High Residual Stresses Research Articles

Investigation of residual stress in epoxy-based coatings using X-ray and FEM-ANN techniques

Residual stresses play a significant role in the properties and performance of epoxy-based coatings, with their origins rooted in various factors encountered during production and application. This study focuses on quantifying residual stresses in three distinct epoxy-based coatings, commonly used as linings for crude oil storage tanks, namely, pure epoxy, Novolac epoxy, and glass-flake-reinforced epoxy. We employ X-ray diffraction to measure these residual stresses and compare them against predicted values obtained through finite element and artificial neural network methods. Our findings reveal notable differences in residual stresses among the three types of epoxy coatings. Specifically, pure epoxy coatings exhibit higher residual stresses, Novolac epoxy coatings display the lowest, and those reinforced with glass flakes fall in between. Utilising the FEM-ANN model for simulations yields results that closely align with experimental measurements obtained via the X-ray method. Test results demonstrate that the coatings cured at high temperatures have high residual stresses compared to those cured at lower temperatures. Increasing the curing temperature from 10 to 50oC will increase residual stresses by 40.81, 11.085, and 56.98% for coatings reinforced with glass-flake, Novolac, and pure epoxy-based coating, respectively.

Residual stress and thickness effects on fracture behavior of trilayer films during uniaxial loading

The effects of residual stress and film thickness on crack evolution in metallic trilayers deposited on polyimide substrate have been investigated during uniaxial tensile loading. Four different systems, consisting of Mo/Cu/Cr layers deposited on polyimide, with nominal thicknesses of 100/100/100 or 100/500/100 nm, were developed using two physical vapor deposition techniques, resulting in opposite as-deposited high residual stress for the top Cr layer (about +2.7 GPa versus -2.6 GPa). First, it was found that the higher the compressive residual stress of the top Cr layer and the thickness of the middle Cu layer, the higher the critical strain to initiate crack propagation in the trilayer film system. Second, it was found that the higher the compressive residual stress of the top Cr layer and the thickness of the middle Cu layer, the lower the maximum crack density in the trilayers. The maximum crack density in the thicker system with Cr in a tensile residual stress was similar to the maximum crack density in the thinner system with the Cr top layer in a compressive residual stress.

Microstructure and wear behaviors of 17-4 PH stainless steel fabricated by laser cladding with post laser shock peening treatment

Laser cladding (LC) technology provides an attractive and cost-effective way to prepare wear-resistant coating on high-value engineering components. However, the thermal effect of coating will lead to a large amount of residual tensile stress in the coating, which seriously affects the wear performance. This paper investigated the influence of laser shock peening (LSP) on the microstructure and wear behaviors of wire-based LC 17-4 PH coating. Results showed that there was no laser remelting phenomenon during LSP process, and all residual austenite in cladding layer transformed into martensite after LSP treatment, accompanied by a more compact and uniform microstructure. LSP can introduce severe plastic deformation to form dislocation structures on the top surface of the coating, which lead to grain refinement through dynamic recrystallization (DRX), and the average grain size decreased from 4.15 μm to 1.81 μm when the laser energy is 9J. Meanwhile, the high strain rate plastic deformation induced by LSP generated high residual compressive stress on the material surface, with a maximum value of 500 MPa. The surface microhardness of the cladding layer increased by 100HV after LSP treatment, reaching to a peak of 458HV at a laser power of 9J, and gradually decreased along the depth direction, presenting a gradient distribution. After LSP treatment, GCr15 steel counter ball was used to simulate the sliding friction of solid particles under real working conditions. Results showed that the friction coefficient (COF) and wear rate of 17-4 PH coating were significantly reduced. Furthermore, the wear type changed from dominative adhesive to slight abrasive wear. The improvement of microhardness and wear properties were attributed to the combined effects of Hall-Petch and Taylor hardening strengthening.

