8Cr4Mo4V钢激光冲击残余应力的演化仿真及其对疲劳性能的影响

图1 热处理和LSP示意图

Fig.1 Schematic diagram of the heat treatment and subsequent LSP treatment (a) heat treatment; (b) hardness test and LSP treatment; (c) rotating bending fatigue test sample (unit: mm)

将热处理后的试样线切割成尺寸为15 mm×10 mm×10 mm的块状试样，依次用240^#、600^#、1000^#和2000^#金相砂纸将其表面研磨并抛光，然后用乙醇清洗待强化表面。在表面粘贴100 μm厚度的黑胶带为吸收层，然后用能量稳定且空间分布均匀的LAMBER-08型固体激光器进行激光冲击强化(图1b)。工艺参数为：脉冲能量8 J，脉宽15 ns，方形光斑尺寸4 mm×4 mm，约束层约为1 mm厚度流水，光斑搭接率为50%。为了确定激光冲击强化后产生的残余压应力对疲劳性能的影响，测试激光冲击强化后试样的旋转弯曲疲劳性能，疲劳试样尺寸如图1c所示。

用维氏硬度计测量冲击面由表及里的硬度梯度(图1b)，载荷为10 N，用截面硬度梯度表征冲击强化后试样的强化层深度。用扫描电镜观察经激光冲击强化后试样的微观组织和旋转弯曲疲劳断裂后试样的断口。使用的腐蚀液配比为：100 mL C₂H₅OH+2.5 g (NO₂)₃C₆H₂OH+5 mL HCl，腐蚀时间约30 s。用线切割从LSP处理表面切取厚度约为0.5 mm的试样，将激光冲击强化的表面粘结在工装上，然后用600#~2000#砂纸打磨线切割面以保证激光冲击强化表面为保留层，打磨后样品的厚度约为40 μm。用电解双喷仪(TenuPol-5) 将薄片试样减薄以获取高质量薄区，工作电压25 V，温度-20℃，电解液为10%HClO₄+90%C₂H₅OH。用JEM2100型透射电镜(TEM)观察试样表面的微观组织。

将疲劳试样研磨抛光，以去除试样表面因激光冲击强化产生的损伤层。用X射线残余应力测试仪检测试样的表面残余应力。用QBWP-10000型旋转弯曲疲劳试验机检测旋转弯曲疲劳极限强度，转速为5000 r/min，试样发生疲劳断裂或转动周次达到1.0×10⁷时停止运行，用扫描电镜观察试样的断口。

1.2 建立数学模型

1.2.1 激光冲击波峰值压力

传输到试样上的激光束对其有两种冲击：热冲击和机械冲击，以机械冲击为主。热冲击是靶材表面热膨胀产生的，机械冲击是激光激发的等离子体引起的冲击波产生的。Fabbro^[24]研究约束模型下的冲击波时，假设激光能量均匀分布，在光斑范围内材料表面均匀受热。约束层和靶材皆为各向同性的均匀物质，等离子体为理想气体，等离子体只在轴向膨胀，如图2所示。

图2

图2 等离子体膨胀示意图

Fig.2 Schematic diagram of plasma expansion

激光的功率密度为

I_{0} = \frac{E}{τ S}

(1)

式中E为激光能量(J)；τ为脉宽(ns)；S为光斑面积(cm²)。

在水约束层下冲击波的峰值压力P与激光功率密度I₀的关系为

P = 0.01 \sqrt[]{\frac{α}{2 α + 3}} \sqrt[]{Z} \sqrt[]{I_{0}}

(2)

式中α为内能转化为热能部分的系数(常取0.1~0.3)；Z为靶材和约束层的声折合阻抗，可表示为

\frac{2}{Z} = \frac{1}{Z_{1}} + \frac{1}{Z_{2}}

(3)

