Experimental And Modeling On Forming Limit Of Cu/Ni Clad Thin Sheets At Mesoscale

The growth in demand for micro-parts has led to a surge in demand for micro sheet parts in electronics,aerospace,medical engineering a nd other modern industrial manufacturing.The size effect makes macroscopic theories inefficient in analyzing deformation behavior at the mesoscale.The main manifestations of this variability are unpredictable material strength,increased uneven deformation,significantly reduced ultimate strain,and changed the fracture mechanism.The formability of the single-layer metal sheets can not meet the requirements of the microforming process effectively.Clad metal sheet can meet performance and cost savings by optimizing the structure and designing the laminate material.The plastic deformation behavior of the clad sheets at mesoscale is coupled by the free surface and interface and the microstructure.The size effects of different metal layers persist and the interface layer effects also affect the material properties.The multifactorial coupling makes its plastic deformation and fracture behavior more complex at mesoscale and how to construct accurate forming limit models for clad sheets at mesoscale has important implications for their application in microforming.The aim of this paper is to analyze in depth the influence of size effects on the plastic deformation and fracture behavior of Cu/Ni clad sheets and to obtain forming limit diagrams of Cu/Ni clad sheets at mesoscale.Finally,a forming limit prediction model for clad sheets at mesoscale is supposed to be established.The core content of this paper is divided into the following three sections.(1)Study of plastic deformation and fracture behavior of Cu/Ni clad sheetsThe true stress-strain curves of Cu/Ni clad sheets with different feature sizes with thickness of 50 μm,100 μm and 500 μm were obtained by uniaxial tensile tests.The law of the feature size effect on flow stress was analyzed and the critical value of the size effect of flow stress was determined.The influence of the feature size on the plastic deformation behaviors and fracture behaviors of the Cu/Ni clad sheets was analyzed by the finite element simulation and the observation of fracture morphology.The results demonstrated that as the feature size decreases,the flow stress gradually decreases and there are two inflection points in curves.The non-uniformity of local strain increases.The intensified uneven deformation leads to the advancement of the necking process time.The observation of fracture morphology indicated that the reduction of feature size makes the dimples in fracture morphology of the Cu layer and the void in fracture morphology of the Ni layer gradually disappears and the fracture mode gradually changes to single crystal slip.(2)Forming limit experiments of Cu/Ni clad sheets at mesoscaleBased on Holmberg and Marciniak experiments,the forming limit diagram at mesoscale of Cu/Ni clad sheets with thicknesses of 50 and 100 μm and different grain sizes is obtained by Digital Image Correlation(DIC).The influence of size effect on the ultimate strain,loading path and deformation process under different loading paths were analyzed.The results demonstrated that the increase in grain size decreases the ultimate strain.The degree of dispersion of the limit strain points increases and the uncertainty of the strain path increases.The strain concentration time during the deformation process advances,leading to the advancement of the necking process.In addition,the set-up method also affects its ultimate strain.The ultimate strain obtained by the Cu-Ni set-up method is higher than that by the Ni-Cu set-up method.(3)Establishment of forming limit model of Cu/Ni clad sheets at mesoscaleFirstly,the forming limit diagram of Cu/Ni clad sheets was obtained based on the macroscopic forming limit criteria.The prediction results of various traditional criteria for forming limit of Cu/Ni clad sheets at mesoscale were analyzed and the optimal model that could accurately describe the deformation process of the thin sheets was determined.By analyzing the evolution process of surface roughening under the influence of size effect in the deformation process of thin sheets,the thickness evolution equation of thin sheets affected by the size effect was obtained.The optimal model was modified to obtain a modified meso-scaled forming limit model of clad sheets.The results demonstrated that compared with the traditional forming limit theory,the prediction accuracy of modified meso-scaled forming limit model was significantly improved.For different loading paths,the segmentation prediction method can improve the prediction accuracy.