Research On The Key Technology Of Multi-coining Process Simulation And Flash Line Defect In Commemorative Coin Manufacturing

Commemorative coins as the cards of our nation are circulating legal tender.Commemorative coins with fine designed imprints are used to carry forward and cultivatethe Chinese civilization with long history and the amazing modern developments to allover the world. The traditional design and manufacture depending on work experience canno longer meet the demands of shortening the R&D cycle, reducing the costs andimproving the qualities. Numerical simulation has been used in coining process in recentyears. As there is little research on the technology, methods and defect mechanism inmulti-coining, simulations with CAE software have many problems including: accuracydeficiency, un-professional, and high computational costs etc., which result in invalidtheoretical guidances. Therefore, it is urgent need to develop professional multi-coiningCAE system and build valid defect prediction model on the basis of intensive study on theplastic deformation mechanism of multi-coining process.This paper is supported by the grant form National Natural Science Foundation ofChina (No.51275184). The key algorithm in coining simulation, multiscale constitutivemodel based on strain gradient plasticity theory, plastic forming principles inmulti-coining process and flash line defect mechanism are intensively investigated. Theresearch results are successfully employed in COINFORM to improve the accuracy andstability in coining analysis, which provide theoretical guidances to coinage.An8-node hexhedral element with selective reduced integration is developed toaccurately analyze the coining process. The standard B matrices are deduced by assumedstrain method to avoid volume locking with nearly incompressible materials.Modifications to shear related items are made to eliminate shear locking in bendingproblems. A physical stabilization algorithm without any artificial parameters is employedto suppress hourglass mode. Then the element is successfully employed in the dynamicexplicit FEM in coining.The forming process is deeply studied to analyze the size effect in coinage. On thebasis of the Conventional Mechanism Strain Gredient (CMSG) theoretical frameworkform the Taylor dislocation model, a multiscale constitutive model with Ag999is built to describe the size effect in coining. Least squares technique is applied in the local area ofintegration points to calculate the effective plastic strain gradient. Then the model and itsnumerical algorithm are employed in COINFORM to enhance the accuracy of coiningsimulation.Simplified coining experiments are designed to investigate the loading and unloadingmechanism in multi-coining process. Trasformation between the equilibrium state and thepseudo-equilibrium state as a result of non-uniform plastic deformation is regarded as thedeformation mechanism of the multi-coining process. Thus the implicit incrementalalgorithm is developed to analyze the unloading process in multi-coining. Moreover, thecombination of dynamic explicit and implicit incremental FEM is successfully employedin COINFORM to carry out complete multi-coining process simulation.The mechanism of flash line defect is studied by the coining experiments and flowanalysis in COINFORM. The radial component of friction in the interface betweencoining dies and workpiece is considered as the main reason to the defect. Then amodified friction model considering the influence of the sliding velocity in the interface isemployed to describe the frictional behaviors in coining more properly. A novel radialfriction work (RFW) model is proposed to predict the tendency of flash line and employedin COINFORM finally.The theoretical system in multi-coining process and defect prediction mechanism arepreliminary constructed in COINFORM on the basis of the achievement in this paper,which will provide valid guidances to improve the design and manufacturing capability incoinage.