式中Z₁=1.6×10⁶g/(cm²∙s)为靶材声阻抗；Z₂=0.165×10⁶g/(cm²∙s)为约束层声阻抗。

1.2.2 激光冲击强化残余应力的形成原理

在激光冲击作用下材料表面产生残余应力场的过程可分成两个阶段：第一阶段为应力加载过程(图3a)。冲击波作用于靶材表面时光斑中心使材料发生塑性变形，光斑内的能量呈高斯分布，处于光斑边缘的应力波压力小于材料的屈服极限，使材料产生弹性变形；在深度方向上由表面至内，随着应力波的衰减表层较浅的部分产生塑性变形，而深层部分产生弹性变形。第二阶段为应力卸载阶段。应力波的作用完成后，冲击面中心和表层较浅的材料发生塑性变形，不能恢复到原来的形态；而处于冲击光斑边缘和次表层的材料发生弹性变形，应力波卸载后有恢复至原来状态的趋势。因此，冲击光斑边缘的材料挤压中心处的材料 (图3b)，产生一个与中心区材料变形方向相反的压力，即产生了一个残余压应力场。

图3

图3 残余应力场形成过程的示意图

Fig.3 Schematic diagram of the formation process of residual stress field (a) loading process; (b) unloading process

当作用在金属材料表面的激光功率密度大于1 GW/cm²时，在激光冲击强化过程中形成近似平面的冲击波，其压力达到GPa量级，为了便于分析将产生的应变简化为一维应变。压力足够大的冲击波，使材料屈服而进入塑性变形阶段。在产生一维应变的冲击压力条件下，材料发生屈服时的正应力

\begin{array}{l} σ_{x} = σ_{H E L} = (1 + \frac{λ}{2 μ}) (σ_{Y}^{d y n} - σ_{0}) = \\ \frac{1 - υ}{1 - 2 υ} (σ_{Y}^{d y n} - σ_{0}) \approx (1.7 ~ 1.9) (σ_{Y}^{d y n} - σ_{0}) \end{array}

(4)

称为Hugoniot弹性极限。式中λ=Eυ/(1+υ)(1-2υ)和μ=E/2(1+υ)为拉曼系数，υ为泊松比，σ₀为表面初始应力(GPa)； $σ_{Y}^{d y n}$ 为材料在高应变率下动态屈服强度(GPa)。

由冲击波峰值压力P和材料Hugoniot弹性极限σ_HEL，可得激光冲击强化金属部件表面塑性变形量

ε_{P} = \frac{- 2 σ_{H E L}}{3 λ + 2 μ} (\frac{P}{σ_{H E L}} - 1)

(5)

当峰值压力P<σ_HEL时材料表面只产生弹性变形，P>σ_HEL时产生塑性变形，有部分弹性恢复。

在材料内传播的激光冲击波逐渐衰减，在材料内P<σ_HEL的深度不再发生塑性变形，此深度为材料的塑性变形深度，即残余压应力的影响层

L_{p} = (\frac{C_{e l} C_{p l} τ}{C_{e l} - C_{p l}}) (\frac{P - σ_{H E L}}{2 σ_{H E L}})

(6)

式中C_el= $\sqrt[]{(λ + 2 μ) / ρ}$ 和C_pl= $\sqrt[]{(λ + 2 μ / 3) / ρ}$ 分别为弹性波和塑性波在材料中的传播速度(mm/s)；ρ为材料密度(g/cm³)。

已知塑性应变ε_P 和残余压应力影响层深度L_P，依据Fabbro模型^[24]激光冲击强化引起的表面残余压应力为

\begin{array}{l} σ_{s u r f} = σ_{0} - [μ ε_{P} (1 + υ) / (1 - υ) + σ_{0}] \\ \cdot [1 - \frac{4 \sqrt[]{2}}{π} (1 + υ) \frac{L_{P}}{a}] \end{array}

(7)

式中 $σ_{0}$ 为表面初始应力值(MPa)；a为方形光斑边长(mm，若为圆形光斑，a可替换为a= $\sqrt[]{2}$ r，r为光斑半径(mm))。

2 结果和分析

2.1 残余应力

使用ABAQUS软件建立有限元模型计算激光冲击强化对8Cr4Mo4V钢残余应力的影响，其单元类型为8节点六面体单元C3D8R(图4a)。将Johnson-Cook(J-C)模型作为材料的本构方程^[24]。试样在激光冲击强化作用下的变形属于高速率应变，J-C模型能较好的模拟材料对高应变率的响应。J-C本构方程可表示为

σ = [A + B ε^{n}] [1 + C l n {\dot{ε}}^{*}] [1 - {(T^{*})}^{m}]

(8)

