Hybrid Laser-MIG welding - DiVA...

LICENTIATE T H E S I S

Luleå University of TechnologyDepartment of Applied Physics and Mechanical Engineering

Division of Manufacturing Systems Engineering

2005:82|: 02-757|: -c -- 05⁄82 --

2005:82

Hybrid Laser-MIG weldingAn investigation of geometrical considerations

Marc Wouters

Hybrid Laser-MIG welding:

An investigation of geometrical

considerations

Marc Wouters Division of Manufacturing Systems Engineering

Luleå University of Technology

Luleå, Sweden

Luleå, November 2005

Keywords: Hybrid welding, laser welding, weld geometry, joint fit-up, weld strength

Supervisors:

Alexander Kaplan1, John Powell1, 2 and Klas Nilsson1

1 Luleå University of Technology, Division of Manufacturing Systems Engineering, S-971 87 Luleå,

Sweden2 Laser Expertise Ltd., Acorn Park Ind. Estate, Harrimans Lane, Nottingham NG7 2TR, U.K.

The present thesis is composed of the following publications:

Paper 1: Fundamental analysis of hybrid laser-MIG welding

A. F. H. Kaplan, M. Wouters, K. Nilsson and J. Powell: Proceedings of the Conference

EUROJOIN 5, Vienna (Austria), 10-14 June 2004, published by The European Welding

Federation

Paper 2: The influence of joint geometry and fit-up gaps on hybrid laser-MIG welding

Y. Yao, M. Wouters, J. Powell, K. Nilsson and A. F. H. Kaplan: Submitted to the Journal of

Laser Applications (Impact Factor 0,61)

Also in modified form:

Wouters, M., J. Powell, A. Kaplan: The influence of weld geometry and fit-up on hybrid

laser-MIG welding, Proceedings of the conference NOLAMP 10, 17-19 August 2005, Luleå

(Sweden) (2005), published by Luleå University of Technology

Paper 3: The influence of joint gap on the strength of Hybrid Nd:YAG laser-MIG welds

Wouters, M., J. Powell, A. Kaplan: Submitted to the Journal of Laser Applications (Impact

Factor 0,61)

Also in modified form:

M. Wouters, J. Powell, A. F. H. Kaplan: The influence of joint gap on the strength of Hybrid

Nd:YAG laser-MIG welds, conference PICALO 2006, Melbourne (Australia), 3-5 April 2006,

to be published by The Laser Institute of America

ii

To my grandfather Andy, my grandmother Mamita, my mother Claire and my sisters

Dora, Cath and Caro…I hope you will enjoy reading this thesis ;-)

iii

Preface

At last the end!! It was not so easy to bring together more than two years of work!!

This licentiate thesis is the result of work carried out since June 2003 at the Division of Manufacturing Systems Engineering at Luleå University of Technology. Most of the experiments have been done in the division’s laser laboratory under the supervision of laser processing experts, namely Professor Alexander Kaplan, Dr. Klas Nilsson and Dr. John Powell. This project has been financed by FERRUFORM AB and by the Research Council of Norrbotten.

First of all, I would like to thank my supervisors Professor Alexander Kaplan, Dr. John Powell and Dr. Klas Nilsson for their valuable help, their advice and their encouragement and for having devoted so much time to pass on their knowledge to me. I also wish to thank all my colleagues at the university and especially at Department of Applied Physics and Mechanical Engineering, for their friendly support in the administrative and technical “meanders” of a PhD student’s life…and for the pleasant atmosphere we have at work.

I am very grateful to all my friends here in Sweden, in France and abroad, for being my friends… Thank you for having always been there when I needed you, for listening to me and for your help and advice throughout my time as a PhD student in Luleå, which has had its bright and dark seasons… Also a special thanks and my affection to Mia for having been at my side these last months.

Finally but not least, thank you with all of my heart to my family, and especially my grandparents, my mother and my three sisters, for your understanding, support, patience, and presence throughout my time in Luleå, in spite of the great distance between us which was not always easy to handle.

Luleå, November 2005

Marc Wouters

iv

Abstract

This thesis presents a collection of theoretical and experimental investigations into various

geometrical aspects of hybrid laser-MIG welding.

The work is divided up into four parts;

1. A review of the hybrid laser-MIG welding technique with brief summaries of the three

papers which make up the remainder of the thesis.

2. A paper entitled “Fundamental analysis of hybrid laser-MIG welding”. This paper provides

a theoretical insight into the hybrid laser-MIG welding process. This fundamental analysis

includes a description of the effect of process parameters on the cross-sectional geometry of

the weld.

3. A paper entitled “The influence of joint geometry and fit-up gaps on hybrid laser-MIG

welding” This paper analyses the effect of the geometry of the pre-welded joint on the final

weld cross-section. This paper specifically investigates the influence of gaps between the

workpieces and their effect on the welding process. From this work, guidelines on the

achievement of successful welds have been developed.

4. A paper entitled “The influence of joint gap on the strength of hybrid Nd:YAG laser-MIG

welds”. This paper looks into a specific feature of weld geometry; the effect of different size

gaps at the root of partial penetration butt welds. It was postulated (and confirmed) that an

optimum range of fit-up gaps gives maximum weld strength. If the gap is smaller than this

optimum then the fit-up gap acts as a sharp “crack” at the base of the weld. If the gap is larger

than the optimum range then the root of the weld takes on a more complex geometry which,

once again, includes stress raising features.

v

TABLE OF CONTENTS

INTRODUCTION.................................................................................................................... 1

1. MOTIVATION BEHIND THIS THESIS ...................................................................... 1

2. METHODOLOGICAL APPROACH ............................................................................ 1

3. STATE OF THE ART OF HYBRID LASER-ARC WELDING.................................... 1

3.1. The concept of hybrid welding ............................................................................... 1

3.2. Laser welding ......................................................................................................... 2

3.3. MIG welding (Metal Arc Inert Gas Welding) ........................................................ 4

3.4. Hybrid laser-MIG welding ..................................................................................... 5

3.5. A history of the development of hybrid laser-MIG welding ................................... 8

4. INDUSTRIAL APPLICATIONS................................................................................. 10

4.1. Overview............................................................................................................... 10

4.2. Automotive............................................................................................................ 11

4.3. Shipbuilding ......................................................................................................... 11

4.4. Pipe lines and offshore installations .................................................................... 11

4.5. Aerospace and aviation industry.......................................................................... 12

4.6. Power generation ................................................................................................. 12

4.7. Off road and heavy vehicles ................................................................................. 12

5. SUMMARIES OF THE PAPERS PRESENTED IN THIS WORK............................. 13

5.1. Fundamental analysis of hybrid laser-MIG welding ........................................... 13

5.2. The influence of joint geometry and fit-up gaps on hybrid laser-MIG welding... 15

5.3. The influence of joint gap on the strength of hybrid Nd:YAG laser-MIG welds.. 18

6. CONCLUSIONS.......................................................................................................... 21

7. SUGGESTIONS FOR FUTURE WORK..................................................................... 22

8. REFERENCES............................................................................................................. 23

PAPER 1: FUNDAMENTAL ANALYSIS OF HYBRID LASER-MIG WELDING...... 26

PAPER 2: THE INFLUENCE OF JOINT GEOMETRY AND FIT-UP GAPS ON

HYBRID LASER-MIG WELDING............................................................................. 36

PAPER 3: THE INFLUENCE OF JOINT GAP ON THE STRENGTH OF

HYBRID ND:YAG LASER-MIG WELDS ................................................................. 50

vi

Marc Wouters Introduction Page 1

INTRODUCTION

1.MOTIVATION BEHIND THIS THESIS

The original motivation behind this thesis came from an industrial requirement

to know how sensitive the laser - MIG hybrid welding process is to changes in fit-up between

the parts being welded. This request directly stimulated the research presented in the

experimental papers of this thesis ( Paper 2;The influence of joint geometry and fit-up gaps on

hybrid laser-MIG welding, Paper 3; The influence of joint gap on the strength of hybrid

Nd:YAG laser-MIG welds). A fundamental overview of the welding process was necessary in

order to establish an understanding of the mechanisms involved and this is presented as the

first paper of the thesis (Fundamental analysis of hybrid laser-MIG welding).

2.METHODOLOGICAL APPROACH

This thesis combines experimental and theoretical work. The experimental work

involved the production of Nd:YAG laser-MIG welds between specially designed

workpieces. The welds were produced over a range of carefully chosen parameters in order to

identify trends in the final results. Optical microscopy and macroscopic examination were

employed to examine the weld cross-sections. Impact testing at ambient and liquid nitrogen

temperatures was carried out on welds produced with different inter-workpiece gaps.

Computer based mathematical analysis of the process was also carried out.

3.STATE OF THE ART OF HYBRID LASER-ARC WELDING

3.1. The concept of hybrid welding

One of the remarkable characteristics of laser welding is the narrow and deep

configuration of the weld. This narrow weld is the result of the high energy concentration of

the process and the high welding speed which result in a low heat input into the workpiece.

