Superplastic forming of a spherical shell out a welded envelope

Superplastic free forming of edge welded titanium envelopes is investigated theoretically and experimentally. The envelope is transformed into a spherical shell without clamping of its edges so that the final diameter of the spherical shell, Ds, produced is less than the initial diameter of the envelope, D0. Mathematical modelling of the process under consideration is based on the principal equations of the membrane theory of shells. Experimental justification of the model suggested is carried out using the welded envelopes of various dimensions made from different commercial titanium alloys. Comparison of the theoretical predictions with corresponding experimental data is carried out obtaining good agreement. It is found that the upper limit of the value of D0/Ds is equal to 1.25, which corresponds to the case of superplastic forming of an isotropic envelope when the influence of the welded joint is negligible. It is shown that superplastic free forming makes it possible to produce spherical shells having more uniform thickness distribution as compared with conventional procedure where the envelope is clamped rigidly along its periphery. Technological operations providing the above-mentioned advantages of superplastic free forming include the suitable choice of D0, the correct arrangement of the sheets and an appropriate method of welding. The model developed enables one to calculate the pressure–time cycle, thickness distribution and initial geometry of the envelope.