Research On Fundamentals Of Hydroforming Of Tailor-Welded Tube With Dissimilar Thickness

Hydroforming of tailor-welded tube (TWT) is that the tube blank is deformed by hydraulic pressure into the desired shape with dissimilar thickness welded tubes. TWT hydroforming generates many advantages in forming hollow components with diffirent thickness, including improvement of design flexibility, optimization of material performance and further weight reductions. The geometry characters and the difference in stress and strain states between thinner and thicker tubes were studied to reveal the plastic deformation during TWT tube hydroforming. The weld-seam movement and the thickness distribution were also deeply investigated.Simulation and experiment were conducted on the analysis of geomatry character and the effects of weld-seam position and thickness difference on the deforming process. The mechanism of the bulging pressure varying was proved. It is indicated that the final shapes of thinner and thicker tubes are ellipsoidal and conical respectively. The expansion ratio of thinner tube is always bigger than that of thicker tube. The difference in expansion ratio between two tubes can be minimized by increasing the length of thicker tube or decreasing the difference in thickness between thinner and thicker tubes. The bulging pressure of TWT increases as the length of thinner tube decreases, but is always bigger than that of the thinner tube and less than that of the thicker tube. The mechanical analysis proves that the curvature radius decrease as the length of thinner tube decreases. Consequently, the bulging pressure required for thinner tube deformation increases, which is help to reduce the difference in expansion ratio between thinner and thicker tubes.During TWT hydroforming, plastic deformation occurs firstly in the middle zone of the thinner tube, and then extends towards two ends simultaneously with pressure increasing. However, as for the thicker tube, plastic region occurs firstly in the end adjacent to weld-seam, and then gradually extends to the other end. Though both stress states of thinner and thicker tubes are biaxial tensile stress, considerable difference in the strain appears. Tensile strain in longitudinal direction occurs in thinner tube. On the contrary, compressive strain occurs in thicker tube because of the lower tensile stress along this direction. Weld-seam position and thickness difference do not affect stress and strain states.During TWT hdyroforming, weld-seam moves from thinner tube to thicker tube all the way through the bulging process. Weld-seam movement appears at the time when thicker tube begins to expand and stops at the time when weld-seam contact with die. The amount of weld-seam movement increases as the thickness ratio increases. When the ratio of thicker tube's length to TWT's length is less than 0.65 or the thickness ratio of thicker tube to thinner tube is bigger than 1.6, most of the weld-seam movement will concentrate to take place during the late period of thinner tube deformation, which can induces notable effects on axial strain distribution. Mechanical analysis reveals that the main reason to induce weld-seam moving is the difference in stress ratios between thinner and thicker tubes due to the dissimilar thickness. During TWT hydroforming, the ratio of axial stress to hoop stress on thinner tube is bigger than 1/2. On the contrary, the ratio of axial stress to hoop stress on thicker tube is less than 1/2. Thus, elongation and compression deformation happen on thinner and thicker tubes respectively. Consequently, weld-seam moves from thinner tube to thicker tube.A Multidiameter tube was formed to investigate the character and the regularity of thickness distribution during TWT hydroforming. It is indicated that the thinning ratio of thinner tube is bigger than that of thicker tube especially at the zone closed to weld-seam. The difference in thinning ratio between thinner and thicker tubes aggravates as thickness difference increases. Weld-seam movement and different strain state between two tubes are the main reason to induce nonuniform thickness distribution. When deformation sequence of thinner and thicker tubes inverses, the deformation condition is improved and two tubes'deformation states are similar. Thus, the the uniformity of thickness distribution is improved.