Several applications take advantage of this narrow weld characteristic and high speed

processing, but for a lot of other applications the laser process is too expensive and its narrow

weld leads to some difficult metallurgical and fit-up problems. To avoid these problems a


hybrid welding technique has been developed which combines the laser welding process with

an arc process, namely hybrid laser-arc welding.

In hybrid laser-arc welding a laser (CO2 or YAG) is combined to an arc process

(TIG, MIG, MAG or plasma). This combination allows us to benefit from the advantages of

both processes. The laser beam offers the possibility of producing deeper welds in one pass,

whereas the arc energy is used to increase welding speed and to fill the fit-up defects between

the pieces to be joined.

In the following sections laser welding and arc welding will be briefly described

separately and in their hybrid conjunction;

3.2. Laser welding

Laser welding involves focusing the beam of a high power laser on the joint

between two workpieces (see figure 1). Nowadays, the power of these lasers is often in the

range of 5-10 kW (up to 50 kW in some cases) for the CO2 lasers, and 0.3-3 kW (6 kW lasers

are available) for Nd:YAG lasers. This energy is very concentrated, with an intensity of

power input at the weld surface of around 106 W/cm2, which is one of the highest among the

different welding processes available. This high energy concentration produces a weld with a

high depth to width ratio and with minimal thermal distortion. The process is also quite fast,

which is of interest when looking at productivity. But this deep and narrow shape of the weld,

which has many advantages, is also one of the main drawbacks to the process because it

requires careful and accurate machining and positioning of the workpieces [1, 2 and 3].


Figure 1: Schematic of the laser welding process

To sum-up, laser welding is an interesting process because of the following

advantages:

Good quality: Narrow, deep weld seam

High completion rates

Low consumable costs (no filler required)

Low but concentrated heat input, which results in low and predictable distortion levels

Reduced post weld rework

No mechanical contact between the laser equipment and the workpieces

Joining of widely dissimilar materials is possible

But laser welding has also some drawbacks which are:

High cost of equipment and maintenance

Poor gap bridging ability, which leads to high requirements on joint preparation

Limited welding positions

Poor electrical efficiency (2 % for CO2 lasers, 10 % for Nd:YAG lasers)

Occasional metallurgical problems due to the high cooling rates


3.3. MIG welding (Metal Arc Inert Gas Welding)

MIG welding is a welding process based on the creation of an electrical arc

between a welding torch (anode) and the workpiece (cathode), see figure 2. Heat is transferred

to the workpiece through a plasma. The intensity of the power input of this process is around

103 W/cm2 (significantly lower than for laser welding), which produces a weld of small depth

and medium width. The welding speed is also lower than the one provided by the laser

process and this can result in some distortion of the workpiece, which often needs to be

machined afterwards. But MIG welding is interesting from an industrial point of view because

it has a good bridging ability, the equipment costs are low compared to laser welding, and this

process is also very energy efficient (60 to 80 %).

Figure 2: Schematic of the MIG welding process

To sum-up, the main advantages of the MIG welding process are:

Excellent gap bridging ability

Low cost of equipment

High efficiency of the process (60-80 %)

But this process has also some drawbacks which are:

Energy density and welding speed lower compared to laser welding. This causes high heat

input to the workpiece and consequent thermal distortions.

Low speed


3.4. Hybrid laser-MIG welding

It has been known for many years that the combination of a laser beam and an

electric arc can produce welds with many of the technical advantages of those made using just

a laser, for example; deep penetration and low distortion. This process where both the laser

and arc act in the same melt pool (see figure 3) gives higher speeds, with even deeper

penetration and greater tolerance to fit-up compared to the laser alone. Hybrid laser-MIG

welding allows high completion rates in comparison with laser processes, with a decrease in

the necessary laser power and a clear improvement of the joining process reliability. Hybrid

welding is thus cheaper than laser welding and retains, or even improves the technical

benefits of laser welding [1, 3 and 4].

Figure 3: Schematic of the Hybrid laser-MIG welding process

Hybrid welding minimizes the drawbacks of both the single laser and the MIG

process to obtain an optimized welding technique. The main advantages of hybrid laser-arc

welding compared to laser welding are:

Lower capital cost, reduction of 30-40% compared to laser alone due to reduction in laser

power requirement.

Higher welding speeds.

Reduction of edge preparation accuracy needs.

Control of seam width.


Control of metallurgical variables through the addition of filler wire

Less material hardening

Improved process reliability

Higher electrical efficiency, up to 50% reduction in power consumption.

However, a large number of parameters have to be correctly set to achieve these

improvements [5, 6 and 7];

Laser power

An increase in laser power will generally increase the weld penetration. In the

case of hybrid laser-arc welding (as opposed with the laser-only process) this phenomenon is

accentuated because the reflectivity of the workpiece metal is reduced when the metal is

heated by the arc.

Welding speed

The weld penetration increases when the welding speed is decreased because the

heat input per unit length of weld is higher. Also the gap filling capability by the filler wire is

improved at lower welding speeds (at constant filler wire feeding). The ratio between welding

speed and filler wire feeding is important to the stability of the keyhole and thus for the

stability of the process itself.

Relative arrangement of the laser and the MIG torch

To get the maximum weld penetration the laser is positioned perpendicularly to

the direction of welding. The leading or trailing position of the arc torch is a determining

factor for the weld characteristics. For mild steels the arc leading configuration is preferred

since an increase in penetration is obtained in this way. Also, the distance between the laser

and the wire tip is one of the most important parameters to control in hybrid laser-arc

welding. A short distance, typically 2 mm between the laser spot and the filler wire tip has

been shown to be favorable for a steady keyhole and for maximum penetration.


Focal point position

The maximum weld penetration for the hybrid laser-arc process is generally

obtained when the laser beam is focused below the top sheet surface (2 to 4 mm).

Angle of electrode

The penetration of the weld increases with the angle of the electrode to the

workpiece surface up to 50 degrees. The gas flow along the welding direction provided by the

arc torch deflects the plasma induced by the laser, and reduces the absorption of the laser

beam by this plasma when CO2 lasers are employed. Therefore the angle of electrode to the

top surface of the workpiece is often set at around 40-50 degrees.

Shield gas composition

The predominant constituent of the shield gas is generally an inert gas such as

helium or argon. A shield gas providing a higher ionisation potential is required since the

plasma can deflect or absorb a portion of the laser energy when CO2 lasers are employed.

Helium is therefore often preferred to argon for laser welding, but its lightness is a

disadvantage and it is often combined with argon which is heavier without substantial

alteration of the weld penetration depth. The addition of reactive gases such as oxygen and

carbon dioxide has been shown to have an influence on the weld pool wetting characteristics

and bead smoothness.

Power modulation of the arc welding source

The arc welding source uses a DC mode rather than an AC mode because the

energy input and density are higher in the first case. The arc source is often operated in a

pulsed mode since this has been shown to reduce the amount of spatter whilst maintaining a

deep penetration of the weld. The welding voltage has been shown not to greatly influence the

weld penetration depth, which is mostly dependant on the laser power, but the weld bead gets

wider if the welding voltage increases, giving a lower depth to width ratio for a same laser

power. The arc voltage (and wire feed rate) will therefore need to be increased for wider fit-up

gaps, to avoid any lack of fusion. The welding current is generally matched to the filler wire

diameter (higher welding current for higher wire diameter). Considering a given wire


diameter and voltage settings it has been shown that an increase in welding current will give a

deeper weld, with a higher depth to width ratio.

Joint gap

For laser welding gaps up to 0.2 mm can be managed. Gaps larger than this will

lead to weld defects such as an incomplete weld bead and undercut. The hybrid laser-arc

process allows us to join workpieces with gaps of 1 mm without any problem and even wider

gaps if the wire feeding is set high enough. This process is therefore more tolerant to

inaccurate joint preparation and joint fit-up as well as thermal distortion of the workpiece

during the welding process. It is also more tolerant to a beam to gap misalignment.

Edge preparation

For conventional laser welding parallel and straight edges with a narrow gap are

required, due to the small diameter of the laser beam. For arc welding a V-shape or other

angled cut are necessary. In the case of hybrid laser-arc welding, the need for edge

preparation is lower than for laser welding, since it is not as sensitive to the presence of

oxides on the edges to joint, as long as the gap is wide enough. An edge groove is generally

machined for material thickness higher than 8-10 mm.

3.5. A history of the development of hybrid laser-MIG welding

1970s

In the late seventies at Imperial College in London a group lead by William

Steen performed the first attempts in combining a laser and an arc (TIG) welding process.

This new hybrid laser-arc welding immediately showed improved characteristics compared to

both the solo laser and the solo arc welding techniques as well as providing increased process

stability, a significantly higher welding speed and a deeper and narrower weld. However, this

innovation did not immediately find practical applications as the laser process itself was not

viable on an industrial scale [4 and 5].


