Residual Stress Depth Profiles Research Articles

Depth Profile of Residual Stresses to Analyze Textures in Extruded A6XXX

The extruded profiles of A6XXX alloys have a textured surface structure, different from those having the inner cube and Goss orientations. This structure possibly contributes to the decrease in bending strength. Although the mechanism of degradation has been discussed for precipitates, it is necessary to reconsider it from the viewpoint of residual stress distribution. In this study, we first determine the optimal method to evaluate the residual stress in a textured structure. The cosa method with 3-axis oscillation successfully yielded highly precise residual stress values even in extruded aluminum alloys. Although the inner layers of A6063 and A6005C had the cube and Goss orientation, the surface layer at a depth of ∼400 μm had a (112) orientation in A6063 and that at ∼200 μm had a (110) orientation in A6005C. The residual stress increased further from the surface, and changes in the preferred orientation corresponded well with those of residual stress, especially in A6005C. It is suggested that the residual stress increases as the tensile strength increases.

Nondestructive residual stress depth profile analysis at the inner surface of small boreholes using energy-dispersive diffraction under laboratory conditions.

Energy-dispersive diffraction under both laboratory and synchrotron conditions was applied to study the hoop stress in the near-surface region of the inner wall of boreholes with a small diameter of 2 mm. By use of different X-ray beam cross sections for the sin2ψ measurements, it is demonstrated that the borehole-to-beam-diameter ratio must be considered in the evaluation. A beam cross section which is comparable to the borehole diameter reduces the slope of the d hkl φψ-sin2ψ distributions and thus invalidates the result of stress analysis. A quantitative relationship is applied, which allows the results obtained under the above conditions to be scaled so that they reflect the actual residual stress state at the measurement position. Owing to the small diffraction angles, energy-dispersive diffraction proves to be the only suitable experimental technique that allows a nondestructive and depth-resolved analysis of the hoop stress component at the inner surface of boreholes with a large length-to-diameter ratio.

Depth Profiles of Residual Stresses in Inconel 718Machined with Uncoated and Coated Tools

Inconel 718 is one of the super-alloy materials, belonging to nickel-chromium alloy family, which has major applications in aerospace sector such as engine parts and turbine components. Durability of these components during engineering performance is affected by residual stresses induced in them in the course of their manufacturing processes. The concept of the present paper is to provide an insight view of induced residual stresses in Inconel 718 work piece, when machined with coated (TiN) and uncoated tools at optimum conditions. For this purpose, turning experiments have been conducted on IN718 material through statistical approach using L9 orthogonal array. Taguchi optimization method is exercised with the emphasis on minimizing the cutting forces resulted during machining. The residual stresses generated in the work piece at the optimum conditions employed for both the tools have been evaluated using XRD method. Conditions such as cutting speed of 60 m/min, feed at 0.068 mm/rev and depth-of-cut of 0.10 mm have been optimized for achieving minimum cutting forces during machining of IN 718 using both coated and uncoated tools. However, tensile stresses on the initial surface layer and compressive stresses in the sub-surface layers are found higher in the work piece material machined with uncoated tool. Surface roughness and temperature developed on the surface of the machined bar are higher in case of uncoated tool than those with coated tool.

A validated modelling technique for incorporating residual stresses in glass structural design

This paper presents the development of a simple parabolic residual stress depth profile model for characterising residual stresses in construction-sector glass. The proposed model requires only the knowledge of the surface residual stress, which is usually available from glass manufacturers. Unlike the complex computational techniques reported in the literature, such as modelling physical, microstructural and mechanical phenomena of glass at different temperatures during manufacturing, the proposed model obviates the need for modelling multi-physics phenomenon of the generation of residual stresses. The proposed model also eliminates the need of sophisticated experimental equipment, such as Scattered-Light-Polariscopes (SCALP), which are usually not available among practicing engineers, in order to characterise the residual stresses. Residual stress predictions from the proposed parabolic model were validated against experimental results reported in the literature. Using the concept of eigenstrains, the paper also extends the results of the proposed parabolic residual stress depth profile model for incorporating the effects of residual stresses in stress analysis of glass structures.

