Development of Vertical Welding Technology for
Heavy-Thick and Ni Steel Plate
Using Hot-Wire Laser Welding Method（ホットワイヤ・レーザ溶接法による厚鋼板およびニッケル鋼
立向き溶接技術の開発）

November, 2019

Sittisak Charunetratsamee
Department of Mechanical Science and Engineering
Graduate School of Engineering
Hiroshima University

Table of Contents
Chapter 1

Introduction……………………………………………………………

1.1 Research background………………………………………………………...
1.1.1 Background of the shipbuilding industry…………………..………..
1.1.2 Background of LNG-fueled ship……………………………………..
1.2 Objectives and construction of the thesis…………………………………..
Chapter 2

1
1
1
3
5

Literature review………………………………………………………..

8

2.1 Introduction...…………………………………………………………………..
2.2 Brief information of single-pass vertical welding process…………………
2.3 Development of steel plate for shipbuilding and LNG fuel tanks………...
2.3.1 High strength and high HAZ toughness steel for large heat input
2.3.2 Evolution of Ni steel plate for LNG tanks…………………………...
2.4 Recent tends of vertical welding processes………………………………..
2.4.1 Shipbuilding……………………………………………………………
2.4.2 LNG tanks……………………………………………………………...
2.5 Laser beam welding and hot wire method………………………………….
2.6 A novel single-pass vertical welding process using hot-wire laser………

8
8
10
10
12
14
14
16
17
19

Chapter 3

A study of welding condition effects of single-pass verticalwelding using hot-wire laser method………………………………...

3.1 Introduction…………………………………………………………………….
3.2 Methodology and welding conditions of single-pass vertical welding
using hot-wire laser method………………………………………………….
3.2.1 Introduction of a joint configuration and equipment of
the novel welding process in the small size jig and specimen..….
3.2.2 Materials used…………………………………………………………
3.2.3 Methodology for investigation of effect of laser power density
and laser energy distribution on welding phenomena…………….
3.3 Optimization of laser-irradiating conditions on welding phenomena andjoint creation……..……………………………………………………….……
3.3.1 Effects of a laser spot size with weaving system………………….
3.3.1.1 Molten-pool temperature distribution during welding andcross-sections…………………………………………………………
3.3.2 High-edged energy density of the rectangular laser beam……….
3.3.2.1 Effects of high-edged energy density of
the rectangular laser beam………………………………...
3.4 Effects of oxygen additions to argon shielding gas………………………..
3.5 Summary……………………………………………………………………….

i

20
20
21
21
26
27
28
29
33
36
38
40
44

Chapter 4

Optimal parameters of the novel welding process in application of
the large size specimen……………………………………………….

4.1 Introduction…………………………………………………………………….
4.2 Introduction of the joint configuration and equipment of the novel
welding process in the large size jig…………………………….…………..
4.2.1 In-situ observation…………...………………………………………..
4.2.2 Materials used…………………………………………………………
4.3 Welding performing of large size specimen and a weld beadappearance…………………………..………………………………………..
4.4 Joint properties of 1-pass vertical welding using hot-wire laser method
4.4.1 Hardness distribution…………………………………………………
4.4.2 Microstructure characteristics of the welded joint…………………
4.4.2.1 Comparison of the welded joints of electrogas arc
welding and hot-wire laser welding…………………………………
4.4.3 Tensile test of the welded joint………………………………………
4.4.4 Charpy V-notch impact test of the welded joint……………………
5.4.5 Crack tip opening displacement (CTOD) test of the welded joint
4.5 Summary……………………………………………………………………….
Chapter 5

Development of vertical welding for heavy-thick steel plateusing twin diode lasers and hot wire…………………………………

5.1 Introduction…………………………………………………………………….
5.2 Optimization of advanced laser beam control of twin diode lasers
for vertical welding…………………………………………………………….
5.2.1 Investigating power distribution on rectangular laser beams
for 50 mm steel plate………………………………………………….
5.2.2 Introduction of a joint configuration and equipment of theproposed process using twin diode lasers for a heavy-thick joint
5.3 Effects of power distribution on a heavy-thick welded joint using
twin diode lasers and hot wire method……………………………………...
5.3.1 Materials used…………………………………………………………
5.3.2 Experimental procedure……………………………………………...
5.3.3 Welding performing of twin diode lasers for a heavy-thick joint
and weld bead appearance…………………………………………..
5.3.3.1 In-situ observation…………………………………………..
5.3.3.2 Weld bead appearances…………………………………
5.3.4 Cross-sections and joint properties of heavy-thick specimens
using the proposed method………………………………………….
5.3.4.1 Macro cross-sections……………………………………….
5.3.4.2 Microstructure characteristics……………………………..
5.3.4.3 Vickers Hardness distribution……………………………..
5.3.4.4 V-notch Charpy impact test………………………………..
5.4 Summary……………………………………………………………………….
ii

45
45
46
47
48
49
53
54
55
57
58
59
63
65

66
66
67
67
68
69
70
71
73
74
75
76
76
78
79
80
84

Chapter 6

The application of single-pass vertical welding for Ni steel
using hot-wire laser welding…………………………………………..

6.1 Introduction…………………………………………………………………….
6.2 Optimization of welding conditions for Ni steel plate using the proposed
welding process……………………………………………………………….
6.2.1 Introduction of the joint configuration and welding conditions of
the novel welding process for Ni steel plate………………………..
6.2.1.1 In-situ observation…………………………………………..
6.2.2 Materials used…………………………………………………………
6.2.3 Experimental procedure…………………………………...…………
6.2.4 Welding performing of Ni steel specimens andweld bead appearances………………………………………………
6.2.5 In-situ observation on the molten pool by a high-speed camera...
6.2.6 Evaluation of macrograph and heat input of welded joints……….
6.3 The properties of 7%Ni steel welded joints using the proposed process
6.3.1 Microstructure characteristics of the welded joints………………..
6.3.2 Hardness distribution of the welded joints………………………….
6.3.3 Tensile test of the welded joints……………………………………..
6.3.4 Charpy V-notch impact test of the welded joints…………………..
6.4 Summary……………………………………………………………………….
Chapter 7

85
85
86
86
87
88
89
92
95
97
99
99
101
102
107
112

Summary and future work…………………………………………….. 113

Acknowledgements…………………………………………………………………..
Reference……………………………………………………………………………….
Published or summited papers in regard to this thesis….………………….…
Presentations…………………………………………………………………………..

iii

116
117
127
128

Chapter 1
Introduction

Chapter 1
Introduction
1.1 Research background1.1.1 Background of the shipbuilding industry
In order to increase the efficiency of maritime transport, a container ship
capacity has dramatically increased over the past decades to minimize thetransportation cost. The transition of the maximum loading capacity of container
ships is shown in Fig. 1.1. The container size is rising from around 800 TEU(Twenty-foot equivalent unit: a 20-foot-long (6.1m) container) in the 60S, andcontainer ships up to 20000 TEU has been constructed [1-2].
Container ships that carry their entire load inside the hull have the hatch coveron the deck and a large opening in the upper deck as shown in Fig. 1.2, and whilesailing on the sea, the ship is subjected to heavy rolling and pitching which cannot
only disturb the cargo but also upset the stability of the ship. Therefore, it isnecessary to ensure the longitudinal strength of the hull against a load of waves that
bend the entire hull and the high strength deck structures including the hatch side
coaming, strength deck, sheer strake, and longitudinal bulkhead are applied.
The heavy thickness of large container ships’ structures is up to 100 mm andthose are as high as a few meters on the ship side shell so it is very difficult to weldin the vertical position with multi-pass welds. For an increase in productivity,
Electrogas Arc Welding (EGW), which is one-pass vertical welding, is an effective
method for vertical welding process with a high deposit rate [3]. However, since
EGW creates large heat input, the decrease in the toughness of coarse grain HAZ(CGHAZ) is a problem. On the other hand, the alloy design in a base metal and the
TMCP process has been also optimized to obtain proper impact toughness in
CGHAZ [4].
For instance, the occurrence and development of a brittle crack in a welded
joint due to lack of fracture toughness might lead to the huge scale accident, andYP40 steel has been used for the hatch side coaming in particular. The hatch side
coaming requires not only the fatigue resistance due to the pitching load but also thearrest performance to stop propagating the brittle crack. Moreover, as the hatch side
coaming is located at the uppermost part of the hull cross section, it is an importantmember also in terms of strength and regarding a recent increase in the size ofcontainer ships, the thickness has risen so the further large heat input welding for
thicker joints has been concerned. Therefore, YP47 steel was newly developed. It
achieved more both strength and toughness which greatly provided to reduce theheavy thickness of components [5-9].

1Chapter 1
Introduction

As described above, a novel vertical welding process is strongly demanded toachieve the higher efficiency and low heat input for obtaining welded joints withhigher strength and higher toughness.

Fig. 1.1 Transition of maximum loading capacity.

Fig. 1.2 Strength deck structure of a container ship.
2

Chapter 1
Introduction

1.1.2 Background of LNG-fueled ship
In recent years, the International Maritime Organization (IMO) has issued
particular emission regulations. Figure 1.3 shows the schedule of SOx emission
regulations. The SOx and particulate matter emission controls apply to all fuel oil,combustion equipment, and devices onboard. These fuel oil sulfur limits are
expressed in term of %m/m. Emission control areas (ECAs) established in Baltic
Sea, North Sea, North America, and United States Caribbean Sea have more limited
the emissions than outside ECA. Therefore, in order to comply with the regulations,the ships which is operated inside these ECA need to use a low sulfur fuel oil(Marine Gasoil: MGO) with a sulfur content of less than 0.1%. However, that issignificantly more expensive than heavy fuel oil and in early April 2016, the price ofmarine gasoil, for example, was more than double that of heavy fuel oil. In addition,
low sulfur fuel oil of Marine gasoil has extremely low viscosity for fuel pumps ofmarine engines designed for a minimum viscosity which leads to inadequatehydrodynamic lubrication causing wear and scuffing. In order to prevent problems
due to low viscosity, the installation of a fuel-oil cooler is newly required.
An exhaust gas cleaning system is an alternative method to remove sulfur
oxides from ship’s engine, but the problem is that the equipment itself is large which
is necessary to require new space, and the initial investment cost is very high. In
addition, regulations of NOx emissions have been limited in 2016, and the emission
standards, for example, is shown in Fig. 1.4 which have been applied in emission
control areas, and it is required to reduce NOx emissions by 80% compared toprimary regulations. In order to respect and follow the regulations, it not only
supports the engine, but also additional external equipment such as Selective
Catalytic Reduction (SCR) and Exhaust Gas Recirculation (EGR) [10-13].
Year

Sulfur Limit in Fuel, % m/m
IMO ECA

2005

1.5

2010

IMO Global
4.5

1.0

2012

3.5

2015

0.1

2020

0.5

Fig.1.3 Schedule of SOx emission regulations.
3

Chapter 1
Introduction

Tier

Year

Tier ⅠNOx Limit, g/kWhn<130130≤n<2000n≥20002000

1745・n-0.2

9.8

Tier Ⅱ

2011

14.4

44・n-0.23

7.7

Tier Ⅲ2016 (ECA)

3.4

9・n-0.2

1.96

Fig.1.4 Schedule of NOx emission standards.

As per the emission regulations above, liquefied natural gas (LNG) is analternative fuel to prove SOx and NOx emission standards. Using LNG as fuelalmost eliminates SOx and particle (PM) emissions completely, and reduces NOx
emission up to 85%. Therefore, the number of ships using LNG as fuel (excluding
LNG carriers) is increasing fast and more. However, because LNG is stored atcryogenic temperatures under -163 °C, the steel for storage tanks needs to havehigh fracture resistance and strength under low-temperature conditions. Aluminumalloys, austenitic stainless steel (SUS 304), and 9%Ni steel are listed as a material
that is resistant to low-temperature brittleness and applicable to LNG tanks. The
9%Ni steel is desirable due to its reasonable price compared with other materials.
There are three-type tanks, a square tank, cylindrical tank and cylindrical tank forcryogenically storing and offloading LNG. The IHI-SPB LNG tank is one of thesquare tanks as shown in Fig. 1.5 [14-16] that has a high capacity and is usually
made of aluminum alloys. On the other hand, the cylindrical tank is made of the lowcost material of 9%Ni steels. Thus, it is currently needed to develop the square tankusing 9%Ni steels.
For the vertical welded joints at the inner wall of the LNG tank, automatic TIG(Tungsten Inert Gas) welding and SMAW (Shielded Metal Arc Welding) are generally
used for 9%Ni steels since filler wires of Ni-base alloys are used to achieve the hightoughness and brittle crack arrestability. However, the automatic TIG welding and
SMAW employ multi-pass welds for a relatively large gap of the joint and long weld
4

Chapter 1
Introduction

length therefore that affect to a construction period and cost. Thus, in this research,
the novel single-pass vertical welding process using a hot-wire laser method is
studied to reduce the construction time and maintain joint properties. In addition, it isfeasibly considered to reduce the deposited of a Ni-based filler wire on the welded
joint by using a small groove area for the reduction of consumable cost.
Furthermore, although 9% Ni steels have been used for several decades as astructural material for the inner wall of LNG tanks, in recent years the steel
manufacturer has developed a new material of a 7% Ni steel as an alternative to the
9%-Ni steel to reduce the cost of additional Ni alloy. In order to maintain properties ofthe base metal and welded joints of the 7%Ni steel, its alloy composition has been
optimized and the latest TMCP (Thermal Mechanical Control Process) technology
has been applied. As a result, the 7%Ni steel has the same excellent performance
as 9%-Ni steel and it has been applied to the LNG tank. The Ni steel has been
applied only the above-ground type LNG tanks so far, however, the number of LNGfueled ships is growing up, in near future, therefore it is expected that the demand of
Ni steel application to LNG tanks for ships would also increase [17-18].

Fig. 1.5 IHI-SPB LNG tank
1.2 Objectives and construction of the thesis
A welding process plays an important role for ferrous or non-ferrous material
fabrication in many industries such as shipbuilding, oil and gas plants, power plantand so on. Since the weld joint can be the weak point as a result of a heat input
during welding, the advanced welding process, therefore, has been developed formaking it more reliable and also optimizing construction cost.
For the shipbuilding industry, a novel single-pass vertical welding process for a
thick steel plate using the combination of a diode laser as a heat source anda hot-wire system was proposed and developed in order to reduce a heat input and5Chapter 1
Introduction

maintain mechanical properties of the weld joint compared with a conventional
single-pass vertical welding as known EGW.
For the LNG fuel tank construction, the novel single-pass vertical welding
process was applied on Ni-steel plates to increase productivity and to reduce agroove area, which is a volume of a welding consumable of a Ni alloy. In addition,
the flexibility of the hot-wire laser welding process to the thickness variation wasinvestigated, and the weld joint properties are evaluatedThis thesis emphasizes the basic ideas and investigations of the proposed
process, and the optimization of welding conditions. Besides, the weld joint
properties are analyzed in many kinds of method and compared with the standard
requirement. Figure 1.6 shows a flow chart of the construction of the thesis.
Chapter 1 discusses the background of research in addition to the objectives
and thesis statements.
Chapter 2 reviews kinds of research and theoretical information that are
relevant to the high-performance structure of steel types, the vertical welding
process for a heavy-thick joint, a fillet welded joint, etc. Furthermore, an innovative
welding process of hot-wire laser method is discussed.
In Chapter 3, effects of a laser irradiating condition, laser energy distribution on
welding phenomena and joint creation were investigated. In addition, the effect of theoxygen content in shielding gas on welding phenomena and joint creation were alsostudied.
In Chapter 4, based on the results of the above investigations, the optimal
welding conditions were obtained in the chapter 3. In this chapter, the obtained
optimal conditions were applied to large size specimens. The hot-wire laser welding
system with a self-propelled base was developed in order to do the trial for the long
weld joint in a practical use. Finally, the properties of the welded joint were
investigated.
In Chapter 5, the principles and basic ideas to improve the laser beam control
using twin diode lasers for a 50-mm heavy thick joint were introduced. The advanced
laser beam control of twin diode lasers with a total power of 12 kW and a hot-wiresystem are used. The combination of double laser irradiating conditions waschanged and its effect was investigated on base metal melting and molten poolformation. Finally, the properties of the welded joint were investigated.
In Chapter 6, the basic ideas and a method of the hot-wire laser welding
process and the large size jig were applied to the 7%Ni steel plates with a long-weld
length. The flexibility of the process to the thickness variation was also investigated.
The welding conditions of a rectangular laser spot size, laser power, welding speed,etc. are optimized to achieve a sound welded joint. Then the welded joint
appearances, cross-sections and mechanical properties are evaluatedFinally, the conclusions of this thesis are summarized in chapter 7.
6

Chapter 1
Introduction

Chapter 1: Introduction and research objectives

Chapter 2: Literature review
Chapter 3: A study of welding condition effects of single-passvertical welding using hot-wire laser method•

Introduction of the novel welding process and its methodology•

Investigations of laser-irradiating conditions on the groove area using a
rectangular spot beam, high-edged energy type and a weaving system•

Elucidation of oxygen-added Ar shielding gas effects on the weld joint

Chapter 4: Optimal parameters of the novel welding process inapplication of the large size specimen•

Introduction of the large size jig for the novel welding process•

To apply optimal parameters in large size specimen in 28.5-mm
thickness•

Joint properties of 1-pass vertical welding using hot-wire laser method

Chapter 5: Development of vertical welding for heavy-thick steel plateusing twin diode lasers and hot wire•

Optimization of advanced laser beam control of twin diode lasers for
50-mm thickness•

Effects of power distribution on a heavy-thick joint•

Cross-sections and joint properties of heavy-thick specimens

Chapter 6: The application of single-pass vertical welding for Ni steel
plate using hot-wire laser welding•

Optimization of welding conditions for Ni steel plates with thickness of
10,15, 20mm•

Evaluation of macrograph and heat input of the welded joints•

The properties of 7%Ni steel welded joints using the proposed process

Chapter 7: Summary and future work
Fig. 1.6 Construction of the thesis.

