Accepted Manuscript

Title: Friction stir welding of small-dimension Al3003 and pure Cu pipes

Author: Binxi Chen Ke Chen Wei Hao Zhiyuan LiangJunshan Yao Lanting Zhang Aidang Shan

PII: DOI: Reference:

S0924-0136(15)00147-8 http://dx.doi.o r g/doi:10.1016/j.jmatprotec.2015.03.044 PROTEC 14361

To appear in: Journal of Materials Processing Technology

Received date: Revised date: Accepted date:

11-10-201426-2-201526-3-2015

Please cite this article as: CHEN, B., CHEN, K., HAO, W., LIANG, Z.,YAO, J., ZHANG, L., SHAN, A.,Friction stir welding of small-dimensionAl3003 and pure Cu pipes, Journal of Materials Processing Technology (2015), http://dx.doi.o r g/10.1016/j.jmatprotec.2015.03.044

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Friction stir welding of small-dimension Al3003 and pure Cu

pipes

Binxi CHEN1,2, Ke CHEN1,2*, Wei HAO1,2, Zhiyuan LIANG1,2, Junshan YAO3, Lanting

ZHANG1,2, Aidang SHAN1,2

1. School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai

200240, China

2. Shanghai Key Laboratory for high temperature materials and precision forming, Shanghai Jiao

Tong University, Shanghai 200240, China

3. Special Welding Technology Center, Shanghai Aerospace Equipment Manufacturer, Shanghai

200245, China

*Corresponding author:

Tel: +86-21-54748974

Postal address: Rm 407, Material Building A, Shanghai Jiao Tong University, 800 Dongchuan

Rd, Minhang District, Shanghai Jiao Tong University, Shanghai 200240, China

E-mail address: chenke83@sjtu.edu .cn

1

Page 1 of 29

Abstract

Small-dimension Al3003 pipe and pure copper pipe of thin wall (Al: 1.5mm; Cu: 1mm) and

small diameter (19mm) were successfully joined by a developed welding method with a

specially-designed friction stir welding (FSW) system. A distinctive temperature history due to

heat accumulation was identified as an important feature for FSW of small-dimension pipes,

leading to distinctive variations of surface condition, macro-/micro- structure along the

circumferential weld seam. Hardness distribution, tensile strength, ductility and fracture modes

were found to change correspondingly along the weld seam.

Keywords: Friction stir welding, Pipe welding, Dissimilar materials, Welding

temperature, Microstructure, Mechanical properties

2

Page 2 of 29

Introduction

Copper (Cu) is one of the most important metals and is widely used in industry (Lipowsky

and Arpaci, 2008). The fast increase of its price calls for its replacement by the cheaper substitutes

in order to save the cost (Weigl et al., 2011). Aluminum (Al) and Al alloys are considered as ideal

candidates not only because their prices and densities are lower than Cu but also they share

similarity with Cu in mechanical properties and electric properties (Miller et al., 2000). Reliable

and efficient joining between Cu and Al is therefore essential and has drawn considerable

attention. Fusion welding techniques, such as brazing (Koyama et al., 2002) and laser welding

(Mai and Spowage, 2004), were used to join these two metals. Koyama et al. (2002) used Al-Si-

Mg-Bi brazing alloy to join Al and Cu by vacuum brazing. Massive Al2Cu and Al2Cu3 formed

in the joints reduced the tensile strength. Fracture preferentially occurred in these

intermetallic compound (IMC) layers (70% in Al2Cu). Mai and Spowage (2004) characterized

dissimilar joints in laser welding of Al4047 and Cu and reported that the fusion zone mainly

consisted of brittle IMCs which induced solidification cracks at high welding speed

(150mm/min). Therefore, the

formation of IMCs is difficult to control during fusion welding, which inevitably weakens the

mechanical properties of joints.

In solid state welding, the formation of IMCs can be well controlled by avoiding the melting

of materials and controlling heat input, resulting in an improved mechanical property. Thus,

solid state welding methods are more feasible solutions for welding Al and Cu. Friction stir

welding (FSW) is a fast developing solid state joining technique since it was invented in 1991

(Thomas et al., 1991), although several disadvantages have been identified, such as keyhole left

after withdrawing the tool, large downward force, heavy-duty clamping required, and less

flexibility and slower traverse rate than some fusion welding techniques. Many previous

studies have reported that sound and defect-free Al to Cu joints with good mechanical

properties can be obtained via FSW. Liu et al. (2008) reported that the tensile strength of the

FSW joint between T2

Cu and Al5A06 reached 100% of Cu and 94% of 5A06. Fotoohi et al. (2013) investigated the butt

joining of Al5083 to commercially pure Cu via FSW and reported the ultimate tensile strength

3

Page 3 of 29

(UTS) of joints close to 96% of the Cu.

