Inherent Residual Stresses Research Articles

Effects of Implemented Residual Stresses on Mechanical Responses and Behavior of the Full-Layered Murine Aortic Medial Ring: A Parametric Finite Element Study

Effects of Implemented Residual Stresses on Mechanical Responses and Behavior of the Full-Layered Murine Aortic Medial Ring: A Parametric Finite Element Study

Influence of Local Electropolishing Conditions on Ferritic–Pearlitic Steel on X-Ray Diffraction Residual Stress Profiling

Layer removal with electropolishing is a well-established method when measuring residual stress profiles with lab-XRD. This is done to measure the depth impact from processes such as shot peening, heat treatment, or machining. Electropolishing is used to minimize the influence on the inherent residual stresses of the material during layer removal, performed successively in incremental steps to specific depths followed by measurement. Great control of the material removal is critical for the measured stresses at each depth. Therefore, the selection of size of the measurement spot and electropolishing parameters is essential. The main objective in this work is to investigate how different electrolytes and electropolishing equipment affect the resulting surface roughness, geometry, microstructure, and consequently the measured residual stress. A second objective has been to establish a methodology of assessing the acquired electropolished depth. The aim has been to get a better understanding of the influence of the layer removal method on the accuracy of the acquired depth. Evaluation has been done by electropolishing one ground and one shot peened sample of a low-alloy carbon steel, grade 1.1730, with different methods. The results showed a difference in stresses depending on the electrolyte used where the perchloric acid had better ability to retain the stresses compared to the saturated salt. Electropolishing with saturated salt is fast and results in evenly distributed material removal but has high surface roughness, which is due to a difference in electropolishing of the two phases, ferrite, and pearlite. Perchloric acid electropolishing is slower but generates a smooth surface as both ferrite and pearlite have the same material removal rates but may cause an increased material removal for the center of the electropolished area. In this work, it is suggested to use perchloric acid electropolishing for the final layer removal step.

Grain boundary engineering activated by residual stress during the laser powder bed fusion of Inconel 718 and the electrochemical corrosion performance

Conventional grain-boundary engineering (GBE) is not directly compatible with the complicated geometric parts fabricated via laser powder bed fusion (LPBF) owing to the multiple thermomechanical processes involved therein. In this study, GBE was activated by the inherent residual stress of an Inconel 718 superalloy produced via LPBF. The grain boundary character distribution and corresponding electrochemical corrosion behaviors were systematically investigated in a 3.5 wt% NaCl solution. The results indicate that numerous Σ3 boundaries (twin boundary, 64.89%) can be obtained via a simple heat treatment, and a favorable grain size can be maintained owing to the segmentation effect of the twin boundary. The corrosion resistance is closely related to the fraction of low special low-coincidence site lattices because the random boundary networks are broken. In addition, samples fabricated using GBE with the highest fraction of the twin boundary exhibited more compact and stronger passive films. The main components (oxide and metallic states) of thicker passive films provided intrinsic protection for the matrix. This study demonstrates a feasible method for GBE in additive-manufactured superalloys for fabricating complicated geometric parts with unique properties.

Gradient microstructure and prominent performance of wire-arc directed energy deposited magnesium alloy via laser shock peening

