Thermal Residual Stress Distribution Research Articles

Structure and property evolution of LaYbZr2O7/YSZ heterogeneous interface in double‐layer thermal barrier coatings after thermal exposure

AbstractDouble‐layer thermal barrier coatings are one of the development directions of thermal barrier coatings technology. LaYbZr2O7 (LYZO) has high toughness and low thermal conductivity, making it promising for use as a high‐temperature thermal barrier coating. However, its relatively poor mechanical properties, especially low fracture toughness, limit its further application. Therefore, we prepared a double‐layer coating of LYZO and Y2O3 partially stabilized ZrO2 (YSZ) using atmospheric plasma spraying, with the inner layer being YSZ and the outer layer being LYZO. The results showed that after spraying, LYZO was in the metastable fluorite phase, while YSZ was in the tetragonal phase. LYZO and YSZ had good chemical compatibility, and an interaction layer formed at the interface between them, with its thickness increasing with the sintering time. Eventually, after 100 h of heat treatment, the thickness reached 12 µm. The hardness of the LYZO/YSZ interface increased with the sintering time, reaching a maximum value after sintering at 1300°C for 10 h. When evaluating the interfacial bonding strength using indentation method, the length of cracks caused by indentation significantly decreased with the increase of sintering time. The increase in bonding strength can be attributed to the formation of a reaction layer and the reduction of thermal residual stresses. Raman spectroscopy was used to measure the thermal residual strain at the LYZO/YSZ interface. Unbalanced distribution of thermal residual stress and M phase was observed near the interface. As the sintering time increased, both thermal residual stress and M phase content decreased. In summary, the double‐layer thermal barrier coating of LYZO/YSZ has good potential for development.

The Effect of Scanning Strategy on the Thermal Behavior and Residual Stress Distribution of Damping Alloys during Selective Laser Melting.

The manufacture of damping alloy parts with stable damping properties and high mechanical performances in the selective laser melting (SLM) process is influenced by temperature evolution and residual stress distribution. Choosing an appropriate scanning strategy, namely the specific trajectory along which the laser head scans powders within given area, is crucial, but clearly defined criteria for scanning strategy design are lacking. In this study, a three-dimensional finite element model (FEM) of the SLM process for manufacturing a WE43 alloy component was established and validated against the published experimental data. Eleven different scanning strategies were designed and simulated, considering variables such as scanning track length, direction, Out-In or In-Out strategy, start point, and interlayer variation. The results showed that scanning strategy, geometry, and layer number collectively affect temperature, melt pool, and stress outputs. For instance, starting scanning at a colder part of the powder layer could lead to a high peak temperature and low melt pool depth. A higher layer number generally results in lower cooling rate, a lower temperature gradient, a longer melt pool life, and larger melt pool dimensions. Changing the start point between scanning circulations helps mitigate detrimental residual stress. This work highlights the potential of analyzing various scanning strategy-related variables, which contributes to reducing trial-and-error tests and selecting optimal scanning strategies under different product quality requirements. This article can assist in the design of appropriate scanning strategies to prevent defects such as element loss due to evaporation, poor bonding, and deformation or cracking from high residual stress. Additionally, identifying stress concentration locations and understanding the effects of geometry and layer number on thermal and mechanical behaviors can assist in geometry design.

Thermo-Mechanical Stress Distributions in a Ceramic Matrix Composites Turbine Vane Coated with Environmental Barrier Coatings

It is of great significance to obtain an accurate stress assessment when replacing traditional metal components with ceramic matrix composites (CMCs) in turbine engines. The current study aims to investigate the stress characteristics of CMCs turbine vanes with multilayer-structured environmental barrier coatings (EBCs) using numerical simulation techniques. A three-dimensional finite element model of CMCs turbine vanes coated with EBCs was formulated. The distribution of thermal residual stresses generated during the manufacturing process of EBCs and the distribution of stresses under different loading conditions were calculated and compared. The results show that the hoop stress (σ11) and spanwise stress (σ22) in the turbine vanes are significantly higher than the through-thickness stress (σ33) under coupled loads. The maximum hoop stress (σ11) is approximately 346 MPa. The thermal residual stress induced during the EBCs manufacturing process reaches a maximum of approximately 360 MPa. The loading conditions significantly influence the stress distribution of EBCs, and the stress distribution of EBCs exhibits certain regularities at different heights under varying loading conditions. These results enable us to gain a deeper understanding of the failure mechanism of CMCs/EBCs turbine vanes and can improve the optimization capabilities for these components.

