Influence of an external applied AC magnetic field on the melt pool dynamics at high-power laser beam welding [original]

IOP Conference Series: Materials Science and Engineering

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Influence of an external applied AC magnetic field

on the melt pool dynamics at high-power laser

beam welding

To cite this article: Ömer Üstünda et al 2021 IOP Conf. Ser.: Mater. Sci. Eng. 1135 012017

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18thNordicLaserMaterialsProcessingConference(18thNOLAMP)

IOPConf.Series:MaterialsScienceandEngineering 1135 (2021)012017

IOPPublishing

doi:10.1088/1757-899X/1135/1/012017

Influence of an external applied AC magnetic field on the melt

pool dynamics at high-power laser beam welding

Ömer Üstündağ1, Nasim Bakir1, Andrey Gumenyuk1,2, Michael Rethmeier3,1,2

1Bundesanstalt für Materialforschung und -prüfung (BAM), Unter den Eichen 87,

12205 Berlin, Germany

2Fraunhofer Institute for Production Systems and Design Technology, Pascalstraße 8-

10587 Berlin, Germany

3Institute for Machine Tools and Factory Management, Technische Universität Berlin,

Pascalstraße 8-9, 10587 Berlin, Germany

oemer.uestuendag@bam.de

Abstract. The study deals with the determination of the influence of an externally applied

oscillating magnetic field on the melt pool dynamics in high power laser beam and hybrid laser

arc welding processes. An AC magnet was positioned under the workpiece which is generating

an upward directed electromagnetic force to counteract the formation of the droplets. To

visualise the melt flow characteristics, several experiments were carried out using a special

technique with mild steel from S355J2 with a plate thickness of up to 20 mm and a quartz glass

in butt configuration. The profile of the keyhole and the melt flow were recorded with a high

speed camera from the glass side. Additionally, the influence of the magnetic field orientation to

the welding direction on the filler material dilution on laser hybrid welding was studied with

variating oscillation frequency. The element distribution over the whole seam thickness wa

measured with X-ray fluorescence (XRF). The oscillation frequency demonstrated a great

influence on the melt pool dynamics and the mixing of the elements of the filler wire. The high

speed recordings showed, under the influence of the magnetic field, that the melt is affected

under strong vortex at the weld root, which also avoids the formation of droplets.

1. Introduction

Despite the availability of high-power laser systems within the range beyond 100 kW on the market, the

use of high-power laser beam welding (LBW) for thick metal sections at industrial scale are still under

discussion. Its application for plate thicknesses greater than 15 mm has certain technological limitations.

One of the limiting factors is the formation of sagging due to the gravitational forces or the hydrostatic

pressure often observed during welding in flat position (1G). For that reason, the arc-based welding

processes are usually implemented for welding of thick-walled structures with wall thicknesses above

15 mm. However, these are less productive as compared with high-power LBW cause of lower

penetration depth. Hence, multi-layer technology is used for welding of thick metal plates. It leads to

high heat inputs and distortion or thermal induced residual stress of the plates as well as reworks

hindrances such as flame straightening thus expend more time and cost.

Several investigations were conducted to identify the maximum boundaries of the single-pass high

power LBW or hybrid laser-arc welding (HLAW) and their challenges and physical backgrounds.

Similar challenges or defects are to be expected for high-power LBW or HLAW. Single-pass HLAW

18thNordicLaserMaterialsProcessingConference(18thNOLAMP)

IOPConf.Series:MaterialsScienceandEngineering 1135 (2021)012017

IOPPublishing

doi:10.1088/1757-899X/1135/1/012017

was produced up to 28 mm metal thickness using a laser power of just 19 kW with an electromagnetic

weld pool support system [1] or HLAW of 25 mm thick plates using cut-wire, which was filled within

an air gap between workpieces, with ceramic or flux backing and also up to 50 mm depth by double

sided welds approach showed successful outcomes [2]. Multilayer high-power LBW, especially HLAW

technique was demonstrated in [3] for one sided steel welds ranging in thickness from 28 mm to 32 mm

in two to five layers. Laser beam welding process under vacuum conditions [4,5], laser submerged arc

hybrid welding [6], or narrow-gap laser-arc hybrid welding [7] were conducted successfully for deep

