a novel method for resistance spot welding between steel
TRANSCRIPT
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A novel method for resistance spot weldingbetween steel and aluminum alloy
Ranfeng Qiu1, 2, Jiuyong Li1, Lihu Cui1, Hongxin Shi1, 2, Yangyang Zhao1
(1. School of Materials Science and Engineering, Henan University of Science
and Technology, Luoyang 471003,China; 2. Collaborative Innovation Center of
Nonferrous Metals, He'nan Province, Luoyang 471003, China)
Abstract: A new jointing method, termed resistance spot welding with
embedded composite electrodes, was tried to weld steel to aluminum
alloy. A reaction layer was observed at the welding interface and its
thickness at the interface presented a bimodal distribution. A tensile
shear load of a maximum of 6.28kN was obtained at a welding current
of 28kA. The results reveal that resistance spot welding with embedded
composite electrodes is an effective method for restraining the interfacial
reaction products growth in the welding central region.
Key words: resistance spot welding; aluminum alloy; steel; embedded
composite electrodes
DOI: 10.7512/ j.issn.1001-2303.2017.13.12
Prof. Ranfeng QiuEmail: [email protected]
0 IntroductionTo reduce pollution and save energy, it is attractive to make car
bodies lighter by introducing aluminum alloy parts as substitutes for
the previous steel structures. Therefore, the joining between steel and
aluminum alloy is unavoidable. However, joining between the two
kinds of materials by fusion welding methods faces to technological
and metallurgical limitations, because of the large difference in physical
and thermal properties between steel and aluminum alloy. Accordingly,
various pressure welding methods have been used to achieve optimal
results when welding aluminum alloys and steel, such as diffusion
welding [1-2], friction welding [3-4], explosive welding [5], friction stir
welding[6-8], ultrasonic welding [9-10], and resistance spot welding [11-16].
As a result, it is well known that brittle reaction products, which formed
at the welding interface, would deteriorate the tensile strength of steel/
aluminum alloy joint.
In the case of resistance spot welding of steel/aluminum alloy, the
authors have found that the thickness of the reaction layer is thin at the
peripheral region and it increases as approaching to the center of the
weld, and that the interfacial reaction layer whose thickness exceeded
1.5 µm can deteriorate the mechanical properties of the joints [17-18].
Therefore, a sound resistance spot welded steel/aluminum alloy joint
would be fabricated by a welding process which is helpful to suppress
the growth of reaction products in the central region of the weld.
In view of this, a new method termed resistance spot welding with
embedded composite electrodes for joining steel/aluminum alloy is
put forward in the present study. The purpose is to better understand
the weldability of steel and aluminum alloy, and to provide some
foundation for improving resistance spot welded steel/aluminum alloy
joint properties.
1 Resistance spot welding with embedded composite electrodesResistance spot welding is a joining process based on the heat
source obtained from Joule’s effect of the resistance and electric current
flow through the sheets held together by the electrode force, in which
the coalescence occurs at the spot area in the faying surfaces. Electrodes
are the important carrier for resistance spot welding. Fig.1(a) and (b)
shows the embedded composite electrode developed in this study.
Its production process is as follows: A tungsten rod was embedded in
drilled cylindrical embryo piece of CuCrZr alloy, and preheated to 680℃
using the high temperature box type resistance furnace. The preheated
copper block with tungsten rod was placed in the mold cavity, upsetting
extrusion was performed. Extrusion force was 50kN; extrusion ratio
was 25:1. After extrusion, the embryo piece was machined until the
shape and dimension to meet the design requirements.
Ranfeng Qiu was born in 1974. He received his PhD in Mechanical Engineering from Kumamoto University in 2009. He is currently a Vice-Professor at the School of Materials Science and Engineering, He'nan University of Science and Technology. His research interests are resistance spot welding of dissimilar materials. He has published over 50 refereed papers in international journals and conferences.
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Fig.1(c) shows the schematic diagram of resistance spot welding
with embedded composite electrodes. Because the resistance of
tungsten at the central region of embedded composite electrode is
higher than copper alloy at the outer edge of embedded composite
electrode, most of the curent would flow through the outer edge
of embedded composite electrode based on Joule’s law during spot
welding. Namely, utilizing the embedded composite electrodes during
resistance spot welidng can make welding zone current gather in ring
shape (henceforth calls ring-gathered current ) as shown in Fig.1(c).
