Non linear finite element simulation of complex bulge forming processes

Bulge forming is a manufacturing process that is becoming increasingly important as a technology that can be used to produce seamless, lightweight and near-net-shape industrial components. The process is being increasingly applied in the automotive and aerospace industries where the demands for increased structural strength and decreased vehicle weight make it a very attractive manufacturing method. This work is concerned with increasing knowledge of the deformation mechanisms during bulge forming processes using numerical simulation. A number of complex bulge forming operations which have not been satisfactorily analysed in published research were identified and subsequently analysed using commercial finite element software. A non-linear explicit solution method was used in each case. The processes chosen for simulation were: hydraulic bulge forming of cross joints, bulge forming using a solid bulging medium, bulge forming of bimetallic tubes and the behaviour of the die during these bulge forming processes. In each case a number of process parameters were varied and their effect on the process identified. Where possible the finite element results were validated against results from experimental trials. It was found that the simulations predicted the experimental results with accuracy, thus indicating that the models developed here can be used with confidence to predict the behaviour of bulge forming operations. From the results of the finite element simulations it was concluded that when designing processes to bulge form cross joint components that compressive axial loading should be used in conjunction with pressure loading where possible, friction between the die and workpiece should be kept to a minimum where maximum branch height is required and greater tube thickness should be used when seeking to reduce stress and thinning behaviour in the formed component. The results also indicate that, where possible a solid bulging medium should be used as it results in much more favorable forming conditions, which can allow the realisation of greater branch heights. The simulations of bulge forming of bimetallic tubes showed that the relative thickness of the two metal layers has a significant effect on the shape of the final component. It was also found that varying the relative strength of the two metallic layers had a significant effect on the branch height obtained. The development of stress in the die during various bulge forming process was detailed at various stages during the process. It was found that generally a stress concentration moves towards the die bend as the process progresses. The effect of using different die materials was examined and it was concluded that certain materials are unsuitable for use as die materials due to the fact that their yield stress is exceeded during the forming process