The effects of some variables on CO2 laser-MAG hybrid welding

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Anna Fellman

THE EFFECTS OF SOME VARIABLES ON CO

2

LASER-MAG HYBRID WELDING

Thesis for the degree of Doctor of Science (Technology) to be presented with due permission for public examination and criticism in Auditorium 1383 at Lappeenranta University of Technology, Lappeenranta, Finland, on the 23^rd May, 2008, at noon.

Acta Universitatis Lappeenrantaensis 306

LAPPEENRANTA

UNIVERSITY OF TECHNOLOGY

Anna Fellman

THE EFFECTS OF SOME VARIABLES ON CO

2

LASER-MAG HYBRID WELDING

Thesis for the degree of Doctor of Science (Technology) to be presented with due permission for public examination and criticism in Auditorium 1383 at Lappeenranta University of Technology, Lappeenranta, Finland, on the 23^rd May, 2008, at noon.

Acta Universitatis Lappeenrantaensis 306

LAPPEENRANTA

UNIVERSITY OF TECHNOLOGY

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Supervisors Docent Dr Antti Salminen

Faculty of Technology

Department of Mechanical Engineering

Lappeenranta University of Technology

Finland

Professor Jukka Martikainen

Finland

Reviewers Dr Richard Martukanitz

Applied Research Laboratory

Pennsylvania State University

USA

Dr Martti Vilpas

Radiation and Nuclear Safety Authority, STUK Finland

Opponents Emeritus Professor William Steen

Department of Engineering

University of Liverpool

United Kingdom

Dr Martti Vilpas

Radiation and Nuclear Safety Authority, STUK

Finland

ISBN 978-952-214-574-1 ISBN 978-952-214-575-8 (PDF)

ISSN 1456-4491

Lappeenrannan teknillinen yliopisto Digipaino 2008

Supervisors Docent Dr Antti Salminen

Finland

Professor Jukka Martikainen

Finland

Reviewers Dr Richard Martukanitz

Applied Research Laboratory

Pennsylvania State University

USA

Dr Martti Vilpas

Radiation and Nuclear Safety Authority, STUK Finland

Opponents Emeritus Professor William Steen

Department of Engineering

University of Liverpool

United Kingdom

Dr Martti Vilpas

Radiation and Nuclear Safety Authority, STUK

Finland

ISBN 978-952-214-574-1 ISBN 978-952-214-575-8 (PDF)

ISSN 1456-4491

Lappeenrannan teknillinen yliopisto Digipaino 2008

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ABSTRACT

Anna Fellman

The effects of some variables on CO2 laser-MAG hybrid welding Lappeenranta 2008

153 p.

Acta Universitatis Lappeenrantaensis 306 Diss. Lappeenranta University of Technology

ISBN 978-952-214-574-1, ISBN 978-952-214-575-8 (PDF) ISSN 1456-4491

The CO2-laser-MAG hybrid welding process has been shown to be a productive choice for the welding industry, being used in e.g. the shipbuilding, pipe and beam manufacturing, and automotive industries. It provides an opportunity to increase the productivity of welding of joints containing air gaps compared with autogenous laser beam welding, with associated reductions in distortion and marked increases in welding speeds and penetration in comparison with both arc and autogenous laser welding.

The literature study indicated that the phenomena of laser hybrid welding are mostly being studied using bead-on-plate welding or zero air gap configurations. This study shows it very clearly that the CO2 laser-MAG hybrid welding process is completely different, when there is a groove with an air gap. As in case of industrial use it is excepted that welding is performed for non-zero grooves, this study is of great importance for industrial applications.

The results of this study indicate that by using a 6 kW CO2 laser-MAG hybrid welding process, the welding speed may also be increased if an air gap is present in the joint. Experimental trials indicated that the welding speed may be increased by 30-82% when compared with bead-on-plate welding, or welding of a joint with no air gap i.e. a joint prepared as optimum for autogenous laser welding.

This study demonstrates very clearly, that the separation of the different processes, as well as the relative configurations of the processes (arc leading or trailing) affect welding performance significantly. These matters influence the droplet size and therefore the metal transfer mode, which in turn determined the resulting weld quality and the ability to bridge air gaps. Welding in bead-on- plate mode, or of an I butt joint containing no air gap joint is facilitated by using a leading torch.

This is due to the preheating effect of the arc, which increases the absorptivity of the work piece to the laser beam, enabling greater penetration and the use of higher welding speeds. With an air gap present, air gap bridging is more effectively achieved by using a trailing torch because of the lower arc power needed, the wider arc, and the movement of droplets predominantly towards the joint edges.

The experiments showed, that the mode of metal transfer has a marked effect on gap bridgeability.

Transfer of a single droplet per arc pulse may not be desirable if an air gap is present, because most of the droplets are directed towards the middle of the joint where no base material is present. In such cases, undercut is observed. Pulsed globular and rotational metal transfer modes enable molten metal to also be transferred to the joint edges, and are therefore superior metal transfer modes when bridging air gaps.

It was also found very obvious, that process separation is an important factor in gap bridgeability. If process separation is too large, the resulting weld often exhibits sagging, or no weld may be formed at all as a result of the reduced interaction between the component processes. In contrast, if the

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processes are too close to one another, the processing region contains excess molten metal that may create difficulties for the keyhole to remain open. When the distance is optimised – i.e. a separation of 0-4 mm in this study, depending on the welding speed and beam-arc configuration – the processes act together, creating beneficial synergistic effects. The optimum process separation when using a trailing torch was found to be shorter (0-2 mm) than when a leading torch is used (2-4 mm);

a result of the facilitation of weld pool motion when the latter configuration is adopted.

This study demonstrates, that the MAG process used has a strong effect on the CO2-laser-MAG hybrid welding process. The laser beam welding component is relatively stable and easy to manage, with only two principal processing parameters (power and welding speed) needing to be adjusted.

In contrast, the MAG process has a large number of processing parameters to optimise, all of which play an important role in the interaction between the laser beam and the arc. The parameters used for traditional MAG welding are often not optimal in achieving the most appropriate mode of metal transfer, and weld quality in laser hybrid welding, and must be optimised if the full range of benefits provided by hybrid welding are to be realised.

Keywords: Laser hybrid welding, CO2 laser-MAG hybrid welding, metal transfer, welding parameters, weld quality

UDC 621.791.725: 621.791.8.01

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PREFACE

The research work reported in this thesis was done in Lappeenranta University of Technology as a part of research projects “Adapli” and “Hiprola” funded by National Technology Agency Tekes and Finnish industry during years 2005-2007.

I am grateful for the supervisors of my study, Docent Antti Salminen and Professor Jukka Martikainen for their guidance and support during the course of this work.

