Failure mechanisms in cobalt welded with a silver–copper filler

Abstract

Cobalt silver–copper (Co–AgCu) weldments approximate the stresses and failure mechanisms of beryllium aluminum–silicon (Be–AlSi) welds, which have strategic importance but are hazardous to study. Failure tests of these surrogate Co–AgCu welds, examined in tension and four-point bending, show that residual stresses and post-welding heat treatment have little or no effect on strength, whereas weld quality and geometry are extremely important. Scanning electron microscopy images reveal abundant defects in poor welds, which usually fail through propagation of preexisting cracks. Fracture surfaces show a variety of morphologies, ranging from dimples in the AgCu filler, to cleavage steps in the CoCu peritectic, and suspected intergranular fracture in the cobalt base. Spatially resolved acoustic spectroscopy reveals significant changes in microstructure near the base–filler interface, whereas wavelength dispersive analysis shows high Cu concentrations in this area.. Contrary to finite element predictions, these welds were found to be stronger during face bending than root bending, likely resulting from the increased number of cracks and imperfections in the Co base. These computations correctly predict that weld strength depends on geometry and that welds fail either in the cobalt base, or along the base–filler interface. Crack compliance measurements show that the largest residual stresses are located along this interface. However, these stresses are unlikely to influence failure due to their direction, whereas stresses in the weld root are too small to have observable effects on failure. The strength of Co–AgCu welds depends strongly on geometry, penetration, and weld quality, but little on residual stresses, and this conclusion is tentatively extended to Be–AlSi welds.