CAPACITOR DISCHARGE RESISTANCE SPOT-WELDING OF SIC FIBER-REINFORCED TI-6AL-4V

Solid-state and fusion welds have been produced between sheets of mono lithic and SiC fiber-reinforced Ti-6Al-4V using capacitor discharge re sistance spot welding. Solid-state welds in monolithic sheet were char acterized by beta grain growth across the weld interface, an alpha-pri me martensite microstructure and the presence of occasional, fine inte rface discontinuities. Despite experiencing tensile-shear fracture alo ng or directly adjacent to the weld interface, average tensile shear s trengths for optimized solid-state welds were comparable to those of c onventional fusion spot welds produced at higher energy inputs which f ailed by nugget pullout. High integrity, solid-state welds were also p roduced in Ti-6Al-4V sheet containing 35 vol-% continuous SiC (SCS-6) fibers. Under optimized conditions, defect-free solid-state welds were produced which exhibited negligible evidence of fiber displacement or degradation. The weld zone was characterized by limited beta grain gr owth across the interface and a fine, martensitic microstructure. The average tensile shear strength for these welds was approximately 60% o f that exhibited by optimized solid-state welds produced in the monoli thic material. This strength reduction was attributed primarily to the initiation of tensile shear fracture in the vicinity of the weld oute r periphery notch and propagation either along the fiber/matrix interf ace in the heat-affected zone (HAZ) directly adjacent and parallel to the weld interface or transverse through the sheet, in both cases remo te from the weld interface. An increase in weld energy input promoted localized melting at the fiber/matrix interfaces. At a sufficiently hi gh energy input, melting extended across the entire weld interface the reby creating a fusion weld. Such melting promoted considerable fiber dissolution and displacement. Solid-state welds produced between the m onolithic and fiber-reinforced sheets exhibited an average tensile she ar strength equal to 70% of that exhibited by optimized solid-state we lds in the monolithic alloy and failed remote from the weld interface along an adjacent and parallel layer of fibers in the weld heat-affect ed zone.