Cutting performance and failure mechanisms of TiB2-TiC-Al2O3 multi-dimensional gradient ceramic tool in intermittent turning of hardened steel

The TiB2-TiC-Al2O3 multi-dimensional gradient ceramic tool (MDGCT), whose composition varied in two directions, were designed and fabricated via hot-pressed vacuum sintering. The cutting performance, including cutting force, cutting temperature and chip morphology, and failure mechanisms of MDGCT with different orientation angles in intermittent turning of SKD11 hardened steel were investigated in comparison with the homogeneous commercial ceramic tool CC650 at different depths of cut and cutting speeds. And the distribution of thermal residual stress of MDGCT was investigated employing FEM analysis. Results showed that the multi-dimensional gradient ceramic tool with orientation angle of 15 degree (TT3A15) had the optimal comprehensive cutting performance. The main failure mechanisms of CC650 tool were chipping and fracture caused by crack propagation, while the crack propagation was effectively inhibited in the TT3A15 tool, and its failure mechanism was chipping at the tool nose. The contribution of multi-dimensional gradient structure of TT3A15 tool to the increasing residual compressive stress of minor cutting edge, the favorable anti-crack capacity of flank face and the heterogeneity of physical and mechanical properties of rake face were responsible for its optimal cutting performance.

Finite element simulation of residual stress distribution during selective laser melting of Mg-Y-Sm-Zn-Zr alloy

Selective laser melting (SLM) has been confirmed as a promising additive manufacturing technology for the formation of lightweight structural parts of magnesium alloy. However, it is noteworthy that residual stress is easily generated during the SLM process of magnesium alloy, and it will induce cracks to reduce the compactness of formed parts. In this study, a three-dimensional (3D) nonlinear transient thermodynamic coupled finite element model (FEM) was built to analyze the effects of the scanning strategy on the thermal stress evolution and residual stress distribution during the SLM process of Mg-Y-Sm-Zn-Zr alloy. Moreover, six groups of simulation tests were performed based on three different scanning strategies (i.e., successive scanning, island scanning, and orthogonal scanning). The stress values of equal scale specimens produced with the same process parameters were examined to verify the simulation results. The equivalent residual stress was the largest under the use of the successive scanning strategy, as indicated by the results. Using the island scanning, the stress distribution in the island was similar to that of successive scanning, whereas there was a high-stress island close to the interface. Besides, the average S11 and S22 residual stresses were 4.90% and 36.13% lower than that of the successive scanning, respectively. Orthogonal scanning was confirmed as the most effective scanning strategy to reduce residual stress. To be specific, the average S11 and S22 residual stresses were 25.19% and 43.12% lower than that of successive scanning when using the intralayer orthogonal scanning. The simulation and experimental verification results indicated that the maximum equivalent residual stress is the lowest (229 MPa), and the stress distribution was most uniform when orthogonal scanning was performed between 4 × 4 island.

Mechanical behavior of austenitic stainless-steel welds with variable content of [formula omitted]-ferrite in the heat-affected zone

δ-ferrite solidified in the fusion zone (FZ) mitigates the microfissuring susceptibility in multi-pass welded joints performed on austenitic stainless steel (ASS). The herein investigation studied welded joints between two 304L grade ASS with different chemical compositions, which varied δ-ferrite content in the heat-affected zone (HAZ). The pulsed-current gas tungsten arc welding (GTAW-P) process was used along with filler metals ER 308L and ER309L. Additionally, an autogenous joint was made with stainless steel (SS) of high δ-ferrite content (10%). A thermo-mechanical finite element (FE) model was applied to estimate the thermal history, residual stress (RS) and distortion distributions on SS joints. Then, numerical results were correlated with the fracture analysis. The tensile test corroborated the increment in ductility due to δ-ferrite content increasing in the HAZ. The enhancement of mechanical strength observed in the autogenous welded joint was associated with the pinning effect produced by δ-ferrite stringers formed in the HAZ, which inhibited grain growth. Despite the high joint efficiency of SS 304L weldments, the FZ was fractured due to the microfissuring and precipitated particles. The FZ rupture starts in the FZ-HAZ interface of the first weld bead related to the ductility loss with the heat input. Microfissures found in the FZ of all weldments were related to the tensile stress concentration caused by the uneven distortion distribution and the ductility increment (up to 39%) provided by the high δ-ferrite content. Some chromium carbides in the FZ acted as stress raisers.

