| The key technology for the operation of PWR nuclear power plants is the manufacturing level of nuclear fuel assemblies.The basic unit of nuclear fuel assemblies is the grid strip.Therefore,it is the forming quality of the grid strip that determines the manufacturing level of nuclear fuel assemblies.However,when using the zirconium alloy sheet strip to punch the grid strip,the punching performance of the strip is poor and it is prone to breakage.Aiming at the problem of "stuck neck" in the punching and forming of the grid strip,this paper takes a new type of the zirconium alloy sheet strip to be used in a nuclear fuel assembly as the research object.Combining the methods of tensile test,simulation forming,single factor test,orthogonal test and punching test,the material parameters and process parameters that affect the punching and forming of the grid strip are preliminarily explored.The punching process of the grid strip is optimized on the basis of the punching performance evaluation,and it offers guidance on theory and support on technology for real production hopefully.Firstly,the three-direction tensile test was carried out on the new zirconium alloy sheet strip to obtain the mechanical property parameters required for the simulation forming.From the yield ratio and hardening index,the formability in the RD direction was better than that in the 45° direction and the TD direction,and the thickness anisotropy coefficient in average is ?=6.34,the plane directionality in average is ?=-1.15.Using the Dynaform forming simulation software,the sheet metal forming process can be numerically simulated.The maximum thinning rate,the maximum principal strain and the safety margin were observed,and the forming limit diagram was used to evaluate the forming effect.Based on these evaluations indicators to explore the material parameters that affect punching,and it is found that when the hardening index is 0.12≤9)≤0.18,it is beneficial to improve the punching quality of the strip;when the thickness anisotropy coefficient in average is 5.15≤?≤8,the forming effect is better;when the poisson’s ratio is =0.32,it is beneficial to the punching of the strip;changing the size of the elastic modulus,the punching effect doesn’t change significantly,and the correlation between the elastic modulus and the strip punching is small.Secondly,the process parameters affecting punching are explored.When the punching speed is too large or too small,it will affect the forming quality of the strip,and a reasonable range should be selected;the blank holder force has little effect on punching;when the friction coefficient decreases,the strip forming quality becomes better;when the concave-convex mold gap increases,the strip forming quality becomes better.Based on the orthogonal experiment,the punching process is optimized.The plan is: the first level is the mold gap,which is set to 1.2t,and the second level is the punching speed,which is set to 500mm/s,and the third level is the friction coefficient,which is set to 0.075,and the fourth level is the blank holder force,which is set to120 KN.After optimization,the maximum thinning rate of the sheet strip is reduced from 8.63% to 8.288%,and the maximum principal strain is reduced from 0.201 to0.186,the safety margin increased from 5.5% to 6%.Finally,using the mold used for punching and forming the grid strip,it is found that after the improvement of the punching process,the rigid-convex feature of the grid strip is not broken.And the thinning rate of the strip after optimization is smaller than that of the strip before optimization,which proves that the forming performance of the strip after optimization is better than that of the strip before optimization.The results of the actual punching test and the simulation test are consistent. |
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