An Analysis of Defects Occurred in the Tube-Hydroforming Process with High Strength Steels

• Main Authors: Sin-Liang Lin, 林信良
• Other Authors: 陳復國
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/g4rm6z

博士 === 國立臺灣大學 === 機械工程學研究所 === 105 === The fracture defect is most frequently present in the tube-hydroforming process. The shapes of fracture could be classified into clear crack and unapparent microgroove. In addition to fracture, the springback defect also becomes more significant as the strength requirement of tube material gets strict. In the present study, both fracture and springback defects occurred in the tube-hydroforming process were investigated by the finite element analysis. As for the crack defect, the localized expansion is considered as the major cause for it. An analysis model was then constructed in the present study to examine the effects of part geometry and material hardening on the local expansion. The simulation results reveal that the part thinning increases as the overall expansion ratio of cross section increases, and the most thinning portion is located at the area where the localized expansion presents. Regarding the effect of work-hardening on the localized expansion, the finite element simulation results indicate that the maximum material thinning locates at the transition area between the strong region and the weak region, but at the weak side. The finite element analysis was also employed to ameliorate localized expansion, and the simulation results indicated that localized expansion can be ameliorated by changing the material distribution of the tube by using a preform or an inner die. These methods substantially reduce the maximum material thinning. Finally, the method was used to improve the formability in the hydroforming with an actual product. The unapparent microgroove defects produced by the torsion beam in the forming process can also cause the material to crack in the bench test, and this study found that the unapparent microgrooves may occur in the preformed process. The finite element software DEFORM was adopted to analyze the occurrence of the microgrooves with the use of damage value as an index. The simulation results found that the corner of radius on the side of V shape and the width of the corner on the top V shape would affect the damage value in the preformng operation. An optimized preformed die face was then proposed in this study. Springback is one of the defects that cause the lower accuracy of the product. This study designed an innovative experimental jig which can be adaptive to the tube specimen shape to conduct the reverse tension-compression tests. A material model which considers the Bausinger effect was adopted in the finite element analysis to investigate the tube bending process and the preformed process in the manufacturing of a V-shape twist beam. The simulation results indicated that the Bauschinger effect affect the springback in these two process significantly. The hydroforming of an actual front sub-frame was performed to validate the finite element analyses conducted in the present study, and the experimental results successfully validated the proposed method and analysis.