I wish to thank the pre-disputation examiners, Dr Richard Martukanitz from PennState University and Dr Martti Vilpas from STUK for their guidance and valuable comments.

I am extremely grateful for Dr John C. Ion for his valuable advice and comments and great work with English of this thesis.

I would like to express my sincere thanks to the guys, who really made this work a reality: Pertti Kokko, Jaakko Moisio, Antti Heikkinen and Juha Turku from LUT. I am also very grateful for Jukka Varrio and Antti Pesola from Hämeenlinna University of Applied Sciences for performing the high frame rate videography and Taito Alahautala, Janne Lehtinen and Petri Harkko from Cavitar Oy for performing the low frame rate videography. Without your work this thesis would have never seen the light. I have really enjoyed the co-operation with you guys. I would also like to thank Marja Kokko for all the support and encouragement she has given.

Financial support provided by the Research Foundation of Lappeenranta University of Technology is gratefully acknowledged. I would also like to thank Risto Laitinen from Ruukki for giving me the steel material for this work.

The real motivators for this work are the two most important persons of my life, my husband Janne and my father Aarno. You have always been there for me, supporting and giving me strength to go on. Your love and care means everything to me and I am extremely happy for having you! Our dog Mithril has also given a lot of cheering in her own way during the course of this work and deserves a lot of scratches and bones for that work.

I cannot forget the support and cheering I have received from my friends and relatives, so big thank belongs to them as well.

Lappeenranta, April 2008

Anna Fellman

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Symbols and abbreviations

Abbreviation Explanation

MIG Metal Inert Gas welding MAG Metal Active Gas welding TIG Tungsten Inert Gas welding

PA Plasma Arc welding

HAZ Heat Affected Zone

IB Inverse brehmstrahlung absorption

G Gravitational force

Fr Reactive force

Fem Electromagnetic force

Femr Radial component of electromagnetic force Fema Axial component of electromagnetic force

Fst Surface tension force

vfw Filler wire feed rate [m/min]

s Thickness of work piece [mm]

wag Width of air gap [mm]

v Welding speed [m/min]

dfw Diameter of filler wire [mm]

Ew Welding energy [kJ/mm]

PL Laser power [W]

U Mean voltage [V]

I Mean current [A]

Meff Melting efficiency [J/mm³] Aw Weld cross-sectional area [mm²] CCD camera Charge coupled device camera

CMOS camera Complementary metal oxide semiconductor camera

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1. Introduction

Laser hybrid welding is a process in which laser welding and traditional arc fusion welding are combined to form a united process in which the two components act in synergy, Fig. 1.1. The processes are arranged such that they act on the same process zone; the laser beam supports the effects of the arc and vice-versa. By combining the processes correctly, the advantages of both can be exploited and the disadvantages avoided. The lasers used are typically of high power (suprakilowatt); CO2, Nd:YAG, diode, disk or fibre lasers being the most common. The arc processes that they are commonly combined with are metal inert gas (MIG)/metal active gas (MAG), tungsten inert gas (TIG) or plasma arc (PA).

Figure 1.1. Principle of the laser hybrid welding process. (Dilthey, U. & Wieschemann, A., 1999) Laser hybrid welding was first investigated at the end of the 1970s. Professor William Steen together with his postgraduate students (Eboo, M. et al, 1979; Steen, W.M., 1980; Alexander, J. &

Steen, W.M., 1980 & 1981) examined the effects of combining an electric arc with a laser beam on the welding process. At the time, maximum available laser powers were on the order of about 2 kW. The main problem encountered in transferring hybrid laser welding to industrial use was the high capital cost of the laser. Therefore the driving force for research in laser hybrid processing was to find ways of reducing the laser power needed for the application such that a lower power laser could be used. It was though then that if the arc was used to preheat the work piece prior to laser welding, a laser of reasonably low power could be used, whilst the advantages of laser welding, namely the high penetration depth and increased welding speed, could be utilised.

In 1982 and 1983 Hamazaki et al., working in Japan published results obtained using a 5 kW CO2

laser combined with TIG and MIG arc sources to create a hybrid welding process. Subsequently the research effort was reduced for a considerable time because available laser sources were still of insufficient power, and it was thought that the combination of two different energy sources was too complex for industrial use. In 1994 Beyer et al. working at the Fraunhofer Institute in Germany drew attention again to hybrid processes. They studied the combination of a 2 kW Nd:YAG laser and a TIG arc as a laser hybrid welding process. Since then many studies have been published demonstrating the advantages of hybrid welding, but the mechanisms of the method have not been

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completely explained. It has become clear that the interaction of many phenomena induces the occurrence of complex behaviours, such as the interaction of the laser beam and arc, the behaviour of the keyhole and molten pool, movement of the pole, and the effect of plasma and assisting gases.

(Abe, N. & Hayashi, M., 2002)

The hybrid welding process is advantageous because synergic effects are created from the two processes; the overall characteristics may be greater than the sum of the individual processes. In general, the arc provides energy and molten metal (if filler wire is added) whilst the laser produces characteristics such as deep penetration, arc stability, and extension of the root. It is thus possible to increase productivity and improve weld quality simultaneously. (Abe, N. & Hayashi, M., 2002) The principal advantages of the hybrid laser-arc process are (Abe, N. & Hayashi, M., 2002), (Dilthey, U. & Wieschemann, A., 1999), (Abe N., et al., 1998b):

- Deeper penetration may be achieved than with either the laser or arc alone.

- The welding speed may be greater than with either the laser or arc alone.

- Via the keyhole, the laser beam helps to ignite the arc and stabilize it and enhances the penetration of energy deep into the material.

- The energy input is decreased compared with arc welding and therefore thermal distortion and post weld residual stresses in the structure are diminished. The arc energy also permits the control of cooling conditions, which means that the cooling time can be increased in comparison with autogenous laser welding and therefore the hardening is reduced in hardenable materials.

- Tolerances for groove preparation and misalignment of the laser beam may be relaxed, without the need for wide grooves typical of arc welding thick materials. This means that the amount of filler metal that needs to be added to the joint can be reduced and therefore the amount of energy introduced into the processing region is reduced. Fabrication time is also reduced.

- Characteristic defects of high energy beam welding (e.g. lack of fusion and incomplete joint filling) may be reduced.

- The mechanical properties of the weld can be improved. For example, the energy introduced by arc welding results in a tempering effect (short-time post heating) of the hard microstructure that may be produced by the laser beam alone. By using filler metal the impact strength and other mechanical properties of the weld can be improved.

- The use of additional molten metal also suppresses underfilling of the weld when a gap is present in the groove.

- Pieces of dissimilar thickness may be welded with a smooth transition between the plates.

The main disadvantages of the hybrid laser-arc process are (Dilthey, U. & Wieschemann, A., 1999):

- The number of variables is large and reference values are not readily inferable from the individual processes.

