation
- ja (13) (entfernen)
Schlagworte
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.
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.
A multi-physics numerical model was developed to investigate the influence of a steady magnetic field aligned perpendicular to the welding direction during partial penetration high power laser beam welding of aluminium in downhand position. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were successfully solved with the finite element differential equation solver COMSOL Multiphysics 4.2. The implemented material model used temperature-dependent properties up to evaporation temperature. Marangoni convection in the surface region of the weld pool, natural convection due to the gravitational field and latent heat of solidliquid phase transition were taken into account. Solidification was modelled by the CarmanKozeny equation for porous media morphology. The flow pattern in the melt as well as the weld bead geometry were significantly changed by the induced Lorentz force distribution in the liquid metal. It reveals that the application of a steady magnetic field to laser beam welding with corresponding Hartmann numbers Ha2 ≈ 104 allows for a suppression of the characteristic wineglass-shape of the weld cross section caused by thermocapillary flow. The numerical results are in good agreement with experimental results obtained with welding of AlMg3 with a 16 kW disc laser. The steady magnetic field was delivered by permanent magnets mounted on both lateral sides of the weld specimen. The maximum magnetic flux density was around 500 mT. It shows, that the applied magnetic field has a predominant dissipating effect on the weld pool dynamics independently of its polarity.
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.
The main characteristic feature of deep penetration laser beam welding is a large temperature difference between the plasma cavity (keyhole) in the centre of the weld pool and the melting/solidification front. Large temperature gradients in the weld pool result in a very intensive thermocapillary (Marangoni) convection. The weld pool surface width becomes very large and unstable. However, an externally applied oscillating magnetic field can stabilize the surface of the melt (the Garnier-Moreau effect, 1983). In the present work this technology was used to stabilize the surface of the weld pool in partial penetration 4.4 kW Nd:YAG laser beam welding of AW-5754 alloyin PA position. The AC magnet was mounted on the laser welding head. The oscillating magnet field was oriented perpendicular to the welding direction. It was found that the AC magnet field can drastically reduce the surface roughness of welds. The analysis of the x-ray images shows a rastic reduction of porosity content in the welds. This effect can be explained as a result of electromagnetic ectification of the melt.
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.
A multi-physics numerical model was developed to investigate the influence of a steady magnetic field during partial penetration keyhole laser beam welding of an aluminum plate in flat position. Three-dimensional heat transfer, fluid dynamics including phase transition and electromagnetic field partial differential equations were solved with the finite element differential equation solver COMSOL Multiphysics. The magnetic field was aligned perpendicularly to the welding direction. The main objective of these simulations was to estimate the critical value of the magnetic field needed to suppress convective flows in the weld pool during high-power (up to 20 kW) laser beam welding of aluminum alloys with up to 20 mm deep weld pool. It reveals that steady magnetic fields with corresponding Hartmann numbers Ha^2 ~ 10^4 based on the half-width of the weld pool can effectively suppress convective flows in the weld pool. Moreover, the typically occurring wineglass-shape of the weld cross section caused by thermo-capillary flow is weakened.
A three-dimensional turbulent 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 stainless steel AISI 304 being modeled as an ideal non-ferromagnetic material. Three-dimensional heat transfer and fluid dynamics as well as the electromagnetic field equations were solved with the finite element package COMSOL Multiphysics 4.2 taking into account the most important physical effects of the process. Namely, the thermo-capillary (Marangoni) convection at the weld pool boundaries, natural convection due to gravity and density differences in the melt volume as well as latent heat of solidliquid phase transitions at the phase boundaries were included in the model.
It is shown that the gravity drop-out associated with the welding of thick plates due to the hydrostatic pressure can be prevented by the application of AC magnetic field between 80 mT and 135 mT for corresponding oscillation frequencies between 1 kHz and 10 kHz below the weld specimen. Experimentally, a value of the magnetic flux density of around 230 mT was found to be necessary to allow for single-pass laser beam welding without sagging or drop-out of melt for a 20 mm thick combination of austenitic stainless steel AISI 304 and ferritic construction steel S235JRC at an oscillation frequency of around 2.6 kHz.
The objective of this study was to investigate the influence of externally applied magnetic fields on the weld quality in laser beam welding. The optimization of the process parameters was performed using the results of computer simulations. Welding tests were performed with up to 20 kW laser beam power. It was shown that the AC magnet with 3 kW power supply allows for a prevention of the gravity drop-out for full penetration welding of 20 mm thick stainless steel plates. For partial penetration welding it was shown that an0.5 T DC magnetic field is enough for a suppression of convective flows in the weld pool. Partial penetration welding tests with 4 kW beam power showed that the application of AC magnetic fields can reduce weld porosity by a factor of 10 compared to the reference joints. The weld surface roughness was improved by 50%.
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.