Microstructure and phase constitution near the ... - Springer Link

Journal of Mechanical Science and Technology 26 (12) (2012) 4089~4096 www.springerlink.com/content/1738-494x

DOI 10.1007/s12206-012-0884-7

Microstructure and phase constitution near the interface of Cu/3003 torch brazing using Al-Si-La-Sr filler† Fei Yan1, Daorong Xu2, S.C. Wu2,3, Qinde Sun2, Chunming Wang1,* and Yajun Wang1 1

School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China 2 Centre for Advanced Materials Joining and Simulations (AMJS), School of Materials Science and Engineering, Heifei University of Technology, Hefei, 230009 China 3 Centre for Advanced Computations in Engineering Science (ACES), Department of Mechanical Engineering, National University of Singapore (Manuscript Received December 2, 2011; Revised July 25, 2012; Accepted August 3, 2012) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Abstract It has been mainly studied in this paper on brazing of Cu to Al using Al–Si filler metal. The optimized scanning rate of 2.5 mm/s is first obtained through simulating the temperature field of Cu-Al brazing process based on ANSYS software. Then the brazing of CuC11000 to Al-3003 using Al–Si–La–Sr filler is carried out by torch brazing technology. It is found that the brazing seam region is mainly consisted of α-Al solid solution and CuAl2 IMC. Further experimental results also show that the rare earth element La in filler metal can not only refine the grain, but also promote the dispersion of intermetallic compounds into the brazing seam, which significantly improves the brazing seam microstructure and mechanical properties of the joints. Keywords: Temperature field; Cu-Al; Torch brazing; Intermetallic compounds; Finite element method ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction With the increasing application of the light weight and high performance components into industry departments, considerable interest recently has been focused on welding nonferrous materials to form compound structure due to the excellent electrical and thermal properties of Cu and Al. Substituting aluminum for copper can not only decrease manufacturing costs, but also make full use of the performance advantages of different materials. So Al-Cu dissimilar non-ferrous metal connectors are widely used in industries of electric power, chemical, refrigeration and aerospace and so on. The melting point, fusion heat and linear expansion of Cu and Al are very different and intermetallic compounds (IMCs) can be easily formed between Cu and Al. It is therefore difficult to weld Cu and Al for the compound structure [1]. Generally, the ultrasonic welding [2, 3], friction stir welding (FSW) [4-6] and laser welding [7] are used to weld the Cu and Al, and these techniques form a joint based on either solid diffusion or transient liquid phase principle. One important issue to weld the copper and aluminum is that the chemical potential difference between copper and aluminum is relatively larger, usually leading to the corrosion of joints. Additionally, there *

Corresponding author. Tel.: +86 13871541964, Fax.: +86 27 87543894 E-mail address: [email protected], [email protected] † Recommended by Associate Editor In-Ha Sung © KSME & Springer 2012

are some brittle intermetallic compounds [6] and crack [4] in the weld, which result in the lower strength, compared with the base material. The effects of the oxide film on the welded joint properties had been investigated by some researches. Ono et al. [8] investigated influence of oxide film on weld, and found excess oxygen supplied from oxide film caused solidification crack as well as porosity and thought the porosity was formed by the Al2O gas through the reaction between Al2O3 and Al. Nigo [9] et al. observed that the thickness of the aluminum oxide film and porosity after welding of an aluminum-copper alloy (2219) increased when an aluminum alloy was exposed to atomic oxygen. There are two main problems in the Cu-Al brazing process. Firstly, a strong corrosive brazing flux needs to remove oxide film on the surface of aluminum, whose slag can make joints corroded after the electrolyte is formed. Secondly, aluminum and copper atoms spread quickly so that fusible brittle eutectic phases arise easily in the joint, leading to lowering joint strength [10]. In this paper, first of all, a copper plate and aluminum plate with respectively 2 mm and 3 mm thickness are modeled and the temperature field of brazing process is simulated for suitable process parameters using the ANSYS code. After that, Cu and Al are brazed by the flame using Al-Si-La-Sr filler and no corrosive brazing flux. The microstructure, microhardness

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F. Yan et al. / Journal of Mechanical Science and Technology 26 (12) (2012) 4089~4096

Table 1. Thermal properties of Cu, Al and the brazed filler. Temperature (°C) -1

3003

200

300

400

500

600

λ(W⋅m⋅°C )

159.1

166.1

174

180

184

190

200

750

810

840

870

900

920

940

ρ (kg⋅m-3)

2680

λ (W⋅m⋅°C )

