【作者】 陈晓晓；

【导师】 赵军；

【作者基本信息】 山东大学 ， 机械制造及其自动化， 2014， 博士

【摘要】 随着航空、航天、汽车、模具、能源动力等制造领域产品复杂程度的提升,复杂曲面的应用越来越多,对关键零部件的加工技术提出了更高的要求。由于多轴数控加工刀轴矢量灵活可控,可优化刀具与工件的接触点位置及相对空间姿态,加工范围广,加工高效性显著,在高端制造领域的应用越来越普遍。多轴铣削技术是多轴数控加工的典型代表,加工表面完整性对复杂曲面零部件的使用性能、寿命及装备的可靠性具有至关重要的影响。本文针对多轴高速铣削加工过程、表面完整性及工艺优化展开研究,具有重要的理论与应用价值。采用计算机辅助建模技术,构建了多轴铣削切屑的三维几何模型,研究了多轴铣削各类倾角工况对应的刀-工接触区变化规律,分析了逆铣工况参切的刀具转角范围随刀具倾角的变化规律。综合考虑切削截面积、截面周长变化及参切转角范围,正前倾角、负侧偏角及负侧偏角和正前倾角复合倾角较理想。若为提高刀具寿命,对切削过程最大切削力和平均切削力有特殊要求,可优先选择负侧偏角、正前倾角或负侧偏角和正前倾角复合倾角；解析描述了刀齿某点的空间运动轨迹,得出了多轴高速铣削刀齿的空间扫掠轨迹和扫掠曲面；构建了基于三维球头铣刀建模的有限元仿真模型,研究了不同倾角对应的刀-工接触线随刀齿转动的变化规律,对比特定倾角对应的切削力试验值与仿真值,分析了切屑形成的异同,提取了各倾角工况对应的瞬时最大切屑温度。研究发现：最大切屑温度随侧偏角增大而增大,这主要是由于参切刃段的有效切削速度随侧偏角的增大而增大；不同前倾角工况对应的最大切屑温度总体变化趋势并未呈现出明显的不同。分析了不同粗加工、半精加工及精加工工况下的切屑形态和颜色,观测、讨论了实际切屑的微观形态特征,检测了切屑自由曲面和背面的元素分布；粗加工和半精加工易发生剧烈的变形和摩擦,进而产生大量的切削热,促进生成较多呈红色的铁的氧化物；剪切区的瞬时热-机械耦合效应会影响最终切屑形貌。切削热的存在会促进切屑与环境中活性气体元素的化学反应。基于正交切削、斜角切削理论、矩阵转换及球头铣刀几何特征分析,探讨了多轴铣削微元刃剪切区切削应力和温度等物理量的时空变化规律,分析了多轴铣削刀具-工件接触区域和切削刃参切条件,仿真了各向切削力的变化规律；建立了多轴高速铣削三维有限元仿真模型,分析了各类倾角条件下最大切屑温度随刀齿转动的变化规律,探讨了不同倾角加工时切屑分离瞬时工件温度场的空间分布状态,对比了各类倾角条件下的能耗和力矩。研究了各类多轴铣削工艺对应的参切刃段有效切削速度；检测并分析了多轴顺/逆铣加工侧偏角、前倾角、复合倾角及切削参数对加工表面残留高度、粗糙度、宏微观表面纹理和形貌、表面损伤的影响规律；重点分析了多轴逆铣复合倾角工况加工特点及加工表面残留高度直方图和表面载重比的变化规律；研究表明：多轴逆铣加工时,有效切削半径及切削速度随负侧偏角绝对值的增大而增大；随正侧偏角增大时,上限切削速度先降低后增大,下限切削速度逐渐增大；其他类型多轴铣削工况的有效切削速度可与多轴逆铣侧偏角工况类比；多轴高速铣削加工表面纹理方向直接决定于靠近最终成形表面的一系列参切微元刃的切削速度方向；加工表面损伤主要形式为凸痕、凹坑、微观裂纹及点蚀等。分析了多轴顺铣加工表面二维轮廓波动情况,理想情况下间歇进给的残留高度仅与球刀半径、切削宽度有关。研究了多轴铣削工艺参数对加工表面残余应力、表面硬度及表面变质层的影响规律。多轴逆铣或顺铣加工时,0°附近的侧偏角和前倾角加工时易产生表面残余压应力,主要是由靠近刀尖参切刃段的挤压作用和较低的有效切削速度引起的。综合考虑各类倾角工况对应加工表面几何特征,针对逆铣加工工况,可优选刀轴转角210°和330°对应的复合倾角,以期在进给和间歇进给方向均获得残余压应力,有利于零部件抗疲劳性能的提高。加工表面残余应力状态无严格要求且需采用顺铣工况的场合,22.5°、315°刀轴转角附近复合倾角可采用,通常可优选正侧偏角和正前倾角组合。逆铣加工时,各侧偏角工况加工表面硬度大于相对应的前倾角加工表面硬度；表面硬度随前倾角增大的变化并不明显；顺铣加工表面硬度随侧偏角和前倾角的增大,总体均近似出现双峰状。针对复合倾角工况,逆铣加工时,综合考虑里氏硬度和显微硬度,刀轴转角60°、120°和210°加工表面硬度较高,刀轴转角180°加工表面硬度较小。顺铣加工时,加工表面硬度随刀轴转角增大总体上呈现出先增大后降低的趋势,315°刀轴转角附近复合倾角加工表面硬度较大。多轴高速铣削表层材料受到切削热和机械载荷的综合作用,会产生组织致密化等现象；最表层铁元素含量急剧增加,同时检测到一些氧元素。基于前述理论分析、几何建模、有限元仿真及试验检测分析,进行了顺铣和逆铣的响应曲面试验。分析了考虑交互作用的各工艺参数对各评价指标的影响规律,提炼了各类目标/约束对应的优化工艺方案,得出了切削过程及加工表面各评价指标与多轴铣削工艺参数间的多项式回归方程,进一步采用遗传算法,构建了目标函数,采用Matlab软件计算出最理想的目标函数值对应的工艺参数组合,可辅助响应曲面法优化工艺方案指导实际加工。理想度最高的多目标协调响应曲面优化工艺方案与最理想的遗传算法优化方案吻合。构建了典型曲面三维几何模型,结合加工实际,依据工艺优化方案,制定了合理的加工策略；借助CAM软件UG生成刀具路径,依据刀轴倾角研究结论,优化控制刀轴矢量,合理控制刀具的参切刃段；进行了典型曲面的加工验证,检测结果表明：样件各区域的加工表面纹理规整、粗糙度较小,且采用推荐的刀具倾角组合易产生较大的硬度和较大的残余压应力,具有较理想的表面完整性。更多还原

【Abstract】 With the increasing of products complexity of manufacturing fields related to mold, automotive, aerospace and astrospace, energy and power equipment, more and more applications of complex curved surface appear, and higher request for the processing technology of some key components is put forward. Due to the multi-axis numerical control machining technology owns wide machining range, high efficiency and flexibility comparing to the traditional processing technology, the multi-axis machining technology is more and more commonly applied in the field of high-end manufacturing. The cutter-axis vector could be flexiblly controlled, and optimal engaged cutting edges could be selected by optimizing the contact point position and relative space position between cuttter and workpiece. Thus, the machining quality, efficiency and precision could be improved. Multi-axis milling technology is a typical representative of multi-axis numerical control machining. Machined surface integrity plays an important role in the performance, life and reliability of the components with complex surfaces. This thesis concentrated on the machining process, the machined surface integrity and process optimization for multi-axis high speed milling operation, and this research has important theoretical and