GH80A镍合金电子束焊接接头旋转弯曲高周疲劳行为研究

随着镍合金电子束焊接在工业中的大量应用，尤其是在航空发动机和燃气轮机等关键长寿命服役设备中的使用，有必要对镍合金电子束焊接接头的高周疲劳属性和断裂机理进行系统的分析研究。本文利用旋转弯曲高周疲劳试验机进行疲劳试验，获得了母材和焊接接头的应力-寿命(S-N)曲线和疲劳断口，同时利用扫描电镜（Scanning Electron Microscope,SEM）对疲劳断口进行了微观特征分析，确定了母材和焊接接头在不同应力幅下的疲劳裂纹萌生区和扩展区，分析了裂纹萌生区位置与应力幅的相互关系。最后，利用有限元分析了焊接接头热影响区微裂纹位置与大小对材料疲劳性能的影响。从现有的试验和模拟结果可以得到：1)母材和电子束焊接接头应力-寿命(S-N)曲线分布趋势一致，但焊接接头疲劳强度要低于母材，在靠近107周次时，两者疲劳强度差距最小；2)在高应力幅（低周疲劳寿命阶段）母材和焊接接头的疲劳裂纹均起源于试件表面并且都是多点萌生断裂，焊接接头疲劳断口位置位于焊接熔合区或热影响区；3)在低应力幅（高周疲劳寿命阶段）疲劳裂纹在试件次表面萌生，焊接接头疲劳断口位于热影响区或焊接母材靠近热影响区处；4) 通过有限元模拟发现微裂纹的存在有利于裂纹的扩展。在拉应力作用下，横向微裂纹更优于纵向裂纹沿着应力方向进行裂纹扩展；随着微裂纹尺寸增大，微裂纹间更易于相互贯通，形成更长的裂纹，从而降低了材料的疲劳性能。综上可知，电子束焊接仅仅影响材料的疲劳强度。疲劳断裂机理和母材一致都为穿晶解理断裂，疲劳裂纹萌生区域位置也和母材一样都受应力幅的直接影响。

Research on Rotary Bending High Cycle Fatigue Behavior of the Electron Beam Welding Joint for GH80A Nickel Alloy

With the application of nickel alloy electron beam welding in industry, especially in critical long service equipment such as aircraft engine and gas turbines etc., it is necessary to systematically analyze and study the high cycle fatigue properties and fracture mechanism of nickel alloy electron beam welding joint. Fatigue tests were carried out using the rotary bending high cycle fatigue testing machine. The stress-life (S-N) curves and fatigue fracture of the base metal and welded joints were obtained from experiment. Then, the fatigue fracture surfaces were examined by scanning electron microscope (SEM). The fatigue crack initiation and extension zones of the base metal and welded joints under different stress amplitude were analyzed and determined, which the crack initiation location was affected by the stress amplitude. Finally, the effect of the location and size of microcracks on the fatigue properties of the welded joint was discussed by the finite element method (FEM). The experimental and simulation results show that: 1) the stress-life (S-N) curves distribution trend of the base metal and welded joints is consistent, and fatigue strength of the welded joint is lower than the base metal, which there is small difference nearby the 107 cycle; 2) with the high stress amplitude (low cycle fatigue life stage), fatigue crack initiates from multiple point at the surface for base metal and welded joints. The fracture position of welded joints is located in the weld fusion zone and heat affected zone; 3) fatigue crack initiation transforms from surface to subsurface in the low stress amplitude (high cycle fatigue stage). The fatigue fracture of the welded joints is located in the heat affected zone and base metal near the heat affected zone; 4) The existence of microcracks is beneficial to the propagation of crack by FEM. Under the tensile stress, the transverse microcracks are better than the longitudinal crack to propagate along the stress direction. With the increase of the microcrack size, the microcracks are easier to cross each other and form longer cracks, to reduce the fatigue performance. The results also show that the electron beam welding only affects the fatigue strength of the material, the fatigue failure property and the fatigue crack initiation position is the transcrystalline cleavage fracture and directly affected by the stress amplitude, respectively, which is consistent with the base metal.