压缩载荷对冷胀孔残余应力的影响

用考虑组合强化、增量理论的有限元法计算了２１２４－Ｔ８５１ 板件开缝衬套冷胀孔的残余应力场及其在压缩载荷作用下的变化情况。结果表明冷胀之后的压载循环将引起冷胀残余应力的衰减。一个足够大的压缩载荷能够使冷胀工艺给疲劳寿命带来的增益完全丧失。文中说明压缩载荷对冷胀的作用可以和超载后半周压载对裂纹的作用相比拟,建议在确定合理胀孔量时应当把它对所承受的载荷谱中压缩载荷的敏感性考虑在内,应当加强有关的疲劳对比试验研究及残余应力场测量研究。     

摩擦液柱成形初始阶段的二维轴对称数值模拟 

基于二维轴对称有限元模型,通过外部施加表面热流强度的手段对摩擦液柱单元成形(FHPP)初始阶段的热力耦合接触摩擦问题进行了数值模拟。结果表明：金属棒接触摩擦表面上的温度上升迅速,当其材料的屈服强度降低到一定值后就会在轴向应力的作用下发生塑性流动,进而对预钻孔底部进行填充;所施加的轴向应力、转速以及金属棒与预钻孔之间的径向间隙对成形过程都有不同程度的影响,轴向应力对成形过程的影响较转速对成形过程的影响略大,而径向间隙对成形过程的影响相对较小。     

Editorial preface for special issue of FAIM 2015 published in IJAMT

Machining of critical components such as turbine compressor and pump parts is required to generate compressive residual stress on the surface layer in order to obtain high fatigue life. As an effective method to generate and improve the compressive residual stress of machined parts, burnishing has been widely used in industry. Despite its importance, few studies have investigated the mechanism of burnishing on surface residual stress. In this paper, the interference effects due to nearby burnishing points were revealed and investigated in the context of an elastic burnishing tool. The interference effects during the burnishing process help to enhance the compressive residual stress and improve the distribution of compressive residual stress on the burnished surface layer. In order to analyze the mechanism behind the interference effect more clearly, a 2D finite element model of the burnishing process was developed. It was found that the interference effect exists and becomes stronger as the feed rate is decreased. Small feed rates show a more apparent effect on the enhancement of interference effects. The results indicate that the interference effect of the workpiece surface is mainly created by the influence of the preceding burnishing points on the future burnished surface.     

Residual stress field of ballised holes

Ballising, involving pushing a slightly over-sized ball made of hard material through a hole, is a kind of cold working process. Applying ballising process to fastener holes produces compressive residual stress on the edge of the holes, and therefore increases the fatigue life of the components or structures. Quantification of the residual stress field is critical to define and precede the ballising process. In this article, the ballised holes are modeled as cold-expanded holes. Elastic-perfectly plastic theory is employed to analyze the holes with cold expansion process. For theoretical simplification, an axially symmetrical thin plate with a cold expanded hole is assumed. The elasticplastic boundaries and residual stress distribution surrounding the cold expanded hole are derived. With the analysis, the residual stress field can be obtained together with actual cold expansion process in which only the diameters of hole before and after cold expansion need to be measured. As it is a non-destructive method, it provides a convenient way to estimate the elastic-plastic boundaries and residual stresses of cold worked holes. The approach is later extended to the case involving two cold-worked holes. A ballised hole is looked upon as a cold expanded hole and therefore is investigated by the approach. Specimens ballised with different interference levels are investigated. The effects of interference levels and specimen size on residual stresses are studied. The overall residual stresses of plates with two ballised holes are obtained by superposing the residual stresses induced on a single ballised hole. The effects of distance between the centers of the two holes with different interference levels on the residual stress field are revealed.     

