Research Of Damage And Fracture For Cross Wedge Rolling Based On Damage Mechanics

Cross Wedge Rolling (CWR), a powerful metal forming technique widely used in manufacturing industry, possesses the advantages of better product quality, higher production efficiency, greater material utilization and so on. A fatal weakness of CWR is the internal defect of products, particularly when the die is worn severely or forming parameters are designed inappropriately. While axle forgings usually suffer higher torque and bending load, having a higher requirements on material performance, therefore it is not allow to have material crack and pore cavity. At present, Mannesmann effect of CWR has already become a key problem restricting the technology and its application.In this paper, the applicability of different ductile fracture criterions in the forming of CWR was carried on deeply and systematically. Meanwhile, from damage mechanical point of view, a ductile evolution equation suitable for CWR process has been established for predicting inner defect of CWR workpiece, which provided a successful method for predicting inner fracture occurrence of workpiece during the forming process of CWR more accurately. The main research content and conclusions of this paper as follows:Using DEFORM-3D FEM software and 45 steel stress-strain data with thermal simulation compression experiment, the applicability of commonly used eight kinds of ductile fracture criterion have been analyzed, which suggested that Oyane criterion is the most suitable ductile fracture criterion for CWR deformation. And combining with fracture morphology of actual workpieces and finite element simulation method, damage threshold of 45 steel under the condition of deformable temperature 1150℃during CWR process has been determined as 2.72.Based on the theory of damage mechanics and the mechanical characteristics of center material of CWR workpiece, a new damage evolution equation has been put forward, correction coefficient A(Z) has been fixed by thermal simulation compression experiment and multi-steps tensile experiment. Using secondary development, this criterion has been embedded in finite element program to calculate the damage distribution field of CWR workpiece, which was in good agreement with actual experimental result. With combination of experiment and finite element method, damage threshold of 45 steel under CWR process condition has been determined as 2.77. Meanwhile, entire process from the generation and development of material damage to the occurrence of fracture has been simulated, which have laid a good foundation for understanding the central cracking mechanism of CWR workpiece and knowing optimization of die design deeply.Stress and strain variation law in the center of workpiece shaped by normal CWR die, CWR die which can produce workpiece with macro defect in the center and Cross Rolling(CR) die have been analyzed. It has been found that positive mean stress in the center of workpiece produced by CWR die which can produce workpiece with macro defect in the center dominates during deformation process, while the mean stress in the center of workpiece produced by normal CWR die fluctuate around zero, but mean stress in the center of Cross Rolling(CR) workpiece remains positive during the whole deformation. So it is reasonable to consider that positive mean stress in the main factor that lead to macro defect in the centre of workpiece during CWR process.