式中A为屈服应力(MPa)；B为应变硬化系数；n为应变硬化指数；C为应变速率系数；m为热软化系数； ${\dot{ε}}^{*}$ = $\dot{ε} / {\dot{ε}}_{0}$ ， $\dot{ε}$ 为动态压缩应变率； $\dot{ε}$ ₀为参考应变率，T^*=(T-T₀)/(T_m-T₀)为无量纲温度；T₀为室温；T_m为材料熔点。

图4

图4 有限元模型和冲击波加载示意图

Fig.4 Finite element model and shock wave loading (a) mesh division of the finite element model; (b) relationship between shock wave pressure and time; (c) schematic diagram of the spatial distribution of shock wave pressure

根据文献[25]，冲击波压力的作用时间约为脉宽的2~3倍，冲击波压力随加载时间的变化规律如图4b所示。施加激光冲击载荷需要同时考虑冲击波压力的时间和空间分布，冲击波压力振幅与空间位置的关系如图4c所示。用显示动力积分算法对冲击波的动态传播进行数值模拟，动态分析时间需长于冲击波压力持续时间以保证模型内部塑性变形达到饱和且动应力趋于稳定。将弹塑性动态分析结果导入ABAQUS/Standard模块，进行静态应力应变分析以计算静态平衡残余应力场。

激光诱导冲击波与8Cr4Mo4V钢表面相互作用，在表面形成稳定的残余应力场。图5给出了激光冲击强化后8Cr4Mo4V钢的残余应力分布。模拟结果表明，表面残余应力的分布类似于正弦分布，边缘区域的残余应力较小，进入冲击区域后发生明显的变化，表面最大残余压应力达到-607 MPa，如图5a、c所示。在截面方向，随着距表层深度的增大残余压应力逐渐减小，在距表面约0.6 mm处残余应力趋于零，如图5b、d所示。其原因是，冲击波接触材料后动能转化为塑性应变能，随着等离子体冲击波的衰减残余应力随着深度的增加而减小^[26]。

图5

图5 用有限元法计算残余应力的分布

Fig.5 LSP residual stress distribution calculated by finite element method (a) and (c) surface residual stress distribution; (b) and (d) cross sectional residual stress distribution

图6给出了激光冲击强化前后8Cr4Mo4V钢的硬度与测定出的残余应力。图6a给出了激光冲击强化前后试样的截面硬度梯度。可以看出，激光冲击强化前平均硬度约为745 HV1，激光冲击强化后表面硬度提高，最大硬度约为776 HV1，随着深度的增加硬度逐渐降低，表面形成约0.75 mm深的硬化层。图6b给出了用有限元数值模拟计算出的和采用X射线衍射法测定的试样表面的残余应力，可见计算结果与实验数据吻合得较好。有限元法计算的残余应力为-607 MPa，实验结果为-584 MPa，两者相差23 MPa，误差相对于实测值仅为3.94%。

图6

图6 LSP前后8Cr4Mo4V钢的硬度和残余应力

Fig.6 Hardness and residual stress of 8Cr4Mo4V steel before and after LSP (a) hardness gradient before and after LSP; (b) simulation and experimental results of residual stress

2.2 微观组织

图7给出了激光冲击强化前后8Cr4Mo4V钢的微观组织和表面碳化物的统计。图7a给出了热处理后试样表面的微观组织，可见表面有共晶碳化物(EC)、短棒状碳化物(SC)和细丝状碳化物(FC)。激光冲击强化使表面的共晶碳化物碎化(图7b)。抛光去除由激光冲击强化导致的表面损伤层后，可见与原始组织(图7a)相比次表面的二次析出碳化物增多(图7c)。观察激光冲击强化后试样的纵截面，可见截面形成了由表及里约16.35 μm厚的致密组织层(图7d)。使用Image-Pro Plus软件统计图7a、c中碳化物的尺寸及其面积百分数，发现激光冲击强化后碳化物的面积比由10.05%增加到14.00%，碳化物的尺寸由0.34 μm减小到0.31 μm(图7e)。

图7

图7 LSP前后8Cr4Mo4V钢的显微组织SEM像和碳化物统计

Fig.7 SEM images of microstructure of 8Cr4Mo4V steel before (a) and after LSP (b~d) and carbide statistics (e)

