Research On The Mechanism Of Compacting Voids In The Cross Wedge Rolling Shafts Based On The Floating-pressure Method

With the popularization of high-speed rails and automobiles at home and abroad,the demand for the automobile shafts and balls gradually increases year by year.Because cross wedge rolling(CWR)technology has many advantages such as high efficiency,energy-saving and material-saving,it is widely applied in forming the billets of shafts and balls,and about 85% of the shafts and balls' billets are formed by CWR technology every year.But due to the limit of the forming conditions,the defects such as loose and void easily appear at the core of rolled parts,weakening the mechanical properties of rolled parts and even directly scrapping the rolled parts.However,these problems to some extent restrict the development of CWR technology in our country,thus it is of great social importance and engineering value to study on the elimination of internal defects.In this paper,the floating-pressure method(FPM),using high temperature and high gas pressure to compact the internal voids in CWR parts,is applied for improving the mechanical properties and the yield of the CWR parts.Firstly,the mechanical model of compacting voids in CWR parts based on FPM was established,and the relationships of each process parameter,the shape parameter of the rolled parts and the metal rheology were deduced.Based on the single factor analysis method,the influences of process parameters and shape parameters on the metal flow,external dimensions of CWR parts were investigated using DEFORM-3D finite element software,and the correctness of the mechanical model was verified.Then,the mechanism of compacting void in CWR parts was obtained through systematically analyzing the stress field,the metal flow laws and the evolution laws of microstructure in CWR parts.Because the equivalent stress is directly related with the healing of microcracks,therefore the uniform test design method with fewer test times was adopted,and the data obtained from the uniform tests was further numerically fitted using MATLAB software to obtain the relationships equation of gas pressure,material temperature,rolled parts' diameter and the equivalent stress at the core of CWR parts,which is helpful for the selection of process parameters.Finally,on the basis of the theoretical analysis and numerical simulation,the experiments of compacting voids artificially set up in the steel balls and shafts by FPM were carried out.The results show that the internal voids in the specimens were effectively closed;the specimens' outer dimension almost remained the same value;the grain sizes were refined.Then,the mechanical properties of the specimens were tested,and it is found that the tensile strength of the specimens significantly increased,but its surface hardness slightly reduced,which can be subsequently improved through the heat treatment.The above results provide a new way for improving the internal quality and yield of CWR parts.Because FPM has good generality,it can also be applied in eliminating internal defects of other forgings to lay theoretical foundation for the development of forming high-precision and high-quality product in China.