IMPROVING FATIGUE STRENGTH OF METALS USING ABRASIVE WATERJET PEENING

Abrasive waterjet (AWJ) peening has been proposed as a viable method of surface treatment for metal orthopedic devices. In this study the influence of AWJ peening on the compressive residual stress, surface texture and fatigue strength of a stainless steel (AISI 304) and titanium (Ti6Al4V) alloy were studied. A design of experiments (DOE) and an analysis of variance (ANOVA) were used to identify the primary parameters contributing to the surface texture and magnitude of surface residual stress. The influence of AWJ peening on the fatigue strength of the metals was evaluated under fully reversed cyclic loading. It was found that AWJ peening results in compressive residual stress and is primarily influenced by the abrasive size and treatment pressure. The residual stress of the AISI 304 ranged from 165 to over 460 MPa. Using the optimum treatment parameters for maximizing the residual stress, the endurance strength of Ti6Al4V was increased by 25% to 845 MPa. According to results of this study AWJ peening is a viable method of surface treatment for applications that require an increase in surface roughness and maintenance or increase in fatigue strength, qualities that most often are not available from a single process.     

喷丸强化因素对钛合金固体粒子冲蚀抗力的影响

探讨了喷丸强化(SP)因素(残余压应力引入、表面粗糙度增大和表面加工硬化等)对Ti6Al4V钛合金固体粒子冲蚀(SPE)行为的影响和作用机制,为充分发挥SP改进航空发动机零部件服役性能的潜力提供依据。结果表明:Ti6Al4V合金表面直接喷丸处理,其SPE抗力无明显改变;SP处理后进行表面抛光,Ti6Al4V合金SPE抗力明显增加。SP造成的表面粗糙度增大导致了钛合金在大小冲击攻角下的SPE抗力的下降;SP引入的表面残余压应力对提高钛合金在90°大攻角下的SPE抗力起了重要作用,原因是SP残余压应力增加了裂纹闭合力和抑制了疲劳裂纹早期扩展;SP引起的表面加工硬化作用对提高钛合金在30°小攻角下的SPE抗力有重要贡献,这归于加工硬化提高了材料表面在小攻角下的微犁削抗力。     

AN EXAMINATION OF ABRASIVE WATERJET PEENING WITH ELASTIC PRE-STRESS AND THE EFFECTS OF BOUNDARY CONDITIONS

Abrasive Waterjet Peening (AWJP) has emerged as a potentially viable method of surface treatment for components requiring compressive residual stress and a rough surface texture. In the present investigation the residual stress distributions resulting from AWJP of Ti6Al4V with load and displacement control flexure pre-stress were compared. An experimental evaluation was conducted to quantify the variations in the residual stress characteristics (magnitude and depth) as a function of boundary conditions over a pre-stress ranging from 0 to 75% of the material's yield strength. In addition, a finite element model for indentation-based surface treatments was developed to investigate the mechanisms contributing to the observed trends in experimental results. Overall, AWJP of the Ti6Al4V resulted in a surface residual stress ranging from approximately 800 to 1600 MPa. Load control pre-stress resulted in a larger surface residual stress (up to 50% greater) than that achieved under displacement control treatments. Although the maximum surface residual stress was obtained at the largest applied pre-stress, the difference between load and displacement control treatments decreased with pre-stress magnitude. Boundary conditions had limited influence on the depth of residual stress achieved, but were important to the depth of plasticized zone beneath the treated surface.     

