Residual Stress Depth Profiles Research Articles

A study of stress, composition and grain interaction gradients in energy-dispersive X-ray stress analysis on materials with cubic symmetry.

The influence of various combinations of residual stress, composition and grain interaction gradients in polycrystalline materials with cubic symmetry on energy-dispersive X-ray stress analysis is theoretically investigated. For the evaluation of the simulated sin2ψ distributions, two different strategies are compared with regard to their suitability for separating the individual gradients. It is shown that the separation of depth gradients of the strain-free lattice parameter a 0(z) from residual stress gradients σ(z) is only possible if the data analysis is carried out in section planes parallel to the surface. The impact of a surface layer z* that is characterized by a direction-dependent grain interaction model in contrast to the volume of the material is quantified by comparing a ferritic and an austenitic steel, which feature different elastic anisotropy. It is shown to be of minor influence on the resulting residual stress depth profiles if the data evaluation is restricted to reflections hkl with orientation factors Γ hkl close to the model-independent orientation Γ*. Finally, a method is proposed that allows the thickness of the anisotropic surface layer z* to be estimated on the basis of an optimization procedure.

Prediction of residual stress depth profiles in case-carburized gears

Residual stresses have a significant influence on the load carrying capacity of gears. High near-surface compressive residual stresses can increase pitting strength and tooth root bending strength up to 50%. Tooth flank fracture (TFF) is a gear fatigue failure mode, which is in contrast to pitting or tooth root breakage not initiated at the surface or close to the surface but in larger material depths beneath the active gear flank. Therefore, the residual stresses in larger material depths are decisive for TFF. In these larger material depths, the residual stress conditions are almost unknown up to now. It is assumed that tensile residual stresses are present in these areas, which can negatively influence the TFF load carrying capacity. So far, no validated methods are known to estimate or predict these tensile residual stresses. As a result, the tensile residual stresses could not be considered in the calculation methods for the risk of TFF at all until recently.This paper presents a method to predict the residual stresses in case-carburized gears based on comprehensive numerical simulations of case-carburizing processes of different-sized gears. The calculated residual stress profiles also consider the tensile residual stresses in larger material depths.

Residual stress depth profiles self-developed in cathodic arc deposited Ti-Al-N coatings prepared at different constant substrate bias values

Ti-Al-N coatings were prepared by cathodic arc deposition on Inconel 718 substrates at different values of constant substrate bias voltage, aiming to produce samples with different self-developed residual stress (RS) depth profiles through the thickness of the coatings. RS profile measurements and structural characterization were performed on a laboratory-scale x-ray diffraction system (x-ray energy of 8 keV) and in a synchrotron x-ray radiation facility (x-ray energy of 15 keV). Mechanical testing to obtain hardness and Young’s modulus values was performed by instrumented nanoindentation. The results indicate higher compressive RS at the film/substrate interface that decays to lower compressive stress or mild tensile stress at the film surface. Surface hardness and the compressive RS value of the coating increase with larger values of the substrate bias voltage. By comparing the stress characterization done on a laboratory scale and at the synchrotron facility, one observes a generally good agreement, indicating that these analyses may be conducted at a smaller scale and with less costly equipment, and still maintain a reliable precision. The work presents and reviews in detail the methodology of the RS depth-profile analysis. The highest hardness of 31.1 GPa and near-substrate compressive RS around −10 GPa were obtained for a bias of −100 V. Transmission electron microscopy results indicate that regions with higher compressive stresses are found to have smaller columns and denser structure, while portions of the same sample with mild compressive or tensile stresses present larger column size and are richer in hexagonal phases. The findings demonstrate the complex interplay between stress, microstructure, and ultimately mechanical properties in industrially produced Ti-Al-N coatings and indicate that any successful strategy to mitigate stress development should consider the inhomogeneous self-developed stress gradients present even in coatings deposited under constant and controlled conditions.