- The accessibility of laser welding is destroyed.

- High investment costs of equipment if compared to traditional arc welding only.

There are many ways to classify the laser hybrid welding process. Classification may be based on:

the power balance between the processes used (arc-assisted laser welding, laser-stabilized arc welding etc.); the process combination (CO2 laser-MIG, Nd:YAG laser-TIG etc.); the orientation of the processes (leading arc/leading laser); and groove type (I, Y or V).

Laser hybrid welding process may also be distinguished based on the distance between the laser and arc processes. When the distance is large, the processes operate with no mutual interaction, acting separately in terms of time or region; this is known as process combination. When the processes are close to one another, acting at the same time in one zone, they mutually influence and assist one another, and the processes are then coupled, Fig. 1.2. (Dilthey, U. & Wieschemann, A., 1999)

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Figure 1.2. Process coupling and combination (Dilthey, U. & Wieschemann, A., 1999)

Whether the laser’s or arc’s character predominates, depends on the selected power input ratio. The processes are operated in succession working close to each other in the same working zone. The application area of laser hybrid welding process depends also on which arc process is used. TIG process can be operated either with or without filler metal addition. The use of TIG process together with CO2 or Nd:YAG laser welding process finds use mainly for small plate thicknesses. This is due a fact, that TIG arc cannot transfer a lot of filler metal to the process area as a part of the heat of the arc is used for melting of filler wire. Plasma arc welding can be used together with laser beam process in such way that the laser beam is surrounded by a concentric plasma arc. The heat of the plasma arc brings about a heat treatment which provides the ability to reduce the cooling rate and decrease the susceptibility to hardening and development of residual stress states. Therefore it is possible to tailor the microstructural condition of weld and HAZ to the application in hand. The third option is to use MIG/MAG together with laser beam welding. By using MIG/MAG (continuous or pulsed arc) process as the arc process in laser hybrid welding, the gap bridging ability can be increased as the addition of filler metal I more controlled and its volume can be higher if compared to cold wire feed used together with plasma and TIG arc. (Dilthey, U. &

Wieschemann, A., 1999)

TIG has been used in many laser hybrid welding studies. As the TIG arc is almost the same as MIG/MAG arc, the difference being the controlled wire feed of MIG/MAG welding, some of the research studies using TIG as arc source in laser hybrid welding are presented in the literature review. As this research work was done using pulsed MAG process together with CO2 laser, no other arc or laser welding processes were not thoroughly presented in the literature review.

1.1 Industrial applications of laser hybrid welding

Laser hybrid welding is currently used in shipbuilding, the automotive industry, vessel manufacturing, and tube and profile production. The process has gained an interest because of its advantages over arc or laser welding alone, outlined above. Interest in its application in industrial production continues to increase.

Probably the most well-known user of laser hybrid welding is the Meyer Werft shipyard in Pappenburg, Germany, where CO2 laser-MIG hybrid welding is used for joining plates together in butt configurations, and joining of stiffeners to decks using T-butt joints. Some of the reasons for their decision to use the process include the higher welding speed and significantly reduced thermal distortion in comparison with conventional arc welding. Reduced distortion leads to increased accuracy, reduction of fitting and straightening in subsequent assembly work, and improved preconditions for outfitting. Another significant reason was the good metallurgical and fatigue

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properties of laser hybrid welds, potentially allowing a reduction in plate thickness for decks, and thus significant weight savings in the ship structure. After adopting hybrid welding in ship hull production the total time for hull production has been reduced by 40%. (Roland, F. et al., 2002) Another popular application of laser hybrid welding is the welding of containers, tanks and pipes.

Figure 1.3 shows an application in which components of an oil tank are joined using CO2 laser-MIG hybrid welding. The material is mild steel S235JR of thickness ranging from 5 to 9 mm. The groove edges are shear cut, creating an air gap that varies between 0 and 1 mm. The pulsed MIG process trails and the welding position is 45° downward. The shielding gas used is an argon-helium-mixture and the filler wire is G3Si1 of diameter 1.2 mm. In this application two or more cylinders and two lids are joined circularly by radial seams as butt joints to form the tank. (Petring, D. et al., 2003)

Figure 1.3. Production of oil tank with CO2 laser-MIG hybrid welding and cross-sections of welds.

(Petring, D. et al., 2003)

Figure 1.4 shows an application of hybrid welding of single-pass longitudinal joints of AISI 304/304L stainless steel tubes. The wall thicknesses in the application vary between 2.4 and 14.4 mm. The CO2 laser-MAG (pulsed) hybrid welding process creates a smooth and regular bead with a sound root. The welding speed achieved is 10 times higher than the conventional welding process used previously, even though the shear cut edge preparation remains unchanged. (Petring, D. et al., 2003)

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Figure 1.4. CO2 laser-MAG hybrid welding of stainless steel tubes and weld cross-sections.

(Petring, D. et al., 2003)

The automotive industry provides numerous applications for hybrid welding, e.g. in joining of tailored blanks and lap joints in the car body. In tailored blank welding it is possible to achieve a smoother junction between plates of varying thickness particularly if filler metal is used. The demands for edge preparation and clamping can be reduced. In automotive applications, the driving forces for the use of hybrid welding are the higher welding speed, low energy input and reduced distortion. (Petring, D. et al., 2003)

Volkswagen has adopted the laser hybrid process for the production of doors of the Phaeton model.

A structure with high stiffness and low geometrical tolerances was required. In order to produce a stiff, low-weight door, the door was made of aluminium sheet, cast and extruded materials. A car body there contains many lap joints. It is very probable that air gaps exist between the sheets – autogenous laser welding results in an incompletely filled groove. In the automotive industry, wire feed is used in some cases, where some of the laser energy is used for melting the wire. By using laser hybrid welding the filler material can be added without consuming laser power and the laser energy can be used to increase penetration into the base material. In addition, the weld bead is widened and the mechanical properties of the weld remain good. (Graf, T. & Staufer, H., 2002) The use of high strength steel has increased in recent years because thinner materials can be used, providing weight savings. Traditionally these materials are welded using conventional MIG/MAG processes, which require multiple passes. This creates a large amount of thermal loading in the structure, and therefore induces residual stresses and distortions and irregular annealing of previous passes, which often reduces the strength and toughness of the weld seam. In addition, the processing times are long. In one application using Nd:YAG laser-MIG hybrid welding, welds were manufactured with one pass in sheet thicknesses of 3-8 mm; the disadvantages mentioned above were avoided and simultaneously the welding speed was increased. The possibility of using filler wire enables control of the weld metal composition according to the requirements of high strength steels. One advantage is also that the process stability and reliability is guaranteed for air gaps up to 1 mm for different types of joint configuration. (Mahrle, A. & Beyer, E., 2006)

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1.2 Emphasis of this study

As mentioned previously, interest in laser hybrid welding processes is increasing in industry because of the advantages listed above. Typically, in the metals industry in which large products are fabricated, welding production is now performed using traditional arc welding processes, which have a large energy input into structures, causing residual stresses and distortions that must be straightened after welding. It is estimated that in some cases the straightening of structures after welding may consume almost half of the total welding production time and require skilled professionals (Ion, J.C., 2005). The use of laser hybrid welding processes can lower the costs for production considerably. The competitiveness of welded products could be improved as non value- adding production costs (e.g. straightening work) may be avoided and welding times reduced. One very important aspect is the good and uniform weld quality achieved.

