喷丸件表面粗糙度及其对疲劳极限的影响

本文对40Cr钢淬火后200℃和550℃回火的两种试样,进行了多种规范的喷丸,然后测定了它们的表面粗糙度和疲劳极限。提出了表面粗糙度引起的应力集中系数的半解析表达式以及疲劳源在表面时疲劳极限的预测方法。     

喷丸强度对不同粗糙度表面超高强度钢疲劳性能的影响

不同方式加工的高强度钢零件具有不同的表面状态,对其采用相同的喷丸工艺是否合理尚不明确。对不同表面粗糙度23Co14Ni12Cr3Mo E超高强度钢作不同强度的喷丸强化。采用扫描电镜及白光干涉仪观察了喷丸试样的表面形貌,采用金相显微镜观察喷丸试样的表面组织,采用旋弯疲劳试验机测试了喷丸试样的疲劳性能,采用X射线衍射残余应力测试仪测试喷丸试样的残余应力。研究了喷丸强度对不同表面粗糙度超高强度钢的表面组织、形貌、残余应力场及疲劳性能的影响。结果表明:喷丸过程可以细化表面组织,引入残余压应力场,改变表面形貌特征,从而引起表面应力集中状况的改变,其改善效果与原始表面状态有关;对于初始表面粗糙度Ra≤0.4μm的表面,喷丸强化过程能有效提高材料的疲劳寿命,促使疲劳裂纹源内移;随表面粗糙度的提高,当应力集中状况过于严重时,表面组织细化和残余压应力对疲劳性能的提升作用会被抑制,喷丸工艺对疲劳寿命的提升效果大幅下降,疲劳裂纹源均位于表面。     

喷丸残余应力场有限元数值模拟分析

为探讨喷丸参数对残余应力大小及分布的影响,利用LS-DYNA软件建立了分批顺序撞击的9丸粒有限元模型,对喷丸残余应力场进行了数值仿真研究,并与已有结果进行对比以验证其准确性。在此基础上研究了弹丸速度、重复打击次数、搭接率以及多丸粒分布对残余压应力的影响。结果表明:增加弹丸搭接率和覆盖率可使各点的残余应力分布趋于一致;重复打击一定次数后将使残余应力增长率减缓并最终趋于饱和;最大残余压应力及其到达深度均随喷丸速度增加而增大。本模型为获得理想残余应力提供了理论依据。     

GH909合金喷丸强化残余应力场的研究

研究了GH909合金不同喷丸强化工艺下的残余应力场和残余应力在550℃的松驰情况，总结了喷丸强化所获得的残余应力场特性。结果表明，GH909合金经适当喷丸强化后获得的有益残余压应力场可改善其疲劳性能。     

关于残余应力的静载松弛与最佳喷丸残余应力场的研究

本文研究了喷丸残余应力场在疲劳加载初期的静载松弛现象及机理,并对最佳喷丸残余应力场进行了探讨。试验结界表明,残余应力在疲劳过程中的静载松弛是工件表层材料剧烈塑性变形的结果。残余应力的静载松弛会使晶界、相界等障碍物处形成一定数量的微裂纹,给工作表层材料带来损伤,降低疲劳裂纹的形核寿命。为了避免疲劳初期由于残余应力的静载松弛所造成的损伤,在喷丸后采用应力松弛低温回火工艺,预先降低残余应力场中的最大残余压应力值,建立最佳残余应力场。这种通过热激活的方式使残余应力发生的松弛属子非损伤性松弛,因而能够有效地提高材料在S—N曲线上的较高交变应力区的疲劳寿命。     

喷丸起落架外筒焊接热影响区的残余应力测定

本文用X射线法测定了喷丸起落架外筒焊缝影响区的残余应力,并从残余应力角度考察了起落架外筒现行工艺路线的优劣。测定结果表明,在相同构件上虽然所测方向和部位不同,但残余应力的分布、大小及符号基本相同。与前起外筒相比,喷丸处理在主起外筒上形成的残余压应力值较低。分析认为消除焊接残余拉应力的合理工序,是两种外筒均应采用焊后热处理,但应防止表面脱碳。     