PVD coating strategies: Developing a combination of AlCrN and AlTiSiN for enhanced surface performance during threading of super duplex stainless steel

Super duplex stainless steel (SDSS) is one of the difficult-to-machine materials due to its high tendency to work-harden and low thermal conductivity. According to recent findings, PVD hard coatings based on Al, Cr, and Ti are recommended for SDSS machining. In this work, three different PVD coating systems were applied for the threading process of SDSS. Monolayer Al50Cr50N, Al60Cr40N, and multilayer Al60Cr40N/Al50Ti45Si5N were deposited on cemented carbide inserts. This paper highlighted the effect of alloying and coating architecture design on the cutting tools' mechanical properties, tribological characteristics, and wear performance. A novel balanced combination of AlCrN/AlTiSiN multilayer coating with a Si content of 5 at. % was proposed, and it exhibited improved adhesion, beneficial mechanical properties, and superior cutting tool life. Furthermore, tribological characteristics under extreme environments were analyzed through a heavy-load, high-temperature tribometer, as well as XPS and AES measurements. The surface integrity of workpiece material machined by various coatings was also examined through microstructure, microhardness, and residual stress measurements. The depth profiles of residual stress reveal that the machining process significantly impacts the outcome, influenced by the tribological properties of the coatings. Specifically, the AlCrN/AlTiSiN coating exhibits a minimal effect on the surface. These investigations enhance the comprehension of the mechanisms that cause material mechanical surface transformation in threading operations, contributing to a deeper understanding of machining processes.

Depth profiling of residual stress distribution in surface treated metallic structures using nonlinear ultrasonics

Robotic hammer peening (RHP) is a cold-working technique to improve the fatigue life of metallic structures by inducing compressive residual stress in the near-surface region. Measuring the depth profiling of residual stress distribution plays an important role in process design towards the advanced manufacturing of metallic structures. This study investigates the use of the nonlinear ultrasonic method to measure the residual stress profiles through the frequency-dependent penetration depth approach. Stainless steel specimens were treated with varying RHP intensities, and their morphological evolutions were characterized with a step-profilometer. The amplitudes of the second and third harmonic components of the longitudinal critically refracted (LCR) and Rayleigh waves were measured and analyzed. The effect of the surface roughness on the acoustic nonlinearity parameters before and after polishing at various peening intensities was briefly discussed. The results show a large variation in the acoustic nonlinearity parameter at the surface layer, indicating the potential of the proposed nonlinear ultrasonics for the measurement of depth profiling of residual stress.

Non-contact measurements of residual stress distribution and grain size in titanium alloys with laser ultrasonic system

In this study, a laser ultrasonic method is proposed for nondestructive non-contact measurement of both the residual stress depth profile and grain size in Ti-6Al-4 V alloy processed by warm shot peening (WSP). The method utilizes a pulsed laser to generate a surface acoustic wave (SAW) along the cross-section of the sample, and a laser interferometer to detect SAW signals. Laser ultrasonic measurements were performed on the samples treated by WSP at different temperatures (32, 100, 200 and 300 °C). A theoretical model for calculating the residual stress profile and a model to correlate the center frequency shift and the mean grain size of the sample were presented. The results obtained by the laser ultrasonic method were compared with those obtained by X-ray stress measurement, center-hole drilling (CHD), and electron backscattering diffraction (EBSD). The proposed method can measure and locate the maximum compressive residual stress (CRS) in a peened surface layer with a high spatial resolution of approximately 30 µm, allowing for precise extraction of the residual stress profile. This method can also provide an estimation of the grain size through the same experiment. The developed method is promising for the quick and nondestructive measurement of the residual stress profiles and grain size of surface treated metallic materials.

Effects of power density on residual stress and microstructural behavior of Ti-2.5Cu alloy by laser shock peening without coating

The present study concerns the micro structural changes of Ti-2.5 Cu after LPwC at various laser intensities (i.e., ≈3 GW cm−2, 6 GW cm−2 and 9 GW cm−2). X-ray diffraction (XRD), electron backscattered diffraction (EBSD), transmission electron microscope (TEM) were used to analyze the microstructure. X-ray photoelectron spectroscopy (XPS) was used to characterize the oxidation behavior of the samples after peening. The depth profile of residual stress was obtained using the sin2ѱ X-ray diffraction technique, and the compressive residual stress increased for all the peened conditions in the surface and sub-surface regions, and the maximum obtained was −402.44 MPa at 50 μm depth for the 9 GW cm−2. Furthermore, the cross-sectional micro hardness of the peened samples improved at a depth of 50 μm with increasing laser intensity. For the highest energy, the maximum hardness at a depth of 50 μm was 439.40 HV (0.1), which was higher than the unpeened sample (315.56 HV0.1). TEM studies showed nano precipitates (50–170 nm), mechanical twins with a width ranging from (27.20–118.51 nm) and cell size dislocation structure. EBSD results conformed the high angle grain boundaries increased to 43% with the formation of mechanical twins at the higher energy.

