Effects of applied pressure on the brazing of superplastic INCONEL 718 superalloy
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I.
INTRODUCTION
THE nickel-base superalloy possesses excellent properties at elevated temperatures and has been widely used in automobile, aerospace, and nuclear industries. However, complex components of nickel-base superalloy can only be obtained with difficulty using conventional mechanical processing. The high-temperature brazing method is an important joining process for manufacturing complex components. There are many filler metals, especially nickel-base filler metals, which have been developed for the brazing of nickel-base superalloys.[1,2] In order to lower the liquidus of nickel-base filler metals, melting temperature depressants such as phosphorus, silicon, boron, and palladium were added. However, intermetallic phases, which are extremely hard and brittle, were formed along the centerline of the joint and decreased the strength of the brazements. Longer brazing time, higher brazing temperature, or narrower joint clearance has been suggested in order to eliminate brittle intermetallic phases. Applying a moderate compressive load to the brazing method has been reported to compare favorably with the conventional brazing method. Rabinkin and Pounds[3] found that for brazing copper with copper-phosphorus filler metal under an applied pressure of 9.8 MPa, its Charpy impact energy was increased and the microstructure of the joint was changed. An ejection model had been proposed to explain the increase in brazement strength. According to this model, the liquid phase enriched with melting temperature depressants was ejected out of the brazing joint, which resulted in the disappearance of brittle intermetallic phases and an increase in ductile phases. A similar beneficial effect from applied pressure in the brazing of a superalloy was also found by Dammer.[4] In this study, the effects of applied pressure on the brazM.S. YEH, Associate Professor, is with the Department of Mechanical Engineering, Chung-Hua Polytechnic Institute, Taiwan, 300, Republic of China. T.H. CHUANG, Professor, is with the Institute of Materials Science and Engineering, National Taiwan University, Taiwan, 106, Republic of China. Manuscript submitted January 2, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
ing of a superplastic INCONEL* 718 superalloy with two *INCONEL is a trademark of INCO Alloys International, Inc., Huntington, WV.
nickel-base filler metals were investigated. For this purpose, the bonding strength, microhardness, and corrosion behavior of the brazements were evaluated after brazing under various pressures. II.
EXPERIMENTAL PROCEDURE
The chemical composition of the superplastic INCONEL 718 superalloy (INCONEL 718SPF), manufactured by INCO Alloys International, Inc., which was used in this study, is given in Table I. This alloy possesses a grain size of about 10 mm and reaches a total strain of about 194.0 pct with the initial strain rate of 1.3 3 1023 s21 at a temperature of 954 7C.[5] Alloy strips of 34 3 15 mm with a thickness of 1.3 mm were cut from the as-received sheet material. The surfaces to be fayed were
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