A study of the role of friction in superplastic blow forming of alloy sheet

Abstract

Superplastic forming (SPF) is a manufacturing process whereby certain materials, under specific temperatures and strain rates, exhibit high ductility resulting in large deformations. Superplastic forming has found a wide application in industry, especially in the aerospace sector for the fabrication of complicated structures, because of its superior advantages of significant cost savings and weight reduction over conventional forming processes. However, failures of SPF components are present in the manufacture of aircraft parts with complex shapes. In particular, severe component thinning or even cracking during a superplastic blow forming process contributes to a major problem for Rolls Royce Plc, which is strongly associated with the poor level of current understanding of friction behaviour in the SPF process. It is thus the aim of this research to investigate the superplastic blow forming process and the role of friction, to minimize uneven component thinning to meet design criteria. The objective was tackled by way of finite element modelling together with experimental validation. The superplastic blow forming of a Pb-Sn alloy sheet into a complex component as well as a cylindrical cup was simulated to investigate the deformation process, pressure versus time curve and thickness distribution. To study the effect on thickness distributions, the role of friction was modelled by specifying a friction coefficient associated with the contact between the dies and the component. The finite element model has been validated using experimental data from literature. It was found that a lower friction coefficient results in a better uniform thickness distribution. The simulations also verified that reverse blow forming resulted in less thinning in the formed component. A set of laboratory test apparatus for performing superplastic blow forming experiments under controllable temperature and pressure was designed and built in order to investigate the friction behaviour and component thinning mechanism. A set of equations were derived theoretically for the calculation of the friction coefficients at different contact regions. The experimental deformation in a superplastic blow forming has been recorded through a transparent die, and the friction coefficients at different contact regions were calculated according to the derived friction equations. The friction coefficients and the experimental parameters were then entered into the validated finite element model. The predicted deformation process is in good agreement with the experimental observation. In addition, the maximum discrepancy between the predicted and experimental thickness distributions is less than 40%. The implementation of the methodology from this study will improve the understanding of the friction mechanism in blow forming process and thus help achieve less component thinning in the industry.