18-8 stainless steel

Materials Research Bulletin 38 (2003) 1493–1499

Vacuum brazing technology and microstructure near the interface of Al/18-8 stainless steel Liu Peng*, Li Yajiang, Wang Juan, Guo Jishi Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education School of Materials Science and Engineering, Shandong University, Jinan 250061, PR China Received 25 October 2002; received in revised form 5 May 2003; accepted 30 June 2003

Abstract The microstructure performance for vacuum brazing interface of Al/18-8 stainless steel was studied by means of SEM, micro-hardness test and X-ray diffraction (XRD). The test results indicate that the adhesion brazing interface is excellent, there is no porosity or crack near the interface. The microstructure of the brazing joint is mainly composed of equiaxial and column crystal. The fracture morphology of the brazing joint consists mainly of a coarse and gray fiber-shape fracture, and the fracture is mainly the mixed fracture of toughness and cleavage. The brazing interface consists of d (Al, Fe, Si) and a-Al (Si), without obvious brittleness phase of the high hardness. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Interface; C. X-ray diffraction; D. Fracture; D. Microstructure

1. Introduction In recent years, the thermal strength and the corrosion resistance of stainless steel and the low density and high heat conductivity of aluminum, the clad structure of aluminum and stainless steel (Al/18-8), has been investigated by many researchers [1–3]. The demand for joints of aluminum and stainless steel has greatly increased in a wide range of industrial applications from automobile, rail, aviation industries to smaller and more commonly used products, such as saucepans [4,5]. Due to the difference of the melting point, coefficient of heat conductivity and specific heat volume, the clad structure can produce thermal stress. The refractory oxide film results in inclusions of the brazing metal. So Al and 18-8 stainless steel are difficult to be welded together. The unfavorable effect of physical state of the brazing interface can be avoided by means of vacuum brazing technology. And the reliable brazing metal can be obtained. In this paper, Al/18-8 dissimilar materials are successfully welded together by means of advanced vacuum brazing technology. The microstructure and hardness distribution of the brazing interface are *

Corresponding author. Tel.: þ86-531-8393538; fax: þ86-531-2609496. E-mail address: [email protected] (L. Peng). 0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0025-5408(03)00176-4

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analyzed by scanning electron microscope (SEM) and micro-hardness test. The phase constitution is analyzed by X-ray diffraction (XRD). The microstructure performance of the brazing joint is further analyzed. The brazing technology parameters are emphasized, which is important for enlarging the application of the clad structure of Al/18-8 stainless steel.

2. Experimental The materials in the test are aluminum (1060) and 18-8 stainless steel (1Cr18Ni9Ti). The brazing filler alloy is Al–Si alloy. Chemical composition and thermo-physical performance of the test materials are shown in Tables 1 and 2. The brazing technology adopted step heating in the test. The technology parameters during the vacuum brazing are shown in Fig. 1 (technology parameters: the heating temperature T ¼ 630 8C, and the holding time t ¼ 10 min). The oxide film of Al and 18-8 stainless steel was cleaned by HF water solution. Parameters in the vacuum brazing equipment were: the heating power 45 kVA, the vacuum degree 5  105 Pa. A series of specimens was cut from the location of the brazing joint of Al/18-8 stainless steel. These specimens were prepared into a series of metallographic samples. These samples were observed and analyzed by means of microscope and SEM.

3. Results and analysis 3.1. Microstructure and micro-hardness in the brazing interface Microstructure of Al/18-8 vacuum brazing joint observed by SEM is shown in Fig. 2. The obvious microstructures of columnar crystal can be observed (Fig. 2a). The columnar crystals grow and stretch Table 1 Chemical composition of the base metal in the test Materials

Al 1060 1Cr18Ni9Ti Al–Si alloy

Chemical composition (wt.%) Al

Cu

Fe

Si

Mn

Mg

Zn

C

Cr

Ni

Ti

99.6 – Other

0.05 – 3.4

0.35 Other –

0.25 1.0 9.15

0.03 2.0 –

0.05 – –

– – –

– 0.12 –

– 17.019.0 –

– 8.011.0 –

– 0.50.8 –

Table 2 Thermo-physical performance of the base metal in the test Materials

Melting point (8C)

Density (g cm3)

Average specific heat volume (J kg1 K1)

Melting potential heat (kJ mol1)

Heat conductivity (W m1 K1)

Al 1060 1Cr18Ni9Ti Al–Si alloy

657 1539 –

2.07 8.03 –

917 451.4 –

10.47 – –

238 71.17 –

Solidus line

Liquidus line

577

582

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Fig. 1. Technology parameters for vacuum brazing of Al/18-8 stainless steel.

from the side of Al into the brazing metal. The morphology of the Al matrix close to the brazing metal is shown in Fig. 2b. The position of the microstructure is parallel in relation to the interface. There are new phases in the surface of the Al matrix. Several dendrites in the brazing interface can be observed (Fig. 2c). The composition measured by the energy dispersive X-ray (EDX) analysis indicates that the bulk and white crystals are primarily Si. The fine and white strip crystals are the eutectic Si, and the black matrix is a-Al (Si) phase. At 577 8C, the largest solubility of the Si in a solid solution is 1.65% [6]. In the centre of the brazing metal, the state of the brazing filler metal is an Al–Si eutectic structure. The a solid solution can be obtained in the brazing interface. During the vacuum brazing a solid solution separates first from the brazing interface, and then separates from the primary crystal Si and the eutectic Si. The a solid solution is favorable for the microstructure performance of the brazing joint.

Fig. 2. Microstructure for the brazing interface of Al/18-8 stainless steel: (a) column morphology of interface; (b) Al matrix close to the brazing metal; (c) crystal microstructure of interface.