7Chapter 2
Literature review

Chapter 2
Literature review
2.1 Introduction
This chapter discusses theoretical backgrounds and literature review related tothis research. It is composed mainly of single-pass vertical welding technology,
development of steel plate and recent trends of vertical welding processes forshipbuilding and LNG tanks, hot-wire laser welding, and latest our research of anovel welding process.
2.2 Brief information of single-pass vertical welding process
Electroslag welding (ESW) and Eectrogas welding (EGW) are currently applied
to thick-section welded joints in a vertical or near-vertical position between retaining
water-cooled copper shoes. Figure 2.1 shows the schematic comparing of two
vertical welding processes. ESW and EGW involve high energy input relative toother welding processes. Those of welding processes are similar to in situ castingwith large volumes of molten pool and heat content.
ESW produces coalescence with molten slag with melts the filler wire and thesurface of base metal. The weld pool is shielded with the molten slag that moves
along with the groove vertically. An electric arc is used to create an initial molten pooland when a molten bath is achieved, the arc is extinguished and during welding, flux
is added continuously to maintain an adequate slag coving over the pool. In case of
low-carbon steel, the temperature of the weld pool is reported to be in the vicinity of
1925oC, while the temperature of the surface is about 1650oC. Figure 2.2 illustrates
that the cooling rate of ESW is very low relative to other arc welding process [19-21].

Fig. 2.1 Schematic comparing of two vertical welding processes (a) electroslag
welding (b) eletrogas welding.
8

Chapter 2
Literature review

Figure 2.3 shows the macrograph of HAZ in an electroslag weldment. As a result of
the extremely slow solidification, it results in a coarse primary solidification structure
and also a very large HAZ [22].
EGW was introduced in the early 1960s [23] to perform single-pass verticalwelding that can be classified as a gas-metal arc welding process. The process isvery similar to ESW except the presence of an arc and the weld pool shielding
mechanism. In most cases, a shielding gas is used together with either a bare or
flux-cored electrode to provide a shielding gas covering the molten pool. However,self-shield flux-cored wires are often used without separate shielding gas. Moreover,
EGW is generally superior to ESW having higher deposition rate that presents thelower heat input, more refined microstructure and improved HAZ properties [21].

Fig. 2.2 Typical thermal cycle of an electroslag weld relative to that of an arc weld.

Fig. 2.3 Macrograph of weld metal and HAZ in an electroslag weldment.
9Chapter 2
Literature review

2.3 Development of steel plate for shipbuilding and LNG fuel tanks
2.3.1 High strength and high HAZ toughness steel for large heat input
As a consequence of the rapid increase in the capacity of container ships, thelarger container ship strongly demands the higher strength and thicker plate rather
than the exists steels in order to carry as many containers as possible. An increasein the thickness, the toughness of welded joints tends to decrease due to larger heat
input. According to this situation, Mitsubishi Heavy Industries, Ltd. (MHI) together
with Nippon Steel Corporation developed steel plate with a yield strength of 47
kgf/mm2 (460N/mm2) known as YP47 steel, which enlarges approximately 20% incomparison with conventional steel plates for ship hull. This steel possesses both
high strength and high toughness. In addition, the reduced plate thickness of highstrength material can decrease the weight in the same size as the container ship.
The first large container ship in the world using this grade was built to Nippon Kaiji
Kyokai also known as ClassNK and delivered in June 2007 [24-27]. Figure 2.4
shows the cross-section and steel grades for hull structure of container ships. In
case container ship size increase from 6000 TEU to 8000 TEU, the EH40 (yieldstrength ≥ 390 N/mm2) and for larger than 13000 TEU container ship, EH47 is
expected to be used [28-29].

Fig. 2.4 Cross-section and steel grades for hull structure of container ships
In the meantime, the steels have been developed to be welded under thegeneral condition of large heat input welding process of the single-pass vertical joint
such as EGW. Steelmakers have improved the strength and toughness property byadding chemical elements and controlling rolling and cooling processes. The
Thermal-Mechanical Control Process (TMCP) was applied to control rolling for the
first time in the 1960s [30] and in 1980 NKK Corporation (now JFE steel) succeeded
in applying an on-line accelerated cooling process for plates called OLAC [31-32]. At
Nippon Steel, TMCP is called the Continuous Online Control (CLC) process [33].
TMCP is the most efficient method to improve mechanical properties throughmicrostructure control. Figure 2.5 [34] shows the schematic illustration on TMCP andmicrostructural change in the process.
10Chapter 2
Literature review

Fig. 2.5 Schematic illustration on TMCP and microstructural change in the process.
For HAZ toughness control technology, the most important point in HAZ
toughness improvement is how to obtain as fine a metallographic structure of HAZ
as possible. Figure 2.6 shows the progress of HAZ microstructure improvement.
Nippon Steel Corporation has succeeded in developing a new technology for HAZ
fine grain steel, which is called Super High HAZ Toughness Technology with Fine
Microstructure Imparted by Fine Particles (HTUFF). As shown in Fig. 2.7, Ɣ grains in
HAZ of HTUFF steel are almost prevented from growing even under temperatures ashigh as 1,400 0C [35-36]. However, TiN steel has been also developed throughcontrolling TiN particles, refining the microstructure of HAZ by using B, Ca andfurthermore, dispersion of martensite-austenite (MA) is controlled as shown in Fig.
2.8 [37-39]. In addition, the developed YP40 and YP47 steels have been studied in
the general high heat input condition to confirm the mechanical properties of welded
joints [40-45].

Fig. 2.6 Progress of HAZ microstructure improvement through HTUFF
11

Chapter 2
Literature review

Fig. 2.7 Comparison of Ɣ grain growth between conventional TiN steel and HTUFF

(b)

(a)

Fig. 2.8 Typical studies for development of TiN steel (a) Effects of Ca content insteels on coarse and fine TiN particles (b) Controlling MA in HAZ by reducing Si.
2.3.2 Evolution of Ni steel plate for LNG tanks
For a reasonable price, 9%Ni steel has been recognized by ASME Code forcryogenic use since 1954 and been widely used around the world as a material forthe inner walls of liquefied natural gas (LNG) tanks in competition with stainlesssteel, other austenitic alloys and aluminum [46]. As shown in Fig.2.9, 9%Nickel
steels are generally heat treated by quenching and tempering processes for highstrength steel [47-48]. Then, 9%Ni steel has been developed in heavy plates up to
50 mm thick plate [49-50]. Properties of both base metal and weld metal of 9%Ni
steel weld joint are investigated in several heat treatment conditions [51-54].

12

Chapter 2
Literature review

Fig. 2.9 Production process of quenched and tempered 9% Nickel steel.
The 7%Ni-TMCP steel plate for LNG tank has been newly developed by Nippon
Steel & Sumitomo metal that applies a Thermal-Mechanical Control Process (TMCP)
to obtain the retain austenite and refine microstructure to reinforce the inferiority of
2%Nickel as shown in Fig. 2.10. That means over 20% Nickel reduction in the nickel
price and saving natural resources [55-56].

Fig. 2.10 Target for development.
13

Chapter 2
Literature review

In addition, Figure 2.11 shows the production process of 9%Ni steel and the
new steel for an LNG tank and its microstructure. It is different from the main processof 9%Ni steel of Reheat-Quenching-Tempering (RQ-T), but the new steel of 7%Ni is
produced by the production process of Thermal Mechanical Control ProcessLamellarizing-Tempering (TMCP-L-T). Fine martensitic microstructure of the 7%Ni
steel is achieved when controlled rolling and direct-quenching in the accelerated
cooling process are applied. 7%Ni-TMCP steel is excellent in brittle crack initiation
suppressing and propagation arresting properties. And all properties are confirmed
equivalent to 9%Ni steel. Furthermore, the 7%Ni-TMCP has been registered for therange of Ni composition 6.0-7.5% in JIS as SL7N590 in JIS G3127, in ASTM and
ASME as Grade G of A841 since 2013 [57-60].

Water
Cooling

Fig. 2.11 Production process and microstructure.
2.4 Recent tends of vertical welding processes
2.4.1 Shipbuilding
In the 1970s, a single-electrode VEGA (Vibratory Electrode-gas Arc Welding)
process was developed by Nippon steel welding production & engineering company
limited to reduce heat input and to improve impact property of the welds. This
process with an oscillating electrode is able to weld in a narrower groove, so thewelding current is reduced due to a smaller diameter of filer wire used. As shown in
Fig. 2.12, in the same thickness of 19 mm the weld of single-electrode VEGA shows
narrow weld bead and heat affected zone. In addition, the much lower heat input canbe achieved, and lack of fusion is not observed in a thickness up to 50 mm [61].

14Chapter 2
Literature review

Fig. 2.12 Comparison of cross-sections of welds by conventional EGW and Singleelectrode VEGA(a) Two electrodes(b) Four electrodes

Fig. 2.13 Schematic layout of VEGA welding process (a) Two electrodes (b) Fourelectrodes

15

Chapter 2
Literature review

However, a trend of shipbuilding is growing in size and thickness. Steel plateswith a thickness of 50 mm or more have been used. Therefore, a two-electrode
VEGA welding process is developed as shown in Fig. 2.13(a) and that is applied tothick plates for shipbuilding applicable to weld on the maximum thickness 80 – 90
mm. Figure 2.14(a) presents sample cross-sections of welds on the steel plates
having 50, 60 and 70 mm. There are no imperfections near the front surface and theback surface of the plate often associated with it. Figure 2.13(b) shows the newly
developed four-electrode VGEA welding process for thicker plates in the range 100 –200 mm. As shown in Fig. 2.14 (b), the macrostructures of weld joints 100 mm and
200 mm even in one-pass welding of a 200-mm-thick plate obtain good troughthickness penetration without any defects [61-62].

(a) Two-electrode VEGA(b) Four-electrode VEGA

Fig. 2.14 Macrostructure of welds with (a) two-electrode and (b) four-electrode VEGA
welding process.
2.4.2 LNG tanks
Regarding low-temperature material, it is used to store liquefied natural gas atunder -163 oC. The toughness property of the weld joint should be considered toensure the safety of LNG storage tanks. The various welding processes such as
tungsten inert gas (TIG), flux cored arc welding (FCAW) and gas metal arc welding(GMAW) affect the fracture toughness in weldments differently [63]. For verticalwelded joint, shielded metal arc welding (SMAW) has mainly been applied in LNG
tanks made of 9%Ni steel overseas, but automatic TIG welding has been used
increasingly in Japan since the 1980s and then remote controlled automatic TIG
welding equipment and a high-efficiency automatic TIG welding method (SEDARTIG) have been developed by IHI engineering [64-65] to use in practice. However, inthe oversea field, SMAW is still mainly used for vertical joint due to the limitation ofequipment available to the subcontractor. In order to increase deposition rate, IHI
developed vertical Submerged Arc Welding (vertical SAW) which has been limited toflat and horizontal positions so far. Figure 2.15 shows the schematic illustration ofvertical SAW. The vertical SAW uses a smaller wire of 1.2 mm in diameter, so thearc current and heat input become lower than a conventional SAW. The crosssection is shown in Fig. 2.16 and no imperfections can be observed [66].

16Chapter 2
Literature review

Fig. 2.15 Schematic illustration of vertical SAW.

Fig. 2.16 Macrostructure and pass sequence.
2.5 Laser beam welding and hot wire method
Laser beam welding (LBW) is a welding process using a laser to joint pieces ofmetal or thermoplastic. The LBW allows extremely concentrating power density oflaser spot which results in narrow and deep weld bead, small HAZ and high-speed
welding that is a low heat input welding process. Moreover, laser keyhole welding
having high laser power requires a very high power density to create deep
penetration welds for welding thick plates in 1 pass [67-70]. However, LBW with theinclined welding positions of horizontal and vertical direction can lead to more
defects of pores and undercuts which are difficult to achieve a sound weld joint
because of action of gravity [71-73].
17

Chapter 2
Literature review

A Hot-wire system is an electrically heated filler wire up to its melting point asshown in Fig. 2.17 [74]. A hot-wire power source supplies electricity from the contact
tip to the workpiece through wire feeding during welding by Joule heating. Figure
2.18 illustrates temperature distributions on filler wire. Wire current of 133A obtains ahigher temperature than the current of 121A [75]. The hot wire is used as deposited
weld metal which is able to increase the deposition rate. In order to increase welding
and reduce heat input, it is therefore investigated welding phenomena and combinedwith conventional welding processes such as GTAW [75-77].

Fig. 2.17 Schematic layout of a hot-wire system

Fig. 2.18 Effect of wire current on temperature distributions of filler wire.

18

Chapter 2
Literature review

Hot-wire laser welding (HLW) is the novel welding process by combination alaser as a main heat source and the hot-wire system to preheat a filler wire during
welding. This process can improve a deposition rate as well as reduce laser powerand heat input. The application of this process has been investigated in many kindsof fields, for instance, the lap joint of the high tensile/ultra-high tensile strength steelswith a wide gap for an automobile body [78], fillet welded joints of medium-thick platefor shipbuilding and bridge [79-80], and a narrow-gap joint to reduce large shrinkage
and residual stress. The reflection of laser irradiation is also investigated that is aparticular characteristic of laser beam welding. Figure 2.19 shows a high-speed
image and a cross-section of the laser beam reflection effect [81-82].

MoltenregionFig. 2.19 Effect of laser beam reflection welded by a laser with a hot-wire feeding.
2.6 A novel single-pass vertical welding process using hot-wire laser
In previous studies, the conceptual idea of the new single-pass vertical welding
method is proposed to reduce the heat input and laser power as shown in Fig. 2.20.
Preliminary investigations in welding phenomena by combining a diode laser with alarge rectangular beam spot and hot-wire feeding system were performed on steel
plates. In addition, the effects of welding conditions such as laser power, theweaving of laser beam spot, gap width, laser irradiating angle, etc. were investigated[83-86].

Fig. 2.20 Schematic illustration of single-pass vertical welding using hot-wire laser.
19

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Chapter 3
A study of welding condition effects of single-pass verticalwelding using hot-wire laser method
3.1 Introduction
Previous studies of single-pass vertical welding using a hot-wire laser method
have presented the conceptual idea and preliminary investigations in welding
phenomena of the combination of the diode laser as a heat source and the hot-wiresystem to a large tolerance gap. There are many conditions varied for making thissystem steady such as laser power, gap width, weaving system, laser beam angle,
the number of hot-wire feedings and so on.
In this chapter, the methodology introduction of a joint configuration andequipment, and optimization on specific welding conditions of the proposed welding
process are presented. A small size jig used for a small specimen is described aconception of a welding operating mechanism to combine laser irradiating and hotwire feeding for a vertical welding process, and it is applied for the investigation of
welding phenomena.
Effects of power density of beams, laser energy distribution and the rectangularlaser spot with high-edged energy density are studied on welding phenomena andjoint creation to optimize laser power and heat input. The laser spot size and its
motion by a weaving system strongly affect the optimization of laser power
distribution and melting phenomena. The effects of oxygen additions in Ar shielding
gas are also investigated on welding phenomena and weld bead creation.

20

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

3.2 Methodology and welding conditions of single-pass vertical welding using
hot-wire laser method
In this chapter, the effects of welding conditions on welding phenomena andjoint creations were investigated in order to verify and optimize a novel welding
process of single-pass vertical welding which is combined with a diode laser
oscillator using a relatively large rectangular beam and single hot-wire system into
the small size jig. The laser power density and laser energy distribution wasconsidered first on a thick specimen with a relatively large gap of 10 mm to achieve
the steady welding phenomena and the sound joint creation of the proposed
process. Therefore, this section describes the basics and apparatuses of hot-wirelaser single-pass vertical welding and emphasizes optimizing the particular welding
parameters.
3.2.1 Introduction of a joint configuration and equipment of the novel welding
process using in the small size jig and specimen
Firstly, the basics of the hot-wire laser single-pass vertical welding and theexperimental set-up of the small size jig and specimen are presented schematically.
Secondly, the materials used in this chapter are shown and then the basic idea aswell as the investigation methodology of melting phenomena in the proposed
process is introduced.

Specimen(a)(b)

Fig. 3.1 Schematic layout of the small size jig and specimen of the proposed process(a) a joint configuration and overall equipment (b) the top view of large rectangularlaser beam irradiation through on a weld joint.

21

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Figure 3.1(a) shows the schematic layout of a diode laser beam, a hot wire, aspecimen and the apparatus of a small size jig. A specimen was set between copper
shoes and moved towards the opposite side in the vertical welding direction by auniaxial electric actuator. The water-cooled copper shoe was used for backing
material. Its design was to keep a weld pool shape of both wire feeding side andback side. The copper shoe had slots for inserting a hot-wire torch, shielding gas,and cooling water. An electrically-heated filler wire up to its melting point as called
hot-wire was fed into the molten pool. A diode laser used as a heat source combinedwith focusing and homogenizing lenses to create an adjustable rectangular shapeand the spot size according to the thickness and gap width of the used plate. A highspeed camera was used to monitor weld pool formation, welding stability, and hotwire feeding during welding. Figure 3.1(b) shows the schematic layout on the topview of the weld groove. The large rectangular laser spot having the length fitting thespecimen thickness was placed along with the groove surface. The width of therectangular laser spot was changed up to the 10-mm gap to vary the laser energy
density. The weaving system was used to sweep the rectangular laser spot in theparallel direction to the gap width when the narrower laser spot width was employed.

Fig. 3.2 Laser head and weaving system for rectangular laser spot.
Figure 3.2 illustrates the constituent of a laser head to create a rectangularlaser beam which is combined with a weaving system. A fiber core leads the laser
beam to a collimator lens in order to create a straight and large size laser spot, thenthe laser beam is conveyed to a homogenizing and focusing lenses which changes
the shape of a laser beam from circle to rectangle. Thereafter the focal length of therectangular laser beam depends on the focusing lens closed with homogenizer.
Moreover, a weaving head was employed to move the beam on a groove area in thisstudy. The laser beam incident on a weaving blade is reflected perfectly and theweaving blade is swept by a harmonic motor. The weaving motion can be controlled

22Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser methodby a controller unit to adjust a weaving waveform, amplitude and frequency. Further
details of the weaving motion are discussed in 3.2.3.1.
Table 3.1 shows the length and width of rectangular laser spots created by the
combination of optical lenses and a fiber core diameter. The combination of the fibercore diameter, collimator lens and focusing lens decides the rectangular beam width
and the beam length can be adjusted by changing the combination of ahomogenizing and focusing lenses. For instance, the combination of a 1-mm fibercore, 100-mm collimator, LL2.35 homogenizer and 400-mm focusing lens creates
the rectangular laser beam having the 26-mm length and 4-mm width at a just focus.
In this study, the laser beam length was adjusted to the base metal thickness.
Table 3.1 Rectangular beam shapes with combinations of optical lenses andfiber core.