Study of Residual Stresses and Austenite Gradients in the Surface after Hard Turning as a Function of Flank Wear and Cutting Speed.

This paper investigates the influence of cutting speed and flank wear on the depth profile of residual stresses, as well as the fraction of retained austenite after hard turning of quenched bearing steel 100Cr6. Residual stress and retained austenite profiles were studied for the white layer, heat-affected zone thickness, and XRD sensing depth. It was found that the influence of flank wear on the white layer and heat-affected zone thickness predominates. On the other hand, residual stresses are affected the cutting speed and the superimposing contribution of flank wear. Moreover, these aspects also alter microhardness in the affected regions. The study also demonstrates that information concerning residual stresses and the austenite fraction is integrated into the white layer, and the heat-affected zone in the surface is produced by the insert of low flank wear since the XRD sensing depth is more than the thickness of the white layer. On the other hand, the pure contribution of the white layer or the heat-affected zone to residual stress and the austenite fraction can be investigated when the affected surface region is thick enough.

Sub-Surface Analysis of Grinding Burns with Barkhausen Noise Measurements

Barkhausen noise (BN) measurements are commonly used for surface characterisation. However, often there is also a need to verify the sub-surface region because detrimental tensile stresses may be present after different manufacturing steps. Especially in a grinding burn, the surface stress may be compressive, but it changes quickly into tensile stress below the surface. The aim of this study was to find out whether regular surface-sensitive BN measurement is also sensitive to the stresses below the surface caused by grinding burns. More specifically, the aim was to study the relationship between BN features and sub-surface stresses and to identify a model that estimates sub-surface stresses. Real samples were collected from an actual process. The samples were cylindrical samples manufactured from commercial alloyed AISI/SAE L6 steel that was through-hardened prior to grinding. Barkhausen noise measurements were carried out for 42 grinding burn locations followed by X-ray diffraction-based residual stress surface measurements and residual stress depth profiles. Depth information was obtained through step-by-step electrolytic removal of thin layers. The stress profiles were pre-processed through interpolation and averaged stress was calculated as a function of depth below the surface. Correlation analysis was carried out to evaluate the relationships between BN features and stress at different depths and among BN features. The main outcome of the analysis was that BN measurement is dominated by the sub-surface tensile stresses rather than the compressive stress at the surface. It was also noticed that BN features form two groups, corresponding to average Barkhausen activity and magnetising field strength leading to maximum Barkhausen activity. Models for stress at different steps were identified systematically. The performance of the models for sub-surface stresses was reasonable with R2 values of around 0.85 and root mean squared error (RMSE) values of around 95 MPa. Based on the results, it is concluded that BN measurement provides information about sub-surface stresses and that stress can be evaluated through straightforward modelling, allowing fast detection of grinding burns.

Coupled Eulerian-Lagrangian model for residual stress prediction in orthogonal cutting of Waspaloy

Residual stress is one of the most critical factors of machined surface integrity, which directly determines corrosion resistance and fatigue life of components. Therefore, it is necessary to accurately predict the machining-induced residual stress, especially for critical aerospace parts. In this paper, a finite element model based on the Coupled Eulerian-Lagrangian method is employed to predict the residual stress in orthogonal cutting of Waspaloy material under different cutting conditions. The depth profiles of residual stress along the cutting speed direction are determined using the X-ray diffraction and electropolishing techniques. The results show tensile residual stresses at the cutting speeds of 64 m/min and 86 m/min with 100 microns uncut chip thickness. The depth at the machined surface influenced by the residual stress is identified and compared from both simulations and experiments. The simulated cutting forces and residual stress distributions from the finite element model are compared with the experimental results to validate the model. In addition, the microstructure property at the machined surface is examined to determine the depth of plastic deformation layers. White layers are identified at the higher cutting speed, and it is shown that the formation of white layers is directly related to the location of the peak tensile residual stress.

