Cavity minimization and uniformity studies on superplastic forming of thin eutectic Pb-Sn sheet by optimum loading and preforming

Superplasticity in materials is characterized by large neck-free elongation under low flow stress when they are formed at temperature exceeding about one half of the melting point. The bulge forming technique is considered the most common practice employed in forming superplastic materials. A common technical problem in superplastic forming is non-uniform wall thickness distribution of formed parts. In this proposed work, eutectic Pb-Sn superplastic sheet materials were considered. Cast sheet blanks were thermo-mechanically treated to obtain superplastic properties. A sophisticated PC-controlled loading system was prepared for the bulge-forming process. An optimum pressure-time profile based on variable pressure was proposed. In addition, the optimum preforming required for improving the uniform thickness distribution and dome height was also analyzed. The thickness distribution after preforming effect was compared with optimum pressure-time performance. This combined method based on variable pressure and preforming gives good results than the variable strain rate method. This work reduces the system complications on the variable strain rate method. The cavitations in the various process samples were also analyzed. The combined method based on optimum variable pressure paths and the preforming gives more reduction in cavitations in formed parts.