A novel approach for predicting the tensile strength of brazed joints
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I.
INTRODUCTION
A brazed joint contains a thin, soft interlayer metal between harder base metals. Deformation and failure of the joint are determined by the geometry of the interlayer metal and the mechanical properties of the interlayer and base metals. Numerous investigators have confirmed that brazed joints have tensile strength that exceeds the bulk strength of their interlayer metals, especially when the interlayer metal has a particularly advantageous thickness.[1–4] Several investigators have theorized that the high tensile strength of brazed joints is due to the mechanical constraint provided by the harder base metals, which restricts transverse contraction of the interlayer metal.[5,6] Despite the critical importance of predicting the tensile strength in brazed joints for the design of brazed joints, few studies have specifically addressed this issue. The purpose of the present study is to propose a method for predicting the tensile strength of brazed joints. The method utilizes a finite element method (FEM) analysis, which includes a failure criterion also derived in this study. The predicted tensile strength is compared with experimental results for titanium joints brazed with aluminum. II.
EXPERIMENTAL PROCEDURES
Commercially pure titanium, 1-mm thick, was butt-joint brazed to aluminum foil of 99.5 wt pct purity at a temperature of 953 K under a vacuum level of 1022 Pa, as shown in Figure 1(a). Before brazing, the titanium base metal was immersed in nitric acid–fluoric acid aqueous solution to remove surface oxides and then rinsed in distilled water. The aluminum foil was degreased in an ultrasonically agitated bath of acetone and then rinsed in distilled water. To vary the microstructure of the joint at each brazing, the aluminum foil thickness was varied from 5 to 100 mm and the holding time was varied from 1 to 3600 seconds. After brazing, two projections, indicated by the shaded regions in Figure 1(b), were cut from the brazed joint and utilized
KUNIMASA TAKESHITA, Associate Professor, is with the Department of Mechanical Engineering, Faculty of Engineering, Fukui University, Fukui 910, Japan. YUHJI TERAKURA, formerly Graduate Student, Department of Mechanical Engineering, Fukui University, is now Assistant Researcher with Die Engineering Research Laboratory, with Hitachi Metals Company, 5200 Mikajiri, Kumagaya, Saitama 360, Japan. Manuscript submitted December 17, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
for microstructural analysis of the joint. The remainder of the brazed joint, the shape and size of which is shown in Figure 1(c), was used as a tensile test specimen. Microstructural examination of the brazed joint was performed using scanning electron microscopy (SEM) and collimated X-ray diffractometry (Cu Ka, collimator diameter: 100 mm) techniques. Tension tests were conducted at room temperature using an Instron-type test machine at a crosshead speed of 1023 mm/s. The separated surfaces of the fractured tensile test specimens were observed using the SEM technique. In addition, c
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