Wire-arc directed energy deposition (DED) has attracted significant interest for the fabrication of large-sized, lightweight Mg-alloy components. However, these components generally exhibit poor mechanical properties and limited corrosion resistance owing to their inherent residual stress and non-equilibrium microstructures. Herein, laser shock peening (LSP) was adopted to successfully modify the stress state and microstructure of AZ31 Mg-alloy fabricated using wire-arc DED. The influence of LSP on the residual stress, mechanical properties, electrochemical behaviour, and microstructural evolution was systematically investigated. The experimental results indicate that, compared with the as-built specimen, the performance of the LSP-treated specimen was notable, with a ≈63.8% decrease in the corrosion current density and ≈30% and ≈13% decreases in the yield strength (YS) and ultimate tensile strength, respectively. The enhanced corrosion resistance can be attributed to the LSP-induced compressive residual stress, nanograins, and nanoparticles. Nanocrystallisation, particle refinement, dense mechanical twins (MTs), and planar dislocation arrays (PDAs) jointly contributed to the enhancement of the YS. The LSP-induced nanocrystallisation was rationalized by the accumulation of PDAs, the intersection of multiple nano-MTs, and the transformation of nano-MTs blocks into sub-grains and then into nanograins owing to continuous dynamic recrystallisation. The particle refinement mechanism involved dislocation proliferation and the development of dislocation slip bands, which eventually led to fragmentation and separation. Therefore, this study introduces a LSP post-treatment technology for the residual stress regulation, microstructural modification, and performance enhancement of Mg alloys fabricated using wire-arc DED. Based on the ability of LSP to tailor the microstructure and performance of Mg alloys, a novel method of wire-arc DED with online LSP treatment is proposed. This method can achieve in-situ surface strengthening and the integrated formation of large-sized components with complex geometries.

Effect of the residual stress induced by surface severe plastic deformation on the tensile behavior of an aluminum alloy

Surface severe plastic deformation (SSPD) improves the mechanical behaviors of materials by generating non-uniform, heterogeneous microstructures on their surfaces. SSPD enhances the mechanical properties of the material, such as resistance to fatigue and yield strength. Herein, a decrease in the yield strength after SSPD, which is contrary to the findings of previous studies that report increased yield strengths, was considered. The main cause of this phenomenon was the internal tensile residual stress, which was inevitably induced by the surface compressive residual stress during SSPD. The inherent residual stress after SSPD was measured and applied in a tensile study of a real Al alloy to verify the phenomenon. Finally, the effects of the residual stress on the tensile mechanical properties were analyzed via analytical and numerical methods. Moreover, the transient behavior in the early stage was determined based on residual stress.

The Residual Stress Relief and Deformation Control of Al Alloy Thin-Walled Antenna Components by Ultrasonic Regulation

The residual stress fields of the initial billet and subsequent machining in the material bring great challenges to the precision machining and geometrical stability of aluminum alloy thin-walled components. To ensure that a certain type of large-sized aluminum alloy thin-walled antenna has a small flatness deformation during forming, this paper firstly employed the ultrasonic critical refraction longitudinal wave (LCR wave) detection method to measure the different depth ranges’ residual stress distribution of 5A06/6061/7075 aluminum alloy plate, both as blanks and after multiple milling. Additionally, the effects of inherent residual stress (IRS) and machining-induced residual stress (MIRS) on the subsequent milling deformation were analyzed. After that, combined with the self-developed ultrasonic stress relief (USR) system, the deformation control effect of a thin-walled plate after eliminating residual stress in each stage was tested. The results show that the ultrasonic stress relief treatment can quickly and efficiently eliminate the IRS and MIRS with small flatness deformation. By introducing the URS treatment in the blank, rough machining, and semi-finishing stages, the components before each subsequent machining are in a low-stress state, and the component deformation can be gradually controlled so that the final thin-walled antenna has a smaller flatness.

Defectometry, distortion analysis and metallurgical properties in the keyhole and conduction mode of laser beam welding of Al–Mg–Sc alloy butt joints

A study on porosity evolution, distortion evolution and metallurgical properties of laser beam welded AA 5024 (Al–Mg–Sc) alloy have been presented here. A comparative assessment was done for two welding types, i.e. deep penetration (keyhole) and partial penetration (conduction) welding were used for joining AA 5024 sheets. Detailed Finite element-based thermo-mechanical simulations have been carried out to simulate the temperature profiles and their temporal evolution of distortion and welding-induced stress for different keyhole and conduction heat sources. Extensive X-ray micro tomography investigation has been done to understand the physics behind the porosity formation. Bands of porosities have been observed in conduction welding at the edge of the weld pool. The inherent residual stress evolution was also mapped, and the reason behind the propensity to failure was investigated. High-cycle fatigue tests were done to check the endurance of these welded joints. The low power and high speed for keyhole welding and low power and low speed for conduction welding have been seen to give rise to minimal distortion. The conduction welded specimens at low speed, i.e. 0.5–1 m/min, and high power 3.5–4 kW region performed better under HCF.