180

188.4

180.0

184.2

186.2

c (J⋅kg-1⋅°C-1)

958

1009

1172

1298

1290

-3

ρ (kg⋅m )

700

2740

-1

C11000

100

c (J⋅kg-1⋅°C-1)

-1

Filler

20

λ (W⋅m⋅°C )

390.3

370.1

355.1

345.4

334.9

320

315.8

310.3

c (J⋅kg-1⋅°C-1)

397.8

418.7

431.2

439.6

452.2

464.7

471.5

477.3

-3

ρ (kg⋅m )

8900

2.2 Simulation parameters

10mm

C11000 30mm

Gassian surface heat source model

0.2mm

Forth

B A

Flow direction

Back

Filler metal 3003

Computational parameters used in brazing are as follows: flame core temperature is about 2500°C, and the effective heating radius is 6 mm. To study the effect of scanning rate on brazed joints, a set of scanning rate is designed: v = 1.0 mm/s, 2.5 mm/s and 4.0 mm/s. The heating of the flame is cycled back and forth as demonstrated in Fig. 1. The surface of brazed components exchanged the heat with air through the convection. The heat conductivity (λ), specific heat (c) and the material density (ρ) are listed in Table 1 [11-13].

3mm

2.3 Simulation results and conclusions Fig. 1. The computational model of torch brazing.

distribution, features and phase constitution in the brazing seam region are analyzed by means of metallography, scanning electron microscope (SEM), X-ray energy dispersive spectrometer and X-ray diffraction (XRD). The results not only work out the problem about fusible brittle eutectic phases, but also provide a favorable basis for the extensive application into the engineering structure.

2. 2 Finite element simulation 2.1 Model formulation In this paper, the ANSYS software is employed to simulate the torch brazing process. Fig. 1 depicts the computational model. The solid70 element is selected for the temperature field. To enhance the numerical efficiency and accuracy, the filler material zone is given a finer grid, and the other regions of both Cu and Al materials are relatively coarse using the free meshing strategy. The Gaussian surface heat source is taken to simulate the flame over the filler metal as shown in Fig. 1. Clear original figures in black and white should be used. Equations should be numbered consecutively throughout the paper and located at the right margin as in Eq. (1) below. Figures and tables should be placed at the top or at the bottom of each column as in Fig. 1 and Table 1.

It should be noted that, for each scanning rate, only one back-and-forth is simulated. Fig. 2 gives the final temperature distributions under three types of scanning rate: v = 1.0 mm/s (upper), v = 2.5 mm/s (middle) and v = 4.0 mm/s (lower). The total time required for each rate is 20s, 8s and 5s, respectively. It can be clearly observed that rapid scanning rate of 4.0 mm/s cannot fully melt the filler metal into the brazing seam, and vice versa the slow scanning rate of 1.0 mm/s will overheat the filler so that it flows out of the Al plate. Moreover, more heat energy will make Al substrate be fused to wear, which severely degrades the quality of brazed joint. It is thus found that the scanning rate of 2.5 mm/s is suitable because the filler metal can fully fill the entire weld according to the temperature distribution of middle plot. Fig. 3 shows the temperature profiles under three types of scanning rate for two sampled points of A (upper) and B (lower) along the brazing seam. Note that only half of brazing time for scanning rate of 1 mm/s is shown in Fig. 3. It can be clearly seen that the slower scanning rate overheats the filler metal to wear, and the peak temperature is about 1902°C after one back-and-forth with the total time of 20s. On the contrary, rapid scanning rate of 4 mm/s cannot provide enough heat energy to melt the filler metal. The upper figure shows that A point under the 2.5 mm/s can melt the braze, and the lower figure implies the farthest point of B can also be melt, which shows that 2.5 mm/s is the suitable rate to

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Al2 O3 +2F- +AlF4 - → 3AlOF2 - .

obtain the good quality of brazed joints.