applied values.The three dimension geometric model of chip of the multi-axis ball end milling operatioin was constructed by CAD modeling technology, and variation of the cutter-workpiece contact zone corresponding to various kinds of inclination angles in multi-axis ball end milling was studied. Furthermore, the variations of engaged rotation angles range with varying various kinds of tool inclination angles under up milling condition were analyzed. Considering the cutting section area, perimeter and engaged rotation angles range of cutting tool, the cutting conditions corresponding to negative tilt angle, positive lead angle, or compound inclination angles with negative tilt angle and positive lead angle are ideal. Comprehensively considering the variations of cutter-workpiece contact zone, the sectional cutting area and perimeter of the uncut chip and the engaged rotation angles range of cutting tool with the increasing of various tool inclination angles (tilt angle, lead angle and compound inclination angles), process the maximum cutting force and average cutting force of cutting with special requirements, the cutting conditions of negative tilt angle, positive lead angle, or compound inclination angles with negative tilt angle and positive lead angle could be chosen to to improve the tool life, or to satisfy the special request for the maximum and mean cutting forces during the cutting process. The spacial motion trail was mathematically described, and swept trajectory and swept surface of cutter flutes in the multi-axis high speed ball end milling were obtained. The finite element simulation model was constructed based on the three dimension modeling of ball end mill cutter, and the variations of the cutter-work contact line for different inclination angles with the increasing tool rotation angle were investigated. The experimental cutting forces and simulated cutting forces under the particular tool inclination angle were compared, and the similarities and differences of chip formation were analyzed. The maximum instaneous chip temperature corresponding to various tool inclination angles was extracted. The research shows that the maximum chip temperature increases with the increasing of tilt angle, which is mainly due to the effective cutting speed of the engaged cutting edge increases when tilt angle increases. The overall trend of the maximum chip temperature corresponding to various lead angles did not present significantly different. The chip morphology and color under the roughing, semi-finishing and finish machining condition were analyzed. The corresponding chip micro morphological characteristics were discussed, and also the element distributions were detected in the chip free form surface and back surface. Due to severe deformation and intense friction action, more cutting heat was produced during the chip formation process, which promotes generate more iron oxide presenting red color. Therefore, the chip presents red under roughing and semi-finishing cutting condition. The generation of the thermal and mechanical coupling effect of the instantaneous shear zone would affect the final chip morphology. The existence of the cutting heat would promote the chemical reaction between the chip and the active gas in the surrounding environment.Based on orthogonal cutting theory, oblique cutting, matrix transformation knowledge and analysis of the ball end milling cutter geometric feature, the variations of shear stress and temperature of the micro engaged cutting elements in the primary cutting zone with the varying time and space position were discussed for the multi-axis ball end milling operation, and the