J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Parameter selection in dissimilar friction stir welding of ODS ferritic steel andRAFM steel F82H
Wentuo HAN a,*, Akihiko KIMURAa, Dongsheng CHENb, Zhexian ZHANGb,
Hisashi SERIZAWAc, Yoshiaki MORISADAc, Hidetoshi FUJIIc
a Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japanb Graduate School of Energy Science, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japanc Joining and Welding Research Institute, Osaka University, Ibaraki, Osaka 567-0047, Japan(Received: 16 September 2014 / Accepted: 19 January 2015)

Dissimilar joints of ODS ferritic steel and RAFM steel F82H would be required for the construction ofadvanced blankets to increase the exible design margin and get the benets from each material in a functional way.
The welding parameter determines the formation of joint, the generation of welding defect and the microstructureof the joint. This study aims to achieve the defect-free joint by optimizing the welding parameters in dissimilar
friction stir welding of the ODS and F82H steels. Four different welding conditions were applied to the welding
processes, and the corresponding effects on defects and microstructures were investigated. Results show that neitherthe δ ferrite nor inter-metallic compound can be generated in the FSW joint of F82H and ODS steels. In order toachieve the defect-free joint and to prevent the stir tool from breaking, the “softer material” F82H should bearranged on the retreating side and the stir tool should be plugged into the F82H side.
Keywords:

friction stir welding,parameter selection,

dissimilar welding, ODS ferritic steel,welding defect analysis

RAFM steel F82H,parameter determines the formation of joint, thegeneration of welding defect and the microstructure ofthe joint, the selection of welding parameter is highlyrequired.
Till now, only few works have been focused on thewelding of ODS steels. Particularly, no related work
concerning the parameter selection of dissimilar FSW ofODS and F82H steels has been reported. The present study
aims to achieve the defect-free joint and characterize theeffects of different parameters on the microstructures anddefects of the dissimilar FS welded joints.

1. Introduction

Nuclear materials demand special standards ofperformance and quality because of their severe
surroundings of the high energy bombardment and theelevated temperature. The ODS ferritic steel, which has
excellent mechanical properties at elevated temperatureand great resistance to the irradiation induced swelling, iswidely accepted as a structure material for advanced fusion
blankets, while the F82H ferritic/martensitic steel has been
considered as a promising candidate materials for the ITER
fusion reactor. Dissimilar welding of ODS ferritic steel andF82H steel would be required for the construction ofadvanced blankets to increase the exible design margin
and get the benets from each material in a functional way[1-4].
However, comparing with the welding of other steels,the dissimilar welding process of ODS and F82H steel ismuch more complicated and difficult. It has been foundthat conventional melting welding methods cannot be used
to weld these two steels, as it can disturb the finedispersion of oxide particles in the ODS steel, as well as
significantly decrease the strength and toughness in F82Hsteel due to the generation of δ ferrite and inter-metalliccompounds. Friction stir welding (FSW), which is a solid
welding technology has been considered to be a promising
way to weld these two materials [5-9]. As the welding

2. Experiment

The materials used in this study were the 15Cr-ODS
ferritic steel and the RAF/M steel F82H with thecompositions tabulated in Table 1. Both of ODS and
F82H Plates were cut into pre-weld specimens with thedimensions of 1.5 mm thickness, 35 mm length and 10mm width, and then subjected to FSW in a butting
configuration.
Table.1 Chemical compositions of ODS and F82H steels (mass%)

author’s e-mail: w-han@iae.kyoto-u.ac.jp

65©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

W. Han et al., Parameter Selection in Dissimilar Friction Stir Welding of ODS Ferritic Steel and RAFM Steel F82H

shifted a 2 mm toward the ODS or F82H side, and did not
penetrate into the other steel.
Specimens welded by different conditions were cut
perpendicular to the welding direction. The investigationsof the welding defects and microstructures wereundertaken on the transverse cross-section of the welds
and carried out by an optical microscopy (OM) and ascanning electron microscopy (SEM, Zeiss, Ultra 55
FE-SEM). Energy dispersive X-ray spectroscopy (EDX)
was performed to determine of the elements in specialstructures (e.g., the mixed structures of ODS and F82H
steels and the lump-like welding defects). Combining
with the EDX results, the demarcating function ofKikuchi lines in the EBSD system of Zeiss Ultra 55
FE-SEM was used to analyze the structure of thelump-like welding defects. In order to remove
deformations and scratches of EBSD specimens, and toobtain highly reflective surfaces, samples were polishedby emery paper, diamond paste and alumina colloidal
silica, sequentially.

Ultra-temperature-resistant W-C based stir tools,
which had shoulder diameter of 12 mm, pin diameter of4mm and 1.3 mm pin length, were used. In the welding
processes, tool tilt-angle was set as 3°. The dissimilar
welding processes totally involved four different welding
conditions, which were labled as FSW1, FSW2, FSW3
and FSW4 in Table 2. In all the conditions, the traverse
Table 2 Welding conditions of the dissimilar welding processes

speed were set as 50 mm/min, while two varied rotation
speeds (200rpm, 250rpm) were used. During all thewelding process, ODS steel was set on the AS (advancingside), and F82H was set on the RS (retreading side),
while the plug positions of the stir tool were different. In
conditions of FSW1 and FSW3, the stir tools wereplugged into the ODS side, while the plugged positions ofFSW2 and FSW4 were in F82H steel, a detailed
schematic illustration of the plugged position of the stir
tool is shown in Fig. 1. In each case, the tool probe was