Thermo-mechanical modelling to evaluate residual stress and material compatibility of laser cladding process depositing similar and dissimilar material on Ti6Al4V alloy

• 3D finite element modelling of laser cladding process has been developed to predict residual stress at clad, substrate and interface. • Effect of Similar and dissimilar materials on the development of residual stress during laser cladding. • Benchmarking the model with experimental results. • Laser materials interaction of alloy and ceramic materials, and their effect on generation of residual stress. • Use of commercial software for any material system to select appropriate clad materials with minimised residual stress. The formation of residual stresses due to thermo-mechanical effect and microstructural transformation in the Laser Cladding process predominantly affects the final product integrity and service life. A 3D finite element transient thermo-mechanical model has been developed to predict thermal profile and residual stress distribution for repair application of Ti6Al4V alloy using a moving heat source. Then the developed model was applied for the deposition of ceramic materials Al 2 O 3 and TiC on Ti6Al4V alloy substrate. The outcome of this model is to predict temperature distribution, cooling rate, melt pool depth, heat affected zone and residual stress. This study mainly highlights the thermal effect on the residual stresses for similar and dissimilar clad/substrate materials and suggests the suitable cladding material with minimum residual stress.

Stress–strain behaviors of cermets with various binder contents and residual thermal stress states

The stress-strain behaviors of WC–Co cermets with various binder contents were investigated, in which the residual thermal stress (RTS) was taken into account. The effects of binder content and WC grain morphology on the RTS and strain responses under applied stress were quantified by finite element modeling based on the real microstructures of cermets. It shows that the RTS of both the hard matrix and ductile binder phases are linear functions of the binder content. High-level RTS is concentrated in the WC region close to WC/WC grain boundaries adjacent to the Co phase, and in the Co region close to the WC/Co interface, respectively. In the cermets with RTS, the accumulation rate of strain partitioning and strain localization in WC–Co cermets decreases with the decrease of Co content. It is found that increasing the proportion of thin Co layers parallel to the compression direction is beneficial to simultaneously strengthening and toughening cermets with low Co contents. This study provides access and mechanisms for modulating RTS distribution to achieve excellent comprehensive mechanical properties of cermets with various binder content.

Numerical investigation of thermal residual stress distribution for the sustainability of laser welded joints of dissimilar materials

In this work, a three-dimensional (3D) time dependent finite element method is used to simulate the thermo-mechanical behavior of laser welded joints. Laser welding of two dissimilar materials (AISI 316L and Ti6Al4V) of 2 mm thickness has been considered while modeling in COMSOL multiphysics with a coupling of heat transfer and solid mechanics module. A total of six temperature dependent physical properties, i.e. density, specific heat, thermal conductivity, coefficient of thermal expansion, Young’s modulus, and Poisson’s ratio have been considered for making the numerical model more realistic. The experimental results show that AISI 316L experiences more than 2 times more maximum stresses than Ti6Al4V near the weld bead region during the process. High value of Von mishes stress in the welded zone can be observed due to high temperature gradients of welds. The simulated value of residual after one entire thermal cycle is near to 80 MPa on a point over weld bead.

Study on toughening mechanisms of pyrolytic carbon interface layer in Cf/ZrB2-SiC composites using in-situ tensile experimental and numerical methods

The toughening mechanism in continuous fiber toughened ZrB2-SiC ceramic matrix (Cf/ZrB2-SiC) composites was studied upon introduction of pyrolytic carbon coating at the fiber/matrix interface. The real-time deformation behavior, surface crack initiation and evolution of Cf/ZrB2-SiC composites under tensile load were studied using in-situ scanning electron microscopy (SEM) to determine the typical damage modes and toughening mechanisms. A refined microscopic representative volume element (RVE) inserting the cohesive zone elements was established to study the PyC interface layer damage by using finite element method. It was found that PyC interface layer damage induced by thermal residual stress (TRS) was one of critical factors affecting the mechanical performance of the Cf/ZrB2-SiC composites. The critical thickness of the interface layer was also further determined by analyzing the effect of interface layer thickness on the distribution of TRS, which can guide the design of PyC interface layer for manufacturing the Cf/ZrB2-SiC composites.