1980s

In the eighties laser welding became a suitable process for industry with the

development of reliable and cheaper high power lasers [8]. It was found that its drawbacks

could be minimised or suppressed by using a laser in conjunction with a conventional arc

welding method. These conventional arc welding methods increased the efficiency of the

process, the gap bridgeability, minimized metallurgical difficulties and the reflectivity of the

material was not longer an obstacle to the success of the welding process [4 and 5].

1990s

The implementation of laser welding and hybrid welding in industry followed

the trend of the eighties with further improvements of high power laser equipment. The

development of the hybrid laser-arc welding process highlighted the drawbacks of the single

laser welding technique (cost, edge preparation and positioning and metallurgical problems).

Possibilities and limitations of the hybrid laser-arc welding technique were investigated all

over the world (USA, Europe and Japan) [6, 9, 10 and 11]. The development of this technique

was accelerated due to the emergence of the tailored blank industry, which required wide,

high quality welds made at high speed. Hybrid laser-arc welding became one of the hot topics

of laser processing. New industries became interested such as the automotive, ship building

and the pipeline industries.

2000s

Today we are in a phase of development of integrated hybrid welding heads

(Exial, Fronius) [5 and 12], the hybrid laser-arc welding technique having proved its

suitability in many industrial applications. Also, a lot of different specific situations have to

be tested and modelled. For example, the material to be joined can be of particular importance

when considering hybrid laser-arc welding. The geometry (thickness and joint type) of the

workpieces is another specific parameter which needs to be systematically investigated.


4. INDUSTRIAL APPLICATIONS

4.1. Overview

The hybrid welding process is involved in a growing number of industrial

applications due to the economic and technical advantages of this technology. Some

important superior features, compared to pure laser welding, are:

A higher welding speed. Productivity is improved through increased welding speed. For

sheet material it is possible to get 40% enhancement of the speed compared to

conventional laser welding without the addition of the arc power.

When using a hybrid combination, the investment cost for the power source is

significantly less and the electrical efficiency is much higher when using a hybrid process

A larger tolerance of the joint configuration due to gap bridging with the added MIG wire.

Processing and joint fit-up tolerances are thus improved.

A good weld quality is obtained with low and predictable distortion, which implies a

reduction in the need for rework. In addition, the potential benefit of improved

dimensional tolerances is generally recognized in heavy industries to be of the order of

20-30% of the labour cost due to a reduction of rectification work.

For hybrid processes, which consume filler wire, it should be possible to weld lower cost,

lower grade steels than those which are usually required for autogenous laser welding

To sum-up, hybrid laser MIG welding combines the advantages of both arc and

laser processes resulting in high joint completion rates with increased tolerance to fit up and

without compromising joint quality and distortion control. The benefits to industry include

increased productivity, simplified set-up procedures and reduced post weld reworking costs.

However, this technology is experiencing only slow growth in today’s industries. Some

reasons for this slow acceptance are the high cost of the investment and the complexity of the

process due to its large number of parameters. The set up of the processing parameters

requires a high degree of skill and accuracy, and these imperatives added to an incomplete

knowledge of the process are limiting factors for the industrial application.

Most of today’s applications for hybrid welding are limited to sheet materials in

the range of 2 to 10 mm, but thick materials may also derive benefits from this joining process

[7]. Hybrid welding is suitable on an industrial scale within the following industries:


4.2. Automotive

Within the automotive industry, Volkswagen and Audi are two particularly well

known examples of companies convinced by the benefits of Hybrid laser-MIG welding [13,

14, 15 and 16]. Each type of car includes many different welding conditions, depending on

the materials and on the joint configuration. In many cases, MIG welding, laser welding and

hybrid laser-MIG welding are combined, depending on the weld seam configuration and

requirements. MIG welding is applied when a high gap bridging ability and a minimum

groove preparation are needed. Laser welding permits low deformation, deep penetration and

high speed, but an accurate adjustment of the parts to join is necessary. And finally, hybrid

welding permits high welding speeds with lower tolerances. In these applications, lasers from

2 to 4 kW are used, and welding speeds of around 4 m/min are possible.

4.3. Shipbuilding

The hybrid welding process is of great interest for shipbuilding industries all

over the world [17, 18, 19 and 20]. The use of this process in European and Asian shipyards is

getting more and more common, whereas its acceptance by U.S. shipyards is quite slow.

Hybrid welding permits significant time and cost savings by the elimination of multi-pass

requirements, taking advantage of the deep penetration offered by this process. This industrial

area often uses very high power lasers of up to 6 kW for Nd:YAG lasers and up to 25 kW for

CO2 lasers. MEYER in Germany, KVAERNER in Finland and FINCANTIERI in Italy are

some examples of shipyards using this technique.

4.4. Pipe lines and offshore installations

Hybrid laser-MAG welding has been investigated to improve weld quality and

reduce manufacturing costs of pipe lines. After comparison with filler-added laser welding

and three-pass MAG welding, this process has been qualified for and implemented in an

industrial environment. Stainless steel pipe lines are now welded with a pore free structure

and no significant hardness increase, at a welding speed of up to 1.2 m/min for a wall

thickness of 5 to 8 mm [21].


4.5. Aerospace and aviation industry

The leading position of Airbus in the aircraft industry is particularly due to its

capacity for innovation. This company is continuously developing new technologies to

produce aircraft with the best compromise between innovation, operational reliability and

economics. This spirit stimulated the Airbus interest in hybrid welding technology, after

having already used for years the laser welding process for the production of aircraft fuselage

sections [22 and 23]. In this application the high quality of hybrid welds has proved

profitable. This technique is also of great interest for these industries (and particularly for

aerospace and military applications) when dealing with titanium alloys. It is a fact that high

quality welds in thin titanium alloys are almost impossible with traditional methods, but

hybrid welding is able to face this challenge with very good results [24].

4.6. Power generation

Power generation, where thick plates of over 15 mm are often used, is a very

suitable area for high energy density beam processes, due to their deep penetration capability.

Particularly, laser welding of these structures has been investigated and implemented [25], but

hybrid welding is progressively getting more and more attention in this application as it would

overcome the disadvantages of conventional laser welding [26]. In this kind of application,

laser welding and hybrid welding are commonly applied in a multiple pass mode [27].

4.7. Off road and heavy vehicles

Laser welding is already a well established technique in the heavy automotive

industry for joining medium section construction steels, and its efficiency has been

demonstrated many times [28]. However, hybrid welding shows superior features compared

to laser welding (reduction of pre- and post-machining for example) and is therefore slowly

taking its place within these industries.

These are just a few applications of the hybrid welding process, which is in its

expansion phase. Among the other possible applications are domestic appliances, railways

and chemical plants (particularly for the joining of tanks and tubes designed for chemical

plants, often made of stainless steel) [29].


5.SUMMARIES OF THE PAPERS PRESENTED IN THIS WORK

5.1. Fundamental analysis of hybrid laser-MIG welding

This paper presents a comparison of a mathematical model of laser hybrid

welding with experimental evidence. Figure 4 shows how many inter-related phenomena are

involved when a laser acts alongside a MIG arc. The arc not only provides energy to the

process but also liquid metal in the form of droplets. Figure 5 provides a schematic of how the

various different variables are accounted for in the mathematical model.

Figure 4: Side view of the complex geometry and physics during hybrid welding

Figure 5: Mathematical model


The paper then presents a comparison of experimental results and calculated

figures which describe the process. For example; figure 6 shows the calculated gas and

vapour flow characteristics of the weld zone with a high speed photo of the actual process.

Figure 7 compares calculated and actual weld cross sections as a function of gap width and

wire feed rate.

(a) (b)

Figure 6: Shielding gas flow, arc and escaping keyhole plasma jet: (a) calculation, (b) high

speed imaging (Naito 2001)

(a) (b) (c)

Figure 7: Calculated weld seam cross section: for varying gap width (a), wire feeding (b),

compared to experiments (c)


The following conclusions were drawn from this paper;

Hybrid laser-MIG welding has gained significant industrial attention

Various joint configurations can be hybrid welded at high speed, even in the presence of a

significant gap

The fundamental physics of hybrid welding has been studied by developing a

mathematical model

In contrast to MIG-welding the torch is inclined, causing a change in energy transfer

The displacement between the laser beam and the arc modifies the weld pool shape

For the arc leading configuration the keyhole is affected by the leading weld pool

For the arc trailing configuration the melt has to move in the welding direction to enable

keyhole production

During hybrid welding the melt pool is significantly enlarged, permitting more extensive

melt flow

Energy transfer is the result of the electrode current, shielding gas convection and laser

beam absorption

The wire is melted via the anode drop zone and current, the hot drop is transferred to the

workpiece in spray mode by detaching a single drop per electric current pulse due to the

magnetic forces

The weld seam shape is governed by the wire feeding rate relative to welding speed and

gap width

5.2. The influence of joint geometry and fit-up gaps on hybrid laser-MIG welding

Most welding processes are known to be sensitive to the geometry and to the fit-

up of the parts being welded. In the case of laser welding, these parameters have been found

to be of particular importance, as a poor fit-up (gaps wider than 1 mm) could have disastrous

consequences on the final weld quality with the appearance of pores and cracks or in some

extreme cases with the collapse of the welding process. The hybrid laser-MIG welding

technique constitutes an improvement of laser welding because it is more gap tolerant and

allows a weld depth comparable or even greater than laser welding. But this technique also

has its limitations when looking at the gap bridgeability. The sensitivity of the weld


penetration to the gap width is a potential problem for the application of hybrid laser-MIG

welding.