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Fig. 3. Microhardness for the brazing interface of Al/18-8 stainless steel.

The micro-hardness of Al/18-8 brazing interface was measured by means of the Shimadzu type microsclerometer, using 15 g loading and a load time of 10 s. The test result of microstructure is shown in Fig. 3. The micro-hardness first increases from the Al side to the 18-8 stainless steel side, and then decreases. The micro-hardness close to the brazing metal is higher than the one away from the brazing metal. When the distance of the brazing metal center close to the 18-8 stainless steel side is about 0.4 mm, the micro-hardness decreases gradually. When the distance of the brazing metal center close to the Al side is about 0.25 mm, the micro-hardness also decreases gradually. There is no obvious brittleness phase of the high hardness. With the composition change of Al and 18-8 stainless steel matrix close to the brazing metal, the micro-hardness of Al/18-8 brazing interface is also changed. At 630 8C (holding time t ¼ 10 min), the Al and Fe diffuse to a small extent. The diffusion of the Al is larger due to the melting point (660 8C) of Al near to the brazing temperature (630 8C). Therefore, the composition of Al matrix close to the brazing metal will be changed. 3.2. Fracture morphology of the brazing joint The obvious cleavage fracture zone can be observed (Fig. 4a) by means of the JXA-840 scanning electron microscope (SEM). The fracture morphology of the brazing metal mainly consists of coarse and gray fiber-shape fracture. The inter-atomic bond between the brazing filler and the matrix is destroyed and produces the trans-granular fracture. The cleavage and second-fracture will be also produced together, but the second-fracture is quite small. The mixed fracture of the toughness and the cleavage can be observed in Fig. 4b, where the proportion of the toughness fracture is quite large. The fracture morphology consists of dimple by means of SEM (Fig. 4c). There are impurity and the secondphase particles in the center of the dimple. The dimple is produced due to the difference of ductility and toughness of the second-phase particle and the matrix. With the increase of ductility deformation and the dimple, hence the gather and joining of the micro-hole produce the fracture.

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Fig. 4. Microstructural morphology for the brazing interface of Al/18-8 stainless steel (SEM): (a) cleavage fracture morphology; (b) local plasticity and cleavage zone; (c) local plasticity zone.

There are different mechanical bond zones and freedom surfaces in the fracture. The dimension of the mechanical bond zone is small. The metal on the two sides is joined closely, and forms the metallurgical bond. The dominant feature of the brazing interface fracture is the mechanical bond and the freedom surface. The test results indicated that the fracture morphology was mainly the mixed fracture of the toughness and a little cleavage. It is favorable for the performance of the brazing joint. 3.3. X-ray diffraction analysis in the brazing interface The sample of Al/18-8 stainless steel was analyzed by means of X-ray diffraction. The X-ray diffraction results of phase constituents for the brazing interface are shown in Fig. 5. The results are comparable with data from the Joint Committee on Powder Diffraction Standards (JCPDS) shown in Table 3. The test results indicated that the brazing metal consists of d (Al, Fe, Si), a-Al (Si) and Al. The existence of d (Al, Fe, Si) increase the hardness of the brazing metal. The chemical composition of

Fig. 5. X-ray diffraction diagram for the brazing interface close to aluminum side.

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Table 3 X-ray diffraction analysis results for the brazing interface of Al/18-8 stainless steel Testing value, d (mm)

Data from JCPDS d (Al, Fe, Si)

0.3149 0.3056 0.2473 0.2347 0.2032 0.1742 0.1484 0.1433 0.1296 0.1221 0.1171

a-Al (Si)

Al

d (mm)

hkl

I/I0

d (mm)

hkl

I/I0

d (mm)

hkl

I/I0

0.312 0.304 – 0.236 0.207 – – – – 0.1228 –

003 200 – 112 114 – – – – 207 –

5 5 – 20 100 – – – – 5 –

– – – 0.234 0.2024 – – 0.1431 – 0.1221 0.1169

– – – 111 200 – – 220 – 311 222

– – – 100 47 – – 22 – 24 7

– – 0.2465 – – 0.1749 0.1486 – 0.1298 – –

– – 220 – – 400 332 – 520 – –

– – 50 – – 50 20 – 20 – –

d (Al, Fe, Si) is (wt.%): Fe 15, Si 20 and Al 65. So the high aluminum content provides ductility and toughness for the brazing joint. The eutectic microstructure of a-Al (Si) increases the strength and hardness of the brazing joint, and improves the corrosion resistance.

4. Conclusions Al/18-8 dissimilar materials are successfully welded together by means of the advanced vacuum brazing technology (the heating temperature T ¼ 630 8C, holding time t ¼ 10 min). The adhesion of the brazing interface is excellent. There is no porosity or crack near the brazing interface. The microstructure of the brazing joints is mainly composed of equiaxial crystals and columnar crystals. According the micro-hardness test, the micro-hardness first increases from the Al side to the 18-8 stainless steel side, and then decreases. The micro-hardness of the brazing metal is higher than the Al matrix. SEM shows that the fracture morphology of Al/18-8 brazing joint consists mainly of a coarse and gray fiber-shape fracture. The fracture morphology is the mixed fracture of ductility and cleavage. X-ray diffraction test indicates that the brazing metal zone consists of d (Al, Fe, Si), a-Al (Si) and Al. The existence of d (Al, Fe, Si) increases the hardness of the brazing metal. The eutectic structures of aAl (Si) provide ductility and toughness for the brazing joint.

Acknowledgements This work was supported by The Foundation of the National Key Laboratory of Advanced Welding Production Technology, Harbin Institute of Technology and the National Natural Foundation of Science (No. 50071028), People’s Republic of China.

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