Formation Process of Joints Brazing with Amorphous Filler Metal

Rare Metal Materials and Engineering Volume 42, Issue 4, April 2013 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2013, 42(4): 0688-0691.

ARTICLE

Formation Process of Joints Brazing with Amorphous Filler Metal Yu Weiyuan,

Lu Wenjiang,

Xia Tiandong

State Key Laboratory for Non-ferrous Metal New Materials of Gansu Province, Lanzhou University of Technology, Lanzhou 730050, China

Abstract: The formation, wetting and spreading process of the liquid amorphous filler metal were investigated. The results indicate that the eutectic phase of the amorphous filler metal is damaged and plenty of high melting-point copper solid solution and copper-nickel compound are generated due to the full solid-phase diffusion before melting. Therefore, relatively less liquid phase is produced by amorphous filler metal than crystalline filler metal and the liquid phase spreading and flow is weak. However, the crystalline filler metal generates a large amount of liquid phase. Spreading and flowing of the liquid phase is the main process of crystalline filler metal brazing. As the amorphous filler metal is very thin, the atomic diffusion distance is very short in the filler metal, which is quite favorable for the rapid dissolution into the base metals. The ordinary filler metal is relatively thick, and it tends to take longer time to dissolve than the amorphous filler metal. The thickness of the residual filler metal decreases with the increase of brazing temperature and dwell time. The thickness of the residual layer of amorphous filler metal is smaller than that of the crystalline filler metal under the same brazing temperature and dwell time. Key words: amorphous filler metal; brazing; residual layer; wetting; spreading

In recent years, the outstanding characteristics of amorphous filler metal (FM) have led to their wide applications as interlayers for brazing[1]. These foils are manufactured by rapid solidification procedure and possess a large number of advantages such as extremely high chemical and phase homogeneity, higher diffusion and capillary activity than crystalline FM, and narrow melting and solidification ranges[2]. However, these advantages will disappear with FM melting when brazing. There are only a few articles to report on the reason why joints brazed with amorphous FM possess excellent performance. DeCristofaro N.J. first reported amorphous filler metal in 1978[3]. He indicated that the high joint strength and good wettability are mainly due to the amorphous FM free-flux, which reduces the oxides contamination. In addition, the most important advantage of amorphous FMs is their flexibility and ductility. A ductile amorphous FM could be made into various shapes to ensure the quality of brazing[4]. The amorphous FM flows more freely upon melting than any powder form[5,6]. A smaller clearance also promotes improved

retention of base metal properties because of curtailed base metal erosion by the use of a smaller volume of FM in the amorphous form[7]. The amorphous FM may be used as a preplaced perform, and there is no need for large brazing gaps, as those used with pastes, to achieve a complete filling of the braze cross section[8,9]. The main aim of this paper is to demonstrate what happens when amorphous FM melt and how joints are formed. Research on the formation process of brazing joints may help understanding the influence of all factors upon the joint performances and revealing the essential reasons why the performance of joint brazed with amorphous FM is superior to that with the crystalline FM.

1

Experiment

In this experiment, the crystalline as-rod and amorphous ribbon FM have the same composition (Cu66Sn9Ni16P7). The melting temperatures for crystalline and amorphous FM are 598 and 597 °C, respectively. The as-rod brazing FM with

Received date: July 19, 2012 Foundation item: The National Natural Science Foundation of China (50965012); The Doctoral Foundation of the Lanzhou University of Technology (BS01200902) Corresponding author: Yu Weiyuan, Ph. D., Associate Professor, State Key Laboratory for Non-ferrous Metal New Materials of Gansu Province, Lanzhou University of Technology, Lanzhou 730050, P. R. China, Tel: 0086-931-2976302, E-mail: [email protected] Copyright © 2013, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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Yu Weiyuan et al. / Rare Metal Materials and Engineering, 2013, 42(4): 0688-0691

the diameter of 20 mm was cut into 0.1 mm chip for use. The base metal for brazing was pure copper. Firstly, surface treatment for samples was performed by grinding. Afterwards, the samples were dipped into acetone for ultrasonic cleaning for 10 min. Then they were blown to dry rapidly. The brazing temperatures were 600 and 630 °C, respectively with dwelling time 10 min. The degree of vacuum was 1.0×10-2 Pa in brazing. After brazing, Ar gas with high-purity was rapidly blown into the furnace to cooling down to room temperature. A metallographic analysis was performed after brazing. The EPMA-1600 electronic probe and the D8 ADVANCE X-ray diffractometer were used for analysis of the joint structure.