Effect of supersonic fine particle bombardment on microstructure evolution and tribological properties of GCr15SiMn bearing steel

The surface of GCr15SiMn bearing steel was strengthened by supersonic fine particle bombardment (SFPB), and the effects of different impact times on the microstructure, mechanical properties and friction and wear properties of the material were studied. The results show that the surface roughness of GCr15SiMn steel samples increases and the grain is refined after SFPB. As the impact time prolongs, the grain size gradually decreases. When the bombardment time is 180 s, the surface grain size of the sample reaches 12.7 nm. The microhardness of the sample gradually increases and tends to stabilize with the increase of the impact time. The residual compressive stress layer is produced on the surface of the sample after SFPB and the residual compressive stress shows a trend of first increasing and then decreasing with rising the impact time. The friction coefficient and wear rate of GCr15SiMn bearing steel decreases after SFPB. The wear mechanism of the sample changes from mixture of adhesive wear and oxidation wear, accompanied by slight abrasive wear (before SFPB) to the abrasive wear, supplemented by adhesive wear (after SFPB). The wear resistance is improved by the of the grain refinement and the formation of a high residual compressive stress layer.

Multi-physics multi-scale simulation of microstructure evolution in entire welds of 2219-T6 aluminum alloy: An MPF-LBM study

The microstructure of fusion welds significantly affects its mechanical properties. Existing studies on the dynamic evolution of welding molten pools have exposed great limitations in considering the flow field and the solid motion of free dendrites. In this paper, a multi-physics multi-scale coupling simulation of thermal-mass-flow was conducted to study the solidification behavior during the welding of 2219-T6 aluminum alloy. The developed multi-phase field lattice Boltzmann model realized the direct full coupling of phase, temperature, solute, solid motion, and flow fields. It is also combined with a continuous nucleation model to simulate heterogeneous nucleation and columnar-to-equiaxed transition (CET), and a transient growth-condition model to simulate the movement and solidification of the entire molten pool. The model and simulation results were verified by welding experiments, electron back-scattered diffraction, and scanning electron microscopy tests. Good agreements are achieved in terms of grain structure, dendrite morphology, and grain size. The undercooling field, fluid flow, solid motion, dendrite morphology, solute distribution, and quantitative parameters during the whole solidification process were analyzed in detail to reveal the true evolution of thousands of dendrites in fusion welds. The results show that solute convection caused by fluid flow significantly affects the dendrite morphology. The solid motion of equiaxed dendrites squeezes the solute enrichment layer and forms a solute hysteresis transport effect. A band of equiaxed dendrites gathers in front of the columnar dendrites and blocks their growth, forming a mechanical blockade mechanism of the CET process. The equiaxed dendrites dominated the subsequent solidification after CET is complete, and the final microstructure consists of brief-growth columnar dendrites and dense equiaxed dendrites. The top tangential flow causes a band of equiaxed dendrites with large gaps in the upper weld, which was presumed to be the source of surface cracks in welds with high residual stress.

Preparation of near fully dense (LaO1.5)x(ErO1.5)x(YO1.5)0.03-0.5x(ZrO2)0.97-x ceramics with restricted grain growth and high surface residual stress by hot pressing sintering at 950 °C

(LaO1.5)x(ErO1.5)x(YO1.5)0.03-0.5x(ZrO2)0.97-x ceramics with high density (98.1 %), high strength (1533 MPa), high surface residual stress (1305 MPa) and fine grain (96 nm) were prepared by ultra-low temperature hot pressing sintering. The results indicate that high pressure significantly promotes densification in the contact area of (LaO1.5)x(ErO1.5)x(YO1.5)0.03-0.5x(ZrO2)0.97-x ceramics particles and no effect on grain growth. Low temperature can effectively reduce the transition from tetragonal to cubic phase (T-C) and limit grain growth. In addition, 1305 MPa compressive stress on the surface of (LaO1.5)x(ErO1.5)x(YO1.5)0.03-0.5x(ZrO2)0.97-x ceramics were the main reason for the improvement of bending strength. This process provides a design idea for obtaining fine-grained fully dense structural ceramics.