弹丸直径和速度对喷丸残余应力分布的影响分析

采用有限元软件分析喷丸强化后的残余应力分布,研究不同直径弹丸的喷射速度对喷丸强化引入的残余压应力场的影响。具体分析在0.2mm～0.4mm弹丸直径范围内不同弹丸速度对残余压应力场的影响,将每个弹丸直径下的最优残余压应力值提取出来与弹丸速度进行Gauss曲线拟合,并建立喷丸强化的"弹丸速度-残余应力值"函数关系式。     

喷丸材料及粒径对300M钢原始表面粗糙度的影响

过去的研究忽视了零件表面原始粗糙度对喷丸后形成的粗糙度的重大影响,而表面粗糙度又是考核喷丸质量的重要指标。采用车削、精车削和磨削3种加工方式在300M钢表面形成3种粗糙度,采用不同喷丸工艺,测定了300M钢喷丸前后的表面粗糙度,观察了材料表面形貌,研究了喷丸对300M钢表面粗糙度的影响。结果显示,铸钢弹丸对材料表面形貌...     

复合喷丸对铝-铜-镁合金表面形貌和硬度的影响

对铝-铜-镁合金试样进行复合喷丸处理,通过观察喷丸处理后试样的表面形貌和测量试样的表面粗糙度、残余应力、衍射峰半峰宽及显微硬度等参数,分析了复合喷丸处理对铝-铜-镁合金表面形貌和力学性能的影响.结果表明:铝-铜-镁合金试样经复合喷丸处理后表面粗糙度变化不大,由0.70μm增加到2.97μm,试样表面整体细密、均匀,完整...     

7A65铝合金喷丸强化表面完整性及疲劳性能

目的 以7A65高强度铝合金为研究对象,研究喷丸强度、弹丸介质(铸钢丸和陶瓷丸)对靶材疲劳性能的影响规律。方法 利用扫描电镜、激光共聚焦显微镜、X射线衍射仪等仪器表征喷丸强化7A65铝合金表面完整性和疲劳失效断口,分析喷丸工艺参数与疲劳性能、断裂模式的相关性。结果 喷丸强化后铝合金表面粗糙化严重,表面粗糙度从初始0.622 μm增加至4.736 μm(铸钢丸、喷丸强度为0.22 mmA),并出现褶皱损伤;在相同喷丸强度下陶瓷丸喷丸表面粗糙度较低,无褶皱损伤。2种弹丸在金属表面引入的残余应力场基本相同,残余压应力层深约300 μm,最大残余压应力值为-480.6 MPa,其产生位置为距离表面75 μm处(喷丸强度为0.22 mmA)。铝合金疲劳性能对铸钢丸介质敏感性较高,当喷丸强度较低(0.11 mmA)时喷丸强化效果最佳,疲劳寿命是原始寿命的5倍多,疲劳源从表面转移至次表面(500 μm);当喷丸强度增至0.22 mmA时,裂纹源向表面靠近,疲劳寿命为原始寿命的2倍。铝合金疲劳性能对陶瓷丸介质敏感性较低,在喷丸强度为0.11～0.22 mmA时疲劳寿命较为稳定,在喷丸强度为0.11 m...     

