Thermal Residual Stresses Research Articles

Development of Low-Residual-Stress Photosensitive Adhesive Materials for Wafer-Scale Microfluidic Device Fabrication

We have previously developed a photosensitive adhesive material that enables microfluidic channel structure formation and subsequent bonding to a cover with the aim of simplifying the process of wafer-scale microfluidic device fabrication. However, the bonding process and a subsequent curing process required heat treatment at 200 ℃ and 180 ℃, respectively, resulting in the generation of relatively high residual thermal stress. This may lead to delamination, warping, and cracking depending on the application environment. Therefore, in the present study, we designed a new material platform in which the patterning, bonding, and curing processes are based on ultraviolet (UV) reaction of polyfunctional acrylate, tackiness of epoxy resin, and UV/thermal reaction of epoxy resin in order to achieve low residual stress through low-temperature processing. Proof of concept was conducted through the evaluation of various physical properties of the system and found that the new type of photosensitive adhesive material, which bonded to a cover and cured at lower temperatures of 100 ℃ and 120 ℃, respectively, had a low residual stress of 10 MPa and good insulation reliability. This is a promising material for applications such as cooling devices for semiconductor chips, where long-term reliability is required.

FEM of stress partition during compression of WC-10 wt.% Co with realistic 3D morphology

A Finite Element Model is developed to represent the mechanical behavior of WC-10 wt.% Co in compression tests. In this research, realistic three-dimensional model is obtained by slicing a sample by Focused Ion Beam and reconstituting it, resulting in two interpenetrated phases. The brittle WC phase is defined as elastic. The Co phase includes plasticity. The post sintering cooling is simulated, followed by a loading–unloading compression cycle. The results are then compared with experimental data obtained by neutron diffraction. The residual thermal stresses in both phases are in good agreement with the data obtained by neutron diffraction, featuring high tensile stress in the Co phase and moderate compressive stress in the WC phase. The elastic strain in the Co phase shows a good agreement in the loading cycle, but presents some differences in the unloading cycle. The plastic model applied to the Co phase is then modified to improve the accuracy of the simulation, showing a final good agreement with the experiment.

Interplay of manufacturing-induced thermal residual stresses and microvoids in damage and failure of fiber-reinforced composites

Fiber-reinforced composites have inherent thermal residual stresses (TRS) and microvoids that are both generated during the manufacturing process. While the two factors are naturally included in experimental measurements as they preexist in the specimens for testing, theoretical and computational modeling of composites usually neglect these two or include only one. Such simplifications inevitably introduce errors to the predicted mechanical properties of composites, which may further influence the reliability of subsequent modeling and design efforts. Using a micromechanics-based failure modeling approach, this work investigates the effects of both TRS and microvoids on the mechanical properties of fiber-reinforced composites. The TRS is generated when composites are cooled down from the curing temperature. The microvoids are assumed to take realistically existing circular, triangular, square, and pentagonal shapes. Damage progression is systematically studied in six representative loading modes, five voids, and two TRS conditions. All results are statistically extracted to account for geometric uncertainties. The study unravels the interplay of TRS and microvoids in determining the mechanical properties of composites along with the corresponding failure mechanisms. This investigation provides high-fidelity predictions and theoretical insights to improve our understanding of TRS and microvoids in composites, mitigate their impacts, and guide future modeling and design efforts.

Evaluation of elastic-plastic deformation in HSS work roll under coupling of residual stress thermal stress and contact stress during hot rolling

Fatigue damage such as crack and spalling on the work roll surface in hot rolling is caused by comprehensive action of residual stress, thermal stress and contact stress. In this work, a HSS work roll was studied based on the actual hot rolling condition, and high temperature performances of the work roll materials are considered. Finite element method was used to obtain the residual stress distribution of the work roll after quenching and tempering. Then, the coupling mechanism of residual stress, thermal stress and contact stress of the work roll in the bite region are analyzed by a transient thermomechanical model. The results showed that, the maximum equivalent strain and equivalent plastic strain appear at the exit of the bite region. And residual stress and thermal stress account for about 90% of the coupling stress. The accumulation of plastic deformation only occurs in the initial rolling, and the initial temperature influences the plastic deformation accumulation. Furthermore, abnormal rolling often leads to large plastic deformation on the roll surface.