cutter-workpiece contact zone and the engaged cuting conditon for cutting edge were analyzed. Furtherly, the cutting forces in three directions were simulatd and verified. Three dimension finite element simulation model for multi-axis high speed ball end milling is established, and the variations of the maximum chip temperature for various tool inclination angles with the increasing tool rotation angle were analyzed. The energy consumption and torque corresponding to various tool inclination angles were compared with each other. The spatial distributions of workpiece temperature field at the moment when the chip separating from the workpiece under different tool inclination angles were discussed.The effective cutting speed of the engaged cutting edges for various kinds of the multi-axis ball end milling process adopting various tool inclination angles are discussed, and the influence of different inclination angles and cutting parameters on the machined surface scallop heights, roughness, macro and micro surface textures and topographies, and surface damages were investigated. Machining characteristic, machined surface scallop height histogram distribution and surface bearing ratio were mainly analyzed for the multi-axis ball end milling under up milling. The research shows that, for up milling, the effectively cutting radius and cutting speed increases with the absolute value of negative tilt angle. The cutting speed of the upper limit firstly decreases and then increases with the increasing of positive tilt angle, while the cutting speed of the lower limit gradually increases with the increasing of positive tilt angle. The effective cutting speed for other cutting conditions could be determined according to the analysis for varying tilt angles in multi-axis ball end milling process under up milling condition. The machined surface textures of multi-axis high speed ball end milling are directly determined by the direction of the cutting speed direction of a series of engaged cutting elements close to the final formed surface, and the machining surface damages mainly inlcude convex trails, concave pits, micro cracks and corrosive pitting. The two-dimensional contour fluctuations of the machined surface of the multi-axis high speed ball end milling under down milling condition were analyzed, Ideally, the scallop height along feed direction is only related with the cutter radius and width of cut.The influence law of process parameters on the machined surface residual stress, surface hardness and the metamorphic layer surface were studied in the multi-axis high speed ball end milling operations. For up milling or down milling condition for multi-axis ball end milling, surface compressive residual stress are easily produced in the cutting conditions with tilt or lead angle nearby0°, which is mainly due to the squeezing effect of the engaged cutting edges close to the tool tip and low effective cutting speed. Considering the machined surface geometry features under all kinds of cutting condition with various tool inclination angles, according to conventional milling processing conditions, inclination angle combinations corresponding cutter axis rotation angle210°and330°could be preferentially chosen to obtain compressive residual stress state in both feed and cross-feed direction for up milling contion, which is helpful for the enhancement of fatigue resistance performance of the component. If suitable down milling conditions need to be adopted and there are no strict requirements for the machinied surface residual stress state, cutter axis rotation angle22.5°and315°could be adopted, and usually inclination angles