Technical note A study on stress, reflection and double shot peening to inrease compressive residual stress

The technique of shot peening is commonly used to increase the fatigue limit of a steel. However, there are many practical difficulties in applying it to very high HV steel and complicated components. To overcome these problems, the authors proposed two new methods: stress double shot peening and stress reflection double shot peening. Both techniques were applied to quench and tempered steel (QT steel) and induction-heated steel (IH steel). The main results were as follows: (a) by double shot peening, the compressive residual stress near the sample surface was increased considerably; (b) by stress shot peening, the maximum compressive residual stress ( σ _max ) and the surface compressive residual stress ( σ _s ) were greatly increased; (c) by stress double shot peening, very high compressive residual stresses ( σ _max = −1710 MPa and σ _s = −1320 MPa) were successfully introduced into a hard steel (HV = 700); (d) in the new method (stress reflection double shot peening), very high compressive residual stresses ( σ _max = −1760 MPa and σ _s = −1460 MPa) were successfully introduced into a hard steel (HV = 700).     

喷丸过程中的能量转化及残余应力分布研究

本文建立了喷丸表面强化过程的三维有限元模型,研究了靶材的力学性能及弹丸喷射速度对能量转化率和残余应力分布的影响规律。通过对有限元计算结果的分析发现,杨氏模量与屈服强度之比E/&;#61555;y较大,应变硬化率较低的靶材喷丸效率较高。屈服强度的提高使得最大残余压应力变大,残余压应力场深度变小。应变硬化率和喷丸速度的提高均会使最大残余压应力及其深度变大,同时会使表面残余压应力减小并向拉应力转变。本文的研究结果从能量转化的角度为喷丸强化件材料的选择提供了理论基础     

Effect of shot peening on fatigue limit of surface flawed samples

The effects of shot peening on the fatigue limit of specimens having a semicircular notch of varied surface length, 2a , are investigated. In the case of un-peened specimens, the fatigue limit of specimens having a notch of a = 0.05 mm was equal to that of the un-notched specimens. However, the fatigue limit of a = 0.3 mm was 46% smaller than that of the un-notched specimens. On the contrary, in the case of peened specimens, the fatigue limit of a = 0.2 mm was equal to that of the un-notched specimens and furthermore, that of a = 0.3 mm was only 5% smaller than that of the un-notched specimens. Multiple non-propagating cracks were observed in peened specimens after fatigue testing. The stress intensity factor of the maximum non-propagating crack size corresponded to that of a = 0.2 mm notch. These results indicate that shot peening increases fatigue limit and decreases the likelihood that a surface flaw will result in failure.     

Mechanism modelling of shot peening effect on fatigue life prediction

A new mechanism modelling is proposed in this paper to explain the shot peening effect on fatigue life predictions of mechanical components. The proposed methodology is based on the crack growth analysis of shot peened specimens, which are affected by the interaction of surface roughness and residual stress produced during the shot peening process. An asymptotic stress intensity factor solution is used to include the surface roughness effect and a time‐varying residual stress function is used to change the crack tip stress ratio during the crack propagation. Parametric studies are performed to investigate the effects of surface roughness and the residual stress relaxation rate. Following this, a simplified effective residual stress model is proposed based on the developed mechanism modelling. A wide range of experimental data is used to validate the proposed mechanism modelling. Very good agreement is observed between experimental data and model predictions.     

A numerical simulation to relate the shot peening parameters to the induced residual stresses

This paper presents a computational modelling of the shot peening process, in which the finite element method was employed to study the elastic–plastic dynamic process of shots impacting on a metallic target, and the discrete element method was used to study the multiple particles dynamics. The results demonstrated the capability of the method to capture realistic behaviour including the interaction between incoming shots and rebounding shots. Statistical analyses of the shots impact data reveal the relationships between peening process parameters and peening quality, which are useful for identifying the parameters which need to be controlled and regulated in order to produce a more beneficial compressive residual stress distribution within the target.     