Fig. 3.3 Diode laser oscillator.
23

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

The continuous wave (CW) diode laser was used in this research. The laser
oscillator as a heat source is shown in Fig. 3.3. The laser oscillator is Laserline LDF
6000-40 with a limited laser power of 6 kW. Four wavelengths of 910, 940, 980 and
1030 nm are employed to irradiate laser power as a high-power heat source.

(a)

(b)

(c)

(d)

Fig. 3.4 Hot-wire equipment (a) Hot-wire power source, (b) Hot-wire torch, (c)Hot-wire setup parameters and (d) Remote controller.
Hot-wire equipment is shown in Fig. 3.4. A hot-wire power source in Fig. 3.4(a),which is Power Assist HI-TIG IV662, supplies a pulse current with its maximum
average current of 165 A. A positive pole is usually connected to a hot-wire torch as
shown in Fig. 3.4(b) and (c). Essential parameters for hot-wire setting are a wire feed
speed (m/min), energized-heated distance (mm), a wire current (A), a wire feedingangle (), and a wire feeding position (mm). All of the essential parameters are drawn
as the schematic illustration in Fig. 3.4(c). The wire feeding speed is adjusted as adepending on a groove size and the welding speed. In the practical use of the
proposed process, it was set at of 1.2-times large value compared to the calculated
value. The energized-heated distance affects the filler wire temperature by Joule
heating and the optimum wire current. The wire feed position and the wire feed angle
24Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

slightly affect wire melting. The wire current that should be adjusted adequately
depending on the feeding speed, a wire diameter, wire material and laser energy on
a wire tip. In this study, the setting of the wire feeding speed and wire current were
adjusted by the hot-wire remote controller as shown in Fig. 3.4(d).

Fig. 3.5 High-speed camera equipment.
Figure 3.5 shows the high-speed camera equipment for an observation during
welding. The camera of Pencil camera V-193-M1 model was used connected with
NAC: MEMRECAM GX-5 module. The high-speed camera was provided to monitor
molten pool creation and hot-wire feeding during welding operation in order to makeclear the welding mechanism.

Fig. 3.6 Shielding gas mixer.
Figure 3.6 shows a shielding gas mixer. In order to mix oxygen (O2) with argon
shielding gas (Ar) for the investigation of the O2 effect on joint creation, a shield gas
mixer (Log MIX, manufactured by Front Corp.) was used. This mixer has a maximum
25

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

flow rate of 10 l/min for Ar and 2 l/min for O2. The flow rates of each shielding gas
condition can be changed by varying argon and oxygen flow rates concurrently.
3.2.2 Materials used
KE-47 steel plate was used in this study. It must be noted that material of YP47
steel is equivalent to KE47 steel. It hereby is called KE47 steel as the base metal.
The dimensions of a small size specimen are 100 (width) x 200 (height) x 26 mm(thickness) as shown in Fig. 3.7. Two specimens were fixed and aligned as a verticaljoint configuration. The square groove was set with a 10-mm gap using a bottom
spacer having dimensions of 10 (width) x 20 (height) x 28.5 mm (depth).
The filler metals of YM-60A (JISZ3312G59JA1UM3M1T) and YM-80A(JISZ3312G78A4UMN5C1M3T) with a diameter of 1.6 mm were used. Those filler
wires are a solid wire for MAG (Metal Active Gas) having 590 MPa-class and 780
MPa-class tensile strength respectively. The chemical compositions of the basemetal and filler metals are presented in Table 3.2.
Table 3.2 Chemical compositions of base metal and filler wire.
Materials

Chemical Composition, wt%
C

Si

Mn

P

S

Al

Cu

Ni

Nb

Ti

Cr

Mo

KE-47

0.09 0.07 1.52 0.007 0.002 0.014 0.32 0.69 0.01 0.01 0.02 0.00

YM-60A

0.06 0.35 1.45 0.010 0.007

-

0.20

YM-80A

0.06 0.40 1.69 0.006 0.003-

0.20 3.01

-

-

-

-

0.05 0.46 0.29

Fig. 3.7 Specimen dimensions used to optimize welding conditions.
26

0.38

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

3.2.3 Methodology for investigation of effect of laser power density and laser
energy distribution on welding phenomena
The principal factor of the proposed welding process to get a sound joint isadequate laser power density in order to keep the molten pool and to melt the basemetal as shown in Fig. 3.8. As the main heat source is only laser power, therectangular beam shape should be controlled. As shown in Fig. 3.9, when the
proposed process is applied to a thick-joint and a large-size laser beam is used to fit
a groove area, the average power density becomes low and spreads the wholegroove stably. It is good at keeping the molten pool but poorly melting the basemetal. In this case, the welded joint needs more laser power to melt the base metal;
however, the increase in a laser power gives higher initial cost and heat input. In the
proposed process, the narrower laser spot than a gap is employed and combinedwith a weaving system to control laser beam motion throughout the gap. Thismethod can increase the heating energy around the groove surfaces and thus canimprove base metal melting, but the heating energy on a middle of molten pool isdecreased. Since the hot-wire system does not need much laser power to melt itself,
the heat energy from the laser irradiation on the middle of the molten pool can be
optimized. Consequently, the novel welding process is developed to reduce laser
power consumption and to achieve a low heat input process.

Fig. 3.8 Basic of welding phenomena.

Fig. 3.9 Effects of laser beam size on welding phenomena.
27

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

3.3 Optimization of laser-irradiating conditions on welding phenomena andjoint creation
The weaving motion is the movement of a narrow rectangular laser beamthroughout a gap. When the laser beam is moving, the laser energy density also is
being changed due to distinctive interaction time. It accords with weaving frequency
and weaving waveform. Figure 3.10, for example, shows the characteristic of theweaving waveform and frequency using the measured actual oscillating signal.
Figure 3.10(a), (b), (c) and (d) are 5-Hz sine wave, 15-Hz sine wave, 5-Hz
exponential wave, and 15-Hz exponential wave respectively. The weaving amplitude
indicates the laser beam moving distance on the gap direction. In this case, the 5mm moving distance from groove center toward both groove sides is applied.
Changing the weaving waveform, sine and exponential waves affects the interaction
time near the groove surface largely.

(a)

(b)

(c)

(d)

Fig. 3.10 Weaving waveform and frequency (a) 5-Hz sine wave (b) 15-Hz sine
wave (c) 5-Hz exponential wave (d) 15-Hz exponential wave.
Table 3.3 presents the interaction time of each weaving condition indicated inFig. 3.10. To clarify the effects of the weaving waveform and frequency on groove
surface melting and molten pool formation, the pulse duration time while holdingnear the groove surface and moving on the middle groove are calculated. An
exponential waveform has longer interaction time on a groove wall than a sine
waveform at the same frequency. On the other hand, the exponential waveform hasa shorter interaction time in the middle zone.
28

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Table 3.3 Interaction time for typical weaving conditions.
Exponential
waveform
5 Hz
15 Hz
Sine waveform
5 Hz
15 Hz

1 cycle Holding time ona groove wall
ms
ms
200
72.5
66.67
25

For 1 s1 cycle Holding time ona groove wall
ms
ms
200
34
66.67
11.5

For 1 s

ms
362.5
375

ms
170
172.5

Moving time in
1cyclems
55
17

For 1 sMoving time in
1cyclems
132
44

For 1 s

ms
275
255

ms
660
660

The evaluation of the laser energy density of a weaving beam and averagelaser power density is shown in equations (3.1), (3.2). E is the laser energy density
irradiation during 1 cycle, Ppeak is the power density on the beam, t is the interaction
time during 1 cycle weaving, f is the weaving frequency and Pave is the average laser
power irradiating during 1 cycle.
E = t x Ppeak , J/mm2(3.1)

Pave = E x f , W/mm2(3.2)

As shown in Table 3.3, the exponential waveform takes longer time near thegroove wall than the sine waveform does on the same weaving frequency. It meansthat the laser irradiation with the exponential waveform obtains higher laser energy
and more average laser power density on the groove walls.
It finds the interaction time during 1 cycle changes largely depending on theweaving frequency even if the same waveform is used. In one cycle, the 5-Hz
weaving frequency has a longer holding time than the 15-Hz weaving frequency, thismeans that the 5-Hz frequency irradiates the higher laser energy near the groovewalls, but in one second both 5 Hz and 15 Hz frequencies have the similar averagelaser power. The previous investigation has cleared that the pulse interaction time inthe one cycle has affected the melting efficiency strongly and it has been consideredrather than the interaction time in one second. Thus, the 5-Hz exponential waveform
was applied in this study.
3.3.1 Effects of a laser spot size with weaving system
Laser-irradiating conditions of a laser spot size, laser power density and laser
spot weaving were investigated on molten-pool formation and weld-bead creation.
Three laser spot sizes of 10 mm  26 mm, 4 mm  27 mm and 2 mm  27 mm wereapplied by changing the optical-lens and fiber-cable combination to investigate theeffects of the laser power density and laser spot width. The weaving-irradiating
method was also applied with a narrow laser spot of 4 mm  27 mm and 2 mm  27
29

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

mm. The effects of laser-irradiating condition were obtained based on high-speed
imaging during welding and cross-sectional observation.
Table 3.4 gives the welding conditions used. Oblique laser irradiation withsingle hot-wire feeding that had been investigated previously was used. A laser
irradiating angle of 15° and a welding speed of 3.33 cm/min were fixed in all
experiments. A 6-kW diode laser oscillator was used as a heat source with combined
focusing and homogenizing lenses providing a long and narrow rectangular shapefor the laser spot.
Table 3.4 Welding parameters.
Laser irradiation method

Stationary

Weaving

Fiber core, mm

1.0

0.4

1.0

Homogenizer

LL2.85

LL2.35

LL2.35Focus lens, mm

1000

400

400

Laser power, kW

6

Laser-irradiating angle, degree

15

Defocus, mm
Spot size, mm x mm

0w

10 x 26

20
l

w

20

2 x 27lw

4 x 27l

Laser power density, W/mm2

23

Weaving frequency, Hz

none

5

Weaving waveform

none

Exponential

111

Filler wire diameter, mm

1.6

Welding speed, cm/min

3.3

Wire feed speed, m/min

5.31

Wire current, A

161

181

Wire feeding angle, degree

45

Wire feeding position, mm

5Shielding gas, 10 l/min

Ar

Pre-irradiation time, s

90

55

164

The joint-configuration layout and apparatus for stationary laser irradiation andweaving laser irradiation is presented schematically in Fig. 3.11. The hot-wirefeeding speed was adjusted to fill the groove, and the wire current was set to heat
the filler wire tip to near its melting point appropriately in consideration of a heat
input from direct laser irradiation under several laser power densities. Argon gas wasused for shielding. A high-speed camera was used to observe the molten-poolformation and stability, and the feeding of the hot wire during welding. An image ofthe molten pool and hot wire was captured from the upper side of the groove. The
shooting conditions of a frame rate of 50 fps and a shutter speed of 1/20000 s wereused with an 810-nm band-pass filter. The image that was captured during welding
30

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

can be used to observe molten-pool formation and hot-wire feeding, and toinvestigate qualitatively the temperature distribution on a molten pool.

(a) Stationary laser irradiation(b) Weaving laser irradiation

Fig. 3.11 Schematic layout of joint configuration and apparatus.
Figure 3.12 shows a schematic of the laser-beam-irradiating conditions withthree laser spot widths in the groove during weaving. The widest laser spot width of
10 mm with the lowest laser power density of 23 W/mm 2 was used for stationary
laser irradiation to fit the groove width (gap). Narrower laser spot widths of 4 and 2
mm with the middle and highest laser power densities of 55 and 111 W/mm 2 usedwith a laser weaving system. The ratio between the laser beam spot width (W L) and
gap width (W G), or the W L/W G ratio, was defined as a key parameter. The W L/W G
ratio is 1.0, 0.4 and 0.2 for the 10-mm, 4-mm and 2-mm laser-beam spot width.

Fig. 3.12 Schematic of laser-beam shapes and laser-weaving conditions.
The weaving system can sweep the large rectangular laser spot parallel to the
gap width. The weaving amplitude can be adjusted to a variable gap and theweaving frequency can also be changed. The weaving amplitude was defined to fit
the 10-mm gap, and a 5-Hz weaving frequency was used. Table 3.3 showsinteraction time of the laser-beam spot motion. The holding time of the laser spotnear the groove surface was ~72.5 ms and the time taken to move through the
opposite groove surface was only ~27.5 ms.
31

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Figure 3.13 illustrates laser beam profiles of narrow rectangular beams used inthis study. At the same laser power, the 2 mm x 27 mm laser spot has a higher-peakpower density. On the other hand, the 4 mm x 27 mm laser spot has a broad power
density. Figure 3.14 (a) and (b) show the schematic layout of the beam moving in thegroove when the gap width was 10 mm and Δt was the moving time in 1 mm ofa 5-Hz exponential waveform. In case of the laser spot width of 2 mm, the moving
distance to a groove surface from the start is 4 mm with moving speed of 0.15 m/s.
In case of the laser spot width of 4 mm, the moving distance is 3 mm with the moving
speed of 0.11 m/s. The laser power was fixed at 6 kW in this research; thereafter theenergy density can be calculated with equation (3.1) in each moving time andaccumulated to summarize the energy distribution on the groove.
4x27 mm2x27 mmFig. 3.13 Laser beam profile.

(a) 2 mm beam width.

(b) 4 mm beam width.
Fig.3.14 Schematic illustration of moving motion and power distribution.

32

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Figure 3.15 indicates the laser energy distributions of three laser spot widths in thegroove with a 10-mm gap. These distributions were calculated in one cycle of a 5-Hz
exponential waveform. A previous report [83] showed that the critical laser power
density to melt the groove surface (base metal) was 35 W/mm2 (7 J/mm2 in 1 cycle of
5 Hz) for the same welding speed of 3.3 cm/min by using the stationary laser
irradiation. From the results, it can be presumed that the weaving laser irradiation byusing the long-length and narrow-width laser beam maintains a higher power density
than the critical value for the groove surface (base metal) melting, whereas
stationary laser irradiation using a long-length wide-width laser beam does not
maintain the power density above the critical value.

Fig. 3.15 Energy distribution on the groove of a weaving beam in one cycle of a 5-Hz
exponential waveform and a stationary beam.
3.3.1.1 Molten-pool temperature distribution during welding and crosssections
Figure 3.17 shows the contour image of the relative radiative intensity on amolten-pool surface transformed from the high-speed image in Fig. 3.16. The
grayscale image that was captured thorough the 810-nm band-pass filter with an 8bit color depth was converted to a color relative-intensity image. The small relative
intensity in blue means a lower temperature on the molten pool because of the lowheat input from laser irradiation. The large relative intensity in red means a higher
temperature because of the high heat input from laser irradiation.
For all laser-irradiation conditions, the intensity at the edge regions of the laser
and wire sides on the molten pool is lower than that at the center. Therefore, thelower temperature at the edge regions on the molten pool induces a high probability
33

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser methodof a lack of fusion compared with the center region. A higher intensity near bothgrooves (base metal) was observed when weaving irradiation was applied with anarrow 2- and 4-mm laser-spot width compared with stationary irradiation without
weaving by using the wider laser-spot width of 10 mm. This high intensity means thata high temperature was created by the higher laser power density of 55 W/mm 2 forthe 4-mm width and 111 W/mm2 for the 2-mm width compared with 23 W/mm2 forthe 10-mm width of the laser spot. The weaving irradiation with an exponential
waveform and a long holding time at the groove boundary of 72.5 ms in the 5-Hz
frequency as Table 3.3 created a higher temperature near both grooves (base metal)
and encouraged base-metal fusion. Although a high intensity along the groove (basemetal) is visible at the laser-irradiating side when the narrowest laser spot width of 2
mm with the highest power density was applied, the relatively lower intensity isvisible at the opposite groove side and center region on the molten pool. It can bepresumed that too narrow a laser-spot width, compared with the groove width (gap)does not maintain an adequate molten-pool temperature over the entire area in the
gap during weaving because this narrow laser spot provides heat input only in thelimited narrow region. Therefore, the temperature near one groove side, where thelaser does not irradiate, drops rapidly during laser-spot irradiation at the opposite
groove side. These results imply that a laser power density and relative laser spot
width to groove width (gap) of 10 mm, or the W L/W G ratio, would affect molten-poolformation and base-metal melting during welding.

Hot wire

Hot wire

Hot wire

Fig. 3.16 High-speed images during welding for three laser-irradiating conditions.

34

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Fig. 3.17 Intensity distribution on molten-pool surface by high-speed image captured
during welding.
Figure 3.18 shows macro-cross-sections of welded for three laser-irradiating
conditions. Sufficient weld metal at the edge of the wire-feeding side and insufficient
weld metal at the edge of the laser-irradiating side result for all three conditions. The
oblique laser-irradiating condition could not provide sufficient heat input at the edge
on the laser-irradiating side in the groove and could not create sufficient temperature
as shown in Figs. 3.16 and 3.17. This inadequate temperature condition resulted ininsufficient base-metal melting and weld-metal creation in this region.
The lowest power density of 23 W/mm2 in the stationary laser-irradiating
condition using the widest laser spot of 10 mm  26 mm did not provide sufficientheat input to the groove surfaces and the temperature was not elevated sufficiently
as shown in Fig. 3.17. Therefore, a large lack of fusion (LOF) of 45% was generated
at the laser-irradiating side and wire-feeding side.
For two weaving laser-irradiating conditions, the lower power-density condition of
55 W/mm2 with the wider laser spot of 4 mm  27 mm creates a larger melting areaand base-metal penetration compared with those under the higher power-density
condition of 111 W/mm2 using the narrower laser spot of 2 mm  27 mm. The laser
irradiation using the laser power density of 111 W/mm2 with the spot size of 2 mm 
27 mm and a W L/W G ratio of 0.2 yields a stable and consistent small melting depthon the entire length without the edge at the laser-irradiating side. The average beadwidth of the laser-irradiating condition is 10.9 mm, but the LOF of 13% is generated
on the edge at the laser-irradiating side. The laser-irradiating condition using thelaser power density of 55 W/mm2 with a spot size of 4 mm  27 mm and a W L/W G of
0.4 yields a larger melting region than that of the aforementioned cases. The
average bead width was 11.9 mm, but the very small LOF of 1% still occurred withon the edge at the laser-irradiating side.

