Materials and Design 31 (2010) 3937–3942

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Materials and Design

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Short Communication

Effect of welding speed on microstructure and mechanical properties of frictionstir welded copper

J.J. Shen, H.J. Liu *, F. CuiState Key Laboratory of Advanced Welding Production Technology, Harbin Institute of Technology, Harbin 150001, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 20 October 2009Accepted 16 March 2010Available online 19 March 2010

0261-3069/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.03.027

* Corresponding author. Tel.: +86 451 8641 3951; fE-mail address: liuhj@hit.edu.cn (H.J. Liu).

The 3-mm-thick copper plates were friction stir welded at a low tool rotation rate of 600 rpm. The influ-ence of welding speed on microstructure and mechanical properties of the joints was investigated. As thewelding speed increased, the grain size of nugget zone first increased and then decreased, the thermo-mechanically affected zone became narrow and the boundary between these two zones got distinct,but the heat affected zone was almost not changed. The ultimate tensile strength and elongation ofthe joints increased first and decreased finally with increasing welding speed, but the effect was littlewhen the welding speed is in the range of 25–150 mm/min. The defect-free joints were produced atlower welding speeds, and the fracture locations were outside the nugget zone on the retreating side.With increasing welding speed, the average hardness of nugget zone decreased first and then increased,but welding speed had little effect on the hardness of the other regions within the joints.

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1. Introduction

Copper has been widely applied in many areas for its highelectrical and thermal conductivities, favorable combinations ofstrength and ductility, and excellent resistance to corrosion [1,2].However, it’s difficult to join commercial pure copper by conven-tional fusion welding processes due to the influence of oxygen,impurity and high thermal conductivity [3–5].

Friction stir welding (FSW), invented by The Welding Institute(TWI) of the UK in 1991 [6], is a relatively new solid-state joiningprocess in which the welding defects formed in the conventionalfusion welds were not observed [7,8]. The FSW process is in rapiddevelopment for aluminum alloys and has been successfullyimplemented into commercial applications [9], but the studies onthe FSW of copper are relatively limited and preliminary [10–13].

Welding heat input required for copper is much higher than foraluminum and its alloys due to the greater dissipation of heatthrough the work-piece [14], therefore a higher tool rotation rateshould be used. For example, Okamoto et al. [15] fabricated a cop-per backing plate for cooling by FSW at a tool rotation rate of1300 rpm together with a welding speed of 170 mm/min; Leeand Jung [10] welded 4-mm-thick copper plate successfully at atool rotation rate of 1250 rpm with a welding speed of 61 mm/min, and Sakthivel and Mukhopadhyay [16] obtained FSW weldsof 2-mm-thick copper sheet at a tool rotation rate of 1000 rpm

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with a welding speed of 30 mm/min. However, due to the genera-tion of coarse grains within the joints resulted from high tool rota-tion rate, the strength efficiency of all the joints mentioned abovewas no more than 87%, which was slightly higher than that of theelectron beam welded joint [17]. Although, Xie et al. [12] weldedcopper under a relatively low tool rotation rate, his research wasonly focused on the nugget zone (NZ), instead of on the heat af-fected zone (HAZ) or thermo-mechanically affected zone (TMAZ).It is well known that the HAZ (or TMAZ) is a key part in the FSWjoint, where is often the minimum-hardness region and fracture al-ways occurs [9,18,19], so some technological processes wereadopted to improve the microstructure and properties of HAZ,including the control of welding heat input and the use of post-weld heat treatment [20–24]. Furthermore, as far as now withinthe studies on the FSW of copper, such as [10,12,15–17], it seemsthat the welding parameter range is quite narrow, and none hasstudied the evolution of microstructure and mechanical propertiesof the joints with welding parameters yet. Besides, on some spe-cific issue, such as the existence of TMAZ, Lee and Jung [10] re-ported that no distinct TMAZ was identified in copper weldswhile the results of Sakthivel and Mukhopadhyay [16] and Anders-son and Andrews [25] exhibited distinct TMAZ, however no reasonwas given.

In order to control welding heat input in FSW process, a 3-mm-thick pure copper plate was friction stir welded using a low toolrotation rate in the present study. Under the definite condition oftool rotation rate, the aim is to investigate the effect of weldingspeed on the microstructure and mechanical properties of copperjoints.

Table 1Width of NZ formed at various welding speeds.

