【作者】 许鸿昊；

【导师】 王珉； 左敦稳；

【作者基本信息】 南京航空航天大学 ， 机械制造及其自动化， 2008， 博士

【摘要】 随着高速加工技术的发展,钛合金的高速铣削技术已经在实际生产中获得应用。但是人们对钛合金高速铣削件的疲劳特性缺乏深入的了解,影响了高速铣削技术的优势在航空工业领域中得到进一步发挥。本文基于拉伸装夹高速铣削的抗疲劳加工新思路,重点研究拉伸装夹条件下TC4钛合金铣削工件的表面残余应力及其疲劳特性。采用自行设计的单向拉伸夹具,在HSM-600U型五轴高速加工中心上开展了TC4钛合金拉伸装夹铣削(Stretching-fixation Milling, SFM)的试验研究,铣削速度从38 m/min到566 m/min,拉伸装夹力从0到6330 N。采用Mahr Sp3表面粗糙度仪对铣削表面的微观几何形貌进行了测量分析,使用MSF-3M型X射线应力分析仪分别测量了铣削表面内与拉伸方向呈0°、30°、90°和120°角的四个方向上的残余应力,并结合电解抛光方法进一步测量了切削方向上铣削表面以下50μm深度内的残余应力分布。基于平面应变的热弹塑性理论分析了SFM过程中应力应变的变化过程,推导了拉伸装夹引起的初始应变和残余应力变化量之间的关系,并提出应变叠加模型,认为拉伸装夹产生的初始应变影响了铣削工件表层的塑性应变,从而使残余应力发生变化。使用有限元软件ANSYS对SFM过程进行仿真,把整个拉伸装夹铣削过程分为装夹施加、金属切削和装夹卸除三部分,分别采用不同的积分方式进行求解。最后,对SFM件进行了高频低周疲劳实验,建立疲劳裂纹监测系统对疲劳裂纹扩展的整个过程进行了在位停机观测,并使用JSM-7001F型场发射扫描电镜观察了疲劳断口。研究结果表明:在每转进给量保持不变的前提下表面粗糙度主要由高速旋转时刀具的动平衡偏心量决定,铣削速度和铣削装夹方式基本不影响表面粗糙度;拉伸装夹铣削工件表层残余压应力的增大与否取决于是否形成初始拉应变,而后者和方向相关;有限元仿真对拉伸装夹铣削工件表层残余应力分布的预测和实测结果基本一致;拉伸装夹铣削可以形成更有利的残余压应力层,对滑移带的形成和疲劳裂纹的萌生产生有效的抑制;与常规铣削试样相比,拉伸装夹铣削试样的疲劳寿命可提高8%~16%,疲劳源区位于铣削表面以下更深处。更多还原

【Abstract】 With the development of high-speed machining technology, the high-speed milling has been applied in Ti alloy machining in practice. However, the fatigue characteristic of Ti alloy workpiece high-speed milled remains unclear, and this makes the high-speed milling technology difficult to further play to its advantages in aviation industry. In this paper, a new approach to anti-fatigue manufacture based on high-speed milling under stretching fixation was proposed, and the residual stresses and fatigue characteristics of TC4 Ti alloy workpiece milled under stretching fixation were studied in detail.Stretching-fixation milling (SFM) of TC4 Ti alloy was performed on five axis high speed machining center of Mikron HSM-600U with unidirectional stretching fixture designed independently,at milling speed of 38~566 m/min and stretching force of 0~6330 N. The surface roughness of the milling surface was measured, and the surface profile was analyzed with Mahr Sp3 surface roughness instrument. The residual stresses in the milling surface were measured at four directions of 0, 30, 90 and 120 degrees from the stretching direction respectively with MSF-3M X-ray stress analyzer, and the residual stress distribution 50μm below milling surface was studied by combining X-ray diffraction and electro-polishing techniques. Moreover, the thermoelastoplastic theory of plane strain was adopted to analyze the variations of stress and strain in the SFM process and to reveal the relationship between the stretching strain and the residual stress. Then, a strain superposition model was proposed, which recognized that the initial strain produced by stretching fixation changed the plastic strain, thereby the residual stress in the milling surface layer. By using general FEA software ANSYS, a finite element model of SFM was developed, in which the entire SFM process was divided into three parts, namely fixation loading, metal cutting and fixation unloading, and each part was solved by using different integral method respectively. Finally, low cycle fatigue test of SFM specimen under high frequency was carried out on PLG-100C high-frequency fatigue testing machine, and during periodical shutdown of the machine a fatigue crack monitoring system designed independently was used to in-situ observe the fatigue crack, while the fatigue fracture section was observed under JSM-7001F field-emission scanning electron microscope.From the results obtained, it is made clear as follows. When the feed per revolution remains unchanged, the micro fluctuation of milled surface is mainly attributed to the dynamic imbalance of high-speed rotary tool, while is little affected by milling speed and stretching force. The variation of the residual stress in SFM surface layer depends on the value of initial strain produced by stretching fixation, and the latter is different in each direction. The residual stress distribution simulated by FEA shows good agreement with the experiment result. SFM produces a more favorable residual compressive stress layer, and delays the formation of slip band and the initiation of fatigue crack effectively. Consequently, stretching fixation in high speed milling causes an 8%~16% growth of fatigue life, and fatigue crack of SFM specimen initiates at deeper layer from the surface.更多还原