Finite element modelling for mitigation of residual stress and distortion in macro-scale powder bed fusion components

Powder bed fusion (PBF) has attracted significant attention in many applications due to its capability of fabricating complex and customized metal parts. However, the potential for high inherent residual stresses that produce distortion in additive manufacturing (AM) components prevents more widespread application of the AM technique. Efficient and accurate prediction of residual stress and distortion at component-level (macro-scale) is a complex task. Although process-level (meso-scale) thermo-mechanical simulations have resulted in accurate predictions for small-scale parts, the computational times (typically weeks) and memory requirements for application of such methods to component-level are prohibitive. The main goal of the current study therefore is to present an efficient and accurate finite element (FE) simulation method with detailed validation for PBF manufacture of a complex 3D Inconel 625 benchmark bridge component (macro-scale). The simulation results are successfully validated against the published benchmark experimental measurements from neutron diffraction, X-ray diffraction (XRD), contour method and coordinate measurement machine (CMM) by the National Institute of Standards and Technology (NIST) laboratory. A key additional novelty of the present work is the investigation of the effects of substrate removal and preheating on mitigation of residual stresses and distortions using the validated model. Ultimately, these results will guide the selection of optimal manufacturing protocols and integration of the FE-based AM modelling for industrial application with complex geometries. The ultimate aim of the present work is to facilitate fatigue life prediction of complex geometry AM components including residual stress effects, for example, conformally-cooled injection moulding dies (for different material than Inconel 625).

Effects of microstructure and inherent stress on residual stress induced during powder bed fusion with roller burnishing

A hybrid metal-additive manufacturing (AM) process that combines laser-based powder bed fusion (PBF-LB) with interlayer burnishing is investigated using a comprehensive modeling framework to provide new insights into how the inhomogeneous microstructure and residual stress from the laser powder bed fusion process affect the induced residual stress field that evolves during interlayer burnishing. Researchers have recently studied changes in microstructure resulting from similar hybrid metal-additive processes, however, it was only hypothesized that the resulting microstructure has some influence on the induced residual stress. In addition, researchers have numerically investigated the influence of burnishing/rolling process parameters on induced stress but neglected the effects of microstructure, thereby making homogeneous, isotropic assumptions. Such practice inhibits the prediction of microstructure-driven anisotropy that can exist in the inhomogeneous fused layer. This paper parametrically examines the influence of microstructure modeling, inherent residual stress mapping, and environment temperature on the induced residual stress during the hybrid metal-additive process. The demonstrated modeling framework incorporates inherent residual stresses that emerge from the laser powder bed fusion process, as well as the predicted microstructure, in a subsequent burnishing simulation to elucidate their individual and combined influences on the burnishing-induced residual stress. Findings reveal that modeling an inhomogeneous PBF-LB microstructure introduces an anisotropic distribution of plastic strain and residual stress along the burnished surface; a periodicity in planar stress components along the treated surface coincides with the PBF-LB scan lines. Effects of inherent residual stress on the burnishing-induced residual stress is less significant, but nonetheless observable. Elevated temperatures not only reduce the magnitude of compressive residual stress induced but also result in less variation of residual stress component magnitudes predicted along scan lines and hatch spaces. The presented framework offers new insights into the decoupled influences of microstructure and PBF-LB residual stress on burnishing-induced stresses that are not distinguishable via experimental techniques. However, trends in averaged residual stress through the depth of the specimen, as well as surface hardness magnitudes after burnishing show good agreements, respectively, with X-ray diffraction and microindentation measurements documented in the literature.

