【作者】 皇攀凌；

【导师】 李剑峰；

【作者基本信息】 山东大学 ， 机械制造及其自动化， 2011， 博士

【摘要】 钛合金以其优异的综合性能广泛应用于航空航天领域,但是由于它具有导热系数低、高温化学活性高和弹性模量小等特点,因此是一种典型的难加工材料。在切削过程中,大幅度的振动导致刀具磨损加快、表面质量难以控制、切削速度难以提高。同时,由于航空钛合金零部件的结构设计特点,大量的材料需要从整块坯料中去除,急需较高的加工效率。因此,效率和质量之间的矛盾成为制约航空航天制造业发展的瓶颈之一。为了解决航空钛合金易产生振动的问题,本课题以Ti-6A1-4V为加工对象,借助于理论分析和试验研究,对具有减振性能的变齿距铣刀减振机理和结构进行系统、深入地研究,该研究对于提供变齿距铣刀结构和加工参数优化具有重要的理论指导意义,对于增强我国航空航天业的竞争力具有重大的实用价值。针对钛合金加工易振动,刀具偏心严重等特点,结合变齿距螺旋立铣刀的结构特点,建立了考虑铣刀偏心和铣刀变形影响的变齿距铣刀三维铣削动力学模型。通过铣削力试验结果,用最小二乘法拟合出铣削力系数,并对铣削力进行了仿真。基于铣削力模型分析了齿间角、螺旋角、轴向切深和径向切深对y向铣削力影响,分析了铣刀偏心对离心力的影响。结果表明：从控制y向铣削力的角度来讲,对于钛合金加工应选择较大的螺旋角、小的径向切深和每转进给量。考虑铣削干扰因素影响,建立了变齿距铣刀的铣削动态位移和动态铣削力模型,并根据稳定性条件得到了变齿距铣刀铣削的极限切削深。为了深入研究铣削系统的稳定性,开展了铣削系统的模态试验,根据铣削系统的频响函数得到了铣削系统的模态参数,结合铣削力仿真数据,利用MATLAB软件绘制出铣削系统的颤振稳定性叶瓣图,分析了铣刀结构参数、加工工艺参数及铣削系统模态参数对极限切深的影响,为优化铣刀结构及加工工艺参数提供理论指导。结果表明：选择大的齿间角差、小的径向切宽,大的铣削系统刚度和阻尼有助于增加系统的极限切深,从而有助于增加铣削系统的稳定性区域。为了分析铣削过程的异常行为,提出了基于时域、频域和时频域分析的铣削力和振动加速度信号分析方法。采用干铣削顺铣方式开展了铣削速度从80m/min增加到360m/min的钛合金铣削试验,并利用信号分析方法分析了不同铣削速度下的铣削力及加速度信号。比较了稳定和非稳定状况下铣削力、已加工表面质量的不同,得到振动对铣削加工的影响以及铣削加工的最佳铣削速度,从而在保证铣刀寿命和加工表面质量的前提下,提高铣削效率。实验结果表明：当颤振发生时,铣削力增加了61.9%-66.8%,表面质量降低,其表面粗糙度增加了34.2%-40.5%。其时频域分析即小波分析结果表明：颤振频率处切削力幅值对铣削力的贡献较大,因此应当避免颤振。基于频谱线能量分布理论分析了变齿距铣刀的减振机理。以芯部直径、螺旋角、前角、后角以及齿间角为影响因子,根据正交试验设计方法设计了25把不同结构参数的螺旋立铣刀,借助于有限元软件ABAQUS进行静力学仿真,利用极差分析得到各结构参数对铣刀应力应变的影响规律；在保证铣刀前角、后角及芯部直径恒定的前提下,改变齿间角及变螺旋角进行静态仿真,得到两者对铣刀应力应变的影响规律。基于频谱线能量分布原则,借助于MATLAB软件仿真优化了变齿距铣刀齿间角分布。结果表明：对于四齿铣刀,最优齿间角为83°-97°-83°-97°和87°-93°-87°-93°；对于直径为20mm的铣刀,铣刀芯部直径在11.4mm-13.4mm之间,铣刀前角为7°-11°之间；铣刀后角不超过20°。变螺旋角360-38°时铣刀有较好的刚度。基于变齿距铣刀结构优化分析结果,设计出10种结构的铣刀,进行了钛合金铣削加工试验,分析了因改变铣刀齿间角、螺旋角和增加变槽深结构对铣削加工的性能影响。通过时域、频域和时频域分析了铣刀的结构参数、铣削速度的变化对铣削力与铣刀振动位移的影响,得到了不同参数对铣削性能的影响规律。结果表明：变齿间角对铣刀的抑振效果最好,齿间角差越大,振动位移越小。当铣削速度为120m/min和150m/min时,铣削力激振频率和机床固有频率接近,造成了共振,从而加剧了铣削振动；铣削速度为150m/min时的颤振峰值最大,所有结构的铣刀在此切削速度下均有较大的铣刀振动,推断此振动为共振行为；当铣削速度为180m/min时,铣刀振动位移较小,铣削力也最小,而且其材料去除率最大,因此可推荐为最佳铣削速度。更多还原

