Synchronously tailoring of residual stress and surface quality of high-strength titanium alloy tube using magnetic field-assisted finishing

Abstract

High-strength titanium alloy tubes are widely used as key materials in hydraulic and fuel systems in aeronautic industries, which are subjected to harsh conditions such as alternating loads and oil erosion during the service process. The residual stress state and the surface quality of the tubes are crucial to their service performance. However, these tubes are primarily prepared through a cold pilgering process, and the finished tube exhibits significant residual tensile stress on the outer surface, which cannot be changed with the variation of the process parameters. Moreover, the conventional shot peening process can only achieve the outer surface treatment while causing surface roughening, thereby reducing the fatigue strength and stress corrosion resistance. Accordingly, this study aims to achieve the synchronous tailoring of the residual stress and surface quality of high-strength titanium alloy tubes by introducing magnetic field-assisted finishing (MAF) technology, as well as to investigate the residual stress evolution mechanisms during the MAF process. The effects of MAF parameters on the residual stress and surface quality of the tubes are revealed firstly. With the increase in magnetic needle diameter and disk rotational speed, the residual compressive stresses exhibit a gradually increasing trend on the inner and outer surfaces of the tubes, with the maximum residual compressive stress reaching −600 MPa. While the surface roughness of the inner and outer surfaces of the tubes firstly decreases and then increases, with the lowest Ra values of 0.24 μm and 0.52 μm for the outer and inner surfaces, respectively. Secondly, the residual stress evolution mechanisms during the MAF process are elucidated. During the MAF process, significant plastic deformation occurs in the region near the surface of the tube with a depth of approximately 90 to 100 μm. The dislocation densities along the wall thickness direction show gradient distribution characteristics, with a gradual decrease trend from the surface to the middle layer. The increase in the difference in dislocation density between the area near the surface and the middle layer results in the distribution characteristics of residual compressive stress on the surface of the tubes after MAF. It can be concluded that MAF can effectively tailor the residual stress on the outer and inner surfaces of high-strength titanium tubes by adjusting the magnetic needle diameter and disk rotational speed, without reducing the surface quality of the tubes, and is of great significance for improving the fatigue service performance of the tubes.