喷丸强化残余应力对疲劳性能和变形控制影响研究进展

作为机械表面强化技术之一,喷丸强化使工件表层发生形变硬化,引入较高的残余压应力,减少了疲劳应力作用下微裂纹的萌生并抑制其扩展,从而显著提高零件的抗疲劳断裂和抗应力腐蚀开裂的能力。基于喷丸残余应力解析计算模型,从余弦函数模型、接触应力模型和球腔膨胀模型三个方面介绍喷丸强化残余应力的产生,进而对喷丸残余应力的仿真预测及影响规律进行论述。为了提高试件疲劳强度而引入的残余压应力会带来影响形位精度的变形,基于此阐述了喷丸残余应力对疲劳性能的影响及其在疲劳过程中的演化,同时论述了喷丸残余应力变形预测及控制的研究现状,最后对喷丸残余应力未来的研究内容与发展方向进行展望。     

Compressive Residual Stress into Titanium Alloy Using Cavitation Shotless Peening Method

Cavitation shotless peening (CSP) method, where impacts are generated by a submerged cavitating jet (without shots), was used to introduce compressive residual stress in titanium alloy, Ti-6Al-4V for the purpose of enhancing the conventional fatigue and fretting fatigue life and strength. This method provided higher compressive stress at surface as well as up to a depth of 40 m from the surface than that with the shot peening method. Further, the surface treated by CSP was considerably less rough compared to that by the shot peening method, which is a highly desirable feature to improve the fretting fatigue performance.     

Analysis of Fretting Fatigue of Cavitation Shotless Peened Ti–6Al–4V

Fretting fatigue behavior of cavitation shotless peened titanium alloy, Ti–6Al–4V coupons was investigated using finite element method and a critical plane-based multi-axial fatigue parameter. Cavitation shotless peening (CSP)-induced compressive residual stress, which was larger at the contact surface than its counterpart from the shot peening (SP). However, compressive residual stress decreased more sharply with distance from the contact surface in CSP than in SP. Analysis using a critical plane-based multi-axial fatigue parameter demonstrated that the crack initiation would occur inside the cavitation shotless peened specimen which matched with the experimental observations. On the other hand, crack initiation would occur on the contact surface in the shot peened specimen which again was in agreement with experiments. The analysis also showed that the crack propagation part of the total fretting fatigue life was longer in the shot peened specimen than in the cavitation shotless peened specimen while the crack initiation part was almost equal from both peening methods. Therefore, CSP could not improve the fretting fatigue life/strength as much as the SP did but it improved relative to the un-peened specimen.     

Numerical Calculation and Experimental Research on Residual Stresses in Precipitation-hardening Layer of NAK80 Steel for Shot Peening

Shot peening can improve fatigue strength of materials by creating compressive residual stress field in their surface layers,and offers a protection against crack initiation and propagation,corrosion,etc.And fatigue fracture and stress corrosion cracking of NAK80 steel parts are improved effectively.Currently there lacks in-depth research in which the beneficial effect of the residual stress may be offset by the surface damage associated with shot peening,especially in terms of the research on the effective control of shot peening intensity.In order to obtain the surface residual stress field of NAK80 steel after shot peening,the samples are shot peened by pneumatic shot peening machine with different rules.The residual stress in the precipitation-hardening layer of NAK80 steel is measured before and after a shot peening treatment by X-ray diffraction method.In order to obtain true residual stress field,integral compensation method is used to correct results.By setting up analytical model of the residual stress in the process of shot peening,the surface residual stress is calculated after shot peening,and mentioning the reason of errors occurred between calculated and experimental residual stresses,which is mainly caused by the measurement error of the shoot arc height.At the same time,micro hardness,microstructure and roughness in the precipitation-hardening layer of NAK80 steel before and after shot peening were measured and surveyed in order to obtain the relation between shot peening strength and surface quality in the precipitation-hardening layer.The results show that the surface quality of NAK80 steel is significantly improved by shot peening process.The over peening effect is produced when the shot peening intensity is too high,it is disadvantageous to improve sample’s surface integrity,and leading to reduce the fatigue life.When arc high value of optimal shot peening is 0.40 mm,the surface quality is the best,and the depth of residual stress in the precipitation-hardening layer reaches to about 450 μm.Numerical calculation is very useful to define the process parameters when a specific residual stress profile is intended,either to quantify the benefits on a specific property like fatigue life or to help on modeling a forming process like shot peen forming.In particular,the proposed parameter optimization in the progress of shot peening and effective control of the surface texture provide new rules for the quantitative evaluations of shot peening surface modification of NAK80 steel.     