Mechanical strengthening and recovery of C/C-Mo joints during thermal cycling

In order to prepare C/C-Mo joints that possess good thermal shock resistant performance between 1100 °C and room temperature, MoNiSi compounds and Ni-based solid solutions were introduced into the interlayer. The as-prepared C/C-Mo joints display good stability and reliability at high temperatures, in particular the unique strengthening and recovery phenomenon of mechanical strength during thermal cycling. After 5 thermal cycles, the joint strength increased by 28.4%, up to 59.19 MPa, showing a thermal shock strengthening phenomenon. When further increasing the cycles, the joint strength first decreases, then begins to recover. The joint strength has recovered to a peak of 52.39 MPa after 50 cycles, which is higher than the original strength. The thermal shock strengthening is ascribed to the release of the residual thermal stress caused by the formation of the microcracks (dominated by the rapid alternating temperature factor), while the recovery phenomenon is related to the filling of the defects caused by the flowing of the phases with low melting points like Ni-based solid solutions (dominated by the thermal treatment factor). This work provides a strategy to obtain the joints with good tolerance to multiple thermal cycles and excellent thermal shock resistant performance.

Preparation of waterborne polyurethane with high surface activity for regulating the interfacial properties of carbon fiber/polyamide 6 composite

The poor wetting and spreading performances of waterborne polyurethane dispersion (WPUD) on carbon fiber (CF) surface limited its application in regulating the interfacial properties of CF reinforced polyamide 6 (PA6) composites. Unlike previous studies, this work diversified into improving the film-forming property of WPUD on CF surface. Two novel WPUD sizing agents named WPUD-Y1 and WPUD-F1 were synthesized by introducing block polyether and organic fluorine into the molecular chain. Results showed that surface energies of the elaborately designed WPUD-Y1 (35.5 mN/m) and WPUD-F1 (32.7 mN/m) were significantly lower than that of ordinary WPUD (40~55 mN/m). Especially, both sizings formed a complete and uniform film on CF surface. Interfacial shear strength (IFSS) analysis from micro-droplet test showed that sizing relieved the thermal residual stress at composite interphase by over 40%, at the cost of reducing its interfacial strength by 20%. More importantly, although sizing had not enhanced interfacial adhesion, the IFSS values between the sized CF and PA6 were still 20% higher than the maximum interlaminar shear strength (ILSS) values reported in literatures.

Dependence of residual thermal stress in cermets on the sintering technique

The effects of representative sintering techniques on the residual thermal stress (RTS) in the as-prepared cermets were investigated using the WC-Co composites as an example. The real processing parameters of the spark plasma sintering (SPS) and sinter-hot isostatic pressing (Sinter-HIP) techniques were introduced to quantify RTS and its mechanisms. The real microstructures of cermets were used in the models for evaluation of RTS distributions in different phases. The macro-mesoscale coupled calculations indicated that during the cooling of both the SPS and Sinter-HIP processes, the WC phase of the sintered cermets has a state of compressive stress, while Co phase has a tensile stress state. The internal stress magnitude of both phases increases linearly with the decrease of temperature. There are larger temperature and displacement fields in the SPSed cermets compared to those prepared by Sinter-HIP. The stress accumulation in the SPS process is faster than that in the Sinter-HIP by 30%. The RTS in the SPSed cermets is highly concentrated at the acute dihedral angles of WC/Co interfaces, which tends to promote the face-centred cubic (fcc) to hexagonal close-packed (hcp) martensitic transformation of the Co phase.

Experimental and Numerical Study of Thermal Residual Stresses on Multimaterial Adherends in Single-Lap Joints.

The presence of residual stresses in composite materials can significantly affect material performance, especially when integrated in bonded joints. These stresses, often generated during the cure process, can cause cracking and distortion of the material, and are caused by differences in the coefficients of thermal expansion or cure shrinkage. In the current research, multimaterial adherends combining carbon-fibre-reinforced polymer (CFRP) and aluminium in a single-lap joint (SLJ) configuration are analysed, allowing us to understand the effect of the thermal residual stresses, developed during the curing process, in the overall performance of the joints. A numerical model resorting to a finite element analysis (FEA) is developed to assess and predict the behaviour of the joints. The use of FML (fibre metal laminates) was found to significantly improve the strength of the joints, as well as the failure mode. The proposed geometry performed similarly to the comparable FML geometry, in addition to a decrease in the joint weight.