Influence of the Measurement Parameters on Depth-Resolved Residual Stress Measurements of Deep Rolled Construction Steel using Energy Dispersive X-ray Diffraction

Abstract In this study, the influence of different measurement parameters of energy dispersive residual stress measurements on the obtained residual stress depth profiles of deep rolled construction steel S355 G10+M was investigated. Especially the diffraction angle θ and afterwards the measuring time t per inclination angle ψ were varied. A diffraction angle of θ = 20° shows an acceptable compromise between achievable information depth and detected total intensity of diffracted X-ray quanta. Furthermore, a measuring time per inclination angle ψ of t = 2,400 s leads to an acceptable standard deviation regarding the determined residual stress states. With these parameters for the energy-dispersive measurement, a comparison between angle-dispersive and energy-dispersive determination of residual stress depth profiles was carried out. Quantitative similarities between these two methods were observed, whereby the energy-dispersive determined residual stress depth profiles are rather discontinuous. A possible explanation could be found in the model used for the calculation of the net-plane-dependent radiographic elastic constants (XEC). In general, the energy-dispersive residual stress measurement was qualified for the determination of residual stress depth profiles of deep rolled construction steel. Based on the findings, a time-efficient non-destructive residual stress measurement can be carried out in the future with the discussed measurement parameters at maximum possible information depth.

Effect of Grit Blasting Parameters on Surface and Near-Surface Properties of Different Metal Alloys

In thermal spray, grit blasting is the standard method used to prepare the substrate surface before coating deposition. This study examines the effect of the grit blasting parameters on the residual stresses, roughness and hardness of three metal alloys with widely different mechanical properties: low-carbon steel, Ti-6Al-4V and Inconel 718. It also estimates the density of dislocations using the Williamson–Hall method. The dislocation structures of low-carbon steel grit blasted at different grit impingement angles were observed under a transmission electron microscope. The surface dislocation density was found to increase with the blasting time and angle of impact. Moreover, the depth profile of the dislocation density was in good agreement with that of the hardness profile of the blasted specimen. The residual stress depth profiles of each material at different blasting pressure showed an increase in the value and depth of maximum compressive residual stresses. Both surface residual stresses and roughness were found to increase with the grit blasting pressure, angle and, to some extent, with time and stand-off distance. The mechanisms of material erosion were found to be microcutting and indentation at lower and higher angles of abrasive impingement, respectively. The extent of damage of the materials was explained on the basis of the Johnson–Cook flow strength model.

Precision of Hole-Drilling Residual Stress Depth Profile Measurements and an Updated Uncertainty Estimator

Measurement precision and uncertainty estimation are important factors for all residual stress measurement techniques. The values of these quantities can help to determine whether a particular measurement technique would be viable option. This paper determines the precision of hole-drilling residual stress measurement using repeatability studies and develops an updated uncertainty estimator. Two repeatability studies were performed on test specimens extracted from aluminum and titanium shot peened plates. Each repeatability study included 12 hole-drilling measurements performed using a bespoke automated milling machine. Repeatability standard deviations were determined for each population. The repeatability studies were replicated using a commercially available manual hole-drilling milling machine (RS-200, Micro-Measurements). An updated uncertainty estimator was developed and was assessed using an acceptance criterion. The acceptance criterion compared an expected percentage of points (68%) to the fraction of points in the stress versus depth profile where the measured stresses ± its total uncertainty contained the mean stress of the repeatability studies. Both repeatability studies showed larger repeatability standard deviations at the surface that decay quickly (over about 0.3 mm). The repeatability standard deviation was significantly smaller in the aluminum plate (max ≈ 15 MPa, RMS ≈ 6.4 MPa) than in the titanium plate (max ≈ 60 MPa, RMS ≈ 21.0 MPa). The repeatability standard deviations were significantly larger when using the manual milling machine in the aluminum plate (RMS ≈ 21.7 MPa), and for the titanium plate (RMS ≈ 18.9 MPa). The single measurement uncertainty estimate met a defined acceptance criterion based on the confidence interval of the uncertainty estimate.