35

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

The spot size of 2 mm  27 mm provided a much higher laser power density of
111 W/mm2 than the critical value of 35 W/mm2, which achieved stable fusion of thegroove surface (base metal), whereas the small W L/W G ratio of 0.2 did not maintainthe molten-pool temperature stable over the entire area and contribute to heatconduction fusion of the groove surface (base metal). Therefore, the laser irradiating
condition with the 2 mm  27-mm spot size created a uniform and small fusion area
through the plate thickness. The spot size of 4 mm  27 mm provided a higher laser
power density of 55 W/mm2 than the critical value of 35 W/mm2, and the large
WL/W G ratio of 0.4 kept the molten-pool temperature over the entire area stable tocontribute heat-conduction fusion of the groove surface (base metal). As a result, thelaser-irradiating condition with the 4 mm  27-mm spot size produced a large melting
area especially on the center region in the thickness direction.
The above result indicates that the laser power density and W L/W G ratio affect thegroove surface melting. A laser power density above the critical value of 35 W/mm 2
is required to achieve stable melting of the groove surface and to avoid defectformation. An adequate W L/W G ratio, which is 0.4 in this research, contributes to amore efficient and stable creation of groove surface fusion by maintaining a uniform
and sufficient molten-pool temperature.

Wire Side

Laser Side

Fig. 3.18 Macro cross-sections of welded specimens in various beam widths.
3.3.2 High-edged energy density of the rectangular laser beam
In the former investigations, when the uniform rectangular laser beam wasused, the imperfection occurred on the edges of both groove sides as shown in Fig.
3.19. It was the result of the low power density on the uniform beam edges. In orderto improve this problem and to optimize laser power density, the rectangular laser
beam with high energy at edges was applied. The laser beam is created by a special
36Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

optic lens of LL2.35-edge irradiates the concentrated laser power at both edges ofthe laser beam where lack of fusion can occur easily. Figure 3.20 shows the beamprofile and power density of the beam at 6-kW laser power. The rectangular beam of
26-mm length and 4-mm width (just focus) was made by a 1-mm fiber core, 100-mm
collimator lens and 400-mm focus lens. The beam has higher power energy on
groove edges and it can be assumed that this beam profile can reduced the lack offusion on edges.

At edges lack of fusion
can occur easily

Fig. 3.19 An example of the lack of fusion on the edges of the weld joint.

Fig. 3.20 Laser beam profile of high-edged energy type and its power energy.

37

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Fig. 3.21 Schematic layout of a weaving laser beam and energy density distribution.
3.3.2.1 Effects of high-edged energy density of the rectangular laser beam
The methodology and joint configuration of this study are shown in theschematic layout of single-pass vertical welding in Fig. 3.11(b). The oblique laser
irradiation with tilt angle of 150 and single hot-wire feeding was used. Because in theapplication of large size specimen the laser head could not access on the top ofgroove, the oblique laser was employed instead of the perpendicular laser
irradiation.
Table 3.5 shows the welding conditions used in this study. The continuous
wave (CW) of diode laser with laser power of 6 kW and welding speed of 3.3 cm/min
were fixed in this experiment. The laser head of 1-mm fiber core was assembled witha collimator lens of 100 mm, a focus lens of 400 mm and a homogenizer.
Two homogenizers were used in this experiment such as LL2.35 and LL2.35edge. Both of them could create the same dimension of the rectangular laser beam,
but the LL2.35-edge homogenizer, which is the special optic lens, could concentratehigh energy density on laser beam edges.
For the 10-mm gap, the rectangular laser spot size had a length of 28.5 mm
and a width of 4 mm at 20-mm defocus. The laser spot length fitted thickness ofbase metal. Although the laser spot width was narrower than the gap, the weaving
laser irradiation of 5-Hz exponential waveform was applied to distribute the laser
power density sufficiently and keep high heating on groove walls. Figure 3.21 shows
the schematic layout of a weaving laser beam with high-edged energy and theenergy distribution on the groove. Only one beam can concentrate energy density inthe edges and groove surfaces where the melting region is need.

38Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Table 3.5 Welding parameters.
Gap width size, mm

10Laser type

Diode laser

Fiber core, mm

1.0

Homogenizer

LL2.35, LL2.35-edge

Focus lens, mm

400

Laser power, kW

6

Laser irradiation angle, degree

15

Defocus, mm

20w

Spot size, mm x mm

4 x 27 , 4w x 28.5l

Laser distribution type

Uniform energy, High energy on edges

Weaving frequency, Hz

5Weaving waveformlExponential waveform

Filler wire diameter, mm

1.6

Welding speed, cm/min

5

Wire feed speed, m/min

7.6

Wire current, A

213

Wire feeding angle, degree

45

Wire feeding position, mm

4Shielding gas, 10 l/min

Ar

Pre-irradiation time, s

90

Figure 3.22 shows macro cross-sections of two laser rectangular beam
distribution types that identify %lack of fusion ration, %dilution ratio, penetrationarea, and HAZ width. When the uniform energy density in the rectangular beam wasused, it was considered that the temperature at the corner of the laser beam was
relatively less than the inside of that. Therefore, un-melted groove surface could
occur easily and became the lack of fusion (remarked in the red line). When the highenergy density at edges in the rectangular beam was used, it was found thedecrease in lack of fusion, higher dilution ratio and more penetration than uniform
energy density. It is as the result in improving the complete welded joint and that
reduces laser power consumption. Consequently, the beam having the high-edged
type and weaving system could optimal power distribution on the groove area.

39Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Fig. 3.22 Marco cross-section of uniform and high-edge energy density of therectangular laser beam.
3.4 Effects of oxygen additions to the argon shielding gas
In past decades, a number of studies investigated the influent of oxygen in weld
metal in conventional arc welding processes such as GMAW and GTAW [87-90].
Oxygen additions in shielding gas play an important role as an active element in arc
welding. The oxygen in the weld pool has positive effects on the weld penetration,
the shape of the weld and the number of nonmetallic inclusions or the volume ofacicular ferrite. In addition, the filler we used is the solid wire of GMAW. Its weld
metal requires the oxygen content of 200 – 250 ppm to obtain a high absorbed
energy property. Therefore, this section would like to study the effects of oxygen
added to argon shielding gas in the proposed welding process.

40

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Table 3.6 Welding parameters.
Gap width size, mm

10Laser type

Diode laser

Fiber core, mm

1.0

Homogenizer

LL2.35-edge

Focus lens, mm

400

Laser power, kW

6

Laser irradiation angle, degree

15

Defocus, mm

20w

Spot size, mm x mm

4 x 28.5l

Laser distribution typeHigh energy on edges

Weaving frequency, Hz

5Weaving waveform

Exponential waveform

Filler wire diameter, mm

1.6

Welding speed, cm/min

3.3

Wire feed speed, m/min

5.5

Wire current, A

163

Wire feeding angle, degree

45

Wire feeding position, mm

4Shielding gas, 10 l/min

Ar, Ar+5%O2, Ar+10%O2, Ar+20%O2

Pre-irradiation time, s

90

Table 3.6 shows the welding conditions used. The continuous wave (CW) of
diode laser with laser power of 6 kW and welding speed 3.3 cm/min were fixed in thisexperiment. The laser head of 1-mm fiber core was assembled with a collimator lensof 100 mm, focus lens of 400 mm and homogenizer of LL2.35-edges to create
4x28.5 mm rectangular laser beam at defocus 20 mm. 5 Hz exponential waveform
was used. In this experiment, the shielding gas was varied an amount of oxygen as
5%, 10%, and 20% to study effects of oxygen content on welds compared with pureargon shielding gas.
Figure 3.23 shows oxygen content in the weld metal of the proposed welding
process. It finds that the oxygen added in argon shielding gas can be added in theweld once an amount of oxygen in argon shielding gas rises to 5%. However, theoxygen content does not significantly change when oxygen added in argon shielding
gas increases from 5%O2 to 20%O2. The proposed welding process is not similar to
arc welding process. The oxygen cannot be added in the molten pool by an effect ofarc mechanism and the shielding gas is mainly used for protecting molten pool.
Since oxygen is an active element, oxygen additions to argon shielding gas affectthe surface tension and further change the Marangoni convection direction in the41

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser methodmolten pool that may result in the welding dilution in this welding process. Figure
3.24 shows the cross-sections when the ratio of O2 in the Ar shielding gas changes.
In case of the pure Ar shielding gas, the lack of fusion is largely generated at groove
surface of wire feeding side, but when the ratio of O2 in the shielding gas increases,
especially it is up to be 10% and 20%, the cross sections showed higher dilution ratio
and the imperfections greatly reduces. However, oxygen generates the oxide film on
the weld pool surface and a large number of added oxygen could generate heavy
oxide layer. Thus, the oxygen added in argon shielding gas should be optimized.

Fig.3.23 Oxygen content in weld metal of mixed shielding gas.

Fig. 3.24 Macro cross-sections of mixed shielding gas.

42

Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Figure 3.25 shows the Vicker hardness test results through the weld metal, HAZ
and base metal at the thickness center when pure Ar, Ar+5%O2, Ar+10%O2, andAr+20%O2 were used as a shielding gas. All the weld joints have no significant
difference in hardness distribution. The weld metal obtains high mean hardness
value about 300 HV that is much higher than the base metal hardness of 200 HV on
account of the different material strength between the filler wire and the base metal.
It also shows the softening degree in HAZ.

Fig. 3.25 Hardness distribution on weld metal and HAZ.

Fig. 3.26 Microstructure of weld metal.
43Chapter 3
A study of welding condition effects of single-pass vertical welding using hot-wire laser method

Figure 3.26 shows the microstructure of weld metal in each shielding gas. The
weld metal of pure Ar shielding gas reveals upper bainite structure and themicrostructure of Ar+5%O2, Ar+10%O2 and Ar+20%O2 shielding gas can be
confirmed that an increase in the amount of acicular ferrite in upper bainite structure
is formed compared with the pure Ar shielding gas in accordance with raising oxygen
content in weld metal.
3.5 Summary
In order to reduce the heat input of single-pass vertical welding joint, the hot-wirelaser welding method was investigated to replace high heat input of a conventionalarc welding process. Moreover, the proposed process could optimize power density
distribution to reduce laser power and it achieved a sound joint and steady welding
phenomenon. The optimal conditions can be drawn as follows:
1) A diode laser beam irradiated a controllable rectangular beam to optimize power
density distribution on the groove combined with a hot-wire system. The narrow
beam having high power density with a weaving system played an important rolein optimizing laser power to keep the molten pool and to melt the base metal.
2) The spot size of 2 x 27 mm and a W L / W G ratio of 0.2 and the spot size of 4 x 27mm and a W L / W G ratio of 0.4 with a 5-Hz exponential waveform having different
power density were studied. The sound W L / W G ratio, which was 0.4 in thisresearch, achieved the largest base-metal fusion area because it maintained anadequate uniform and stable temperature of the molten pool. The small W L / W G
ratio, which was 0.2 in this research, maintained a smaller fusion area because it
produced a sudden temperature drop and temperature fluctuation of the molten
pool with laser spot weaving.
3) High-edged energy beam type made by a particular optic lens could concentrate
more power on the beam edges which increased dilution and reduced lack offusion on the groove edges. Therefore, the combination of a weaving system andhigh-edge energy type was an optimal condition for one beam.
4) Furthermore, mixed shielding gases were studied by adding 5%, 10% and 20%
oxygen into Argon shielding gas. In this experiment increasing oxygen had apositive effect on higher penetration and decreasing lack of fusion. However, it
could generate an oxide film on the weld pool or some slag between the copper
shoe and the weld bead. An amount of oxygen, therefore, should be optimized.

44

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

Chapter 4
Optimal parameters of the novel welding process in applicationof the large size specimen
4.1 Introduction
In the aforementioned investigations in the Chapter 3, laser irradiation with aweaving system and oxygen-added Ar shielding gas were optimized to achieve asound joint. In the practical shipbuilding field, however, vertical welding must be
applied in the extremely large-size and heavy-thick plate. Therefore, in this part, theproposed process used for the large size jig is developed and investigated thewelding phenomena and mechanical properties of the welded joint.
In the first part, the development of the hot-wire laser system, the welding
configuration for large size jig used for a long-weld specimen similar to EGW in orderto do the trial large in a practical use are presented. Then the obtained optimal
welding conditions as investigated in the first part are applied to analyze the effectsof the proposed welding process on long weld length as same as in the actual fieldwelding compared with a EGW weld joint.
In the second part, the weld joint properties of the novel welding process oflarge size specimen performed in the above part such as weld bead appearance,
macro cross-section, hardness distribution, microstructure, tensile test, and fracture
toughness test are evaluated in accordance with the standard requirement.
Accordance to a limited diode laser of 6 kW, an adequate laser power densitywith the 10-mm gap width is only up to 28.5-mm thickness. However, the actual
thickness of this structure is more than 50 mm. Consequently, in the Chapter 5, twin
diode lasers employed for heavy-thick steel plate are discussed.

45

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

4.2 Introduction of the joint configuration and equipment of the novel welding
process in the large size jig

Fig 4.1 Schematic layout of the large size jig of the proposed process.

For single-pass vertical welding with a long-welded joint in a practical use of the
shipbuilding, laser irradiation and hot-wire feeding can access only one side similar
to EGW. Figure 4.1 presents a schematic layout of the joint configuration for a largescale specimen. The specimen is set vertically on a copper plate making a back side
groove. The obtained optimal conditions are applied to large size specimens. The
hot-wire laser welding (HLW) with a self-propelled base is developed in order to trial
for the long-welded joint in a practical use. The hot wire, oblique laser irradiating, acopper shoe and a high-speed camera, which access from the front side, are moved
together. In this jig, the laser head angle and defocus length is fixed to create theproper laser spot size with a weaving system. Moreover, the shielding gas andcooling water are applied through the inside of the copper shoe.

46

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

4.2.1 In-situ observation
A high-speed image of the welding phenomena was captured by a high-speed
camera. The camera of pencil camera V-193-M1 model connected with NAC:
MEMRECAM GX-5 module with a focus lens of 35 mm with a diameter of 25 mm
and an 810-nm bandpass filter as shown in Fig. 4.2. The capturing parameters are
the frame rate of 50 fps, shutter speed of 1/1000 s, and closed aperture. Table 4.1
shows the monitoring condition. For setting a camera perspective to see the melting
phenomena on the groove wall surfaces, it was perpendicular to groove wall
surfaces and 80 degrees from the weld pool plane.

Fig 4.2 Transmission curve of 810 nm bandpass filter.

Table 4.1 Monitoring conditions of in-situ observation.

High-speed camera
Frame rate, fps
Aperture
Focus lens, mm
Band pass filter, nm/FWHM
Lighting

GX-5
50
Closed35
810 / 10
N/A

Shutter speed, s

1/100047

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

4.2.2 Materials used
KE 47 steel plate, available in the ClassNK standard, was used as base metalmaterial that thickness is reduced to 28.5 mm. There were two sizes of specimens(Medium: 250w x 500h and Large: 500w x 1300h mm) as shown in Fig. 4.3. Plates
were fixed and aligned as a vertical joint configuration. The square groove was set
with a 10-mm gap using a bottom spacer having dimensions of 10 (width) x 20
(height) x 28.5 mm (depth). YM-60A (JIS Z3312 G59JA1UM3M1T) with a diameterof 1.6 mm was used for hot-wire rod. The chemical compositions of base metal andfiller metal are illustrated in Table 4.2.

Table 4.2 Chemical compositions of base metal and filler wire.
Materials

Chemical Composition, wt%
C

Si

Mn

P

S

Al

Cu

Ni

Nb

Ti

Cr

Mo

KE-47

0.09 0.07 1.52 0.007 0.002 0.014 0.32 0.69 0.01 0.01 0.02 0.00

YM-60A

0.06 0.35 1.45 0.010 0.007

-

0.20

-

-

0.7

Fig. 4.3 Specimen dimensions used for large size jig.

48

-

0.38

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

4.3 Welding performing of large size specimen and a weld bead appearance
Figure 4.4 shows the photograph showing vertical hot-wire laser welding during
operating. The large size specimen was fit up with the 10-mm gap placed on a backside copper plate. The self-propelled unit carried a laser head, a hot-wire torch,water-cooled copper shoe, and a high-speed camera to move together along a guiderail that is similar to EGW. The copper shoe was cooled by water flow inside to form
a weld bead in front side and it supplied shielding gas to protect the weld pool. For
long-welded length, it was possible to acquire high heating accumulation on hot-wireduring welding and then a filler wire would be melted inside a ceramic guide. Thus,
the hot-wire torch was covered by the water-cooled torch holder. The high-speed
camera was used to observe weld pool phenomena on the top of the self-propelledunit and its parameter was shown in Table 4.1.

Fig. 4.4 Photograph showing the proposed welding process.

49

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

Table 4.3 gives the optimal welding conditions used for large size jig. The
maximum of 6-kW laser power and mixed shielding gas of Ar+20%O2 were used.
The laser head of 1-mm fiber core was assembled with collimator lens of 100-mm,
focus lens of 400 mm and LL2.35-edged homogenizer to create 4x28.5-mm
rectangular laser beam at 20-mm defocus and 5-Hz exponential waveform wasused. This experiment performed with two welding speeds due to the effect of large
size specimen. The welding speed was varied in 2.5 and 3.3 cm/min.
Table 4.3 Welding parameters.
Gap width size, mm

10Laser type

Diode laser

Fiber core, mm

1.0

Homogenizer

LL2.35-edge

Focus lens, mm

400

Laser power, kW

6

Laser irradiation angle, degree

15

Defocus, mm

20w

Spot size, mm x mm

4 x 28.5l

Laser distribution typeHigh energy on edges

Weaving frequency, Hz

5Weaving waveform

Exponential waveform

Filler wire diameter, mm

1.6

Welding speed, cm/min

2.5

3.3

Wire feed speed, m/min

5.2

5.92

Wire current, A

144

163

Wire feeding angle, degree

45

Wire feeding position, mm

4Shielding gas, 10 l/min

Ar+20%O2

Pre-irradiation time, s

90

50

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

In the previous study of optimizing welding conditions in a small size specimen,
there was no any imperfection formed up to the surface of a welded joint which was
produced with a laser power of 6 kW and a welding speed of 3.3 cm/min. As regards
a large specimen of YP 47 steel, the heat of the molten pool tends to distribute moreon the surface because the size of the test piece becoming larger has highconductivity, and the molten pool is cooled faster. In order to keep the molten pool,increasing an amount of heat input is needed by enlarging laser power or reducing
welding speed. Electrogas arc welding (EGW), which is currently used, is able toperform the single-pass vertical welding with a welding speed of 5 cm/min. The
proposed process can be competitive by an increase in laser power. However, themaximum of a laser oscillator used in the experiment is 6 kW. We decided to reduce
the welding speed. Therefore, in this chapter, we changed the welding speed from
3.3 to 2.5 cm/min and fixed the 6-kW laser power. The welded joint of welding speed
2.5 cm/min was evaluated in various characteristics.