Finite element analysis of shot-peening effect on fretting fatigue parameters

Shot peening is widely used to improve the fretting fatigue strength of critical surfaces. Fretting fatigue occurs in contacting parts that are subjected to fluctuating loads and sliding movements at the same time. This paper presents a sequential finite element simulation to investigate the shot peening effects on normal stress, shear stress, bulk stress and slip amplitude, which are considered to be the controlling parameters of fretting damage. The results demonstrated that among the modifications related to shot peening, compressive residual stress has a dominant effect on the fretting parameters.     

预应力切削钛合金的有限元分析

为揭示预应力切削对钛合金Ti6Al4V加工表面残余应力的调整机理,探讨切削时锯齿形切屑的形成过程,基于预应力切削原理建立了钛合金的预应力切削有限元模型,模拟了0、280 MPa和560 MPa这3种预应力下的锯齿形切屑形成过程以及已加工表面的残余应力分布。结果表明:采用预应力切削方法可以调整钛合金已加工表面的残余应力状态;预应力对锯齿形切屑的形成过程和切屑特征无明显影响;在材料弹性极限内施加越大的预应力,表面层残余压应力效果越显著,次表层最大残余压应力值越高,残余压应力层分布也越深。     

Process characterization of vibrostrengthening and application to fatigue enhancement of aluminum aerospace components��part I. Experimental study of process parameters

Vibrostrengthening is a fatigue-enhancement process, originally developed by the Russian aviation industry (Rumyantsev et al. 2004). A potential alternative to shot peening, currently a standard industrial surface treatment for fatigue enhancement, vibrostrengthening offers the potential for shorter processing times and uniform treatment of the surface, especially when dealing with fragile parts and complex part geometries. Vibrostrengthening is a modification of a vibratory finishing process in which the parts or workpieces and a medium of hard granular particles are vibrated together in a processing tub causing the particles to mechanically work the surface of the workpiece. In vibrostrengthening, the workpiece is fixed inside a vibratory tub, increasing the relative velocities between the particles in the medium and the workpiece. This gives rise to more aggressive mechanical working of the workpiece surface. The resulting plastic deformation at the surface produces a sub-surface compressive residual stress, which together with a better surface finish, is conjectured to improve the fatigue strength of workpiece. This paper is an experimental study of vibrostrengthening of aluminum components for fatigue life enhancement. The effects of various process parameters on the fatigue strength of a specimen are studied to experimentally characterize the process. These experiments also demonstrate that the vibrostrengthening process produces significant fatigue enhancement on experimental samples produced by machining. Further, these experiments verify that, in fact, fatigue enhancement in the vibrostrengthening process is a result of the combined effect of inducing a compressive residual stress field within the material and improving the material??s surface finish. Fatigue tests indicate that the fatigue enhancement of this process is comparable to, if not better than, shot peening. One important reason for such a favorable comparison, given the lower levels of residual stress that result from this process, is the superior surface finish it produces. A companion paper (Sangid et al. 2010) presents a study involving process visualization to understand and explain the process mechanics; further, a computational model is produced to characterize the fatigue enhancement of the process through the compressive residual stress field and surface topography.     

铸态SiCW/Al复合材料的表面喷丸强化

对铸态20％SiCw/6A02Al复合材料进行了表面喷丸处理，测量了表层基体残余应力的分布情况， 建立了复合材料表层基体屈服强度的X射线试验方法，探讨了其表层喷丸强化效应。结果表明：经喷丸处理后复合材料表层基体呈现出较大残余压应力状态，X射线衍射峰半高宽度即显微组织强化效应明显增大，导致复合材料表层基体屈服强度提高。     