3. Materials and experiments The materials used in this paper are copper (C11000) and aluminum alloy (3003). The chemical compositions are listed in Table 2. Cu and Al samples are machined into the dimensions: 100 mm × 10 mm × 3 mm and 100 mm × 10 mm × 3 mm. The filler metal is Al-Si-La-Sr, which is made up of eutectic Al-Si alloy and a handful of elements: Sr and rare earth La, and the radius of welding wire is 2.0 mm or so. The brazing flux is taken as NOCOLOK with no corrosiveness，chemical compositions of which are KalF4 and K3AlF6. It is applied as flux slurry [14]. The flux melts have an ion exchange dissolving role for aluminum oxidation film. The reactions between the brazing flux and the oxide film are as follows: Al2 O3 +AlF63- → 3AlOF2-

(1)

(2)

The technological parameter during the torch brazing is taken as: brazing temperature T = 582~613°C. Before torch brazing, the oxidation film and greasy dirt on the surface of substrates and filler are eliminated by a series of mechanical and chemical methods. The bonding area of lap-joint is 100 mm2. After brazing, the samples are cut from the brazing joint using a line cut machine. All samples are ground using different types of sand paper, polished and finally etched with solutions including 5% HNO3 mixed with CH3CH2OH. Metallographic examinations are first conducted to examine the microstructure feature near the Cu/Al interface. The microhardness distribution and phase constitution of the Cu/Al brazing joint are then analyzed respectively by means of the JSM-6490LV and X-Ray Diffractometer (XRD).

4. Results and analyses 4.1 Microstructure feature of Cu/Al joint

Table 2. Chemical compositions of 3003 and C11000 (in wt%). Chemical compositions (wt%)

Materials Mn

Mg

Fe

Si

Cu

Zn

Al

3003

1.0~1.6

0.05

0.70

0.60

0.20

0.1

bal.

C11000

—

—

0.005

—

≥ 99.9

—

—

Microstructure feature near the interface zone of Cu/Al brazing joint can be observed using the optical metallographic microscope (Fig. 4). It is found that the interface zone includes the transition region on the Cu side, the middle brazing seam region and the transition region on the Al side. The boundary of brazing seam region near the 3003 substrate can be seen

v = 1 .0 mm/s

v = 4 .0 mm/s 1500

1450 1500

1600

1750

1600

1700 1650

1900 1850 1800 1750 1700 1650 1600 1550 1500 1450 1400 1350

400

450

450

500

600 500

650 500 450

1600 400

1550 1550

t = 20s

Fig. 2. Temperature distribution after two-pass scanning.

Fig. 3. Temperature history of filter metal on A (upper) and B (lower) points.

t = 5s

950 900 850 800 750 700 650 600 550 500 450 400

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( a)

Brazing seam zone

3003

(b ) Fig. 5. Al-Cu phase diagram.

Brazing seam zone

Fig. 6. The fracture of the lap joint.

(c )

C11000

the strength of joints; otherwise excessive infiltration may result in the decrease of corrosion resistance of joints. According to the Al–Cu phase diagram (Fig. 5), the generated prismatic structure is α-Al solid solution and thanks to the diffusion of Si atom in Al substrate [10], concentration of Si element increases gradually from the root to the top of the prismatic structure. The proper growth of Al prismatic structure is welcome to improve the mechanical property of brazed Cu/Al joint [15].

Brazing seam zone

4.2 Strength of the joint

Fig. 4. Microstructure in interface zone of Cu/Al brazing joint: (a) 3003 side; (b) Brazing seam region; (c) C11000 side.

easily from Fig. 4(a), and bulk prismatic structure is formed perpendicular to Al surface. The width of transition region on the side of Al substrate is about 0.2 mm. From Fig. 4(b), there are a lot of fine grains arising in the brazing seam region, which is relatively dense. The boundary of transition region on the side of Cu substrate can also be seen in Fig. 4(c), and the width is 0.15 mm. In addition, it can be seen that a lot of filler-elements penetrate into the base material. The formation of dispersive distribution of fine particles among the grain boundary of base metal could be explained by the dissolution of copper into filler metal rather then the infiltration of filler metal into the base metal. It is known that an appropriate amount of filler metal element infiltration along the base material of grain boundary can enhance

To measure the mechanical property near the joints of Cu/3003 torch brazing, a shearing test can be conducted using an electronic universal testing machine. Welding slag left in the weld surface of the lap-joint must be eliminated before conducting the test. Five groups of samples were tested in this experiment. The results indicate that the base material 3003 near the weld shrank and split in Fig. 6. In this case, we know that the shearing strength of the joint is higher than 54 M Pa, and therefore the area of 3003 near the weld is the weak point. There are two explanations for the fracture that occurred in 3003 side near the weld. Examining the microstructure of the joint in Fig. 8(a), we determined that the spilt in 3003 side of near the weld was the result of the Cu element excessive infiltration. The shearing strength of joints can be is greatly affected by the microstructure in the weld. The uneven stress concentration in 3003 side can also lead to fracture of the base metal.

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Table 3. EDS result of element composition in the brazing zone.