Residual Stress Analysis of Laser Welding Between Output Needle and Helix

In order to obtain the welding stress of the output needle and helix, a finite element model is established and the mesh at the weld is refined to evaluate the welding residual stress. ANSYS parameter design language is used to simulate the motion load of welding heat source. Based on the exact calculation of the temperature field, the corresponding thermal units are converted into structural units to calculate the welding residual stress. Finally, the thermal stress distribution curve and residual stress distribution trend are obtained after post-treatment.

Effects of heat treatment on microstructure and mechanical properties of 2.5D Cf/BN/pyrocarbon composites by chemical vapor infiltration

In this study, 2.5D carbon fiber reinforced pyrocarbon matrix composites with a BN interphase (Cf/BN/PyC) were prepared by chemical vapor infiltration. Through comparing the distribution of thermal residual stress and the evolution of the interface microstructure of composite before and after heat treatment, the reason why the structure and orientation of the BN interphase in the Cf/BN/PyC composite deviate from the layered structure required for the expected compliant interface was analyzed. The results show that the crystalline skin of Cf affects the order of BN near the Cf/BN interface, thereby forming BN interphase with a structural gradient from the beginning. The infiltration process of pyrocarbon interacts with the growth and rearrangement process of BN, resulting in incomplete crystallization of BN. After heat treatment at 1600°C, BN interphase with high crystallinity in the nano-domain but disordered orientation is formed, although it can also debond the interface according to He and Hutchinson's model. The BN grain orientation deviates from the fiber axis mainly due to multi-factor coupling, in which the axial residual thermal stress generated at the Cf/BN/PyC interface is the main influencing factor. Understanding the influence of the heat treatment (HT) on the atomic-domain orientation change and structural evolution during the preparation of boron nitride (BN) interphase by chemical vapor infiltration paves us an effective way to modify the preparation of interphase with low fracture energy.

Investigation of thermal residual stress of borided steel: finite-element simulation

Although a technologically interesting characteristic of boriding is the production of a hard, wear-resistant layer on the treated steel parts, thermal residual stresses may lead to crack formation in both the iron boride (FeB) layer and the vicinity of the iron boride–diiron boride (Fe2B) interface. In this study, the thermal residual stress distribution in borided tool steel without temperature gradients was investigated systematically by using finite-element analysis. A transient analysis was performed to obtain the stress change with cooling time. The models were cooled down from 823 to 293 K under steady-state conditions. The maximum compressive residual stress is about 690 MPa and tends to decrease with increasing layer thickness. For the case of the double-layer model with 60 μm thick diiron boride, the tensile residual stress occurs on the surface of the iron boride layer and its value changes within the range of 1870–2025 MPa with the iron boride layer thickness. However, the magnitude of the shear stress at the steel–diiron boride interface is not crucial in the model with a single boride layer. The shear stress at the iron boride–diiron boride interface in the double-layer model increases dramatically with the iron boride layer and exceeds 1050 MPa for a high percentage of iron boride (52%) in the total layer thickness.

Finite element analysis of the influence of interphase on the thermal residual stress level and distribution of SiCp/6061Al composites

ABSTRACT The influence of interphase on the level and distribution of thermal residual stress (TRS) in SiCp/6061Al composites was studied by using a three-dimensional (3D) interface-phase model. The thermal expansion coefficient and elastic modulus of interphase components were the main factors affecting the TRS level in SiCp/6061Al composites. The formed SiC-MgO-6061Al interphase has highly concentrated TRS, and the formed SiC-Si-6061Al interphase had the lowest TRS level. With the increase of interphase thickness, for most interphase components, the TRS in SiCp/6061Al composites showed a trend of first decreased and then increased. When the interphase thickness was 1.2 μm, it had the lowest TRS level. When the SiC particle aspect ratio was large, the TRS was mainly concentrated in the sharp corner region jointly formed by the reinforcement and interphase. As the SiC particle aspect ratio decreased, the sharp corners of interphase become passivated, the level of TRS in the reinforcement and interphase of SiCp/6061Al composites also gradually decreased. The research results were based on the 3D interface-phase model and generally consistent with the actual experimental analysis.

