Investigation of the complexities inherent in manufacturing near-unconstrained superplastic parts by experiments and simulation

Abstract. Superplastic forming is a sheet metal forming process that has found widespread use in the aerospace industry. It produces parts that are free of residual stresses, dimensionally accurate, and with strains unobtainable using conventional forming methods. When combined with diffusion bonding, a phenomena where under similar processing conditions the material involved will produce a near flawless weld with itself, a variety of reinforcements and internal structures can be produced. In most superplastic parts the material is blow formed up to a die and the material takes on the dimensions of the die with small variations in thicknesses. In this work, however, we investigate a process unique to superplastic forming with diffusion bonding using four sheets. The two outer sheets are formed up to the die while the two inner sheets form a complex sandwich structure. These inner sheets are completely unsupported except at the edges of the part. Superplasticity is stress-history dependent and somewhat chaotic in nature. Therefore, the inner sheets have a large variance in geometry due to the fact that they have only limited constraints and are free to shift and translate as the forming operation progresses. Without rigid constraints on the inner sheets small variations in geometry can be magnified to create large changes in final geometry. The variances in forming are quantified using a variety of techniques to measure the major features of the process including cell wall measurements, gas pathway measurements, and computer vision-based geometry analysis. Two- and three-dimensional finite element simulations of the inner sheet forming process were used to compare the characterization results with idealized geometry. The results of the analysis provide insights into the complexities of manufacturing such internal structures.