【作者】 刘玉梅；

【导师】 李兆前；

【作者基本信息】 山东大学 ， 机械制造及其自动化， 2012， 博士

【摘要】 大型复杂结构的薄板、薄壁结构件在加工过程中,由于受到装夹力、切削力、切削热以及力热耦合作用,发生变形；且加工过程中产生的残余应力会引起残余应力变形。这两种变形都会导致加工精度降低。本文针对薄板件加工过程产生的残余应力和变形控制问题,综合考虑切削过程、装夹布局及夹紧力等影响因素,以铝合金6061为实例,利用切削实验与模拟仿真方法研究薄板件切削回弹变形机理及变形控制方法,从而为薄板件的加工变形控制提供理论依据。实验研究了铝合金6061表面切削应力的状态,得出了表面切削应力属于二维平面应力；应用响应面原理建立了铣削参数与表面切削应力之间的关系模型；综合考虑表面切削应力与表面粗糙度,对铣削参数进行了优化,并分别对面支撑和点支撑装夹方式下的薄板件进行了表层铣削实验。测试和分析结果表明,面支撑装夹方式的薄板件表层铣削完成后,去除装夹时的回弹量是影响加工精度的主要因素；点支撑装夹方式的薄板件表层铣削完成后,夹紧力施加位置沿薄板件板长方向相对支撑点位置变化时,对变形及残余应力都有重要影响。应用ABAQUS有限元软件,建立了薄板件表层切削过程及去除装夹后回弹过程的二维仿真模型。分析了不同装夹方式的薄板件去除装夹前后,薄板件变形、回弹变形和内部应力分布情况。结果表明,面支撑装夹方式的薄板件表层切削完成后,去除装夹时存在较大的回弹变形量；此时薄板件的主要变形为回弹变形；内部应力主要是切削力、切削热耦合作用产生的切削应力,沿薄板件板厚方向的进给方向的应力分布值远大于垂直进给方向的应力值和剪应力值,因此后续研究仅考虑进给方向的切削应力。针对点支撑与夹紧点正对装夹的薄板件进来了仿真计算结果表明,支撑点位置对薄板件切削后的变形、回弹变形、内部应力分布及大小都有重要影响。而薄板件的变形和残余应力分布及大小之间没有直接的关系。研究了薄板件沿板厚方向的切削回弹变形分析方法。应用微观位错原理对点支撑装夹方式的薄板件在表层切削过程中的变形区进行了划分。构建了基于微观位错的回弹变形分析方法。对面支撑装夹方式的薄板件,应用宏观弹塑性变形原理进行推导。分析表明内部应力重新分布是薄板件回弹变形的主要原因。建立了切削参数与轴向切深方向的切削应力之间的关系模型。研究结果表明,切削应力在表面主要表现为拉应力,沿板厚方向接近表面处迅速转化为最大压应力,之后慢慢减小到零。应力主要集中于表层0-0.3mm内。采用逐层铣削方法,测量了薄板件沿板厚方向的切削应力值,结果与预测结果一致。应用该模型建立了切削应力与切削应力引起的薄板件变形挠度之间的关系。针对支撑点位置与夹紧力施加位置正对装夹的薄板件,采用BP神经网络建立了预测模型,映射薄板件板长方向的内部残余应力均值、板厚方向的变形量均值与板长、板厚、夹紧力、支撑点位置和板长比之间的关系。针对薄板件的支撑点位置,设计了基于遗传算法的变形均值最小和残余应力均值最小的多目标优化方法。通过BP神经网络预测模型计算适应度值,采用适应度惩罚措施,在非支配前沿等级划分和小生境计算的基础上应用二元联赛选择机制,对种群进行优劣排序,得出优化结果集合。针对支撑点位置与夹紧力施加位置正对装夹的薄板件表层切削过程中产生的“欠切”问题,应用应变叠加原理设计了基于装夹的预变形、预应力补偿方法,通过夹紧力值变化及夹紧力施加位置沿薄板件板长方向相对支撑点位置变化产生预变形与预应力,补偿薄板件变形及残余应力。更多还原

【Abstract】 Thin parts tend to be deformed under the clamping force, cutting force, cutting heat and thermo-mechanical effect in machining. The internal re-sidual stress and the as-machined residual deformation deteriorate the final machining precision. In this study, aluminum alloy6061is used to examine the cutting deformation mechanism of thin parts. Specifically, the residual stress and deformation generated during the machining process are studied based on fixture layout and clamping force. The study is meaningful for the control of cutting deformation of thin parts.The surface residual stresses in milling aluminum alloy6061were measured with Doelle-Hauk method. The measurement shows that the prin-cipal plane is approximately parallel with the machined surface of the workpiece, and stresses are under two-dimensional plane stress state. The model was established between milling parameters and surface residual stress by the range analysis and orthogonal experiment method, establish the relationship model by the means of Response surface principle. The parameters were optimized based on overall consideration of surface re-sidual stress and surface roughness. Then the experiment of milling surface of thin parts was conducted under different fixing schemes. Based on the result and analysis of experiment, the thin part with fixing under surface support features a large springback after removing fixture, which heavily affects the precision of thin parts. For fixing of point support, the defor-mation varies as the support points change.To simulate the cutting surface and springback of the parts, a two di- mensional model of was built with Package Abaqus. Deformation and in-ternal stress distribution is studied for different fixing. For surface support, the thin part has a large deformation caused by springback, and the spring back is the main factor of deformation of thin parts. For surface support, the internal stress of thin part along the thickness mainly concentrates on the surface. Meanwhile the stress along feed direction is far greater than the stress along the thickness and shear stress. Therefore, subsequent studies focus on the stress along the feed direction. For point support, under the condition of the position of clamping force and point support on the oppo-site, the springback, deformation and the distribution were severely affected by the position of point support and surface support. Meanwhile the de-formation and internal stress had no direct relation.The deformation mechanism was studied for the springback of thin parts along the thickness. The deformation zone was divided based on the dislo-cation theory. And the springback is analyzed with force between each other for surface support. At last the main reason for springback is the re-distribution of internal stress.The model of relation between cutting stress along the thickness and cutting parameters was established using Response surface principle and Polynomial fitting principle. By comparison of predicted stress and ex-periment stress, the model is robust for predicting cutting stress along the thickness. Basically, the cutting stress is tensile stress on the surface, then quickly decreased to the maximum compressive stress, then slowly in-creased to zero.For the point support, a prediction model was established using BP Neural Network between the deformation, internal stress and the thickness, length, clamping force, the ratio of the position of point support and the length of thin part. The position of point support is important for the de- formation and distribution of internal stress through analysis using the prediction model. Thus, the position is optimized using genetic algorithm, in which the sorting method was designed for guaranteeing the diversity of population and the elitist strategy was used for preserving the lost of ex-cellent individuals in the process of evolution, for the thin part with fixed size based on the minimization of deformation and internal stress.For undercut, the strain superposition principle is used, using which the predeformation and prestress is added to the thin part through optimization of position of clamping force.更多还原