Welding speed (mm/min) Width of NZ (mm)

25 5.2650 5.13

100 5.01150 4.66200 4.27

3938 J.J. Shen et al. / Materials and Design 31 (2010) 3937–3942

2. Experimental procedure

The base metal (BM) used in the experiment was a pure copperplate (under the 1/2H condition) of 3 mm thickness. The plate wascut and machined into rectangular welding samples of 200 mmlength by 50 mm width, and then the samples were butt weldedperpendicular to the rolling direction using a welding machine(FSW-3LM-003). The rotation tool was made of high-speed toolsteel, with a pin (/3 � 2.85 mm) having standard right-handthreads and a shoulder (/12 mm) having a concave profile. FSWwas conducted at a constant rotation rate of 600 rpm together withdifferent welding speeds of 25, 50, 100, 150 and 200 mm/min.

The specimens for microstructural evaluation were sectionedfrom the FSW joints transverse to the welding direction, polishedand then etched with a solution of 15 ml hydrochloric acid,100 ml distilled water, and 2.5 g iron chloride. Microstructural fea-tures were examined by optical microscopy (OM, Olmpus-PGM3).Vickers hardness measurements were performed on the cross sec-tion at mid thickness of the welds with a load of 500 g for 10 s. Ten-sile specimens with a gauge length of 50 mm and a width of12 mm were machined perpendicular to the FSW direction. Thetensile test was carried out using a universal testing machine (In-stron-1186) at a constant crosshead speed of 1 mm/min. The fac-ture surfaces were analyzed using scanning electron microscopy(SEM, Hitachi-570).

3. Results and discussion

3.1. Microstructural characteristics

Fig. 1 shows the typical cross-sectional photographs of thejoints welded at a tool rotation rate of 600 rpm. It can be seen thatno welding defect was detected in the joints welded at lower weld-ing speeds, and a high welding speed results in cavity defect (seeFig. 1c). The joint exhibits several distinct microstructural regions,i.e. the NZ at the weld centre, the HAZ surrounding the NZ, theTMAZ between the NZ and HAZ, and the BM. Besides, the size ofNZ decreases with the increase of welding speed. The width ofNZ at mid thickness of the welds is listed in Table 1, in which itis indicated that the width of NZ decreases more remarkably at ahigher welding speed.

The microstructures of NZ formed at various welding speeds areshown in Fig. 2a–c. Compared with the BM (see Fig. 2d), the NZ hasmuch smaller equiaxed grain due to dynamitic recrystallization.With the increase of welding speed, the grains become coarse firstand then get small, which is similar to those reported in Ref. [9]. It’swell known that a high welding speed (tool rotation rate is a con-stant) results in a decrease in both the degree of deformation andthe peak temperature of thermal cycle. On one hand, the decreasein the degree of deformation during FSW results in an increase inthe recrystallized grain size according to the general principlesfor recrystallization [26]. On the other hand, the decrease in peak

1mm a

c

NZ TMAZ

HAZ

TMAZ

HAZ

Fig. 1. Cross-section macrograph of copper FSW joint: (a) 25 mm/

temperature of FSW thermal cycle leads to generation of smallrecrystallized grains. Therefore, the variation of recrystallized grainsize with welding speed in FSW depends on which factor is domi-nant. Some literatures [9,27] appear to indicate that the peak tem-perature of FSW thermal cycle is the dominant factor indetermining the recrystallized grain size. Thus, the recrystallizedgrain size in FSW generally decreases with increasing weldingspeed. But in this study, it is shown that the degree of deformationis the dominant factor at first, and with the further increase ofwelding speed, the peak temperature of FSW thermal cycle is thedominant factor.

Fig. 3 shows the microstructure of HAZ obtained at variouswelding speeds. Compared with the BM (see Fig. 2d), the grainsin the HAZ grow to some extent, but the grain size almost doesnot change with the welding speed from 25 mm/min to 200 mm/min (see Fig. 3a–c). This implies that the welding speed has littleeffect on the grain size in the HAZ. Besides, the grain size of HAZon the RS is bigger than on the AS (see Fig. 3d).