Mechanical Properties and Residual Stress Depth Profiles of Plasma Sprayed Ceramic Coatings Deposited Under Comparable Particle Temperature and Velocity

Mechanical Properties and Residual Stress Depth Profiles of Plasma Sprayed Ceramic Coatings Deposited Under Comparable Particle Temperature and Velocity

Understanding the role of in-flight particle temperature and velocity on the residual stress depth profile and other mechanical properties of atmospheric plasma sprayed Al2O3 coating

This report deals with the influence of particle temperature and velocity on the microstructure, mechanical properties and residual stress depth profile of plasma sprayed alumina coating. The coatings were produced by varying the particle temperature while maintaining a relatively fixed particle velocity, and vice versa. Residual stress profiles were acquired by measuring the stress in successive layers using X-ray Sin2ψ technique. A substantial increase in hardness and indentation modulus by 76% and 64%, respectively were observed with an increase in particle temperature. With an increase in velocity, coating porosity was found to decrease initially. However, at a high velocity, porosity again increased to a limited extent. The mechanical properties were found to depend strongly on porosity. An increase in particle temperature resulted in an increase in the tensile residual stress in the coatings. Moreover, partial recovery of the grit blasted compressive substrate occurred during spraying owing to an annealing effect.

Calibration and Validation of Micromagnetic Data for Non-Destructive Analysis of Near-Surface Properties after Hard Turning

Abstract Micromagnetic non-destructive (NDT) methods offer a great potential for the analysis of near-surface properties after machining due to potential time and cost reduction as well as the option to be implemented into the machining process. As a result, the development of soft sensor concepts including micromagnetic NDT methods is in focus of current investigations in order to eventually improve the surface integrity of machined components and, thus, service life and reliability. However, a preceding calibration based on empirical data as well as a reliable validation is often referred to as one of the main challenges of micromagnetic NDT methods. The present study provides insights into the calibration and validation of a micromagnetic 3MA-II system for NDT analysis of the near-surface properties, with a focus on the residual stress depth profiles after hard turning of 51CrV4 specimens. Different calibration functions as well as a combination of different NDT methods are taken into consideration. The results and the potential of the 3MA system as well as open challenges are critically discussed.

Residual Stresses from Incremental Hole Drilling Using Directly Deposited Thin Film Strain Gauges

BackgroundCommonly, polymer foil-based strain gauges are used for the incremental hole drilling method to obtain residual stress depth profiles. These polymer foil-based strain gauges are prone to errors due to application by glue. For example zero depth setting is thus often erroneous due to necessary removal of polymer foil and glue. This is resulting in wrong use of the calibration coefficients and depth resolution and thus leading to wrong calculations of the obtained residual stress depth profiles. Additionally common polymer foil-based sensors are limited in their application regarding e.g. exposure to high temperatures.ObjectiveThis paper aims at a first step into the qualification of directly deposited thin film strain gauges for use with the incremental hole drilling method. With the directly deposited sensors, uncertainties regarding the determination of calibration coefficients and zero depth setting due to the absence of glue can be reduced to a minimum. Additionally, new areas of interest such as the investigation of thermally sprayed metallic layers can be addressed by the sensors due to their higher temperature resilience and their component inherent minimal thickness.MethodsFor the first time, different layouts of directly deposited thin film strain gauges for residual stress measurements were manufactured on a stainless steel specimen. Strain measurements during incremental hole drilling using a bespoke hole drilling device were conducted. Residual stress depth profiles were calculated using the Integral method of the ASTM E837 standard. Afterwards, strain measurements with conventional polymer foil-based strain gauges during incremental hole drilling were conducted and residual stress depth profiles were calculated accordingly. Finally the obtained profiles were compared regarding characteristic values.ResultsThe residual stress depth profiles obtained from directly deposited strain gauges generally match the ones obtained from conventional polymer foil based strain gauges. With the novel strain gauges, zero depth setting is simplified due to the absence of glue and polymer foil. With the direct deposition, a wide variety of rosette designs is possible, enabling a more detailed evaluation of the strain field around the drilled hole.ConclusionsThe comparative analysis of the obtained residual stress depth profiles shows the general feasibility of directly deposited strain gauges for residual stress measurements. Detailed investigations on uncertainty sources are still necessary.

