Article type: Article

Title: Characterisation of Quasi-Stationary Temperature Fields in Laser Welding by

Infrared Thermography

Titel: Charakterisierung von quasi-stationären Temperaturfeldern beim

Laserstrahlschweißen durch Infrarot Thermographie

Authors: J. A. Francis1*, S. Gach2*, S. Olschok2, A. Haeusler3, A. Gillner3, R. Popawe3, U.

Reisgen2.

*Corresponding Authors: E-mail: (john.francis@manchester.ac.uk, gach@isf.rwth-aachen.de)

1School of Mechanical, Aerospace and Civil Engineering, The University of Manchester,

Sackville Street, Manchester M13 9PL, United Kingdom

2RWTH Aachen University, Welding and Joining Institute (ISF), Pontstraße 49, 52062

Aachen, Germany.

3RWTH Aachen University, Institute for Laser Technology (LLT), Steinbachstraße 15, 52074

Aachen, Germany

Abstract: In this work, high-speed thermography is shown to effectively capture quasi-

stationary temperature fields during the laser welding of steel plates. This capability is

demonstrated for two cases, with one involving the addition of a ferritic-bainitic filler wire,

and the other involving the addition of a low-transformation-temperature (LTT) filler wire.

The same welding parameters are used in each case, but the temperature fields differ, with the

spacing between isotherms being greater in the case where the LTT filler material is added.

This observation is consistent with the differences in the extent of the heat-affected zone in

each sample, and the shape of the weld pool ripples on the weld bead surfaces. The

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characterization of temperature fields in this way can greatly assist in the development of

novel methods for reducing residual stresses, such as the application of LTT filler materials

through partial-metallurgical injection (PMI). This technique reduces or eliminates tensile

residual stresses by controlling the temperature fields so that phase transformations take place

at the optimum times, and success can only be guaranteed through precise knowledge of the

temperature fields in the vicinity of the welding heat source in real time.

Abstract (Deutsch): In dieser Arbeit wird Hochgeschwindigkeits-Thermographie zur

effektiven Dokumentation quasi-stationärer Temperaturfelder während des

Laserstrahlschweißens von Stahl vorgestellt. Gezeigt wird dies am Beispiel von zwei

repräsentativen Schweißungen, wobei einerseits ein ferritsch-bainitischer Zusatzdraht und

andererseits ein Low-Transformation-Temperature (LTT) Zusatzdraht Verwendung findet. In

beiden Fällen werden die gleichen Schweißparameter verwendet. Im Vergleich wird eine

merkliche Abweichung der Temperaturfelder deutlich. Bei Einsatz des LTT-Zusatzdrahtes

vergrößern sich die Abstände zwischen den Isothermen. Makroschliffe stützten diese

Beobachtung und zeigen, dass bei Verwendung des LTT-Zusatzdrahts die Aufmischung des

Grundwerkstoffs steigt. Die Charakterisierung der Tempe-raturfelder kann auf diesem Weg

die Entwicklung neuartiger Methoden zur Eigenspannungs-reduzierung unterstützen, wie

beispielsweise die Applikation von LTT-Zusatzmaterial durch die Anwendung der partiellen

metallurgischen Injektion (PMI). Diese Technologie reduziert Zugeigenspannungen durch

gezieltes Applizieren von Wärmefelder, so dass die martensiti-sche Phasenumwandlung zu

einem optimalen Zeitpunkt abläuft. Eine Voraussetzung zur er-folgreichen Anwendung sind

präzise Kenntnisse über die Temperaturfelder in der direkten Umgebung der

Schweißwärmequelle in Echtzeit.

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Keywords: cooling rate, heat-affected zone (HAZ), phase transformation, residual stress,

temperature measurement, welding thermal cycle.

Schlüsselwörter: Abkühlrate, Wärmeeinflusszone (WEZ); Phasenumwandlung,

Eigenspannungen, Temperaturmessung, Schweißwärmezyklus

1 Introduction

Residual stresses in welds tend to be in the order of the yield stress of the material, due to the

steep temperature gradients that are generated [1], which in turn cause highly localised

(constrained) expansion and contraction of the material, and hence localised plastic

deformation. In the absence of phase transformation effects, peak residual stresses in welds

are normally tensile, so they can exacerbate problems associated with cracking [2] and fatigue

[3]. The mitigation of residual stresses in steel welds is therefore of great engineering interest.