Fig. 4.5 An example of weld bead appearance of the 250W x500L mm specimen with awelding speed of 2.5 cm/min.

51

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

Figure 4.5 shows the welded joint appearance of the specimen prepared withthe welding speed of 2.5 cm/min using the proposed welding process. The weld
bead achieves complete penetration on both the front side and back side, and it
obtains a sound welded joint. There are no visible imperfections that can beobserved. Because of adequate power distribution and hot-wire system, a stablewelding process was achieved. The large size jig of the novel welding process could
continue smoothly even though the heat source of diode laser and hot-wire feeding
moved during welding.
Figure 4.6 shows the macro cross-sections of two long-welded specimens withdifferent welding speed of 3.3 cm/min and 2.5 cm/min. Both welded joints obtain low
dilution and narrow HAZ as a result of low heat input welding process. However, in
3.3 cm/min specimen a little lack of fusion on the edge was found sometimes
regarding the large size specimen and during welding, an amount of heating molten
pool could spread more rapidly in the large size specimen than the small size
specimen. In this experiment, we tried to increase heat by reducing welding speed
from 3.3 to 2.5 cm/min and a heat input also enlarged from 109 to 144 kJ/cm. The
heat input was calculated by the laser power and the welding speed not including theabsorption factor. The result showed that 2.5-cm/min specimen could achievecomplete penetration and there were no imperfections. Consequently, the laser
output is 6 kW for the large specimen with the welding speed at 2.5 cm/min. It ishighly possible to produce a joint without welding defects.

Fig. 4.6 Macro cross-sections of the 28.5tx250Wx500L mm specimen produced by thelarge size jig using HLW.

52

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

Fig 4.7 High-speed image and schematic layout of the proposed welding processduring welding.
The welding phenomena during welding of the long-welded specimen with 2.5
cm/min welding speed were observed by a high-speed camera and its parameters
are shown in Table 4.1. Figure 4.7 shows the schematic layout and a clear
explanation of weld pool perspective compared with a high-speed image of weld pool
phenomena of the proposed process during welding. The schematic layout describes
a rectangular laser beam of 4w x 28.5L mm with a weaving system which is placedalong with joint thickness moves throughout the gap width between groove surfaces.
High-speed image of the large size specimen using 6-kW laser power and 2.5cm/min welding speed with hot wire shows stable welding phenomena. The molten
pool could penetrate the whole welded groove on both wire feeding side and backside continuously.
4.4 Joint properties of 1-pass vertical welding using hot-wire laser method
This part is to study the welded-joint properties of large size specimens using
the proposed welding process. The long-welded joints that achieved complete
penetration presented in previous part in the length of 500 mm and 1300 mm were
investigated the properties such as macro/microstructure, hardness distribution,
tensile test, and fracture toughness test according to the standard specification.

53

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

4.4.1 Hardness distribution
A micro Vickers hardness tester (MHT-1, manufactured by Matsuzawa Seiki
Co., Ltd.) was used for the measurement. The load was 1 kgf and the holding timewas 10 s in accordance with JIS Z 2244. The Vickers hardness defined in thefollowing formula was calculated with equation (4.1) from the average value of themeasured indentation diagonal length.
HV = 0.1892 ×

F
𝑑2

(4.1)

Hereby, HV: Vickers hardness, F: load [N], d: average value [mm] of the diagonallength of the indentation.
Figure 4.8 shows the hardness distribution through the base metal, HAZ andweld metal on the middle of thickness. The result finds that the mean weld metal
hardness of 226 HV is relatively higher than the base metal hardness of 206 HV due
to a higher strength material and it also shows that the softening degree in HAZ issmall as 188 HV and the softened width is narrow as 5 mm.

Fig. 4.8 Vickers hardness distribution in the middle of the welded joint.

54

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

4.4.2 Microstructure characteristics of the welded joint
This section is to investigate microstructure characteristics of the large size
specimen that was mentioned above. 6-kW laser power and 2.5-cm/min welding
speed results in heat input of 144 kJ/cm. Filler wire and base metal materials of YM60A (JIS Z3312 G59JA1UM3M1T) and KE-47 were used respectively. The crosssection was polished firstly with various grit sizes of sandpaper number 100, 500,
1200, 2000 and 3000 in order and then polished with diamond paste with a particle
size of 6, 1 μm and magnesium oxide (MgO) powder. In addition, it was etched by
3% nital solution to reveal microstructure and that was observed by optical
microscopic (1LM21 lasertec)
Figure 4.9 shows the microstructure of HAZ of the welded specimen. Coarse
grain heat affected zone (CGHAZ) is a part of HAZ caused by heat during welding
process. Its microstructure changing in prior austenitic grain size affects toughness
and properties of a welded joint. Therefore, a decrease in heat input can obtainnarrow CGHAZ and improve the welded joint properties. The proposed welding
process achieves the narrow CGHAZ as 0.84 mm in 28.5-mm-thick specimen as aresult of the low heat input welding process.
Figure 4.10 illustrates the microstructure of weld metal and fusion boundary.
The results show that fine lower bainite is generated in the weld metal. A large
fraction of ferrite grain boundary and upper bainite are found in the fusion line. In
addition, coarsening of crystal grain size in HAZ is observed pro-eutectoid ferrite andlower bainite.

Fig. 4.9 Coarse grain heat affected zone region of large size specimen with a heat
input of 144 kJ/cm (6kW, 2.5 cm/min).
55

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

Fig 4.10 Microstructure of fusion boundary and weld metal.

56

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

4.4.2.1 Comparison of the welded joints of electrogas arc welding and hotwire laser welding
Figure 4.11 shows the cross-sections of EGW and HLW welded joints having
the same thickness. By comparison with the EGW process currently used in the field,
although the welding speed of the EGW joint is 2 times higher, the HLW joint obtains
extremely smaller dilution ratio, penetration area and narrower heat affected zone. In
accordance with the microstructure as shown in Fig. 4.12, in the microstructure of
weld metal, coarse ferrite is found, and bainite and acicular ferrite are also partly
generated in the weld metal of the EGW joint. In the HLW joint, it reveals fine lower
bainite. In the microstructure of HAZ, the EWG joint shows coarse grain boundary
ferrite including bainite and acicular ferrite partly formed. In the HWL joint,intergranular ferrite, upper bainite and acicular ferrite are found, and it can be
confirmed that the coarse grain boundary ferrite of the HLW joint is smaller than thatof the EGW joint even in lower welding speed. Consequently, the HLW process
which achieves an efficient welding process and the sound weld joint is relatively
better than the EGW process as a result of a low heat input.

Fig. 4.11 Cross-sections of EGW and HLW.

57

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

Fig. 4.12 Microstructure of weld metal and heat affected zone.
4.4.3 Tensile test of the welded joint
The large size specimen was cut to prepare a tensile specimen as shown in
Fig. 4.13. The standard specimen size was made in accordance with JIS Z 2201
no.14B and the thickness became 26.9 mm after machining.
The tensile test result is shown in Fig. 4.14. Regarding the material strength offiller wire and base metal, the tensile specimen is broken in base metal and greatly
elongated. The tensile strength over 570 MPa, the yield stress over 460 MPa andelongation over 17% meet the requirement of the ClassNK standard.

Fig. 4.13 Tensile test specimen.

58

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

Fig. 4.14 Tensile test result.
4.4.4 Charpy V-notch impact test of the welded joint
Figure 4.15 shows the cutting plan of the welded joint and notch locations. The
stand specimen size for Charpy impact test is 10 mm x 10 mm x 55 mm according to
JIS Z 2242. The Charpy test equipment has a heavy pendulum of 25.31kg and arm
length of 0.7 m in this test.
This impact test is a method to evaluate absorbed energy of material for brittle
fracture at low temperature. In this study, we varied the test temperatures to find out
the ductile and brittle transition temperature (DBTT) and the notch locations were at
weld metal (WM) and fusion line (FL).

Weld Metal

Fusion Line

Fig. 4.15 Cutting plan and notch locations.

59

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimeno64J (-20 C)

Fig. 4.16 Absorbed energy VS test temperature at weld metal and fusion line.
The Charpy V-notch impact test results of the notch location at weld metal andfusion line is shown in Fig. 4.16. The weld metal shows high absorbed energy attemperature of -20 oC, which satisfied the ClassNK standard (64 J at -20 oC). The
fracture surface of the Charpy impact specimens is shown in Fig. 4.17(b). At 0 oC,
the fracture surface indicates completely plastic deformation. At -20 oC, shows
plastic deformation with a little brittle deformation and at -60 oC, shows a brittle
manner without plastic deformation. Thus, the DBTT is between -20 oC to -60 oC.
However, Fig. 4.16 shows that the fusion line obtained the unstable absorbed
energy even in temperature above -20 oC. Large grain size and grain boundary
ferrite observed in the fusion line microstructure are mentioned in Fig. 4.10 that
affects the unstable absorbed energy although the low heat input can be achieved.
The fracture surface of the Charpy impact specimens is shown in Fig. 4.17(a).
Almost all specimens showed a brittle deformation in a straight direction. Then the
microstructure of the fracture path in a low absorbed energy specimen (no.F112) isobserved in Fig. 4.18. The crack path propagates along with a fusion boundary thathas a fraction of grain boundary ferrite. Moreover, the cleavage fracture surface isobserved by SEM.

60

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen(a) Fusion line(b) Weld metal
Fig.4.17 Fracture surface and direction of Charpy impact test specimen in(a) fusion line and (b) weld metal.

61

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

200µm

o

No.F112: 33 J, -20 C

Fig. 4.18 Microstructure of fracture path and fracture surface in Charpy impact
specimen at fusion line.

62

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

4.4.5 Crack tip opening displacement (CTOD) test of the welded joint
The CTOD test is one fracture toughness test that is used when some plastic
deformation can occur prior to failure. The schematic diagram of a test piece isshown in Fig. 4.19. A specimen given the tip of a crack is bent by an external force.
While the test is in process, the crack tip opening displacement (Vp) and load arerecorded automatically and the CTOD value (δc) is calculated from Vp. The higher
the CTOD value is, the higher the fracture toughness is.
In this experiment the location of notch in the weld metal using two specimens.
The CTOD test results are shown in Table 4.4. The weld metal exhibits the highresistance to brittle propagation at -10 0C with the CTOD values exceeding 0.25 mm.
According to the load/displacement chart in Fig. 4.20, both charts show plastic
deformation before failure and the fracture surface in Fig. 4.21 is to ensure that thecrack opening straightly propagates.

Fig. 4.19 Schematic illustration of CTOD test.

Table 4.4 CTOD test results.

63

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

Notch-opening displacement, Vp (mm)Notch-opening displacement, Vp (mm)Fig. 4.20 Load VS Crack opening displacement (Vp).

YM60-1

YM60-2

Fig. 4.21 Fracture surface of CTOD specimen.

64

Chapter 4
Optimal parameters of the novel welding process in application of the large size specimen

4.5 Summary
In order to improve the proposed welding process for a practical use, thevertical hot-wire laser welding system with a self-propelled base was developed inorder to do the trial for the long weld joint similar to the field welding based on theprevious investigations of the optimization of welding conditions, and then theproperties of the welded joint were investigated.
1) The optimal welding conditions of laser power distribution, hot-wire system,shielding gas, etc. were developed to apply for the large size specimen of 500
mm and 1300 mm in length. During welding, this process achieved stable welding
phenomena continuously the whole welding period and a sound welded joint
could be observed even the heat source of laser head and hot-wire system
moved.
2) The HLW joint of long-welded specimen obtained low dilution and narrow heataffected zone as a result of the low heat input welding process and the heat inputof the HLW welded joint is much lower than that of the EGW welded joint at thesame thickness of base metal.
3) According to properties of the HLW welded joint, the tensile strength met therequirement of the specification. The V-notch Charpy impact test result of notch
location at weld metal showed high absorbed energy at -200C according to CTOD
test results that exhibited the high resistance to brittle propagation at -100C.
However, the fusion line obtained the unstable absorbed energy because grainboundary ferrite could be observed in the fusion line.

65

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

Chapter 5
Development of vertical welding for heavy-thick steel plateusing twin diode lasers and hot wire
5.1 Introduction
In the previous chapter, the conceptual idea of the new welding method andpreliminary investigations in welding phenomena were presented. The combinationof a diode laser with a rectangular beam spot and a hot-wire feeding system were
performed on steel plates with its thickness up to 28.5 mm. In addition, the effects of
welding conditions such as the laser beam with high-edged energy, weaving motion,
shielding gases and so on were investigated. In this chapter, the twin diode laser
system is applied to increase a total laser power for thicker plates with 50-mm
thickness. Moreover, optimization of laser irradiation conditions is studied using atwin diode laser system with special optics systems such as a weaving head and ahigh-edged energy lens.
Firstly, the principles and basic ideas to improve the laser beam control using
twin diode lasers for 50-mm thickness are presented and then this part illustrates
introducing the methodology of a joint configuration and apparatus applied forinvestigating the advanced laser beam control of twin diode lasers with a total powerof 12 kW and the hot-wire system. The power density distribution is determined to be
sufficient for melting the base metal and keeping the molten pool by using a highedged energy beam and a weaving beam. In addition, double stable beams without
weaving are also investigated.
Secondly, the weld bead appearances and cross-sections of heavy-thick joints
in varied laser power distribution as described in the previous section are observed
Thirdly, this part shows the properties of vertical welding joints in 50-mm plates
using twin diode lasers with the hot-wire system. The microstructure, Vickers
hardness test, and Charpy impact test are presented.

66

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

5.2 Optimization of advanced laser beam control of twin diode lasers forvertical welding
Because of the limited power of a 6-kW diode laser, one laser beam isn’t
sufficient for melting the base metal and keeping the molten pool in a heavy-thickjoint. Therefore, this part would like to improve the laser beam control of twin diodelasers and to apply for 50-mm-thick specimen.
5.2.1 Investigating power distribution on rectangular laser beams for 50 mm
steel plate
As shown in Table 3.1, the rectangular beam shape can be changed by
combining the optical lenses of a laser head. In case of 50-mm thickness, in threewelding conditions the beam length fits the thickness based on the optimal focal
length and accessibility for setting a stable beam and a weaving beam. The laser
beam shapes and their intensity distribution used in this chapter are shown in Fig 5.1
and Fig 5.2. Figure 5.1 shows large rectangular laser beams fitting a groove area.
These beams irradiate into the groove stably without weaving. Figure 5.1(a) is a
uniform energy beam that is excellent to keep power density on a whole area of amolten pool steady. On the other hand, a high-edged energy beam in Figure 5.1(b)made by a special homogenizer can concentrate more power density on the beamedges and in the meantime, it can keep power density on the molten pool so that we
can improve dilution and reduce lack of fusion on groove edges. Figure 5.2 shows
the narrow and middle rectangular laser beams. Both beams are narrower than agap width and combined with the weaving head to move them throughout the gap
area. The beam motion of weaving waveform and frequency is controlled anddescribed in Table 3.3. The average power density of the narrow beam in Figure 5.2(a) is higher than that of the middle beam in Figure 5.2 (b) and therefore canconcentrate more power density on groove surfaces.

8 x 51 mm

10 x 48 mm(a) Uniform energy beam(b) High-edged energy beam

Fig. 5.1 Large rectangular laser beams fitting a groove area.
67Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

2 x 56 mm

5 x 56 mm(a) Narrow beam(b) Middle beam

Fig. 5.2 Narrow and middle rectangular laser beams combined with a weaving
system.
5.2.2 Introduction of a joint configuration and equipment of the proposed
process using twin diode lasers for a heavy-thick joint(a) Stable and weaving laser beams(b) Double stable laser beams

Fig. 5.3 Schematic layout of the experimental set-up for single-pass verticalwelding using twin diode lasers.

68Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

The schematic layout of the experimental set-up for single-pass vertical welding
using twin diode lasers and a hot wire is shown in Fig. 5.3. Two plates of a specimen
were fixed inside a copper shoe and moved vertically by a uniaxial electric actuator.
The covering copper shoe had a water cooling system inside used for keeping theweld pool shape of both a wire feeding side and the opposite side. Moreover, it hadother slots for inserting a filler wire and a shielding gas to protect the molten pool. A
filler wire deposited into the molten pool was electrically heated up to its melting
point as called hot wire. Two high-power diode lasers were used as the main heatsources. For rectangular laser spot, a process fiber cable assembled with focusing
and homogenizing lenses created an adjustable rectangular shape of a beam spot
size according to a thickness and gap width, and a high-speed camera was applied
to monitor weld pool formation, welding stability, and a hot-wire feeding during
welding. Figure 5.3 (a), (b) shows the schematic layout of the proposed processusing stable and weaving beams, and double stable laser beams respectively.
The welding phenomena during welding were monitored by a high-speed
camera. Pencil camera V-193-M1 model was connected with NAC: MEMRECAM
GX-5 module shown in Fig. 3.5. Table 5.1 shows the monitoring condition. The
camera has a focal length of 35 mm with 25 mm diameter combined with a bandpass
filter of 750 nm. Besides, the frame rate is 50 fps and the shutter speed is 1/20000 s.
Table 5.1 Monitoring condition of in-situ observation.
High-speed camera

GX-5

Frame rate, fps

50

Aperture

ClosedFocus lens, mm

35Band pass filter, nm/FWHM

750 / 10Lighting

N/A

Shutter speed, s

1/20000

5.3 Effects of power distribution on a heavy-thick welded joint using twin diodelasers and hot wire method
This part is to apply the combinations of two rectangular beams in different
power distributions into the 50-mm-thick specimen. In order to achieve a sound joint,
both diode lasers irradiate rectangular beams with special optics systems such as aweaving head and high edged energy lens to optimize power distribution and laser
power that would be able to keep the molten pool and to melt the base metal stable.