2

Temperature/℃

50 MPa (7250 psi)

L+γ-Cu (L+A(B)) b

800

600

400

L

γ-Cu (A(B)) c

d

a γ-Cu+(Cu,Ni)3P (A(B)+A3B)

10

20

0 Cu Cu8(O) Cu5(P) C(F)Cubr(O) Cubr(P) C at%(Sn+Ni+P) (A) (B)

Fig.2

Results and Discussion

°F 1832

1472 1112

752 CA3B

Schematic diagram of the pseudo-binary phase. C0 is the original percentage of the FM, Cbr is the percentage of the

2.1 Melting and liquid formation of FM Fig.1 shows the micrographs of the brazing joints with crystalline FM. In Fig.1, the structure of residual FM consist of the black blocks (Cu, Ni)3P phase and the γ-Cu (Sn+P+Ni)+(Cu, Ni)3P eutectic. The residual FM and the base metal form a white diffusion-solution layer across the interface. The thickness of crystalline residual FM at 630 °C is obviously smaller than that at 600 °C and the eutectic structure in residual FM is also smaller than that at 600 °C and mainly concentrates on the top of the residual FM. Reduction of the residual FM is mainly caused by generation and flow of the liquid FM. Fig.1c reveals the microstructure of the spreading foreland of liquid FM after rapid cooling down. From Fig.1c, it can be see that nearly all the brazing FM are eutectic structure. Fig.2 is the Cu1-X(Sn+P+Ni)X pseudo-binary phase diagram[10]. As seen from Fig.2, the formation and spreading process of liquid FM is easily observed. As the FM used in this paper is eutectic structure, the base metal and FM are regarded as a binary eutectic system. The base metal Cu is referred to component A, and all the alloy elements inside the a

b

Ambient pressure

1000

c

50 μm Fig.1 Microstructures of crystalline filler metal: (a) middle of filler metal (brazing at 600 °C), (b) middle of filler metal (brazing at 630 °C), and (c) foreland of filler metal (brazing at 630 °C)

liquid FM after melting, and CS is the maximum solid solubility of component B (Sn+P+Ni) in the solid metal A (Cu)

FM (Sn, Ni and P) make up another component B with the content of X= XNi+XSn+XP. When the brazing temperature exceeds the eutectic temperature, the eutectic structure of FM begins to melt and liquid phase is generated. At the same time, convergence of liquid phase occurs. With the continuous increasing of temperature, more liquid phase is generated and migrated towards the surfaces of the FM. The liquid phase starts to flow and spreads towards the base metal surface under the surface tension, rapidly reducing the thickness of the FM. If cooling down immediately, a large amount of eutectic structure will appear in spreading layer on the surface of base metal. Schematic of FM melting and forming liquid is shown in Fig.3. The experimental results of the melting and migratory process can be explained by the liquid phase channel effect[11]. When the temperature exceeds the solidus temperature of the FM, the eutectic structure begins to melt and the grain boundary melts into the liquid eutectic. If much eutectic liquid phase is generated inside the FM, dissolution across the grain boundary will be faster and a network of liquid phase inside the brazing seam is formed. Obviously, the distribution of liquid phase may speed up the material exchange and continuous flow between liquid phases. These generated liquid phases will migrate towards the surface of the FM under the surface tension. As for butt joints, only a part of the liquid phase is inside the brazing seam under the capillary action. This part of the liquid phase may interact with base metal, namely the dissolution and diffusion. In the subsequent cooling process, the liquid phase solidifies inside the brazing seams. 2.2 Dissolution of base metals and flowing-spreading of liquid phase FM When the brazing temperature (Tbr) rises, all the FM will melt and the liquid FM begin to spread and flow. Spreading and flow of liquid FM include two processes. One is corrosion of the base metal, and the other is the flow of the liquid FM and rapid reduction of the brazing seams. When liquid FM is