penetration weld joints. As reported in the studies dealing with single-pass high-power LBW, the

formation of sagging or drop-outs is a major challenge during welding. Guidelines for preventing the

drop-outs were reported in [8], where the increase of the welding speed or laser power is recommended

for the prevention of those defects. The main physical background of this recommendation lies in the

fact, that with increasing welding speed the geometrical sizes of the seam is changed to thinner seam

width. This leads to the increase of the surface tension which counteracts the hydrostatic pressure on the

root part. Hence, the stability criterion is dependent on geometrical dimensions of weld seams and is

defined in conformance with [9] as shown in Equation 1:

h x w < 2 lcap2, (1)

where h is the plate thickness, w is the root width, lcap = [γ/(ρg0)]1/2 is the capillary length, γ is the surface

tension coefficient, ρ is the density of the melt, and g0 is the gravitational acceleration. The geometrical

sizes on the root side of the weld pool and the stability criterion for liquid steel is shown in Figure 1

according to [9].

Figure 1. Geometrical sizes on the root side of the weld pool (left) stability criterion

for liquid steel (right) [9].

Another typical problem associated with the application of high-power HLAW or wire feed LBW at

deep penetration is the inadequate and non-uniform distribution of filler wire elements over the entire

seam depth which may deteriorate mechanical properties especially in the root part. The high-power

LBW seams are characterized by a high cooling rate, why deteriorated mechanical properties can be

observed, especially at LBW of high-strength steels. So, the use of a filler wire and a homogeneous

mixing of the filler wire plays an integral role to achieve the required mechanical properties. In [10] it

was reported that the Charpy impact toughness of a single-pass hybrid laser-arc welded 25 mm thick

structural steel plate is decreased up to 60 % in the root part (laser-dominated zone) compared to the top

part (arc-dominated zone). Thereby, the decrease in cooling time over depth, the resulting microstructure

and grain size, together with the inhomogeneous mixing have a great impact on the mechanical

properties. Studies dealing with the improvement of the dilution are already known. In [11] the influence

of the arc mode during HLAW on the dilution was studied, where a pulsed or modified spray modes

were recommended. Another effective method to improve the filler wire dilution is to weld thick plates

with a small air gap of 0.3 mm to 0.4 mm as reported in [12,13]. The effectiveness of trailing GMAW

configuration in the mixing of the melt during the welding of 10 mm thick plates was noted in [14]. The

same approach was shown in [15]. Additionally, the shielding gas containing more than 2 % O2 produces

a more homogenous distribution of alloying elements [15]. The impact of the welding parameters such

as laser beam power, wire feed speed and resulting arc power, distance between the two heat sources

18thNordicLaserMaterialsProcessingConference(18thNOLAMP)

IOPConf.Series:MaterialsScienceandEngineering 1135 (2021)012017

IOPPublishing

doi:10.1088/1757-899X/1135/1/012017

and the configuration was studied in [16]. It was found that with increasing laser power and arc current,

the filler wire mixing was improved. With a distance of 2 mm to 6 mm between the wire tip extension

and the laser beam, promising results could be achieved.

As the state-of-the-art shows, the challenges during high-power LBW or HLAW of thick-walled

steels, especially the formation of drop-outs and inhomogeneous filler wire mixing is well known.

Methods with the adaptation of the welding parameters for the prevention of an inhomogeneous mixing

were developed for welds with a thickness of up to 15 mm. For thicker plates an additional external

force may be needed to influence the melt flow dynamics for a better mixing behaviour. Therefore,

external magnetic fields were applied in some studies such as in [17-21]. A magnet with a low frequency

in the range of 10 Hz to 20 Hz was applied for the improvement of the element distribution for LBW of

3 mm thick aluminum plate [17-19]. In [20] an external magnetic field was applied for wire-feed LBW

of 10 mm thick austenitic stainless steel to improve the mixing behaviour. It was found that a bulging

region is formed, which narrows the metal transfer channel from the top to the bottom region. A change

of the magnetic field orientation of 10° to 20° to the welding direction was recommended numerically

as well as experimentally to eliminate the bulging phenomenon, thus providing a downward transfer

channel for the melt flow. An approach to prevent sagging and to improve the filler wire mixing at

single-pass HLAW of steel plates with a wall thickness of 20 mm was demonstrated in [21]. Therefore,

an AC magnet was positioned under the workpiece and operated with a frequency of approx. 1.2 kHz.