Optimization of Butt-joint laser welding parameters for elimination of angular distortion using High-fidelity simulations and Machine learning

The high temperature gradient and high cooling rate induced in the butt-joint laser welding process generate a high residual stress, which can result in significant distortion of the final weld. Previous researches usually employed experimental approach to search for optimal parameters which is costly and time consuming. Additionally, there was several studies that used simulation to compute the distortion and to find optimal parameters, however, these studies neglect the effect of solid cooling rate and size of the heat-affected zone (HAZ) in the process of finding optimal parameters. Accordingly, the present study proposes a new systematic framework based on finite element (FE) simulations, regression analysis, and two artificial neural networks (ANNs) for determining the laser welding parameters (i.e., the laser power and welding speed) which minimize the angular distortion of the laser-welded butt joint and enhance the mechanical properties. In the proposed approach, to determine the variables of the selected double-conical heat source model for any combination of the laser power and welding speed is studied within the feasible design space. The variables are then imported into a three-dimensional COMSOL Multiphysics FE model to predict the melt pool dimensions, solid cooling rate, and size of the HAZ as a function of the laser power and welding speed. Finally, the processing map produced by the ANN is screened by four quality criteria based on the melt pool depth, angular distortion, cooling rate, and HAZ width, respectively, to determine the optimal laser welding parameters which minimize the angular distortion and maximize the mechanical properties. It shows that SS316L tensile test specimens fabricated using the optimal parameters have a tensile strength (662 MPa) which is higher than that of the base metal (660 MPa). Furthermore, the angular distortion of the butt-joint samples is less than 564 µm. Thus, the effectiveness of the proposed optimization procedure is confirmed.

Overload and variable amplitude load effects on the fatigue strength of welded joints

Different load spectra and individual load peaks might substantially relax high residual stresses as well as induce compressive residual stresses in welded components and, consequently, affect the fatigue performance of these joints. Consideration of peak loads and resulting relaxation of residual stresses in fatigue analyses can substantially enhance the accuracy of life prediction. The aim of the current study is to experimentally investigate the fatigue strength of welded joints subjected to different levels of overloads and variable amplitude (VA) loads and to develop local fatigue assessment method to account for the relaxation of residual stresses via a mean stress correction using the 4R method. The 4R method applies a local stress ratio for the mean stress correction considering material strength, residual stresses, applied stress ratio of external loading and local weld geometry in elastic–plastic material behaviour. Fatigue tests were carried out on fillet-welded longitudinal gusset joints made of S700 high-strength steels under applied stress ratio R = 0–0.1. A mild strength steel (S355) and ultra-high-strength steel (S1100) were selected as reference steel grades for the fatigue testing to study the material strength effects. Numerical analyses were conducted to evaluate the fatigue notch factors using the effective notch stress concept with the reference radius of rref = 1.0 mm and theory of critical distance (TCD) using the point method. The experimental results indicated that a substantial improvement in the fatigue strength capacity can be claimed in the joints subjected to tensile overloads, particularly in the studied S700 and S1100 steels. The higher-level overload (0.8fy), corresponding to the nominal cross-sectional area, improved the mean fatigue strength of the welded joints manufactured of high-strength S700 steel by approximately 60%, while the lower overload (0.6fy) improved the mean fatigue strength by 20%. In addition, a use of equivalent nominal stresses for the joints subjected to VA loads resulted in conservative assessments when employing S–N curves obtained for the CA loading. The 4R method, via the local mean stress correction for individual cycles, provided higher accuracy for the fatigue assessments.

Integrity Assessment of Stress Corrosion Cracking Susceptibility of Duplex UNS S32205 and Austenitic UNS S31653 Stainless Steel Reinforcements

Herein, the stress corrosion cracking (SCC) susceptibility of duplex (UNS S32205) and austenitic (UNS S31653) stainless steel (SS) reinforcements was evaluated using integrity assessment criteria. Mechanical properties were analyzed and compared by different SCC susceptibility factors. The integrity assessment was conducted applying Cosenza, Creazza, and Ortega ductility criteria, following three different standards (ACI 318-19, ASTM A615, and FIB). A conventional carbon steel (UNS G10080) reinforcement was also evaluated for comparative purposes, whose high residual stress value (>280 MPa) promoted a high corrosion growth rate. Duplex UNS S32205 SS grade showed a significant decrease in elongation, leading to failure after ductility assessment at high chloride concentrations. Fractographic analysis of both SS grades, duplex and austenitic, revealed less than 40% brittle areas at 8 wt.% Cl−, while UNS G10080 had over 85% at 4 wt.% Cl−.