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Yu Weiyuan et al. / Rare Metal Materials and Engineering, 2013, 42(4): 0688-0691

a

b

c

d

e

f

Fig.3 Schematic of FM melting and forming liquid: (a) original condition, (b) part of GB melt, (c) GB melt completely, (d) GB thicken and inner particles melt, (e) liquid moving to surface, and (f) liquid layer becoming thicker

generated, the solid base metal reacts and continues to melt into the liquid FM until reaching the maximum saturated concentration and the dissolution stops. The reaction equals to the following: LC0+A→LCbr (1) When the base metal dissolves into FM, the ratio between the solid solution (XA(B)) and the eutectic phase (Xeut) can be obtained by the Leverage Law, as seen: X A(B) Cbr − C0 (2) = X eut Cs − Cbr The calculated value of the ratio for the brazing joint Cu/FM (Cu-Ni-Sn-P)/Cu is 0.45. While this ratio obtained from the metallographic photo is about 0.5, which basically coincides with the actual measurement values. The erosion process of base metal, flow of the FM and reduction of the brazing seams can be evaluated by a feature time parameter (τ1). We make the following assumptions in order to calculate the solution time (τ1): 1) The layer of liquid FM of the solid-liquid phase close to the base metal surface stays static (the diffusion boundary layer is static) and the thickness is δ (Fig.4). 2) Diffusion at the boundary layer is stable, namely, it varies with the distance instead of the time. 3) Element diffusion is full in the liquid FM outside the diffusion boundary layer. It is free liquid, namely, concentration in all places residuals is the same. Suppose the density of the liquid phase FM is ρ, its volume is V and the solid-liquid phase effect area is S, and suppose the initial concentration of the base metal element inside the liquid FM is C0 and the solubility limit is Cs. After a period of time (t) of dissolution, the concentration of the base metal element in the brazing FM is Cbr. We can obtain the dissolved amount by Eq.3, and the speed of dissolution can be obtained from Eq.4 at a constant temperature: (3) Q = ρV (Cbr − C 0 ) Diffusion bound

⎛ C (T ) − C 0 ⎞ S ln ⎜ s t ⎟ = K (T ) ρV ⎝ Cs (T ) − C br ⎠

(4)

Here, V is the volume of the liquid FM, S is the contact area of solid with liquid, and K is the constant, dissolution rate of the base metal element in the brazing FM (cm/s). The reaction with diffusion as the control procedures are concerned in Eq.5: D K= (5)

δ

where D is the diffusion coefficient and δ is the thickness of the effective melted boundary layer. The diffusion coefficient of Cu in the liquid metal has been reported [12], and the extrapolation method is used to obtain the self diffusion coefficient of Cu in liquid metal. When the brazing temperature reaches 875 K, the diffusion coefficient of Cu in liquid metal of Cu-Ni-Sn-P is 1×10-4 cm2/s. It is obvious that δ is smaller than h/2 (h is the brazing gap), so we select δ=h/2. Table 1 is data for calculation. According Table 1, the dissolution time is calculated as τ1=0.12 s. As regarding the crystal brazing FM, the FM’s thickness is 0.2 mm in the present research. The dissolution time τ1=1 s. Since the amorphous FM is very thin, the atomic diffusion distance is very short in the FM; therefore it is quite favorable for the rapid dissolution of the base metal. However, the ordinary FM is relatively thick. It tends to take longer time to dissolve than the amorphous FM. As for either amorphous FMs or ordinary FMs, the base metal dissolution time is much shorter than the brazing dwell time and even shorter in actual brazing process. Therefore, corrosion actually occurs within a very short period of time and saturation occurs shortly after corrosion. Compared with crystalline FM, the amorphous FM may generate relatively less liquid phase when brazing. In order to study the relations of the liquid phase generated during the brazing process, 9 brazing temperatures were selected, including 580, 590, 600, 610, 620, 630, 640, 650 and 660 °C with the dwell time of 10 min. After brazing, the thickness of the brazing seam diffusion layer and brazing seam residual layer at different temperatures are measured and shown in Fig.5. As the brazing seam residual layer is formed by liquid FM after solidification, changes of the residual layer thickness may reflect the changing of the amount of liquid FM. Fig.5 shows the relation of the residual layer thickness and brazing temperature of the FM. The whole curve consists of three stages: in the first stage (580~590 °C), the thickness of the residual layer changes little. It is the solid diffusion stage before melting of the FM. In the second stage (590~630 °C), the residual layer changes significantly, because of the liquid phase flow and spreading. The third stage is 630~660 °C

