表面粗糙度对Ti-6Al-4V合金超高周疲劳性能的影响

EFFECT OF SURFACE ROUGHNESS ON VERY HIGH CYCLE FATIGUE BEHAVIOR OF Ti-6Al-4V ALLOY

采用自行研制的超声疲劳实验装置,研究不同表面粗糙度下Ti-6Al-4V合金的超高周疲劳性能.结果表明,当表面凹痕宽深比a/c（a为凹痕宽度,c为凹痕深度）在2～10之间时,Ti-6Al-4V合金的临界凹痕深度在0.49～1.10μm之间.当表面凹痕深度小于临界深度时,表面粗糙度对Ti-6Al-4V合金的超高周疲劳性能没有影响.当表面凹痕深度大于临界深度时,Ti-6Al-4V合金疲劳寿命随表面粗糙度的增加而下降,并且随着循环周次的增加,Ti-6Al-4V合金疲劳性能对表面粗糙度的敏感性下降.随着表面粗糙度的增加,Ti-6Al-4V合金超高周疲劳裂纹的萌生方式发生变化.超高周疲劳裂纹源有由一个向多个、由内部向次表面转移的趋势;当表面凹痕深度增加到一定程度后,在超高周疲劳寿命区间,Ti-6Al-4V合金疲劳寿命随粗糙度的增加而大幅下降.疲劳裂纹全部从合金表面凹痕根部处萌生,没有内部萌生的情况.

Ti-6Al-4V alloys are widely used in aero engine blades for their unique properties, such as high specific strength, high specific stiffness and high fatigue strength. Aero engine blades usually suffer a variety of cyclic loading during the period of services, which finally results in fatigue failure. Fatigue life of materials is known to highly depend on the surface quality. Consequently, more and more researches about the influence of machined surface roughness on the fatigue behavior of materials have been carried out in the last decades. However, there are less relevant results about the relationship between surface roughness and very high cycle fatigue（VHCF） properties of Ti-6Al-4V alloy. To investigate the effects of surface roughness on fatigue properties of Ti-6Al-4V alloy under very high cycle fatigue regimes, ultrasonic fatigue tests were conducted at the conditions of 20 k Hz and stress ratio R1=- 1 at room temperature in air. During ultrasonic fatigue testing, each specimen was water- cooled. The specimen surfaces were cut and grinded which gave different surface roughnesses. The surface roughness was characterized using profilometry. In order to explain the high dependence of stress- fatigue life curves on the surface roughness, an approach based on the finite element analysis of measured surface topography was proposed. The results show that the VHCF property of Ti-6Al-4V alloy was significantly affected by surface roughness. The critical flaw size was 0.49～1.10 mm when the ratio between spacing and height of circumferential grooves was between 2～10. When surface roughness was smaller than the critical flaw size, surface roughness exerted no influence on fatigue life. While surface roughness was greater than critical flaw size, fatigue life decreased with increasing surface roughness. Surface roughness played a more important role in long life regime than that in VHCF regime in which with the growth of surface roughness, the crack initiation site changed from single one to two or more ones, as well as changed from inside to subsurface. When the surface roughness was large enough, all cracks initiated from surface even in super long life regime.