喷丸对单晶合金中温疲劳性能的强化机制

Strengthening Mechanism of Shot-Peening on Medium-Temperature Fatigue Property of Single-Crystal Superalloy

为研究喷丸对单晶合金中温疲劳性能的强化机制,采用陶瓷弹丸对单晶合金进行喷丸强化,研究了喷丸单晶合金表面形貌、截面微观组织、铸造微孔和650℃旋转弯曲疲劳性能,并观察了疲劳断口。结果表明:喷丸强化后650℃/Kt=1.7和650℃/Kt=3旋转弯曲疲劳极限分别提高21.3%和12.7%。首先,喷丸强化消除了表面尖锐的加工痕迹,形成了圆滑弹坑,将表面应力集中系数由1.44降低到1.38,这是表面形貌优化机制。其次,喷丸单晶合金表面形成最表面的严重塑性形变层–次表面的线型形变层–里层的小塑性形变层–基体梯度塑性形变层。形变层最表面硬度较基体提高42%,是形变强化机制。此外,在交变载荷下,2~10μm的铸造微孔成为疲劳裂纹源;喷丸强化后铸造孔洞闭合或椭圆化,减小了孔洞位置的应力集中,喷丸对铸造孔洞的形变作用是第3个疲劳强化机制。

Ceramic shot were employed to shot-peen the single-crystal superalloy(SX)to analyze the strengthening mechanism of medium-temperature fatigue property.Surface morphology,cross-sectional micro-structure,casting porosities and 650℃rotating-bending fatigue property were researched,and the fatigue fractures were observed.The results show the 650℃/Kt=1.7 and 650℃/Kt=3 rotating-bending fatigue limit increase by 21.3%and 12.7%after shot peening respectively.Firstly,shot peening eliminated sharp machining masks at the bottom and formed a smooth crater,which reduced the surface stress concentration factor from 1.44 to 1.38.This is the surface topography optimization mechanism.Secondly,a gradient plastic deformation layers were formed at the surface by shot peening,composed of the severe deformation layer at the outer-surface,the deformation layer with the“line structure”at the subsurface,the small deformation layer at the inferior surface and the matrix.The micro-hardness of the deformation layer was 42%higher than that of the matrix,which is the deformation strengthening mechanism.Furthermore,the as-received casting porosities with the diameter 2–10μm,could be the fatigue crack source during alternating stress.The micro-holes closed at the outer-surface and ovalized at the subsurface and inferior surface after at shot peening,which reduced the stress concentration at the casting porosities.The deformation effect on the casting micro-holes of shot peening is the third strengthening mechanism..