Laser welding is an accurate welding process but requires stringent tolerances on part manufacture and fixturing. In many cases involving joining of large structures it is impossible to attain the accuracy requirements needed for laser welding, not to mention maintaining them during welding.

Because the laser hybrid welding process allows larger joint tolerances, the scope of use of laser welding in production may be widened.

This thesis provides explanations of the mechanisms of the CO2 laser-MAG hybrid welding process and describes how the process can be made even more efficient for the production of high quality welds. Most of the experiments are performed using a butt joint with an air gap, because this simulates real-world production techniques in which an air gap is present at some point in the joint.

The steel used for the study was a common low-carbon structural steel, type S355K2G3 (EN 10025), with a thickness of 6 mm. Methods of improving process efficiency are studied by examining how the welding speed can be increased through joint preparation whilst maintaining high quality welds. The study concentrates on the effects of several parameters on weld quality:

joint preparation, welding speed, process separation and torch/arc orientation. The effects of arc welding process parameters on weld quality are found to be remarkable, and play an important role in process behaviour. Many of the experiments were performed to examine the effects of arc parameters (pulse parameters and potential difference) on weld quality as well, but fell outside the scope of the thesis. Pulsed MAG welding was used because of its relatively low energy input into the joint region and the deeper weld penetration when compared with a continuous arc.

In the CO2 laser-MAG hybrid welding of structural steel S355K2G3 (thickness 6 mm) with 4.7-5.2 kW laser power and pulsed MAG welding, the following arguments can be invoked based on this study:

- Joint and air gap:

By introducing an air gap into the joint, the maximum welding speed for which full penetration is achieved may be increased by approximately 80%. Wider air gaps than those used in autogenous CO2 laser welding may be accommodated, but if the air gap is too wide it becomes difficult to achieve a good quality weld because of arc instabilities (high welding speed and arc power).

- Process separation:

The process separation has an effect on the gap bridgeability. The process separation, welding speed and torch location have a significant effect on droplet formation and its trajectory, which defines the metal transfer type.

- Metal transfer and molten metal movements:

By using the synergetic arrangements defined for MAG welding, the type of metal transfer may be different from that expected with MAG welding because the laser beam has an effect on the melting of filler wire. The MAG torch location in relation to the laser beam also has an effect on

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molten metal movement and therefore the bridging of an air gap and weld quality. The bridgeability of an air gap depends on the metal transfer type produced during welding.

2. Theoretical background

2.1 Laser beam-material interaction mechanisms during CO₂ laser beam welding In laser keyhole welding a high power density laser beam impinges the work piece to be welded.

Initially the surface reflects a large fraction of the laser beam, but as soon as the surface starts to heat up, it begins to absorb the laser power more efficiently. Figure 2.1 shows the absorptivity of the laser beam for various ferrous alloys as function of temperature. As absorption increases, the high power density laser beam starts to evaporate material and forms a keyhole, Fig. 2.2. The keyhole is traversed through the material with the molten walls sealing up behind it, forming a weld. The keyhole remains open because of the pressure of the vapour generated, which pushes the walls sideways. The laser beam radiation enters the keyhole and is subject to multiple reflections inside the keyhole. The absorption of laser energy in the keyhole depends on the laser beam wavelength, the chemistry of the material being welded, and the temperature of the vapour.

Absorption has a large effect on the shape of the fusion zone and the integrity and efficiency of the laser welding process. Absorption of the laser beam can be divided into two categories: Fresnel and inverse Brehmsstrahlung (inverse braking, IB) absorption. In Fresnel absorption the laser beam is absorbed by the work piece surface and IB is an absorption mechanism where the photon energy is absorbed by electron or ions. (Steen, W.M., 2003), (Metzbower, E.A., 1997)

Figure 2.1. Absorptivity of the laser beam as a function of temperature for ferrous alloys. (Hügel, H. & Dausinger, F., 1995)

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a) b)

Figure 2.2. Formation mechanism of keyhole in laser welding process. a) Laser beam forms a keyhole which is kept open by a hydrostatic pressure balance, b) molten metal flows inside the keyhole downwards and around the keyhole. (Ion, J., 2005; Steen, W., 2003)

2.1.1 Metal vapour and plasma plume formation in CO₂ laser welding

As the terminology concerning vapours and plumes in CO2 laser welding are varying in different references, Figure 2.3 presents the most commonly used definition of terminology, which are also used in this thesis.

In high power laser welding, when a focused laser beam impinges the metal surface to melt and vaporise it, metal vapour is formed above the surface of the work piece, see Fig. 2.3. The vapour is produced by high power density laser beam irradiation, and is composed of vaporised neutral atoms, clusters, ionised atoms and free electrons. Each of these particles can result in power attenuation, in the form of absorption of the beam by the metal vapour, scattering of incident laser power by vapour particles and the re-radiation of absorbed laser power from the plasma plume, thus complicating the overall laser-vapour interactions. It has been proposed that the formation of plasma plume (see Fig. 2.3) is associated with “inverse Brehmsstrahlung” absorption. The partially ionised gases in the plasma plume absorb the infrared wavelength of a CO2 laser beam by the inverse Brehmsstrahlung mechanism. The absorbed energy from the laser beam accelerates the electrons in the metal vapour. The plasma plume develops when the metal vapour temperature is high enough to produce ionisation. The plasma plume detaches from the surface and moves upwards towards the focusing optics, see Fig. 2.3. The increasing electron density results in increased absorption of the laser beam by the metal vapour. Depending on the shielding gas composition, the power density and wavelength of the laser beam, the plasma plume rises above the material surface until the power density of the expanding beam is insufficiently high to sustain the plasma. Once the plasma is extinguished, a second plasma plume is generated from the material surface. This periodic formation of plasma interrupts continuous laser power input to the material, causing the laser welding process to be unstable, which influences the laser beam energy utilization and weld quality. When welding with low intensities, the plasma is confined to the surface region, and aids in thermal coupling of the beam to the work piece. (McCay, M.H. et al., 1988), (Xie, J., 1999), (Chiang, S., 1991), (Kim, K.R. & Farson, D.F., 2000)

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Laser beam

Filler wire

Droplet

Laser plume Metal vapour Plasma

plume

Work piece

Figure 2.3. Terminology of plumes and vapour used in this thesis.

