Hot flow behavior characterization for predicting the titanium alloy TC4 hollow blade surface Sinkage defects in the SPF/DB process

Titanium hollow blade is applied in aircraft turbo-machine to gain higher thrust-to-weight ratio. The typical combined superplastic forming and diffusion bonding (SPF/DB) technology has been widely used in manufacturing complex multi-layer hollow structures such as the titanium hollow blade. This study introduced a TC4 hollow blade with internal reinforcing ribs fabricated with a series of hot forming operations including diffusion bonding, hot twisting, stamping and gas bulging. During the forming process, the blade outer surface will sink at the cavity locations due to lack of internal support. The gas pressure bulging process is necessary to repair the defect and bulge the caved surface to the required profile. In order to predict the TC4 blade deformation during the SPF/DB process, its hot flow behavior in the DB and SPF were separately investigated by carrying out isothermal tensile tests at corresponding temperature range and strain rate range. And taking the initial microstructure influence in consideration, samples which experienced the DB heating history were used to characterize the flow behavior in the SPF processes. A strain hardening form power law equation and a hyperbolic-sine law equation were employed to describe the constitutive relations during the DB and SPF respectively. Both models were calibrated with the hot flow curves and applied in the corresponding forming step finite element (FE) simulations. The blade outer surface profiles extracted from each forming step simulation showed good correlation with the experimental measurement. It proved that the hollow blade surface sinkage defect during the SPF/DB process can be accurately predicted by the FE simulation with the calibrated constitutive models. And the effectiveness of the gas bulging process for repairing the sinkage defect were verified in both simulation and experiment.