The aim of this experimental investigation was to determine how the workpieces

geometry and fit-up influence the hybrid laser-MIG welding process. In a typical industrial

production and in the particular case of a fillet weld between two thick plates, the upper plate

(symbolised by the part A in this study) and the base plate (symbolised by part B) may be

separated (before welding) by a distance of 0 to 1 mm. It was therefore proposed to introduce

a chamfer at the edge of the upper plate which would present the welding process with a joint

gap in all cases. By introducing chamfers in T-joints between two mild steel plates and

varying the gap between them, the penetration, filling capability and the stability of the

process were investigated. A T-joint geometry without chamfer was compared to other T-

joints with different chamfer geometries for gaps at the root of the weld varying between 0

and 1.5 mm (see figure 8).

Figure 8: Schematic of the joint geometry experimentally tested, (a) without chamfer and (b)

with chamfer(X=3 or 6 mm, and Y=1 or 2 mm)

A CO2 laser (Rofin-Sinar RS6000) with a laser power of 5000 W was used to

run these experiments, in combination with MIG equipment (type ESAB MEK 44C).


Figure 9: Weld cross-sections for three different MIG parameters for different chamfer

geometries (all the welds shown here had a pre-weld fit-up gap of 1.0 mm)

Cross sections of each weld were polished, etched, and gathered in matrices to

get a general and qualitative valuation of the weld geometry as a function of the chamfer on

the one hand, and of the gap on the other hand (see figure 9). All welds were measured to get

a quantitative evaluation of their strength. The weld throat was defined (see figure 10) as the

best geometrical representation of the strength of this kind of weld.


Figure 10: A typical weld cross section with its weld throat depth measurement

When the MIG parameters were optimized for each type of chamfer the

following conclusions could be drawn;

If a particular weld throat depth is required when using the laser-MIG hybrid welding

process then it is important to carry out a survey to establish the range of fit up gaps

expected. The MIG parameters should then be set at a high enough level to fill and bridge

this range of gaps during welding.

The penetration depth of the weld (which is generally closely related to the throat and thus

the weld strength) is generally controlled by the laser (type, power and process speed).

Surplus MIG power or wire will cause a widening of the weld cross-section but will have

only a modest effect on the throat depth.

Chamfers can be used to reduce the sensitivity of the process to gaps.

The laser-MIG welding process is rather insensitive to gaps below 3mm as long as the

MIG parameters are set high enough to bridge those gaps.

5.3. The influence of joint gap on the strength of hybrid Nd:YAG laser-MIG welds

This paper presents the results of an experimental and theoretical analysis of the

effect of joint gap on the strength of hybrid laser-MIG welds. The welds were of the partial

penetration butt type with various joint gaps but identical weld penetration. Impact testing

established that a zero gap gave a weak weld because the weld geometry contained the

equivalent of a sharp crack where the unwelded parts met each other (see figure 11).


Figure 11: The cross sectional geometry of a partial penetration weld automatically includes

a “crack” at the root of the weld.

This paper presents the results of an experimental program designed to

investigate the principle that increasing the gap between the workpieces would produce

stronger welds because the “crack” would be made less sharp. A number of otherwise

identical partial penetration welds were produced (by hybrid Nd:YAG laser-MIG welding)

with different gaps between the workpieces. The welds were sectioned to produce impact test

pieces and it was discovered that optimum weld strength was achieved when small gaps were

introduced into the weld geometry.

(a) (b) (c) (d)

Figure 12: Photographs of weld root geometry with increasing gap width (a) : 0.0 mm, (b) :

0.5 mm, (c) : 1.0 mm, (d) : 1.5 mm

Figure 12 shows the cross section of a group of welds with different gaps.

Figure 13 shows that welds with gaps of 0.5 and 1.0 mm perform approximately 50 % better

than those with zero or 1.5 mm gaps in impact tests.


0,4

0,5

0,6

0,7

0,8

0,9

1

0 0,5 1 1,5Gap (mm)

Surf

ace

Ener

gy A

bsor

bed

(J/m

m2)

Figure 13: Impact test results for the weld types shown in figure 12.

The improvement of impact strength from zero gap to 1 mm gap was expected

from the “sharp crack” argument. The fall of impact strength from 1 mm to 1.5 mm gaps can

be explained by reference to figure 14.

Figure 14: The change in the geometry of the root of the weld with increasing gap size.

As the gap is widened above 1 mm the geometry at the root of the weld changes

because the liquid in the weld zone penetrates downwards into the gap. This produces two

small radius “cracks” at the base of the weld (see figure 14(c)) which reduce the impact

testing performance of the weld.


Figures 15 and 16 confirm the idea put forward in figure 14.

(a) (b) (c) (d)

Figure 15: Higher magnification views of the root of the welds shown in figure 12 with

increasing gap width (a) : 0.0 mm, (b) : 0.5 mm, (c) : 1.0 mm, (d) : 1.5 mm

Figure 16: A photo of a 1,5 mm wide gap weld after impact testing at room temperature

From the above evidence it is clear that there is an optimum range of workpiece

fit-up for partial penetration welds. This optimum is reached when the gap is as wide as

possible without allowing the melt to slump into it. For any welding application this optimum

range could be identified by cross sectional analysis.

6.CONCLUSIONS

From the work presented in this thesis the following general conclusions can be reached;

a. If the laser-MIG welding process is to be effective then the MIG parameters must be set

high enough to bridge any forecast gap between the workpieces. In this thesis gaps of

up to 3mm were accommodated by the process.


b. If condition “a” is satisfied then the depth of the weld will be largely dependent on the

laser power and the weld width will be controlled by the MIG parameters.

c. Very small (i.e. less than 0.5mm) and large (e.g. more than 2mm) gaps between the

workpieces to be welded should be avoided in partial penetration welds as they

introduce stress raisers into the weld geometry.

d.Chamfers on the parts to be welded can be used to improve the reliability of the welding

process.

7.SUGGESTIONS FOR FUTURE WORK

The following ideas could form the basis for a continuation of this research;

a. Mechanical testing of the weld types investigated in paper 2 (“The influence of joint

geometry and fit-up gaps on hybrid laser-MIG-welding”).

b.Tensile testing of the weld types presented in paper 3 (“The influence of joint gap on

the strength of hybrid laser-MIG welds”).

c. The use of special welding rods to strengthen partial penetration welds.

d.The extension of the theoretical model presented in paper 1 to include a forecast of the

strength of the welds produced.


8.REFERENCES

[1] Hügel, H., C. Schinzel, Handbook of laser technology and applications, Vol. 3,

Applications, Part D, Welding, edited by Colin E. Webb and Julian D.C. Jones, Bristol,

Institute of Physics, 2004

[2] Duley, Walter W., Laser welding, New York, Wiley, 1999

[3] Steen, William M., Laser material processing, 3rd edition, London, New York,

Springer, 2003

[4] Seyffarth, P., I. V. Krivtsun, Laser-arc processes and their applications in welding and

material treatment, London, Taylor & Francis, 2002

[5] Bagger, C., Flemming O. Olsen: Review of laser hybrid welding, Journal of Laser

Applications, Vol. 17, no. 1, Feb. 2005

[6] Magee, K. H., V. E. Merchant, C. V. Hyatt, Laser assisted gas metal arc weld

characteristics, Proceedings of the Laser Materials Processing - ICALEO '90, Nov 4-9

1990, Boston, MA, USA, LIA (Laser Institute of America), v 71, 1991

[7] P. Jernström, Hybrid welding combines the benefits of laser and arc welding, 19th

Nordiska Svetsmötet, Stockholm, 3-5 Sept. 2003

[8] Ready, John F., Industrial Applications of Lasers, 2nd edition, Academic press, London,

1997

[9] Beyer, E., R. Imholff, J. Neuenhahn, K. Behler, New aspects in laser welding with an

increased efficiency, Proceedings of the Laser Materials Processing - ICALEO '94,

Orlando, FL, USA, LIA (Laser Institute of America), 1994

[10] Dilthey, U., A. Wieschemann, Prospects by combining and coupling laser beams and

arc welding processes, International Institute of Welding, IIW Doc. XII-1565-99, 1999

[11] Ishide, T., S. Tsubota, M. Watanabe, Latest MIG, TIG, arc-YAG laser hybrid welding

systems for various welding products, First International Symposium on High-Power

Laser Macroprocessing, SPIE, 2002

[12] Dinechin (de), G., F. Briand, K. Chouf, P. Lefebvre, Soudage hybride arc / laser avec le

procédé EXIAL, Matériaux, 2002

[13] Staufer, H., LaserHybrid welding and laserBrazing at Audi and VW, IIW, Doc. IV-847-

