Formation mechanism and elimination method for surface groove of the hollow lattice structure by superplastic forming

Abstract. The titanium alloy hollow lattice structures can be widely applied on the advanced aircrafts due to their excellent mechanical properties and light weight. Superplastic forming and diffusion bonding (SPF/DB) process is an advanced process to manufacture these hollow lattice structures with two outer plates and one inner hollow plate. To further decrease the weight of the structures, thinner outer plate is preferred when compared with the inner plate, however, it would result in the occurrence of surface groove at the intersects between the outer plate and the hollow stiffener during superplastic forming process, leading to the failure of the structures. In this paper, the formation characteristics of the surface groove and the influence of the outer/inner sheet thicknesses on the occurrence of surface grooves was investigated through a well-developed finite element model. Furthermore, an innovative method was presented to eliminate the surface groove during SPF/DB, which makes use of double-sided dynamically adjusted gas pressure on the inner and outer of the plate during superplastic forming process. Based on the developed numerical models, the effects of assistant outer gas pressure parameters on groove formation have been investigated. The effectiveness of the proposed new forming method and the optimized parameters have been validated by performing a SPF/DB forming tests of a typical lattice structures. The method developed in this study provides a new way to improve the forming quality and accuracy of hollow parts by SPF/DB.