Microstructure and integrity of multilayer ceramic coating with alumina top coat on silicon carbide by laser chemical vapor deposition

Thin double-layer coatings comprising an alumina top coat and mullite bond coat were deposited on SiC substrates by laser chemical vapor deposition (CVD). The effect of the presence of mullite bond coat and the phases of alumina top coat on the structural integrity against the thermal residual stress loaded by the high-temperature CVD process was examined by microstructural characterization and simulative consideration. A laminated layered structure having a dense γ-alumina top surface with a cone-like morphology are grown at a deposition temperature of 1323 K, whereas an α-alumina top layer comprising densely packed faceted grains was grown at 1473 K. The interfaces between the SiC and mullite layers were coherent owing to the formation of a thin transition layer. The γ-alumina layer formed an adhesive interface bordering the mullite layer, whereas small residual defects were formed in the α-alumina layer bordering the mullite layer. Spacings of surface cracking induced by the high-temperature deposition process in the double-layer coatings were approximately half of those in the coatings without mullite layers. As simulative results by finite element method suggested, the double-layer coatings were experimentally verified to be more tolerant to the formation of surface cracks and interfacial delamination, compared to single-layer coatings without the mullite bond layers.

Modeling of thermal damage and residual stress in slicing of silicon wafers using wire-electrical discharge machining: Comparison with experiments

Wire electrical discharge machining (wire-EDM) is a potential technique to fabricate high-quality ultra-thin silicon (Si) wafers. However, its universal integration at the industrial scale remains impeded by the inherent limitation of induced thermal damage and residual stress. It is understood that the process parameters play a significant role in determining these subsurface damages. Therefore, a single-pulse thermal model accounting for plasma heating through a surface heat flux is developed in the present study. The applied heat flux encompasses the heat transferred to the workpiece through sparking during pulse on-time. The heat flux post spark incorporates the heat dissipation via expanding plasma channel. The governing equations of the 2D computational model are solved using the finite element method (FEM) in ABAQUSTM. The proposed model is in good agreement with the experimental results of thermal damage (10 %–13 %), peak residual stress (5 %–10 %), and residual stress damage depth (15 %–20 %). It is observed the wire-EDM slicing of silicon wafers can induce thermal damage of 10–34μm, peak residual stress of 95–180 MPa, and residual stress damage depth of 13.6–55μm. The open voltage is found to have a more significant influence on thermal damage and residual stress than pulse on-time. The model reveals that thermal damage depth could be reduced to as low as 10μm, residual stress damage depth up to 13.6μm, and peak residual stress up to 95 MPa.

Fabrication of ceramic scintillators by laser sintering: the case of Lu3Al5O12:Pr

The fabrication of ceramic scintillators by laser sintering is briefly reviewed and current limitations discussed. The experimental work focused on the fabrication and characterization of undoped and Pr-doped Lu3Al5O12 (LuAG). X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the structure of the sintered ceramics, with XRD results suggesting the absence of residual thermal stresses. Collectively, Raman results suggested the incorporation of Pr to affect the structure and its dynamics. Broadening the peaks of the ceramics in relation to those from the single crystal revealed the presence of structural disorder. Scanning electron microscopy revealed intergrain porosity thus explaining the lack of optical transparency. Energy-dispersive X-ray spectroscopy (EDX) measurements showed Pr to be homogeneously distributed. Radioluminescence measurements under X-ray excitation as a function of the temperature were used to investigate intrinsic defects of the host, including anti-sites and F-type defects.

Modeling Deformation and Fracture of Boron-Based Ceramics with Nonuniform Grain and Phase Boundaries and Thermal-Residual Stress

A phase field framework of elasticity, inelasticity, and fracture mechanics is invoked to study the behavior of ceramic materials. Mechanisms addressed by phase field theory include deformation twinning, dislocation slip, amorphization, and anisotropic cleavage fracture. Failure along grain and phase boundaries is resolved explicitly, whereWeibull statistics are used to characterize the surface energies of such boundaries. Residual stress incurred by mismatching coefficients of thermal expansion among phases is included. Polycrystalline materials of interest are the ultra-hard ceramics boron carbide (B4C) and boron carbide-titanium diboride (B4C-TiB2), the latter a dual-phase composite. Recent advancements in processing technology enable the production of these materials via spark-plasma sintering (SPS) at nearly full theoretical density. Numerical simulations invoking biaxial loading (e.g., pure shear) demonstrate how properties and mechanisms at the scale of the microstructure influence overall strength and ductility. In agreement with experimental inferences, simulations show that plasticity is more prevalent in the TiB2 phase of the composite and reduces the tendency for transgranular fracture. The composite demonstrates greater overall strength and ductility than monolithic B4C in both simulations and experiments. Toughening of the more brittle B4C phase from residual stress, in addition to crack mitigation from the stronger and more ductile TiB2 phase are deemed advantageous attributes of the composite.