In this way, restraining the interfacial reaction products growth in the
welding central region would be realized.
2 Experimental materials and proceduresThe materials used in this study were 1.0 mm thick mild steel Q235
sheet and aluminum alloy A6061 sheet with thickness of 2.0mm.
The nominal compositions are listed in Table 1. The sheets were cut in
the size of 100 mm×30 mm. After washing by anhydrous ethanol and
drying, lap joint configuration was prepared. They were welded using
DM-200 moderate frequency inverter resistance spot welding machine.
The welding current was changed every 2kA between 10 and 32kA
at the fixed electrode force of 2kN and welding time of 0.2 s in the
welding process. The tip diameter of embedded composite electrode
used in this experiment was 6mm, and the diameter of inlaid tungsten
rod was 4mm.
After welding, the tensile shear tests were performed on AG-1250kN
tensile testing machine under a cross-head velocity of 1.7×10-5m/s
at room temperature. The weld joints were cut perpendicular to the
faying surface through the weld center and cross-section observation
experiment was conducted after grinding and polishing. The interfacial
microstructure of the joint was investigated using a scanning
electron microscope (SEM). And the chemical compositions of the
reaction products were determined using an energy dispersive X-ray
spectroscopy (EDS). Besides, the tensile shear load and nugget diameter
were evaluated by the average value of five specimens per condition.
3 Results and discussion
Fig.2 shows the optical micrograph of the cross-section of joint
welded under the condition of welding current of 22kA. The following
characterizations can be seen. First, a white acetabuliform nugget was
observed at the side of aluminum alloy. Although few welding current
flowed through the center of welding zone where correspond with
the tungsten rod contained in the electrode during spot welding, there
still happened remelting at the aluminum alloy side. This is because the
thermal conductivity of aluminum alloy is larger and its melting point is
lower. The heat is easy diffused to the center of welding zone from the
peripheral region by heat conduction, which caused aluminum alloy
also melted at there. Since the generated heat was larger due to higher
resistance at the steel side, the nugget diameter of aluminum side
adjacent the welding interface is larger. However, its diameter is getting
smaller away from the interface. Therfore, the acetabuliform nugget
present at the aluminum alloy side of joint.
Second, two kinds of zone with various contrast was observed at
the steel side. One is an ashen zone at the center of welding zone,
where no remelting indication is observed. Its size is coincident with the
diameter of tungsten rod contained in the electrode. The other is a zone
with deep contrast on both sides of the ashen zone, which is a fusion
zone. In geometry aspect, the deep zone is a ring shape. Henceforth it
is called ring-nugget. At the steel side, the formation of ring-nugget is
considered to be due to ring-gathered current during resistance spot
welding. Compared with aluminum alloy, the thermal conductivity of
steel is lower and its melting point is higher. At the center of welding
zone, the heat got by conduction is not so enough as to melt steel. This
is main reason for the formation of ring-nugget.
Third, the diameter of nugget at welding interface was larger
than the electrode tip diameter (6mm) as shown in Fig.2. Under the
electrode pressure, the base metal sheets undergoed deformation
during welding, which caused the increase of contact area between
Fig.1 Side view (a) and top view (b) of embedded composite elec-
trode; schematic diagram of resistance spot welding with embedded
composite electrodes (c)
Table 1 Chemical composition of materials (wt.%)
Matterial w (Cr) w (Cu) w (Ti) w (Mg) w (V) w (C)
A6061 0.04 0.3 0.15 1 — —
Steel — — — — 0.06 0.14
Matterial w (P) w (S) w (Si) w (Mn) w (Fe) w (Al)
A6061 — — 0.6 0.15 — Bal.
Steel 0.04 0.02 0.4 1 Bal. —
Fig.2 Appearance of cross-section joint
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have nearly the same chemical composition with an Al:Fe atomic
ratio of approximately 3:1, which means the thinner reaction layer is
composed of FeAl3. The interfacial characterization found in this study is
similar to the results obtained in the previously literature [17, 19].
