Increasing Aging Temperature Research Articles

Influence of Aging Temperature on the Electrochemical Corrosion Behavior of an Age-Hardening 7xxx Aluminum Alloy

In this paper, the microstructure changes of an Al–6.8Zn–2Mg–2Cu–0.1Zr–0.2Sc alloy for shipbuilding under different T6 states were investigated. The effect of aging temperature on the electrochemical corrosion behavior of the alloy was analyzed by means of SEM, EDS, and TEM, and the corrosion mechanism was revealed. The results show that the bean-shaped Al3(Sc, Zr) phase is formed in the T6 alloy. The matrix-precipitated phase is mainly the GP zone at 120 °C. At 150 °C, part of the GP zone is transformed into the η′ phase, and at 180 °C, it is mainly η′ phase + η phase. After electrochemical testing in a 3.5 wt.% NaCl solution, it was found that the Cu content in the grain boundary η phase increased with the increase in aging temperature, the potential near the grain boundary increased, and the corrosion resistance increased. At the same time, the grain boundary precipitates were coarsened and distributed intermittently, which hindered the formation of corrosion channels and improved the corrosion resistance of the alloy. The corrosion mechanism of the alloy after aging at 120 °C/150 °C was mainly intergranular corrosion and pitting corrosion, while the corrosion mechanism after aging at 180 °C was mainly pitting corrosion.

Unveiling microstructural evolution and its effect on mechanical performance in a Cu-9Ni-6Sn alloy

Cu-9Ni-6Sn alloy exhibits profoundly potential as a new environmental-friendly conductive elastic material. In this work, the formation and growth mechanism of discontinuous precipitation, as well as its effect on mechanical properties of Cu-9Ni-6Sn alloy are systematically studied. The investigation indicated that the discontinuous precipitation does easily initiate from random grain boundaries but showing opposite result for Σ3 boundaries. The lower frequency Σ3 boundaries relatively, the more intense the solute diffusion, resulting in a higher volume fraction of discontinuous precipitation. The increasing aging temperature and time will accelerate the grain boundary discontinuous reaction, and the fine-grained samples exhibit a higher volume fraction of discontinuous precipitates and smaller lamellar spacing due to the more nucleation sites and increased interfacial energy. The strengthening mechanism of Cu-9Ni-6Sn alloy mainly focus on dislocation strengthening and precipitation strengthening, in which the D022 or L12-γ′ phases exhibit more significantly precipitation strengthening effect but is difficult to guarantee ductility. The localized grain boundary discontinuous precipitation detrimentally affect both the tensile strength and ductility.But the nano-lamellar discontinuous precipitation is beneficial to the strength-ductility trade off when it occupies the entirely Cu matrix. This work establishes a robust foundation for the microstructural optimization and multi-component design of Cu-9Ni-6Sn alloy.

Effect of Precipitation Heat Treatment on a Mechanically Alloyed Al‐Based Composite Reinforced with Metallic Glassy Powder

An Al‐based composite reinforced with titanium‐based metallic glassy particles is synthesized by the powder metallurgy method through mechanical alloying followed by hot extrusion. The effect of precipitation heat treatment on the structure and properties of the composite is studied. During aging of the solution‐treated composite, crystallization of the metallic glassy reinforcement becomes more pronounced as the solutionizing temperature, solutionizing time, aging temperature, and aging time increase. A reaction layer has formed at the interface between the reinforcement and the matrix after the aging treatment. It is found that the addition of metallic glassy particles promotes precipitation of the nanophase in the matrix. With appropriate heat treatment, the compressive strength, yield strength, and fracture strain of the composite have increased to 942, 635 MPa, and 27%, respectively. Herein, it is demonstrated that the mechanical properties of these composites can be further enhanced through an appropriate heat treatment process, thereby having significant potential for a variety of industries, including aerospace, automotive, and defense, where high‐strength, lightweight materials are critical for performance and safety.

Hydrolysis of poly(ester urethane): In-depth mechanistic pathway determination through thermal and chemical characterization

