Finite element modeling and experimental investigation on manufacturing TA18 alloy pipes via hot free bending forming technology: Forming characteristics and process optimization

Free bending forming (FBF) technology is widely used for manufacturing complex-shaped Ti pipes without changing the forming die. Nevertheless, the characteristics of Ti alloys, such as low ductility and high yield stress, pose challenges in achieving high-quality components at room temperature. Therefore, the hot FBF technology is used to manufacture and optimize the process parameters to produce complex-shaped TA18 pipes. Combining theoretical analysis, FE modeling, and experimentation, the forming characteristics of TA18 alloy pipes at elevated temperatures were revealed, and the process parameters were optimized. The Johnson-Cook constitutive model was established in this study and coupled with FE modeling providing accurate simulations of the hot FBF process under various conditions. The FE model of the TA18 alloy pipe was verified by comparing its results with the actual TA18 pipe trials. The hot FBF process can improve the wall thickening and cross-sectional distortion of TA18 bending components and reduce the bending radius of the pipe by weakening the dynamic rebound at the end of the bending and forming of titanium pipes. The increase in heating length is also generally conducive for forming TA18 pipe, and the component's external wall thickness reduction and cross-sectional distortion can be inhibited when the heating length is 250 mm. Moreover, the increase in axial propulsion speed improves the thickening of the inner wall thickness of the TA18 bending components but also increases the thinning of the wall thickness and the distortion of the cross-section. Finally, the process parameters were optimized, and the forming quality of the complex-shape TA18 pipe can be improved. The microstructure of the TA18 pipe hardly changes during the hot FBF process.