Table 2 EDS analysis results (at.%)
Element w (M) w (N) w (P) w (Q)
Al 71.59 76.81 73.98 76.02
Fe 28.41 23.19 26.02 23.98
The observation results show that the thickness variation of reaction
layer along the interface direction is non-monotonous and complex.
The reaction layer is getting thicker from A to C spot at the interface
as shown in Fig.2, and then turning thinner. The thickness of reaction
layer at the A, C and E spot is approximately 1.0, 3.25 and 0.75µm,
respectively. Therefore, the thickness of reaction layer at the interface
presents a bimodal distribution, if seen together with the left half of
the interface. Fig.3(f) shows the diagrammatic drawing of reaction
layer thickness distribution at the interface. Here, the zone where the
aforementioned thicker reaction layer is named for ring-zone. From the
point geometric position, the ring-zone and the ring-nugget formed in
the steel is corresponding. In other words, the ring-zone locates in the
interface under the ring-nugget as shown in Fig.2. Apparently, the
thickness of reaction layer formed in the ring-zone is larger, and
the thickness of reaction layer formed in the center and periphery
region of the joint interface is thinner. The distribution of reaction
layer at the interface obtained in this study is different from that
of the Al/steel joint welded by commonly electrodes [16, 20]. This is
considered to be due to the role of composite electrodes during
welding. It is well known that interfacial reaction layer thickness
is a function of interaction time and temperature and that is
described by the Arrhenius equation. The reaction layer thickness
variation in this study may be analyzed using the Arrhenius
equation, although thermal phenomenon during resistance spot
welding is complex, in which interaction time depends on the position
base metal sheet and electrode, and then resulted in the formation of
larger nugget.
Fourth, several voids were found at the center of the acetabuliform
nugget. This is considered to be due to the shrinkage result from
the solidification of molten metal. During the heating process in
resistance spot welding, the expansion of molten aluminum metal was
constrained by the surrounding solid metal and subjected to shrinkage
strain, Since the shrinkage strain cause insufficient aluminum in the
molten weld cavity, the subsequent solidification of molten metal
formed the voids at the nugget center, which is the last solidified area.
The welding interfacial zone was observed by SEM. Fig.3 shows SEM
images of the interfacial region of the A6061/Q235 joint. Images (a)
to (e) indicate the typical morphology of the A6061/Q235 interface at
the positions (A to E ) in Fig.2, respectively. As shown in Fig.3, a reaction
layer was observed at the welding interface.
As shown in the Fig.3, the thickness of the reaction layer was
varying along the welding interface. The reaction layer, which formed
in the interfacial area corresponding to the ring-nugget, is thicker. In
this case, the reaction layer adjacent to the steel exhibits a tongue-like
morphology and adjacent to the aluminum alloy shows a serrated-like
morphology as shown in Fig.3(b), (c) and (d). On the other hand, the
reaction layer generated in the periphery and center zone of the joining
interface is thinner. Its both side are relatively flat as shown in Fig.3(a)
and (e).
In order to determine the chemical composition of of the reaction
layer, EDS spot analysis was carried out on four spots as shown in
Fig.3. Table 2 gives the atomic ratio of Al to Fe based upon EDS analysis
results. It clearly indicates the M point adjacent to steel has the chemical
composition of Fe2Al5, whereas the N point adjacent to aluminum alloy
has the chemical composition of FeAl3 in the thicker reaction layer.
Namely, the thicker reaction layer is composed of tongue-like Fe2Al5
adjacent to the steel and serrated-like FeAl3 adjacent to the aluminum
alloy. The corresponding EDS results indicate that the P and Q spots
Fig.3 (a~e) SEM images of the interfacial zone, (f) diagrammatic drawing of reaction layer thickness distribution at the interface
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at the welding interface and the temperature of every point at the
welding interface varies with welding time. The ring-zone was a heating
zone in the study, because the welding current (ring-gathered current)
flowed through the zone during spot welding. The temperature was
higher and the interaction time was longer. Therefore, the reaction layer
formed at the zone is thicker.
At the center zone of the joining interface, few current flows
through the zone during welding because the resistance of tungsten
at the central region of embedded composite electrode is higher than
copper alloy at the outer edge of embedded composite electrode. The
zone was heated by the conductive heat diffused from the ring-zone.