03, 2003


[14] Brettschneider, C., A8 meets DY, Eurolaser (Zeitschrift für die Industrielle

Laseranwendung), September 2003

[15] Beyer, E.; B. Brenner, R. Poprawe, Hybrid Laser Welding Techniques for Enhanced

Welding Efficiency, ICALEO Proceedings, Section D, Detroit, USA, 1996

[16] Graf, T.; Staufer, H.: Laser-Hybrid Welding drives VW Improvements, Welding

Journal, Vol. 82, 2003

[17] Laser-GMA hybrid welding advances, Industrial laser solutions, December 2004

[18] Merchant, V., Shipshape laser applications, Status report, Industrial laser solutions,

August 2003

[19] Denney, P., Hybrid Laser Welding for Fabrication of Ship Structural Components,

Welding Journal, vol. 81, Sept. 2002

[20] Dilthey, U.; A. Wieschemann, H. Keller, CO2-laser GMA hybrid and hydra welding:

innovative joining methods for shipbuilding, LaserOpto, Vol. 33, 2001

[21] Thomy, C., T. Seefeld, F. Vollertsen, Hybrid welding of line pipes, Industrial laser

solutions, September 2003

[22] Eine frage der Grösse, Laserstrahlschweissen macht Flugzeuge leichter, Euro Laser,

December 2004

[23] Dzelnitski, D., Wire feed systems for laser welding, Industrial laser solutions,

Application report, September 2004

[24] Shinn, R. W., A. P. Joseph, P. E. Denney, Hybrid welding of titanium, Industrial laser

solutions, Technology report, April 2004

[25] Jokinen, T., V. Kujanpää, High power Nd:YAG laser welding in manufacturing of

vacuum vessel of fusion reactor, Elsevier Science, 2003

[26] Jokinen, T., M. Karhu, Welding of thick austenitic stainless steel using Nd:yttrium-

aluminum-garnet laser with filler wire and hybrid process, Journal of laser applications,

Vol. 12, number 4, November 2003

[27] Jokinen, T., M. Karhu, V. Kujanpää, Hybrid welding in the manufacturing of vacuum

vessel for fusion reactor, Industrial systems review, 2005

[28] Nilsson, K., H. Engström, J. Flingfeldt, T. Nilsson, A. Skirfors, M. Miller, High power

laser welding of constructions steels, Svetsen, vol.59, no 1, 2000


[29] Kaierle, S.; K. Bongard, M. Dahmen, R. Poprawe, Innovative hybrid welding Process in

an industrial application, ICALEO Proceedings,Section C, Dearborne, USA, 2000

Marc Wouters Paper 1: Fundamental analysis Page 26

PAPER 1: FUNDAMENTAL ANALYSIS OF HYBRID LASER-MIG WELDING

A. F. H. Kaplan, M. Wouters, K. Nilsson and J. Powell,

Proceedings of the Conference EUROJOIN 5, Vienna (Austria), 10-14 June 2004


Paper 1: Fundamental analysis of hybrid laser-MIG welding

Alexander Kaplan1, Marc Wouters1, Klas Nilsson1, John Powell1,2

1Luleå University of Technology, SE-971 87, Sweden 2Laser Expertise Ltd., Acorn Park Industrial Estate, Harrimans Lane, Nottingham NG7 2TR,

UK

Abstract

Hybrid welding combines a laser beam with an electric arc. A mathematical

model has been developed, accompanied by fundamental experiments, in order to improve the

physical understanding of this highly complex process. The mechanisms of drop detachment,

arc development, heat conduction and pulsing conditions have been investigated. Also, the

positional arrangement of the two energy sources and the resulting seam geometry for various

joint configurations has been studied. The differences between hybrid welding and the

individual techniques of laser welding and MIG welding are also discussed.

1. Introduction

Hybrid welding combines the techniques of laser welding and arc welding.

While hybrid welding was originally invented for combined laser-TIG welding [Steen 1979],

the present study will focus on hybrid laser-MIG (or MAG) welding owing to its greater

industrial relevance.

During hybrid welding the laser beam creates a vapour capillary, the keyhole,

that enables a deep welding effect where the beam energy is distributed throughout the

workpiece depth. An electric arc is created between the MIG-wire (as the anode) and the

workpiece that melts the feeding wire. Droplets are detached and transferred to the workpiece

to fill any gap and to create the desired weld top shape. Both the arc and the keyhole create a

plasma, thus producing a highly complex physical situation. As two techniques are combined,

the number of process parameters is large and therefore difficult to optimize in order to

achieve the desired weld.


Laser welding [Duley 1999] has been studied with respect to keyhole and

plasma plume formation [Kaplan 1994, 2002] and melt pool flow. Investigations on arc

welding [Lancaster 1986] often focus on the melt pool shape, particularly with respect to

Marangoni convection, on the arc formation [Choo 1990, Dowden 1994] and droplet

detachment [Jones 1998]. Most fundamental studies on hybrid welding [Dilthey 2000]

concern laser-TIG welding [Naito 2001]. Mathematical modelling, Finite Element Analysis

and high speed imaging have often been applied to investigate the complex phenomena

involved.

Initially, commercial interest in hybrid welding was low but it has gained

industrial attention during the last decade as it combines the advantages of both techniques.

Laser welding causes a deep, narrow weld seam and permits high welding speeds. MIG

welding in turn is highly tolerant to gaps and different joint configurations and delivers extra

energy thus increasing the speed. Today hybrid welding is used in several industrial

applications ranging from the automotive industry (Audi A2) to shipbuilding (Odense

shipyard). However, as the technique is complex it requires feasibility studies and an

improved fundamental understanding in order to optimize and control the welding process.

The present study will introduce a mathematical model and fundamental experiments

investigating the basic physical mechanisms involved.

2. Experiments

Feasibility experiments and fundamental parameter studies have been conducted

for a variety of potential industrial applications. Typical weld seam cross sections and an

experimental set-up are shown in Fig. 1.


Fig. 1: Hybrid welds for different joint configurations for 6-8 mm heavy section steel (a)-(e)

and 2 mm thin sheet steel (f); (g) typical arrangement of hybrid welding by combining a laser

beam with a MIG-arc

Different joint configurations, materials and material thickness, Fig. 1(a)-(f),

have successfully been investigated [Nilsson 2002, 2003], i.e. achieving high speeds with a

favourable crack- and pore-free weld shape of sufficient strength. Industrial feasibility studies

have led to satisfactory results for various applications, such as for automotive sub-assemblies

in thin sheet welding and truck beams, crane profiles and ship building for heavy sections.

Either a 3 kW Nd:YAG-laser or a 6 kW CO2-laser has been used, the former utilising a 0.6

mm fibre with an industrial robot. As shown in Fig. 1(g), the processing head of the laser

contains the focusing optics and the shielding gas is provided by the MIG-torch. The optics


were protected from spatter by a cross jet. The MIG-torch is inclined at an angle and its axis

is off-set from the laser beam axis by a distance x. Both the leading and the trailing

configuration of the torch were studied, and the latter was found to be preferable.

3. Mathematical Model

Fig. 2: Side view of the complex geometry and physics during hybrid welding

Fig. 3: Mathematical model


A mathematical model has been developed which is schematically explained in

Fig. 2 and 3. In contrast to FEA, the model is semi-analytical, thus enabling short computing

times and flexible interaction of the many different phenomena. A model of laser welding

[Kaplan 1994, 2002], considering most mechanisms such as keyhole geometry, plasma

properties, heat conduction and pore formation, has been extended by integrating an electric

arc. Phenomena like the plasma properties in the keyhole and in the arc, wire melting and

drop transfer have been taken into account.

4. Results and Discussion

In the following section the modelling results will be presented and briefly

discussed. The interaction of the MIG-shielding gas with the escaping keyhole plasma is

shown in Fig. 4(a) (calculated from the model) and a high speed image is presented in Fig.

4(b) [Naito 2001]. The model and the high speed image both describe a general flow of gas

and plasma from right to left in these images. The force of the MIG gas flow deflects the laser

induced keyhole plasma back along the line of the weld.

(a) (b)

Fig. 4: Shielding gas flow, arc and escaping keyhole plasma jet: (a) calculation, (b) high

speed imaging [Naito 2001]

For calculation of the temperature field and melt pool shape the integrated point

source model [Resch 1998] is most suitable (as it is the most flexible). Fig. 5(a) and (b) show

the melt pool shape viewed from the side resulting from pure laser and arc welding


respectively and super-imposes these images with the arc pool trailing or leading the laser

pool by 2 mm. While the laser creates a deep, narrow melt geometry, the arc causes a shallow

melt pool. Superposition of both temperature fields leads to different weld pool shapes, shown

in Fig. 5(c). In particular the arc leading configuration causes a leading edge geometry of the

pool that affects the keyhole. High speed X-ray imaging [Naito 2001] shows a typical hybrid

(laser-TIG) weld melt pool in cross section (fig. 5(d)). The agreement between this shape and

that calculated by the model (fig. 5(c)) demonstrates the accuracy of the calculation method.

