Surface and Bulk Residual Stress in Ti6Al4V Welded Aerospace Tanks

Abstract

The residual stress (RS) in two curved plates cut from a large welded propellant tank for spacecrafts was investigated nondestructively by neutron and laboratory x-ray diffraction. Each plate had two weld beads symmetric to a central monoblock reinforcement. One plate had received a post-weld heat-treatment. The two nondestructive test techniques successfully determined both the bulk (thickness averaged) and the surface stress state, due to the highly different penetration of these radiations in metals. In the as-welded tank, both neutrons and x-rays show a stress level (both in the axial and hoop directions) higher in the heat affected zone (HAZ) than in the weld pool (300 against 160 MPa). A considerable degree of relaxation annealing was observed by neutron diffraction after the application of the heat treatment. In this case, the hoop stress in the HAZ relaxes from about 300 to about 100 MPa. X-rays also permitted the separate determination of the α and β-phase stresses and the calculation of the macro-RS. The latter showed the bending deformation resulting from the cut of the plates from the original tank. The average stress measured by x-rays was found to be very similar to the RS obtained by neutron diffraction technique.