Wire Arc Additive Manufacturing: A Study of Process Parameters Using Multiphysics Simulations

In this work, analyses focus on understanding the effects of the scanning pattern and speed on the thermal profile, phase transformation, and residual stress generation in the WAAM deposition. An FE numerical model is constructed that takes into account the phase evolution and transformation plasticity using the ABAQUS user subroutine, UMAT. The results show that the scanning pattern significantly affects the heat accumulation and the cooling rate during the AM deposition, and, eventually, the generation of residual stresses. According to the simulation results, the highest residual stress is generated in the case of the out–in scanning, while the alternate pattern leads to the lowest residual stress. The influence of the scanning speed on the thermal profiles and residual stress are also examined. The analyses show that an increase in the scan speed leads to a decrease in the peak temperature and an increase in the cooling rate, which result in an increase in the martensite volume fraction of the deposition.

Synergistic anti-wear performance of zinc-rich epoxy coating on shot peening strengthened Q345 steel

The synergy of tribology design and surface engineering is vitally important for fulfilling the long-term anti-wear requirements of mechanical equipment. Here, Q345 steel was processed by shot peening (SP) for forming a volcano-like hardened layer (77.72 HV) with high surface roughness (3.77 μm) and low residual tensile stress (210.75 MPa). Then, a zinc-rich epoxy coating was painted on SP-strengthened Q345 steel to construct the double-layer protection. The double-layer system shows excellent tribological behaviors, especially wear resistance being reduced by 76.50% and 38.75%, respectively, with Q345 steel and that sprayed by epoxy coating as a comparison. Crucially, SP layer plays a role in mechanical support, while epoxy coating acts as a cushion to friction force, thus achieving the synergy for enhancing the anti-friction/wear abilities.

Impact of laser powder bed fusion processing on the cyclic and fatigue properties of Ni20Cr alloy

Additive manufacturing is a new-age technology specialising in intricate fabrication in the manufacturing industry. However, extremely high cooling rates and far-from-equilibrium kinetics produce heterogeneous microstructure and induce high initial dislocation density, porosity, and residual stress. These unique microstructural features are known to severely impact cyclic and fatigue properties of alloys. In such a context, we investigated the cyclic and fatigue properties of a Ni-20 wt%. Cr alloy manufactured via laser powder bed fusion process (LPBF) and compared with its cast counterpart. The fatigue testing was carried out with three different strain amplitudes depicting Low, Medium and High Cycle Fatigue (LCF, MCF, HCF). LPBF samples exhibited higher fatigue resistance than that of cast samples, however experienced early failure in all the conditions. Despite strong planar glide, LPBF samples exhibited softening behaviour; the degree of softening is similar for LCF and MCF and less pronounced for HCF. The cast samples did not undergo any softening whatsoever indicating huge differences in cyclic strain mechanisms. Masing analysis was proposed to graphically analyse this softening behaviour. Further, the flow stress was categorised into backstress and effective stress for LPBF and cast samples. Majority of softening in LPBF samples for LCF and MCF occurred via backstress, whereas effective stress is associated to HCF. To investigate the origin of this softening for LPBF samples, microstructure characterisations were performed during the softening and at fracture. Post-fatigue microstructure indicates a clear modification of the dislocation structures from those inherited from LPBF to those linked to fatigue. This gradual change in microstructure is expected to induce the cyclic softening.

Mitigating Stress Corrosion Cracking of 304L and 316L Laser Welds in a Salt Spray through Micro-Shot Peening

Two austenitic stainless steel (ASS) plates, 304L and 316L, were cold-rolled (304R and 316R) with a 10% reduction in thickness and then subjected to laser welding. Cold rolling caused slight surface hardening and introduced residual tensile stress into the ASS plates. The susceptibility to stress corrosion cracking (SCC) of the welds (304RW and 316RW) was determined using the U-bend test pieces in a salt spray. To highlight the stress concentration at the weld’s fusion boundary (FB), the top weld reinforcement was not ground off before bending. Moreover, micro-shot peening (MSP) was performed to mitigate the SCC of the welds by imposing high residual compressive stress and forming a fine-grained structure. Cold rolling increased the susceptibility of the 304R specimen to pitting corrosion and intergranular (IG) microcracking. Moreover, pitting corrosion and SCC were found more often at the FBs of the 304RW. The corrosion pits of the peened 304RW (304RWSP) were finer but greater in amount than the those of the un-peened one. The results also indicated that the 316L ASS welds with MSP were resistant to the incidence of pitting corrosion and SCC in a salt spray. The better reliability and longer service life of dry storage canisters could be achieved by using 316L ASS for the construction and application of MSP on it.