Fretting Fatigue Behavior of Cavitation Shotless Peened Ti–6Al–4V

Fretting fatigue behavior of cavitation shotless peened (CSP) titanium alloy, Ti–6Al–4V was investigated. Constant amplitude fretting fatigue tests were conducted at several maximum stress levels, σ_max, ranging from 400 to 555 MPa with a stress ratio of 0.1. Test results showed that the fretting fatigue life was enhanced by CSP treatment as compared to the unpeened specimen, but the enhancement was not as large as that from the shot-peening treatment. Residual stress measurements by X-ray diffraction method before and after fretting test showed that residual compressive stress was relaxed during fretting fatigue. Before fretting, CSP specimen had higher compressive residual stress on the surface than the shot-peened specimen. However, greater residual stress relaxation occurred in CSP specimen such that the relaxed compressive residual stress profile near the contact surface of CSP specimen was lower than that of shot-peened specimen. This lower compressive residual stress from fretting fatigue was the reason for shorter fretting fatigue life of CSP specimen as compared to shot-peened specimen at the applied stress level.     

Shot peening simulation based on SPH method

Shot peening is a complex surface-treating process which is usually employed to improve the fatigue strength of metallic part or members. In dealing with shot peening simulation, existing literatures apply finite element method (FEM) to establish only a single shot or several shots models, thus the effect of a mass of shots impacting repeatedly and the interaction among adjacent shots are ignored. To overcome these defects of FEM models, smoothed particle hydrodynamics (SPH) coupled FEM modeling is presented, in which the shots are modeled by SPH particles and the target material is modeled by finite elements. Contact algorithm is used to simulate the interaction between shots and target. Utilizing this model, material model for shots is established, the relationships between compressive residual stress and peening frequencies, coverage, and velocities are analyzed. Steady compressive residual stress can be gotten by multiple peening; higher coverage can improve the compressive residual stress; faster velocities can induce greater and deeper maximum residual stress in target subsurface. The simulation results agree well with the existing experimental data. The study will not only provide a new powerful tool for the simulation of shot peening process, but also be benefit to optimize the operating parameters.     

喷丸三维残余应力场的有限元模拟

运用大型有限元计算软件ABAQUS建立了模拟喷丸残余应力场的三维有限元模型,预测了在相同喷丸强度下玻璃丸和钢丸两种类型弹丸喷射所产生的残余应力场。模拟过程中,分析了线性减缩积分单元的沙漏参数、材料的应变硬化率、喷丸覆盖率以及初始残余拉应力等因素对304不锈钢靶材残余应力分布的影响。从计算结果可以看出,钢丸喷丸产生的残余压应力层较深,但在高覆盖率时,玻璃喷丸产生的残余压应力的平均值比钢丸喷丸处理后产生的大。在有初始残余拉应力(250 Mpa)存在的情况下,两种类型的喷丸处理均能使304不锈钢靶材表面形成残余压应力层,这说明喷丸工艺可以提高奥氏体不锈钢焊接构件的抗应力腐蚀开裂能力。本研究成果为进一步探讨喷丸强化不锈钢焊接头抗应力腐蚀性能的机理奠定了基础。     

Effect of shot peening coverage on fatigue limit in round bar of annealed medium carbon steel

Shot peening is an effective and economical technique for improving the fatigue strength of metallic components by inducing compressive residual stress and hardening the layer near the surface. The effect is generally evaluated by main two parameters: coverage and peening intensity. However, the valuable coverage for improving the fatigue strength depends on the shape of the target material. In this study, the effect of coverage on fatigue limit in round bar of annealed medium carbon steel was experimentally studied. The fatigue limits for shot peened round bar specimens with 140–2300% coverage increased 14–25% by comparing those for non-peened round bar specimens. The valuable range of coverage was 280–60% in the used material and shot peening condition for improving the fatigue limit in short time. The result indicates that the valuable coverage of the round bar material is higher than full coverage to improve the fatigue limit of the material due to the effect of incident angle on round bar, even though the degree depends on the materials and shot peening conditions.     