(d)

Fig. 5: Melt pool geometry (side view): laser and arc pool separately for (a) trailing and (b)

leading position of the arc (moving right), (c) combined hybrid pool for both cases, (d) X-ray

pool imaging (TIG leading) [Naito 2001]

In our model the joint gap and wire addition by the MIG-torch are taken into

account by a mass balance. The calculated results in Fig. 6(a) and (b) show the influence of

the gap width and wire feed rate, respectively, on the weld seam cross section. According to

the mass balance the wire speed has to be kept proportional to the welding speed and gap

width to maintain the desired weld seam shape (top profile convex). The calculated weld

seam cross section area is shown in Fig. 6(c) together with actual experimental results. There

is a close correlation between the trends of the two curves but the experimental values for the

cross sectional area are lower than those predicted by the model. This difference is due to the

fact that the model does not account for material lost to the weld pool as a result of spatter and

evaporation.


(a) (b) (c)

Fig. 6: Calculated weld seam cross section: for varying gap width (a), wire feeding (b),

compared to experiments (c)

From the model fundamental aspects for a process theory can be derived, as

illustrated in Fig. 7.

Fig. 7: Process theory (side view): (a) physical fields involved, (b) energy transfer, (c)

material transfer

Fig. 7(a) illustrates the physical fields occurring and interacting, such as the

(usually pulsed) electric and magnetic fields, the electric current, the heated shielding gas

flow, the keyhole plasma flow field and the laser beam. The energy transfer contributing to

the welding process (shown in fig. 7(b)) is composed of the anode (wire) and the cathode

(workpiece) being heated by the electric arc current, radiation and convection from the arc

and laser beam absorption in the keyhole. The droplets are transferred from the wire to the

workpiece, as shown in Fig. 7(c). Hybrid welding is usually operated in the spray mode, with


a pulsed current, detaching one droplet per pulse. For the arc trailing configuration, as in Fig.

7(c), an interesting question arises as to how a stable liquid layer can form in front of the

keyhole in the case of a joint gap.

5. Conclusions

Hybrid laser-MIG welding has gained significant industrial attention

Various joint configurations can be hybrid welded at high speed, even in the presence of a

significant gap

The fundamental physics of hybrid welding has been studied by developing a

mathematical model

In contrast to MIG-welding the torch is inclined, causing a change in energy transfer

The displacement between the laser beam and the arc modifies the weld pool shape

For the arc leading configuration the keyhole is affected by the leading weld pool

For the arc trailing configuration the melt has to move in the welding direction to enable

keyhole production

During hybrid welding the melt pool is significantly enlarged, permitting more extensive

melt flow

Energy transfer is the result of the electrode current, shielding gas convection and laser

beam absorption

The wire is melted via the anode drop zone and current, the hot drop is transferred to the

workpiece in spray mode by detaching a single drop per electric current pulse due to the

magnetic forces

The weld seam shape is governed by the wire feeding rate relative to welding speed and

gap width

Acknowledgements

The authors acknowledge the financial support from the Research Council of

Norrbotten and VINNOVA.


Literature

Steen, W.M., M. Eboo: Arc augmented laser welding, Metals Constr., II, 7, pp 332-336

(1979).

Duley, W. W., Laser welding, Wiley, New York (1999).

Kaplan, A.: A model of deep penetration laser welding based on calculation of the keyhole

profile, J. Phys. D: Appl. Phys., 27, 1805-1814 (1994).

Kaplan, A. F. H., M. Mizutani, S. Katayama, A. Matsunawa: Unbounded keyhole collapse

and bubble formation during pulsed laser interaction with liquid zinc, J. Phys. D: Appl. Phys.,

35, 1218-1228 (2002).

Lancaster, J. F.: The physics of welding, Pergamon, Oxford (1986).

Choo, R.T.C, J. Szekely, R.C. Westhoff: Modeling of high current arcs with emphasis on free

surface phenomena in the weld pool, Weld. J., September 1990, pp. 346s-361s (1990).

Dowden, J., P. Kapadia: Plasma arc welding: a mathematical model of the arc, J. Phys. D:

Appl. Phys., v 27, pp 902-910 (1994).

Jones, L. A., T. W. Eagar, J.H. Lang: A dynamic model of drops detaching from a gas metal

arc welding electrode, J. Phys. D: Appl. Phys., v 31, n 1, pp 107-123 (1998).

Dilthey, U., Wieschemann, A. Prospects by combining and coupling laser beam and arc

welding processes, Welding in the World, v 44, n 3 (2000).

Naito, Y., S. Katayama, A. Matsunawa: Liquid flow in molten pool and penetration

characteristics in laser-arc hybrid welding, Proc. JWS, n 69, pp 8-9 (2001).

Nilsson, K., H. Engström, A. Kaplan: Influence of butt and T-joint preparation in laser arc

hybrid welding, Annual IIW Assembly, Copenhagen, WG IV, 27 June 2002 (2002).

Nilsson, K., A. F. H. Kaplan: CO2-laser/pulsed MIG hybrid welding of high strength steel,

Proc. 9th NOLAMP, Trondheim (N), 4-6.August 2003, Ed.: E. Halmøy (2003).

Resch, M., A. F. H. Kaplan: Heat conduction modelling of laser welding, Lasers in Eng., 7,

229-240 (1998).

Marc Wouters Paper 2: Joint geometry and fit-up gaps Page 36 36

PAPER 2: THE INFLUENCE OF JOINT GEOMETRY AND FIT-UP GAPS ON

HYBRID LASER-MIG WELDING

Y. Yao, M. Wouters, J. Powell, K. Nilsson and A. F. H. Kaplan

Submitted to the Journal of Laser Applications (Impact Factor 0,61)


Paper 2: The influence of joint geometry and fit-up gaps on hybrid laser-MIG welding

Y. Yao1, M. Wouters2, J. Powell2,3, K. Nilsson2 and A. F. H. Kaplan2

1 Jilin University and China FAW Group Cooperation R&D Center, Changchun, China 2 Luleå University of Technology, Division of Manufacturing Systems Engineering, S-971 87

Luleå, Sweden Phone: +46 920 49 1733, E-mail: [email protected] 3 Laser Expertise Ltd., Acorn Park Ind. Estate, Harrimans Lane, Nottingham NG7 2TR, U.K.

Abstract

This paper presents the results of an experimental program to investigate the

sensitivity of the hybrid laser-MIG welding process to gaps between the work pieces being

welded. It was concluded that the minimum throat depth of the welds remained surprisingly

stable for fillet welds as long as the MIG parameters were set high enough to fill the gaps

involved (up to a gap size of approximately 3 mm). If the MIG parameters were insufficient

then the welding process collapsed with increasing gap widths.

1. Introduction

It is well known that most welding process are sensitive to the geometry of the

joint involved and to the “fit up” between the parts being welded (poor “fit up” indicates a

gap between the parts to be welded). In the early days of laser welding it was discovered that

one of the main disadvantages of the process was the need for excellent fit up between the

parts to be welded. A joint gap of a millimeter or more would be considered satisfactory for

most welding techniques but could have a disastrous effect on the laser welding process. This

situation lead to the development of the hybrid laser-MIG welding process [1, 2 and 3], which

has been successful in combining the gap filling capabilities of the MIG process with the

depth of penetration associated with laser welding [4, 5 and 6]. For example, full penetration

welds have been achieved with gaps over 1mm using a 2kw CO2 laser and 2.7kw MIG

equipment [7], also, it has been demonstrated that T joints and fillet joints can be produced

with gaps of 1mm in 2.5 mm mild steel [3]. In Plasma Arc Laser Welding, a higher tolerance

to beam-gap misalignment (from 0.15 to 0.5 mm at 2m/min, 50A) has been noted [8].

mailto:[email protected]


It is, however, obvious that the laser-MIG hybrid technique has a limit to the

size of gap which can be successfully welded [9 and 10]. Where, for example, a cast part

needs to be welded to another component it is possible that the joint gap may vary from zero

to one or two millimeters across a single joint. In this case the weld cross section may vary

profoundly along the weld.

This paper presents the results of an experimental program to investigate the

effect of joint gap and geometry on the morphology of the weld produced.

2. Experimental materials and set up

The range of joint geometry investigated is described in figure 1.

Fig .1. Joint geometry (a) Non-chamfered joint (b) Chamfered joint (X=3 or 6 mm, Y=1 or 2

mm)

As figure 1 demonstrates, the weld geometry involved three different joint gap

widths (0, 0.5, 1.0mm) and a comparison of chamfered and non-chamfered work pieces. The

length of Part A and B was 137mm.


A typical example of a completed weld is shown in figure 2.

Fig .2. A typical weld cross section

The steels being welded together had the chemical composition given in table 1.

