Study on effect of time parameters of laser shock peening on residual stresses using FE simulation

Laser shock peening (LSP) is the newest and most innovative surface treatment technique. LSP residual stress distribution is affected by many parameters. Of them, the parameters are main factors that determine the convergence of finite element analysis (FEA) and characteristic of pressure pulse of laser system. The parameters, related to the convergence of FE simulation, are stability limit time for the stable convergence of results, and solution time for dynamic analysis. The other parameters, related to characteristics of pressure pulse of laser system, are pressure pulse duration time and laser pulse interval time for multiple LSP. In the present work, we have conducted to confirm the influence of time parameters of LSP system on residual stress results using FEA, and we have also predicted optimized range of time parameters.     

激光冲击钛合金板料的有限元模拟

激光冲击强化技术（LSP）是一种新型的表面处理技术,它利用激光冲击波作用靶材表面而产生残余压应力场.通过有限元软件模拟（FEM）可以分析激光冲击强化处理后靶材的残余压应力场分布,分析材料表面和深度方向的残余应力场的分布情况.先分析了材料的本构模型、激光冲击波的峰值压力的计算、有限元单元类型的选取、边界条件的处理等条件;再通过有限元软件ABAQUS对激光冲击TC4钛合金板料进行了数值模拟,分析了残余应力场的分布特点.     

Optimization of peened-surface laser shock conditions by method of finite element and technique of design of experiments

This paper presents a numerical simulation of the laser shock peening (LSP) process using the finite element method. The majority of controlling parameters of the LSP process have been taken into account. The LSP loading has been characterized by the use of a repetitive time Gaussian increment pressure applied uniformly at a circular impacted zone. The utilized model of the treated material behaviour law is the Johnson-Cook’s visco-elastic-plastic coupled with damage. The proposed model leads to determine the LSP surface modifications: (i) the in-depth residual stresses, (ii) the induced plastic strains and (iii) the superficial damage. These modifications can be significantly induced in few cases, particularly when the operating conditions are not well optimized. An application is carried out on the laser peened titanium aero-engine super alloy Ti-6Al-4V. A satisfactory correlation between the computed and experimental results is observed. Also, it is noted that the computed superficial damage values increase with the growth of the maximal peak pressure of the laser spot, which are physically consistent. Otherwise, in order to optimize the laser peening operating conditions, a design of experiments is established. It allows having surface-response relationships between the operating parameters and the three announced induced effects.     

Wear and friction of 6061-T6 aluminum alloy treated by laser shock processing

Laser shock processing (LSP) is becoming an important surface treatment to induce a compressive residual stress field, which improves fatigue and fracture properties of components. In this work, we examine the effect of laser shock processing on the wear and friction behavior of 6061-T6 aluminum alloy. Wear rate and friction coefficient evolution are investigated for different process parameters of LSP. Roll-on-flat tribometer is used with different loading conditions. Hardness and residual stresses are assessed as well. It is observed that wear rate decreases as pulse density increases; this is explained in light of residual stress distribution.     

Effect of input variability on the quality of laser shock processing

Laser shock processing (LSP) involves high-energy laser radiation combined with suitable overlays to generate high-pressure pulses on the surface of the metal. The stress wave generated due to high pressure pulses propagates into the material causing the surface layer to yield and plastically deform, and thereby, develop a significant residual compressive stress in the surface region of the substrate material. The developed compressive stress field is beneficial to improve surface properties such as fatigue, wear, and corrosion. To improve the understanding of the shock hardening process, investigation into the physical processes involved is necessary. In the first part of this paper, the temporal variation in the pressure intensity and spot size is calculated by using a two-dimensional recoil pressure prediction model. Using an explicit non-linear FEA code, ANSYS LS-DYNA, the deformation behavior and residual stresses in the substrate material are predicted. In the second part, a probabilistic approach to the modeling and analysis of LSP is presented in this paper. Various factors that affect the probabilistic performance of the LSP are grouped into categories and a select number of factors known to be significant, for which the variability could be assessed, are modeled as random variables (such as recoil pressure, laser beam spot size, substrate material properties and others). The potential of the probabilistic approach in predicting the structural integrity of the laser-shocked components is addressed.     

基于ANSYS的磨削残余应力场仿真研究

作为影响工件表面完整性的关键因素之一,残余应力影响工件强度,在制造时会导致产生变形和开裂等工艺缺陷,同时在制造后的自然释放过程中也会导致材料的疲劳强度、应力腐蚀等力学性能降低。本文借助有限元分析软件ANSYS,采用热—力顺序耦合方法,建立了平面磨削残余应力场的有限元模型。该模型能够动态反映磨削加工过程中工件表层残余应力的变化情况。在此基础上,分析了不同磨削参数对工件表层残余应力的影响,从计算机仿真角度对磨削加工工艺参数进行了优化。仿真结果表明,在磨削过程中,工件表层同时存在着残余拉应力和残余压应力,与其他磨削参数相比,磨削深度对残余应力的影响最为显著。     