Liquid

Solid

Table 1 Data for calculation

δ

Cbr0 /at%

Fig.4 Interface schematic between liquid FM and solid matrix

82

C0/at% 77.5

D/cm2·s-1 1×10-4

δ/μm 20

690

Residual Layer Thickness, h/μm

Yu Weiyuan et al. / Rare Metal Materials and Engineering, 2013, 42(4): 0688-0691

160

diffusion distance is very short into the filler metal. It is quite favorable for the rapid dissolution into the base metal. 2) Less liquid phase is generated by amorphous filler metal than crystalline filler metal. Therefore, the spreading and flowing process of the liquid phase are weak. 3) The residual layer thickness of the amorphous filler metal is smaller than that of the crystalline filler metal under the same brazing temperature and dwell time.

Amorphous FM Crystalline FM

140 120 100 80 60 40 20 0

References 580

600

620

640

660

Brazing Temperature/°C Fig.5

Relation between residual layer thickness, diffusion layer thickness and temperature

1 Rabinkin A. Proceedings of the 3rd International Brazing and Soldering Conference[C]. San Antonio, Texas: ASM International, 2006: 148 2 De Cristofaro N, Datta A. Rapidly Quenched Metals[J], 1985, 1: 1715

after the melting of FM, when the residual layer changes relatively small. Comparison of the changes of the amorphous FM with the crystalline FM within the second stage shows that amorphous FM residual layer changes little while crystalline FM changes remarkably in this stage. It proves that when the liquid phase generates obvious spreading and flowing, the process of the crystalline FM is fully completed. It is the major process for brazing of the crystalline FM. Due to the full solid-phase diffusion before melting, the eutectic structure of the amorphous FM is damaged and generates plenty of high melting-point copper solid solution and copper-nickel compound so that relatively less liquid phase is generated by amorphous FM than crystalline FM. Therefore, the spreading and flowing process of the liquid phase appear weak.

3 Conclusions

3 De Cristofaro N, Henschel C. Welding Journal[J], 1978, 57: 33 4 Rabinkin A. Issue of Advanced Materials & Processes[J], 2001, 7: 23 5 Shapiro A. Welding Journal[J], 2003, 82: 36 6 Datta A, Rabinkin A, Bose D. Welding Journal[J], 1984, 63: 14 7 Nakahashi M, Shirokane M. Journal of the Japan Institute of Metals[J], 1990, 54: 826 8 Miyazawa Y. Journal of the Japan Welding Society[J], 1992, 10: 50 9 Naka Masaaki, Okamoto Ikuo. Transactions of JWRI[J], 1985, 14: 185 10 Jyrki M. Calphad[J], 2001, 25: 67 11 Gao Feng. Research on Formation Mechanism of Aluminum-Heat Exchange Composite Brazing Filler Metal Board Joints [D]. Harbin: Harbin Institute of Technology, 2003 12 Yamamura T, Ejima T. Japan Institute of Metals[J], 1973, 37: 901

1) As the amorphous filler metal is very thin, the atomic

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