Evolution of residual stresses in micro-arc oxidation ceramic coatings on 6061 Al alloy

Most researches on micro-arc oxidation mainly focus on the application rather than discovering the evolution of residual stresses. However, residual stresses in the surface coatings of structural components have adverse effects on their properties, such as fatigue life, dimensional stability and corrosion resistance, etc. The micro-arc oxidation ceramic coatings are produced on the surfaces of 6061 aluminum alloy by a homemade asymmetric AC type of micro-arc oxidation equipment of 20 kW. A constant current density of 4.4___0.1 A/dm2 and a self-regulated composite electrolyte are used. The micro-arc oxidation treatment period ranges from 10 min to 40 min, and the thickness of the ceramic coatings is more than 20 Bin. Residual stresses attributed to 7-A1203 constituent in the coatings at different micro-arc oxidation periods are analyzed by an X-ray diffractometer using the sin2~u method. The analysis results show that the residual stress in the ceramic coatings is compressive in nature, and it increases first and then decreases with micro-arc oxidation time increase. The maximum stress value is 1 667_＋20 MPa for period of 20 min. Through analyzing the coating thickness, surface morphology and phase composition, it is found that the residual stress in the ceramic coatings is linked closely with the coating growth, the phase composition and the micro cracks formed. It is also found that both the heat treatment and the ultrasonic action release remarkably the residual compressive stress. The heat treatment makes the residual compressive stress value decrease 1 378 MPa. The ultrasonic action even alters the nature of the residual stress, making the residual compressive stress change into a residual tensile stress.     

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.     

THE SCATTER OF SURFACE RESIDUAL STRESSES PRODUCED BY FACE-TURNING AND GRINDING

Conventional studies on residual stresses induced by manufacturing processes have focused on the average residual stress value over the processed surface area. However, what dictates the fatigue life of a manufactured surface is its weakest point. Thus, it is not the average value of the stress but the local extreme that is most relevant for safety considerations. Therefore, it is very important to study the variations of residual stresses over the machined surface. This paper is the continuation of the work [1] investigating the magnitude of surface residual stress scatter between the face-turned and ground samples. The objective of this research is to test the hypotheses that the scatter of surface residual stresses over the faced samples is smaller than that of the ground ones and that the scatter of surface residual stresses varies significantly among ground samples while it does not vary in a statistically significant sense among faced specimens for the given cutting conditions. In order to compare the surface residual stress variations, two sets of the specimens of Ti 6Al-4V bar are ground while the other two sets faced. The residual stresses over a small surface area (5mm × 8 mm) are measured at four locations of each machined sample using an X-ray diffraction technique. Statistical analysis of the measured residual stresses shows that the proposed hypotheses hold. Experimental data also show that a different number of grinding passes may induce a different scatter of microhardness. The possible causes and ramifications of the foregoing results are discussed. It is suggested that the variations of residual stress be included as a surface integrity parameter, joining its average value.     

In Situ Observation of Wax-in-Oil Flow in Rough Soft Contact

The paper presents the results of experimental study into the microstructure and changes in residual stresses resulting from sliding and rolling/sliding loaded interaction between metallic surface coatings with embedded PTFE reservoirs and various counter faces. It was found that before testing surface coatings had compressive residual stresses. Molybdenum coating with all types of PTFE reservoirs displayed, as a result of testing, decrease in residual stress level unevenly distributed within the contact path. On the other hand, aluminum–bronze coating, regardless the shape of PTFE reservoirs embedded into it, did not show any appreciable change in the residual stress level due to testing. Residual stresses generated under testing conditions in counter materials were typically of the compressive nature.     

Residual stresses in TiN, DLC and MoS2 coated surfaces with regard to their tribological fracture behaviour

Thin hard coatings on components and tools are used increasingly due to the rapid development in deposition techniques, tribological performance and application skills. The residual stresses in a coated surface are crucial for its tribological performance. Compressive residual stresses in PVD deposited TiN and DLC coatings were measured to be in the range of 0.03-4 GPa on steel substrate and 0.1-1.3 GPa on silicon. MoS₂ coatings had tensional stresses in the range of 0.8-1.3 on steel and 0.16 GPa compressive stresses on silicon. The fracture pattern of coatings deposited on steel substrate were analysed both in bend testing and scratch testing. A micro-scale finite element method (FEM) modelling and stress simulation of a 2 μm TiN-coated steel surface was carried out and showed a reduction of the generated tensile buckling stresses in front of the sliding tip when compressive residual stresses of 1 GPa were included in the model. However, this reduction is not similarly observed in the scratch groove behind the tip, possibly due to sliding contact-induced stress relaxation. Scratch and bending tests allowed calculation of the fracture toughness of the three coated surfaces, based on both empirical crack pattern observations and FEM stress calculation, which resulted in highest values for TiN coating followed by MoS₂ and DLC coatings, being K_C = 4-11, about 2, and 1-2 MPa m^1/2, respectively. Higher compressive residual stresses in the coating and higher elastic modulus of the coating correlated to increased fracture toughness of the coated surface.     