In recent years, most studies of FSW between Al and Cu focused on the effects of welding

parameters on mechanical properties of the joints and the formation of IMCs. By studying the

influence of the fixed location, pin offset and tool rotation rate (TRR) on the microstructure and

mechanical properties of the joints, Xue et al. (2011) concluded that sound and defect-free joints

could be produced only when Cu was placed at the advancing side (AS) and tensile properties of

the joint were poor when pin offset was too large and/or TRR was too low due to insufficient

reaction between the Cu bulk/pieces and Al matrix. Bisadi et al. (2012) studied the effect of TRR

and welding speed on the microstructures and mechanical properties in friction stir lap welded

Al5083 and commercially pure Cu joints and concluded that joint defects formed at either very

low or extremely high welding temperature. Esmaeili et al. (2011) found that the gradual

formation of IMC during FSW of brass and Al1050 was initiated at the interface, followed by

thickening and development of IMC layers consisted of Al2Cu and Al4Cu9 with further increase

of

rotation speed. Xue et al. (2010) investigated the effect of the thin IMC layers on mechanical

properties of Al/Cu FSW joint. They reported that excellent metallurgical bonding with a

bonding strength higher than 210 MPa was produced at the Al/Cu interface due to the

formation of

continuous and uniform Al-Cu IMC layer with a proper thickness of ∼1 μm.

Besides, most of previous researches on FSW focused on butt or lap welding of flat-surface

sheets or plates. There were only a very limited number of studies on the joining of pipes.

Lammlein et al. (2012) presented a FSW process method for joining Al-6061T6 pipes with a

diameter of 107mm and a wall thickness of 5mm; high-strength pipe joints with sound internal

and superficial appearance were obtained successfully. Doos and Wahab (2012) studied the

feasibility of welding Al pipes by FSW and obtained a maximum weld strength of 179 MPa with

a weld efficiency of 61.7%. Furthermore, Packer and Matsunaga (2004) developed FSW

equipment and process method for joining X65 pipes (outer diameter: 324mm) and

successfully obtained

fully consolidated weld joint with superior tensile and impact properties. Peterson et al. (2011)

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Page 4 of 29

also reported a system for FSW of small diameter tubes or pipes.

Different from above mentioned studies, welding between small-dimension pipes of

dissimilar materials and with different geometries was studied in this research, namely, Al3003

alloy and pure Cu pipes with small diameter (19mm) and thin wall (1mm for Cu and 1.5mm for

Al3003). The technology of welding Al pipes to Cu pipes has potential application in heating and

air-conditioning systems, where Cu pipes are currently used extensively. The partial replacement

of Cu pipes by Al pipes enabled by this technology can significantly reduce the cost of material.

There are several unique challenges in developing this technology. First, dissimilar FSW of Al

and Cu is difficult because of their large difference in physical properties. For example, the

melting temperature of Cu is nearly 400 K higher than Al. Second, the highly curved surface of

the circumferential welding seam suggests that the interaction between the FSW tool and pipes is

very different from the case in flat-surface welding. Hence, the material flow during welding

should be also quite distinct. Third, the thin wall (~ 1mm) of the pipes results in a challenge in

process control and requires a high controlling accuracy. In order to deal with these challenges, a

welding method was developed and a welding system was designed and fabricated accordingly.

Using this welding system, successful joining between Al and Cu pipes was obtained. A key

feature found for this welding method is the distinct variation of welding temperature,

macro-/micro-structure and mechanical properties along the circumferential weld seam. This

paper introduces the welding

method, presents this feature and discusses its formation.

5

also reported a system for FSW of small diameter tubes or pipes. Page 5 of 29

Experimental

1. Materials and FSW tool

Al3003 alloy pipes and pure Cu pipes with an outer diameter of 19 mm were used in this

study. Wall thicknesses of the Al pipes was 1.5 mm while that of Cu pipes 1 mm. Chemical

composition of Al pipes is listed in Table 1. The UTS of Al and Cu base metals were 197MPa

and

315MPa, respectively. A FSW tool made of H13 steel with concave shoulder (10°) and

cylindrical pin was used. Shoulder diameter (SD) was 6 mm, pin diameter (PD) 2 mm and pin

length (PL) 0.7 mm.

