Effects of superplastic deformation on the diffusion welding of SuperDux 65 stainless steel

The SuperDux 65(1) stainless steel diffusion welded in a nonsuperplastic st ate (880 degrees C, 60 min) required greater pressure, compared to aluminum and titanium alloys, to create a contact area at the weld interface, which will increase the atomic diffusion paths. However, an unsatisfactory weld strength of 45.3 MPa was obtained under the applied pressure of 7 MPa. This alloy deformed easily at its superplastic temperature of 970 degrees C, re sulting in a tight contact surface. The higher welding temperature was also beneficial for atomic diffusion. Both effects were advantageous for diffus ion welding, while the joined workpieces macroscopically deformed markedly. In this study, a two-stage diffusion welding method was proposed. The spec imens were diffusion welded in a nonsuperplastic (or superplastic) state fo r a short time and then further diffusion welded at superplastic (or nonsup erplastic) temperatures for a longer heating period. It was found that the welding strength could be improved drastically using such a two-stage proce ss. The contributions of superplastic deformation on the diffusion welding of th is alloy during the two-stage process were clarified.