The superplastic forming/diffusion bonding of TA7 titanium alloy for manufacturing hollow structure with stiffeners

Abstract

The superplastic forming/diffusion bonding (SPF/DB) process has been proved to be an effective way to fabricate multi-layer integrated structures. During this study, a hollow structure with two stiffeners was manufactured by SPF/DB process using TA7 titanium alloy. The tensile behavior of the alloy was investigated by undertaking uni-axial tensile tests at high temperature, with results showing that the maximum elongation was 199.8% at 870 °C/0.001 s−1. Diffusion bonding experiments utilizing TA7 titanium alloy were conducted at different temperatures. The shear strength of DB joints at 880 °C and 900 °C were 568.4 MPa and 569.1 MPa, respectively. Based on the superplastic properties, a finite element simulation of the hollow structure was carried out, with results showing that this could be fabricated by the DB/SPF process. Finally, fabrication of this hollow structure was successfully demonstrated by DB at 880 °C/2 MPa/1 h and SPF at 870 °C with a target strain rate of 0.001 s−1. Thickness measurements indicated that the maximum reduction in thickness was approximately 10% and occurred at the short stiffener. The experimental result was less than the predicted reduction from the finite element simulation. When comparing the tensile strength and elongation of the initial material, the reduction in the manufactured material was 9.4% and 0.4%, respectively. The microstructure was characterized and the average grain size was measured to increase from 7.5 μm to 14.8 μm.