Coupling between inherent and machining-induced residual stresses in aluminum components

This work investigates the coupling of inherent residual stress (IRS) and machining-induced residual stress (MIRS) on the final-state of residual stress (FRS) and distortion when high-speed machining (HSM) high aspect-ratio aluminum components. Motivation for this work stems from the simplifications in related numerical investigations that give rise to two limitations: First, the mapping of incompatible, incomplete, and/or spatially scarce IRS profiles generates unrealistic distortions and incorrect stress fields during static equilibration. Second, the simulation of machining via element deletion, inactive elements approach, or Boolean subtraction (removal) of material either ignores thermal and MIRS effects, or implements them based on simplified analytical/empirical models. Such practices therefore prevent a thorough understanding of how IRS and MIRS are coupled. Accordingly, two wrought aluminum 7050 blocks having different IRS profiles (based on stress relief) are considered in this work. An iterative stress reconstruction algorithm is implemented to numerically model a spatially-complete and fully-compatible IRS field in each aluminum block using limited data from slitting measurements documented in the literature. A 2D orthogonal cutting model is used to validate the material and damage models employed, as well as to elucidate the influences of IRS and MIRS on FRS. A 3D end milling model, which adopts the validated material and damage definitions, is then applied with different tool paths to reveal the coupled effects of IRS and MIRS on the distortion when HSM a C-channel featuring high aspect-ratio walls. The results reveal that the interaction between IRS and MIRS is nonlinear in nature, thus contrasting assumptions allowing for their superposition, as are widely reported and adopted in the literature. The results also show that the nonlinear coupling between IRS and MIRS varies according to both the component and location of stress within the machined part. Moreover, the findings reveal that the final part distortion is significantly influenced by the nonlinear coupling, as well as the specific machine tool path implemented.

Combined residual stresses and fluid-structure interaction finite element analysis on bent pipes

ABSTRACT The combined inherent residual stress (RS) from manufacturing and in-service loading acted on a bent pipe which could be exerted from the fluid flow and/or thermal loading, a common situation faced by main steam pipes in high temperature plants, needs to be considered to understand their overall impact on the integrity of the pipe. The geometrical influence on a bent pipe’s stress distribution was investigated using a combined RS – Fluid-Structure Interaction (FSI) finite element analysis approach. The reduced stresses along the pipe derived from the room temperature RS-FSI simulation and analysis demonstrated the impact of the fluid and structure interaction, particularly at the locations where geometry varied. A substantial reduction in the stress level observed, especially at the extrados where wall thinning was detected. The combined RS-FSI analysis therefore offers a more realistic approach in predicting a safe allowable stress, thus the integrity or life of bent pipes.

A numerical study of the effect of the thickness parameter on machining distortion for aluminum alloy plates

The deformation produced after the machining of a structural component is known as part distortion. This phenomenon is a consequence of the inherent residual stresses that exist in raw materials. In this study, such phenomenon is numerically investigated in simple plate elements by considering their thicknesses and their corresponding contribution to part distortion. A total number of eleven flat plates were analyzed using a numerical part distortion procedure for finite element models that also considered their machining positions. The results of this study show that part distortion has more impact on slender plates because these present higher loads than thicker plates in which the residual stresses self-balance throughout their section. Consequently, the part distortion phenomena in simple structural flat plates are related the plate thickness, their machining position, and geometrical parameters.

Design & Modelling of Double Cantilever structure by Stainless Steel 316L deposited using Additive Manufacturing Directed Energy Deposition Process

Distortion in the metal additive manufacturing due to inherent residual stress during the separation of the deposited part from the base plate is critical. Distortion gets amplified after removing it from the build platform. Internal stress in the deposited sample during the material deposition and cooling phase is crucial to estimate. Residual stress in the structure influences structural distortion. Numerical analysis of residual stress prior to deposition helps to identify inconsistent stress in the built part and helps to take required measures like optimising process parameters, base plate preheat to make it less significant. Residual stress estimation before deposition using numerical calculation plays a major role in the improvement of the built part and establishing technology for a wide application. A numerical model for thermal history and residual stress estimation has been developed for Directed Energy Deposition (DED). A standard double cantilever model was considered for physical deposition and numerical model verification. Thermal history from Finite Element (FE) Analysis was compared with the in-situ measurement. A close agreement was noticed between in situ measurement and simulation results. A double cantilever was built using austenite stainless steel 316L by the DED technique. Internal stress in the deposited part before and after a partial separation from the base plate was compared. Numerical results comparison before and after separation emphasize the impact of residual stress on distortion.