In developing residual stress mitigation strategies, it is usually important to understand how

the welding process affects the temperature fields that are generated in the parts being joined.

In this work, in-situ thermography is used to examine the quasi-stationary temperature fields

in the laser welding of steel for two distinct cases. The first case involves the addition of a

filler material that normally produces a ferritic-bainitic structure upon cooling, while the

second case involves the application of a low-transformation-temperature (LTT) filler

material. The temperature field in the vicinity of the heat source and on the top surface of the

plate is captured in each experiment. In conducting this comparative study, noticeable

differences in the temperature fields are observed, despite the same welding parameters being

used in each case. Preliminary results are presented, and the potential for applying infrared

thermography when using a novel residual stress mitigation technique is discussed.

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2 State-of-the-art

The residual stresses that arise in the vicinity of the fusion zone in welds are normally tensile.

However, in steels, the situation is made more complicated by the effects of solid-state phase

transformations [4]. It is known that, if austenite decomposes to martensite at a temperature

below ~ 400C [5-7], the strains associated with this phase transformation can mitigate the

development of tensile residual stresses. The martensitic transformation strains include a

dilatational component (i.e. a volume change) and, in the presence of a macroscopic stress,

they may also include an anisotropic component associated with a bias in the crystallographic

variants that form [8]. The overall effect is that the generation of tensile stresses due to

thermal contraction during cooling after welding is counteracted by the increase in volume (as

well as any anisotropic strain) that occurs during the phase transformation. The potential to

mitigate tensile residual stresses in this way has stimulated efforts to design weld filler

materials that simulataneously eliminate tensile stresses while also achieving acceptable weld

toughness [9]. Low-Transformation-Temperature (LTT) filler materials typically have

martensite-start-temperatures between 200 and 300C, and they are already being applied

successfully in arc welding processes [10, 11].

In contrast to arc welding processes, where LTT filler materials can be added in volumes that

are large enough to fill a weld groove, beam processes offer the capability to apply these

materials in smaller volumes and at precise locations. The LTT effect is activated during

welding through careful control of the temperature fields. This concept is called partial-

metallurgical injection (PMI) [12]. To apply this technique in future applications, it is

essential to obtain detailed knowledge of the martensite transformation temperature, as well

the exact locations that transform, and the timing of the transformations in comparison to the

surrounding material. There are several empirical approaches to predicting the martensite-start

temperature as a function of chemical composition. The equation of Steven and Haynes [13]

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provides good estimates for steels containing approximately 10 wt.-% chromium and 10 wt.-

% nickel, and it will be used in this study. However, the main focus of this work is to assess a

novel method for capturing the details of welding temperature fields in real time.

3 Experimental Methodology

The welded plates are made from S235 steel and have dimensions of 100 mm 50 mm 5

mm. Each plate has a groove, 100 mm long, with a 1.6 mm 1.6 mm square cross-section,

running along the centreline. Filler material is added by laying a 100 mm long piece of the

filler wire, which has a diameter of 1.6 mm, in the machined groove prior to welding, Figure

1. The sample is then positioned under the laser optics and the wire is welded to the base

material. A diode-pumped disc laser "TruDisk 16002" is used in the welding experiments.

The output wavelength is 1030 nm. The beam quality of the laser beam source is 8 mm*mrad,

and the diameter of the optical fibre is 200 μm. The processing optics have an imaging ratio of

1:3, resulting in a laser spot diameter of 0.6 mm. The shielding gas is argon 4.6 in both cases,

and the gas flow rate is 10 l/min.

One welded sample is made using a solid filler wire (G3Si1) that normally leads to a ferritic-

bainitic structure at room temperature, while the other is made with a low-transformation-

temperature (LTT) alloy-cored filler wire. LTT filler materials are designed to transform to

martensite at low temperatures in order to be effective in mitigating residual stresses [9]. The

principal alloying elements for the base material and for each of the filler materials are given

in Table 1, while the process parameters for each of the welds are listed in Table 2.

During welding, the quasi-stationary temperature field in the vicinity of the heat source is

captured using infrared thermography. This is achieved by placing an infrared camera so that

it is focussed on the point at which the laser beam is incident upon the plate, Figure 2. The

plate is then translated during welding so that the camera remains focussed on the region in

the vicinity of the molten pool. Images are captured at a frequency of 1 kHz during welding. It

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is possible to adjust the range over which temperatures are measured to improve resolution.