Consequences of Deep Rolling at Elevated Temperature on Near-Surface and Fatigue Properties of High-Manganese TWIP Steel X40MnCrAl19-2

Due to pronounced work-hardening induced by the complex interplay of deformation mechanisms such as dislocation slip, twinning and/or martensitic phase transformation, high-manganese steels represent a class of materials well-suited for mechanical surface treatment. In the present study, the fatigue behavior of a high-mangsanese steel showing twinning-induced plasticity (TWIP) effect at room temperature (RT) was investigated after deep rolling at 550 °C. Results are compared to a former study discussing the behavior after RT deep rolling. Evolution of the near-surface microstructure was analyzed by scanning electron microscopy (SEM), microhardness measurements and residual stress depth profiles obtained by X-ray diffraction (XRD). Both uniaxial tensile tests and uniaxial tension-compression fatigue tests have been conducted in order to rationalize the macroscopic material behavior. Following deep rolling at 550 °C, SEM measurements employing electron backscatter diffraction (EBSD) revealed a heavily deformed surface layer as well as localized deformation twinning. Specimens showed inferior hardness and residual stress depth profiles when compared to RT deep rolled counterparts. Tensile tests indicated no difference between the conditions considered. Fatigue properties however were improved. Such behavior is rationalized by a more stable residual stress state induced by dynamic strain aging.

Redistribution of carbon and residual stress in low-temperature gaseous carburized austenitic stainless steel during thermal and mechanical loading

The effect of thermal annealing and mechanical loading on the redistribution of carbon- and residual stress-depth profiles in low-temperature carburized 316 L austenitic stainless steel was investigated. Gaseously carburized 316 L austenitic stainless steel was strained with a custom-built four-point bending device and isothermally annealed at temperatures in the range 300–500 °C for up to 200 h. Experimental results show that the carburized case is stable at 300 °C and that carbides precipitate only at the highest temperature investigated. At temperatures above 300 °C, the carbon atoms in the carburized case continue to diffuse inwardly, causing an increase of the case depth. Consequently, the surface concentration of carbon, the surface hardness and compressive residual stress change with time. Four-point bending during isothermal annealing accelerates the diffusion-controlled redistribution of carbon over the carburized case for both tensile and compressive side of the strained specimen. Since the carbon-induced volume expansion of austenite during carburizing is accommodated elasto-plastically, a substantial reduction of the carbon content in the surface-near region leads to a change from compressive to tensile residual stresses in the surface region.

Fracture mechanism of TiN coatings on Ti-6Al-4V substrates: Role of interfaces and of the residual stress depth profile

The performance and integrity of coated engineering components rely on the time required for the appearance of defects and cracks and their propagation, leading to delamination of the coating and degradation of the substrate. Optimizing the coating's mechanical properties such as hardness, residual stress (RS), and adhesion is essential to delay crack onset and propagation. In the present work, we investigate the mechanical properties, and the failure mechanisms of model TiN coatings reactively sputtered onto Ti-6Al-4V substrates while comparing three interface engineering approaches to enhance the system durability: a) Argon plasma treatment, b) plasma surface nitriding, and c) Titanium implantation. The TiN coatings possessed a hardness of ~29 GPa and a Young's modulus of ~350 GPa. Multi-reflection grazing incidence X-ray diffraction was used to assess the compressive RS depth profile. Each interface engineering process induced RS variation in the coating and the adjacent interfacial area and in the substrate's near-surface layer ranging from −1 GPa to −2.2 GPa with different gradients. Cohesive failure, crack evolution and fracture toughness were studied using micro-scratch and micro-tensile tests, while the surface grain deformation behaviour and surface cracks on fractured Ti-6Al-4V with various interface treatments and TiN coatings were identified by SEM image and elasto-plastic property analyses. We found a close correlation between the RS of the coatings or the interface layers and the fracture mechanism. Specifically, higher compressive RS led to an enhanced interfacial shear strength and a critical energy release rate of around 550 to 790 MPa, and of ~18 J/m2, respectively.