69

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

5.3.1 Materials used
EH 40 steel that is available in shipbuilding standard was used as base metalmaterial having a thickness of 50 mm. Its dimensions are shown in Fig. 5.4. The
specimen size was 100w x 200h mm. Two plates were fitted up with a square groove
having 10 mm gap, and a bottom spacer used for creating an initial molten pool had
dimensions of 10w x 20h x 50d mm. JIS Z 3312 G78A4UM5C1M3T (a trade name of
YM-80A, equivalent to ER110S-G) with a 1.6-mm diameter was used as a hot-wirefiller material. Table 5.2 shows the chemical compositions of these materials.
Table 5.2 Chemical compositions of base metal and filler wire.
Materials

Chemical Composition, wt%
C

Si

Mn

P

S

Al

Ni

Nb

Ti

Cr

Mo

EH 40

0.12 0.31 1.34 0.007 0.003 0.012 0.01 0.01

-

-

-

-

YM-80A

0.06 0.40 1.69 0.006 0.003

-

-

Cu

0.20 3.01

Fig. 5.4 Specimen dimensions.

70

0.05 0.46 0.29

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

5.3.2 Experimental procedure
Table 5.3 shows the welding parameters of twin diode lasers and hot wire. As
the intensity of laser beams in Fig. 5.1 and 5.2, there are three welding conditions
used to investigate the effects of power distribution on the 50-mm-thick joint. Figure
5.5 (a), (b) and (c) illustrate both rectangular laser beams irradiated on a groove areain three conditions. The first diode laser (LD1) irradiated a large rectangular beam of
51 mm and 8 mm having a high energy density on both edges in its beam length.
The beam was irradiated stably without weaving to keep power density on a wholearea of a molten pool surface and to concentrate a high energy density on edges
near plate surfaces, and this beam was used in all of the three welding conditions.
Table 5.3 Welding parameters of twin diode lasers and hot wire.
Laser type

LD 1

Laser head type

StableWeaving

Weaving

StableFiber core, mm

1.0

0.4

1.0

0.4

Collimator, mm

100

100

100

100

2.35-edge

2.20

2.20

2.85+2.20

Focus lens, mm

800

500

500

500

Laser power, kW

6

6

6

6

0

0

0

Homogenizer, LL-line

Laser irradiation angle
Defocus, mm

35

LD2

o

Just focusw

Just focuslw

Just focusw

Just focusl

5 x 56l

10w x 48l

Spot size, mm x mm

8 x 51

Laser distribution typeHigh-edged

Uniform

Uniform

Uniform

Weaving frequency, Hz

None

5

5

None

Weaving waveform

None

Exponential

ExponentialNone

2 x 56

Filler wire diameter, mm

1.6

Welding speed, cm/min

3.3

Wire feed speed, m/min

10

Wire current, A

228

Wire feeding angle

45

Feeding position, mm

10Shielding gas, 10 l/min

80%Ar and 20%O2

Pre-irradiation time, s

80

According to a study of power distribution in the groove area, the second diodelaser (LD2) irradiated a varied beam size in three types of power distribution in agroove. The first condition in Fig. 5.5 (a), The LD2 irradiated a long and narrowrectangular beam of 56-mm length and 2-mm width and the second condition in Fig.
5.5 (b), the LD2 irradiated a middle rectangular beam of 56-mm length and
5-mm width. Thus, both laser beams that were narrower than gap width applied with71

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wirea weaving system using a 5-Hz exponential waveform. Figure 5.6 shows theschematic of laser beam motion and weaving amplitude in one cycle. In one cycle ofa 5-Hz exponential waveform, the holding time of the laser spot on each groove
surface is about 72.5 ms and the moving time between is about 27.5 ms.

(a) First condition(b) Second condition(b) Third condition

Fig. 5.5 Illustration of twin laser irradiations in three welding conditions.

Fig. 5.6 Schematic layout of laser motion and amplitude at a 5-Hz exponential
waveform.
In case of 2-mm beam width, the moving distance is 3 times wider than the laser
spot width. Therefore, the average of a laser power density on the middle of groove
was very low compared with it on near groove surfaces, and regarding narrow beam
having high power density, the longer holding time generates more heat energy
where it is close to groove surfaces. In case of 5-mm beam width, the beam moves
slower due to shorter moving distance and it is placed on a half of a gap area, so this72

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

beam can divide some power density into the weld pool and in the meantime when it
is holding on the groove surface, there is some laser reflection to the groove surface.
The third condition in Fig. 5.5 (c), the LD2 irradiated a large rectangular beam of 48mm length and 10-mm width with uniform power density fitting groove area. The
beam was irradiated stably without the weaving system. Therefore, this beam cankeep heating on the molten pool, but it is poor to melt the base metal as a result of
low laser reflection.
5.3.3 Welding performing of twin diode lasers for a heavy-thick joint and weld
bead appearance(a) Stable and weaving laser beams(b) Double stable laser beams

Fig. 5.7 Photographs showing experimental set-up of twin diode lasers and hot wire.
Figure 5.7 shows photographs of the actual experimental set-up of verticalwelding using 2 diode lasers and the hot-wire system for the 50-mm-thick specimen.
Because of the access of both laser heads to the specimen and their focal length,
only the small size jig can be employed. Figure 5.7 (a) shows diode lasers of a stable
head (LD1) and a weaving head (LD2) which were applied for the first and second
welding conditions as the aforementioned procedure. LD1 irradiated a large
rectangular beam obliquely at a 350 angle on the opposite side of the wire feeding. In
addition, the beam had a high-edged energy type, and LD2 irradiated a narrower
rectangular beam straightly into the groove area. Figure 5.7 (b) shows double stable
73

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

diode lasers of the third welding condition. LD1 irradiated the same beam as first andsecond welding conditions, and LD 2 irradiated a large rectangular beam having auniform energy type stably without weaving. Moreover, the whole welding period thehigh-speed camera monitored weld pool phenomena and hot-wire feeding. The
specimen mentioned in Fig. 5.4 was fixed on a base plate by tack welding before
inserted in a copper shoe and only one hot-wire feeding was used.
5.3.3.1 In-situ observation
The welding phenomena during welding were monitored by a high-speed
camera. Pencil camera V-193-M1 model connected with NAC: MEMRECAM GX-5
module. The camera had a focal length of 35 mm with a 25-mm diameter and a
bandpass filter of 750 nm. Besides, the frame rate was 50 fps. Before hot-wirefeeding, the 6-kW weaving laser and 6-kW stable laser irradiated onto the groove
area on the spacer surface to generate an initial molten pool for 80 s since a large
quantity of the weld metal volume was deposited into the heavy-thick joint, and themolten pool by the pre-irradiation in the groove area could help to melt and distributethe feeding wire steadily. Figure 5.8 shows high-speed images taken before hot-wirefeeding of the 2 x 56 mm weaving and 8 x 51 mm stable beams using the proposed
process. In pre-irradiation images before hot-wire feeding, the 1.32 s image shows
the weaving laser held on a groove surface and high-edged stable laser irradiation.
In the 1.36 s image, the weaving laser is moving to the other side and then the 1.42s image shows the weaving laser is held on the other side. This laser spot motion
repeats during welding. The 80.00 s image shows the stable molten pool by preirradiation before hot-wire feeding on the whole groove area.

Fig. 5.8 High-speed images in pre-irradiation of the 2x56 mm weaving and
8x51 mm stable beams.

74

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

Figure 5.9 shows a schematic layout and a clear explanation of a weld pool
perspective compared with a high-speed image of weld pool phenomena of the 2x56
mm weaving and 8x51 mm stable beams during welding. The image shows anexample of a large rectangular weld pool generated by twin diode lasers and hotwire feeding. The hot wire is fed into the molten pool and the specimen moves
downward vertically. The steady weld pool formation and wire feeding wereobserved continuously for the whole welding period. The single hot-wire feeding
could deposit the weld metal sufficiently for the heavy-thick joint, as a result thesufficient weld metal could be achieved both on the back side and wire feeding side.

Fig. 5.9 High-speed images and schematic layout during welding of the 2x56 mm
weaving and 8x51 mm stable beams.
5.3.3.2 Weld bead appearances
According to the weld bead appearances in Fig. 5.10, all of three welding
conditions could carry on the vertical weld joints using the combination of twin diodelasers and the hot-wire system. The stable welding phenomena until the finish could
be observed. In other words, the proposed process achieves a steady welding
process continuously regarding the adequate power density distribution and hot-wiresystem. The weld beads obtain complete penetration and there is no visible lack offusion that could affect the properties of the welded joint.

75

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

Fig. 5.10 Weld bead appearances of heavy-thick welded joints.
5.3.4 Cross-sections and joint properties of heavy-thick specimens using the
proposed method
5.3.4.1 Macro cross-sections
Figure 5.11 shows the macro cross-sections. The cross-sections of the welded
joint by using the proposed process were derived from different power distributions
on the heavy-thick grooves cut at the same level of 70 mm from the welding start.
Accordance to the cross-sections of stable and weaving beams, the stable laser
beam with the high-edged energy of LD1 that could concentrate more power density
on edges of a welded joint and keep energy the whole groove combined with theweaving beam of LD2. The 2-mm beam width with the weaving system provided
higher energy density and greater laser reflection to the groove surfaces comparedwith the 5-mm beam width, but the 5-mm beam width distributed higher energy
density on the molten pool near groove surfaces and in the same time the peak ofenergy density could be reflected to the base metal. Therefore, both laser beams
could melt the base metal sufficiently. The cross-sections of 2x56 mm beam and5x56 mm beam even having different power density show similar weld beads and nolack of fusion can be found. The low dilution and narrow HAZ can also be observed,that is, this process is a low heat input welding method.
In case of double stable lasers, both laser beams were irradiated stably withouta weaving system in the whole welding time. Regarding this combination, power
distribution spread on an entire groove area that was excellent to keep the molten
76Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wirepool, but poor at melting the base metal owing to low laser reflection on the groovewalls. The cross-section shows very low dilution and reveals some lack of fusionnear the wire feeding side. As shown in Fig. 5.12, the bead width on the wire feedingside is relatively small since the filler wire cut the laser beams above the molten pool.
Therefore, the actual laser power density in this area become lower and dilution issmaller in the wire feeding side than in the middle and back side of the welded joint.
Consequently, the stable laser beam with the high-edged energy type and theweaving beam can optimize the laser power distribution in a heavy-thick groove. The
power density concentrated on edges of a welded joint and groove wall surfaces canincrease base metal dilution and reduce lack of fusion. Moreover, the hot-wiresystem can heat a filler wire up to its melting point, therefore the power density tomelt a large amount of the filler wire deposited in the middle groove can be reduced.

Fig. 5.11 Macro cross-sections of heavy-thick specimens.

Fig. 5.12 Measured bead width of the welded joints.
77

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

5.3.4.2 Microstructure characteristics
Figure 5.13 (a), (b), (c) show microstructure of HAZ and FL of the welded
specimens. The proposed process achieved the narrow CGHAZ about 1.5 mm in aheat input of 218 kJ/cm for the 50-mm specimen. As the study of the previous
chapter, the 28-mm specimen using the heat input of 144 kJ/cm had 0.83-mm
CGHAZ, therefore the heat input is the main effect to enlarge CGHAZ. The CGHAZ
region of three specimens is not significantly different even though the weaving andstable lasers obtain the higher dilution. Consequently, the advanced laser beam
control can change the power density distribution and also affect the melting of basemetal, but not has an influence on the CGHAZ region. At the fusion boundary, afraction of ferrite and bainite are found along with fusion line. At the CGHAZ, proeutectoid ferrite and lower bainite are observed.
Figure 5.14 shows an example of weld metal microstructure. The results show
fine lower bainite and ferrite generated in the weld metal(a) Microstructure of weaving (2x56mm) and stable beams.

(b) Microstructure of weaving (5x56mm) and stable beams.
Fig. 5.13 Microstructure of heat affected zone and fusion boundary.
78

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire(c) Microstructure of double stable lasers.
Fig. 5.13 Microstructure of heat affected zone and fusion boundary (continued).

Fig. 5.14 Microstructure of weld metal.
5.3.4.3 Vickers Hardness distribution
A micro Vickers hardness tester (MHT-1, manufactured by Matsuzawa Seiki
Co., Ltd.) was used for the measurement. The load was 1 kgf and the holding timewas 10 s in accordance with JIS Z 2244.
Figure 5.15 shows the hardness distribution through the base metal, HAZ andweld metal on the middle of thickness. The results find that all of the welding
conditions are alike. The mean weld metal hardness of 270 HV is much higher thanthe base metal hardness about 160-170 HV due to a high strength material of filler
wire and it also shows that the softening degree in HAZ was 150-160 HV and similar
to the base metal.

79Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

Fig. 5.15 Vickers hardness distribution in the middle of the welded joints.

5.3.4.4 V-notch Charpy impact test
The toughness property of the specimen of 2x56 mm weaving and 8x51 mm
stable beam was investigated. Figure 5.16 shows the cutting plan and notch
locations of test pieces. The V-notch is located at weld metal and fusion line with thetest temperature of -20oC. The standard specimen size for Charpy impact test is
10mm x10mm x55mm according to JIS Z 2242. In this test, the Charpy test
equipment has a heavy pendulum of 25.31kg and arm length of 0.7 m.
The V-notch Charpy impact test result of the notch location at weld metal andfusion line is shown in Fig. 5.17. According to the Japan Marine Standard of theClassNK, the minimum average impact test value of the welded joint is 64 J at -20
o
C. The weld metal obtains high absorbed energy more than 64 J. However, thefusion line shows the low absorbed energy having the mean value of 40 J less than
the specification requirement. Because grain boundary ferrite is observed in thefusion line as shown in Fig. 5.13 which affects the unstable absorbed energy
although the low heat input can be achieved.
The fracture surface and crack direction are shown in Fig. 5.18. For fusion line,
almost all specimens show a brittle deformation in a straight direction and in two
specimens, the cracks propagate from fusion line to base metal showing highabsorbed energy as called fracture path deviation (FPD). Then the microstructure offracture path in a low absorbed energy specimen is observed in Fig. 5.19 to confirm
that the crack path propagate along with fusion boundary having a fraction of grainboundary ferrite. Moreover, SEM observes the cleavage fracture surface.

80Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire(a) Cutting plan.

(b) Test pieces and notch location.
Fig. 5.16 Preparing specimen for V-notch Charpy impact test.

81Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

Fig. 5.17 Charpy impact test results at a test temperature of -20oC.

(a) Weld metal.
Fig. 5.18 Fracture surface and crack direction of test pieces.
82

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire(b) Fusion line.
Fig. 5.18 Fracture surface and crack direction of test pieces (continued).

Notch location:
FL-Middle (19.6 J)

Fig. 5.19 Microstructure of fracture path and SEM of fracture surface in FL specimen.
83

Chapter 5
Development of vertical welding for heavy-thick steel plate using twin diode lasers and hot wire

5.4 Summary
Since limited power laser of a diode laser is 6 kW, the twin diode lasers and a
hot-wire system was studied and applied to the heavy-thick joint with 50-mm
thickness in the vertical direction. The results of optimizing power distribution andinvestigating welded joint properties can be drawn as follows:
1) The twin laser system with single hot-wire feeding was investigated power
distribution on the heavy-thick groove. The large rectangular beam having highedged energy was irradiated stably combined with either the narrow rectangularbeam with weaving system or the stable large rectangular beam that could carryon the welding continuously until the end based on adequate the power
distribution and hot-wire system.
2) The weld bead appearances showed complete penetration in both wire feedingside and back side without visible lack of fusion. Nevertheless, the cross-sectionof double stable laser beams showed very low dilution and narrow bead widthand revealed the lack of fusion near the wire feeding side. Therefore, the stablebeam with high-edged energy and the weaving beam of either 2-mm or 5-mm
beam widths could keep stable molten pool and melt base metal. That could
improve dilution on edges near plate surfaces and groove walls where the lack offusion easily occurred in order to achieve a sound joint. Consequently, the novel
single-pass vertical welding method can be applied to a heavy-thick joint andaccomplish a high efficient method, the reductions in a total of laser power andheat input.
3) Microstructure revealed the narrow CGHAZ region of all welding conditions eventhe different weld dilution as a result of low heat input. The toughness results ofnotch location at weld metal showed a high mean value at -200C, but the fusionline obtained low average value as the faction ferrite could be found along withthe fusion line.

84Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

Chapter 6
The application of single-pass vertical welding for Ni steel
using hot-wire laser welding
6.1 Introduction
In recent years, the fabrication of the LNG fuel tank in the ships has been
demanded increasingly instead of oil fuel in order to reduce SOx and NOx
emissions. Due to the amount of heat energy for propulsion, its capacity is twice ofthe conventional fuel. Therefore, the LNG fuel tank capacity becomes larger. The
materials such as aluminum, stainless steel (SUS304), 9%Ni and %7Ni steel arehigh resistant to low-temperature brittleness used for storing cryogenic which has avery low boiling point of -163 0C. The conventional welding process for a vertical joint
is automatic TIG that employs multi-pass layers to fulfill a thick-welded joint.
According to multi-pass welding, it spends a great deal of time in the construction
and deteriorates welded joint properties. Thus, we would like to apply the novel
vertical welding process using hot-wire laser welding method on Ni steel plate to cut
down the time spent in multi-pass layers. Moreover, the heat input of the proposed
welding process could achieve much less than that of a conventional single-passvertical welding method, EGW and get an efficient welded joint.
The first part is the introduction of the methodology and joint configuration of
the proposed welding process. The basic ideas in the Chapter 3 and the large size
jig in the Chapter 4 are applied for the long-welded length of 7%Ni steel plate. The
specimens in this study consist of three thicknesses 10 mm, 15 mm and 20 mm. To
achieve a sound welded joint, the welding conditions such as a rectangular laser
spot size, laser power, welding speed and so on are optimized. Then the welded
joint appearances and cross-sections are observed to evaluate the effects of theheat from welding on the dilution in base metal, heat affected zone andmicrostructure.
The second part is to investigate the mechanical properties of welded joints ofthe proposed welding process such as hardness distribution, tensile test, and Vnotch Charpy impact test compared with standard requirements.

