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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.
An electromagnetic weld pool support system for 20 mm thick duplex stainless steel AISI 2205 was investigated numerically and compared to experiments. In our former publications, it was shown how an AC magnetic field below the process zone directed perpendicular to the welding direction can induce vertically directed Lorentz forces. These can counteract the gravitational forces and allow for a suppression of material drop-out for austenitic stainless steels and aluminum alloys. In this investigation, we additionally adopted a steady-state complex magnetic permeability model for the consideration of the magnetic hysteresis behavior due to the ferritic characteristics of the material. The model was calibrated against the Jiles-Atherton model. The material model was also successfully tested against an experimental configuration before welding with a 30 mm diameter cylinder of austenitic stainless steel surrounded by duplex stainless steel. Thereby, the effects of the Curie temperature on the magnetic characteristics in the vicinity of the later welding zone were simulated. The welding process was modelled with a 3D turbulent steady-state model including heat transfer and fluid dynamics as well as the electromagnetic field equations. Main physical effects, the thermo-capillary (Marangoni) convection at the weld pool boundaries, the natural convection due to gravity as well as latent heat of solid–liquid phase transitions at the phase boundaries were accounted for in the model. The feedback of the electromagnetic forces on the weld pool was described in terms of the electromagneticinduced pressure. The FE software COMSOL Multiphysics 4.2 was used in this investigation. It is shown that the gravity drop-out associated with the welding of 20 mm thick duplex stainless steel plates due to the hydrostatic pressure can be prevented by the application of AC magnetic fields between around 70 mT and 90 mT. The corresponding oscillation frequencies were between 1 kHz and 10 kHz and the electromagnetic AC powers were between 1 kW and 2.3 kW. In the experiments, values of the electromagnetic AC power between 1.6 kW and 2.4 kW at oscillation frequencies between 1.2 kHz and 2.5 kHz were found to be optimal to avoid melt sagging or drop-out of melt in single pass fullpenetration laser beam welding of 15 mm and 20 mm thick AISI 2205.
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
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.
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.