Effects of deposition temperature on the mechanical and structural properties of amorphous Al–Si–O thin films prepared by radio frequency magnetron sputtering

Aluminosilicate (Al–Si–O) thin films containing up to 31 at.% Al and 23 at.% Si were prepared by reactive RF magnetron co-sputtering. Mechanical and structural properties were measured by indentation and specular reflectance infrared spectroscopy at varying Si sputtering target power and substrate temperature in the range 100 to 500 °C. It was found that an increased substrate temperature and Al/Si ratio give denser structure and consequently higher hardness (7.4 to 9.5 GPa) and higher reduced elastic modulus (85 to 93 GPa) while at the same time lower crack resistance (2.6 to 0.9 N). The intensity of the infrared Si-O-Si/Al asymmetric stretching vibrations shows a linear dependence with respect to Al concentration. The Al–O–Al vibrational band (at 1050 cm−1) shifts towards higher wavenumbers with increasing Al concentration which indicates a decrease of the bond length, evidencing denser structure and higher residual stress, which is supported by the increased hardness. The same Al–O–Al vibrational band (at 1050 cm−1) shifts towards lower wavenumber with increasing substrate temperature indicating an increase in the average coordination number of Al.

Support Structures Optimisation for High-Quality Metal Additive Manufacturing with Laser Powder Bed Fusion: A Numerical Simulation Study.

This study focuses on Metal Additive Manufacturing (AM), an emerging method known for its ability to create lightweight components and intricate designs. However, Laser Powder Bed Fusion (LPBF), a prominent AM technique, faces a major challenge due to the development of high residual stress, resulting in flawed parts and printing failures. The study's goal was to assess the thermal behaviour of different support structures and optimised designs to reduce the support volume and residual stress while ensuring high-quality prints. To explore this, L-shaped specimens were printed using block-type support structures through an LPBF machine. This process was subsequently validated through numerical simulations, which were in alignment with experimental observations. In addition to block-type support structures, line, contour, and cone supports were examined numerically to identify the optimal solutions that minimise the support volume and residual stress while maintaining high-quality prints. The optimisation approach was based on the Design of Experiments (DOE) methodology and multi-objective optimisation. The findings revealed that block supports exhibited excellent thermal behaviour. High-density supports outperformed low-density alternatives in temperature distribution, while cone-type supports were more susceptible to warping. These insights provide valuable guidance for improving the metal AM and LPBF processes, enabling their broader use in industries like aerospace, medical, defence, and automotive.

The microstructure and mechanical properties of He charged Al films fabricated by HiPIMS/DCMS co-sputtering

Although magnetron sputtering is an effective method to prepare helium (He) charged films, due to the shadowing effect and low plasma density, the films prepared by this method are high porosity and residual stress. In order to solve this problem, the microstructure and properties of He charged aluminum (Al) thin films deposited by the direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS)/DCMS co-sputtering were investigated carefully. The results show that, even with the same concentration of He content, He charged film deposited by HiPIMS/DCMS show relatively lower hardness than that of He charged films by DCMS, which the difference is about 0.75 GPa. This is mainly because increased gain size and decreased the number of dislocation loops, due to strong migration ability on the growth surface in HiPIMS/DCMS process. A denser, less columnar structure of He charged film was prepared by the HiPIMS/DCMS co-sputtering, which might help to promote the research on the properties of metal materials containing He bubbles.

Revealing corrosion behavior of B4C/pure Al composite at different interfaces between B4C particles and Al matrix in NaCl electrolyte

In the present work, the corrosion behavior of the B4C/pure Al composite in a Cl− containing environment is investigated. Thereby, the B4C/pure Al composites have been subjected to the different heat treatments to highlight the impact of the corrosion behavior at the different interfaces. The results of current sensing atomic force microscope (CSAFM) and scanning Kelvin probe microscopy (SKPFM) show that the dissolution of the Al matrix is relevant to the micro-galvanic corrosion between anodic Al matrix and cathodic B4C particles. The as quenched (AQ) composites with a higher volta potential difference (VPD) show inferior anticorrosion property than that of the as produced (AP) composites and the as annealed (AA) composites. By combining X-Ray diffraction (XRD) and geometric phase analysis (GPA), the higher residual stress and obvious interfacial strain between the B4C particles and the Al matrix of the AP samples is detected, which result in increasing the driving force of micro-galvanic corrosion.

