Research On The Evolution And Elimination Of Void Defects In Large Ingots During Hot Forging

As the key parts for heavy machine and equipment,large forgings are widely used in the fields of nuclear power generation,metallurgy,shipbuilding,aerospace and national defense industry.The quality requirements of large forgings are extremely strict because the large forgings should be responsible to guarantee the high safety and reliability when sustaining poor working conditions,large loads and long working time.However,the void defects,such as shrinkage cavities and porosities,inevitably exist in the large ingot due to the non-uniform solidification of the materials during casting.These void defects destroy the strength continuity of the products severely,and cause the concentration of stress and the initiation of cracks,which result in the loss of serving life and the discarding of the large forgings.Therefore,investigating the void evolution law and exploring the effective forging processes to close the voids are of great significance to improve the products quality and achieve the “shape & property” controlling of the large forgings.To research the evolution of the large amount of tiny voids in the large ingot,a general 3-d void evolution model was proposed,on the basis of theoretical modeling,numerical calculation and experiment study.By integrating this model into the finite element code,a numerical simulation method was established to predict the void closure behavior during the hot forging process of large ingots.The main contents of this paper are as following:1.The representative volume element(RVE)was introduced to investigate the relationship between the deformation of small voids and the macroscopic loading conditions in large ingots.Considering the extreme difference between the dimensions of small void defects and large ingots,the RVE can be treated as an infinite region to the void,while as a point in the large ingot.According to solving the velocity fields in RVEs under different stress-strain conditions,the relationship between the void evolution behavior and the macroscopic loading variables was established.Generally,three-dimensional deformation of void will occur during the actual forming processes,because the loading condition of the materials is complex and versatile.Therefore,the evolution behavior of the three-dimentional ellipsoidal void was investigated,by using a void shape index to descritbe the three-dimentional void shape.Firstly,referring to the work of Eshelby(1957),a semi-analytical expression was deduced to evaluate the deformation of ellipsoidal void in linear viscous material.Then,for the non-linear viscous materials,a rigid visco-plastic finite element(FE)procedure was applied to solve the RVEs under different stress states.According to the calculated data,a void evolution model was established,in which the void radius changing rate was expressed as a function of the void shape index,the macroscopic stress and strain-rate.Meanwhile,the evolution of void orientation was also studied when the void principal axis was not aligned with the principal direction of macroscopic stress states.2.A numerical simulation method was established by intergrating the void evolution model into FE software,and it is applied to predict the void closure behavior in the forging process.The FE results provide the evolution of macroscopic stress,strain and strain-rate,and then the void evolution model is used to calculate the changes of void shape and volume in each step of the deformation history.It can be found that the predicted results agree well with the experiment measurements and numerical simulations with embedded void shapes,which demonstrates that this method can be appropriately used to predict the void evolution during the deformation process.3.The internal void closure efficiency in the cogging process of the large ingot was studied by using the numerical simulation method for void evolution.According to the comparison of void closure behaviors in different processes,to close the spherical void in the ingot center,the efficiency of upsetting and extend-forging is similar.However,to close the void which locates near the two ends of the ingot and the prolate ellipsoidal void which has a long principal axis aligned with the ingot axial direction,the upsetting is less helpful but the extend-forging is more efficient.Therefore,using the extend-forging as the first step in cogging process is more efficient to close the voids,considering the morphology of the real voids in the ingot.Moreover,the effects of different processing conditions,such as the height-diameter ratio of the ingot,height reduction,die shape and die-width ratio,were investigated to determine the appropriate parameters to enhance the void closure efficiency.4.In order to investigate the evolution of the porous defects in large ingot,the void volume fraction was introduced to modify the presented void evolution model.By integrating the modified void evolution model and G-T constitutive model to the FE calculation,the coupling relation between the macroscopic mechanical performance and the mesoscopic void evolution in porous defects was explored,which provides a method to investigate the multi-scale interactions between the mechanical response of materials and the evolution of small voids during the forging process of the large ingots.