Residual stress and cold work assessment in shot-peened IN718 using a dual-mode electromagnetic technique

This paper investigates the feasibility of a novel dual-mode electromagnetic NDE technique based on broadband Hall impedance and eddy current conductivity measurements for characterization of IN718 coupons shot-peened at Almen 4A−12A intensities. The measured data were inverted into residual stress and cold work depth profiles using an iterative inversion procedure and elastic and plastic gauge factors obtained by independent calibration tests conducted under uniaxial stress. Comparison between the inverted dual-mode NDE results and residual stress and cold work depth profiles obtained by destructive XRD testing showed good qualitative agreement. However, the quantitative agreement was found to leave much to be desired as the residual stress levels were significantly underestimated while the cold work levels were slightly overestimated. To mitigate this problem caused by simple but inaccurate approximations used in the inversion, an empirical correction was applied that reduced the elastic gauge factors by 32% and increased the plastic gauge factors by 32%. This single-parameter empirical correction significantly improved the accuracy of the inverted NDE residual stress and cold work depth profiles. To check if this method worked in thermally relaxed states, preliminary dual-mode NDE data were obtained from some of the shot-peened IN718 coupons after thermal exposure to 600 °C for 100 h. The measured data showed trends that were different from those expected based on linearized gauge factors and indicated that the highly accelerated thermal relaxation used in these tests led to thermally activated microstructural changes in the highly cold worked near-surface regions of the shot-peened IN718 coupons. Therefore, future work should be focused on thermally relaxed IN718 coupons exposed to less accelerated thermal relaxation that leads to less severe thermally activated microstructural changes.

Depth-profiling of residual stress and microstructure for austenitic stainless steel surface treated by cavitation, shot and laser peening

While the general characteristics of various peening techniques have been established, there have been few comparative studies. Here we compare the variation of the residual stresses and microstructural characteristics with depth for 316L austenitic stainless steel treated by cavitation peening (CP), shot peening (SP) and laser peening (LP) all peened to similar intensity levels. While the plastically affected depths were similar in all cases (~400 μm), the SP specimen showed the most extensive near surface plastic deformation, deformation twinning, dislocation density and compressive residual stress. To counterbalance this, the compressive residual stresses extended deeper for the LP and CP. Across the three treatments, a similar dependency was found between diffraction peak broadening and hardness. The dislocation density at the surface determined by the diffraction line profile analysis (LPA) for the SP specimen (4.9 × 10 15 m −2 ) was approximately 2.5 times that for the CP and LP specimens (2.0 × 10 15 and 2.1 × 10 15 m −2 ). Electron backscatter diffraction (EBSD) shows that the extensive work introduced by the SP had generated planar defects near to the surface. The increase in yield stress estimated from the hardness corresponded with the increase in dislocation density obtained by the LPA. • Shot peening (SP) introduces larger near-surface stresses while cavitation peening and laser peening penetrate deeper. • SP gives the highest nearsurface hardness of the three treatments. • SP also introduces a larger number of dislocations and planar defects than the others. • The hardness at a given FWHM is broadly the same regardless of the kind of the peening treatment. • The increase in yield stress estimated from the hardness correlates with the increase in dislocation density.

Controlling Thermal Diffusivity, Residual Stress and Texture in W/Cu Nano-Multilayers by Magnetron Chamber Pressure Variation

The effect of magnetron deposition pressure during manufacture of W/Cu nano-laminates on their thermal diffusivity, residual stress depth profiles and texture was analysed. A strong correlation between these properties and the Ar working pressure was confirmed experimentally via a combination of Transient Grating Spectroscopy, Focused Ion Beam ring drilling, and cross-sectional high resolution SEM and EDX characterisation. The underlying mechanisms controlling thermal diffusivity and residual stress can be related to the defect evolution during sputter deposition, which is largely controlled by the interaction of the working gas and the sputtered metal atoms.