85

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

6.2 Optimization of welding conditions for Ni steel plate using the proposed
welding processThis part is to investigate welding conditions of the proposed welding process
for Ni steel plate used as inner walls of the LNG storage tank. Because the thickness
of LNG tanks in the field is variable regarding the capacity, in this experiment there
are three different thicknesses of specimens such as 10, 15 and 20 mm which were
employed to study the efficiency of the proposed welding process in several
thicknesses.
6.2.1 Introduction of the joint configuration and welding conditions of thenovel welding process for Ni steel plate

Fig. 6.1 Schematic layout of the large size jig of the proposed process.
In this experiment, test specimens were performed the long-welded length.
Figure 6.1 shows the schematic layout of the joint configuration for the large-scalespecimen. All of the equipment such as a laser power source, hot wire machine, aweaving system and so on follow in the Chapter 3. The two plates were set vertically.
The hot wire and a diode laser with a weaving system were applied from the front
side and the high-speed camera was attached to observe from the top side. All ofthem were moved vertically by a self-propelled system to control welding speed. A
copper shoe kept the molten pool and supplied the shielding gas and cooling water
inside.
86

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

6.2.1.1 In-situ observation
A high-speed image of the welding phenomena was captured by a high-speed
camera. The pencil camera V-193-M1 model connected with NAC: MEMRECAM
GX-5 module is shown in Fig. 3.5. A focus lens was 35 mm with a diameter of 25
mm and an 810-nm bandpass filter. The capturing parameters were the frame rate of
50 fps, shutter speeds of 1/1000 s and 1/2000 s, and closed aperture. Table 6.1
shows the monitoring condition. For setting a camera perspective to see the melting
phenomena on the groove wall surfaces, it was perpendicular to groove wall
surfaces and 80 degrees from the weld pool plane.

Fig. 6.2 Transmission curve of 810 nm bandpass filter.

Table 6.1 Monitoring conditions of in-situ observation.

High-speed camera
Frame rate, fps
Aperture
Focus lens, mm
Band pass filter, nm/FWHM
Lighting

GX-5
50
Closed35
810 / 10
N/A

Shutter speed, s

1/1000, 1/2000

87

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

6.2.2 Materials used
7%Ni-TMCP steel plate was used as a base metal material. There are three
thicknesses of 10, 15 and 20 mm as well as three different grades of A841 grade G
Class 9, Class 10 and SL7N590 respectively. However, all grades are the 7%NiTMCP steel having similar chemical compositions as shown in Table 6.2 and thedifference in the standard requirement and mechanical property range. The size ofthe specimen is shown in Fig. 6.3 having dimensions of 250w x 500h mm. Plates
were fixed and aligned as a vertical joint configuration. The square groove was set
with a 10-mm gap using a bottom spacer having dimensions of 10 (width) x 20
(height) x 10, 15 and 20 mm (depth). YGT9Ni-2 with a diameter of 1.2 mm wasdeposited as hot-wire filler wire and its chemical composition is shown in Table 6.2.
Table 6.2 Chemical compositions of base metal and filler wire.
Materials

Chemical Composition, wt%
C

Si

Mn

P

S

Al

Cu

Ni

W

Cr

Mo

Fe

A841 Gr.G 0.05 0.07 0.98 0.003
Class 9

-

-

-

6.33

-

Add

Add

Bal.

A841 Gr.G 0.05 0.07 0.98 0.003
Class 10

-

-

-

6.33

-

Add

Add

Bal.

Add

Bal.

SL7N5900.05 0.07 0.98 0.003 0.0003--

6.32 -Add

YGT9Ni-2

0.02 0.08 0.07 0.001 0.001-

0.8

75.1 3.1-

Fig. 6.3 Specimen dimensions of 7%Ni steel.
88

19.7 1.0

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

6.2.3 Experimental procedure
There are three welding conditions regarding the thickness of specimen variedin 10, 15 and 20 mm. The welding parameters of the proposed welding process
should be considered to optimize power density. A laser spot beam, laser power,
and welding speed should be adjusted properly when the thickness changes. In thisexperiment, we have already optimized the welding conditions of three specimens.
For 10-mm thickness, Table 5.3 shows the optimal welding conditions used forlarge size jig of 10-mm thickness. The laser power of 4.5 kW with a welding speed of
6.7 m/min was used. The argon shielding gas with 30-l/min flow rate was used. The
previous study in applying the weaving system obtained better dilution in base metal
and less lack of fusion. Therefore, the laser head of 0.4-mm fiber core wasassembled with a collimator lens of 80 mm, focus lens of 400 mm and homogenizerof LL2.85 to create 2x11 mm rectangular laser beam at just focus. 11-mm length oflaser spot fitted the thickness of specimen and the weaving system of 5-Hz
exponential waveform was applied. The laser spot beam on the groove area isshown in Fig. 6.4 (a).
Table 6.3 Welding parameters for 10-mm 7%Ni steel.
Gap width size, mm

10Laser type

Diode laser

Fiber core, mm

0.4Collimator, mm

80

Homogenizer

LL2.85Focus lens, mm

400

Laser power, kW

4.5

Laser irradiation angle, degree

15

Defocus, mm

0w

Spot size, mm x mm

2 x 11l

Laser distribution type

Uniform distribution

Weaving frequency, Hz

5Weaving waveform

Exponential waveform

Filler wire diameter, mm

1.2

Welding speed, cm/min

6.7 (4m/h)

Wire feed speed, m/min

9.53

Wire current, A

89

Wire feeding angle, degree

45

Shielding gas, 30 l/min

Ar

Pre-irradiation time, s90

89

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

For 15-mm thickness, Table 6.4 shows the optimal welding conditions used forlarge size jig of 15-mm thickness. The laser power of 6 kW with a welding speed of
6.7 m/min was used. The argon shielding gas with 30-l/min flow rate was used. The
previous study in applying the weaving system obtained better dilution in base metal
and less lack of fusion. Therefore, the laser head of 1-mm fiber core was assembled
with a collimator lens of 100 mm, focus lens of 400 mm and homogenizer of LL2.54
to create 4x17 mm rectangular laser beam at just focus. 17-mm length of laser spot
fitted the thickness of specimen and the weaving system of 5-Hz exponential
waveform was applied. The spot beam on the groove area is shown in Fig. 6.4 (b).
For 20-mm thickness, Table 6.5 shows the optimal welding conditions used forlarge size jig of 20-mm thickness. The laser power of 6 kW with a welding speed of
4.17 m/min was used. The shielding gas of argon with 30-l/min flow rate was
employed. The previous study in applying the weaving system obtained better
dilution in base metal and less lack of fusion. Therefore, the laser head of 1 mm fibercore was assembled with a collimator lens of 100 mm, focus lens of 400 mm andhomogenizer of LL2.35 to create 4x26 mm rectangular laser beam at just focus. 26mm length of laser spot fitted the thickness of specimen and the 5-Hz exponential
waveform was applied. The spot beam on the groove area is shown in Fig. 6.4 (c).
Table 6.4 Welding parameters for 15-mm 7%Ni steel.
Gap width size, mm

10Laser type

Diode laser

Fiber core, mm

1Collimator, mm

100

Homogenizer

LL2.54Focus lens, mm

400

Laser power, kW

6

Laser irradiation angle, degree

15

Defocus, mm

0Spot size, mm x mm

4w x 17l

Laser distribution type

Uniform distribution

Weaving frequency, Hz

5Weaving waveform

Exponential waveform

Filler wire diameter, mm

1.2

Welding speed, cm/min

6.7 (4m/h)

Wire feed speed, m/min

14.1

Wire current, A

105

Wire feeding angle, degree

45

Shielding gas, 30 l/min

Ar

Pre-irradiation time, s90

90

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

Table 6.5 Welding parameters for 20-mm 7%Ni steel.
Gap width size, mm

10Laser type

Diode laser

Fiber core, mm

1Collimator, mm

100

Homogenizer

LL2.35Focus lens, mm

400

Laser power, kW

6

Laser irradiation angle, degree

15

Defocus, mm

0Spot size, mm x mm

4w x 26l

Laser distribution type

Uniform distribution

Weaving frequency, Hz

5Weaving waveform

Exponential waveform

Filler wire diameter, mm

1.2

Welding speed, cm/min

4.17 (2.5 m/h)Wire feed speed, m/min

11.8

Wire current, A

91.6

Wire feeding angle, degree

45

Shielding gas, 30 l/min

Ar

Pre-irradiation time, s

90

(a)

(b)

(c)

Fig. 6.4 Schematic layout of weaving laser spot placed on gap area of 7%Ni steel
specimens (a) 10-mm (b) 15-mm and (c) 20-mm thicknesses.
91

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

6.2.4 Welding performing of Ni steel specimens and weld bead appearances
The experimental setup of the proposed welding process for Ni steel plate isshown in Fig. 6.5. The photograph shows that the plates were set vertically with asquare groove having a 10-mm gap. The test specimen was placed on a back-side
copper plate on the large-size jig and was fixed by clamp locks. To protect the gap
shrinkage during welding, the spacer of 10-mm thickness was inserted on the top of
the groove. This jig was similar to EGW. It was accessible from only the front side.
The self-propelled unit carried a laser head, a hot-wire torch, a copper shoe, and ahigh-speed camera to move together along a guide rail. The copper shoe was cooledby water flow inside to form a weld bead in the front side and it supplied shielding
gas to protect the weld pool. The high-speed camera was used to observe weld pool
phenomena on the top of the self-propelled unit and its parameter was shown in
Table 6.1.

Fig. 6.5 A photograph showing the experimental set up using the proposed process.
92

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

(a)

(b)

(c)
Fig. 6.6 Weld bead appearance of 7%Ni steel welded joints (a) 10 mm(b) 15 mm (c) 20 mm.
93Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

Fig. 6.7 Marco cross-sections of 7%Ni steel welded joints.
Figure 6.6 shows the weld bead appearances of 7% Ni steel specimens
performed by the proposed welding process with the large size jig. The long-welded
joints with changing the thickness of the specimen were investigated. Fig. 6.6(a)shows the weld bead appearance of 10-mm thickness. The weld bead achieves full
penetration of both sides without visible lack of fusion and the weld bead is smoothand sound, but in the top of the specimen it had a trouble of hot-wire feeding during
welding. Fig. 6.6(b) shows the weld bead appearance of 15-mm thickness. The weld
bead shows the obvious lack of fusion on the back side in the first period of welding
start because unstable weld pool phenomena could occur when a hot-wire rod was
fed into the molten pool of pre-irradiation in a weld groove, the molten pool flowed
turbulently for a while thereafter the molten pool became stable. The weld bead isthen smooth and sound. There is no visible lack of fusion and it obtains full-filled
penetration. Fig. 6.6(c) shows the weld bead appearance of 20-mm thickness. The
weld bead indicates the a little lack of fusion on the back side in the first period which
is similar the problem to 15-mm specimen.
Figure 6.7 shows the macro cross-sections of 7%Ni steel welded joints using
the proposed welding process. The cross-sections were cut from the bottom and topof each specimen. The top cross-section of specimens shows less dilution than thebottom cross-section due to the heat accumulation and heat distribution during
welding. All cross-sections reveal sound joints without lack of fusion inside thewelded joints. However, some cross-sections at the bottom of 15-mm and 20-mm
specimens indicate small lack of fusion on the edge.

94Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

6.2.5 In-situ observation on the molten pool by a high-speed camera
Refer to the Chapter 3 and 4, a narrow rectangular beam with a weaving
system could optimize the power density distribution and higher dilution wasachieved in this beam than in a stable large rectangular beam. As shown in Fig. 6.8,
the weaving motion of 5 Hz-exponential waveform is used. In one cycle, the weaving
beam is held on each groove surface about 72.5 ms and moving time between eachgroove surface is about 27.5 ms, so the average energy density on near the groove
surfaces is much higher than on the middle of the groove.

Fig. 6.8 Schematic layout of laser motion at 5 Hz exponential waveform and an
example of high-speed images in pre-irradiation.
Before hot-wire feeding the weaving beam was irradiated into the groove area
on a spacer surface to create an initial molten pool for 90 s because a large amountof the weld metal volume was deposited and the molten pool by pre-irradiation could
help to melt and distribute the filler wire steadily. Figure 6.8 shows an example of95

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

high-speed images in pre-irradiation. For instance, the 0.38 s image shows theweaving laser held on a groove surface. In the 0.46 s, the weaving laser is moving tothe other side and then the 0.48 s image shows the weaving laser held on the otherside. This laser spot motion repeats during welding. The 90 s image shows the stable
molten pool by pre-irradiation before hot-wire feeding on the whole groove area.
Figure 6.9 shows a schematic layout and high-speed images of a weld pool
during welding of 10-mm, 15-mm and 20-mm specimens. The hot wire was fed into
the molten pool and moved vertically in the upward direction. The steady weld poolformation and wire feeding were observed continuously for the whole welding period
in all of three welding conditions. The single hot-wire feeding could be deposited theweld metal sufficiently. As a result, the sufficient weld penetration could be achieved
both on the back side and the wire feeding side.

Molten pool during welding

Schematic layout

Fig. 6.9 Schematic layout and high-speed images of the proposed process during
welding of 7%Ni steel specimens 10mm, 15mm, and 20mm thickness.
96Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

6.2.6 Evaluation of macrograph and heat input of welded joints
Figure 6.10 shows the macro cross-sections of 7% Ni steel welded joints of the
proposed welding process. According to the top cross-section of each specimen in astable welding period, the cross sections having different thicknesses such as 10mm, 15 mm and 20 mm illustrate complete weld penetration which is achieved on
both of wire feeding side and the opposite side. Any imperfections are not observed
and low dilution is achieved in all welded joints as shown in Fig. 6.10. These joints
could derive small distortion from low dilution even though a heat input of the
proposed process is higher than that of the conventional TIG welding process due tousing the single-pass welding joint in the proposed process.
The measured weld bead widths of three cross-sections from the wire feedingside to the back side are shown in Fig. 6.11. The gap of specimens was set with 10
mm. The results show that the penetration depth of both groove surfaces is rather
small and not higher than 2 mm. The smallest width appeared to groove edges
where the lack of power density occurs. Additionally, on the wire feeding side filler
wire cut the laser beam above that causes the actual-low laser power density.
Then, the heat input of the proposed process is calculated simply by using only
the heating from laser power as a heat source, not including a small effect of the hotwire system. The equation hereby is (laser power (kW)/welding speed (cm/min))*60
and the heat input was calculated at the maximum value with the efficiency factor of100%. The results are plotted in graph to show the relationship between an amountof heat input and thickness plate as shown in Fig. 6.12 as follows 40, 54 and 86
kJ/cm respectively. Regarding increasing thickness, the heat input is rise optimally ina linear relationship and the approximate value of the heat input per thickness of the
proposed process is about 4 (kJ/cm.mm).

Fig. 6.10 Top-side macro cross-section analysis of three specimens.
97

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

Thickness = 10 mm

Thickness = 15 mm

Thickness = 20 mm

Fig. 6.11 Measured bead width on the cross-sections of three specimens.

Fig. 6.12 A quantity of heat input of the proposed process on thicknesses

98Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

6.3 The properties of 7%Ni steel welded joints using the proposed processThis part presents the results of welded joint properties of 7% Ni steel plateusing the proposed welding process. In the stable welding state of long-welded
specimen, we obtained complete-full penetration without imperfections and then thatwas investigated in microstructure, hardness distribution, tensile test, and Charpy Vnotch impact test.
6.3.1 Microstructure characteristics of the welded joints.
The cross-sections of 7%Ni steel welded joint with filer wire of 75% Ni-basealloy was polished firstly by various grit sizes of sandpaper number 100, 500, 1200,
2000 and 3000 in order and then secondly by diamond paste with a particle size of 6,
1 μm and magnesium oxide (MgO) powder. There are two etching procedures; first,
it was etched by 3% Nital solution to reveal the microstructure of HAZ and second,according to 75% Ni of filler wire etched by electro-etching, 10V with 10% oxalic acidwas employed to reveal the microstructure of weld metal observed by optical
microscopic (1LM21 lasertec).
Figure 6.13 shows the weld metal microstructure of 7%Ni welded joint.
According to the filler wire material of 75%Ni-20%Mo (YGT9Ni-2), its microstructure
can indicate the weld metal solidification with columnar dendrite and find equiaxed
dendrite.
Figure 6.14 shows the FL and HAZ microstructure of the 20-mm-thick welded
joint and its chemical composition of 7% Ni steel is shown in Table 6.2. FL and FL+1
images of CGHAZ region, it consists of the coarsened microstructure of prioraustenite grains and martensite laths. On the other hand, the FL+3 and FL+5 images
indicated that the fine prior austenite gains and the martensite laths.

Fig. 6.13 Weld metal microstructure of the 7%Ni steel welded joint.

99

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

FL and HAZ

Fig. 6.14 Microstructure of fusion line and heat affected zone in the 20-mm thicknessof the 7%Ni steel welded joint.

100Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

6.3.2 Hardness distribution of the welded joints
Micro Vickers hardness testing of the specimens follows the method in 4.4.1
and the hardness of weld metal, heat affected zone and base metal are measured inaccordance with JIS Z 2244.

Hardness distribution

10 mm

7% Ni steel welded joint

15 mm

HAZ

20 mm

HAZ

HAZ

Fig. 6.15 Vickers hardness distribution of the 7%Ni steel welded joints.

101

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

Figure 6.15 shows the hardness distribution through the base metal, heataffected zone and weld metal of three specimens. The cross-sections of 10 mm and
15 mm were measured the hardness on the middle of thickness; however, the 20mm cross-section was measured in three locations such as near a surface, a quarterof thickness and a half of thickness. The results show the same hardness distribution
in all the welded joints based on their microstructures. The base metal of 7%Ni steel
has the mean hardness of 270 HV and the mean weld metal hardness of 210 HV issofter than that of the base metal because the filler wire material strength is less thanthe base metal strength. The high hardness degree in the HAZ was 330 HV due tothe martensite structure and the hardened widths of the HAZ are about 13, 12 and
14 mm respectively.
6.3.3 Tensile test of the welded joints
Figure 6.16 shows the tensile test specimen of three-thickness welded joints.
Each thickness was prepared with 2 tensile test coupons. Figure 6.16 a) shows thetensile specimens of 10-mm welded joint. After machining, the actual thicknesses
were 8.8 and 8.6 mm and their cross-sectional areas were 2.22 x 10-4 and 2.17 x 10-4
m2 in order. Figure 6.16 b) shows the tensile specimens of 15-mm welded joint. The
tensile test coupons had the actual thicknesses of 14.1 and 14.1 mm and their crosssectional areas were 3.53 x 10-4 and 3.53 x 10-4 m2 in order. Figure 6.16 c) shows thetensile specimens of 20-mm welded joint. The tensile test coupons had the actual
thicknesses of 19.3 and 19.2 mm and the cross-sectional areas were 4.86 x 10-4 and4.76 x 10-4 m2 in order.

a) 10-mm specimen.
Fig. 6.16 Tensile test specimens of the 10, 15 and 20 mm welded joint.