图8给出了LSP前后8Cr4Mo4V钢的微观组织。由图8a可以看出，LSP处理前8Cr4Mo4V钢表面有尺寸较大的球形碳化物。LSP处理使碳化物的尺寸减小，说明原始球状碳化物发生了碎化。同时，在碳化物附近产生了大量位错，表明LSP使位错密度提高，且在冲击剪应力作用下位错沿多个方向形成了位错缠结(图8b)。

图8

图8 LSP前后8Cr4Mo4V钢的显微组织TEM像

Fig.8 TEM images of 8Cr4Mo4V steel before (a) and after (b) LSP

2.3 旋转弯曲疲劳性能

图9给出了未经激光冲击强化和激光冲击强化后去除不同表面损伤层深度时，8Cr4Mo4V钢旋转弯曲疲劳周次超过1.0×10⁷未断裂的旋转弯曲疲劳强度。可以看出，未经激光冲击强化的8Cr4Mo4V钢旋转弯曲疲劳强度为740 MPa；经激光冲击强化并去除表面损伤层后的疲劳强度为1080 MPa，与未经激光冲击强化样品相比提高了340 MPa，提高幅度达到45.95%，表明激光冲击强化使8Cr4Mo4V轴承钢的疲劳强度显著提高。

图9

图9 未经激光冲击强化和经激光冲击强化且去除不同深度的表面损伤层8Cr4Mo4V钢的旋转弯曲疲劳强度

Fig.9 Rotating bending fatigue strength of 8Cr4Mo4V steel before LSP and after LSP with different thicknesses of damaged layers removed

图10给出了未经激光冲击强化和经激光冲击强化且去除不同深度表面损伤层后8Cr4Mo4V钢旋转弯曲疲劳断口的形貌。由图10a1~d1可以看出，未经激光冲击强化处理的试样(图10a1)裂纹源区与瞬断区占的面积较大，而经激光冲击强化后裂纹源区与瞬断区的面积减小，且随着表面损伤层去除深度的增加裂纹源与瞬断区的面积减小(图b1~d1)。对断口的裂纹源区观察发现，未经激光冲击强化试样(图10a2)的疲劳裂纹起源于试样表面，经激光冲击强化后裂纹源转移到次表面。激光冲击强化后试样表面分别去除25、50和75 μm，发现疲劳裂纹源产生于次表层，距表面的距离分别为11.10 μm(图10b2)、28.44 μm(图10c2)、41.80 μm(图10d2)，且随着表面损伤层去除深度的增加裂纹源距表面距离增大。观察裂纹扩展区的组织(图10a3~d3)可见，未经激光冲击强化试样其裂纹扩展路径较为分散，存在准解理小面和大量撕裂棱，属于脆性断裂(图10a3)。激光冲击强化后试样的裂纹扩展路径较为紧密，产生大量韧窝，具有较好的韧性，从而降低了裂纹扩展速率(图10b3~d3)。

图10

图10 未经激光冲击强化和经激光冲击强化且去除不同深度表面损伤层8Cr4Mo4V钢的旋转弯曲疲劳断口形貌

Fig.10 Rotating bending fatigue fracture morphology of 8Cr4Mo4V steel before LSP and after LSP with different thicknesses of damaged layers removed (a1~a3) without LSP; (b1~b3) surface removal of 25 μm after LSP; (c1~c3) surface removal of 50 μm after LSP; (d1~d3) surface removal of 75 μm after LSP; (a1~d1) macroscopic fracture morphology; (a2~d2) crack initiation zone; (a3~d3) crack propagation zone

3 讨论

3.1 LSP对微观组织的影响

激光冲击强化不仅使表层的共晶碳化物碎化，还诱导了次表层二次碳化物析出(图7c)。图11给出了激光冲击强化8Cr4Mo4V钢表面碳化物的碎化和二次碳化物的析出过程。由图11a可见，激光冲击强化前试样表面有不同形状和大小的共晶碳化物。激光冲击强化产生的等离子体冲击波压力可达到GPa级别，其峰值压力高于材料的动态屈服强度。在冲击波的作用下产生位错(图8b)，进一步滑移使8Cr4Mo4V钢表面发生塑性变形。碳化物是脆性相，应变和应变率的急剧增加使碳化物内部产生位错滑移带^[27](图11b)，而碳化物中的微裂纹在滑移带处萌生并沿滑移带扩展。激光冲击强化使微裂纹横穿碳化物最终使碳化物碎裂(图11c)。次表面二次碳化物析出是因为激光冲击强化时材料在超高应变率诱导的塑性变形过程中碳原子的扩散能力提高，促进了碳元素的长程扩散能力而使碳化物析出(图11d)。同时，激光冲击强化使表面产生的压应力和变形层中储存的高能量都有助于碳元素析出生成碳化物。