Surface Integrity Variations of Stainless Steel 304 upon Severe Shot Peening

Shot peening is one of the most powerful mechanical peening techniques applied in surface engineering to improve the mechanical properties of metal alloys by introducing compressive residual stresses. During shot peening process, surface integrity, such as microstructures, surface roughness, hardness and residual stress can be varied by adjusting the operating parameters of shot peening. To facilitate the understanding of the microstructure and properties variation, in our latest work, shot peening with high coverage percentage has been employed to create severe plastic deformation on stainless steel 304 (SUS304). Morphological and topological study on the shot peened coupons were carried out using microscopes and 3D optical profiler. Residual stress depth profiling was evaluated using X-ray diffraction technique. Microstructures, phase distribution and crystal orientation were analyzed by electron backscatter diffraction technique. The results show that phase transition γ - α’ and grain refinement occur during this mechanical peening process. A nanocrystalline layer with preferred orientation formed near the surface, and dislocations accumulated within sub-surface area, which can be attributed to the high energy input from the peening process. The maximum compressive residual stress, which is around 1000-1200 MPa, occurred beneath the top surface. All these findings will provide guideline for surface engineering at various scales and designing of the surface enhancement process via mechanical peening for achieving optimum surface integrity of metallic alloys.

Influence of Grit Blasting on Residual Stress Depth Profile and Dislocation Density in Different Metallic Substrates

This work deals with the effect of grit blasting on the surface and in-depth properties of a number of commercially available metals/alloys, i.e., low carbon steel, C45 steel, SS316, Ti-6Al-4V, Inconel 718, and Hastelloy X. The residual stress depth profile and dislocation density of the specimens were determined using X-ray diffraction. A substantial increase in dislocation density was observed after grit blasting. A transmission electron microscope was utilized to observe the dislocation structure in a grit-blasted specimen. Strain hardening was also observed at the blasted surface owing to dislocation interaction and entanglement. The hardness profile followed a trend similar to that of dislocation density along the depth of grit-blasted low carbon steel. Moreover, compressive residual stress is induced in the blasted surface having a maximum value at a certain depth. The maximum induced compressive residual stress was found to have a good correlation with the Johnson–Cook flow stress. Moreover, the depth of the affected layer and the increase in hardness were found to depend on the yield strength and the strain hardening exponent of the materials, respectively. A plastic deformation-induced phase transformation from austenite to martensite was also identified in the case of SS316.

Production-Related Surface and Subsurface Properties and Fatigue Life of Hybrid Roller Bearing Components

By combining different materials, for example, high-strength steel and unalloyed structural steel, hybrid components with specifically adapted properties to a certain application can be realized. The mechanical processing, required for production, influences the subsurface properties, which have a deep impact on the lifespan of solid components. However, the influence of machining-induced subsurface properties on the operating behavior of hybrid components with a material transition in axial direction has not been investigated. Therefore, friction-welded hybrid shafts were machined with different process parameters for hard-turning and subsequent deep rolling. After machining, subsurface properties such as residual stresses, microstructures, and hardness of the machined components were analyzed. Significant influencing parameters on surface and subsurface properties identified in analogy experiments are the cutting-edge microgeometry, S¯, and the feed, f, during turning. The deep-rolling overlap, u, hardly changes the residual stress depth profile, but it influences the surface roughness strongly. Experimental tests to determine fatigue life under combined rolling and rotating bending stress were carried out. Residual stresses of up to −1000 MPa, at a depth of 200 µm, increased the durability regarding rolling-contact fatigue by 22%, compared to the hard-turned samples. The material transition was not critical for failure.

Nano-Scale Residual Stress Profiling in Thin Multilayer Films with Non-Equibiaxial Stress State.