Numerical Simulation and Experimental Study on Residual Stress in the Curved Surface Forming of 12CrNi2 Alloy Steel by Laser Melting Deposition.

The performance and service life of the nuclear emergency diesel engine shaft made of 12CrNi2 alloy steel is very important for the safety of nuclear power. Laser melting deposition (LMD) is a challenging camshaft-forming technology due to its high precision, rapid prototyping, and excellent parts performance. However, LMD is an unsteady process under the local action of laser, especially for curved surface forming, which is more likely to generate large residual stress on components, resulting in cracks and other defects. At present, the stress research on LMD curved surface forming is relatively insufficient. In the present paper, material parameter testing, high-temperature mechanical properties analysis, single-track sample preparation, and heat source checks are conducted. At the same time, the ABAQUS software and the DFLUX heat source subroutine are used to compile the curved double-ellipsoidal moving heat source, and the effects of the temperature-dependent thermophysical parameters and phase change latent heat on the temperature field are considered. A three-dimensional finite element model is established to analyze the thermal stress evolution and residual stress distribution of multi-track multi-layer on a curved surface by LMD, and the effect of the scanning method and interlayer cooling time on the residual stress of the formed components is studied. The results show that with the increase in temperature, the strength of the material reduces, and the fracture morphology of the material gradually transitions from ductile fracture to creep fracture. The material parameters provide a guarantee for the simulation, and the errors of the width and depth of the melt pool are 4% and 9.6%, respectively. The simulation and experiment fit well. After cooling, the maximum equivalent stress is 686 MPa, which appears at the junction of the substrate and the deposited layer. The larger residual stress is mainly concentrated in the lower part of the deposited layer, where the maximum circumferential stress and axial stress are the tensile stress. Compared with the axial parallel lap scanning method, the arc copying lap scanning method has a relatively smaller maximum thermal stress and residual stress after cooling. The residual stress in the deposited layer is increased to some extent with the increase in the interlayer cooling time.

Modeling the Interfacial Debonding Behavior Between Steel Wire and Adhesive

Abstract A polyethylene pipe reinforced by winding steel wires (PSP) has been widely used in the petroleum, chemical, and water supply industries. The PSP has outstanding mechanical properties due to its unique composite structure. However, interfacial debonding between steel wire and adhesive sometimes occurs when the temperature and inner pressure increases to some extent in the application. In this study, the interfacial behavior between steel wire and adhesive was investigated and the interfacial failure process was analyzed. First, to acquire test data of interfacial failure, pull-out tests were conducted using specimens consisted of steel wire and adhesive. Specimens were prepared per the PSP manufacturing process, and a temperature change occurred in the specimens' preparation. Second, as the details of failure process could not be observed directly, finite element models were established to represent the mechanical behavior of the steel-polymer interface in-order to reproduce the debonding failure process. The thermal preload was taken into account in the model, and its influence on interfacial behavior was discussed. Contact surface with cohesive behavior was utilized to characterize the interfacial property. Finally, the interfacial failure process including stick–slip interaction and frictional sliding interaction was modeled in the simulation. The simulation result agreed well with the experimental data. Based on the finite element model, the cause and the distribution of thermal residual stress in pull-out specimen were illuminated. Further, it is discussed that how the stress distribution changes along the adhesive interface.

Mechanical properties optimization of Si3N4 ceramics by in-situ introduction of core-shell structural W-Fe5Si3

It is one inevitable problem that optimizing properties of Si3N4 ceramics by introducing second phase would produce thermal mismatch stress, which may lead to interfacial bonding and strength deterioration. In this work, a novel strategy for regulating the residual thermal stress and inhibiting the interfacial debonding in Fe5Si3/Si3N4 ceramics was proposed, which results in the simultaneous increase of strength and toughness of Si3N4 ceramics. Instead of Fe5Si3 particle, the core-shell structural W-Fe5Si3 particles were in situ generated in the Si3N4 matrix by introducing a new WSi2/FeSi2 powder directly. The maximum principal stress, residual radial and tangential stress around the core-shell structural W-Fe5Si3 and Fe5Si3 particle were compared by using the finite element method. The W core inhibits the radial contraction of the Fe5Si3 shell during the cooling process and alters the residual thermal stress distribution. Both the Griffith fracture equation and Lange's energy model indicate that the core-shell structural W-Fe5Si3 inhibits the interfacial debonding, which is consistent with the experiment result. As a result, with the increase of WSi2/FeSi2 addition from 0 to 1 wt%, the fracture toughness of Si3N4 ceramics increases from 7.1 to 7.7 MPa m1/2, and the strength improves from 1016 to 1173 MPa.