Table 1 Chemical composition of Al3003 alloy (mass %)

Material Al Mn Fe Si Cu Zn

Al alloy Balance 1.0~1.5 <0.7 <0.6 0.05~0.2 <0.1

2. Welding

The welding system and the schematic diagram of the welding are shown in Fig. 1(a) and (b),

respectively. This welding system is very different from that used for butt or lap FSW of flat

plates. Instead of a backing plate used in FSW of flat plates, a fixture was specially designed (as

shown in Fig. 1(a) and (b)) for the FSW of small diameter and thin wall pipes. In order to fix

the pipes during welding, a pair of expandable inner mandrels (EIM) were used, similar

to the corresponding part of the welding system for larger diameter pipes (107mm) in Lammlein

et al.’s (2012) work. After fixing the pipes on the fixture, they were placed on the supporting

seats and clamped by the driving head. The Cu pipes were placed to the AS in this study. The

welding tool was offset in two directions, Offset1 (O1) and Offset2 (O2), as shown schematically

in Fig. 1(c) and (d). O1 is parallel to x-axis, which offset the tool to Al side. O2 is

parallel to y-axis (perpendicular to O1 direction), which was applied for obtaining a “tilt angle”,

similar to that in FSW of flat plates. As schematically shown in Fig. 1(d), ID is the height

difference (along z-axis) between the top of the pipes and the tip of the pin after the insertion of

the tool. Its optimal value

was found to be closely related to O2. Other important welding parameters include TRR and pipe

rotation rate (PRR), as shown in Fig. 1(b).

6

Page 6 of 29

(b)

Manu

(a)

(c)

(d)

Fig. 1 The welding system: (a) the setting for pipe welding; (b) schematic diagram of the

welding parameters; (c) schematic diagram showing O1; (d) schematic diagram showing O2.

(PRS stands for pipe rotation rate; TRR for tool rotation rate; O1 for offset1; O2 for offset2; ID

for insertion depth)

The welding started with the rotation of the tool at a pre-set TRR. The tool was then

inserted into the pipes to a fixed ID. After several seconds of dwell time, the pipes were driven

by the driving head to rotate at a pre-set PRR. The circumferential welding can be finished by

360° rotation of the pipes. However, in our current study, the pipes rotated 400° rather than 360°,

which means that there was a 40-degree overlap region. The reason for such a setting will be

explained later. After the circumferential welding, the tool was lifted up, shifted to zero O2, and

re-inserted into the pipes to perform a linear welding with the tool traversing in the direction of

x-axis. The whole welding procedure was finished by leaving the keyhole in the thicker Al side.

Using different combinations of parameters, a large number of test welding have been

conducted. A common feature was found that weld surface finish, macro-/micro-structure and7

Page 7 of 29

mechanical properties vary along the circumferential weld seam. In order to present this critical

feature, a typical welded joint is selected with the values of the dominant welding parameters

listed in Table 2. Other secondary welding parameters, such as dwell time and traverse distance

in linear welding, were not as dominant as those parameters listed in Table 2 and therefore not

considered in this study.

Table 2 Welding parameters

Welding parameter TRR PRR O1 O2 ID

Unit rpm rpm mm mm mm

Value 2400 2 1.3 1.5 1.45

3. Temperature measurement

The objective of temperature measurement in the current study was to find the welding

temperature variation rather than accurate temperature values. Even through welding temperature

is commonly measured by thermocouples embedded into the workpieces, this technique cannot

record the temperature of the welding zone due to the fixed locations of thermocouples. By

contrast, thermal infrared camera can conveniently record the temperature of the welding zone

continuously. Therefore, a thermal infrared camera (FLIR A615) with 50Hz of sampling

frequency was used to measure the welding temperature in this study. The camera forms thermal

image by detecting infrared radiation which is related to the temperature of the object. The

resolution of this camera is 640*480 pixel which means the spatial resolution is about 0.3mm.

The surface conditions were not carefully calibrated for the temperature measurement. However,

the deviation should be within 10%. The peak surface temperature of the pipes in front of the tool

was recorded to show the variation of the welding temperature during FSW. It is worth noticing

that the peak temperature obtained from the thermal imager is not the actual peak welding

temperature and it can be much lower than the temperature near the tool, since the temperature

gradient of the materials in front of the tool was reported to be large (Song and Kovacevic,

2003).

4. Mechanical property testing and metallographic sample preparation

Longitudinal tensile specimens were cut from the welded pipe joint according to ASTM

E8/E8M-11 (2000) as schematically shown in Fig. 2(a). The dimensions for the cut specimens

are

8

Page 8 of 29

(c)

Manu

(b)given in Fig. 2(b). Based on the measured peak welding temperature variation (given later in Fig.

3), four specimens (Fig. 2(c)) were sliced from different positions of the welded pipe. Tensile

tests were conducted on a ZWick testing machine (BTC-T1-FR020 TN.A50) with grips having

surface contour corresponding to the curvature of the pipes, at a crosshead speed of 1 mm/min.

Because the thicknesses of two pipes were different, force instead of stress was used as the

indicator for

tensile strength.