Fig. 4 shows the TMAZ produced at various welding speeds. Fol-lowing observations can be made from this figure. First, the TMAZis not so distinct as that in aluminum FSW joint because no elon-gated or rotated grains adjacent to the NZ are observed. The grainsin the TMAZ are bigger than those in the NZ, but smaller than in theHAZ. Second, there is a distinct boundary between TMAZ and NZ onthe advancing side (AS), while the transition between TMAZ andNZ is smooth on the retreating side (RS), which can also be ob-served in Fig. 1. Such a result is attributed to the difference of metalplastic flow state between the two sides in FSW process, which hasbeen explained in detail in Ref. [11]. Third, when the welding speedis high, e.g. 200 mm/min, the TMAZ is quite narrow and there is asharp boundary between the TMAZ and NZ (see Fig. 4c). As thewelding speed decreases, the TMAZ becomes wide and the bound-ary is obscure. So whether the TMAZ is distinct or not dependsmainly on the welding parameters, and Lee’s [10] and Andersson’s[25] results reflect two aspects of this effect respectively.

3.2. Mechanical properties

Fig. 5 shows the tensile properties of the joints welded at vari-ous welding speeds. It can be seen that the ultimate tensilestrength (UTS) and elongation have a similar variation trends. Bothincrease first and decrease finally with increasing welding speed. It

1mm

1mm

b

min, (b) 100 mm/min, and (c) 200 mm/min (AS on the right).

20µm

20µm 20µm

20µm

dc

a b

Fig. 2. Microstructure of NZ: (a) 25 mm/min, (b) 100 mm/min, (c) 200 mm/min, and (d) BM.

40µm 40µm

40µm 40µm

dc

a b

Fig. 3. Microstructure of HAZ: (a) 25 mm/min, (b) 100 mm/min, (c) 200 mm/min (a–c on the RS), and (d) 200 mm/min (AS).

J.J. Shen et al. / Materials and Design 31 (2010) 3937–3942 3939

is noted that the UTS and elongation of joints obtained at the weld-ing speed range of 25–150 mm/min have little change, and theirhighest values are corresponding to the welding speeds of50 mm/min and 100 mm/min, respectively.

Fig. 6 shows the fracture locations of the joints welded at vari-ous welding speeds. From the front face graph (see Fig. 6a), signif-icant necking exists around the fracture locations when thewelding speed is not higher than 150 mm/min, which means thatthe macroplastic deformation occurs in the joints during tensiletest. However, there are no significant necking in the joints ob-tained at the welding speed of 200 mm/min. In addition, the jointis fractured on the RS for the former situation, and it is on the ASfor the latter. The specific fracture position can be observed from

Fig. 6b. The fracture occurs at the cavity defect of the joint whenthe welding speed is 200 mm/min, and the fracture path of thejoints passes through TMAZ, HAZ and BM when the welding speedis not higher than 150 mm/min.

The fracture surface of the joint welded at the welding speed of25 mm/min can be divided into two regions, as marked with 1 and2 in Fig. 7a, and their magnified SEM graphs are shown in Fig. 7band c. Figs. 8 and 9 show the fractograph of the joints welded atthe welding speeds of 100 mm/min and 200 mm/min, respectively.It is noted that the fracture surface of the joint formed at the weld-ing speed of 200 mm/min can be divided into four regions, asmarked with 1–4 in Fig. 9, according to their low magnificationfracture pattern. This interesting phenomenon is similar to that re-

40µm 40µm

40µm 40µm

dc

a b

Fig. 4. Microstructure of TMAZ: (a) 25 mm/min, (b) 100 mm/min, (c) 200 mm/min (a–c on the AS), and (d) 200 mm/min (RS).

0 50 100 150 200200

225

250

275

300

Tensile Strength Elongation

Welding speed (mm/min)

Ten

sile

Str

engt

h (M

Pa)

0

5

10

15

20

Elongation (%

)

Fig. 5. Tensile properties of the joints.

3940 J.J. Shen et al. / Materials and Design 31 (2010) 3937–3942

ported in Ref. [11]. The fracture characteristics and micro-fracturemechanism are listed in Table 2. Obviously, the fracture mecha-nisms of the FSW joints are influenced by the welding speed.

In order to explain the relation between tensile features andwelding speed, the transverse hardness distribution of the joints

25mm/min

50mm/min

100mm/min

150mm/min

200mm/min

RS AS RSa b

Fig. 6. Fracture locations of the joints: (

for various welding speeds are measured, as shown in Fig. 10. Withthe increase of welding speed, the average hardness of NZ first de-creases and then increases, identical to the variation trend of grainsize of the NZ (see Fig. 2), and this comparability is in accordancewith the Hall–Patch equation. In detail, the hardness exhibits thehighest value in the NZ and gradually falls down from the NZ totwo sides when the welding speed is 25 mm/min or 200 mm/min, and the average hardness of the NZ is approximately equalto the base metal when the welding speed is 100 mm/min or150 mm/min. And the regions except NZ have almost the samehardness value whatever the welding speed is.