3. Results and discussion

The overviews of welding defects within the joints
welded by different parameters are shown as Fig. 2. Inthese welding conditions, only the condition FSW4
(250rpm, plunged in RS-F82H side) can generate thedefect-free joint, while different types of the welding
defects (e.g., wormholes, dis-bonded defects and lump-like
uncertain structures) can be found in all the other threejoints by observing their macrostructures.
Remarkably, the wormhole defect can be detected in
all the defective joints. Particularly, wormholes prevalently
appear near the lump-like structures in FSW1 and FSW3.
Comparing with the surrounding material, these lump-likestructures are obviously different in microstructures (Fig.
3a, d and e), as well as the hardness of these structures areextremely high with an average value about 890 Hv. These
lump-like structures seem belong to the inter-metalliccompound. However, as the FSW process is far below themelting points of F82H and ODS steels, generation of theinter-metallic compound is unreasonable and illogical. TheEDX analysis (Fig. 4) of the lump-like structure (Fig. 3e)shows that this structure has the W content about 82 wt%.
Although F82H and ODS steels contain W, it can be firmly
speculated this W enriched lump is not formed by theelement segregation of the welding, because the W
contents in the base metals (i.e., 1.9 wt% in ODS, 2 wt% inF82H) are too limited to form the lump like structures insuch enriched W content and in such tremendous size. Inaddition, it is also impossible to occur such large-scalesegregation during the transient welding process. The
further analysis of the Kikuchi pattern of this lump-likestructure (Fig. 5) shows that rather than the single tungsten,
the lump has a W-C structure, as the detected Kikuchi

Thickness: 1.5m

Fig. 1 Schematic diagram of the dissimilar FSW processes: (a)stir tool plunged into ODS steel on AS, corresponding to theconditions of FSW1 and FSW3; (b) stir tool plunged into F82H
steel on RS, corresponding to the conditions of FSW2 and FSW466

W. Han et al., Parameter Selection in Dissimilar Friction Stir Welding of ODS Ferritic Steel and RAFM Steel F82H

Fig. 2 Overview of the joints welded by different conditions; symbols (circle, squire and triangle) present different defects

pattern has the very high superposition with the criterion ofW-C pattern. As the stir tool is made by the W-C basedmaterial, it can be firmly speculated that the lump-likestructures are the fragments of the stir tool.
It is also mentionable that the lumps can only befound when stir tools are plunged into the ODS steels (i.e.,in the conditions of FSW1 and FSW3). This characteristic
should be attribute to the different welding force andtemperature generated by varied plunged positions of stirtool. The comparison of FSW1 and FSW2 can be taken as
an example. Although the rotation and traverse speeds ofFSW1 and FSW2 are same, FSW1 processes much higher
welding resistance impacted on the stir tool, because theplunged position of tool is set on the ODS side, and theODS steel processes more outstanding mechanicalFig. 4 EDX analysis of the lump-like structure㸦a㸧㸦b㸧

Tungsten CI=0.06

㸦c㸧

W-C CI=0.8

Fig. 3 Microstructures of the welding defects:(a)Wormholes near the lump structure in FSW1; (b) Dis-bonded defect inFig. 5 Verification of the Kikuchi pattern: (a) the lump-likeFSW2; (c) Wormholes in RS-TMAZ in FSW2; (d) and (e) detailedstructure; (b) Kikuchi pattern of W; (c) Kikuchi pattern of W-C

microstructures in FSW367

W. Han et al., Parameter Selection in Dissimilar Friction Stir Welding of ODS Ferritic Steel and RAFM Steel F82Hin FSW4 joint also testifies that no evidence of δ ferrite
and inter-metallic compound can be discovered. In view of
the above comparison and analysis, during the dissimilar
FS welding of ODS and F82H steels, the softer material
F82H steel should be arranged on the retreating side andthe stir tool should be plunged into the RS side.

properties than the F82H steel at the evaluated temperature.
Due to the higher welding resistance in FSW1, the friction
force between the tool and the material can besimultaneously increased, consequently the friction heatand the welding temperature are higher in FSW1.
Comparing with FSW1, the welding temperature andwelding force of FSW3 are further enhanced, due to theenhanced rotation speed (250 rpm). According to theseanalyses, among these three defective joints, FSW3
processes the highest values in the welding temperature
and welding resistance force, while FSW2 has the lowest
values. Those differences cause the damage of the stir toolin conditions of FSW1 and FSW3, and generate the lumpstructures.
As the welding temperature and the imposed force ofstirring tool in FSW2 are low, during the welding process
the material flowing is extremely insufficient. The
insufficient flowing material results in the visible
wormhole and dis-bonded defects in of FSW2. Particularly,
in all these defective joints, the wormholes mostly emerge
on the retreating side. This characteristic is considered tobe related to the unique welding force of the FSW process.
Fig. 6 shows that on the AS the rotational stress has the4. Conclusion

The effect of welding conditions on the appearance,
defect and microstructure of the dissimilar joints of ODS
and F82H steels welded by FSW were investigated in thepresented study. The main results are summarized asfollows:(1) The FSW can achieve defect-free joint of ODS andF82H steels.
(2) As the FSW is a solid welding process, neither the δ
ferrite nor inter-metallic compound can be generated
in dissimilar FSW joint of F82H and ODS steels.
(3) The dis-bonded defect is apt to appear in the conditionof the insufficient heat input, while this kind of defectcan be eliminate by properly enhancing the rotation
speed.
(4) In order to achieve the defect-free joint and to preventthe stir tool from breaking, the “softer material” F82H
should be arranged on the retreating side and the stir
tool should be plugged into the F82H side.

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Fig. 6 Schematic diagram of the mechanical force in FSW
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