(a)

(d)

Fig. 2 (a) Schematic diagram showing how the tensile specimens were cut from the joint; (b)

dimension of tensile specimens; (c) obtained tensile specimens; and (d) schematic diagram

of weld region division

As shown in Fig. 2(d), four specimens were cut from the circumferential seam for

macro-/micro-structural characterization and hardness testing of each regions (FR, MR, LR and

OR). Their cross sections were mechanically polished and observed. However, due to the short

length of OR, there was not enough weld seam remained for metallographic study after taking the

tensile specimen. Furthermore, cross section of the OR specimen showed an obvious influence

from the final linear welding. Therefore, macro-/micro-structure and hardness of OR were not

presented in the following sections. Hardness testing was carried out using a Zwick/Roll

hardness

tester with 100g load and 10s holding time. A FEI scanning electron microscope (Sirion 200) was

(c)9

Page 9 of 29

used in backscattered electron imaging (BEI) mode and operating at an accelerating voltage of

5kV. For optical microscopy (OM), a Leica optical microscope (DM4000) was utilized and the

samples were etched by a solution consisted of 10g NaOH and 50 ml H2O for 20s.

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ccepted

Results

Using the welding system developed in this study (Fig. 1), Al3003 and Cu pipes were

successfully welded with the parameters given in Table 2. The welding temperature, weld

surface, macro-/micro-structure, and the mechanical properties of the pipe joint were studied in

order to understand the feature of this method.

1. Welding temperature

Figure 3 shows the variation of the peak surface temperature along the weld seam measured

by the thermal infrared camera. During the tool insertion into the pipes, the temperature

increased at a very high rate (~100°C/s) and reached over 200°C before the start of

circumferential welding. As the pipes rotated from 0° to ~220°, the temperature increased at a

relative steady rate (about

3°C/s) with some small fluctuations. In the last 40° of circumferential welding, there was a

declining stage from ~250°C to ~220°C. After that, temperature decreased and rose back quickly

when the tool was extracted from and re-inserted into the pipes. The welding temperature

maintained at about 200°C during the linear welding and finally decreased to room temperature

slowly after the welding was finished.

Fig. 3 Peak welding temperature variation of the weld seam surface

(FR: former region; MR: middle region; LR: later region; OR: overlap region)

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Page 11 of 29

(a)

(c) (d)Based on this featured welding temperature history, the whole weld seam were divided into

four regions as shown in Fig. 2(d), in order to better study the macro-/micro-structural variation

and the associated variations in mechanical properties. The extra 40-degree rotation of pipes after

a complete turn generated the overlap region (OR). For the OR, the initial 40-degree rotation of

the circumferential welding (1st pass) was overlapped by the welding of the final 40-degree

rotation (2nd pass). OR is followed by former region (FR). For the FR, pipes rotated 90 degrees

and peak welding temperature increased from ~200°C to ~230°C (Fig. 3). The following region

is designated as middle region (MR), where the peak welding temperature was measured to

increase another ~30 degrees during 90-degree pipe rotation (Fig. 3). In the later region (LR),

the peak welding temperature was relatively stable at around 260°C (Fig. 3). The locations of

these four regions along the circumferential weld seam are shown schematically in Fig. 2(d).

2. Weld surface variation

The weld surfaces of the four regions (FR, MR, LR, and OR) showed certain differences (Fig.

4). Even though FR, MR, and LR were all defect-free, the side flash increased and became more

continuous from FR to LR. OR is the region experienced 2 passes of welding. Such an overlap

welding was intentionally performed in order to remove the defects formed in the initial region of

weld seam, because weld defects were found to form most easily during the initial ~40 degrees

of pipe rotation. In OR (Fig. 4(d)), it can be seen that the weld defect was not eliminated

completely. Such a weld surface variation suggests that a steady-state welding was not reached

during the welding of small-dimension pipes and the welding condition/conditions were varying

during the welding even though the welding parameters were fixed. Such a change in the weld

surface should

be attributed to the variation of welding temperature (Fig. 3).

(b)

12

(a)

Page 12 of 29

cepted

Fig. 4 Surface variation along the circumferential weld seam: (a) FR; (b) MR; (c) LR; (d)

OR (refer to Figure 2(d) for the locations of these regions)

3. Macro-/micro-structure variation

Not only the surface conditions changed along weld seam, but also the macrostructure (FR,

MR and LR) were different, as shown in Fig. 5. First of all, thickness reduction for Al3003

increased from ~25% to ~40% from FR to LR. Secondly, the tilting orientation of the Al bulk/Cu

bulk interface is different in the three regions. In FR, the interface was ~45° tilted to the surface

of pipes. However, in MR and LR, the Al bulk/Cu bulk interfaces were both relatively vertical

and close to the initial faying surface (marked by dash line). Finally, only in the nugget of FR

were

large Cu pieces and defects observed.