Simulation of the thermo-mechanical behaviour of tread braked railway wheels by means of a 2D finite element model

The friction heat generated at the wheel-shoe interface during tread braking operations leads to a temperature rise of the wheel tread surface. Fast cooling as well as residual thermal stresses and strains can cause damages to the wheel surface, thus negatively affecting the vehicle dynamic behaviour and strongly reducing the wheel life. Modelling the thermo-mechanical interaction of the wheel-block pair is thus of paramount importance to improve the reliability of braking manoeuvres. The paper shows the development of a decoupled thermo-mechanical model and its implementation in ANSYS Mechanical APDL, which can compute the contact pressure at the wheel-shoe interface and the resulting wheel temperature field produced during braking operations, considering friction heating, air convection cooling and the rail chill effect.

X-ray diffraction measurements and computational prediction of residual stress mitigation scanning strategies in powder bed fusion additive manufacturing

The repeated localised heating-melting-cooling-solidification processes during laser beam powder bed fusion (PBF-LB) additive manufacturing (AM) induce intricate thermal residual stress (RS) in manufactured parts. Non-destructive characterisation using X-ray diffraction was used to measure the RS of Ti-6Al-4V square plates that were manufactured by using six different scanning strategies. Computational modelling was used to interpret the experimental stress measurement results. It was revealed that an inclined scanning strategy is beneficial for reducing the average through-thickness RS because the inclined scanning strategy can mitigate the non-uniform thermal profile and corresponding residual thermal stresses in successive layers of material. Among all the different scanning strategies that were analysed in this work, the 45° inclined 90° rotation scanning resulted in the lowest RS. The thicker parts have a greater gradient of RS than the thinner parts, after base plate removal. This research outcome can help the AM industry to design or optimise the process parameters of the PBF-LB aiming to minimise the RS of metal parts.

BÉZIER BASE EXTENDED ISOGEOMETRIC NUMERICAL METHOD FOR THERMO ELASTIC-PLASTIC ANALYSIS OF CRACK PROPAGATION IN CRACKED PLATE UNDER WELDING RESIDUAL STRESS AND THERMAL LOAD

A new procedure in the field of Bézier base extended isogeometric method (XIGA) has been introduced to analyze the effect of welding residual stress and thermal load on crack propagation rate and fatigue life. This new procedure is based on the constitutive thermoelastic plastic equation. The main parts of this procedure are using the B´ezier base XIGA method to calculate the redistribution of welding residual stress due to crack growth and to compute the value of stress intensity factor (SIF) in the welding residual stress field. For this purpose, the grid points of Bézier elements (with C0-continuity) around the crack line and the crack tip are identified by the level set representation. Then, discontinuous enrichment functions are added to the isogeometric analysis approximation. Thus, this method does not require the re-meshing process. The results show that there is a good agreement between the results of proposed numerical method and the Hole-Drilling Strain-Gage method. The interaction integral method has been used to extract SIF. The effects of welding residual stress and thermal load on the SIF are considered using the superposition method. Also, the Walker equation has been modified to calculate the fatigue life caused by thermal loading and welding residual stress. The results display a good agreement between the proposed method and the finite element method. Due to the advantages of the Bézier based XIGA method, which eliminates parametric space and allows the precise addition of enrichment functions to the basis functions of cracked elements (crack line or crack tip), the obtained results are highly accurate that shows this method is effective for analyzing discontinuous problems.

Distribution of Cutting and Welding Thermal Residual Stress in Steel Bone with and without Holes in the Partially Encased Composite Wall

In order to ensure the mechanical reliability of a partially encased composite wall (the PEC wall), the distribution law of the thermal residual stress of the steel bone of the PEC wall needs to be deeply studied. In this study, we first conducted a cutting thermal conductivity analysis and stress analysis of the flange and webs. Second, based on the cutting residual stress of different sheet steel, the welding thermal conductivity analysis and stress analysis of I-shaped calculation units with and without a hole in the web were carried out. Finally, the difference in the distribution of the stress on the section during welding (or cutting) and after the cooling of the welding (or cutting) was clarified, the shape and size of the residual stress distribution in the two types of steel bone formed by the cutting and welding thermal processing were evaluated, and the approximate distributions of the residual stress of the flange and webs were obtained, respectively.