Finite Element Modeling of the Technology of Multiple Laser Shock Processing of Materials Using the Eigenstrain Method

The laser shock processing (LSP) of material is an efficient modern technology of processing of metal materials, during which significant compressive residual stresses contributing to an increase in their strength and tribological and operational characteristics are generated in the subsurface area. The finite element modeling of the technology of multiple laser shock processing is carried out using the eigenstrain method. The level of the compressive residual stresses arising under LSP is determined. It is shown that the residual stresses on the surface of the VT-6 alloy grow from 510 to 830MPa with an increase in the number of pulses from 1 to 4, and the depth of the zone of the compressive residual stresses increases respectively from 1.26 mm after the first pulse to 1.60 mm after the fourth pulse.     

Finite element analysis and design optimization of low plasticity burnishing process

In the present study, Low Plasticity Burnishing (LPB®) process on the half-space specimen has been simulated using a 3D explicit nonlinear finite element model. The developed finite element model is then used to investigate the effect of main parameters including ball diameter, burnishing force, burnishing feed, and number of passes on the resultant profile of residual stress and plastic deformation. Due to high computational cost associated with the nonlinear finite element model and in order to practically conduct design optimization of the LPB process, the design of experiment combined with the response surface methodology has been used to develop smooth response functions to efficiently and accurately approximate the residual stress profile and plastic deformation over the entire design space. Finally in order to improve the LPB process, a design optimization using the developed response functions has been formulated to obtain the optimum set of parameters such that a deep residual compressive stress with small plastic deformation is generated throughout the thickness of component.     

钛合金激光冲击强化层的残余应力及显微组织

对TC6钛合金进行了激光冲击强化(LSP),对强化层的残余应力分布进行了测试,应用透射电子显微镜对强化层的显微组织进行了观察。结果表明:TC6钛合金LSP的最佳功率密度为4GW.cm-2,LSP能在材料表层产生高的残余压应力场,表面残余压应力可达530.4 MPa;LSP可在钛合金表层产生高密度位错和纳米晶,纳米晶尺寸在10～100nm。     

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.

     

钛合金薄板激光焊接和TIG焊接残余应力数值模拟

基于有限元分析软件ANSYS，以激光焊接和TIG焊接温度场模拟为基础，对钛合金薄板的焊接残余应力进行了数值模拟，并分析了不同焊接工艺参数对激光焊接和TIG焊接残余应力分布的影响。数值模拟中考虑了材料参数的温度相关性，并与小孔释放法测试的焊接残余应力进行比较，结果表明：计算结果和测试结果吻合较好。     

TiC颗粒增强镍基合金激光熔覆工艺仿真

通过有限元分析方法对不同激光功率、扫描速率以及光斑直径下TiC/Inconel 718复合材料制造过程中的热-力学特征进行了研究。通过线性组分公式确定复合材料的热物理性能参数,选用半球高斯热源模拟激光温度载荷,利用生死单元技术实现金属粉末增材过程。采用间接法进行激光熔覆热-力耦合分析,基于温度分析结果转换单元类型进行热应力计算。研究表明个工艺参数与温度、温度变化率及残余应力的变化存在一定的规律,且激光加工功率在225～250 W之间、扫描速度在1.0～1.5 mm/s之间、光斑半径在2.5～3.0 mm之间达存在最佳加工参数,可以达到较好的熔覆效果。     

高档数控机床基础件低应力制造特性的研究

围绕高档数控机床基础件的低应力制造问题,研究了残余应力的分布规律,提出从优化制造过程工艺参数出发的降低残余应力方法.明确了铸造、机加工是两个对残余应力的产生起主导作用的工艺环节,采用有限元法对某加工中心床身进行了残余应力分析,得出了该床身在铸造与机加工环节的残余应力大小及分布情况.据此优化铸造残余应力振动时效处理的工艺...     

Application of 3-D finite element method using Lagrangian formulation to dilution control in laser cladding process

In this study a three-dimensional finite element model is presented for precisely simulating laser cladding process with a focus on dilution control. Dilution is referred to as an important quality index in the laser cladding process, indicating the contamination level of the properties of clad layer by substrate metals. As regards a good quality of laser clad layer, low dilution as well as metallurgical bond of interface are prerequisite, so the dilution control is essential in the process. Unfortunately despite of its importance, to date, any practically usable computation techniques have not been reported, which is a motivation of this work. In the paper, proposing a simulation procedure, we develop and verify a code of finite element method with Lagrangian view-point. Following the procedure, the dilution control is precisely estimated. The effects of process parameters on the dilution of clad layer are quantitatively discussed by the numerical and experimental means.     

