- 學位論文
高強度鋼管件液壓成形製程缺陷之研究
An Analysis of Defects Occurred in the Tube-Hydroforming Process with High Strength Steels
摘要
在管件液壓成形製程中破裂是最常見的缺陷之一，依破裂的程度其又可分為明顯的開裂與隱藏式的裂紋；而隨著高強度鋼的使用，回彈現象亦成為管件液壓成形製程中另一主要的缺陷。本論文即是針對管件液壓成形製程中之破裂與回彈缺陷進行深入之探討。在破裂缺陷方面，局部脹形為液壓脹形過程中造成破裂缺陷之主因，本論文為了探討局部脹形現象對於液壓成形之影響，首先利用有限元素分析軟體針對局部脹形現象建立了一基礎分析模型，藉由此模型探討幾何參數與材料硬化參數對產生破裂的影響。由研究結果得知，當管材膨脹率增加時，減薄率也跟著增加，且最大減薄率皆位在材料於液壓前離模距離最遠處。而在材料硬化參數方面，本論文利用偏移材料應變之方法建立不同材料硬化之曲線以探討材料硬化對液壓脹形之影響，結果顯示當脹形處若為材料應變偏移較多之部分，則最大減薄率位置會處於材料強弱不同之交界處，並且位於應變偏移較少側。 為了改善局部脹形的產生，本論文利用基礎分析模型提出預成形改善、內部模具改善、與低壓成形改善等三種方法，由模擬分析結果得知三種方法皆可改善局部脹形現象，其改善機制為改變液壓脹形前之材料分佈狀態，使液壓前材料於模穴內之分佈狀態更貼近模面。本論文將上述改變液壓前材料分佈之方法，實際應用於汽車結構載具之液壓成形製程以改善局部脹形現象，並藉以提高管件液壓之成形性，同時亦驗證了本論文所提改善方法之可行性。 針對隱藏式裂紋的探討，本論文選擇汽車V型封閉扭力樑於管件液壓成形製程中所產生之細微裂紋缺陷(亦稱魚尾紋)作為研究載具，該裂紋的產生會導致扭力樑於台架實驗中產生疲勞破裂。本論文由實際製程中觀察到扭力樑於預成形後即產生魚尾紋現象，因此，本論文透過修改不同之預成形模面，搭配有限元素法軟體DEFORM進行分析，並使用破壞值(Damage Value)作為魚尾紋產生之參考指標。由模擬分析結果得知，V型側邊R角與V型R角寬度對於預成形後材料之破壞值有極大之影響，本論文經研究分析後提出了較佳之預成形模面造型設計。 隨著產品強度之增高，管件材料性質對液壓成形回彈的影響更為明顯，其中包辛格效應(Bauschinger effect)為一主要材料參數。為探討管材之包辛格效應並建立有限元素模擬分析所需之材料模型，本論文針對管材試片設計出一組可進行反覆拉伸與壓縮實驗之夾治具。由實驗結果得知，高強鋼管材之包辛格效應甚為明顯，且彎管成形以及V型扭力樑預成形之回彈現象皆會受到包辛格效應之影響，若鋼種強度上升，則包辛格效應對回彈現象之影響會更加嚴重，而採用具包辛格效應之材料模型進行回彈模擬分析則更有其必要性。 本論文最後利用汽車前副車架之管件液壓成形驗證本論文模擬分析之正確性，將實際產品之厚度分布與造型特徵和有限元素模擬結果進行比較，兩者間之差異甚微，確認採用含有包辛格效應材料模型之有限元素模擬分析結果更具準確性。
並列摘要
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.