400

Microhardness (HV)

350

Elements Prismatic structure

Point C

wt%

at%

wt%

at%

Al

65.54

77.70

83.70

89.18

83.09

88.58

Cu

34.46

22.30

16.30

10.82

16.34

10.73

0.67

0.69

Si

200

Point B

at%

300 250

Point A wt%

Cu 150

Brazing seam zone

3003

100 50 0 -0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Distance (mm) Fig. 7. Microhardness near the interface of Cu/3003.

4.3 Microhardness near the joint The microhardness of Cu substrate, brazing seam region and Al substrate is measured, and the results are demonstrated in the following Fig. 7. The test instrument and parameters taken are: the Shimadze microsclerometer, 25gf loading and a load time of 10s. It can be clearly observed that the microhardness of prismatic structure is higher than that of 3003 substrate (about 40 HV). The problem arises in the diffusion of Si element in the eutectic reaction. The higher Si element concentration is, the more microhardness of the prismatic structure is [15]. The microhardness of brazing seam region is above 160 HV, and the peak is about 350.5 HV. The main reason is that the brazing process generates a lot of α-Al solid solution and Cu-Al intermetallic compounds, whose microhardnesss is very high. In contrast, the microhardness of Cu substrate is relatively low (about 60 HV), while the microhardness of the transition on Cu side is distinctly higher, reaching the 333 HV. The main reason is that there are a mass of fine grains distributing on the grain boundary of copper matrix, which have played a role in strengthening the matrix. Under the environment of torch brazing, there are various reactions proceeding between Al and Cu, and the element concentration changes in a linear relationship with the distance point A to point B.

4.4 Element distribution near the interface of Cu/Al torch brazing The Al-Si filler metal for brazing copper and aluminum is very good solder. To some extent, quality of Cu/Al joint depends on reactions of the Al-Si filler metal and the base metals. There are multiple elements in the brazing seam zone. The element distributions can be measured by X-ray energy dispersive spectrometer. For further studying element distributions in Cu substrate, brazing seam region and Al substrate, constituents in three points A, B and C are measured in Fig. 8, and the results are showed in Table 3.

Fig. 8. EDS result of element composition in the brazing zone.

According to Al–Cu phase diagram (Fig. 5) and Table 3, the phases in point A are CuAl2 and eutectic structure (α-Al +

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F. Yan et al. / Journal of Mechanical Science and Technology 26 (12) (2012) 4089~4096 18000

■ ●: Al ■: Cu

15000

Intensity

▲: CuAl2 ★ :Al4Cu9 ◆ : Cu5Si

◆ ★ ▲

12000

■

9000

★ 6000

■ 3000

0 30

●

▲ ▲ ● 40

◆ ◆● ▲ ★

★ ● 50

60

70

80

▲ ■

90

2θ(degree )

Fig. 9. XRD result of brazing zone of Cu/3003 brazing joint.

CuAl2), and the phases in point B areα-Al and eutectic structure (α-Al + CuAl2). The phases in point C are the same as the ones in point B. The microstructure of the base material 3003 near the weld contains some fine CuAl2 phases, which have played an important role concerning mechanical property of the Cu/Al brazing joint. In the brazing process, the activity of Al element is higher than that of Cu, so CuAl2 is formed according to the following equation between Cu and Al: 2Al + Cu → CuAl2.

(3)

In the brazing seam zone, the eutectic reaction occurs between a part of CuAl2 phase and Al matrix so that a binary eutectic phase (α-Al + CuAl2) can be formed. It is shown in Ref. [16] that most fractures on the Cu side take place inside the CuAl2 phase. Therefore, it is very salutary for improving the performance of the Cu/Al joint to take the necessary measures by means of reducing the amount of formed CuAl2 IMC or changing the form of formed CuAl2 IMC, such as shortening the treatment time at high temperature, refining the grains of CuAl2 IMC.

4.5 XRD analysis near the interface To clarify the phase constitution near the interface of Cu/3003 torch brazing, the XRD analysis is further carried out under certain conditions. Fig. 9 shows the XRD profile of the brazed samples on the interface. According to the XRD results, the brazing seam region mainly consists of α-Al solid solution and CuAl2 IMC, which is in good agreement with the EDS results. Moreover, Cu5Si and Cu4Al9 phases are formed in the brazing seam region, and these phases appear in the form of small granular structure. The granular compounds are dispersed into the brazing seam region so that it can play an important role in the strengthened alloy. However, no matter what brittle IMCs are formed, the microhardness of brazing seam region would arise and the brittleness would increase. Brittle phase arising in the brazing seam region can seriously weaken the strength of the joints. βCu phase is a kind of solid formed by means of aluminum soluble in copper. Because the structure of solid solution has

Fig. 10. Microstructure analysis of Cu/3003 brazing joint zone.