102

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser weldingb) 15-mm specimen.

c) 20-mm specimen.
Fig. 6.16 Tensile test specimens of the 10, 15 and 20 mm welded joint (continued).
The tensile test results are shown in Table 6.6 and photographs and stressstrain curves are exhibited in Figure 6.17. There are 2 tensile specimens in eachthickness. All specimens are broken at weld metal as the filler wire’s strength is lessthan the base metal’s strength. In the 10-mm welded joint, the tensile strength is690, 691 MPa. In 15-mm welded joint, the tensile strength is 756, 737 MPa and the
20-mm welded joint have the tensile strength of 800, 794 MPa. Therefore, all of thetest coupons meet the requirement having the tensile strength more than thespecification of the filler wire material.

103

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

Regarding the breaking point in the weld metal where the material is softer. As
shown in the photograph specimens, in the 10-mm and 15-mm specimens, theplastic deformation only occurs in the weld metal, so the elongation after fracture
became low. Accordance to the tensile strength results, even though the entire
welded joints are broken in the weld metal, 10-mm specimen obtains less tensile
strength than that of 15-mm and 20-mm specimens and the tensile strength is less in
the 15-mm specimen than in the 20-mm specimen. Soft/Hard ration in the weld metal
could be studied because the welded joints have the same gap width as 10 mm, butdifferent thicknesses. Soft/Hard ratio in weld metal can be defined in equation (6.1).
A surface area of weld metal (soft) is divided by a cross-sectional area of base metal(hard). However, the joints are set with a square groove. The gap width divided bythickness can be applied in these joints and the result is shown in Table 6.7. The 10mm specimen has the highest Soft/Hard ration of 1, the 15-mm specimen of 0.67
and the-20 mm specimen of 0.5 in order that results in the tensile strength of thewelded joint. Consequently, in case the weld metal is softer than the base metal, thehigh Soft/Hard ratio would behave low tensile strength because that is able to havethe weld metal easily plastically deformed.
Table 6.6 Tensile test results.
Specimen name

7%Ni_10 mm

7%Ni_15 mm

7%Ni_20 mm

Thickness, mm

8.8, 8.6

14.1 , 14.1

20, 19Tensile strength,MPa690, 691

756, 737

800, 794

Elongation after
fracture, %16, 1412, 1020, 19A841 G Cl10(750 – 885 MPa)

SL7N590(690 – 830 MPa)

Specification
Base metal

A 841 G Cl9(690 – 825 MPa)

Filler wire

Soft/Hard ratio =YGT9Ni-2 (660 min. MPa)
Weld surface area (soft)
Cross sectional area (hard)=

Gap x htxh

Table 6.7 Soft/Hard ratio.
Specimen

Soft/Hard ratio

10 mm

1

15 mm

0.67

20 mm

0.5
104

=

Gap
Thickness(6.1)

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding(a) 10-mm specimen.

(b) 15-mm specimen.

(b) 20-mm specimen.
Fig. 6.17 Photographs and stress-strain curves of tensile test results.

105

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding(a) 10-mm thickness.

(a) 15-mm thickness.
Fig. 6.18 A relation between the tensile strength of base metal and welded joints.
The aforementioned photographs of 10 and 15 mm tensile specimens indicatethat the plastic deformation only occurs in weld metal resulting in less elongation
after rupture. Figure 6.18 (a), (b) shows the relationship between the tensile strengthof base metal and welded joints in the specimen thickness of 10 and 15 mm in order.
For 10-mm thickness, the yield strength of 720 MPa in base metal is higher thanboth the tensile test results of 690 MPa and 691 MPa in the welded joints, so only in106

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

the weld metal did the plastic deformation appear. For 15-mm thickness, the basemetal shows the yield strength of 750 MPa which is nearly the tensile strength of thewelded joints, 756 and 737 MPa. As a result, the plastic deformation in the basemetal of the welded joints could not be observed.
6.3.4 Charpy V-notch impact test of the welded joints
Figure 6.19 shows the test specimen and notch locations. The stand specimen
size for Charpy impact test is 10mm x10mm x55mm according to JIS Z 2242 and thespecimens are notched at weld metal, fusion line, fusion line+1 and fusion line+2. A
test temperature is evaluated at -1960C and the values shall exceed the minimum
impact energy of 27 J specified in JIS 8285. In this test, the Charpy impact
equipment has a heavy pendulum of 25.31kg and arm length of 0.7 m.
Figure 6.20 (a), (b) and (c) show the cutting plan and notch locations in eachspecimen, 10, 15 and 20 mm.

Fig. 6.19 Test specimen and notch locations.

(a) 10-mm specimen.
Fig. 6.20 Cutting plan.
107

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding(b) 15 mm specimen.

(C) 20 mm specimen.
Fig. 6.20 Cutting plan (continued).

108

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

Figure 6.21 shows the toughness values and fracture specimens of FL andFL+1 in the 10-mm-thick welded joint. The fusion line and fusion line+1 exhibit anextremely high absorbed energy when it is compared with the requirement of 27 J.
The test pieces indicate a ductile fracture surface without imperfections. Therefore,
this welded joint is satisfied for the toughness property.

Fig. 6.21 Charpy impact test results of 10-mm-thick welded joint.

109

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

Figure 6.22 shows the Charpy impact test results and test pieces of FL, FL+1,
and FL+2 in the 15-mm-thick welded joint. The very high toughness values areobserved in all of the fusion line, fusion line+1 and fusion line+2 more than thespecification of 27 J so it satisfies the standard requirement and specimens show aductile fracture surface without imperfections.

\Fig. 6.22 Charpy impact test results of 15-mm-thick welded joint.

110Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

Figure 6.23 shows the Charpy impact test results and test pieces of WM, FL in
the 20-mm-thick welded joint compared with the welded joint of GTAW in 32-mm
thickness. The toughness values of weld metal and fusion line of the proposed
process are high and much more than 27J. The welded joint satisfies therequirement, but the average toughness value has little lower in the HLW welded
joint than in the GTAW welded joint.

Fig. 6.23 Charpy impact test results of 20-mm-thick welded joint.

111

Chapter 6
The application of single-pass vertical welding for Ni steel using hot-wire laser welding

6.4 Summary
This chapter is to apply the single-pass vertical welding process using hot-wirelaser for Ni steel plate. The proposed process is investigated to increase productivity
and to reduce welding consumables compared with the conventional arc welding
process. The summary results can be drawn as follows:
1) Ni steel plate range is from 6 to 50 mm usually used for LNG tanks in the field.
This research applied the proposed process to 10, 15 and 20 mm thickness of
7% Ni steel plate in order to find the optimal welding conditions in different
thicknesses. A controllable narrow and rectangular beam with a weaving system
was used. Then the laser power and welding speed were optimized in eachthickness to derive sufficient heat energy to continue the welding steadily, so theaverage heat input per thickness was about 4 (kJ/cm.mm).
2) The proposed process achieved a stable weld pool formation and steady hot-wirefeeding during welding. However, some lack of fusion could be observed on theback side in the first period because the molten pool did not flow to an edge ofthe back side. Thereafter the complete penetration could be obtained and thecross-sections showed full penetration without any imperfections.
3) The joint properties of tensile test and Charpy impact test met the standard
requirements. In case of the tensile strength, the breaking point was at the weld
metal due to the softer material, and Soft/Hard ration also affected the tensile
strength result. At the same gap width, the 10-mm specimen having higher
Soft/Hard ration indicated the lesser tensile strength. In case of Charpy impact
test, very high absorbed energy was found in all test pieces and the notch was
located at WM, FL, FL+1, FL+2.

112

Chapter 7
Summary and future work

Chapter 7
Summary and future work

The aim of this research is to develop a novel welding process by using a diodelaser and a hot-wire system. The hot-wire laser welding (HLW) was applied to asingle-pass thick weld joint in a vertical direction so as to reduce the heat input andto replace the conventional welding process such as EGW of the large heat input
process. The combination of a controllable rectangular beam as a heat source andhot-wire system was investigated. Adequate power distribution plays an importantrole to achieve a sound joint. The laser beam type and weaving system were
optimized the power density distribution to reduce laser power at the most.
At first, the background statement, the objectives, and construction of the thesis
were discussed and then recent researches on the vertical welding relevant to thisstudy were summarized.
For the single-pass vertical welding, the principle ideas for optimizing power
distribution are to keep the molten pool and melt the base metal. A controllable
rectangular beam with a weaving system was studied. 2-mm and 4-mm beam widths
with a 5-Hz exponential waveform having different power density were used. The 2mm beam width showed high power density on the groove surfaces that could not
keep heating on the middle of groove and some heat was lost by high reflecting laser
on above the weld pool. In other words, the 4-mm beam width could keep the molten
pool and melt the base metal better than the 2-mm beam width. In addition, thebeam could concentrate more power on the beam edges by using a high-edgedenergy beam type made by a particular optic lens that could increase dilution andreduce lack of fusion on the groove edges. Consequently, the beam having the highedged type and weaving system could optimal power distribution on the groove area.
Moreover, Oxygen additions in Ar shielding gas protecting the molten pool
affected weld pool phenomena in a positive way that was able to increase dilution in
the base metal. On the contrary, a high among of oxygen content could generate athick oxide layer between the copper shoe and the weld bead. In the experiment, theoptimal shielding gas provided better dilution by using 20%Oxygen-added Ar
shielding gas than using pure Ar shielding gas.
The optimal welding conditions using a 6-kW rectangular beam were applied toa large size specimen and its thickness of 28.5 mm. The HLW joint with the longwelded specimen obtained low dilution and narrow heat affected zone as a result of
the low heat input welding process and the heat input of the HLW joint had much
lower than that of the EGW at the same thickness of base metal. Regarding theproperties of the welded joint, the tensile strength met the requirement of the113Chapter 7
Summary and future work

specification and the V-notch Charpy impact test result of notch location at weld
metal obtained high absorbed energy at -200C according to CTOD test results thatexhibited the high resistance to brittle propagation at -100C. However, the fusion line
showed the unstable absorbed energy because grain boundary ferrite could beobserved in the fusion line.
As a limited diode power is 6 kW, twin diode lasers to increase a total of 12 kW
were applied to a thicker plate of 50 mm. The large rectangular beam having highedged energy was irradiated stably combined with either the narrow rectangularbeam with the weaving system or the stable large rectangular beam. Based onadequate power laser and hot-wire system, the stable welding process could beobserved during welding in three welding conditions. However, the cross-section of
double stable laser beams showed very low dilution and narrow bead width andrevealed the lack of fusion near the wire feeding side. Consequently, the stablebeam with high-edged energy and the both weaving beams of 2-mm and 5-mm
widths that achieved a sound joint could improve dilution on edges around platesurfaces and groove surfaces where the lack of fusion easily occurred.
Next, the novel single-pass vertical process was studied in Ni steel plate and
EGW is not generally used for Ni steel as a result of large heat input that toughness
property could deteriorate at a very low temperature of the used material and could
generate severe distortion. Regarding Ni steel plate in the 6-50 mm range, it isusually used for LNG tanks in the field. This research applied the proposed process
to 10, 15 and 20 mm of 7% Ni steel plate to find the optimal welding conditions indifferent thicknesses such as a laser spot shape, laser power, a welding speed, ajoint configuration, etc. The HLW achieved a stable weld pool formation and steady
hot-wire feeding during the whole welding period in all specimens. The complete
penetration could be obtained in the steady state and the cross-sections showed full
penetration without lack of fusion. The joint properties of tensile test and Charpy
impact test met the standard requirements. In case of the tensile strength, thebreaking point was located at the weld metal due to the softer material, andSoft/Hard ration also affected the tensile strength value. At the same gap width, the
10-mm specimen having higher Soft/Hard ration showed the low tensile strength
value.
The heat input of the single-pass vertical weld joint between the convention arc
process and HLW can be summarized in Fig. 7.1. SEGARC and VEGA processes
used for EGW of the vertical joint are developed [41, 61, 91-92] to increase
deposition rates and to reduce heat input. The heat input of HLW was calculated at
the highest value using only laser power and welding speed with the efficiency factorof 100% because of the unknown absorption factor of the proposed process.
Therefore, the actual heat input should be much lower than the calculation. The
graph showed that the heat input of the developed EGW processes is lower than thatof the conventional EGW, but it is higher than the heat input of HLW about 30% in
10-20 mm thickness. However, the heat input trend of the developed EGW exhibits
vary high slope when the thickness is more than 30 mm. The HLW trend obtains low

114

Chapter 7
Summary and future work

slope and relatively straight line because the HLW could control the power density
distribution precisely and efficiently for joining a heavy-thick joint.

Fig. 7.1 Comparison of the heat input in a vertical joint between EGW and HLW.
For the shipbuilding industry, the novel vertical welding process achieved the
optimal welding conditions such as power density distribution, the hot-wire systemand so on. The 6-kW rectangular laser beam with the high-edged energy andweaving system could be applied to the large scale size in the thick steel plate, but
its thickness was up to 28.5 mm. Consequently, the twin diode lasers with a total of
12 kW were investigated to a thicker plate of 50 mm and the optimal welding
conditions were used in a small size jig. In future work, it is essential that the twin
diode lasers should be developed to a large scale specimen like the field. A new jig
has to be designed based on focal length and beam shape of both lasers, andthermal behaviors on the large-scale specimen would be realized during welding.
Moreover, the weld joint property of Charpy impact test at fusion boundary should bestudied in the chemical composition and solidification on fusion line to find more thecause of brittleness and be studied in changing groove from square groove to V
groove.
For LNG tanks, since the steel plate is currently used in the high range
thickness depending on tank capacity, the proposed process of the vertical joint wasevaluated for the plate thicknesses of 10, 15 and 20 mm. In the future work, thethicker plate up to 50 mm should be studied and in case the plate is thinner than 10mm, a narrower gap such as 8 mm should be studied so as to obtain high tensile
strength.

115

Acknowledgements

Acknowledgements
Firstly, I would like to express my sincere gratitude to my advisor Prof.
Motomichi Yamamoto for continuous support of my Ph.D study and my living inJapan, for his patience, motivation, and immense knowledge. His guidance helped
me in all the time of research and writing of this thesis. I could not have imagined
having a better advisor and mentor for my Ph.D study. Furthermore, I would like tothank Prof. Kenji Shinozaki. He gave me so much positive help and instructive
discussion in my study, and I have learned so much from him, especially the greatsuggestion on research strategy and metallurgical perspective.
I would like to thank the rest of my thesis committee: Prof. Atsushi Sugata, Prof.
Eiji Shintaku and Prof. Hino Ryuutarou for their insightful comments andencouragement, but also for the hard question which incented me to widen myresearch from various perspectives.
I would like to thank the past members of my research group: Mr. Masuda
Kouhei, Mr. Shota Sekiguchi and Mr. Kodama Kazuki. I would also like to thank all
the past and present members of the Materials Joining Science and Engineering
Laboratory, in the Department of Mechanical Science and Engineering, Hiroshima
University, for their enthusiastic help to both my life and study.
I would like thank my parents for their support and encouragement from myyouth to my time as a doctoral student. Their teaching is gratefully appreciated andhas been invaluable in my life.

116

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81) R. Phaoniam, K. Shinozaki, M. Yamamoto, K. Kadoi, S. Tsuchiya, A. Nishijima :“Development of a highly efficient hot-wire laser hybrid process for narrow-gap
welding-welding phenomena and their adequate conditions”, Weld World,
Vol.57(2013), p.607-613.
82) R. Phaoniam, M. Yamamoto, K. Shinozaki, M. Yamamoto, K, Kadoi :“Development of a Heat Source Model for Narrow-gap Hot-wire Laser Welding”,
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plate using hot-wire laser welding method”, Doctoral dissertation, Hiroshima
University, 2015.
86) M. Kouhei : “Development of vertical welding process for thick steel plate using
hot-wire laser welding method”, Master thesis, Hiroshima University, 2016.
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126

Published or Submitted Papers in Regard to This Thesis

Published or Submitted Papers in Regard to This Thesis
1. S. Charunetratsamee, M. Yamamoto, K. Shinozaki, H. Yajima, N. Oda, H.
Aono : “Development of Vertical Welding Process for Heavy-thick Steel Plateusing Hot-wire Laser Welding”, Proceedings of The 8th Asia-Pacific IIWInternational Congress, Thailand, 2019, p.45-48. (Chapter 5)
2. S. Charunetratsamee, E. Warinsiriruk, K. Shinozaki, M. Yamamoto :“Optimization of Laser-Irradiating Conditions for Vertical Welding on Thick Steel
Plate using Hot-Wire Laser-Welding Method”, Quarterly J. JWS, Vol. 37(4),2019, p.187-192. (Chapter 3)
3. S. Charunetratsamee, E. Warinsiriruk, K. Shinozaki, M. Yamamoto : “Melting
by Reflected Laser Beam during Vertical Welding via Hot-Wire Laser-Welding
Method”, Quaterly J. JWS, 2019. (Accepted) (Chapter 3)

127Presentations

Presentations
1. S. Charunetratsamee, M. Kohei, M. Yamamoto, K. Shinozaki, H. Yajima, T.
Fukui, S. Nakayama, K. Kamita, S. Kodama, H. Aono : “Optimization of Vertical
Welding Conditions for Thick Steel Plate using Hot-wire Laser Welding Method”,
National Conference of JWS, Japan, Vol.101(2017-9), p.424-425. (Chapter 3)
2. S. Charunetratsamee, S. Sekiguchi, M. Yamamoto, K. Shinozaki, H. Yajima, Y.
Funatsu, S. Nakayama, K. Kamita, N. Oda, H. Aono : “Properties ofSingle-pass Vertical Welded Joint of Thick Steel Plate using Hot-wire Laser
Welding”, National Conference of JWS, Japan, Vol.103(2018-9), p.380-381.
(Chapter 4)
3. S. Charunetratsamee, M. Yamamoto, K. Shinozaki, H. Yajima, N. Oda, H.
Aono : “Development of Single-pass Vertical Welding on Heavy-thick Steel
Plate using Hot-wire Laser Welding”, The 8th International Congress on Laser
Advanced Materials Processing, Japan, 2019, p.128. (Chapter 5)128