Forming of pure titanium tubes by combined spinning-ultrasonic surface rolling process: Microstructure and mechanical properties

Spinning-ultrasonic surface rolling process (S-USRP) combined forming technique, as a potential method for manufacturing thin-walled tubes with high performance, not only forms gradient nanostructures on the surface of materials, but also has advantages of reducing surface roughness and improving mechanical properties. In this work, the S-USRP was applied to manufacture thin-walled pure titanium tubes, and the surface morphologies, microstructure and mechanical properties of titanium tube formed by different forming methods (including spinning forming, USRP, and S-USRP) are investigated and compared. The results indicate that spinning ripples on the surface of CP-3 Ti tube can be improved obviously after introducing USRP treatment. Compared to USRP, the titanium tube formed by S-USRP has thicker gradient microstructure layer and finer grains, which is attributed to the spinning pre-deformation, resulting in decrease of strain threshold of grain refinement during USRP treatment. Microhardness after S-USRP varies more slowly between the plastic deformation layer and matrix compared with that after USRP. Tensile strength of titanium tube after S-USRP is promoted by 26.2 % owing to the grain refinement, deformation twins strengthening and synergy strengthening of gradient structured layer. However, the elongation reduced significantly because titanium thin-walled tube has the characteristics of coarse grains on one side and nanocrystalline on the other, leading to premature instability in the tensile process. In addition, the tensile strength and fracture strain of titanium tube after spinning-two passes ultrasonic surface rolling process are promoted compared to the spinning forming alone. This indicates that gradient microstructure with a certain thickness on the surface of components can achieve good strength-ductility synergy. The research in this article is of great significance to the development of high-performance forming processes for thin-walled tubes.