Table 1. The chemical composition of the steel being welded

The laser used in these experiments was ROFIN-SINAR RS6000. The MIG

equipment used was ESAB MEK 44C.

Throughout these experiments the welding speed was kept constant at 1.5

m/min. The laser power was also kept constant at 5000 W. The inclination of the laser beam

to the top surface of Part B was 10°, and the inclination of the MIG torch with the surface of

Part B was 20°. The laser beam was positioned 1.5 mm in advance of the arc during welding.

The MIG power ranged from 37 to 50 kW and the wire filler rate was between 8 and 17

m/min. The filler wire, OK 12.51, of composition C<0.1%; Si<1.2%; Mn 1-2%; Cu<0.5 and

Fe balance, had a diameter of 1 mm. The weld shield gas in all cases was 65He30Ar5CO2

with a flow rate of 24 l/min.


3. Results

Fig.3 Matrix of macrographs of weld cross sections


Figure 3 presents a matrix of macrographs which demonstrate the changes in

weld cross section with different gaps and type of chamfer. The Figures beneath each cross

section show the MIG process parameters for each weld as follows:

Taking, for example, the zero gap, zero chamfer weld figures of 8, 37 and 3.85:

These numbers represent a filler wire speed of 8 m/min, a MIG voltage of 37 Volts and a

MIG power of 3.85 KW.

From Fig. 3 it is possible to get an approximate measure of the relative strengths

of the welds involved although this involves some analysis of the weld cross section. Most of

the welds shown in Fig. 3 are combination of two types of weld shown in figure 4.

(a) fillet weld

(b) partial penetration groove weld

Fig.4 The fillet weld and the partial penetration groove weld


Fig.5 Weld throat depth measurement

The geometries of the welds presented in figure 3 do not lead themselves to

either of the throat depth measurements given in Figure 4. It was decided, for the purpose of

this investigation, to measure the throat depth from the root of the weld to the surface by the

shortest path “AB” as shown in figure 5. Although this line generally passes through weld

metal and the heat-affected zone (HAZ) it does provide an easy comparator of the strength of

the welds. The results of the throat depth measurements are given in table 2.

Table 2. Throat depth measurement (mm)

The results of table 2 are presented graphically in figure 6 and 7. Figure 6

demonstrates that the 6x1 chamfer gives the best result and the zero chamfer gives the

smallest throat depths. Figure 7 makes it clear that a 0.5 mm gap gives better results than a

zero gap. The 1.0 mm gap results are also very good for the 3x1 and 6x1 chamfer geometries.

Figure 6 and 7 also reveal that the geometrical features which give the best throat depths (i.e.

0.5 mm gap and/or 6x1 chamfer) also provide conditions which make the process least

sensitive to change. For example, the 0.5 mm gap results have a spread of only 6% whereas

the zero gap results have a spread of 20% and the 1.0 mm gap results have a spread of slightly


more than this. The results for the 6x1 chamfer are also rather insensitive, showing only a 6%

variation over the range of gap from zero to 1mm.

Fig.6 Weld throat depth as a function of chamfer geometry

One of the most surprising aspects of figure 6 and 7 is the stability of the

process over this fairly wide range of weld geometries. One reason for this stability is the fact

that the MIG wire speed and voltage were increased incrementally for each of the types of

chamfer from zero to 6x2 (Figures for MIG wire speed and voltage are noted by the

macrographs in figure 3). In order to test the true stability of the process with fixed MIG

parameters, several additional weld runs were carried out. Matrices of welds similar to the one

presented in figure 3 were produced with the process parameters listed in table 3.


Fig.7 Weld throat depth as a function of gap width

Table 3. Process parameters for weld sets produced with fixed MIG parameter

Figure 8 presents the macrographs of zero chamfer welds for Matrix A (low

MIG) and Matrix C (high MIG).


Fig.8 Macrographs of zero chamfer welds showing the influence of high and low MIG

conditions on three gap sizes

Table 4 presents measurement of the throat depths of the welds shown in figure 8.

Table 4. Throat depth measurements (mm) of the welds shown in figure 8

To discuss these results it is helpful to refer to figure 9, which is a stylized

diagram of a typical weld geometry.


Fig.9 A typical weld geometry

For all the welds shown in figure 8 the throat depth was equal to the dimension

“a” in figure 9.

If we consider the zero gap results we can see that, although the increase in MIG

wire supply and power have increased the melt cross section, this has had only a small effect

on dimension “a”(Fig 9). Thus, the increased MIG parameters have only acted to spread the

weld by increasing dimensions b and c in figure 9. This spreading of the weld will not

necessarily increase its strength, which is usually related to the shortest path across the weld

(dimension “a” in this case). As the gap between the work pieces increases, the beneficial

effect of the increase in MIG parameters becomes clear; the additional melt provided by the

excess MIG power and wire supply helps to maintain the throat depth by filling the gap

between the work pieces.

In the case of the low MIG 1.0 mm gap, the limited supply of melt from the

MIG wire has resulted in a much smaller throat depth than that which is possible in the high

MIG case. The “filling and spreading” role of the increasing MIG parameters is made very

clear in figure 10, which compares the results of the three different MIG parameters for the

various chamfer geometries with 1.0 mm work piece gap.


Fig.10 Welds cross-section for three different MIG parameters for different chamfer

geometries.(All the welds shown here had a pre-weld fit up gap of 1.0mm)

The throat measurements for the welds shown in figure 10 are given in table 5.

Table 5. Throat depth measurement (mm) for the welds shown in figure 10


The information given in figure 10 and table 5 tells a clear story that; if the MIG

parameters are sufficient for the range of gaps or chamfers involved, the throat depth is rather

insensitive to changes in gaps or to increases in MIG settings. If, however, the MIG

parameters do not provide enough melt to the weld, the welding process collapses completely.

This collapse has obviously taken place in the case of the 6x1 and 6x2 chamfers for the low

MIG parameters in figure 10. If insufficient melt is provided by the MIG process the laser

passes straight through the gap between the work pieces and a failed weld is the result.

4. Conclusions

1. If a particular weld throat depth is required when using the laser-MIG hybrid welding

process then it is important to carry out a survey to establish the range of fit up gaps expected.

The MIG parameters should then be set at a high enough level to fill and bridge this range of

gaps during welding.

2. The penetration depth of the weld (which is generally closely related to the throat and thus

the weld strength) is generally controlled by the laser (type, power and process speed).

3. Surplus MIG power or wire will cause a widening of the weld cross-section but will have

only a modest effect on the throat depth.

4. Chamfers can be used to reduce the sensitivity of the process to gaps.

5. The laser-MIG welding process is rather insensitive to gaps below 3mm as long as the MIG

parameters are set high enough to bridge those gaps.

References

1. E. Beyer, R. Imholff, J. Ncuenhahn and K. Bebler: “New aspects in laser welding with an

increased efficiency”, Proceedings of ICALEO 1994, Orlando, FL (Laser Institute of

America), pp. 183-192.

2. U. Dilthey and A. Wieschemann: “Prospects by combining and coupling laser beam and

arc welding processes”, International Institute of Welding, IIW Doc, XII-1565-99, 1999.

3. T. Ishide, S. Tsubota and M. Watanabe: “Latest MIG, TIG arc-YAG laser hybrid welding

systems for various welding products”, Proc. SPIE 4831, 2002, pp. 347-352.

4. T. Graf and H. Staufer: “Laser-hybrid welding drives VW improvement”, Weld Journal,

2003, vol.82, pp. 42-28.

5. S. Herbert: “Laser-Hybrid Welding of Ships”, Weld journal, 2004, vol.83, pp. 39-43.


6. J. Matsuda, A. Utsumi, M. Katsumura and M. Hamasaki: “TIG or MIG arc augmented laser

welding of thick mild steel plate”, Joining & Materials, 1988, No.1, pp. 31-34.

7. M. Kutsuna and L. Chen: “Interaction of both plasma in CO2 laser-MAG hybrid welding of

carbon steel”, International Institute of Welding, IIW Doc, XII 1708, 2002.

8. J. R. Biffin and R. P. Walduck: “Plasma arc augmented laser welding”, Proceeding

EUROJOIN 2, 1994, pp. 295-304.

9. C. Bagger and F. O. Olsen: “Review of laser hybrid welding”, Journal of Laser

Application, 2005, No.1, vol.17, pp. 2-14.

10. J. Tusek, M. Suban: “Hybrid welding with arc and laser beam”, Science and Technology

of welding and joining, 1999, No.5, vol.4, pp. 308-311.

Marc Wouters Paper 3: Influence of joint fit-up on the strength Page 50

PAPER 3: THE INFLUENCE OF JOINT GAP ON THE STRENGTH OF HYBRID

ND:YAG LASER-MIG WELDS

M. Wouters, J. Powell and A. F. H. Kaplan

Submitted to the Journal of Laser Applications (Impact Factor 0,61)


Paper 3: The influence of joint gap on the strength of hybrid Nd:YAG laser-MIG welds

M. Wouters1, J. Powell1, 2 and A. F. H. Kaplan1

1 Luleå University of Technology, Division of Manufacturing Systems Engineering, S-971 87

Luleå, Sweden Phone: +46 920 49 1733, E-mail: [email protected] Laser Expertise Ltd., Acorn Park Ind. Estate, Harrimans Lane, Nottingham NG7 2TR, U.K.