Many structure/property relationships of hydrolyzed poly(ester urethane) (PEU) – a thermoplastic – have been reported. Examples include changes in molecular weight vs. elongation at break and crosslink density vs. mechanical strength. However, the effect of molecular weight (or molar mass) reduction on some physical, thermal, and chemical properties of hydrolyzed PEU have not been reported. Therefore, a large set of hydrolyzed PEU (Estane®5703) samples were obtained from two aging experiments: 1) accelerated aging conducted under various environments (air, nitrogen, moisture) and at 64 °C and below for almost three years, and 2) natural aging conducted under ambient conditions for more than three decades. The hydrolyzed samples were characterized via multi-detection gel permeation chromatography (GPC), thermogravimetric analysis (TGA), modulated differential scanning calorimetry (mDSC), UV–vis spectroscopy, nuclear magnetic resonance (NMR), and Fourier-transform infrared (FTIR) spectroscopy techniques. Hydrolysis of ester linkages in the soft-segments decreases both the molecular weight (Mw) and the melting point (Tm) of Estane (from ∼55 °C to 39 °C). Aging above this Tm, increased mobility of polymer chains and water diffusivity in the PEU matrix alter the PEU degradation pathway from those expected at aging temperatures below this Tm and have significant bearing on the critical molecular weight (MC) at which the physical, chemical, thermal, and mechanical properties of Estane change abruptly. While a MC value of 20 kDa is found for PEU hydrolysis at mild temperatures (e.g., as low as 39 °C), the value of MC increases with increasing aging temperatures. To complement the existing structure/property relationships reported in the literature, more correlations are obtained, which include the effect of Mw on polydispersity, intrinsic viscosity (Mark-Houwink equation), UV extinction coefficient, and dn/dc (GPC analysis) values. Furthermore, we seek to bolster previously reported aging models for PEU by developing a practical model with which the extent of degradation and material performance can be predicted based on aging under different temperature ranges both above and below the melting point of Estane.

Investigation on effect of L12 and B2 phases on tensile properties and mechanisms of Al0.3CoCrFeNi high entropy alloy through aging treatment

Al0.3CoCrFeNi high entropy alloys (HEAs) have excellent mechanical properties due to the large tunability of microstructures, which makes them one of the most promising candidates for engineering application. In the aspect of enhancing tensile properties, precipitation strengthening was verified effective in this HEA and more studies are needed to deepen relative understanding. In this work, Al0.3CoCrFeNi HEAs with coarse-grained single FCC (A) and fine-grained triplex FCC + B2 + σ (B) structures were aged at different temperatures and times to precipitate second phases, and the tensile properties and mechanisms of aged specimens were studied. The results showed that aging of A resulted in the main precipitation of L12 phases throughout FCC matrix and increasing aging temperature obviously increased L12 size, whereas increasing annealing time had minor effect on phase coarsening. At small L12 size of ∼2.0 nm, stacking fault shearing mechanism was activated during tension and planar dislocation configurations were formed, leading to simultaneous increase of strength and ductility. With increasing L12 size to ∼10.0 nm, the deformation mechanism was transformed into anti-phase boundary shearing and Orowan bypassing and wavy dislocation configurations predominated, which resulted in the significant increase of yield strength from ∼263 MPa to ∼433 MPa without sacrificing notable ductility. On the other hand, aging of B induced the formation of fine B2 phases at residual dislocations and enhancement of tensile yield strength from ∼705 MPa to ∼759 MPa. The ductility decreased slightly from ∼32 % to ∼28 %, which was related to the plastic deformation of B2 phases. However, it was found that the tensile strengths could not be continuously increased by further increasing aging temperature or time. These findings provide insights into the development of precipitation strengthened HEAs with good mechanical properties.

Tailoring strain rate sensitivity and creep behavior of CoCrNi multi-principal-element alloys by aging

While simulations predict that local chemical order in multi-principal-element alloys (MPEAs) renders anomalous dislocation behaviors and thereby varied strain rate sensitivity (SRS) and creep behaviors, the experimental demonstration of such an effect is still under exploration due to the challenges in the manipulation of local chemical order. Here we investigate SRS and creep behavior of CoCrNi MPEAs subjected to long-term aging at various temperatures using nanoindentation. Our results reveal that the SRS and creep resistance of the alloys enhance significantly with increasing aging temperatures, with the SRS notably surpassing that of elemental and solid solution counterparts. In particular, unusual magnitude of creep stress exponent is observed at room temperature and the possible mechanisms are discussed. We suggest that long-term aging could be a straightforward method to tailor the microstructural ordering and mechanical properties of MPEAs, aiming at bridging the gap between simulations and experiments.

Study on the Continuous Whole Process Preparation of Peptizable Pseudoboehmite by High Gravity Strengthening Reaction of CO2 and Heat Transfer.