Silver-based low-emissivity (low-E) coatings are applied on architectural glazing to cost-effectively reduce heat losses, as they generally consist of dielectric/Ag/dielectric multilayer stacks, where the thin Ag layer reflects long- wavelength infrared (IR), while the dielectric layers both protect the Ag and act as an anti-reflective barrier. The architecture of the multilayer stack influences its mechanical properties and it is strongly dependent on the residual stress distribution in the stack. Residual stress evaluation by combining focused ion beam (FIB) milling and digital image correlation (DIC), using the micro-ring core configuration (FIB-DIC), offers micron-scale lateral resolution and provides information about the residual stress variation with depth, i.e., it allows depth profiling for both equibiaxial and non-equibiaxial stress distributions and hence can be effectively used to characterize low-E coatings. In this work, we propose an innovative approach to improve the depth resolution and surface sensitivity for residual stress depth profiling in the case of ultra-thin as-deposited and post-deposition annealed Si3N4/Ag/ZnO low-E coatings, by considering different fractions of area for DIC strain analysis and accordingly developing a unique influence function to maintain the sensitivity of the technique at is maximum during the calculation. Residual stress measurements performed using this novel FIB-DIC approach revealed that the individual Si3N4/ZnO layers in the multilayer stack are under different amounts of compressive stresses. The magnitude and orientation of these stresses changes significantly after heat treatment and provides a clear explanation for the observed differences in terms of scratch critical load. The results show that the proposed FIB-DIC combined-areas approach is a unique method for accurately probing non-equibiaxial residual stresses with nano-scale resolution in thin films, including multilayers.

Correlation of Microstructure and Properties of Cold Gas Sprayed INCONEL 718 Coatings

In the cold gas spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at temperatures well below their melting point. The high particle impact velocities and corresponding peening effects can lead to high compressive residual stresses in cold spray coatings. This can be advantageous with regard to mechanical properties as fatigue life and hence, cold spray is an ideal process for repair applications. In this study, INCONEL 718 particles were cold sprayed by using nitrogen as propellant gas. The deposited coatings with different thicknesses were characterized using electron microscopy techniques to study grain refinement and precipitates in the coating. In addition, depth-resolved residual stress measurements have been performed by the incremental hole drilling method. The residual stress depth profiles in the coatings indicate compressive residual stresses of several hundred MPa which are hardly influenced by the coating thickness. In addition, large compressive stress levels are found in surface-near regions of the substrate due to the grit blasting process. Furthermore, a post-heat treatment analysis was performed to investigate its influence on residual stresses and bonding strength. These findings are used to develop a consistent explanation of the dependence of strength values on thickness.

Nano-scale residual stress depth profiling in Cu/W nano-multilayers as a function of magnetron sputtering pressure

Residual stresses in thin films and multi-layered coatings fabricated by physical vapour deposition largely affect their mechanical and thermal reliability during operation in numerous fields of applications. By changing the argon working pressure in between each multilayer planar DC magnetron sputter deposition step, it is possible to control the residual stress distribution within coatings. A combination of FIB-DIC ring-core strain analysis, synchrotron XRD analysis based on the sin2(Ψ) method and micro-cantilever deflection analysis is used to reconstruct the in-plane stress state of multilayer coatings at different deposition pressures, with a residual stress depth profile resolution of 50 nm. A clear transition from compressive to tensile residual stresses is observed with an increase of working pressure, with pronounced stress peaks near the substrate-coating interface. These peak stresses resolved by FIB-DIC ring-core analysis exceed the average XRD stress measurements significantly, thus providing a reasonable explanation for multilayer failure. Experimental results are presented and comprehensively discussed in the context of deposition conditions for different thin film applications.

Residual stress depth profiling of a coated WC-Co hardmetal – Part II of II: Regression methods for stress depth profile reconstruction from diffraction data

Reconstruction of residual stress depth profiles from diffraction data depends crucially on the underlying assumptions regarding X-ray elastic constants, stress state, and generic shape of the stress depth profile. This article addresses two issues: first, how to account for X-ray elastic constants varying according to different crystallographic planes by rearranging the underlying equation system such that it becomes amenable to linear regression again; second, how to construct the residual stress depth profile via inverse Laplace transformation of a piecewise linear approximation function, thereby obtaining maximum flexibility in the description of the generic shape of the profile. The methods are discussed by means of typical examples.