Fig. 5 Optical images of the cross-sections of FR, MR and LR

Microstructure of similar locations for those three regions was further studied. The locations

were all close to the Al bulk/Cu bulk interfaces, as marked by yellow arrows in Fig. 5. In FR

(Fig.

6(a)), it was found that a large number of Cu lamellae (thickness < 5 μm) were embedded in Al

matrix. The Cu lamellar were strongly deformed and some of them enwrapped the second phase

particles (SPP) of Al3003 base metal. Surrounding the Cu lamellae, thin gray IMC layers

(thickness < 1 μm) can be found, similar to previous observation made by Galvão et al. (2012)

that Al2Cu and Al4Cu9 formed in the mixed zone. These microstructures shown in Fig. 6(a)

can be

regarded as laminated composites with Al3003 matrix enhanced by Cu lamellae and the

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Page 13 of 29

surrounding IMC layers. In MR (Fig. 6(b)), only a few Cu fragments surrounded by IMCs and

Manusc

cepted

SPP of Al3003 were observed in the Al matrix. However, very few Cu pieces/IMCs could be

found in LR (Fig. 6(c)) and this microstructure was very similar to Al3003 base metal (mainly Al

and irregular shaped micro-scale SPP).

(a) (b)

(c)

Fig. 6 SEM images of the stir zone near the Al/Cu interface in 4 regions with the locations

marked in Figure 5 by red rectangles: (a) FR, (b) MR, and (c) LR

Detailed EDS analysis was performed on a large Cu piece and the surrounding IMCs in the

weld nugget of MR. Positions investigated are marked in Fig. 7 and the results are shown in

Table

3. P1 showing bright contrast was proven to be Cu. The composition of P2 (69.08 at.% Al and

30.92 at.% Cu) was very close to that of Al2Cu. The grey region embedded between two Cu

lamellar (P3) was found to be lightly rich in Cu (39.64 at.%) compared with the stoichiometric

composition of Al2Cu (33.33 at.% Cu). It suggested a combination of Al2Cu and a few Cu in P3

because P3 was located between two Cu lamellae. Likewise, the result of P4 suggested a

combination of Al2Cu and Al phases because P4 was located in the IMC particle surrounded by

14

surrounding IMC layers. In MR (Fig. 6(b)), only a few Cu fragments surrounded by IMCs andPage 14 of 29

the Al matrix. Therefore, Al2Cu is possibly the main IMC phase in the laminated composites.

Man

Fig. 7 Intermetallics around Cu pieces in MR, with the EDS tested at 4 positions. EDS results

are shown in Table 3

Table 3 EDS results (at.%) for the measurements marked in Figure 7

Position P1 P2 P3 P4

Al K 2.69 69.08 60.36 78.47

Cu K 97.31 30.92 39.64 21.00

Mn K 0 0 0 0.53

Phases Cu Al2Cu Al2Cu+Cu Al2Cu+Al

4. Variation of mechanical properties

4.1 Microhardness

Figure 8 shows Vickers micro-hardness distributions along the middle height line (marked

by red arrows in Fig. 5) of the metallographic cross-sections of FR, MR, and LR. Outside the

intermixing nugget, hardness distributions of all the three regions were similar as shown in Fig.

8(a). Hardness of Cu decreased gradually from ~130 HV to ~100 HV at AS and that of Al3003

from ~60 HV to less than 50 HV at RS, respectively. Such a variation was most possibly due to

the effect of welding thermal cycle. Inside the intermixing nugget, hardness distributions of the

three regions were quite different. As shown in Fig. 8(b), there were two hardness peaks in FR

and the one near Al bulk/Cu bulk interface showed a hardness of 228 HV which was much higher

than the hardness of base Cu. Besides the two peaks, small fluctuation between ~50 HV and

~90 HV

was observed. As shown in Fig. 8(d), fluctuation was also found in LR with an obvious trough

15

the Al matrix. Therefore, Al2Cu is possibly the main IMC phase in the laminated composites.

Page 15 of 29

reaching a hardness value similar to Al3003 base metal. However, the hardness in the nugget of

MR was relatively steady, varying in a small range of 80 HV~90 HV as shown in Fig. 8(c).

The highest hardness peak in FR corresponded to the region with a microstructure similar to

that shown in Fig. 6(a), containing high density of Cu lamellar and IMCs. Tan et al. (2013) also

observed a hardness peak that reached 195.3HV in the nugget near the Al bulk/Cu bulk interface.