Empirical modeling of shot peening parameters for welded austenitic stainless steel using grey relational analysis

The attempt of this paper is to present an effective approach for the optimization of the shot peening process of welded AISI 304 austenitic stainless steel with multi performance characteristics using Grey relational analysis (GRA) based on Taguchi orthogonal array. Twenty-seven experimental runs are performed to determine best process parameters level. An analysis of variance (ANOVA) is carried out to identify significant peening parameters. The response tables are obtained for analyzing the optimal levels of shot peening parameters and major factors that affect the quality function. The multiple performance characteristics including tensile strength, surface hardness and surface roughness are the quality functions considered for the optimization. Further mathematical models are developed using regression analysis for the tensile strength, surface hardness and surface roughness. It will be very helpful to the engineers in deciding the levels of the shot peening parameters for desired performance characteristics.     

1Cr12Ni2W1Mo1V铣削过程参数对残余应力影响研究

针对汽轮机末级动叶片在实际应用中的抗疲劳性能问题,文章通过对1Cr12Ni2W1Mo1V不锈钢进行不同参数下的加工试验,研究了过程参数对叶片表面残余应力的影响机理及规律。将不同组间的试验数据进行对比得出了结论,并对产生原因进行了分析,认为提高主轴转速有利于残余压应力的加大,而温度热应力对残余拉应力起着主导作用。可在不同加工参数条件下提高叶片表面的残余压应力,减少了后续的喷丸表面处理工序。     

Kinematic modeling and deformation mechanics in shot peening of functional ceramics

The applications of functional ceramics are significantly limited by the brittleness and low reliability. Recent studies have shown that compressive residual stress can be created in ceramics by shot peening, which improves the contact strength and fatigue of ceramic components. However, the formation mechanism of residuals stress in shot peening is yet to understand. In this study, a pressure-dependent plasticity model has been incorporated into a finite element simulation model of shot peening to understand the process mechanism underpinning the residual stress formation. Since shot velocity is the key process parameter to dominate the impact energy which determines the deformation state of the peened surface and the resultant residual stress, a new kinematic model of shots has also been developed by incorporating air drag and travel distance inside and outside the peening nozzle. The results have shown that the shot velocity model can be used to predict shot velocity. The experiment-based model may help understand the process mechanism underpinning the residual stress formation.     

FEM of residual stress and surface displacement of a single shot in high repetition laser shock peening on biodegradable magnesium implant

Laser shock peening (LSP) is one of the prominent surface processing techniques to improve the mechanical characteristics by inducing compressive residual stress on the specimen surface. Generally, LSP is performed using high energy, low repetition pulsed laser. Recently, High repetition laser shock peening (HRLSP) on biodegradable magnesium alloys has been reported. Increased speed and reduced operating costs are the key highlights of HRLSP. This work is aimed towards understanding of the residual stress profile beneath the specimen surface, where a Finite element method (FEM) has been proposed to show the ability of a tightly focussed nanosecond laser pulse for peening magnesium. The depth of maximum compressive residual stress of 48 MPa at 28 mm beneath surface was the result of the simulation. Also the Von Misses stress was analytically found to be 31.5 MPa, which is similar to the value from FEM at 30 MPa. Furthermore, the plastic displacement of FEM at 4.02 µm compares reasonably well with the experimental result at 3.698 µm, thereby validating the Finite element model. If increase in CRS can be created by single shot of laser pulse, it can be concluded that the same can be done beneath the entire magnesium surface using appropriate scanning protocols.

     

钛合金铣削加工表面残余应力研究

金属切削加工引起的残余应力对零部件变形、疲劳性能有重要影响,通过选择合适的加工参数对其进行控制是可能的。目的在于研究钛合金铣削加工中铣削参数对已加工表面残余应力的影响,利用X射线衍射技术测量进给方向和铣削方向的残余应力。为保证X射线应力测量精度,采用摇摆法和峰拟合定峰方法。试验结果表明,钛合金顺铣加工表面残余应力表现为压应力,主要是由于刀具后刀面与已加工表面的挤光效应起主导作用所致。沿铣削方向和进给方向,切削参数对工件表面残余应力影响具有差异性。通过控制切削参数有可能生成定制化的应力。