Study On Effects Of Different Surface Layer Properties On Mechanical Charactors In Micro Forming Process

Micro forming for metal foil has been an important part of metal micro forming and micro manufacture.In micro forming,the sample geometry size decreases and the instinct size remains unchanged.As a result,mechanical properties in micro forming is different from what in traditional metal forming,which is known as size effects.Because of size effects,classical mechanical models and empirical formulas are no more accurate for describing micro forming behaviors.As a result,it is very important to find out the characteristics of micro forming and to predict and control size effects.In this paper,pure aluminum foils with two kinds of surface layers were employed as research objects to investigate effects of different surface layer properties on mechanical characters in micro forming.Mainly work carried out is listed in the following:Material mechanical tests were carried out.Testing samples were heat treated in air and nitrogen respectively to obtain samples with oxide layer and samples without oxide layer.Both microstructure and crystallographic texture were detected to get grain size and grain orientation distribution diagrams.Samples with different grain sizes and thicknesses were chosen to carry out uniaxial tension tests.Effects of different surface layer properties on flow stress and its size effects during tension tests were analyzed.Generally,samples without oxide layer show size effects of “smaller is weaker”,and samples with oxide layer show size effects of “smaller is stronger”.Besides,two critical turning points appear in variation curves of flow stress with grain number across thickness direction for samples without oxide layer,and the value of two critical turning points are fixed for a given material.An analytical model was proposed to describe flow stress and its size effects for samples with different surface layers,which is consisted of reference flow stress-strain relationship and size-related item.The reference flow stress-strain relationship was modeled by experimental data of samples with thickness at macroscopic scale.Size effects are very different for samples with oxide layer and without oxide layer.Consequently,the size related items are different.For samples without oxide layer,an arc tangent function was adopted to describe size effects of flow stress,which is “smaller is weaker”.For samples with oxide layer,a combination of exponential function and power function was adopted to describe size effects of flow stress,which is “smaller is stronger”.Besides,the specific value of effects of surface layer on size effects depends on sample thickness,grain size and grain number across thickness,so the three size-related factors were adopted into the size-related item in analytical model as arguments.Comparison of predicted results by the proposed model and experimental data shows that both flow stress and its size effects can be predicted accurately.To predict size effects on fracture strain,a classical damage model was modified based on the proposed analytical model.The modified model for fracture strain prediction is size related.Plastic deformation strain energy at fracture is the threshold for judging whether fracture occurs.Size dependent map of plastic deformation strain energy at fracture can be obtained through the modified model to visually check fracture strain for samples with different grain sizes or thicknesses.After comparing the predicted fracture strain with experimental data,it can be found that different surface properties has a great influence in fracture energy but a little influence in fracture strain.In order to investigate the physical mechanism of roles played by surface layer in causing flow stress size effects,dislocation density evolution based crystal plasticity model was employed,and dislocation density evolution equation was modified.Grain orientation coefficient was developed and adopted to describe the interactions between dislocations and interfaces like grain boundaries,surface without oxide layer and surface with oxide layer.Grain orientation coefficient is an adjustment parameter of dislocation mean free path related item in dislocation density evolution equation.Take grain boundaries as an example,the greater the difference of grain orientation between two sides of grain boundaries,the smaller the value of grain orientation coefficient.As a result,dislocation mean free path is smaller,and it is harder for dislocations to penetrate grain boundaries.The modified dislocation density evolution based crystal plasticity model can effectively simulate both “softening” phenomenon in surface grain of samples without oxide layer and “hardening” phenomenon in surface grain of samples with oxide layer,as well as the difference of size effects of thickness,grain size and grain number across thickness caused by different surface layer.In order to further verify the accuracy of the modified theoretical model,the theoretical model was applied to simulate the micro bending process.First,the micro bending test was carried out using pure aluminum foils.Effects of different surface layer on bending behavior,especially bending moment and springback angle was analyzed.Second,the proposed theoretical model was adopted for simulating.Comparison between simulated results and experimental data shows that obvious “hardening” phenomenon appears in surface grain of samples with oxide layer,but no obvious “softening” phenomena or obvious “hardening” phenomenon appears in surface grain of samples without oxide layer because surface grains experience more serious deformation than inner grains.Springback angles predicted by the modified model are closer to experimental data than those calculated by classical models.By adding the effects of strain gradient on springback angle,springback angle calculated by modified model agrees very well with experimental data.It can also be seen through simulated results that obvious neutral layer appear for samples with more grains across thickness,and the smaller grain number across thickness the larger effects of grain orientation on micro bending.In conclusion,the modified dislocation density evolution equation in crystal plasticity model can effectively simulate micro bending deforming behavior.