【作者】 张明哲；

【导师】 刘纯国；

【作者基本信息】 吉林大学 ， 材料加工工程， 2024， 博士

【摘要】 喷丸成形是使用具有动能的介质高速撞击工件表面,促使工件在新的应力平衡中发生变形的成形方法。成形效果由介质的物理性质、动能大小和数量等参数共同决定。成形灵活性高,制造成本低,适用于生产尺寸大、变厚度、薄壁轻量的整体壁板零件。喷丸成形技术的关键在于对喷丸成形参数和喷丸路径进行合理的规划。在规划过程中,难点在于针对确定的一组喷丸参数和喷丸路径,如何能精确的预测其喷丸成形效果。本文的目的是使用解析计算揭示弹丸高速撞击下铝合金大应变速率局部变形的机理;在随机弹丸撞击模型中提出分区统计、厚向分类的平均诱导应力计算方法,以获得更精确的平均诱导应力,并探究喷丸强度和覆盖率对条带试样喷丸变形的影响;基于离散元-有限元耦合计算方法建立喷丸参数和平均诱导应力之间的量化关系,应用于铝合金整体壁板喷丸成形的路径规划与参数确定,为铝合金整体壁板类零件的喷丸成形预测提供理论依据和实现方法。本文的主要研究内容如下:(1)建立以喷丸速度和覆盖率为变量的弹丸撞击解析应力场,为选择合理的喷丸速度和覆盖率成形板料提供理论依据。分弹性、塑性和卸载三个阶段计算单弹丸撞击后的诱导应力。基于球腔扩展模型,用应变率对变形的影响表征弹丸高速撞击时材料的力学响应。推导出弹丸速度与诱导应力和载荷位移关系的函数表达式。综合考虑弹丸分布的随机性和多弹丸应力场的复杂性,提出将多弹丸撞击的平均诱导应力近似表示为单弹丸中心线应力和覆盖率的函数。(2)为了获得更为准确的平均诱导应力,提出一种考虑边界条件的平均应力计算方法。首先对喷丸区域进行区域划分,再对指定区域内所有积分点的应力按照深度坐标进行平均,通过对比不同区域的平均应力的大小,获得计算平均应力的最佳区域。随后,使用随机弹丸撞击的有限元模型讨论了路径数量、试件形状、撞击顺序和网格大小等边界条件对平均应力的影响,进一步完善了此方法。最后,使用该方法探究了喷丸速度和覆盖率对平均应力的影响。(3)探究撞击诱导的晶粒细化,提出一种基于晶粒尺寸直接预测喷丸应力的数值模拟方法。通过电子EBSD(电子背散射衍射)实验,表征弹丸撞击区域局部的表面形貌,得到晶粒尺寸与晶粒取向。基于位错密度演化模型,使用数值模拟的方法计算不同参数喷丸后晶粒尺寸的大小,并与实验进行对比。基于拉伸试验结果,使用晶体塑性有限元的方法,对代表性体积单元进行单向拉伸,以标定材料的晶体塑性参数。使用Hall-Petch公式考虑了晶粒尺寸对铝合金壁板应力应变关系的影响。基于晶粒尺寸修正塑性增量理论的应力更新算法,得到晶粒细化对诱导应力的影响,进一步预测弹丸撞击下的诱导应力。(4)提出一种组合的DEM-FEM喷丸成形模拟方法以预测大尺寸试件喷丸成形的成形效果,用于铝合金整体壁板喷丸成形的参数确定。首次使用粒子生成器的运动模拟条带喷丸中喷嘴的行进,能够还原真实喷丸成形中应力场连续叠加与演变的过程。由于DEM-FEM耦合方法中,可生成的弹丸数量众多,因此需要考虑弹丸间的相互碰撞对撞击速度的影响并分析弹丸在不同质量流量和进料速率下撞击速度的分布。随后,根据模拟结果拟合得到了诱导应力的估算方程。将诱导应力作为初始应力引入试件的喷丸区域,使试件在新的应力平衡中完成成形效果的预测。提取试件的表面形貌与实验结果进行对比,模拟曲率半径与实验结果吻合较好,验证了该方法的有效性。(5)首次提出基于等差主动加载的多点预应力加载方法与夹具,该方法由于能够多道次逐步提供复杂预应力,从而能够成形出结构复杂、变形量大的铝合金整体壁板。使用DEM-FEM耦合模型为用数值模拟方法直接计算多点预应力喷丸应力场提供了可能。设计了特定喷丸参数进行预应力喷丸成形模拟,将模拟结果作为正交训练的样本向量,利用神经网络实现预应力喷丸的成形预测。以上分析方法已应用于某型机翼壁板喷丸成形实验,以高筋整体壁板为例,完成了喷丸参数优化和喷丸路径规划,实现了不同喷丸参数下喷丸成形效果的准确预测,保障了成形精度。更多还原