Application of quenching to create highly triaxial residual stresses in type 316H stainless steels

It has been proposed that highly triaxial residual stress fields may be sufficient to promote creep damage in thermally aged components, even in the absence of in-service loads. To test this proposal, it is necessary to create test specimens containing highly triaxial residual stress fields over a significant volume of the specimen. This paper presents results from an experimental and numerical study on the generation of triaxial residual stresses in stainless steel test specimens. Spray water quenching was used to generate residual stress fields in solid cylinders and spheres made from type 316H stainless steel. A series of finite element simulations and measurements were carried out to determine how process conditions and specimen dimensions influenced the resulting residual stress distributions. The results showed that highly compressive residual stresses occurred around the surfaces of the cylinders and spheres and tensile residual stresses occurred near the centre. Surface residual stresses were measured using the incremental centre hole-drilling technique, while internal residual stresses were measured using neutron diffraction. Overall there was good agreement between the predicted and measured residual stresses. The level of triaxiality was found to be very sensitive to the heat transfer coefficient, and could be controlled by adjusting the cooling conditions and changing the dimensions of the steel samples. This differed from other processes, such as welding and shot-peening, where the magnitudes and distributions of residual stresses are ill-defined and the volume of material subjected to a triaxial residual stress state is relatively small.     

Residual stress evaluation in dissimilar welded joints using finite element simulation and the L CR ultrasonic wave

Subsurface stresses in welded structures increase the likelihood of fatigue cracks and environmental induced material degradation. The ability to evaluate stresses at the surface as well as in the interior of welded structural members would substantially increase the accuracy of structure life estimation. The longitudinal critically refracted (L _CR) wave is a bulk longitudinal mode that travels within an effective depth underneath the surface. It may be used to detect in-plane subsurface stresses in the structures. This paper presents a three dimensional thermo-mechanical analysis to evaluate welding residual stresses in dissimilar plate-plate joint of AISI stainless steel 304 and Carbon Steel A106-B type. After finite element simulation, the residual stresses were evaluated by L _CR ultrasonic waves. Finally the results of two methods were compared and verified by hole-drilling method. This paper introduces a combination of ??Finite Element Welding Simulation?? and ??Ultrasonic Stress Measurement using the L _CR Wave?? which is called as ??FEL _CR??. The capabilities of FEL _CR??. The capabilities of FEL _CR in residual stress measurement are confirmed here. And also this paper evaluates residual stresses of dissimilar welded joints by LCR ultrasonic waves. It has been shown that predicted residual stress from three dimensional FE analyses is in reasonable agreement with measured residual stress from LCR method and also the results of both are verified with hole-drilling experimental measurements.     

Micro-Raman depth analysis of residual stress in machined germanium

Raman spectroscopy has proved to be a useful nondestructive technique for measuring residual stresses in semicondutors. The Raman microprobe is used to investigate the effects of machine parameters on residual stresses in single point diamond turned germanium (Ge). A profiling technique that provides a method of obtaining the residual stress information as a function of depth with depth resolutions of 10.0 nm is discussed. This method is used to analyze the asymmetrically broadened and shifted spectral features in the machined samples. Residual stresses are sampled across ductile feed cuts in (100) Ge wafers, which were single point diamond turned using various feed rates (12.5, 25 nm/rev), rake angles (0°, −10°, −30°), and clearance angles (6°, 16°). In general a region of plastically deformed material that shows slight compressive stresses exist near the surface of the diamond turned sample. The compressive surface stress increases to a maximum at a depth of ≈ 50 nm beneath the surface at which point the stress rapidly changes sign. The rapid sign change is indicative of the transition from plastic to elastic deformation. Deeper probe regions exhibit increasing tensile stresses, which reach a maximum and then relax to zero at greater depths in the sample. A related study of the stress field occurring around Vicker's hardness indents provides a link between theoretical and experimental stress profiles and demonstrates the accuracy of the micro-Raman profiling technique.     

