Development of a low-melting-point filler metal for brazing aluminum alloys
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I. INTRODUCTION
OWING to their high specific strength, low cost, and superior corrosion resistance, aluminum alloys have been widely used in aerospace, automobile, and construction industries. Also, by virtue of good thermal conductivity, they are often employed in heat exchanger manufacturing. In industrial application, the bonding problem with these aluminum alloys has always been a serious consideration. Among a variety of techniques, brazing has been adopted as a reliable method for the bonding of aluminum components,[1] wherein a eutectic Al-12Si (wt pct) alloy is recognized as the most popular filler metal.[2,3] Although a sound joint can be obtained with this traditional filler metal, its brazing temperature is high relative to the melting point of pure aluminum due to the high eutectic point of the Al-12Si alloy at about 577 8C, which can bring about degradation of mechanical properties or even localized melting in some engineering aluminum alloys when brazed. Therefore, the development of a low-melting-point filler metal with a satisfactory bonding strength is an important task for the aluminum industry. For this purpose, a series of efforts have been made in the past few years. Humpston et al.[4] and Jacobson et al.[5] had developed an Al-20Cu-2Ni-5Si filler metal with a melting temperature range between 518 8C and 538 8C. For the brazing of a 3003 Al alloy with this filler metal, a shear strength over 75 MPa was obtained. Suzuki et al.[6] reported on a eutectic Al-4.2Si-40Zn filler metal with a melting point T.H. CHUANG, Professor, L.C. TSAO, Doctoral Candidate, and T.C. TSAI, Postdoctor, are with the Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, Republic of China. M.S. YEH, Associate Professor, is with the Department of Mechanical Engineering, Chung-Hua University, Hsin-Chu 300, Taiwan, Republic of China. C.S. WU, Engineer, is with the Metal Industries Research and Development Center, Kaohsiung 811, Taiwan, Republic of China. Manuscript submitted November 18, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
of 535 8C. The disadvantage of that filler metal lies in the fact that the vapor pressure of zinc is very high and thus unsuitable for the vacuum brazing process. Kayamoto et al.[7] found a low-melting-point Al-Ge-Si-Mg filler metal. When that filler metal was applied to braze a 6061 aluminum alloy at 575 8C for 60 minutes, the brazed joints provided sufficient joint strength equal to that of the base alloy. However, germanium is approximately 400 times the price of aluminum, making the alloy excessively expensive for most applications. In this present study, an Al-7Si-20Cu-2Sn-1Mg filler metal with a low melting temperature range of 501 8C to 522 8C has been developed. When applied to braze the 6061-T6 aluminum alloy, this filler metal displayed excellent wettability and a high bonding strength of 196 6 19 MPa. II. EXPERIMENTAL In order to develop a low-melting-point and high strength filler metal for the brazing of aluminum alloys, a series of Al-
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