图11

图11 激光冲击强化使碳化物碎化和促进二次析出的示意图

Fig.11 Schematic diagram of LSP leading to carbide fragmentation and promoting secondary precipitation process of 8Cr4Mo4V steel in different states (a) no LSP; (b) formation of dislocation slip bands; (c) carbide fragmentation after LSP; (d) carbide precipitation on sub-surface after LSP

3.2 LSP对疲劳性能的影响

图12给出了激光冲击强化前后8Cr4Mo4V钢在旋转弯曲疲劳实验中裂纹萌生和扩展行为的示意图。可以看出，激光冲击强化前裂纹起源于试样表面并呈扇形向周围扩展(图12a)，因为表面的大尺寸且不规则的碳化物在疲劳循环应力作用下在其周围产生的应力集中使裂纹萌生。在激光冲击强化过程中材料在等离子体冲击波作用下产生剧烈的塑性变形，使材料间相互挤压和发生晶格畸变。而这些晶体学变化在激光冲击强化后不能完全恢复，从而形成了较大的表面残余压应力(图6b)。激光冲击强化使疲劳裂纹起源于次表层(图12b)，8Cr4Mo4V钢的旋转弯曲疲劳性能的显著提高主要与表面硬度和残余应力的提高以及二次碳化物的析出有关。

图12

图12 LSP前后8Cr4Mo4V钢在旋转弯曲疲劳过程中裂纹的萌生和扩展示意图

Fig.12 Schematic diagram of crack initiation and propagation in 8Cr4Mo4V steel before and after LSP during rotating bending fatigue process (a) no LSP; (b) after LSP

激光冲击强化在材料表层一定深度内产生了残余压应力较高的薄层。对图7和图8的分析表明，实测时最大残余压应力层位于次表层，因为激光冲击强化使表面产生碳化物碎化和脱落后形成尺寸较小的凹坑，这些表面缺陷使表面残余压应力得以释放。另一个原因是，激光冲击强化使材料表面产生较多的位错，位错运动使表面应力集中程度降低而使应力松弛^[28]。较高的残余应力减缓了裂纹源的萌生并降低了裂纹的扩展速率，使材料的疲劳抗力和疲劳强度提高^[29]。同时，表层较高的残余压应力使裂纹源区从表面转移到次表面，在相同的旋转弯曲疲劳实验条件下次表层萌生的裂纹受到的应力更小，也抑制了其进一步向材料内部扩展。激光冲击强化虽然使表层碳化物损伤，但是次表层二次析出了更多与基体紧密结合的细小碳化物，降低了应力集中水平。去除表面损伤层后，表面的压应力也有助于提高试样的疲劳性能。

4 结论

(1) 用有限元模拟激光冲击强化8Cr4Mo4V钢的表面残余压应力，其结果与实验检测数据相当(误差为3.94%)。数值模拟结果表明，激光冲击强化后残余应力随着距表层深度的增加而逐步减小，影响深度约为0.6 mm。激光冲击强化能提高8Cr4Mo4V钢的表面残余压应力。

(2) 激光冲击强化，使表层产生塑性变形和近表面的微观结构发生变化。等离子体爆炸冲击，引起表面碳化物碎化和促进次表面二次碳化物的析出。加工硬化和组织变化使钢的硬度由表及里呈梯度变化，形成了厚度约为0.75 mm的硬化层。

(3) 激光冲击强化使8Cr4Mo4V钢的表面残余压应力和表面硬度提高，抑制了疲劳裂纹的萌生并使裂纹源从表面转移至次表面，次表层析出更多的二次碳化物，降低了裂纹的扩展速率，使旋转弯曲疲劳强度从740 MPa提高到1080 MPa。

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