The Forming Mechanisms Of Zirconium Alloy Strip And The Optimizations Of The Stamping Process

Nuclear energy is a medium and long-term national strategy for China’s energy development.After the 14 th Five-Year Plan,the nuclear power construction has ushered in a rapid development trend in our country.However,the number of advanced reactors in China accounts for only 6% of the world’s total,which affects the localization of the nuclear power industry.One of the key technologies is stamping technology of the strip,which is a basic unit of the nuclear fuel assembly grid.The strip is an essential component that provides radial and axial support and positioning of the fuel rods.The functions of strips are mainly include maintaining reasonable spacing and uniform clamping force,ensuring the uniformity of water and heat conduction,and avoiding fuel pipe rupture and nuclear fuel leakage.Therefore,the requirements for consistency and stability of crucial indicators such as dimensional accuracy,geometric accuracy and forming quality of the strip are very strict.Due to the complex internal working environment of the reactor,stringent requirements are put forward for the compatibility of nuclear working conditions and the corrosion resistance under high temperature and pressure conditions of the raw materials of the strips.Zirconium alloys have been used as the main materials of the strips due to its outstanding properties of the corrosion resistance,low absorption cross section for neutrons as well as satisfied strength.However,the material has the characteristics of strong anisotropy,low plasticity,small forming margin as well as complex plastic deformation mechanism,which leads to frequent occurrence of defects such as local thinning,cracking and spring-back during the stamping process of zirconium alloy strips.At present,the criteria applicable to the plastic deformation of zirconium alloys are still to be studied,and the relevant researches on the forming properties of zirconium alloy materials by heat treatment process and punching process parameters are still in its infancy.Therefore,in this investigation,a systematic study on the forming performances of zirconium alloy strips and the optimization of stamping process parameters are conducted.The plastic deformation mechanism of zirconium alloy materials,the influence of the annealing process and punching process on the forming margin of the material are also investigated.The main contents of this study are shown as follows:1.The plastic deformation mechanism of zirconium alloy materials for strips is studied.A yield surface exponent suitable for zirconium alloys has been proposed,and the Barlat 89 anisotropy criterion is revised.With the proposed exponent,it can accurately describe the plastic deformation behavior of zirconium alloy materials and enhanced the universality of the criterion;Based on the improved M-K micro-defect instability model,the forming limit curve(FLC)of the zirconium alloy has been predicted,and the prediction results were verified by the Nakazima bulging test;The Nakazima experimental digital model of the zirconium alloy sheet has been established,the strain paths of the necking element under different yield surface exponent states are analyzed,and the mechanisms of the yield surface index on the plastic deformation of the material have been revealed.2.The effects of the annealing processes of the zirconium alloy material for strips on its stamping performances has been studied.The punching experiments of the strip feature are conducted and the strain paths of the necking elements during the strip forming process are calculated;Based on the numerical calculation results of the forming limit curves of the zirconium alloy,quantitative evaluations and analyses of the forming margin of the zirconium alloy sheet under different annealing conditions have been executed;The grain size of the zirconium alloy,the proportion of the small-angle grain boundary and the basal texture strength of the material have been analyzed.The influences of the annealing process on the forming properties of the zirconium alloy sheet material are explored.3.The influence of the stamping process parameters of zirconium alloy strips on its forming performances have been explored.The punching experiments of the typical key feature of the strip have been conducted under different processing boundary conditions.The correlations between the processing parameters(punching speed,blank holder force and stamping clearance)and the maximum thinning ratio(characterizing the risk of cracking failure)have been analyzed;Based on the PSO-SVR algorithm,a prediction model of the zirconium alloy strips stamping process is constructed,and its prediction results are verified;Based on the established predictive model,the impacts of the stamping process parameters on the risk of zirconium alloy strip stamping cracking have been evaluated,and the influences of stamping process parameters on the forming performance of zirconium alloy strips are deeply explored,which lay a theoretical foundations for the optimization of the stamping process of strips.4.The multi-objective optimizations on the process parameters of zirconium alloy strip punching are investigated.Based on the Bayesian-Random Forest algorithm,a quantifiable mathematical model of spring-back and stamping process parameters during the forming process of the strip key features is constructed.The spring-back model and the thinning ratio model with respect to the processing parameters are used as implicit objective functions of the multi-objective problems;Based on the NSGA-II algorithm,the process parameters of the two targets of spring-back and maximum thinning ratio are optimized globally and the Pareto optimal solution set is obtained while the optimal solution is experimentally verified;The optimizations of the strip punching process parameters further guarantee the punching qualification rate of the strips for nuclear fuel assemblies,and have an important guiding significance for the production of the strips.