A parametric study on residual stresses and forging load in cold radial forging process

In this work, a comprehensive study of radial forging process is presented through 2-D axisymmetric and 3-D finite element simulations while considering internal tube profile. The tube used in this investigation has four internal helical grooves along its length. The material is modeled with the elastic-plastic behavior, and sliding-sticking friction model is utilized to model the die-workpiece and mandrel-workpiece contacts. The numerical results in the 2-D case are compared with available experimental data. Residual stresses in the forged product, stress concentration around the grooves, pressure distribution on the hammers and mandrel and maximum forging load are studied. The effects of process parameters such as workpiece and die geometries, percentage of deformation, and workpiece motions on residual stresses and applied pressures on the hammers and mandrel are investigated. The results provide a valuable insight into the parameters affecting radially forged products and provide a useful tool for better design of this process.     

某型航空发动机涡轮盘低循环疲劳寿命分析

确定发动机零部件的最大应力应变循环是进行零部件寿命研究的重要内容之一.弹塑性有限元分析常用于计算最大应力应变循环,但是由于各种载荷、约束等条件考虑不全面,得到的应力应变循环往往偏大.同时,某些零部件的瞬态温度场是决定其疲劳强度和使用寿命的重要因素,而获得准确的瞬态温度场是非常困难的.文中对某型发动机的高压涡轮盘进行疲劳试验条件下弹塑性有限元分析,对一台涡轮盘的残余应力进行测试,利用稳态温度场计算涡轮盘危险点最大应力应变循环,并根据弹塑性有限元分析和通过残余应力测试得到的最大应力应变循环进行低循环疲劳寿命预测.研究结果表明,弹塑性有限元分析法预测的寿命偏低,由残余应力可以较准确地确定最大应力应变循环.     

喷丸残余应力场有限元模拟研究进展

喷丸强化处理工艺可以显著提高金属材料的抗疲劳和抗应力腐蚀等性能,这与喷丸后在金属表面层形成的残余应力场紧密相关,因此对喷丸残余应力的大小及分布进行预测具有重要意义.对近年国内外喷丸残余应力场的有限元模拟进行评述,总结出6种典型的残余应力分析模型,分别是二维轴对称模型、四对称面模型、三对称面模型、双对称面模型、单对称面模...     

Effects on mechanical properties in electron beam welding of TC4 alloy by laser shock processing

The surface of TC4 titanium alloy welding line by electron beam welding (EBW) was processed by high power Q-switched and repetition-rate Nd: glass laser. Effects of laser power and spot diameter on residual stress and microhardness of the TC4 alloy welding line by laser shock processing (LSP) have been analyzed. Results show that residual stresses almost do not change as laser power is 45.9 J, spot diameter is ϕ9 mm; While laser power is 45.9 J, spot diameter less than ϕ3 mm, the distribution of residual stress in welding line occurs obvious variation, which residual stress increase obviously with spot diameter decrease. When power density is bigger than 1.8 × 10¹⁰ W/cm², residual stresses of electron beam welding line occur change by LSP, which improve obviously residual stress distribution; while laser power is bigger than 1.2 × 10¹⁰ W/cm², the surface micro-hardness of electron beam welding line occurs change by LSP, which improve obviously micro-hardness distribution. Mechanical properties of TC4 titanium alloy welding line will be improved by LSP, which provides experimental foundation for further controlling the distributions of residual stress and micro-hardness during laser shock processing. __________ Translated from Journal of Jiangsu University (Natural Science), 2006, 27(3): 207–210 [译自: 江苏大学学报 (自然科学版)]     

激光冲击强化对TC4钛合金单面修饰激光焊接接头疲劳性能的影响

对TC4钛合金单面修饰激光焊接接头进行激光冲击强化,对比强化前后焊接接头的疲劳寿命,在光学显微镜和扫描电镜下观察断口疲劳断裂特征,并从焊接接头的显微硬度、微观组织、残余应力分布等方面综合分析激光冲击强化对TC4钛合金单面修饰激光焊接接头的强化机理。试验结果表明：未强化和强化试样均在焊缝咬边处萌生疲劳裂纹,强化试样疲劳寿命是未强化试样疲劳寿命的3.77~9.15倍,强化试样焊缝咬边处马氏体细化,显微硬度提高,焊缝表面呈残余压应力分布,焊缝咬边处残余压应力达-564.37±9.85MPa。晶粒细化和高幅值残余压应力综合作用下抑制了焊缝咬边处疲劳裂纹的萌生,且增大了裂纹扩展阻力,从而提高了焊接接头疲劳性能。     

扫描方式对激光金属沉积成形过程热应力的影响

为改善金属试样的成形质量,降低沉积过程的热应力,研究不同扫描方式下激光金属沉积成形过程中热应力的动态分布规律具有非常重要的意义。根据有限元中的“单元生死”技术,通过ANSYS参数化设计语言 (ANSYS parametric design language, APDL) 编程研究了沿长边平行往复、沿短边平行往复以及层间正交变向平行往复等填充扫描方式对整个成形过程热应力的影响,详细探讨了各种扫描方式下Von Mises热应力、x方向热应力、y方向热应力以及z方向热应力的动态分布规律,并分析热应力产生和分布的原因。在与模拟过程相同的条件下,实际成形试验所得结果与模拟结果吻合较好。