UC San Diego

A new method of joining Co by braze-welding it with an AgCu filler was developed in order to better understand the residual stresses in Be-AlSi weldments. The constituents of this new welding system were selected to replicate the physical properties, crystal structures, and chemical behaviors of the Be-AlSi welds; "welding surrogacy" is used to describe this process. Final welds are five pass manual tungsten inert gas (TIG), with He top -gas and Ar back-gas. Final welds exhibit full penetration melting of the cobalt base, while microscopy indicates that cracking is minimal and not through thickness. Welds are composed of five separate regions consisting of the unaffected Co base, the heat affected zone, the melted Co base, the AgCu filler, and the CoCu peritectic. Failure tests of the surrogate welds show that residual stresses have little or no effect on strength, whereas weld quality and geometry are extremely important. Crack compliance measurements show that the largest residual stresses are located along the Co-AgCu interface, as is expected. Residual stresses in the weld root are too small to observably effect failure, whereas stresses in the CoCu region cannot influence failure due to their location and direction. The strength of Co-AgCu welds depends strongly on geometry, penetration, and defects, but little on residual stresses, and this conclusion is tentatively extended to Be-AlSi welds