Wall thickness distribution of Cu–Al bimetallic tube based on free bending process

Due to the complicated material and structural characteristics of bi-layered non-homogeneous bellows (BNBs) in hydroforming, the bellow deviates easily from its designed profile and this inevitable phenomenon results in a low forming precision. Therefore, it is important to study the springback behavior of bellows for precision forming. Based on finite element (FE) analysis, comparative studies on profiles of single-layered bellows, bi-layered homogeneous bellows (BHBs), and BNBs with two expansion ratios (k, the ratio of outer-to-inner diameter) k = 1.2 and k = 1.6 as well as two materials 304SS and Inconel 718 are implemented. The springback behaviors of different simulation settings are investigated, and several conclusions are drawn: (1) after springback, the U-shaped convolution profile is changed to tongue shape accompanied by a 2.5~38.5% axial elongation and a 0.1~0.6% radial shrinkage; (2) the springback tendency grows with the increase of number of layers, the improvement of mechanical properties of material, and the decrease of expansion ratio.

Springback behaviors of bi-layered non-homogeneous bellows in hydroforming

Large diameter thin-walled tubes are prone to wrinkle due to a low stability of anti-wrinkles, non-uniform material flow, and contact sequence to the die cavity during hydroforming process. Although wrinkling defects can be controlled by reducing the diameter of the initial tube to some extent, the severe thinning will occur. A method of using local constraints was proposed to eliminate wrinkling defects that occurred on thin-walled hydroformed tubes. The effect of axial constraint, hoop constraint, and axial-and-hoop constraint on wrinkling behavior of large diameter thin-walled tubes has been investigated by using numerical simulation and experiment methods. In addition, the trajectory of principal stress of the wrinkling zones with different local constraints has been discussed. The results show that axial compressive stress of the two wrinkling zones can be reduced by 6.8 and 23.0% by using axial constraint, respectively. Hoop constraint reduces axial compressive stress by 11.7% at the two wrinkling zones. And the axial compressive stress remarkably decreased by 22.3 and 31.8% using axial-and-hoop constraint. The results reveal that the application of local constraints can effectively restrict the occurrence of wrinkle by decreasing axial compressive stress during hydroforming of large diameter thin-walled tubes. Finally, a desired part without defects has been manufactured successfully by axial-and-hoop constraint.

Study on wrinkling behavior in hydroforming of large diameter thin-walled tube through local constraints

The hydro-bending process of a double-layered tube is an effective approach for the manufacture of large-diameter, ultra thin-walled curved tubes. The hydro-bending of a mild/stainless steel double-layered tube is investigated here using numerical simulation and experiments. The thickness distribution of the inner tube is investigated in depth through a combination of numerical simulation and theoretical analysis. We found that maximum thinning occurred not at the center of the outer arc but at a nearby point and the maximum thinning ratio of the outer arc was 14.3 %. The reason the maximum thickness reduction occurred at a nearby point rather than the center point of the outer arc was well illustrated and was that the point with the biggest axial stress moved towards the tube ends. The analysis provides further theoretical foundation for the hydro-bending process of ultra thin-walled components.

Analysis of wall thickness variation in the hydro-bending of a double-layered tube

Bending of a thin-walled tube with small relative bending radius is a hot topic which has aroused extensive attention in recent years. The main disadvantage of tube parts with small bending radius fabricated by available bending approaches is non-uniform wall thickness, large springback, poor fatigue performance, and wrinkle that is difficult to control for thin-walled tube bending. In order to solve this problem, a Bend-Bulge Forming (BBF) process combining rotary draw bending and hydroforming that realizes the fabricating of thin-walled elbows with small bending radius has been put forward in this study, and both FEM simulations and experiments were carried out. Meanwhile, in order to reveal the deformation behavior and the failure during BBF process, theoretical and FEM analysis were conducted by free expansion with bent tubes. The results show that the axial feeds play a prominent role in the hydroforming process. It can promote the tube wall outside of the bend contact with hydroforming die and avoid the excessive thinning. At the same time, the tube wall inside of the bend which is thickened during bending operation can be deformed sufficiently. Finally, the final parts fabricated by the BBF process have approximately constant wall thickness and high precision.

A novel hydroforming approach in manufacturing thin-walled elbow parts with small bending radius

Bending of an aluminum alloy thin-walled tube with larger diameter-to-thickness ratio is more difficult than that of a mild steel or stainless steel tube due to its low elastic modulus and yield strength. To solve this problem, experiments were conducted on hydro-bending of bi-layered tubes, which consist of an outer mild steel tube and an inner aluminum alloy thin-walled tube. The wrinkling behavior, springback, cross-sectional non-circularity, and wall thickness variation of the inner aluminum alloy tube were analyzed. It has been shown that the internal pressure plays a prominent role in preventing the wrinkling due to an additional axial tensile stress being introduced to the inner tube through welding the inner tube ends onto the surface of the outer tube. By increasing the internal pressure to a critical value, it is helpful to reduce the magnitude of compressive stress at the inner side of bent tubes, so the onset of winking is prevented. The springback and the cross-sectional non-circularity become smaller, while the thinning rate gets bigger with pressure increasing. Finally, a sound aluminum alloy thin-walled tube with diameter-to-thickness ratio of 63 and relative bending radius of 2.2 was successfully formed under the internal pressure of 25 MPa.

Deformation analysis of hydro-bending of bi-layered metal tubes

The role of boron in the creep properties and grain boundary characteristics in a new Ni-Fe-based weld metal suitable for advanced ultra-supercritical (A-USC) coal-fired power plant applications has been investigated. Ni-Fe-based filler wires without boron and boron-doped (50 ppm wt% boron) were prepared for this study. Boron-doped weld metals exhibited longer rupture lives and lower steady creep rates during the creep rupture tests at 750 °C / 380 MPa and 750 °C / 210 MPa. This study explains the improvement mechanism of boron on creep resistance from the perspective of the effect of boron on M23C6. Boron increased the nucleation rate of M23C6 and participated in the formation of M23(C, B)6 type boron-carbides. After creep deformation, boron still existed stably in the M23(C, B)6. The higher density of discrete M23C6 particles due to boron addition could restrain grain boundary crack propagation and grain boundary sliding, and thereby improve the creep fracture resistance of the GH984G weld metal at 750 °C / (380/ 210 MPa).

Wencai Xie

Papers

Deformation analysis of hydro-bending of bi-layered metal tubes

Effects of boron addition on the microstructure and creep properties of a Ni-Fe-based superalloy weld metal alloy