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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: qiurf1221@163.com

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).

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