【作者】 王倩；

【导师】 董湘怀；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2016， 博士

【摘要】 金属薄板的微成形工艺是金属微加工和微制造中非常重要的一部分。在微成形中,材料的宏观尺寸减小,特征尺寸达到微米级,而材料内部微观结构的尺寸仍然保持不变,因此材料表现出与传统塑性成形不同的力学行为。例如,金属薄板在单向拉伸实验中的流动应力会随着厚度或晶粒尺寸的变化而改变,这种由于尺寸变化引起的材料力学性能改变的现象就是所谓的尺寸效应。尺寸效应使得经典的力学公式和传统加工中的经验公式不能直接用来描述材料在微成形中的力学行为,因此,有必要弄清微成形的特点,对尺寸效应进行预测和控制,从而达到控制微成形效果的目的。本文以具有不同表面层的纯铝薄板为对象,研究了不同表面层特性对微成形力学性能的影响。主要开展了以下工作:采用纯铝薄板进行了材料力学性能试验。将切割好的试样分别在空气和氮气的保护中进行热处理,得到带氧化层试样和无氧化层试样。检测了两组试样的微观结构和晶粒取向分布。为了研究两种表面层特性对材料流动应力及其尺寸效应的影响,分别选取具有不同厚度和不同晶粒尺寸的试样进行了单向拉伸试验。研究发现不同的表面层对厚度、晶粒尺寸和厚向晶粒个数引起的尺寸效应有很大的影响。总体来说,无氧化层试样表现出“越小越弱”的尺寸效应,而带氧化层试样表现出“越小越强”的尺寸效应。其中,无氧化层试样的流动应力在随着厚向晶粒数目的减小而降低的过程中还出现了两个明显的转折点,并且这两个转折点的值是确定的。基于不同表面层特性对试样流动应力的影响规律,提出了适用于描述带氧化层试样和无氧化层试样的流动应力的解析模型。解析模型主要由两部分组成:参考流动应力-应变曲线和尺寸相关项。虽然两组试样的流动应力-应变曲线虽然在宏观尺度下非常接近,但随着厚度的减小又表现出截然相反的尺寸效应。因此,在解析模型中采用宏观尺度试样的流动应力-应变曲线作为解析模型中的参考流动应力-应变曲线。对于尺寸相关项,采用反正切函数来描述无氧化层试样的流动应力-应变曲线相对于参考流动应力-应变曲线的“越小越弱”的尺寸效应,采用指数函数和幂函数来描述带氧化层试样的流动应力-应变曲线相对于参考流动应力-应变曲线的“越小越强”的尺寸效应。由于表面层特性对材料流动应力的影响的具体大小取决于材料厚度、晶粒尺寸及厚向晶粒数目,因此,引入了厚度、晶粒尺寸(或厚向晶粒数目)作为解析模型中尺寸相关项的变量。采用解析模型对拉伸变形过程进行了模拟,结果表明,该解析模型可以很好地预测具有不同表面层的纯铝薄板在单向拉伸中的流动应力及其尺寸效应。为了进一步预测破裂应变的尺寸效应,本文基于已提出的流动应力的解析模型,对经典损伤模型进行了改进。改进后的破裂准则对破裂应变的预测是尺寸相关的,采用破裂时的塑形变形能作为判断材料是否发生破裂的判断准则。通过改进的破裂准则可以得到尺寸相关的破裂应变曲面图,用于直观地查看不同尺寸试样的破裂应变。计算结果还表明,表面层特性虽然会极大地影响破裂时的塑形变形能的取值,但对破裂应变的取值没有很明显地影响。为了从微观角度更准确地描述表面层特性对变形的影响。采用基于位错密度演化的晶体塑性模型来描述材料的力学行为。为了能够描述两种表面层的不同作用,本文对位错密度晶体塑性模型中的位错密度演化方程进行了改进:采用晶粒取向系数描述晶内位错与各界面之间的交互作用,这些界面包括相邻晶粒间的晶界、无氧化层表面和氧化层表面。使用晶粒取向系数对位错密度演化方程中的位错平均自由程相关项进行了修改。将改进的位错密度晶体塑性模型应用于模拟和预测纯铝薄板的流动应力-应变曲线,结果表明,改进的模型不仅能够有效地模拟出具有不同表面层的试样的变形,即无氧化层试样的表层晶粒出现了明显“软化”现象,而带氧化层试样的表面则出现了明显的“硬化”现象,而且也能很好地预测不同表面层特性导致的晶粒尺寸、厚度和厚向晶粒个数引起的尺寸效应之间的差异。为了进一步验证理论模型的准确性,将理论模型应用于模拟微弯曲成形。采用纯铝薄板试样进行了微弯曲实验。分析了两种表面层特性对微弯曲成形性能的影响,以及表面层对厚度、晶粒尺寸、厚向晶粒数目引起的尺寸效应的影响,特别是弯矩和回弹角的影响。采用改进的位错密度晶体塑性模型对微弯曲成形进行了模拟。结果表明,改进的模型可以有效地描述两种表面层对试样微弯曲变形的影响,即带氧化层试样的表层晶粒出现了明显的“硬化”现象;而无氧化层试样中,虽然表层晶粒相对于内部晶粒来说更容易发生变形,但在弯曲过程中表层晶粒的变形又比内部晶粒剧烈,因此,表层晶粒没有出现明显的“软化”现象。改进模型预测的回弹角比经典塑性力学公式计算的回弹角更接近实验值。加入应变梯度导致的额外增大的回弹角度之后,改进模型计算的回弹角与实验数据非常吻合。通过模拟结果还可以看出,厚向晶粒数目越少,晶粒取向对微弯曲成形的影响也越大;厚向晶粒数目较多时,弯曲试样厚向出现了明显的中性层。说明本文对晶体塑性中的位错密度演化的改进可以有效地模拟出微弯曲变形的特点。更多还原

【Abstract】 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.更多还原