The microhardness of different IMC phases in the annealed friction-welded Al-Cu bars was

measured previously by Braunovic et al. (1994). They reported the hardnesses of AlCu2, Al2Cu3,

Al3Cu4, AlCu, and Al2Cu to be 35 HV, 180 HV, 624 HV, 648 HV, and 413 HV, respectively.

The microstructure shown in Fig. 6(a) contained a high proportion of Cu and IMCs (mainly

Al2Cu as suggested by the EDS study, Table 3) with Al3003 as the matrix. Given that the

hardness of Al2Cu is 413 HV, it is possible for this region reaching a high hardness above 200

HV. Based on the above reasoning, the hardness peaks were due to the existence of high

hardness IMC phase. As for the fluctuation of hardness in the intermixing nugget, it can be

reasonably attributed to the microstructure inhomogeneity. The trough shown in Fig. 8(d) was,

on the other hand, located in the region comprising mainly Al. This also indicates that there was

less Cu stirred into the nugget of LR, in agreement with the SEM observation in Fig. 6(c).

Therefore, the hardness in the

intermixing nugget was strongly affected by the intermixing of materials and IMC formation.

16

reaching a hardness value similar to Al3003 base metal. However, the hardness in the nugget ofPage 16 of 29

Manu

cceptedFig. 8 Hardness distributions at the middle thickness along the horizontal dash lines in the cross

sections shown as insets for each region: a) overlap of all distributions; distribution for b) FR;

c) MR; d) LR

4.2 Tensile strength

The tensile curves of the four regions are shown in Fig. 9(a). LR shows very different tensile

17

Page 17 of 29

properties compared with those of FR, MR and OR. It is obvious that LR had the highest tensile

strength of 1191 N (the rest three: 737 N for FR, 849 N for MR and 753 N for OR). Moreover,

strain at fracture of LR was also the largest, which reached 3%. In comparison, barely any plastic

deformation stage can be found in the curves of FR, MR and OR and their strains at fracture were

less than 0.3% from the inset of Fig. 9(a). Therefore, the region experienced the highest welding

temperature, i.e. LR, possessed the best mechanical properties.

Figure 9(b) shows fracture locations of the four regions. Similarly, the fracture location of

LR was very different from the others, locating inside the nugget to the Al side with an obvious

necking occurred before fracture. FR, MR and OR fractured in the Al/Cu intermixing zone of the

nugget near the Al bulk/Cu bulk interface and the fracture paths mainly propagated along or

perpendicular to the band structures in nugget. Xue et al. (2011) investigated that the stacking

layered structures formed in Al/Cu FSW joint was the easy path for the crack propagation during

bending test. Bisadi et al. (2012) also concluded that the brittle IMC was the main reason for the

fracture in tensile shear test. As shown above, abundant IMCs were found in the intermixing

zone of the nuggets of FR, MR and OR, and of banded morphologies (Figs. 5 and 6). Therefore,

relatively low tensile strength as well as poor ductility for these regions can be ascribed to the

easy crack initiation and propagation in this type of microstructure. Unlike FR, MR and OR, the

microstructure of the intermixing zone in the nugget of LR was very similar to that of Al3003

base metal as mentioned previously. Hence, the resistance to fracture of the intermixing zone in

LR should be better than the other three regions. Fracture of the LR specimen therefore occurred

inside the nugget to the Al side where the stressed area was the minimum, as shown in Figs. 5

and

9(b), resulting in the highest tensile strength and best ductility.

Since the weak zone in the weld was identified to locate in the nugget close to Al bulk/Cu

bulk interface, in order to compare the strengths of the weak zones in each region with the effect

of thickness excluded, a rough treatment was made to estimate the peak strength of each region

based on the peak tensile forces and the cross-sectional area of this zone of each tensile

specimen. Thicknesses of this zone in four regions were measured (measurement locations are

marked by red solid lines in Fig. 9) and given in Table 4. The peak strengths were then

approximated to be 160

MPa, 166 MPa, and 213 MPa for weak zones of FR, MR and OR, respectively (refer to Table 4).18

Page 18 of 29

For LR, since the fractured location was not in the weak zone, the peak strength of this zone in LRshould be larger than the calculated value, i.e. 176 MPa, which was ~89% of the base Al3003. It

is also very interesting to find that the strength of the weak zone in OR (213 MPa) was even

higher than the 3003Al base metal (197 MPa). In addition, the strength of the Al bulk/Cu bulk

interface should be higher than all above strength values since none of these regions fractured

along Al bulk/Cu bulk interface. Even though the reason for the high strength of intermixing

zone and Al bulk/Cu bulk interface remains for further detailed study, this observation is

particularly interesting since it suggests effective Al/Cu joint with high strength and/or high

ductility could be

produced via FSW.

