喷丸强化DD6单晶合金低周疲劳寿命预测

Low Cycle Fatigue Life Prediction of DD6 Single Crystal Superalloy by Shot Peening

目的实现喷丸强化后DD6单晶合金低周疲劳寿命的准确预测。方法开展了喷丸强化后DD6圆棒件低周疲劳试验,分析了喷丸强化对单晶合金疲劳寿命的影响机理。在此基础上,建立了各向异性材料喷丸强化工艺有限元模型,获取了喷丸强化所致残余应力分布与粗糙度。基于连续介质损伤力学,考虑残余应力与粗糙度对低周疲劳寿命的影响,建立了喷丸强化DD6单晶合金低周疲劳寿命预测模型。结果喷丸强化后不同载荷下DD6单晶合金的低周疲劳寿命均得到提高,最大可提高108%;高温环境下残余应力松弛导致强化效果与试验温度成反比。喷丸强化工艺有限元模拟得到残余应力分布在试件表面深约130μm的区域,表层残余应力为–380.16 MPa,应力集中系数为1.193,残余应力影响下的八面体Schmid应力幅值降低了10%左右。DD6低周疲劳试验结果在预测结果的2倍分散带以内。结论喷丸强化可以有效提高DD6低周疲劳寿命,对低周疲劳寿命的影响机制为残余应力的引入与粗糙度的改变。所建立的喷丸强化单晶合金DD6低周疲劳寿命预测模型具有较好的准确性。

In order to achieve accurate prediction of low cycle fatigue life of nickel-based single crystal superalloy DD6 after shot peening,low cycle fatigue experiments on DD6 round bar parts after shot peening were carried out,and the mechanism of the effect of shot peening on fatigue life of single crystal superalloy was analyzed.On this basis,a finite element model of the shot peening process for anisotropic materials was established to obtain the residual stress distribution and roughness due to shot peening.Based on the damage mechanics of continuous media,the low cycle fatigue life prediction model of shot peening nickel-based single crystal superalloy DD6 was established considering the influence of residual stress and roughness on the low cycle fatigue life,which was used to predict the low cycle fatigue experiment results.The surface roughness of the specimens increased after shot peening,and the average surface roughness after shot peening was 5.10 times of that before shot peening.The original matrix phase of DD6 single crystal alloy before shot peening was good,and the strengthened phase showed a good cubic structure;however,the boundary of the two phases gradually disappeared after shot peening due to the violent plastic deformation of the specimen surface.The low cycle fatigue life of DD6 single crystal alloy under different loads was improved after shot peening,and the maximum increase was 108%.The residual stress relaxation in the high-temperature environment caused the peening effect to be inversely proportional to the experiment temperature.Due to the effect of residual stress,most of the fatigue cracks initiated on the subsurface of the specimens,while a few initiated at the location of deeper craters on the surface.Considering that the single crystal alloys are anisotropic materials,Hill anisotropic yielding criterion was used to simulate the process of the single crystal superalloy shot peening.The finite element simulation of the shot peening process yielded residual stress distribution in a region 130μm deep on the surface of the specimen.The surface residual stress was‒380.16 MPa,and the maximum residual compressive stress was located at the depth of 41.9μm,reaching‒780.46 MPa.The stress concentration factor was calculated as 1.193 by equating the uneven craters to semicircular notches.The residual stress introduced by the eigenstrain method in the finite element model was generally the same as the simulation result.The residual stress mainly affected the stress conditions in the surface layer of the specimens,resulting in an about 10%decrease of the surface octahedral Schmid stress amplitude.The DD6 low-cycle fatigue experiment results were within twice of the predicted life.Conclusions indicate that the shot peening can effectively improve the low cycle fatigue life of DD6,and the mechanism of influence on the low cycle fatigue life is the introduction of residual stress and the change of roughness.The finite element model can accurately simulate the single crystal alloy shot peening process.The developed model for predicting the low-cycle fatigue life of shot peening nickel-based single crystal superalloy DD6 has good accuracy.