【Abstract】 Titanium alloys are extensively used in aerospace industry due to their excellent performance in aerospace environment. However, it is difficult to machine for their poor thermal conductivity, low elastic modulus and high chemical activation. Vibration and high cutting temperature during machining leads to fast wear of tools and gives poor surface quality of the processed parts at high cutting speed. Simultaneously, most materials need to be removed from roughcast due to the characteristics of component design, so machining cost becomes very high when these parts are processed at low cutting speed. These are some of the constraints in the development of aerospace industry. In order to overcome these constraints, vibration characteristics in machining of titanium alloys Ti-6A1-4V are considered in this study. Vibration minimizing mechanism and structure parameters optimization of variable pitch mill which may significantly support in structural optimization of mills are presented.A three dimensional force model for dynamic milling which includes mill eccentricity and deflection is proposed and simulated. Least square method is used for obtaining milling force coefficients by fitting the cutting force data of experiments. Furthermore, the effect of pitch angles, helix angles, the axis depth and the radial depth on milling force is also analyzed; then, the effect of eccentricity on centrifugal force is analyzed. Results show larger helix angle, larger difference value of adjacent pitch angles, small radial depth of cut and small feed are preferred to produce smaller cutting force Fy.Dynamic models of displacement and force for variable pitch and variable cutting depth are considered. The effect of different parameters on the limit cutting depth is also analyzed and stability lobe diagram is obtained from MATLAB software. Results indicate that large difference value of adjacent pitch angles, large stiffness, large damping, small radial depth and small feed are significant to produce higher depth of cut. Milling force in time, frequency and time-frequency domain is also analyzed. Experiment is carried out at dry, downing milling at variable milling speeds from 80m/min to 360m/min. Quality of titanium parts is compared at stable and unstable cutting and milling speed is optimized by the experimental analysis. The experiment results show that when the chatter occurrence, milling forces were found to increase dramatically by 61.9%-66.8% compared with that of at stable cutting, machining surface quality became poor and machined surface roughness increase by 34.2% -40.5% compared with that of at stable cutting.Vibration minimizing mechanism of variable pitch mills is analyzed in frequency domain.25 different types of mills are designed with orthogonal design method. The effects of the core diameter, helix angle, rake angle, clearance angle and pitch angle on the stress and strain are analyzed in FEM simulation software ABAQUS. Pitch angles of variable pitch mills are optimized based on frequency domain in MATLAB. Optimized pitch angles of 83°-97°-83°-97°and 87°-93°-87°-93°are obtained for mills containing 4 cutting edges. Furthermore, the core diameter between 11.4mm and 13.4mm and the rake angle between 7°and 11°for 20mm’diameter mill are found.According to the above analysis 10 types of mills with variable pitch angle, variable helix angle and variable groove depth structure are also designed for high-speed milling experiment. The effects of structural parameters of mills and milling speed on milling force and vibration displacement of mills are compared after time domain, frequency domain and time-frequency domain analysis. At last, the effective laws of structural parameters on cutting performance and the optimal milling speed are also presented. Results show that variable pitch have best vibration minimizing effects, and the vibration displacements is smaller when the difference value of adjacent pitch angles is larger; the resonance was produced because the excitation frequency of cutting force close to the machine natural frequency when milling speed are 120m/min and 150m/min, thereby cutting vibration was increased.更多还原