Macro And Micro Evolution Rule Of Surface Area Of Metals During Bulk Plastic Forming Processes

There is a complex interaction between the metal material's geometric characteristic parameters which are closely related to plastic deforming,such as volume,surface area and surface state and so on.According to classical plasticity theory,volume of compact metal in the plastic deforming process remains unchanged,which has been verified in practice,and has been applied in theoretical analysis of plastic finite element simulation,and calculation of the billet in production etc.But so far,there are few studies on the evolution of the surface area of the metal in plastic deforming process and some basic issues such its macro and micro evolution mechanism lack of deep understanding.In fact,the blank surface area is not only mutative but also obvious in bulk forming such as forging and extrusion,which has a direct impact on plastic flow of metal,the surface friction and lubricant,blank heat exchange with the environment,the surface oxidation state and the part surface quality.In this paper,the variation rule of surface area of metal in bulk plastic forming process has been studied,in order to understand the basic law of the change of blank surface area from the macro and micro scale.Firstly,this paper summarized the research status of evolution of main geometric parameters in the metal plastic deformation process;secondly,for the basic deformation process--cylinder upsetting,it studied the macro area surface evolution rule based on the numerical simulation and physical experiment;then,through physical experiment it studied the change mechanism of compression and tensile process of micro surface state;finally,for a practical production vehicle crankshaft forging,it researched the changes of the surface area.The main conclusions are as follows:(1)Through the analysis of the typical regular geometry cylinder upsetting process,found that the total blank surface area is firstly decreased and then increased.When the ratio of height to diameter k is equal to 1,it is closest to the sphere,the total surface area of the smallest,which accords with the geometric theory that the sphere surface is smallest under a certain volume.When there is no friction in the blank and the slab,the blank shape always is a cylinder,and because of the unchanged volume,it can be deduced that the total surface area S is a quadratic function of the diameter of the cylinder.When there's friction,the blank is drum due to the friction.But the surface area changes similarly.At the same time,the greater the blank shape is deviated from the sphere(the geometric size of a dimension is greater than or less than the other direction),the larger the surface area.(2)The results of numerical simulation and physical experiments of cylinder upsetting show that the total surface area is firstly decreased and then increased,and the free surface area S1 decreased and the end surface area S2 increased.And when the surface area is minimum,the each direction size of the part is relatively close to each other.Besides,when the friction coefficient of the blank and the slab is increased,the variation quantity of total surface area S will be reduced,the variation quantity of free surface area S1 will increase,and the variation quantity of end surface area S2 will decrease,and the larger the friction is,the smaller the surface area change will be.(3)Before a certain amount of deformation,the surface area of the cylinder firstly decreases and then increases.The microscopic mechanism is mainly due to the fold of free surface of the crystal and so on.(4)During the drawing process,the surface area of sheet metal is increased,and its microscopic mechanism mainly includes the crack of free surface of the crystal boundary and so on.(5)Through the analysis for the complicated forging process of the mini car crankshaft forging,it can be proposed that : for most of the volume plastic forming process,the total surface area is increased because that the difference between the size of some directions and the other direction is growing,resulting in the shape of the blank on the whole is increasingly deviating from the sphere,so the surface area always increasing.In addition,the numerical simulation can well reflect the surface area change of bulk plastic forming process.