An X-ray diffraction study of the triaxial normal residual stresses in worn aluminum bronze surfaces

In metallic systems subjected to sliding wear material loss often results from the formation and subsequent delamination of a transfer layer. This phenomenon has previously been observed in copper-rich aluminum bronzes. The evidence obtained indicates that wear occurs by the delamination of the layer parallel to the surface in the vicinity of the layer-substrate interface.In general, copper-rich aluminum bronzes exhibit increasing wear rates with increasing aluminum content. Since wear results from the failure of the transfer layer, it is expected that factors which affect the stability of the layer should be related to the observed wear rate. One factor could be the residual stress pattern induced during the wear process. In a previous study the in-plane residual stresses were measured on worn copper-rich aluminum bronze samples. The residual stresses were found to be compressive and of increasing magnitude for increasing wear rate (increasing aluminum content). In this investigation the stresses normal to and at some distance below the surface were measured on samples of compositions similar to those used previously. The normal stresses were found to be tensile and increased in magnitude with increasing wear rate. It is suggested that the development of residual stresses normal to the surface may be an important wear-controlling parameter.     

A comparative study of residual stress and affected layer in Aermet100 steel grinding with alumina and cBN wheels

Residual stresses induced by finish machining processes have significant effect on fatigue strength of ultra-high strength steel in large structures. In this study, an experimental investigation was carried out to explore the residual stress and affected layer in grinding Aermet100 by using a resin bond white alumina (WA) wheel and cubic boron nitride (cBN) wheel, respectively. The grinding force and temperature were measured, and then the affected layer of residual stress, microhardness, and microstructure by a WA and a cBN wheel was obtained. The comparisons of surface residual stress studies and thermal–mechanical coupling mechanism on the affected layer were discussed in light of the current understanding of this subject. Experimental results show that grinding with cBN wheel can provide compressive residual stress and a smaller affected layer owing to its better thermal conductivity; the coupling effect of wheel speed and grinding depth plays a more significant role on surface residual stress; when grinding with parameters v _w?=?18 m/min, v _s?=?14 m/s, and a _p?=?0.01 mm, compressive residual stress and hardening effect appeared on ground surface, and the depth of residual stress layer is 40~50 μm; the depth of hardened layer is 30~40 μm and the depth of plastic deformation layer is 5~10 μm.     

Influence of material microstructure changes on surface integrity in hard machining of AISI 52100 steel

Residual stresses are a consequence of thermo-mechanical and microstructural phenomena generated during the machining operation. Therefore, for improving product performance in machined hardened steels, material microstructure changes (commonly referred to as white and dark layers) must be taken into account. This paper presents a finite element model for white and dark layers formation in orthogonal machining of hardened AISI 52100 steel. In particular, a hardness-based flow stress and empirical models for describing the white and dark layers formation were developed and implemented in the finite element code. A series of experiments was carried out in order to validate the proposed simulation strategy and to investigate the influence of material microstructure changes on residual stresses. As main results, it was firstly demonstrated by surface topography analysis as both the white and dark layer are the result of microstructural alterations mainly due to rapid heating and quenching. Furthermore, it was found as both the presence of white and dark layers influence the residual stress profile. Particularly, the former significant impacts on the magnitude of maximum residual stress and on the location of the peak compressive residual stress; the latter reduces the compressive area.     

Transient and residual thermal stresses in quenched cylindrical bodies

To predict residual and thermal stresses which occur during water quenching of solid cylindrical rod and ring cross-sectioned steel tubes, a finite element technique has been used. The variations of residual stresses on different surfaces and cross-sections, e.g. in the radial, axial and tangential directions have been examined, and the effect of internal diameter of tubes on residual stress was investigated. The results show that the residual stresses act as a compressive force along the cooling surface and then by moving away from the surface begin to decrease and reverse their sign, near the centre of the cylinder are subjected to tensile stresses. Because of the reversal of the sign of stress, the effective stress goes to a minimum at some distance from the frontal surface and this may be vital since lower plastic deformation may cause cracking failure. As in solid cylinder, in cylindrical tubes also, the frontal and the upper cooling surface has significant effect on the stress distribution. From the comparison of the residual stress distributions of solid cylinder and cylindrical tubes and using their individual stress maps it was seen that they vary considerably along different cooling surfaces, especially at the frontal surface.     