The dynamic behaviour of metal vapour and plasma during CO2 laser welding with helium shielding has been examined by Xie (1999). He found that the metal vapour grew in size continuously to a maximum height (2-3 mm for steel), then becoming smaller with respect to elapsed time, Fig. 2.4. When the vapour was small enough, the vapour started to grow again to start another cycle of the fluctuation; occasionally the metal vapour disappeared completely. The fluctuations of vapour are explained such that metal vapour blocks and absorbs a portion of the laser beam. The vapour expands as the laser beam vaporizes and ionizes metal from the work piece continuously. The plasma plume formed partially blocks the laser beam by absorbing and defocusing it. When the plasma plume is large enough, the laser beam is not able to reach the work piece surface and therefore no additional materials are available from the work piece to form the plasma plume. Therefore the plasma plume gets smaller as the ions and electrons dissipate away.

When the vapour is again small enough the laser can penetrate the metal vapour and vaporize the metal. Then as the metal vapour grows again another cycle of the plasma plume fluctuation is begun. (Xie, J., 1999)

Figure 2.4. The fluctuation of metal vapour and plasma plume in CO2 laser welding. Metal vapour and plasma plume are seen as a bright cloud. The size of the cloud varies in time and in cycles. CO2

laser, 6 kW, 1.25 m/min, bead on plate, 1045 steel, helium flow rate 20 L/min. (Xie, J., 1999) It is shown that there is a hot spot produced in a keyhole near its opening in the CO2 laser welding of mild steel sheets. It is considered to behave in a different way depending on whether or not the keyhole is sealed at its base, when the hot spot of the metal vapour is heated by absorbing the laser

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beam via an inverse Brehmsstrahlung mechanism. In partial penetration, the hot spot is moved slightly above the surface since the base of the keyhole prevents expansion of the laser-induced plasma core, Fig. 2.5a). In full penetration welding, there is no base of the keyhole to impede the expansion of the hot metal vapour, and so it propagates both upwards and downwards, Fig. 2.5b).

When the laser beam impinges the keyhole near the opening, molten metal is pushed downwards by the recoil force of evaporation, and then the metal vapour propagates into the keyhole. In full penetration welding, the metal vapour in the keyhole can escape from the rear side of the keyhole, but in partial penetration welding the metal vapour remains on the top side of the keyhole. When welding steel with increasing welding speed, the change in height of the metal vapour is reduced and complete disappearance of the vapour is no longer observed. (Mori & Miyamoto, 1997), (Miyamoto, I. et al., 1997), (Xie, J., 1999)

Figure 2.5. Fluctuation of metal vapour in CO2 laser welding. a) In case of partial penetration the hot spot of metal is fluctuated in surface area and b) in full penetration welding the hot spot propagates both upwards and downwards (Mori & Miyamoto, 1997).

2.1.2 The dynamic behaviour of keyhole

The shape and size of the keyhole opening change greatly within a short period of 10 ms or less, even when the laser power remains the same. Molten metal moves from the front of the keyhole to the rear side of the molten pool. The metal vapour develops vertically upwards when the keyhole opening is large, and tilts behind when keyhole size is small. (Matsunawa, A. et al.., 1999), (Matsunawa & Katayama, 2002)

It is stated that metal vapour fluctuation is related to keyhole instability and the fluctuation is probably caused by variations in the opening of the keyhole. The size of the keyhole opening varies at the same frequency as the vapour fluctuates. It is stated that vapour fluctuation is caused by the formation of swelling due to the irregular movement of molten metal in a keyhole, Fig. 2.6. When the plasma plume blocks the laser beam from penetrating the keyhole, the vapour pressure inside the keyhole decreases, making it more difficult for the vapour pressure to push the swelling out of the keyhole. Therefore the probability of keyhole collapse is increased. And so, the keyhole is more unstable when a plasma plume is formed during welding. These effects are stated to be a result of the metallic plasma formed immediately above the keyhole area. The energy reduction in the keyhole area decreases evaporation in the keyhole and therefore the keyhole width and depth

A)

B)

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decrease. When the metallic plasma becomes smaller, the plasma is reduced and evaporation inside the keyhole increases. This again increases the metal vapour and forms a wider and deeper keyhole and larger plasma plume. (Xie, J., 1999), (Seto, N. et al., 2000), (Matsunawa, A. et al., 1999), (Matsunawa & Katayama, 2002)

If the keyhole instability or plasma plume fluctuation are suppressed during laser welding the laser weld quality is improved significantly. Keyhole instability may result in particular weld defects such as porosity, spatter, underfill, blowholes and irregular beads. (Xie, J., 1999)

Figure 2.6. Keyhole instability and plasma fluctuation. (Xie, J., 1999)

It has been postulated that the keyhole is not simply cylindrical, but has a wide opening and a narrow waist below (Miyamoto, I. et al., 1997). It is stated that the cylindrical keyhole filled with metal vapour is maintained because of the balance between the recoil force of evaporation and the surface tension of the molten metal. The reason for the wide keyhole opening is believed to be the heating of metal vapour in the keyhole due to laser absorption. The energy transfer to the keyhole wall near or just above the opening becomes less effective than the middle, and therefore the temperature of the metal vapour near the opening becomes higher than that of the middle. The hot metal vapour near the opening is heated to a high temperature by laser absorption due to inverse Brehmsstrahlung. The hot metal vapour enlarges the keyhole diameter near the opening because of the energy transfer.