. good strength and shaping, it is helpful to the joints. Thus the technological parameters such as the holding time, and the brazing temperature can be controlled properly to limit the Al diffusion into Cu substrate and the formation of any brittle IMC in the brazing seam region. Finally, it is also feasible to take steps to diffuse the distribution of brittle phases.

4.6 Morphology analysis of the joint zone In order to further examine the morphology of brazing seam structure, this joint of the SEM analysis is observed and the


Fig. 11. Microstructure of Cu/Al brazing joint zone with no La in the filter metal.

. results can be found in Fig. 10. It can be found that the grains in brazing seam region are very small, and the microstructure is also very dense (Fig. 10(a)). A more dense intermediate layer made up of eutectic phase (α-Al + CuAl2) between the brazing seam region and the base material in the joint, which has been formed, can make electrode potential transition more subdued from the base metal to the brazing seam center, so that anti-corrosion of the joint can be greatly improved. The grains on 3003 side grow up in the form of columnar grain. The fine grains can penetrate into the inter-columnar crystals and disperse in the columnar crystal. They can also penetrate through the diffusion the Cu grain boundaries within the matrix. Appropriate amount of intergranular penetration can effectively improve the performance of joint area, however too much intergranular penetration usually reduces the strength of the joint and leads to the intergranular corrosion. Small acicular structure is the eutectic structure (α-Al + CuAl2), the black phase is α-Al, and the gray bulk phase is mainly copper and intermetallic compounds (Fig. 10(c)). Intermetallic compounds are diffused distribution of in the solder, which can greatly reduce the brittleness of brazing seam structure and improve the strength of the joint. Comparing with the microstructures in Fig. 4(a), the ones brazed by the filter with no La are bulky in Fig. 11 [15]. So it can be determined that a small amount of rare earth elements La in the brazing filler metal can refine grains and improve the brazing seam of the structure. Copper and aluminum compounds diffused in the brazing seam region in the form of herringbone texture (Fig. 10(d)), not only can improve the hardness of the brazing seam zone, but also can increase the strength of the joint.

5. Conclusions From the above numerical and experimental studies of the microstructure and phase constitution near the interface of Cu/Al torch brazing, the following conclusions can be drawn: (1) The brazing temperature field has been successfully obtained, which implies that the scanning rate of 2.5 mm/s of the

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flame is suitable to form the good quality of brazed joint. And a good joint can be achieved under the help of this simulation parameter. (2) The interface of Cu/Al brazed samples includes a transition region on Cu side, middle brazing seam region and a transition region on Al (3003) side. Layers of Cu9Al4 and CuAl2 are formed between the brazing seam and the Cu substrate. Average width of layered Cu9Al4 and CuAl2 is about 0.15 mm. Microhardness of the layers is higher than that of the matrix. On the transition region of 3003 side, the width is 0.20 mm. Thanks to the diffusion of Si element, microhardness changes in a linear relationship with the distance. (3) The XRD results indicate that the brazing seam region mainly consists of α-Al solid solution and CuAl2 IMC. Moreover, Cu5Si and Al4Cu9 appear in the form of small granular structure, which can strengthen the matrix after dispersing into the brazing seam region. In the brazing seam region, α-Al + CuAl 2 binary eutectic phase is formed. (4) The rare earth element La can refine the grain in the brazing seam. Diffusing copper and aluminum compounds in the brazing seam region can improve the brazing seam structure and mechanical properties of the joints.

Acknowledgment This work is partially supported by the Ph.D project with No.: GDBJ2009-025 and A*Star, Singapore, and China Postdoctoral Science Foundation with No.: 20080440930. It is also partially supported by the Open Research Fund Program of the State Key Lab of Advanced Technology of Design and Manufacturing for Vehicle Body, Hunan University, PR China under the grant number 30915005 and 80915001. The authors also give sincerely thanks to the support of the project supported by National Nature Science Funds with No.: 51005068.

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Fei Yan received his Master's Degree in Material Processing Engineering from Hefei University of Technology, China, in 2010. He is currently pursuing a Ph.D degree at the School of Materials Science and Engineering at HUST in Wuhan, China. His research is focused on laser materials processing. Chun-ming Wang is an Associate Professor at the School of Materials Science and Engineering, Huazhong University of Science and Technology (HUST), China. His research interest is laser materials processing. He is the author of 20 published papers.