【Abstract】 Shot peen forming is a forming method that uses a medium with kinetic energy to impact the surface of a workpiece at high velocity,promoting deformation of the workpiece in a new stress balance.The forming effect is determined by the physical properties,kinetic energy,and quantity of the medium.It has high forming flexibility,low manufacturing cost,and is suitable for producing large-sized,variable thickness,thin-walled and lightweight integral wall panel parts.The key to shot peen forming technology lies in the rational planning of shot peening parameters and peening paths.In the planning process,the difficulty lies in how to accurately predict the shot peening forming effect for a set of determined peening parameters and peening paths.The purpose of this article is to use analytical calculations to reveal the mechanism of high strain rate local deformation of aluminum alloy under high-velocity impact of shots;Propose a partition statistics and thickness classification method for calculating the average induced stress in a random shot impact model to obtain more accurate average induced stress,and explore the effects of peening intensity and coverage on the deformation of strip specimens during shot peening forming;A quantitative relationship between peening parameters and average induced stress is established based on the discrete element finite element coupling calculation method,which is applied to the path planning and parameter determination of shot peening forming of aluminum alloy integral wall panels.This provides a theoretical basis and implementation method for predicting shot peening forming of aluminum alloy integral wall panel parts.The main research content of this article is as follows:(1)Establish an analytical stress field for shot impact using peening velocity and coverage as variables,providing a theoretical basis for selecting reasonable peening velocity and coverage for forming sheet metal.Calculate the induced stress after a single shot impact in three stages: elastic,plastic,and unloading.Based on the spherical cavity expansion model,the influence of strain rate on deformation is used to characterize the mechanical response of materials during high-velocity impact of shots.Derive a functional expression for the relationship between shot velocity,induced stress,and load displacement.Taking into account the randomness of shot distribution and the complexity of multiple shot stress fields,it is proposed to approximate the average induced stress of multiple shot impacts as a function of the centerline stress a single shot and the coverage.(2)In order to obtain a more accurate average induced stress,a method for calculating the average stress considering boundary conditions is proposed.Firstly,divide the peening area into different regions,and then average the stresses at all integration points within the specified area according to the depth coordinates.By comparing the average stresses in different regions,the optimal region for calculating the average stress is obtained.Subsequently,the finite element model of random shot impact is used to discuss the effects of boundary conditions such as path number,specimen shape,impact sequence,and mesh size on the average stress,further improving this method.Finally,this method is used to investigate the effects of peening velocity and coverage on average stress.(3)Explore the impact induced grain refinement and propose a numerical simulation method for directly predicting peening stress based on grain size.By conducting electron backscatter diffraction(EBSD)experiments,the local surface morphology of the shot impact area is characterized,and the grain size and orientation are obtained.Based on the dislocation density evolution model,numerical simulation is used to calculate the grain size after peen forming with different parameters,and compared with experiments.Based on the results of tensile tests,the crystal plasticity finite element method is used to perform uniaxial tension on representative volume units to calibrate the crystal plasticity parameters of the material.The influence of grain size on the stress-strain relationship of aluminum alloy wall panels is considered using the Hall Petch formula.A stress update algorithm based on grain size correction plasticity increment theory is used to obtain the effect of grain refinement on induced stress,and further predict the induced stress under shot impact.(4)Propose a combined DEM-FEM shot peening forming simulation method to predict the forming effect of large-sized specimens during shot peening,and use it to determine the parameters for shot peening forming of aluminum alloy integral wall panels.The motion simulation of the nozzle in strip peening using particle generators for the first time can reproduce the process of continuous superposition and evolution of stress fields in real shot peening forming.Due to the large number of shots that can be generated in the DEM-FEM coupling method,it is necessary to consider the impact of collisions between shots on impact velocity and analyze the distribution of impact velocity of shots under different mass flow rates and feed rates.Subsequently,an estimation equation for induced stress is fitted based on the simulation results.Introduce induced stress as the initial stress into the peening area of the specimen to predict the forming effect in the new stress equilibrium.The surface morphology of the extracted specimen is compared with the experimental results,and the simulated curvature radius is in good agreement with the experimental results,verifying the effectiveness of this method.(5)For the first time,a multi-point prestressing loading method and fixture based on equal difference active loading is proposed.This method can gradually provide complex prestressing through multiple passes,thus forming aluminum alloy integral wall panels with complex structures and large deformation.The use of the DEM-FEM coupled model provides the possibility of directly calculating the stress field of multi-point prestressed shot peening forming using numerical simulation methods.Specific peening parameters are designed for simulation of prestressed shot peening forming,and the simulation results are used as sample vectors for orthogonal training.A neural network is used to predict the formation of prestressed shot peening forming.The above analysis methods have been applied to the shot peening forming experiment of a certain type of wing wall panel.Taking the high rib integral wall panel as an example,peening parameter optimization and peening path planning are completed,achieving accurate prediction of shot peening forming effect under different peening parameters and ensuring forming accuracy.更多还原