Design, Simulation, Fabrication, and Characterization of an Electrothermal Tip-Tilt-Piston Large Angle Micromirror for High Fill Factor Segmented Optical Arrays.

Micro-electromechanical system (MEMS) micromirrors have been in development for many years, but the ability to steer beams to angles larger than 20° remains a challenging endeavor. This paper details a MEMS micromirror device capable of achieving large motion for both tip/tilt angles and piston motion. The device consists of an electrothermal actuation assembly fabricated from a carefully patterned multilayer thin-film stack (SiO2/Al/SiO2) that is epoxy bonded to a 1 mm2 Au coated micromirror fabricated from an SOI wafer. The actuation assembly consists of four identical actuators, each comprised of a series of beams that use the inherent residual stresses and coefficient of thermal expansion (CTE) mismatches of the selected thin films to enable the large, upward, out-of-plane deflections necessary for large-angle beamsteering. Finite element simulations were performed (COMSOL v5.5) to capture initial elevations and tip/tilt motion displacements and achieved <10% variance in comparison to the experiment. The measured performance metrics of the micromirror include tip/tilt angles of ±23°, piston motion of 127 µm at sub-resonance, and dynamics characterization with observed resonant frequencies at ~145 Hz and ~226 Hz, for tip/tilt and piston motion, respectively. This unique single element design can readily be scaled into a full segmented micromirror array exhibiting an optical fill-factor >85%, making it suitable for optical phased array beam control applications.

Investigation of significant factors on deformation with powder bed fusion system

Powder bed fusion (PBF) is one of the most popular techniques in additive manufacturing (AM). The PBF technique of selective laser melting (SLM) consolidates powder layer by layer using a laser as the energy source. This technique ensures the processes capability of fabricating components with internal and external complex geometries, which could be challenging to make with conventional manufacturing methods. However, the cyclic heating and cooling inherent in this process give rise to the buildup of residual stresses, which can distort or completely deform the part. In this work, a screening build with nine factors was designed to investigate the effects of component size, support structure, and energy input on the build completion and average distortion induced by the inherent residual stress. Experimental results indicated that support hatch spacing, part thickness, and support contact spacing played dominant roles in the final quality (i.e. resultant deformation) of the built parts. The identified significant factors from this study can be carefully selected to increase the success rates of single builds and improve the qualities (i.e. geometric accuracy) of the final products.

Ramification of thermal expansion mismatch and phase transformation in TiC-particulate/SiC-matrix ceramic composite

This article presents a microstructural study on the role of incipient residual stress relaxation in TiC-particulate/SiC-matrix ceramic composite toughened by thermal expansion mismatch and phase transformation toughening. Exhaustive microstructural studies was undertaken using scanning electron microscopy and transmission electron microscopy following a wear test. It was found that the superposition of hydrostatic tensile stress induced at the surface following the sliding contact on the inherent residual stresses locked in the composite led to a relaxation and/or reduction in the residual stresses. Stress relaxation presented a wider implication for the tribological properties of this ceramic matrix composite (CMC) in the form of a grain-scale rippling microstructural phenomena.

