Preform Design for Forging and Tube Hydroforming Processes

In the metal forming process, preforming is widely used to deform the work piece into a suitable shape that can form the required product successfully without forming defects and excessive waste of material in the final process. A proper preforming stage can improve the product quality, reduce forming stages and minimize manufacturing cost. Preform design problems are encountered in various metal forming processes. However, this research focuses on the preforming problems in the forging process and Tube Hydroforming process.;Preform Design in Forging Process: Preform design is critical for multi-stage forging processes to ensure the production of defect-free parts. Moreover due to the geometry and material flow complexities in forging processes, finding the optimal preform shapes could be difficult and time consuming.;An efficient design methodology based on geometrical resemblance is proposed for the preform design in forging process. The premise of this methodology is such that the initial and subsequent simulations are carried out by constructing a slightly larger part which geometrically resembles the desired part. Initial FEA simulation of the larger part is performed with reasonably guessed preform shape which may allow the occurrence of forming defects or flash formation. Then a series of intermediate resembling parts between the largest part and the desired part are constructed. The undeformed shape corresponding to the intermediate part could be obtained by backward tracing of material flow from the simulation results of the larger part. This undeformed shape is then taken as the preform shape of the intermediate part. The procedure is repeated until the intermediate part is geometrically close to the desired part, which leads to the preform shape. In order to verify this preform design methodology, several case studies on forging and extrusion processes have been carried out. The methodology has been proven to be computationally efficient since it requires fewer numbers of iterations.;Preform Design in Tube Hydroforming Processes: In the last decade, the tube hydroforming process has received increased attention in the automotive and aerospace industries owing to its advantages such as part consolidation, weight reduction, and better part quality over stamped and welded assemblies. However, an incoming straight tube may need to be preformed due to the shape complexity of the tubular part and material flow limitation. One such preforming process, axial crushing, is used to accumulate the material in the die cavity by forming axisymmetric wrinkles on the tube. However, the knowledge of how to axially crush the tube in the tube hydroforming process is still limited.;In this research, a two-stage preforming process based on the wrinkle formation is developed for the Tube Hydroforming process to accumulate material in the forming zone so that the thinning rate can be reduced and formability can be improved. In preforming stage one, the wrinkle onset is triggered with limited axial compression. In preforming stage two, the wrinkle grows stably and uniformly to a certain height. Then the preformed wrinkles are flattened to conform to the die shape in the final tube hydroforming process. An analytical model based on the bifurcation analysis and post-buckling analysis of the elastic-plastic circular cylinder under internal pressure and axial compression is used to study wrinkle evolution characteristics. The analytical results offer guidance to the process design of the two-stage preforming process. To validate this methodology, preform die sets for three axisymmetric parts were designed and tube bulging experiments were carried out on SS 304 tubing and Al 6061 tubing. Through this methodology, expansion rates of the bulging process are significantly improved.