In CO2 laser welding, the blocking effect of plasma plume is reduced when using gases that have a high ionisation potential. Therefore helium is normally used as the plasma control gas, even though it is four times the price of argon outside the United States. The higher the laser power, the greater the potential for the plasma plume to block the laser beam. At slow speeds, helium is superior in diminishing plasma plume formation, and therefore penetration is increased, but with higher welding speeds argon may be better in achieving greater penetration. This is explained by the fact that metal vapour/ plasma plume may be both beneficial and deleterious in aiding absorption. If the metal vapour lies near the surface or inside the keyhole it is beneficial. But if the metal vapour becomes thick or leaves the surface (ionizes to become plasma plume) its effect is to block or disperse the laser beam. If the gas is slightly reactive with the weld metal, a thin film of oxide may be formed, which enhances optical coupling. (Steen, W.M., 2003)

Normally in laser welding a side jet is used to blow the plasma plume away. The flow rate of the gas must be high enough to remove the plasma. If the flow rate is too high, it affects the weld pool flow and the melt pool can be disturbed resulting in poor weld bead quality. (Steen, W.M., 2003)

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2.2 The characteristics of pulsed MIG/MAG welding

MIG/MAG welding is a commonly used arc welding process, which uses a continuously fed electrode as filler metal. The arc is shielded from atmospheric contamination by the shielding gas delivered through the welding torch and cable assembly. MIG/MAG welding is a direct current process that is operated mainly in the direct current electrode positive (DCEP) mode. MIG uses inert shielding gas (pure argon, helium, nitrogen or their mixtures), whereas MAG uses a active shielding gas (mixtures contains CO2 and O2). (Welding Handbook, 1978)

2.2.1 Properties of welding arc

In general, an arc is a discharge of electricity in an ionized gas. The welding arc forms between the negative (cathode) and positive (anode) electrodes making a circuit that conducts current, which is a flow of electrically charged particles (electrons and ions). The arc consists of the cathode and anode, the arc column, and anode and cathode spots, Fig. 2.7. The cathode region is a very narrow area close to the negative electrode. Electrons are released from the cathode as a result of thermionic emission, via a strong electric field and concentration of a positive charge around the cathode. The cathode surface that emits electrons is called the cathode spot. The arc column is located between the cathode and anode regions. The potential difference gradient in the arc column is lower than those in the cathode and anode regions. Current density, electric field intensity and the cross-section of arc column depend on the degree of thermal ionisation in the gases comprising thermionic plasma in the arc. The arc column has a high temperature and high plasma flow velocity.

The high temperature is maintained by ohmic resistance and dissociation of atoms and molecules, and decreases with losses through energy radiation, conduction and convection. The anode region is a thin layer on the positive electrode. Electrons emitted from the cathode that pass through the cathode region and the arc column have already released a part of their energy, another part being released at the anode surface, which is heated almost to boiling point. The welding arc is a part of the electric circuit surrounded by its own magnetic field. The electrical resistance of the arc depends on current intensity, the setting of the current source, and the degree of ionisation of the shielding gas used. The degree of ionisation may be increased or reduced by external factors, such as the laser beam in laser hybrid welding. (Tusek, J. & Suban, M., 1999)

Figure 2.7. Components of a free burning arc with a positive non-consumable electrode (TIG welding). (Tusek, J. & Suban, M., 1999)

2.2.2 The metal transfer mechanisms in pulsed MIG/MAG welding

The use of the spray molten metal transfer mode in thin sheet applications is not suggested because of the high currents needed to produce a spray arc. The work thickness limitation is largely overcome by the use of special power sources, which produce controlled wave forms and frequencies that pulse the welding current. Two levels of current are normally used: a constant low

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background current which sustains the arc without providing enough energy to cause drops to form on the wire tip; and a superimposed pulsed current with an amplitude greater than the transition current necessary for spray transfer, Fig. 2.8. When the pulse forms, one or more droplets are formed and detached at the same time. The frequency and amplitude of the pulses control the energy level of the arc. For pulsed welding, many variations of such power sources are available.

The simplest provide a single frequency of pulsing and independent control of the background and pulsed current levels. Synergic power sources are also available that automatically provide the optimum combination of background and pulse for any given setting of wire feed speed. (Welding Handbook, 1978) In this thesis the phase using background current is called as background period.

Figure 2.8. Welding current characteristics in pulsed spray arc transfer. (Welding Handbook, 1978)

Pulsed MIG/MAG welding applies waveform control logic to produce very precise control of the arc through a broad wire feed speed range. Because the arc dynamics are precisely controlled, the process can be used as a fast-follow process at high travel speeds or as a high deposition rate, fast- fill process. The use of a pulsed arc enables spray transfer in procedure ranges that would normally be either globular or short-circuiting. The arc is very stable and has a lower energy input than spray transfer. Pulsing transfers small droplets through the arc; normally one droplet per pulse. Pulsing leads to stable spray metal transfer and the formation of a uniform bead shape with shallow penetration. (Sadler, H., 1999), (Kim, Y.-S. & Eagar, T.W., 1993)

Pulsing of the welding current means that there are additional operational parameters to adjust.

These include peak and background current, peak pulse duration and of course the parameters of DC welding, including electrode extension, welding current and potential difference. When one droplet per pulse is detached, the pulse frequency region is optimum. The droplet transfer mechanism depends on the pulse frequency, peak current and background current. All must be adjusted to the correct levels in order to achieve one droplet per pulse transfer. Background current and pulse duration play a significant role in controlling the droplet size. (Kim, Y.-S. & Eagar, T.W., 1993), (Subramaniam, S. et al., 1998)

As the current is increased in DC welding in an argon-rich atmosphere, the metal transfer mode changes from globular to spray. If the current is further increased, the anode spot size increases in size until it begins to rise up the sides of the wire. This causes further melting of wire edges. As the current is high enough, the solid electrode tip becomes tapered. If one liquid metal drop with a size similar to the electrode diameter for every pulse is desired, the operating conditions must be such that there is no tapering at the tip of the electrode. If tapering occurs, the pulsed current process degenerates into a streaming metal transfer mode in which the droplet size is too small to control.

Tapering can be suppressed by using shielding gases consisting of argon and helium. If CO2 is used as the shielding gas in pulsed MAG welding, the metal transfer is known as repelled transfer – pulsing of the current does not then produce projected spray transfer. With helium as the shielding

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gas, the metal transfer may be projected spray transfer in pulsed MIG welding if the peak current is increased above the transition current of the repelled-projected transition. (Kim, Y.-S. & Eagar, T.W., 1993)

If the droplet to pulse frequency ratio is higher than one, the natural frequency becomes larger than the pulsing frequency, hence the pulse frequency is insufficient, and multiple-droplet detachments are observed when the peak time is too long or when the background current is high and the background time is long. Figure 2.9 shows droplet detachment during one pulse in a case when the background current is high and the time is long, and the peak current is high. In cases in which the peak current is high and the time is long, streaming transfer is formed in which there is considerable tapering of the electrode, Fig. 2.10. The taper elongates and a stream of droplets is formed. This type of transfer may result in spatter, and so these conditions are undesirable. If the background current and peak time are low, the molten metal will become a pendant (a liquid cylinder at the wire end after the change from peak current to background current) and no pinch effect is produced to detach the droplet immediately. Subsequently, the liquid cylinder fragments into multiple drops. If the peak current is maintained for a slightly longer time, a continuous streaming of droplets is achieved. (Kim, Y.-S. & Eagar, T.W., 1993), (Subramaniam, S. et al., 1998)

Figure 2.9. Multiple droplet detachments in a single pulse cycle. Peak current 400 A, background current 150 A, frequency 50 Hz.