The accuracy of the temperature measurements is +-2% of the measurement range under

consideration, for temperatures > 100 °C. During welding, the emitted radiation changes due

to physical changes within the material, and due to factors such as the viewing angle and

surface quality. These sources of uncertainty must be considered when interpreting the results.

3 Results and Discussion

Macrographs for the welds made with the alloy-cored LTT filler wire and the solid G3Si1

filler wire are shown in Figure 3. It can be seen that the welds are partial penetration welds,

and that the filler wires do not completely fill the machined grooves. In the bottom corners of

the grooves, lack of fusion is visible, especially in the weld made with the G3Si1 wire.

Nevertheless, adequate fusion is generally achieved. The macrograph for the specimen welded

with the G3Si1 filler wire shows a smaller depth of penetration and a slightly higher bead

width, when compared to the LTT specimen. The hardness of the base material is about 150

HV, but this rises rapidly across the heat-affected zone (HAZ) to values of approximately 350

HV for the G3Si1 specimen. For a steel with less than 0.1 wt.-% carbon, hardness values in

this range would suggest that the microstructures in the HAZ and fusion zone comprise a

mixture of bainite and martensite. The LTT-welded specimen shows similar behaviour, with

hardness values reaching ~ 450 HV. In analogue investigations involving electron beam

welding, similar hardness values are shown to correspond to martensitic microstructures [14].

Analysis carried out by energy dispersive X-ray spectroscopy (EDX) reveals the changes in

chemical composition across the LTT-welded specimen, Figure 4. The iron concentration

decreases rapidly when crossing the fusion line in favour of the major alloying elements in the

LTT-filler wire, namely nickel and chromium, which both reach concentrations of ca. 7 %.

The manganese concentration remains approximately constant across the weld, since the

differences between the manganese concentration in the base material and in the filler

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materials are not large, as can be seen in Table 1. The carbon concentration is not

measureable by EDX, so a range of possible Ms-Temperatures must be calculated using the

carbon concentrations in the base material and in the LTT filler material (Table 1) as limiting

values. According to the equation of Stevens and Haynes [13], this will lead to an Ms-

Temperature for the LTT fusion zone in the range between 265 and 288°C.

The quasi-stationary temperature fields in the vicinity of the heat source for each weld are

shown in Figure 5. These images correspond to the same position in each weld, near to the

mid-length position on each sample. On the right, a top-view of a welded specimen is shown.

Scribe lines (horizontal lines) are marked on the top surface of the specimens at intervals of 5

mm to enable the positions of the isotherms to be compared. Isotherms can be considered to

be lines along which the colour in a thermographic image remains constant. Thus, isotherms

might appear as pink, red or yellow lines, depending on the temperature. Vertical lines can

also be seen in the image for the LTT specimen, and these are associated with undercut,

which is caused by incomplete filling of the groove by the filler wire, Figure 3.

The images in Figure 5 are typical for the temperature fields that were observed throughout

the majority of the weld run. The region that appears in white is at a temperature greater than

400C, a temperature that corresponds to the martensite-start (Ms) temperature in many steels.

Frequently, globules of hot metal appear in the weld area, disturbing the analysis.

Nevertheless, it can be seen that when the same welding parameters are used, the isotherms

are noticeably further apart in the sample made with the LTT filler material. Furthermore, the

isotherms appear to span a greater distance along the welding direction, behind the heat

source, when the LTT filler material is used.

Checks can be carried out to confirm that the differences in the measured temperature fields

are credible. The hardness traverses in Figure 3 suggest that the width of the HAZ is similar in

each sample. However, hardness traverses only provide information along a single line. When

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image analysis software is used to measure the cross-sectional area that falls within the outer

boundary of the HAZ, which appears as the outer boundary of the light-etching region in

Figure 3, the enclosed area is noticeably larger in the LTT specimen (5.9 mm2 ) than it is in the

G3Si1 specimen (4.9 mm2 ). The results are similar when regions of underfill and lack of

fusion are excluded from the analysis. A further check on the credibility of the thermography

results is obtained by measuring the length of the weld pool ripple for each sample, Figure 6.

The length of the ripple for the LTT sample is 2.3 mm, whereas for the G3Si1 sample it is 1.7

mm. This suggests that the length of the weld pool in the welding direction is greater in the

LTT sample, which is consistent with the shapes of the isotherms obtained by thermography.

