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# 钛合金薄壁构件激光冲击残余应力稳定性研究

1 空军工程大学等离子体动力学重点实验室 西安 710038
2 华东理工大学机械与动力工程学院 上海 200237

# Study on Stability of Residual Stress Induced by Laser Shock Processing in Titanium Alloy Thin-Components

HE Weifeng1, LI Xiang1, NIE Xiangfan12, LI Yinghong1, LUO Sihai1

1 Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, Xi'an 710038, China
2 School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, China

Abstract

Because the compressor thin-blades of aero-engine often fractured in service, laser shock processing was suggested to be applied as a surface strengthening technology. Aim at the problem of compressive residual stress relaxation in laser-peened compressor thin-blades, TC11 titanium alloy thin-components were treated by laser shock processing and then conducted in axial tensile-tensile fatigue test and thermal insulation in vacuum. X-ray diffraction tests were carried out to obtain the relaxation rules of residual stress under fatigue loading and thermal stress loading. In addition, the relaxation mechanisms of residual stress were indicated. Experiment results demonstrate that surface compressive residual stress relaxes by 53%, and 95% of stress relaxation occurs in the previous 5 fatigue cycles under the fatigue loading (maximum stress σmax=500 MPa, stress ratio R=0.1). The surface relaxation degree and severely-relaxed depth increase with fatigue loading, and the relaxation mechanism is that plastic deformation of local area material results in residual stress redistribution. Surface compressive residual stress relaxes by 3%, 29% and 48% respectively after thermal insulation for 120 min under the constant temperature of 200 ℃, 300 ℃ and 400 ℃. Surface compressive residual stress relaxes by 18% and 58% respectively after thermal insulation for 120 min under the altering temperature of 200 ℃+400 ℃ and 300 ℃+400 ℃. The relaxation all occurs in the previous 60 min. There is a similar trend with temperature in the aspect of severely-relaxed depth. The relaxation mechanism under thermal stress loading is that dislocations and grain-boundaries are activated to move and annihilated, and then plastic deformation recovery occurs. Due to the distinction of relaxation mechanisms, there is an obvious superimposed effect under the combined action of fatigue loading and thermal stress loading.

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HE Weifeng, LI Xiang, NIE Xiangfan, LI Yinghong, LUO Sihai. Study on Stability of Residual Stress Induced by Laser Shock Processing in Titanium Alloy Thin-Components[J]. Acta Metallurgica Sinica, 2018, 54(3): 411-418 https://doi.org/10.11900/0412.1961.2017.00135

## 1 实验方法

### 1.1 实验材料

TC11钛合金原材料由中航发西安航空动力股份有限公司提供,依据某型涡扇发动机压气机叶片的热处理工艺和加工工序进行处理,其化学成分(质量分数,%)为：Al 5.8~7.0, Mo 2.8~3.8, Zr 0.8~2.0, Si 0.2~0.35, Fe 0.25, Ti余量;热处理制度双重退火：950~980 ℃、1~2 h、空冷;530 ℃、6 h、空冷。室温拉伸性能为：抗拉强度σb=1030 MPa、屈服强度σ0.2=930 MPa、断面收缩率ψ=30%、延伸率δ=9%。根据GB 26076-2010标准,经过线切割、打磨、抛光等工序,将TC11钛合金制作成轴向疲劳实验标准试件,如图1所示。

### 1.2 激光冲击强化实验

Fig.1   Schematics of dimensions and laser shock processing sketch of TC11 titanium alloy thin-component

### 1.3 应力测试

Fig.2   Programs of thermal insulation treatment and residual stress test under constant temperature thermal insulation (a), altering thermal insulation between 200 ℃ and 400 ℃ (b), and altering thermal insulation between 300 ℃ and 400 ℃ (c)

## 2 实验结果与分析

### 2.1 疲劳载荷下的应力松弛

$σrseff=σrs-σrsrelax$(1)

Fig.3   Relaxation curves of surface residual stress during the previous 5 cyc (a) and during the latter cycles (b) (N—fatigue cycle)

$σwork=σftensile+σrsbalance$(2)

Fig.4   Residual stress distribution curves on section before and after 10000 cyc (d—the depth of test point)

### 2.2 热应力载荷下应力松弛

Fig.6   Residual stress distribution curves under different insulation temperatures in surface (a) and on section (b) (t—insulation time)

Fig.7   Residual stress distribution curves under different insulation temperatures in surface (a) and on section (b)

Fig.8   TEM images of surface microstructure in TC11 titanium alloy samples before (a) and after (b) thermal insulation at 350 ℃ for 10 h and the SAED patterns (insets)

### 2.3 疲劳载荷与热应力载荷复合作用下的应力松弛

TC11钛合金激光冲击薄壁试件在400 ℃保温120 min后进行轴向拉-拉疲劳实验(σmax=500 MPa,R=0.1),图9为不同载荷状态下截面残余应力分布曲线。由图可知,表面残余压应力仅在疲劳载荷作用后由强化状态540.5 MPa松弛至253.3 MPa,仅在400 ℃下保温120 min后松弛至279.0 MPa,但在先保温再疲劳后松弛至142.9 MPa,松弛率达到74%,且截面严重松弛深度由0.6~0.7 mm提高至1.0 mm,说明残余压应力在疲劳载荷与热应力载荷复合作用下应力松弛呈现出明显的叠加效应。2种载荷条件下应力松弛之所以呈现出叠加效应,是因为不同载荷作用下应力松弛机理不同,在400 ℃下保温120 min时残余压应力会由于位错、晶界等组织结构的运动和消亡而发生松弛;但在后续疲劳过程中,只要 $σwork$超过材料屈服强度仍旧可以造成塑性变形而使应力进一步松弛。

## 3 结论

(1) 10000 cyc (σmax=500 MPa,R=0.1)下表面残余压应力由540.5 MPa松弛至253.3 MPa,松弛率为53%,其中前5 cyc占了95%,其松弛机理是叠加的实际工作拉应力导致局部材料发生塑性变形而引起应力场重新分布。

(2) σmax为300和700 MPa时,表面残余压应力分别为350.5和214.8 MPa,截面严重松弛区域深度分别为0.5和1.0 mm,表明随着疲劳载荷增大,表面和截面上的应力松弛程度都会增大。

(3) 在200、300和400 ℃下保温120 min后,表面残余压应力松弛率分别为3%、29%和48%,且松弛主要发生在前60 min内,其松弛机理是热应力激活位错、晶界等结构的运动和消亡而引起塑性回复,其稳定临界温度在200~300 ℃之间。

(4) 200 ℃+400 ℃间交变保温120 min后应力松弛了18%,而300 ℃+400 ℃间交变保温120 min下应力松弛了58%,且严重松弛深度也随之增大,这是因为最低温度提升至300 ℃后进一步提高了位错、晶界等结构运动、消亡的时间和强度。

(5) 400 ℃下保温120 min后再进行疲劳后松弛率达74%,且深1.0 mm内都发生了严重松弛,说明由于应力松弛机理不同,疲劳载荷与热应力载荷复合作用下应力松弛呈现出叠加效应。

The authors have declared that no competing interests exist.

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