Phenomenological Analysis of Thermo-Mechanical-Chemical Properties of GFRP during Curing by Means of Sensor Supported Process Simulation

Inherent process-induced deformations (PID) and residual stresses impede the application of composite parts. PID lead to a geometrical mismatch in assemblies and require subsequent work for tolerance compensation. Unknown residual stresses cause overweighted structures resulting from unnecessary high safety factors. To counteract the deformations, the tool design needs to be modified until the component geometry meets the specifications. This process is mostly carried out empirically and is time and cost intensive. To improve the efficiency of the development process, an in-deep comprehension of the manufacturing processes is mandatory. Therefore, experimental and simulation-based methods are increasingly applied and enhanced. The object of this work is to investigate the development of process-induced strains as well as the material behaviour during the manufacturing for a GFRP plate. The process-induced strains are monitored by optical fiber Bragg grating (FBG) sensors. The change of the material phases is detected by dielectric sensors. Furthermore, a detailed process simulation considering viscoelastic effects and reaction kinetics is performed. Finally, the measurements are correlated with the simulation data to validate the simulation approach. A very good correlation for both the reaction kinetics as well as the process-induced strains is observed.

An optimization of part distortion for a structural aircraft wing rib: an industrial workflow approach

Inherent residual stresses in the raw material of large monolithic structural components cause distortion when parts are machined. This is a frequent problem in the manufacturing life cycle of aircraft parts that has resulted in recurring concession, rework or scrap, which cost millions of euros to the aerospace industry. In this study an industrial workflow was implemented to predict and optimize part distortion after the machining of a representative structural aircraft-component. This was carried out by means of the following steps: i) characterization of inherent residual stresses of a material using a modified layer removal method; ii) generation of residual stress profiles using part-distortion measurements; and iii) optimization of distortion using Finite Element Analysis to derive an optimal part location. The workflow was validated with three experiments from two aluminum 7050-T7451 specimens with different residual stress states. A prediction of distortion with a Finite Element model resulted in 0.44mm from the higher residual stresses while from the lower residual stresses the maximum distortion was 0.36mm. The experimental testing was carried out in a CNC center to validate the numerical results. Good agreement from the numerical results in regards experimental tests led to propose the optimal part location which decreased the distortion up to 0.12mm.

Residual stresses prediction in machining: Hybrid FEM enhanced by assessment of plastic flow

Manufactured components are submitted to thermo-mechanical loads that can blemish their surface integrity and change the inherent residual stress distribution. The discontinuous machining process is relevant to the integrity conception because of their potential application at the end of the manufacturing chain. In order to consolidate a method to predict the residual stress state with a reduced CPU effort, this paper addresses the prediction of residual stress fields resulting from the milling process. The approach is using the hybrid FEM approach combined with the analysis of the plastic flow. The results obtained point out to the validity of the combination of the hybrid method and the visualization of the equivalent stress and equivalent strain rates. Moreover, a direct correspondence between the references and experimental dataset was observed, even when the input data for the model is associated with macro-loads obtained by piezoelectric platform measurements. The determination of the stress state as residual stress in mainly associated to the mesh convergence and the time to equilibrate the plastic flow. A conclusion is drawn regarding the viability of applying the same combination for other manufacturing processes.

Review of Wire Arc Additive Manufacturing for 3D Metal Printing

Wire arc additive manufacturing (WAAM) is a crucial technique in the fabrication of 3D metallic structures. It is increasingly being used worldwide to reduce costs and time. Generally, AM technology is used to overcome the limitations of traditional subtractive manufacturing (SM) for fabricating large-scale components with lower buy-to-fly ratios. There are three heat sources commonly used in WAAM: metal inert gas welding (MIG), tungsten inert gas welding (TIG), and plasma arc welding (PAW). MIG is easier and more convenient than TIG and PAW because it uses a continuous wire spool with the welding torch. Unlike MIG, tungsten inert gas welding (TIG) and plasma arc welding (PAW) need an external wire feed machine to supply the additive materials. WAAM is gaining popularity in the fabrication of 3D metal components, but the process is hard to control due to its inherent residual stress and distortion, which are generated by the high thermal input from its heat sources. Distortion and residual stress are always a challenge for WAAM because they can affect the component’s geometric accuracy and drastically degrade the mechanical properties of the components. In this paper, wire-based and wire arc technology processes for 3D metal printing, including their advantages and limitations are reviewed. The optimization parametric study and modification of WAAM to reduce both residual stress and distortion are tabulated, summarized, and discussed.