4 Conclusions

Infrared thermography is applied to measure the temperature fields in the vicinity of a laser

weld for two cases, with one involving a filler metal that normally produces a ferritic-bainitic

structure at room temperature, while the other involves a low-transformation-temperature

(LTT) filler material. Noticeable differences are observed in the quasi-stationary temperature

fields associated with each filler material, despite the same welding parameters being used in

each case. The observed differences in the temperature fields are consistent with observations

of the extent of the heat-affected zone and of the shape of the weld pool, as observed from the

ripples on the weld bead surface in each sample. The ability to measure welding temperature

fields in great detail, and in real time, will be of great value to the development of residual

stress mitigation techniques such as partial metallurgical injection, which requires careful

monitoring and control of welding temperature fields.

Acknowledgements

The presented investigations were carried out at RWTH Aachen University within the

framework of the Collaborative Research Centre SFB1120 “Bauteilpräzision durch

Beherrschung von Schmelze und Erstarrung in Produktionsprozessen” and funded by the

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Deutsche Forschungsgemeinschaft e.V. (DFG). The sponsorship and support is gratefully

acknowledged.

References

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2008, 3598-3612.

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Technology, 23 (9), 2007, 1009-1020.

[5] A. OHTA, N. SUZUKI, Y. MAEDA, K. HIRAOKA and T. NAKAMURA:

International Journal of Fatigue, 21, 1999, S113-S118.

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Materials Science and Technology, 18 (6), 2002, 527-531.

[7] H. Murakawa, M. Beres, C. M. Davies, S. Rashed, A. Vega, M. Tsunori,

K.M.Nikbin and D. Dye: Science and Technology of Welding and Joining, 15, 2010,

393-399.

[8] H.K.D.H. BHADESHIA: Materials Science and Engineering A, 378 (1-2), 2004,

34-39.

[9] R. J. Moat, H.J. Stone, A.A. Shirzadi, J.A. Francis, S. Kundu, A.F. Mark,

H.K.D.H. Bhadeshia, L. Karlsson and P.J. Withers: Science and Technology of

Welding and Joining, 16 (3), 2011, 279-284.

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[11] Y. Mikami, Masahito. Mochizuki, Masao. Toyoda, Yasushi. Morikage. Welding

International. 2007, (21(8)), pp. 547-560. DOI: 10.1080/09507110701637833

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[12] U. Reisgen, S. Olschok, S. Gach, Mat.-wiss. u. Werkstofftech. 2016, 47, 589-599.

[13] W. Steven and A.G. Haynes, J. Iron Steel Inst., London. 1956, 349.

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Figure 1. Schematic representation of the weld configuration, showing a cross-section of the

plate with a 1.6 mm diameter filler wire lying in a slot, which has a square cross-section with

the same nominal dimension. All dimensions are in millimetres.

Figure 2. Schematic representation of the experimental set-up, showing the weld test piece

(bottom), placed on the sample manipulator, the laser optics (top) and the infrared camera (on

the left).

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Figure 3. Macrographs for the welded samples made with the low-transformation-

temperature filler wire and the G3Si1 filler wire (left), and the results of hardness testing at

intervals of 0.2 mm (right). Note that the weld groove is not completely filled.

Figure 4. Results of analysis, by energy dispersive X-ray spectroscopy, across the fusion zone

of the specimen welded with the LTT filler wire. The iron concentration decreases at the

fusion boundary in favour of the LTT-alloying elements, nickel and chromium.

Figure 5. Thermographic images captured at the same stage of the welding process, when

welding with a G3Si1 filler material (left) and with an LTT filler material (center). On the

right a specimen welded with the G3Si1 filler wire is shown. Scribe lines are marked on the

surface at intervals of 5 mm to enable the locations of isotherms to be compared. Equivalent

isotherms span a greater area when an LTT filler material is used.

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Figure 6. Measurement of the length of weld pool ripples in the welding direction. A greater

weld ripple length is considered to indicate that the molten weld pool is longer, when

measured in the welding direction.

Table 1. Chemical composition of base steel and parent materials (wt.-%)

Element C Si Mn Cr Ni Mo

Base plate 0.076 0.05 0.95 0.39 0.03 0.01

G3Si1 0.10 0.85 1.50 - - -

LTT filler 0.05 0.35 0.93 10.1 10.9 0.25

Table 2. Process parameters for the welding experiments

Laser Power

[kW]

Travel Speed

[m/min]

Oscillation

Waveform

Amplitude

[mm]

Frequency

[Hz]

Focal Position

[mm]

1.5 0.9 Triangle 1.4 450 0

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