(Subramaniam, S. et al., 1998)

Figure 2.10. Multiple droplet detachments in a single pulse cycle showing tapering of molten metal. Peak current 400 A, background current 100 A, frequency 50 Hz.

(Subramaniam, S. et al., 1998)

When the droplet/pulse-ratio is less than one, many pulses may be required for droplet detachment.

Since the pulse frequency is high, droplets are formed at the wire tip after every pulse. As the peak current increases, the width of the optimum pulsing frequency region increases. The transfer mode in such situations is known as pulsed globular, and it occurs at high frequencies and low background currents. This means that the pulse duration is small and there is not enough energy to detach the droplet in each pulse. Therefore the droplet grows during each pulse cycle and surface tension is the predominant force preventing detachment. When the droplet finally detaches, it occurs through its own weight, Fig. 2.11. This example shows that the peak time must reach a

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minimum value to detach one droplet for every pulse. (Kim, Y.-S. & Eagar, T.W., 1993), (Subramaniam, S. et al., 1998)

Figure 2.11. Pulsed globular transfer mode. Peak current 250 A, background current 50 A, frequency 400 Hz. (Subramaniam, S. et al., 1998)

If the pulse current lies below a critical limit (area 1 in Fig. 2.12), the pendant droplet swells into a large globe independent of pulse duration and arc behaviour, becoming unstable and accompanied by much spatter because the electromagnetic force applied to the molten electrode tip is not directed towards the base metal. If the peak current limit is achieved, but the pulse duration is too short (area 2 in Fig. 2.12), the pulse energy is too small to create droplet transfer in one pulse, and the pendant droplet swells into a large globe during several pulses. The current should be maintained at the peak value for a longer time to exercise its electromagnetic force over the droplet. If the current falls too fast, the electromagnetic pinch force decreases and the pendant droplet never transfers. If both peak current and pulse duration lie in the optimum range (area 3 in Fig. 2.12), one droplet with a diameter close to the electrode diameter is detached after every pulse and arc behaviour is stabilised. At a constant wire feed speed, a large frequency is preferable for practical use, because:

the frequency of metal transfer increases; the weld bead is uniform; the fluctuation of arc length caused by the metal transfer is small; and the arc length can be set to such a short value that the welding speed can be increased without spatter. If the peak current is attained, but the pulse duration is too long (area 4 in Fig. 2.12), the transfer mode is of the spray type, but the molten electrode tip is prolonged (tapering) and touches the base metal causing much spatter. Also, as the pulse duration increases, the frequency decreases with a constant feed rate of electrode, resulting in incessant fluctuations of the arc length. In this case the next droplet starts to form before the previous one has detached. (Ueguri, S. et al., 1985)

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Figure 2.12. The effect of pulse duration and peak current on metal transfer mode. (Ueguri, S. et al., 1985)

With trailing torch welding, penetration decreases and the weld bead becomes wider and flatter than with leading torch welding. A leading torch also produces a more convex, narrower bead, a more stable arc, and less spatter on the work piece. (Welding Handbook, 1978)

2.2.3 Forces affecting molten metal droplet detachment

The transfer of a metal droplet across the arc may occur in a free flight mode (spray and globular transfer) or in short circuiting mode. In free flight mode, the droplet detached from the wire end flies along the arc, whereas in short circuiting transfer the molten wire end dips into the weld pool.

The detachment of molten metal from the electrode tip has been explained using static force balance theory. According to the theory the principal forces involved are (Fig. 2.13):

- gravitational force, G

- reactive force, Fr

- electromagnetic force, Fem - surface tension, Fst

- plasma drag force (the effect on droplet detachment is minor) (Norrish, J., 2003), (Kim, Y- S. & Eagar, T.W., 1993b)

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d

_fw

F

_st

F

_{em r}

G F

_r

d

_d

d

_fw

F

_st

F

_{em r}

F

_{em a}

F

_em

G F

_r

d

_d

Figure 2.13. Forces affecting a molten metal droplet. (Grubic, K., 1998)

The gravitational force, G, is simply created by the mass of the drop and acts as a detaching force when welding in a flat position. The electromagnetic force, Fem, is created by the magnetic field induced by current flow through the molten droplet. This force is also called the Lorentz force, which may have radial and axial components. The radial force, Femr, which is also called the pinch effect, results from pressure acting perpendicularly to the axis of a conductor through which the current is passing. The pinch force is a direct function of welding current and wire diameter. This pinch force, together with the other forces, deforms the electrode tip and forms a neck in the end of the wire. This neck contracts as long as the droplet detaches from the electrode tip. Only the pinch force forms the neck, with the axial component of the electromagnetic force helping in the detachment of the droplet. The axial force, Fema, is greater than zero if the arc column on the droplet surface has a large diameter, which depends on the current, polarity and shielding gas composition.

With positive polarity, the higher the current, the larger the diameter of the arc column. Shielding gas has a significant effect on detachment: with argon the diameter of arc column is larger than with carbon dioxide, and therefore the axial force with argon is positive and the droplets are smaller.

With CO2 the axial force is negative and therefore the droplet is pushed up from the wire end, typically tilting to the side, Fig. 2.14. This is simply because of differences in the thermal conductivity of gases. When the axial force equals zero, the current level lies in the short-circuiting range and when the force is positive, molten metal transfer occurs in free flight mode. The reactive force is generated by metal vaporization at the electrodes. This force, formed by vapour, is in the opposite direction to metal transfer. The plasma drag force has no effect on droplet detachment from the wire tip; it acts towards the direction of droplet transfer when the droplet is travelling from the wire end to the melt pool. Because of plasma flow there is pressure created on the droplet that enhances droplet movement and transfer. The surface tension force acts to retain the liquid drop at the electrode tip; it depends on the type of filler metal, the temperature of the droplets, and the gas atmosphere in which welding is performed. For example, oxygen decreases the surface tension whereas nitrogen increases it, which means that the droplet can be detached more easily if oxygen is added to shielding gas. (Kim, Y-S. & Eagar, T.W., 1993b), (Grubic, K., 1998)

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Figure 2.14. Droplet formation and forces acting on the droplet with different shielding gases. a) with argon shielding the axial force is positive because the arc column diameter is larger than that of the droplet, b) and c) with CO2 shielding the axial force is negative because the arc column diameter is less than that of the droplet. (Grubic, K., 1998)

In the case of globular mode transfer, which operates with low current values, the gravitational force is the predominant detachment force whilst the vapour or plasma jet forces and surface tension act against transfer. As the current increases, the electromagnetic pinch force becomes significant, causing constriction of the molten metal neck and a downward droplet detachment component. In the case of short circuiting transfer, the predominant detachment forces are surface tension and electromagnetic pinch. (Norrish, J., 2003)

2.3 Interaction between the laser and arc welding processes in hybrid welding The effectiveness of the laser hybrid welding process is based on the fact that the laser beam and arc interact with each other. This was initially observed and introduced at the end of 1970s by Professor Steen and his research group (Eboo, M. et al, 1979; Steen, W.M., 1980; Alexander, J. & Steen, W.M., 1980 & 1981).