Dependence of mechanical properties on microstructure of high-textured pyrocarbon prepared via isothermal and thermal gradient chemical vapor infiltration

In order to explore the dependence of mechanical properties on the microstructure of high-textured (HT) pyrolytic carbon (PyC), the microstructure and properties of HT PyC specimens prepared by isothermal isobaric chemical vapor infiltration (ICVI) and thermal gradient chemical vapor infiltration (TCVI) processes were investigated. The HT PyC of ICVI was uniform and exhibited a more distinct long-range order compared to that of TCVI. Besides, a spatial gradient in texture existed within the HT PyC of TCVI, demonstrating the micro-nonuniformity of the whole matrix. The orientation angle (OA) of HT layers in TCVI increased gradually from the fiber surface to outside, whereas the OA of HT layers in ICVI with better microcrystalline was nearly similar. According to the results of three-point bending and micro-indentation, the flexural strength, interfacial fracture strength, and micro-hardness of TCVI specimens were slightly higher than those of ICVI specimens. Furthermore, the residual thermal stress distribution of TCVI specimens was complex owing to the spatial gradient in temperature of TCVI process, while the residual thermal stress distribution of ICVI specimens was relatively uniform. This work provides meaningful guidance for the process selection and industrial applications of C/C composites.

The role of CNT in improving the mechanical strength retention rate of C/C composites during heat treatment

Graphitization is an important post-treatment of carbon/carbon (C/C) composites. Carbon nanotubes (CNT) as a means of reinforcement have been explored extensively in C/C composites. In this study, CNTs were grafted on the carbon fiber and subsequently densified to delve into the effect of CNT on microstructure, residual thermal stress evolution, as well as the mechanical strength retention rate (MSR) in the course of the heat treatment. Pure C/C and CNT-C/C composites were heat-treated at three temperatures, namely, 1600 °C, 2100 °C and 2450 °C. As suggested from the results, CNT facilitated the transition from the turbostratic structure to the ideal graphite structure. According to the results of measurement and finite element analysis, the presence of CNT decreased and concentratedly distributed the residual thermal stress. In pure C/C composites after undergoing 2450 °C heat treatment, the MSR of out-of-plane compression strength (OCS), in-plane compression strength (ICS) and interlaminar shearing strength (ISS) reached 53.07%, 45.27% and 41.16%, respectively, while these in CNT-C/C composites were 69.69%, 66.33% and 69.70%, respectively. CNT noticeably mitigated the effect of heat treatment on mechanical properties; they also reduced the effect on the anisotropy of mechanical properties. By impacting the microstructure and residual thermal stress distribution of the composites, CNT had further effects on the loading capacity and fracture mode, and finally protected the composites during the high-temperature heat treatment and up-regulated the MSR of CNT-C/C composites.

A new method to estimate the residual stresses in additive manufacturing characterized by point heat source

Residual stress in additive manufacturing (AM) is one of the key challenges in terms of structural integrity and the finish quality of printed components. Estimating the distribution of residual stresses in additively manufactured components is complex and computationally expensive with full-scale thermo-mechanical FE analysis. In this study, a point heat source is utilized to predict the thermal field and residual stress distribution during the manufacturing processes. Numerical results show that the residual stress at a single material point can be expressed as a function of its spatial position and the peak nodal temperature it has experienced during thermal cycles. The distribution of residual stress can be divided into three segments according to the peak nodal temperature. The peak nodal temperature only depends on the heat flux and the distance to the point heat source center. A semi-analytical approach to predict the peak nodal temperature and residual stresses, once the heat flux is known, is proposed. The proposed approach is further validated by a numerical case study, and a very good agreement has been achieved. Compared with traditional thermo-mechanical FE analysis of additive manufacturing, the proposed method significantly improves the computational efficiency, showing great potential for prediction of residual stresses and distortion.