Experimental Study On The Limit Of High-speed Plastic Forming Of Metals Based On Electromagnetic Expansion

The plastic forming limit of a material is a material plastic index that describes the plastic forming ability of a sheet,The study found that high-speed forming technologies such as electromagnetic forming are more efficient than quasi-static plastic forming,and the apparent forming limit has been significantly improved,showing a "plasticizing" phenomenon.There are many controversies on the mechanism of high-speed forming limit increase,which is affected by various factors such as geometry size,strain rate effect,inertia effect,etc.,and it is worthy of further discussion.In this paper,the electromagnetic driven high-speed expansion of thin-walled cylindrical shell experimental technology is used for research,Combined with the finite element to design the experiment,and the high-speed electromagnetic drive uniform expansion experimental loading device and test analysis system of the aluminum alloy thin-walled tube are established to test and analyze the expansion deformation process of the cylindrical shell to obtain the locality of the cylindrical shell under different strain rates during the high-speed deformation,discuss the mechanism and influencing factors of hyper-plasity of high-speed forming limit.The main research contents include:(1)In this paper,the magnetic field distribution and deformation characteristics of metal cylindrical shell specimens with different heights driven by electromagnetic coils are analyzed by ANSYS multi-physics coupling field finite element and experimental test.The finite element results show that the radial electromagnetic force distribution and expansion deformation along specimen height are not uniform,which will lead to warping deformation of thin-walled cylindrical shell in different degrees.The inhomogeneity of the electromagnetic force is related to the relative ratio of the specimen height and the coil height.When the height ratio of the specimen to the coil is about 0.73,the electromagnetic force and deformation are relatively uniform.The research results provide a reference for the design of electromagnetic expansion experiments.(2)Established a test and analysis system for the electromagnetic expansion experiment of thin-walled cylindrical shells,and conducted a series of expansion and failure experiments on aluminum alloy thin-walled cylindrical shells,The expansion process was recorded using a high-speed camera,and the evolution of the strain field of the cylinder shell expansion failure was analyzed in conjunction with the digital image correlation method(DIC),The results show that:(a)The wall cylinder shell deformation and failure process undergoes three stages of uniform expansion,uneven dispersion and localized instability deformation;(b)During the expansion process,the critical strain of dispersion instability is basically unchanged as the loading rate increases,but the formation of localized critical strain and fracture strain increases significantly.(3)The analysis believes that the existence of inertial effects makes the plastic localized instability more dispersed,and forms more localized necking,This phenomenon leads to an increase in fracture strain,which increases the high-speed forming limit and the material exhibits hyper-plasity.In addition,the "plasticization" of the material is also related to the structural size of the sample...