超声滚压工艺对TC4钛合金表面残余应力的影响

钛合金关节轴承是航空发动机的重要部件之一,但由于其润滑条件较差,极易产生磨损从而导致失效事故。采用超声滚压工艺对钛合金试件表面进行强化试验,以有效增强试件表面耐磨性能和抗疲劳特性,并着重研究静压力、滚压次数和主轴转速等超声滚压强化工艺参数对钛合金表面残余压应力与剪切应力的影响规律。试验结果与理论分析结果表明,残余压应力随着静压力、滚压次数和主轴转速的提高而增大,剪切应力随着静压力、滚压次数和主轴转速的提高而总体减小;滚压后试件表面可获得-2 000～-500 MPa的残余压应力和-600～-300 MPa的剪切应力。超声滚压技术可以有效提高钛合金材料表面的残余压应力,并有效降低表面的剪切应力。     

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...     

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.     

Optimization of cutting conditions for minimum residual stress,cutting force and surface roughness in end milling of S50C medium carbon steel

Residual stresses are usually imposed on a machined component due to thermal and mechanical loading. Tensile residual stresses are detrimental as it could shorten the fatigue life of the component; meanwhile, compressive residual stresses are beneficial as it could prolong the fatigue life. Thermal and mechanical loading significantly affect the behavior of residual stress. Therefore, this research focused on the effects of lubricant and milling mode during end milling of S50C medium carbon steel. Numerical factors, namely, spindle speed, feed rate and depth of cut and categorical factors, namely, lubrication and milling mode is optimized using D-optimal experimentation. Mathematical model is developed for the prediction of residual stress, cutting force and surface roughness based on response surface methodology (RSM). Results show that minimum residual stress and cutting force can be achieved during up milling, by adopting the MQL-SiO₂ nanolubrication system. Meanwhile, during down milling minimum residual stress and cutting force can be achieved with flood cutting. Moreover, minimum surface roughness can be attained during flood cutting in both up and down milling. The response surface plots indicate that the effect of spindle speed and feed rate is less significant at low depth of cut but this effect significantly increases the residual stress, cutting force and surface roughness as the depth of cut increases.     

Experimental Determination of Critical Fields of 90-Degree Domain Wall Displacement in Plastically Deformed Low-Carbon Steels

Experimental data are presented for the field dependences of differential magnetic permeability μ_d (H₀) of plastically deformed low-carbon steel-St3 samples both in an unloaded state and under tensile stresses. It is shown that applying tensile stresses drastically changes the shape of curves μ_d (H₀ ) a fact that indicates compensation of internal residual compressive stresses in the samples by external tensile stresses. A new technique is proposed for the experimental determination of the critical fields of displacement of 90-degree domain boundaries based on dependences μ_d (H₀ ). Residual compressive stresses in plastically deformed St3 steel are estimated.     

Abrasive wear in ceramic laminated composites

Laminated ceramic structures in the system Al₂O₃/Al₂O₃ + 3Y-TZP (A/AZ) were prepared using a tape casting technique in order to obtain ceramic layers with different compositions and thicknesses. Piezo-spectroscopy was used to evaluate the residual stresses arisen from a calibrated mismatch in thermal expansion coefficients of the layers during the sintering process of the composite. The dependence of the residual stresses in the A and AZ layers on their thickness ratio was established. A microscale ball cratering method was used to investigate the influence that the surface compressive stress can play on the abrasive wear resistance of the composite structures. The results were compared with those obtained with an unstressed reference material prepared either by lamination of pure alumina green-sheets or by cold isostatic pressing of alumina powder. The experimental results have shown that the abrasive wear resistance is higher for samples with compressive residual stresses within the surface regions.     

FE-simulation of the effects of machining-induced residual stress profile on rolling contact of hard machined components

Compared with grinding, hard turning may induce a relatively deep compressive residual stress. However, the interactions between the residual stress profile and applied load and their effects on rolling contact stresses and strains are poorly understood, and are difficult to measure using the current experimental techniques due to the small-scale of the phenomena. A new 2-D finite element simulation model of bearing rolling contact has been developed, for the first time, to incorporate the machining-induced residual stress profile instead of only surface residual stresses. Three cases using the simulation model were assessed: (a) measured residual stress by hard turning, (b) measured residual stress by grinding, and (c) free of residual stress. It was found that distinct residual stress patterns hardly affect neither the magnitudes nor the locations of peak stresses and strains below the surface. However, they have a significant influence on surface deformations. The slope and depth of a compressive residual stress profile are key factors for rolling contact fatigue damage, which was substantiated by the available experimental data. Equivalent plastic strain could be a parameter to characterize the relative fatigue damage. The magnitudes of machining-induced residual stress are reduced in rolling contact. The predicted residual stress pattern and magnitude agree with the test data in general. In addition, rolling contact is more sensitive to normal load and residual stress pattern than tangential load.     

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.