This resulted in the temperature was lower at the center zone of the
joining interface. Therefore, the reaction layer formed at the zone is
thinner. Similarly, the reaction layer formed at the periphery zone of the
joining interface is also thinner.
The reaction layer thickness distribution at the interface reveals that
restraining the interfacial reaction products growth in the welding
central region can be realized by use of resistance spot welding with
embedded composite electrodes. Although the results demonstrate
that the expected goal in the study has been achieved, the relationship
between the width of ring-zone and the diameter of tungsten rod
embedded composite electrode is still need to discuss in the future.
Fig.4 shows the effect of welding current on the nugget diameter
and tensile shear load of the joint. Here, the nugget diameter was
measured on the fractured surface of aluminum alloy side after the
tensile shear testing of joint. The nugget diameter increased with the
increasing of welding current. In resistance spot welding process, the
heat input increased with the increasing of welding current based on
Joule’s law. This resulted in increasing of nugget diameter. In this study,
the nugget diameter is in the range from 6.24 to 12.9 mm and meets
the relevant standards requirement of D > 4t 0.5 (D is nugget diameter, t
is thickness of sheet) [21].
As shown in Fig.4, the tensile shear load increased with the
increasing of welding current. But the increase rate was larger in the
welding current range from 10 to 22kA, and then became slighter
above the welding current of 22kA. The maximum tensile shear load
of 6.28kN was obtained with a welding current of 28kA. Moreover,
the fracture type of the joints varied depending on the welding current.
Shear and plug fracture were observed in the range of 10~22kA and
24~32kA of the welding current, respectively. The fracture occurred
in aluminum alloy side in the case of plug fracture. When the welding
current is low, the tensile shear load is rapidly increased since
nugget diameter increased with the increasing of welding
current. With increase in welding current, the nugget diameter
still increased but the thickness of aluminum alloy sheet in the
welding area decreased. The former would facilitate the increase
of joint tensile shear load but the latter is the opposite [22], the
combined action of both is the reason why tensile shear load changed
slightly in range of high welding current.
Fig.4 Effect of welding current on the nugget diameter and tensile
shear load of the joint
4 ConclusionsIn the present study, steel and aluminum alloy were welded using
the novel method of resistance spot welding with embedded composite
electrodes. The results were evaluated by studying the interfacial
microstructure and tensile-shear load of joints. The salient results
obtained from this study are as follows:
(1) At the side of aluminum alloy and steel of joint welded by
resistance spot welding with embedded composite electrodes, a
acetabuliform nugget and ring-nugget were observed, respectively.
(2) A reaction layer was observed at the welding interface of
A6061/Q235 joint welded by resistance spot welding with embedded
composite electrodes; its thickness at the interface presented a bimodal
distribution.
(3) The maximum tensile shear load of 6.28kN was obtained with
welding current of 28kA.
(4) Restraining the interfacial reaction products growth in the
welding central region can be realized by use of resistance spot welidng
with embedded composite electrodes.
AcknowledgementsThis work was supported by the Natural Science Foundation of China
(U1204520), Henan Province Support Plan of Universities and Colleges
Innovation Talents (16HASTIT050), Henan Province International
Science and Technology Cooperation Projects (162102410023).
References:Ogura Tomo, Umeshita Hidetaka, Saito Yuichi, et al. Characteristics
and estimation of interfacial microstructure with additional
elements in dissimilar metal joints of aluminum alloys to steel.
Quarterly Journal of the Japan Welding Society 2009,27(2): 174s-
178s.
Shi Hongxin, Qiao Shuang, Qiu Ranfeng, et al. Effect of Welding
Time on the Joining Phenomena of Diffusion Welded Joint
between Aluminum Alloy and Stainless Steel, Materials and
[1]
[2]
· ·150
Manufacturing Processes 2012, 27: 1366–1369.
Taban Emel, Gould Jerry E, Lippold John C. Dissimilar friction
welding of 6061-T6 aluminum and AISI 1018 steel: Properties and
microstructural characterization. Materials & Design 2010(31):
2305–2311.
Kannan P, Balamurugan K, Thirunavukkarasu K. Influence of
silver interlayer in dissimilar 6061-T6 aluminum MMC and AISI
304 stainless steel friction welds, Int. J. Adv. Manuf. Technol.