The goal of this study is to describe the melt flow characteristics in detail and to show the influence

of the magnetic field orientation and oscillation frequency on the filler wire mixing during high-power

LBW and HLAW.

2. Experimental Setup

2.1. Laser beam welding experiments in steel-glass configuration

The high-power fibre laser IPG YLR-20000 with a maximum output power of 20 kW was used as the

laser beam source. The emission wavelength and beam parameter product were 1070 nm and

11 mm x mrad, respectively. The laser radiation was transmitted through an optical fibre with a core

diameter of 200 µm. A laser-processing head BIMO HP has been selected, which provides a

magnification of 2.8 so that the laser beam can be focused into a spot with a diameter of 560 µm. To

detect the keyhole during high-power laser beam welding a special setup was necessary. Welding trials

in butt joint configuration of 25 mm thick structural steel plate (S355J2) and quartz glass were

conducted. A groove with the dimensions of 80 mm x 8 mm x 0.5 mm was milled on the steel plate and

filled with an austenitic powder 316L-Si with Ni as tracing element for the later evaluation. Side views

of the molten pool were taken with help of a highspeed camera Fastcam 1024PCI and interference band-

pass filter at 808 nm and band width of 20 nm. The frame rate and the frame size were 2000 fps and

1200 pixels to 1200 pixels, respectively. An oscillating magnetic field generated by an AC

electromagnet was applied to the root side of the weld specimen, where the magnetic field was

perpendicular and induced electric current parallel to the welding direction. The schematic

representation of the experimental setup is shown in Figure 2. The LBW experiments on steel-glass

configuration were performed using a laser beam power of 18.7 kW at a welding speed of 0.9 m min-1

and a focal position of -7 mm. The AC magnet was operated at an oscillation frequency of 1.2 kHz and

a magnet power of 2.1 kW ± 200 W. To protect the arc on the top side from a deflection due to the

oscillating magnetic field, the frequencies have been selected in the kHz range, where the skin layer

depth is less than the plate thickness. For the evaluation of the high-speed recordings the displacements

in the melt-glass interface were estimated and then the velocities were calculated using the optical flow

algorithm according to Lucas-Kanade method.

18thNordicLaserMaterialsProcessingConference(18thNOLAMP)

IOPConf.Series:MaterialsScienceandEngineering 1135 (2021)012017

IOPPublishing

doi:10.1088/1757-899X/1135/1/012017

Figure 2. Experimental setup for LBW in steel-glass configuration.

For metallographic inspection, longitudinal sections of the laser beam welded samples were cut.

After polishing and etching the samples using 2 % nital solution, the mixing of the powder over the

thickness was evident.

2.2. Hybrid laser-arc welding experiments

The HLAW experiments were performed using the same laser source as described in Section 2.1.

Additionally, a welding machine Cloos Quineo with a maximum current of 600 A was used as an arc

power source. The laser optics and GMAW torch were mounted on the robot arm, where the laser axis

was positioned 90° to the weld specimen surface and the GMA torch was tilted 25° relative to the laser

axis. The processing head and the magnet remained in a fixed position during the welding, where the

specimens were moved by an external axis. The experiments were carried out on 20 mm thick structural

steel plates (S355J2) in butt joint configuration with an arc leading position and a distance of 4 mm

between the two heat sources and under the following welding parameters: laser beam power of

17.7 kW; welding speed of 1.3 m min- 1; focal position of the laser beam of -5 mm; wire feed speed of

13 m min-1; stick-out of 18 mm; shielding gas mixing consisted of 18% CO2 in Ar with a volume flow

rate of 20 m min-1. A Ni-based solid wire Thermanit625 (ERNiCrMo-3 according to AWS A-5.14) with

a diameter of 1.2 mm was used as a filler wire. Figure 3 shows the setup for the HLAW experiments.

Different magnetic field orientation and oscillating frequencies were tested to determine the influence

of the magnetic parameters on the filler wire mixing.

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