(a)

19

For LR, since the fractured location was not in the weak zone, the peak strength of this zone in LR Page 19 of 29

Man

(b)

Fig. 9 (a) Tensile curves of 4 regions; (b) traverse cross sections of fractured joints after

tensile test showing two fracture modes

Table 4 Calculation for the strength of the weak zone

Region Tensile Force Thickness Area Strength

Unit N mm mm2 MPa

FR 737 0.73 4.60 160

MR 849 0.81 5.10 166

LR 1191 1.09 6.87 >173

OR 753 0.56 3.53 213

20

Page 20 of 29

Discussion

From the above results, successful welding between small-dimension Al and Cu pipes was

achieved using the welding method developed in this study. A feature found for this method very

different from the conventional FSW of flat plates is that the surface finish,

macro-/micro-structure, and mechanical properties of the joint change markedly along the

circumferential weld seam. The variation of mechanical properties has been related to the

macro-/micro-structure variation in last section. The main issues to be discussed here are the

formation of the welding temperature variation and its effect on the macro-/micro-structure.

After that, possible measures for improving the current welding method will be discussed.

The temperature increase during the circumferential welding (Fig. 3) was affected by the

accumulation of heat which should be closely related to the fixture fitted inside of the pipes in the

current welding system. In conventional FSW of flat plates, temperature variation in different

locations of weld seam was seldom considered. It is because the welding temperature stabilizes

within a short distance relative to the whole long weld seam. Zhang et al. (2013) showed a

significant effect of the thermal conductivity of the backing plate on the heat accumulation and

hence the welding temperature during FSW. In the current study, the fixture could be regarded as

a backing plate. The heat was difficult to dissipate through the fixture because of its limited size.

Also, the heat dissipation rate decreased with the increasing of the fixture temperature.

Therefore, an obvious heat accumulation resulted and led to the rising of the peak welding

temperature, as shown in Fig. 3. The welding temperature declining during the final 40° rotation

of the circumferential welding, which overlapped the initial welding seam, can be otherwise

attributed to

the thickness reduction produced in 1st pass, as the heat input in FSW would decrease with the

decrease of ID. Therefore, it was the heat accumulation caused by the fixture and the thickness

reduction in the overlapped welding region that produced this featured welding temperature

history shown in Fig. 3 for the present welding of small-dimension pipes.

The formation of macro-/micro-structures in the weld nugget, as shown in Figs. 5 and 6,

should be highly influenced by the intermixing and chemical reaction between Al and Cu and the21

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material flow during welding, all of which can be linked to the welding temperature. For the

current study, because the pin was offset completely to the Al side (O1 = 1.3 mm, PD = 2 mm),

the extensive shear of Cu pieces into the nugget depended on the interaction between the

flowing metals and the Cu at the Al bulk/Cu bulk interface. When the welding temperature was

relatively low in FR, Cu was easily sheared into pieces due to its poor deformability and

brought into the nugget, which resulted in the formation of large Cu pieces embedded in the

nugget and the inclined Al bulk/Cu bulk interface (Fig. 5(a)). Low welding temperature may also

lead to the defect formation in FR, similar to the finding in a previous study on the FSW of Al

to Cu which reported that defects easily formed under low heat input welding condition (Xue et

al., 2011). The increase of welding temperature in MR and LR improved the deformability and

softening of both metals. Hence, less Cu pieces were sheared into the nugget and Al bulk/Cu bulk

interfaces became more vertical (Fig. 5(b) and (c)). Moreover, the softening of the materials

under higher temperature might also increase the actual ID (the actual ID was normally slightly

smaller than the set value due to the imperfect rigidity of the welding system). As a result,

thickness reduction became larger in MR and LR, as shown in Fig. 5.

Based on the different Cu amount in the nuggets among FR, MR and LR as shown in Fig. 6,

it can be obtained that the stirred Cu in the intermixing zone became less with the increase of

welding temperature, which is also in agreement with the observation in Fig. 5. The more

intermixed Cu lamellae in FR made it easier to form IMCs in FR than MR and LR, even though

the welding temperatures of MR and LR were higher than FR. Therefore, under the welding

parameters in this study, the temperature was already high enough for forming Al2Cu. The key

factor affecting the formation of IMCs should be the amount of Cu sheared into the intermixing

zone, which is however still affected by the welding temperature. Therefore, it was mainly due to

the distinctive welding temperature variation that the macro-/micro-structure of the weld

changed along the circumferential weld seam.