When the welding speed is 100 mm/min or 150 mm/min, allthe regions within the joint have almost the same hardness, butthe grain size of HAZ is the biggest and the microstructure in theNZ is more uniform, accordingly the joint is easy to fracture in anuneven multi-component zone, i.e. TMAZ, HAZ and BM (seeFig. 6b). When the welding speed is 25 mm/min or 200 mm/min,the hardness of NZ is the highest, therefore the joint will mostlikely fracture in the lower hardness zones, i.e. TMAZ, HAZ andBM if no defects exist in the joint, or the fracture will be originatedfrom the defect location if defects exist in the joint (see Fig. 6b). Asmentioned above, the grain size of HAZ on the RS is bigger than onthe AS, and thus the fracture occurs on the RS of the defect-freejoints.

25mm/min

100mm/min

200mm/min

AS

a) front face, and (b) cross-section.

-10 -8 -6 -4 -2 0 2 4 6 8 100

20

40

60

80

100

120

140

25mm/min 100mm/min 150mm/min 200mm/min

Har

dnes

s (H

v)

Distance from weld centre (mm)

Fig. 10. Microhardness distribution of FSW joints formed at various weldingspeeds.

1

2

a b cFig. 7. SEM images of the tensile fracture surface: (a) low magnitude, (b–c) magnified of region 1–2 marked in (a) (welding speed: 25 mm/min).

a bFig. 8. SEM images of the tensile fracture surface: (a) low magnitude, and (b) highmagnitude (welding speed: 100 mm/min).

2

1

3

4

Fig. 9. SEM image of the tensile fracture surface (welding speed: 200 mm/min).

Table 2Fracture characteristics and mechanism of typical joints.

Fig. Region Fracture characteristics Fracturemechanism

Low magnification High magnification

7 1 Tear ridge and snake-like pattern

Smooth and fewflat dimples

Slipping

2 Tear ridge and holes Dimples Microvoidcoalescence

8 – Tear ridge and holes Dimples Microvoidcoalescence

9 1 Tear ridge and snake-like pattern

Smooth and fewflat dimples

Slipping

2 Tire pattern Dimples Microvoidcoalescence

3 Smooth and regular River pattern Cleavage4 Tear ridge and holes Dimples Microvoid

coalescence

J.J. Shen et al. / Materials and Design 31 (2010) 3937–3942 3941

All in all, in regard to the defect-free joints, the fracture occursin the multi-component zone composed of TMAZ, HAZ and BM onthe RS during tensile test. When the welding speed is changed from25 mm/min to 150 mm/min, there is little hardness variation inthis zone on the RS (see Fig. 10), therefore the tensile propertiesof joints have little change, as shown in Fig. 5.

4. Conclusions

(1) No welding defect was detected in the joints welded atlower welding speeds, and the size of NZ decreases withthe increase of welding speed.

(2) As welding speed increased, the grain size of NZ increasedfirst and then decreased, while TMAZ became narrow andthe boundary between TMAZ and NZ got distinct, but thegrain size of HAZ has little change.

(3) The ultimate tensile strength and elongation of the jointsincreased first and decreased finally with increasing weldingspeed, but the welding speed almost had no effect on thetensile properties of the joints when the welding speed isin the range of 25–150 mm/min.

(4) The defect-free joints were produced at lower weldingspeeds and the fracture path of the joints passed throughTMAZ, HAZ and BM on the RS, therefore the fracture surfaceshowed a microvoid coalescence and slipping mechanism oronly microvoid coalescence mechanism.

(5) With the increase of welding speed, the average hardness ofNZ first decreased and then increased, but the welding speedhad little effect on the hardness of the other regions within

3942 J.J. Shen et al. / Materials and Design 31 (2010) 3937–3942

the joints. Such hardness distributions can be used toexplain the tensile features of the joints.

Acknowledgements

The research was sponsored by the National Key TechnologyResearch and Development Program No. 2006BAF04B09, Ministryof Science and Technology, PR China, and was also supported bythe Program of Excellent Team in Harbin Institute of Technology,PR China.

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