Research And Application Of SPF/DB Techolonogy In Temperature Resistant Structure

Superplastic forming and superplastc/diffusion welding are advanced materials forming techniques in the field of aerospace. Titanium and nickel alloys have excellent strength, corrosion resistence, elevated temperature performance, etc., but the superplasticity of the coarse grain materials is low. If the grain size can be refined to equiaxial and microcrystalline grains, thses alloys will show good plasticity at proper temperature and can be formed into complex shape and/or structrure.In this thesis superplastic forming and superplastc/diffusion welding techniques of multilayer TA15 and TC4 titanium alloy sheets were investigated. The superplastic tensile properties of TA15 alloy was tested, including the maxium elongation, optimum tensile rate and forming temperature and strain rate sensitivity index m. The experimentals results show that TA15 alloy sheet exhibits large elongation in the temperature range of 880-930℃at the tensile rate of 0.6 mm/min and the as-obtained maxium elongation was 1500% at 930℃, the value of m is up to 0.6mm/min at 900℃when the tensile rate is kept at 0.3 mm/min. Based on these results, the optimum parameters were selected as temperature 900℃and tensile rate 0.6 mm/min, at which the flow stress is about 35MPa.The weldability of dissimilar Ti alloy sheet of TA15 and TC4 were studied basing on the disscusion of thermodynamics and metal diffusion theories. The welding processing parameters were determined and the diffusion welding parts were prepared.The superplastic forming processing of multilayer Ti alloy sheets was investgated and the optimum processing parameters were obtained, which are diffusion temperature 930℃, pressure stress 2⁵MPa, diffusion time 30min, vacuum above 6.5×10^-2Pa, superplastic forming temperature 930℃, forming time 35min and forming stress 0.5MPa. The effects of microstructures of the dissimilar alloys on the weld seam and the thickness distribution in the as-formed sheets were dicussed.The superplastic forming defects in the parts were dynamically analysed, the planar distorsion models for asymmetry die cavity and compression stress transfer-planar wrinkling model were estabilished. It was founded that the complex stress is the key factor leading to surface wrinkling. The author believe that the static processing parameters of materials are not enough in the superplastic forming technique, more attensions should be paid to the dynamic analyzing of the processing and precision theoretical models covering detailed constraint condition should be established to solve the forming problems.