As revealed above, the macro-/micro-structure and the final mechanical properties of the

joints were highly influenced by the welding temperature. Among all four regions, LR exhibited

the highest tensile strength and the best ductility (Fig. 9) with the least IMC formation in the weld

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(Fig. 6). From Fig. 3, the welding temperature reached a plateau in LR, relatively stable at around~260°C. Thus, controlling the welding temperature is critical for improving the current welding

method. Three possible types of methods are proposed here: 1) heat dissipation control; 2)

auxiliary heat input; 3) welding parameter control. For the first type, it is actually the material

selection for the fixture. A correct choice of material with proper thermal conductivity may

stabilize the welding temperature within a shorter time and at an optimum temperature. For the

second type, rather than a single one heat source from the welding tool, an additional auxiliary

heat source can be applied to compensate for the lack of heat to reach the optimum welding

temperature. For the third type, it is known that the welding temperature during FSW strongly

depends on the combination of the tool rotation speed (ȼ) and the travel speed (v), either in the

way of ω2/v (Arbegast and Hartley, 1999) or ω/v (Hashimoto et. al, 1999). Backer et al. (2014)

used a temperature feedback controller that modifies the FSW tool rotation speed to maintain a

constant welding temperature on a FSW robot and successfully obtained stable welding

temperature. Similarly, PRR or TRR in the current method could be adjusted in real time by a

temperature feedback controller so that the welding temperature would be maintained stable at an

optimum value. However, in this case, other welding conditions, e.g. material flow, shearing

effect of the tool, and Al/Cu intermixing, should change as a result of the variation of PRR or

TRR. The macro-/micro-structure and the final mechanical properties should be also affected.

Therefore, it will be interesting to investigate the effect of the variation of PRR or TRR at a

constant welding

temperature on the macro-/micro-structure and the mechanical properties.

23

(Fig. 6). From Fig. 3, the welding temperature reached a plateau in LR, relatively stable at around

Page 23 of 29

Conclusions

Small-dimension Al3003 pipe and pure copper pipe with thin wall and small diameter were

welded via FSW. The main conclusions reached based on the current study are summarized as

follow:

1. A FSW method for welding small-dimension dissimilar pipes was developed by using a

specially-designed FSW system.

2. Welding temperature kept on increasing during the first ~220° rotation of the circumferential

welding and was relatively stabilized for the following ~140° rotation. For the last 40°

rotation of circumferential welding, welding temperature decreased due to the thickness

reduction caused by the 1st

welding pass. Such a distinctive welding temperature history

contributed significantly by the heat accumulation was identified as an important feature for

FSW of small-dimension pipes.

3. Circumferential variations of weld surface condition, macro-/micro-structure and mechanical

properties were also observed and found to be related to the welding temperature variation.

4. None of the tested tensile specimens fractured along the Al bulk/Cu bulk interface. Two

different fracture modes were observed: 1) brittle fracture in the nugget close to Al bulk/Cu

bulk interface with the fracture paths mainly propagating along or perpendicular to the band

structures in nugget; and 2) ductile fracture in the nugget to the Al side with a ductility of

~3%.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No.

51204108), the Shanghai Committee of Science and Technology (Grant No. 11ZR1418100), and

the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education

Ministry.

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Figure and Table Captions

Fig. 1 The welding system: (a) the setting for pipe welding; (b) schematic diagram of the

welding parameters; (c) schematic diagram showing O1; (d) schematic diagram showing

O2. (PRS stands for pipe rotation rate; TRR for tool rotation rate; O1 for offset1; O2 for

offset2; ID for insertion depth)

Fig. 2 (a) Schematic diagram showing how the tensile specimens were cut from the joint; (b)

dimension of tensile specimens; (c) obtained tensile specimens; and (d) schematic diagram

of weld region division

Fig. 3 Peak welding temperature variation of the weld seam surface (FR: former region; MR:

middle region; LR: later region; OR: overlap region)

Fig. 4 Surface variation along the circumferential weld seam: (a) FR; (b) MR; (c) LR; (d) OR

(refer to Figure 3(d) for the locations of these regions)

Fig. 5 Optical images of the cross-sections of FR, MR and LR

Fig. 6 SEM images of the stir zone near the Al/Cu interface in 4 regions with the locations

marked in Figure 5 by red rectangles: (a) FR, (b) MR, and (c) LR

Fig. 7 Intermetallics around Cu pieces in MR, with the EDS tested at 4 positions. EDS results are

shown in Table 3

Fig. 8 Hardness distributions at the middle thickness along the horizontal dash lines in the cross

sections shown as insets for each region: a) overlap of all distributions; distribution for b)

FR; c) MR; d) LR

Fig. 9 (a) Tensile curves of 4 regions; (b) traverse cross sections of fractured joints after tensile

test showing two fracture modes

Table 1 Chemical composition of Al3003 alloy (mass %)

Table 2 Welding parameters

Table 3 EDS results (at.%) for the measurements marked in Figure 7

Table 4 Calculation for the strength of the weak zone

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