Effect of multiple weld repairs on fatigue strength of bogie frame of railroad vehicle

In the welded structure of railroad vehicles, the bogie frame requires a service life more than 25 years, cracks occur in the weld zone owing to the application of repetitive fatigue loading during operation. When cracks occur and propagate in the weld zone, they are removed through gouging, and gas metal arc welding is performed for repair. After the first weld repair, the second weld repair process is performed if crack is visible in the same area. In this study, the effects of repetitive repair welding on the fatigue characteristics were investigated using materials applied in railway vehicle structures. Three types of specimens (weld specimen, first weld repairs specimen, and second weld repairs specimen) were prepared, and fatigue and residual stress tests were performed. The results indicated that the weld specimen had the longest fatigue life, and the first weld repairs specimen had the shortest fatigue life. The fatigue life of the second weld repairs specimen was longer than that of the first weld repairs specimen. Residual stress tests and finite-element analysis results indicated that the first weld repairs specimen had the shortest fatigue life among the specimens tested, because the highest residual stress was generated.

In-plane quasi-static compression deformation of Ti6Al4V double arrow-headed lattice structures fabricated by electron beam powder bed fusion process: Build orientation, scan speed and failure mechanism

The 2D double arrow-headed (DAH) lattice structures, which are promising cellular structures for impact mitigation, remain relatively unexplored in terms of their compression response when manufactured using the powder bed fusion process with Ti6Al4V (Ti64) alloy. This study aims to investigate the effects of build orientation and beam scan speed of Electron Beam Powder Bed Fusion (PBF-EB) process on the energy absorption of 2D Ti64 DAH lattice structures. Additionally, potential microstructural variations due to adjusted process parameters can be linked to different levels of energy absorption. For the compressions, the lattice structures were manufactured at two build orientations (0° and 45°), using three different beam scan speeds: speed function (SF), low speed (LS), high speed (HS). In micro-characterizations, the unit cells of 0deg-LS exhibited the lowest micro-porosity level at 0.12 %. Based on KAM values, thin struts at unit cells had higher residual stresses than thick struts, contributing to the initiation of failure locations. The compressions revealed that the 0deg-LS group absorbed 21.6 % and 24 % more energy than 0deg-SF and 0deg-HS groups, respectively, at compressions of 33 %. 45° samples absorbed approximately 10 % more energy than 0° samples except HS groups. The lowest micro-porosity of 0deg-LS contributed to having the highest energy absorption among 0deg samples. As the residual stresses in KAM values did not differ strongly with varying beam speed, varied energy absorptions were not linked to them. An optimization of the numerical compressions helped obtain designs with higher energy absorption and less relative volume. This study provides valuable insights into Ti64 cellular applications constrained with 2D-type designs.

Impact of Plate Thickness and Joint Geometry on Residual Stresses in 347H Stainless Steel Welds

Weldments of 347H stainless steel are potentially susceptible to stress relaxation cracking at elevated service temperatures. Mitigation of stress relaxation cracking susceptibility within a multipass weld requires a good understanding of welding practices and manufacturing techniques to control high tensile residual stresses. In this work, the dependence of residual stress distribution in 347H stainless steel on base plate thickness, joint geometry design, and preheating condition was systematically investigated by using three-dimensional finite element models. The finite element models were validated through good agreement between neutron diffraction measurements and calculated elastic strains. The single-V-groove welds with and without a preheating step all produced similar peak von Mises residual stresses, above 450 MPa, within both the fusion zone and heat-affected zone (HAZ). In plates thicker than 0.5 in. (12.7 mm), high tensile residual stress could be observed in a relatively large area, from the middle of the plate thickness to underneath the top surface. A double-V groove shifted the high tensile stress area to the middle thickness of the weld. A single-J-groove weld was able to confine the residual stress to a very small region near the middle thickness within the fusion zone and suppressed the von Mises residual stress within the HAZ to below 400 MPa.