Residual stress depth profiling of a coated WC-Co hardmetal - Part I of II: Equi-penetration grazing incidence X-ray diffraction (EP-GIXD) method

The damage behavior of hard coated structures such as used for metalworking tools is influenced by residual stresses since they are superimposed with the load stresses that arise in application. Information on the depth gradient of these residual stresses in a coating's substrate is crucial to understand the evolution of damage in the substrate-coating composite. In the current work, the equi-penetration grazing incidence X-ray diffraction (EP-GIXD) method was adapted for the determination of the residual stress depth profile in the WC-Co hardmetal substrate below an AlCrN-based hard coating. The limits of this method in terms of coating thickness, up to which the EP-GIXD method is applicable to determine the residual stress state in the substrate, were investigated by recording the X-ray absorption in the hard coating in rocking curves. To this end, the diffraction intensity was recorded as a function of the path length in the coating that depends on the incident angle of the X-rays. Subsequently, the obtained residual stress profile was determined as a function of various mean penetration depths selected via the variation of the angle of incidence. The EP-GIXD method facilitates a significant reduction of experimental effort compared to e.g. synchrotron-based approaches for the future study of the residual stress state of substrates in substrate-coating composite structures.

Effect of shot peening on residual stresses and crack closure in CVD coated hard metal cutting inserts

Chemical vapour deposited TiCN-Al2O3 hard coatings applied on hard metals may remain in a residual tensile stress state after production and sometimes they show a surface crack network. Shot peening treatments are used to introduce compressive residual stresses in the coatings and hard metal. The current work investigates dry shot peening treatments of hard metal coated by TiCN as base layer and α-Al2O3 as upper layer by means of finite element modelling. The simulations use experimentally parameterized material models for substrate and coatings. The influence of the coating roughness on plastification and stress development after shot peening is regarded and a number of 2D simulations with various combinations of speed, impact angles, and particle sizes are carried out. Additionally, the effect of compressive residual stresses and associated crack closure is investigated. The study reveals a number of recommendations for effective shot peening in terms of introduced compressive stresses and crack closure. Residual stress depth profiles from the calculations and experiments show good agreement for a chosen set of reasonable process parameters.

Surface and microstructure characterization of low-frequency vibration-assisted drilling of Ti6Al4V

A continuous curled chip formation with a high thermal load is a critical challenge during conventional drilling process of Ti6Al4V. In addition, adverse side effects will take place on the drilled hole quality, as well as the drill tool performance, which is not acceptable in the aerospace industry. Chip segmentation using low-frequency vibration-assisted drilling (LF-VAD) is a promising technology aimed at reducing the difficulties associated with conventional drilling. Compressive residual stresses, lower cutting zone temperature, higher hole size accuracy, and smoother surface finish are of the main advantages that could be achieved with the interrupted cutting technique. The primary objective of this paper is to investigate the effect of LF-VAD machining parameters on the Ti6Al4V residual stresses, microstructure, and surface integrity, for a wide range of modulation amplitude, feed, and cutting speed, for a new frequency range for the first time. The effect of LF-VAD on the chip morphology and subsurface examination of microstructure, surface, and depth profile of residual stress are examined. The experimental results showed that LF-VAD has a significant influence on the induced residual stress. A 35.8% reduction in the cutting temperature has been observed at the exit surface. This method has improved the residual stress to compressive which has a significant impact on the part fatigue life.

Surface integrity in grinding of C-250 maraging steel with resin-bonded and electroplated CBN grinding wheels