Multiscale modelling of curing-induced z-pin/composite interfacial cracks

The curing process generates thermal residual stress in z-pinned composites, leading to partial or complete cracking along z-pin/composite interfaces. In this study, we applied a novel multiscale framework to evaluate the delamination behaviors of z-pinned composites considering the effects of z-pin/composite interfacial cracks. Unit cell models were used to represent the typical microstructures and micromechanisms of z-pinned composites with diverse interfacial conditions and to predict z-pin pullout behaviors. The derived z-pin bridging laws are then used to simulate the behaviors of individual z-pins in macroscale models. The load-versus-displacement relationships of z-pinned composites under mode-Ⅰ loading conditions are predicted using macroscale models and then verified experimentally. A parametric study reveals that interfacial cracks can affect the failure behaviors of z-pinned composites by changing the microscale bridging mechanisms of individual z-pins. It is theoretically feasible to customise z-pin/composite interfacial bonding properties, z-pin surface roughness, and curing temperature to optimise the delamination properties of z-pinned composites.

Resistance brazing of C/C composites-metallized SnAgCu-Ti powder to T2 copper using AgCuTi filler metal

Carbon/carbon (C/C) composites and T2 copper (Cu) alloy were successfully welded by the following two-step process: the pre-metallization on C/C composites was successfully prepared by the vacuum brazing welding using SnAgCu-xTi(x = 1, 2, 3, 4, and 5 wt%) metal powder and the subsequent reliable C/C composites-Cu joints were obtained by resistance brazing welding with AgCuTi filler metal as the interlayer in this paper. The typical interface morphology of the metallization layer on the surface of C/C composites, microstructure, mechanical properties, and fracture mechanism of resistance brazing welded joints were investigated by the relevant analysis method. The smooth metallization layer was generated on the surface of the C/C composites in the pre-metallization process successfully. The formed excellent bonding between the metallization layer and C/C composites in this stage was prepared for the subsequent welding process. Meanwhile, a strong interfacial bond was obtained due to the formation of the TiC reaction layer in the subsequent resistance brazing welding process. Molten solder infiltrated the pores of the C/C composites and formed a ‘pinning structure’. The residual thermal stress and mismatch of CTE could be relieved by the gradient layer and the shear strength could reach 19.68 MPa.

Disclosing residual thermal stresses in UHT fibre-reinforced ceramic composites and their effect on mechanical behaviour and damage evolution

The reusability of CMCs with inherently high-temperature capabilities and ablation resistance goes through understanding of the effects of residual thermal stresses (RTS) caused by thermal expansion mismatch between fibre and matrix. In this work, the RTS of ultra-high temperature ZrB2–SiC based ceramic reinforced with carbon fibres was dependent on fibre anisotropy and fibre coating. A detailed microstructural analysis about the state of matrix (i.e. residual porosity and cracks) and fibre (i.e. porosity within the fibre bundles, volumetric content of the fibres and their degree of dispersion) was performed. The different level of RTS, among the composites, was studied by comparing stiffness-displacement curves of the bending test and the cyclic dilatometric curves. The correlation between the level of RTS and thermomechanical behaviour was also evaluated by flexural tests at 1500 °C and after thermal shock. The results showed that RTS not only reduced flexural strength, but also influenced damage evolution and stiffness linearity. As a result, the release of RTS can alter not only the pristine geometry, obtained after sintering or machining, due to the formation of inner freed fibres (IFFs) but also the Young's modulus of the matrix and the stress and strain at failure onset of the investigated CMCs.

Investigation of interfacial thermal stabilities of Nbf/TiAl composites with and without Al2O3 coating in interface

Continuous Nb fiber-reinforced TiAl-matrix composites have tremendous potential for replacing brittle TiAl alloy. However, at elevated temperature, interfacial reaction between fiber and matrix is aggravated, causing instability of the interfacial microstructure and mechanical property of Nbf/TiAl composite. Fortunately, the problem could be solved by introducing fiber coating. Consequently, in this study, Al2O3 coating was prepared on Nb fiber by cathodic plasma electrolytic deposition (CPED) technique. Concurrently, interfacial microstructure changes of the composites were explored during thermal treatment at 1100 and 1200 °C. The results show that, during heat treatment at 1100 °C for up to 100 h, for the composite without Al2O3 coating, the interfacial microstructure changes are as follows: Nb/σ/α2/TiAl→Nb/σ/B2/α2/TiAl→Nb/σ/TiAl, which leads to serious thermal residual stress and cracks in interface. Inversely, the introduction of Al2O3 coating can prevent the formation of brittle σ phase during preparation and the 1100 °C heat treatment. However, under 1200 °C heat treatment, the Al2O3 coating gradually fads away and be replaced by α2-Ti3Al phase. Finally, the interfacial phases including σ and α2 are formed. The existence of the Al2O3 coating can delay the formations of the brittle phases and avoid cracks for a long time, which greatly improves interfacial thermal stability of the Nbf/TiAl composite.