Continuous Strip Casting, Microstructure and Mechanical Properties of Ag-32 percent Cu Brazing Alloy

摘要

Ag-base brazing alloys, i.e. BAg-series according to AWS specification, are the major group of brazing materials for general use. The present study aimed to examine the suitability of the continuous strip casting process as a new fabrication method for the Ag-32 percent Cu brazing alloy. The effect of aging heat treatment on the microstructure and mechanical properties of an Ag-32 percent Cu strip was also investigated through tensile test, hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations, and electron probe microanalysis (EPMA). Three continuous strip casting processes such as planar flow casting, melt dragging, and twin roll strip casting were applied to the manufacture of the Ag-32 percent Cu strip. Among those processes, twin roll strip casting was found to represent the best strip shaping ability and microstructural homogeneity. 1 mm-thick Ag-32 percent Cu strip was successfully produced as a continuous strip of 1 charge of melts. The heat-treated strip exhibited the eutectic microstructure feature of an Ag-rich matrix and a Cu-rich phase, with the microstructure clearly evolving with longer aging time. Tensile results showed gradually decreasing strengths and simultaneously increasing elongation with longer aging time. The aging condition of 670 deg C/2hrs also realized appropriate T.S. strength of 283 MPa and significantly enhanced plasticity of 41 percent, due mainly to the homogeneous distribution of fine Cu-rich phases less than 0.8 mu m. Finally, twin-rolled strips could be cold-rolled smoothly after aging heat treatment for 2hrs at 670 deg C to a very thin strip (50 approx 100 mu m thickness).