Influence of residual stresses from shot peening on fretting fatigue crack growth

One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental fretting life results from specimens in a Titanium alloy with and without shot peened surfaces were evaluated numerically. The residual stresses were measured at different depths below the fretting scar and compared to the corresponding residual stress profile of an unfretted surface. Thus, the amount of stress relaxation during fretting tests was estimated. Elastic–plastic finite element computations showed that stress relaxation was locally more significant than that captured in the measurements. Three different numerical fatigue crack growth models were compared. The best agreement between experimental and numerical fatigue lives for both peened and unpeened specimens was achieved with a parametric fatigue growth procedure that took into consideration the growth behaviour along the whole front of a semi‐elliptical surface crack. Furthermore, the improved fretting fatigue life from shot peening was explained by slower crack growth rates in the shallow surface layer with compressive residual stresses from shot peening. The successful life analyses hinged on three important issues: an accurate residual stress profile, a sufficiently small start crack and a valid crack growth model.     

先进喷丸表面改性技术研究进展

综述了先进喷丸表面改性技术研究现状及应用,阐述了微粒子喷丸、激光喷丸、超声/高能喷丸、高压水射流喷丸的基本原理,与传统喷丸对比发现,微粒子喷丸可提高材料的耐磨性,激光喷丸可精确控制定位,超声/高能喷丸可实现材料表面纳米化,高压水射流喷丸可承受半柔性冲击并减少应力集中.此外,对各种喷丸技术综合分析并进行对比,认为超声喷丸、复合喷丸(高能-微粒喷丸,激光-机械喷丸)综合性能最佳,并展望了未来应重点开展的工作.     

喷丸强化对Ti6Al4V半椭圆表面裂纹J积分和裂纹扩展速率的影响

采用修正的J积分计算方法,考虑残余应力、残余应变和残余应变能,定量计算和分析喷丸强化对半椭圆表面裂纹前沿J积分参数的影响规律。对喷丸强化工艺进行有限元建模仿真,通过改变约束条件生成疲劳裂纹并施加远场载荷,计算J积分和裂纹扩展速率。考虑不同深度的半椭圆表面裂纹和不同丸粒速率对断裂参量的影响。结果表明:丸粒速率一定时,与未喷丸相比喷丸后J积分值的降幅随裂纹深度的增加而减小,喷丸强化有益于抑制疲劳浅裂纹的扩展。当裂纹深度为0.3mm时,裂纹最深点的J积分值由4.25N/mm降低到2.99N/mm,降幅约30.1%。裂纹深度一定时,J积分值随丸粒速率的增大而降低,提高丸粒速率对抑制裂纹扩展更有益。     

Laser peening and shot peening effects on fatigue life and surface roughness of friction stir welded 7075-T7351 aluminum

The effects of laser peening, shot peening, and a combination of both on the fatigue life of Friction Stir Welds (FSW) was investigated. The fatigue samples consisted of dog bone specimens and the loading was applied in a direction perpendicular to the weld direction. Several laser peening (LP) conditions with different intensities, durations, and peening orders were tested in order to obtain the optimum peening parameters. The surface roughness resulting from various peening techniques was assessed and characterized. The results indicate a significant increase in fatigue life using LP compared to shot peening when tested on their native welded specimens.     

Influence of shot peening parameters on high‐cycle fatigue strength of steel produced by powder metallurgy process

In this study, the effect of shot peening parameters on fatigue strength of steel manufactured by powder metallurgy (PM) was investigated. Steel material obtained from Höganas ASC 100.29 in chemical composition of Fe–0.5% C–2% Cu was produced by using a single action press PM process. To determine the effect of shot peening parameters on fatigue performance, fatigue tests were performed on 20 unpeened and 80 shot‐peened samples, which were machined from sintered steel. Furthermore, shot‐peened samples were peened at different peening intensities, 100% and 200% saturation and full coverage conditions. Fatigue performance of steel, produced by PM process, was improved by surface peening process. For the studied PM steel, the best fatigue performance was obtained with the samples that were shot peened at 20 Almen intensity and 100% saturation. Fatigue strength and limit of the samples, however, were reduced after a certain cold work level. Higher intensity and saturation levels of peening process thus deteriorated the beneficial effect on fatigue strength and limit.