表面完整性对钛合金弯扭疲劳寿命的影响

Effect of surface integrity on bending and torsional fatigue life of titanium alloys

摘要:

为提高某型车辆承载轴在弯扭载荷下的疲劳寿命,针对承载轴材料TC11钛合金,建立了包含表面粗糙度与表层残余应力的微观有限元模型,基于临界平面法建立TC11弯扭疲劳寿命预测模型,研究表面粗糙度与轴向/周向残余应力对疲劳寿命的影响规律；分别使用车削与超声滚压工艺加工疲劳试样,对比两种工艺下试样的表面完整性,进行疲劳实验,验证预测模型准确性,并通过分析断口形貌研究断裂机理。研究表明:表面粗糙度和轴向残余压应力对寿命的影响显著,随着表面粗糙度S_a由1.6 μm下降至0.4 μm,寿命提高了135%；轴向残余压应力由100 MPa增大至400 MPa,寿命提高了123%,周向残余应力的影响较小。滚压试样具有更低的表面粗糙度、更高的加工硬化程度以及更高的表层轴向/周向残余压应力,疲劳寿命更高,建立的寿命预测模型可在25%的误差范围带内预测TC11弯扭寿命,为工艺优化提供理论依据。滚压后试样断口疲劳辉纹间距由0.8~0.9 μm减小至0.3~0.4 μm,裂纹扩展速率降低,在加工影响层主要断裂形式由沿晶断裂转变为穿晶断裂。

Abstract:

To improve the fatigue life of a vehicle bearing shaft under bending-torsion loading a microscopic finite element model containing surface roughness and surface residual stress was established for the bearing shaft material TC11 titanium alloy. A bending-torsion fatigue life prediction model of TC11 was established based on the critical plane method to study the effects of surface roughness and axial/circumferential residual stress on the fatigue life. The fatigue samples were processed by turning and ultrasonic rolling respectively, and the surface integrity of the samples was compared under both processes. Fatigue tests were carried out to verify the accuracy of the prediction model, and the fracture mechanism was studied by analyzing the fracture morphology. The results show that the influence of surface roughness and axial residual compressive stress on life is significant. As the surface roughness S_a decreases from 1.6 μm to 0.4 μm, the life is increased by 135%, and the axial residual compressive stress increases from 100 MPa to 400 MPa, the life is increased by 123%, and the influence of circumferential residual stress is small. The rolling specimens have lower surface roughness, higher work-hardening degree, higher axial/circumferential residual compressive stress, and higher fatigue life. The SWT model can accurately predict the bending and torsional life of TC11 within the 25% error scatter band, which provides a theoretical basis for process optimization. After rolling, the fatigue grain spacing decreases from 0.8-0.9 μm to 0.3-0.4 μm, the crack propagation rate slows down, and the main fracture mode transitions from intergranular fracture to transgranular fracture in the machining-affected layer.