Study On Fatigue Performance Of Ti Alloy High-speed-milled Under Stretching Fixation

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.