A sustainability comparison between drilling and milling for hole-enlargement in machining of hardened steels

Abstract

Hole-making is one of the most important processes of metal shaping domain. Although, drilling is a commonly used approach to cut holes in metallic parts, the process cannot be completed with the cutting action of one drill bit if the work material is hard and diameter of the hole is large. Usually, a drill having diameter equal to the required diameter of the hole is utilized to enlarge a predrilled hole of a smaller diameter. In this work, we have investigated sustainability of using another method of enlarging a pre-drilled hole, namely side and end milling and compared it with the drilling-based approach. The work material used in the study is a high carbon steel, which is heat-treated to two distinct levels of surface hardness. Besides process type and work material hardness, the other two parameters tested in the investigation are cutting speed and depth of hole. A total of 16 experiments were performed to generate data regarding the sustainability measures, namely hole surface roughness, specific cutting energy and tool wear. Process choice (drilling or milling) for hole-enlargement was found to possess a significant effect on all the measured responses. Analyses carried out on the experimental data revealed that although the drilling-based option led to an immensely better surface finish, the milling-based option performed better with respect to the other measures of economic and environmental sustainability.     

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.     

Measurement of residual stress in thick section composite laminates using the deep-hole method

The deep-hole method is a method of measuring residual stress in large metallic components. In this paper, an extension to the deep-hole method is described to allow the residual stresses in thick section composite laminated plates to be evaluated. The method involves first drilling a small hole through the laminate perpendicular to the surface. The material around the hole is then machined away, resulting in a change in diameter of the hole due to the release of residual stress. This change in diameter is measured and used to calculate the residual stress. The calculation requires the evaluation of coefficients that depend on the properties of the composite. In this work, the finite element method is used to evaluate these coefficients. Using this method, the residual stresses in a 22 mm thick carbon/epoxy composite plate are measured and reported.     

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.     

7075铝合金板淬火残余应力模拟及实验研究

铝合金板在淬火过程中产生较大的残余应力,导致构件出现变形、翘曲。采用准耦合法对7075铝合金板在淬火过程中的温度场和应力场进行数值模拟,利用盲孔法对淬火后的残余应力分布进行实验测量。结果表明,7075铝合金板淬火后残余应力呈现出外压内拉的分布状态,淬火过程中形成的较大温度梯度造成的不均匀塑性变形是应力产生的主要因素,铝板表层和心部冷却速度的变化导致了应力分布状态的转变。实验结果验证了准耦合模拟方法的可行性,研究结论为残余应力控制提供重要依据。     

轴承套圈强化研磨表面残余应力试验研究

结合疲劳裂纹扩展原理,分析残余压应力对材料表面的疲劳裂纹扩展速率的影响。采用电磁无心夹具对轴承套圈进行定位,利用新型轴承强化研磨机对轴承套圈表面进行加工,基于X射线衍射法测定轴承套圈加工后的表面残余应力。试验结果表明,强化研磨加工使轴承套圈表面的残余压应力增大,提高工件服役可靠性。     

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.     

GH742合金激光冲击强化后的表面残余应力

采用X射线衍射法对GH742合金激光冲击强化后的表面残余应力进行了测试,采用云纹干涉结合盲孔法对残余应力随深度的分布进行了测试。结果表明:GH742合金经激光单点冲击后,表面残余压应力最高可达1 180MPa,且残余压应力层深度达到1.2mm;50%光斑搭接率强化后的表面残余压应力约为1 100MPa。     

Experimental study on grinding-induced residual stress in C-250 maraging steel

Residual stress plays a significant role in the performance of a part, while the residual stress in the ground maraging steel, which is often used in the manufacture of precision parts, is rarely mentioned. In order to understand the variations of residual stress in ground maraging steel and provide insight into the controlled-stress grinding process of the steel, the surface and subsurface residual stress distributions in ground C-250 maraging steel (3J33) were studied. The results show that the mechanical effects dominate the thermal effects in the dry grinding process, indicated by only compressive residual stress generated in the ground workpiece. Furthermore, more insights into the residual stress distribution were provided by proposing four residual stress distribution parameters including surface residual stress, peak compressive residual stress, the depth of peak compressive residual stress, and residual stress penetration depth. The variations of these parameters were comprehensively studied. Results show that the surface residual stress and peak compressive residual stress depend greatly on the grinding speed and higher grinding speed generates larger compressive residual stress, while the depth of peak compressive residual stress varies slightly with the grinding parameters. The residual stress penetration depth increases with the increase of the grinding speed and grinding depth, and decreases with the increase of the workpiece speed. The results in this study can be used to assist in controlled-stress grinding applications for high performance critical parts of maraging steel.     