Abstract

This paper presents the results of an experimental and theoretical analysis of the

effect of joint gap on the strength of hybrid laser-MIG welds. The welds were of the partial

penetration butt type with various joint gaps but identical weld penetration. Impact testing

established that a zero gap gave a weak weld because the weld geometry contained the

equivalent of a sharp crack where the unwelded parts met each other.

A small gap between the workpieces improved the weld impact strength as the

sharp crack effect became dissipated. Further increases in gap width resulted in a weakening

of the joint and this is the subject of a discussion on joint gap optimization.

1. Introduction

Laser welding and hybrid laser-MIG welding [refs. 1, 2 and 3] are occasionally

used to produce welds which do not fully penetrate the materials to be joined. Partial

penetration welds may be produced deliberately (see fig. 1) or on occasions where a reduction

in power has reduced the penetration of a weld accidentally.

mailto:[email protected]


Figure 1: A type of weld geometry which could employ partial penetration welds

In either case the lack of full penetration can have serious consequences on the

strength of the weld because the weld geometry now includes a “crack” at the weld root (see

fig. 2).

Figure 2: The cross sectional geometry of a partial penetration weld automatically includes a

“crack” at the root of the weld.

“Cracks” of this type are stress raisers which can initiate failure of the weld

under load [ref.4 and 5]. A calculation of the strengths of 4 mm deep welds in 4 mm and 6

mm thick material can be used to demonstrate the weakening effect of the partial penetration

“crack” (see fig. 3).

By graphical estimation [see ref.4, chart 2.30a], the stress concentration factor

Ktn in bending is increased from 1.0 for the full penetration weld (weld 1) to 2.84 for the


partial penetration weld (weld 2) and the impact strength of weld 2 is thus theoretically at

least 65 % lower than the strength of weld 1.

Increasing the width of the crack to 1.2 mm reduces its stress raising properties

and weld 3 is therefore stronger than weld 2 (weld 3 is 47 % as strong as weld 1). In these

calculations the material is assumed to exhibit linear elastic behaviour and the metallurgical

effects of welding have been ignored in the interests of simplicity.

Weld 1: 4 mm deep weld

in 4 mm thick material

(full penetration),

Ktn = 1.0



(non full penetration with

a 0.6 mm wide “crack”),

Ktn = 2.84



(non full penetration with

a 1.2 mm wide “crack”),

Ktn = 2.12

Figure 3: Three different weld geometries. If the strength of weld 1 is taken as 100 % then

welds 2 and 3 have strengths of 35 % and 47 % respectively

In summary, close contact between the workpieces should result in the weakest

welds if the welds do not fully penetrate the material.

This paper presents the results of an experimental program designed to

investigate this principle. A number of otherwise identical partial penetration welds were

produced (by hybrid Nd:YAG laser-MIG welding) with different gaps between the

workpieces. The welds were sectioned to produce impact test pieces and it was discovered

that optimum weld strength was achieved when small gaps were introduced into the weld

geometry.


2. Experimental procedure

Laser conditions:

Laser: RS 6000, Nd YAG

Laser power (measured at the material surface): 2800 W

Lens focal length: 275 mm

Focal point position: on the material surface

MIG conditions:

Equipment : ESAB - ARISTO LUD 450W / MEK 44C

Pulse current: 32 A

Pulse frequency: 130 Hz

Pulse length: 2 ms

Background current: 60 A

Shield gas: 65% He, 30% Ar, 5% CO2, 24 l/min

Electrode wire: OK 12,51, 1 mm diameter

Material:

RAEX 420 MC (plate), by Rautaruukki (10 mm thick)

Variables:

Welding speed: 0.5-2.0 m/min

Wire feed rate: 4.5-20 m/min

MIG voltage: 35-50 V


The geometry of the laser and MIG arrangement is given in figure 4.

Figure 4: The geometrical arrangement of the laser and MIG torch

Impact testing:

The welded samples were sectioned and machined to produce the type of

samples shown in figure 5.

Figure 5: Impact test sample geometry, X = gap width, Y = weld depth

The samples were then broken in an A.B. ALPHA (Sundbyberg, Sweden)

Impact testing machine type 1H539 (with the pendulum striking the top surface of the weld).


Two sample groups were tested; one at room temperature and another after immersion in

liquid nitrogen.

Before and after impact testing the samples were examined by optical and SEM

microscopy.

3. Results and discussion

Figure 6 shows macrographs of a typical weld cross section in its welded,

machined and post impact testing states.

(a) (b) (c)

Figure 6: Macrographs (x15 magnification) of a typical weld sample in its (a) welded, (b)

machined and (c) impact tested condition (liquid N2 cooled sample)

0,40,50,60,70,80,9

1

0 0,5 1 1,5Gap (mm)

Surf

ace

Ener

gy

Abs

orbe

d (J

/mm

2)

0,1

0,15

0,2

0,25

0,3

0 0,5 1 1,5Gap (mm)

Surf

ace

Ener

gy

Abs

orbe

d (J

/mm

2)

(a) (b)

Figure 7: Impact test results: Surface Energy absorbed as a function of gap at (a) room

temperature and (b) cold temperature


The results of the impact tests are given in figure 7. Each data point represents

an average of more than five samples.

Figure 7 demonstrates that zero (or very small) gaps between the workpieces

produce welds which break easily. As the gap widens the “crack” in the weld geometry

becomes less sharp and has a reduced stress raising effect. In both the cold and room

temperature tests an optimum gap width was found to be between 0.5 and 1.0 mm. Welds

with a 0.5 mm gap were stronger than the zero gap results by 57 % in the room temperature

case. For the low temperature samples the optimum gap width was 1.0 mm and this gave a 33

% improvement in impact resistance. These results agree with the argument posed earlier with

reference to figure 3.

When the gap was further increased to 1.5 mm the impact test results were

inferior to those of the 1.0 mm gap welds. This result can not be explained by reference to the

macro-geometry of the joint discussed in figure 3. The source of the weakness of the 1.5 mm

gap can, however, be identified by a microscopic examination of the root of the weld.

Figure 8 shows how the geometry of the weld changes with increasing gap

width.

Figure 8: The change in the geometry of the root of the weld with increasing gap size.


Figure 9 and 10 support figure 8 with photographs of the actual roots of the

welds in each type of case.

(a) (b) (c) (d)

Figure 9: Photographs of weld root geometry with increasing gap width (a) : 0.0 mm, (b) :

0.5 mm, (c) : 1.0 mm, (d) : 1.5 mm

(a) (b) (c) (d)

Figure 10: Higher magnification views of the root of the welds shown in figure 9 with

increasing gap width (a) : 0.0 mm, (b) : 0.5 mm, (c) : 1.0 mm, (d) : 1.5 mm

Figure 8, 9 and 10 reveal that, if the gap exceeds a certain dimension, the liquid

in the weld penetrates vertically downwards into the gap. This gives rise to a new (and

weaker) geometry which involves small radius sharp “cracks” on either side of the weld root.

Figure 11 shows how, in practice, either one of these small radius features can become the

crack initiation point during impact testing.

Figure 11: A photo of a 1,5 mm wide gap weld after impact testing at room temperature


Up to a 1 mm gap it is clear that the capillary forces on the weld melt inhibit

melt penetration into the crack but, once the gap exceeds a certain value, gravity takes control

of the weld geometry in this area. Once the liquid has solidified it subtends an acute angle to

the original workpiece edge (see fig 8(c), 9(d) and 10(d)). The stress raiser thus produced acts

as a site for crack initiation and subsequent failure (see fig. 11).

4. Conclusion

From the above evidence it is clear that there is an optimum range of workpiece

fit-up for partial penetration welds. This optimum is reached when the gap is as wide as

possible without allowing the melt to slump into it. For any welding application this optimum

range could be identified by cross sectional analysis.

References

1. Seyffarth, P., I. V. Krivtsun, Laser-arc processes and their applications in welding and

material treatment, London, Taylor & Francis (2002)

2. Magee, K. H., V. E. Merchant, C. V. Hyatt, Laser assisted gas metal arc weld

characteristics, Proceedings of the Laser Materials Processing - ICALEO '90, Nov 4-9 1990,

Boston, MA, USA, LIA (Laser Institute of America), v 71 (1991)

3. Bagger, C., Flemming O. Olsen: Review of laser hybrid welding, Journal of Laser

Applications, Vol. 17, no. 1 (Feb. 2005)

4. Pilkey, Walter D., Peterson's stress concentration factors, 2nd. edition, New York,

Chichester, Wiley (1997)

5. Young, Warren Clarence, Roark's formulas for stress and strain, 7th. Edition, New York,

McGraw-Hill (2002)

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