2015(81): 1743–1756.
Li Xuejiao, Ma Honghao, Shen Zaowu. Research on explosive
welding of aluminum alloy to steel with dovetail grooves.
Materials and Design 2015(87): 815–824.
Lan Shuhuai, Liu Xun, Ni Jun. Microstructural evolution during
friction stir welding of dissimilar aluminum alloy to advanced high-
strength steel, Int. J. Adv. Manuf. Technol. 2016(82): 2183–2193.
W.B. Lee, M. Schmuecker, U.A. Mercardo, et al.Interfacial
reaction in steel–aluminum joints made by friction stir welding,
Scripta Materialia, 2006(55): 355–358.
A. Yazdipour, A. Heidarzadeh. Effect of friction stir welding on
microstructure and mechanical properties of dissimilar Al 5083-
H321 and 316L stainless steel alloy joints, Journal of Alloys and
Compounds, 2016(680): 595-603.
Fujii Hiromichi T, Goto Yuta, Sato Yutaka S, et al. Microstructure
and lap shear strength of the weld interface in ultrasonic welding
of Al alloy to stainless steel. Scripta Materialia 2016; 116: 135–138.
Mirza F A, Macwan A, Bhole SD, et al. Effect of welding energy
on microstructure and strength of ultrasonic spot welded
dissimilar joints of aluminum to steel sheets. Materials Science &
Engineering A 2016(668): 73–85.
Sun X, Stephens EV, Khaleel MA, et al. Resistance Spot Welding
of Aluminum Alloy to Steel with Transition Material-From Process
to Performance-Part I: Experimental Study. Welding Journal 2004;
83: 188s-195s.
Arghavani M R, Movahedi M, Kokabi AH. Role of zinc layer in
resistance spot welding of aluminium to steel. Materials and
Design, 2016(102): 106–114.
Chen Nannan, Wang Hui-Ping, Carlson Blair E, et al.
Fracture mechanisms of Al/steel resistance spot welds in
lap shear test, Journal of Materials Processing Technology
2017(243): 347-354.
Zhang W H, Qiu X M, Sun DQ, et al. Effects of resistance spot
welding parameters on microstructures and mechanical properties
of dissimilar material joints of galvanised high strength steel and
aluminium alloy. Science and Technology of Welding and Joining
2011, 16(2): 153-161.
Ling Zhanxiang, Li Yang, Luo Zhen, et al. Resistance Element
Welding of 6061 Aluminum Alloy to Uncoated 22MnMoB Boron
Steel. Materials and Manufacturing Processes 2016, 31(16): 2174-
2180.
Qiu Ranfeng, Iwamoto Chihiro, Satonaka Shinobu. Interfacial
microstructure and strength of steel/aluminum alloy joints welded
by resistance spot welding with cover plate. Journal of Materials
Processing Technology 2009, 209(8): 4186-4193.
Qiu Ranfeng, Shi Hongxin, Zhang Keke, et al. Interfacial
characterization of joint between mild steel and aluminum alloy
welded by resistance spot welding. Materials Characterization
2010, 61(7): 684-688.
Qiu Ranfeng, Satonaka Shinobu, Iwamoto Chihiro. Effect of
interfacial reaction layer continuity on the tensile strength of
resistance spot welded joints between aluminum alloy and steels.
Materials & Design 2009; 30(9): 3686-3689.
Wan Z, Wang H P, Chen N, et al. Characterization of intermetallic
compound at the interfaces of Al-steel resistance spot welds.
Journal of Materials Processing Technology 2017(242): 12–23.
Wang N, Yamaguchi T, Nishio K. Interfacial microstructure and
strength of aluminum alloys/steel spot welded joints. The Japan
Institute of Metals and Materials 2013; 77(7): 259-267.
American National Standard, 1997.Weld button criteria,
recommended practices for test methods for evaluating the
resistance spot welding behavior of automotive sheet steel
materials. ANSI/AWS/SAE/D8.9-97, Section 5.7.
Satonaka S, Kaieda K, Okamoto S. Prediction of tensile-shear
strength of spot welds based on fracture modes. Weld in the
World 2004, 48(5/6): 39–45.
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]