The use of ultrasonic cavitation peening to improve micro-burr-free surfaces

Advances in machining technology, particularly in the field of micro-machining, have led to the design and creation of miniature components suitable for use in the precision engineering industry. However, the need to contain ubiquitous burrs still exists and has to be addressed. Previous studies on deburring have mostly focused on the parametric investigations of orientation, temperature, type of liquid media and abrasives, frequency, deburring time and power. It is hypothesized that by inducing compressive residual stresses on a pre-machined workpiece surface, the resulting burrs caused by machining can be minimized or even eliminated. The paper presents the findings of an investigative study into the possibility of inducing compressive residual stresses on machined surfaces by the use of ultrasonic cavitation, with the aim of reducing or eliminating burr formation. The paper also briefly reviews the development of ultrasonic cavitation and covers published work on deburring by ultrasonic cavitation. Experimental results are presented on the performance of ultrasonic cavitation peening on the residual stress in Stavax stainless steels and on micro-burr formation.     

Laser hole cutting into Ti-6Al-4V alloy and thermal stress analysis

Laser hole cutting into Ti-6Al-4V alloy is carried out. Temperature and stress fields during the cutting process are predicted using the finite element code. Temporal variation of surface temperature in the region close to the kerf edge is monitored by a thermocouple and compared with the predictions. The residual stress formed in the cutting region is obtained from the XRD technique and compared with the predictions. The morphological changes around the kerf surfaces are examined incorporating optical and scanning electron microscopes. It is found that von Mises stress attains slightly higher values at the top circumference as compared to that corresponding to the bottom circumference of the hole cut. The prediction of temperature variation agrees well with the thermocouple data. The residual stress predicted also agrees with the results of the XRD technique.     

Effects of the indentation process on fatigue life of drilled specimens: optimization of the distance between adjacent holes

The generation of permanent compressive stresses around the holes is recognized as a valuable mean to delay the onset and propagation of the defects and to extend the fatigue life of the mechanical components. In the work, a bilateral indentation process, performed on each side of the component, is widely used in order to create a residual circumferential stress field around the area to be drilled and that persists after the drilling operation. In order to evaluate the process parameters and to identify optimal geometric solutions, several static and fatigue tests are conducted on AW 6082-T6 aluminum alloy specimen where two holes are created. In particular, experimental tests on only drilled specimens (OD) and specimens subjected to Indentation process (IP) varying the indentation depth and the center-tocenter distance of the holes are performed. Several numerical analyses, conducted in ANSYS APDL environment with explicit solver, allow to determine the influence of the distance between two adjacent holes and the effect of the indentation depth on the residual stress distribution.     

Experiment research on grind-hardening of AISI5140 steel based on thermal compensation

The grind-hardening process utilizes the heat generated to induce martensitic phase transformation. However, the maximum achievable harden layer depth is limited due to high grinding forces, and the tensile residual stress appears on the ground surface in the grind-hardening process. This paper proposes a new grind-hardening technology using thermal compensation. The workpiece of AISI5140 steel is preheated by electric resistance heating, and ground under the condition of the workpiece temperature 25°C, 120°C, 180°C and 240°C. The grinding force, harden layer depth and surface quality including residual stress on ground surface, surface roughness and micro-hardness are investigated. The experimental results show that a deep harden layer with a fine grain martensite can be obtained with the thermal compensation. The ground workpiece surface produces a certain compressive residual stress, and the residual compressive stress value increases with preheating temperature. As the preheating temperature increases, grinding force slightly decreases, while there is slightly increment of surface roughness. Compared with the conventional grind-hardening process, both the harden layer depth and residual stress distribution are significantly improved.

     

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.     

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.     

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.