combinations of tilt angle and positive lead angle are the optimal selection. For up milling condition, the surface hardness under various tilt angle are greater than the corresponding lead angle, and the variation of surface hardness does not change significantly with the increasing lead angle. For down milling condition, variations of the surface hardness with tilt angle and lead angle approximatly present the overall trend of bimodal shape. The surface hardness regarding the cutting conditions with tool inclination angle combinations is analyzed as follows. Comprehensively considering the magnitude of hardness and microhardness under up milling processing, the machined surface hardness under cutter axis rotation angle60°,120°, and210°are larger, and surface hardness under cutter axis rotation angle180°is small. Under down milling condition, surface hardness firstly increases, and decreases with the increasing cutter axis rotation angle, generally showed a trend of reducing the increase. Surface hardness under the corresponding inclination angle combinations near315°cutter axis rotation angle is larger. The phenomenon such as material organization densification will be produced by the combination of cutting heat and mechanical load under multi-axis high speed milling condition. Iron content increases dramatically at the top surface layer, and certain oxygen content is also detected in the surface layer materials.Based on the theoretical analysis, geometric modeling, finite element simulation and experimental detection and analysis for multi-axis high speed ball end milling, the response surface experiments under up milling and down milling condition were designed and implemented. Considering the interaction between various process parameters, influence of the various process parameters on the machined surface ingegrity indexs were analyzed, and the optimization plans corresponding to various target constraints were refined. The polynomial regression equations between the evaluation indexs of cutting process and the machined surface and the process parameters were obtained for multi-axis ball end milling. Furtherly resorting to genetic algorithm, the objective function was constructed, and the optimal process parameters combination could be calculated using Matlab software, which could assist the response surface optimization plans to guide the practical machining process. The process optimization plans using multi-objective coordinated response surface optimization method with the highest desirability value coincides with the ones arised from ideal genetic algorithm optimal objective function values. Comprehensively considering the specific applications requirements and recommended process optimization schemes, the reasonable processing strategies to guide the practical machining could be formulated. The three dimension geometric model of typical curved surface was constructed. Combined the practical machining operation and the process optimization scheme, the reasonable processing strategy were formulated. The tool paths were generated by using UG CAM software. According to the research results with regard to the cutter axis vector optimization control, the engaged cutting edge of the cutting tools could be reasonably chosen. The machining verification of the typical curved surface and detection of specific regions in the machined surface were carried out. The orderly surface textures and small surface roughness could be obtained, and larger surface hardness and larger compressive residual stress are probably induced after machining operation. Finally, the ideal comprehensive surface integrity could be achieved.更多还原