Maraging steels are widely used in the aerospace industry for the manufacture of precision parts. However, surface integrity of ground maraging steels has received little attention. In order to explore the surface integrity of ground maraging steels and help contribute to develop the strategy of grinding these steels, surface integrity components such as roughness, surface defects, microstructure, and residual stresses in grinding of C-250 maraging steel (3J33) were studied. Grinding experiments were performed using resin-bonded and electroplated CBN grinding wheels with two different grain size (180# and 320#) abrasives in the cases of dry and wet grinding conditions. The results show that under the dry grinding conditions, the electroplated CBN grinding wheel is more suitable for grinding of C-250 maraging steel than the resin-bonded CBN grinding wheel. Compressive residual stress depth profiles (maximum value is up to − 200 Mpa in grinding direction), a good surface finish (Ra = 0.56 μm), and a minimum deformation layer thickness (less than 2 μm) can be achieved when dry grinding using the electroplated CBN grinding wheel with 320# grit size. Under the wet grinding conditions, benefitting from the fact that the cooling lubricant is more effective in improving the grinding performance of the resin-bonded CBN grinding wheel than in improving that of the electroplated CBN grinding wheel, the resin-bonded CBN grinding wheel achieved similar surface integrity to the electroplated CBN wheel in grinding of C-250 maraging steel.

Reassessment of evaluation methods for the analysis of near-surface residual stress fields using energy-dispersive diffraction

In this paper two evaluation methods for X-ray stress analysis by means of energy-dispersive diffraction are reassessed. Both are based on the sin2ψ measuring technique. Advantage is taken of the fact that the d ψ hkl –sin2ψ data obtained for the individual diffraction lines E hkl not only contain information about the depth and orientation dependence of the residual stresses, but also reflect the single-crystal elastic anisotropy of the material. With simulated examples, it is demonstrated that even steep residual stress gradients could be determined from sin2ψ measurements that are performed up to maximum tilt angles of about 45°, since the d ψ hkl –sin2ψ distributions remain almost linear within this ψ range. This leads to a significant reduction of the measuring effort and also makes more complex component geometries accessible for X-ray stress analysis. Applying the modified multi-wavelength plot method for data analysis, it turns out that a plot of the stress data obtained for each reflection hkl by linear regression versus the maximum information depth τψ=0 hkl results in a discrete depth distribution which coincides with the actual Laplace space stress depth profile σ(τ). The sensitivity of the residual stress depth profiles σ(τψ=0 hkl ) to the diffraction elastic constants ½S 2 hkl used in the sin2ψ analysis can be exploited to refine the grain-interaction model itself. With respect to the universal plot method the stress factors F ij which reflect the material's anisotropy on both the microscopic scale (single-crystal elastic anisotropy) and the macroscopic scale (anisotropy of the residual stress state) are used as driving forces to refine the strain-free lattice parameter a 0 during the evaluation procedure.

Generalised residual stress depth profiling at the nanoscale using focused ion beam milling

The study of Residual Stress is gaining more and more attention due to its importance in design for structural integrity. At present a lot of emphasis is placed on understanding the origins of mechanical failure that lie at the nano-/micron-scale. This leads to the evident need for evaluating residual stress distributions at increasingly smaller scales, and the search for modern tools capable of accomplishing this task. Prior state-of-the-art methodologies mostly required expensive and time-consuming sample preparation and examination processes to evaluate residual stress, e.g. the study of thin TEM lamellae. The recent advent of Focused Ion Beam methods opened up methods suitable for direct application at sample surface, yet allowing the observation and quantification of stress relief phenomena at the nano-scale. In the last decade, technical aspects of FIB-based method(s) have seen significant development. On the other hand, the calculation framework employed to analyse the experimental outcome remained largely conventional, in most inconvenient for high precision analysis of challenging problems.In the present paper, the eigenstrain-based method previously presented by the authors for the depth-resolved evaluation of equi-biaxial residual stress, is generalised to non-equi-biaxial distributions of residual stress. This extends the applicability of the method to a much wider class of problems. The use of cylindrical ring-core shape in FIB-DIC analysis allows reconstructing the full in-plane residual stress tensor as a function of milling depth. We report formulae for calibrated influence functions that have very broad applicability, and can be used in the overwhelming majority of cases. Their derivation is based on an extensive set of FEM simulations that allowed reliable identification of the limitations of this approach, and highlight the importance of making appropriate selection of ring-core diameter(s). Finally, experimental validation of the method is presented that involves the reconstruction a known non-equibiaxial residual stress depth profile, confirming the validity and reliability of the present approach.