2.3.1 Rooting of arc and arc contraction

In arc welding, the root of the arc on the base material side must move from the molten pool, where it can easily exist, to the cold base material where it cannot exist so easily. This is exacerbated as the welding speed increases. The greater the welding speed, the main part of the arc becomes tilted backwards. Therefore the movement of the root of the arc on the base metal side tends to lag behind the wire. When the welding speed is further increased, the root cannot move smoothly and the arc becomes unstable. In the case of hybrid welding it has been observed that even with a welding speed of 5 m/min, the root of the arc is seen to be located under the wire. (Abe, N. & Hayashi, M., 2002)

c) b)

a)

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In laser hybrid welding the arc is stabilised by the laser-generated plasma and/or the laser-created hot spot. The preheating that occurs in laser hybrid welding enhances arc ignition. This is due to a fact that the arc moves easily to the localized hot spot on a preheated plate, providing a path of lower resistance for the arc when compared with the cold anode plate directly. Also, since the arc will tend to operate at the lowest potential difference possible, it moves to a hot spot or laser generated plasma, both of which contain a greater electron density than neighbouring regions, and therefore offer the line of least resistance or lowest potential drop. The arc prefers to root at the interaction zone of the laser and work piece and the laser-generated plasma acts as a favourable rooting zone for the arc. Strong evaporation of metal increases the concentration of metal ions in the plasma. Metal atoms have a lower ionisation energy than the argon atoms in the shielding gas. This is why the arc is forced to burn predominantly in regions of high metal vapour concentration;

therefore the arc roots to the location at which the laser beam impinges the surface. This stabilises and constricts the arc and lowers the arc voltage. The arc contracts to the dimensions of the plasma, approximately, Fig. 2.15. And so, the path that the arc follows is either straight to the metal or down the keyhole where the plasma exists. It has been stated that for an iron work piece only 130 and 65 W are required to create these effects in the arc for focused CO2 and Nd:YAG laser beams, respectively. These phenomena partly explain the mutual interaction between the processes. (Eboo, M. et al., 1979), (Steen, W.M., 1980), (Wieschemann, A. et al., 2003), (Makino, Y. et al., 2002), (Paulini, J. & Simon, G., 1993)

Figure 2.15. Mechanism of arc discharge in MAG and hybrid welding. (Ono, M. et al., 2002) By using a laser beam with arc welding, arc ionization and the number of electric charge carriers is increased, and carrier conductivity is improved, reducing the arc voltage drop. It is observed that the current intensity in the arc increases whilst resistance decreases. By combining a laser beam and an arc, the temperatures of the anode and cathode spots increase considerably. Therefore emission of electrons and positive ions is facilitated. When welding in the electrode positive mode the motion of the cathode spot on the work piece surface is less intensive and limited to a much smaller area than in traditional arc welding. Another observation is arc contraction, which is stronger the greater the temperature difference between the arc centre and the environment. The laser beam action in the arc centre greatly increases the temperature of the arc. (Tusek, J. & Suban, M., 1999), (Diebold, T.P. &

Albright, C.E., 1984), (Steen, W.M., 1980)

When the arc impinges the work piece at the same interaction point as the laser beam, the arc contracts in width to almost the diameter of the laser beam. An unstable arc can thus be stabilised and the resistance of a stable arc can be reduced, Fig. 2.16. This means that there is a physical relationship between the energy sources. The weld fusion profiles are similar to laser weld profiles, which indicates that the arc root is narrowed by the laser plasma and that the arc enters the keyhole, causing the arc energy to be used more efficiently. (Steen, W.M., 1980)

It is observed that the arc voltage decreases after switching on the laser beam. The reason might lie in the metal vapour produced when the laser beam is initiated. The metal vapour enhances current conduction and reduces arc voltage because metal vapour possesses a relatively low ionisation potential compared with a shielding gas. (El Rayes, M. et al., 2004)

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a) b)

Figure 2.16. Effect of laser beam on TIG arc current/voltage relationship. a) decrease in arc column resistance and b) stabilization of an unstable arc caused by presence of laser beam. (Steen, W.M., 1980)

In the case of a naturally burning arc initial contraction of the expanding arc column at the cold anode surface is observed, caused by conductive heat loss from the plasma column to the base metal, which causes a fall in the gas temperature at the outer edges of the plasma column. The temperature fall increases the gas resistance and forces the arc periphery inwards towards the hotter arc core so that a small arc contraction takes place. The contraction increases the current density and thus the magnetic field strength, which then produces an inward radial force on the arc plasma, which is balanced by the pressure gradient. Because the arc is not cylindrical, a pressure gradient along the axis of the arc is produced, which induces gas flow – the anode plasma jet. The gas flow entrains surrounding cooler gas into the anode root area and causes further cooling and arc root contraction. The laser plasma, which is high in temperature and ion concentration, and has small dimensions may cause further contraction of the arc anode root via this natural arc mechanism. This is because the laser superimposes a hot plasma core which would itself induce contraction. It has been argued that the arc has a tendency to contract to the point of greatest ionisation and vaporisation, and it has been shown that a change in the electrical and thermal properties of the gas due to the presence of vaporised elements of low-ionization potential can also result in contraction of the arc root. As the arc energy is squeezed into a narrow range, the energy is concentrated. This is why filler wire is easily melted and the arc length is short (the potential gradient is large), causing small droplet transfer to the base metal at high frequency. (Steen, W.M., 1980), (Ono, M. et al., 2002)

2.3.2 The effect of parameters on weld quality in laser hybrid welding

Since two processes are combined in a single process zone during laser hybrid welding, many process parameters must be adjusted to obtain good quality welds. The parameters comprise those of the individual processes and those originated through the combination the processes, Fig. 2.17.

The effects of some variables on CO2 laser-MAG hybrid welding

Anna Fellman

THE EFFECTS OF SOME VARIABLES ON CO

LASER-MAG HYBRID WELDING

Acta Universitatis Lappeenrantaensis 306

Anna Fellman

THE EFFECTS OF SOME VARIABLES ON CO

LASER-MAG HYBRID WELDING

Acta Universitatis Lappeenrantaensis 306

ABSTRACT

Contents

Symbols and abbreviations

1. Introduction

2. Theoretical background

Laser beam

Filler wire

Droplet

Laser plume Metal vapour Plasma

plume

Work piece

d

F

F

G F

d

d

F

F

F

F

G F

d