The preparation of highly peptizable pseudoboehmite faces challenges such as slow reaction rate, prolonged aging times leading to poor peptization performance, and difficulty in achieving continuous production. In this study, a novel approach was proposed involving a high gravity reactor to enhance carbonation reaction control for pseudoboehmite nucleation, a high gravity steam heating process, and a continuous aging process in pipelines. By coupling these three processes, the continuous whole process production of highly peptizable pseudoboehmite under high gravity conditions was achieved. First, the effect of high gravity reactor enhancement on the reaction and the resulting solid phase was investigated. Second, the impact of aging temperature and time on the solid phase formed at different reaction end points was studied using high gravity and steam heating of the liquid phase. Furthermore, the influence of synthesis conditions on the peptization performance of pseudoboehmite was extensively examined. Finally, the nucleation and growth mechanisms of pseudoboehmite under high gravity conditions were analyzed. It was found that increasing high gravity factor, CO2 gas flow rate, and CO2 content accelerated carbonation reaction rate, promoting pseudoboehmite crystal nucleation. Increasing aging temperature and time facilitated the growth of pseudoboehmite nuclei and improved peptization performance. The high gravity device altered the gas-liquid phase contact state of traditional kettle-type equipment, reducing the reaction time and heating time from minutes to seconds, thus achieving the continuous whole process production of pseudoboehmite with a peptization index of 100% from sodium aluminate solution by kiln flue gas.

Influence of Annealing and Aging Parameters on the Microstructure and Properties of 1200 MPa Grade Cold-Rolled Dual-Phase Steel

With the rapid development of the automotive industry, the requirements for bodywork materials are not only focused on high strength but also on improved forming properties. To develop a new generation of automotive steels with higher strength–plasticity matching, a high elongation 1200 MPa grade V-Nb microalloyed cold-rolled reinforced formable dual-phase steel was developed in this experiment through rational compositional design and precise process machining. The properties of the test steel are improved by varying the over-aging temperature as well as the annealing temperature to achieve a good strength–plasticity balance. The results show that as the aging temperature increases, the tensile strength and yield strength of the test steel decrease, while the elongation continues to increase. At an aging temperature of 310 °C, the steel exhibits not only high strength but also better ductility. As the annealing temperature increases, the tensile strength and yield strength of the test steel initially increase and then decrease, while the elongation continues to increase. When the heat treatment process involves an annealing temperature of 860 °C and an over-aging temperature of 310 °C, the test steel achieves the best strength–plasticity balance.

The evaluation of the aging process in 15-5 PH materials in terms of formability and microstructure

In this study, the effects of aging heat treatment of 15-5 PH stainless steel material on the mechanical properties, microstructure, and formability were investigated. Instead of cooling in air, cooling in the oven spontaneously was applied in the oven for the first time. Experimental studies show that aging heat treatment was applied for H900, H1025, and H1150 conditions at temperatures of 482 °C, 552 °C, and 621 °C, respectively, and 60 min for H900 and 240 min for H1025 and H1150. It has been observed that controlled cooling in the oven increases the mechanical values of the material obtained by cooling in the air by 8%–10%. The microstructures of the conditions after heat treatment were also examined, and it was observed that the cooling aging heat treatment in the oven caused precipitation hardening in the microstructure of the material. In addition, when evaluating the formability of the results using V-bending, it was observed that materials which could not withstand bending beyond 75° in Condition A (Con A), the initial state of the material, were smoothly shaped in all conditions and at all angles as a result of the cooling process in the oven. A decrease in spring-back angles has been detected due to the decrease in mechanical properties caused by increasing aging temperature. The highest spring-back value was also observed on a 75° bending die with 10.948° in the H900 condition which has the highest mechanical properties.

Synergy of γ′ phase, MC carbide and grain boundary phase on creep behavior for nickel-based superalloy K439B

The synergy of γ′ phase, MC carbide and grain boundary phase on 815 °C/379 MPa creep behavior for nickel-based superalloy K439B was investigated by adjusting the time and temperature of the first-stage aging treatment. Increasing aging time from 4 h to 6 h, the total creep life is reduced by 35.8% due to the increased bimodal γ′ phase size. The bimodal γ′ phase transforms into small unimodal γ′ phase when increasing aging temperature from 1000 °C to 1080 °C. In addition, the content of M23C6 carbide increases, and the total creep life is increased by about 1.3 times. Small-size γ′ phase with bimodal distribution redissolves into γ matrix during creep deformation. Dislocations pile up at the γ/γ′ interface, and partial dislocations enter into large-size γ′ phase, mainly forming isolated parallel stacking faults. By comparison, the crossed stacking faults within small unimodal γʹ phase form Lomer-Cottrell locks, APB coupled dislocation pairs shear into γʹ phase, and more granular M23C6 carbide precipitates at the grain boundary, which further decreases the minimum steady creep rate and improves the creep life. At the beginning of creep deformation, MC carbide prevents the movement of dislocations. MC carbide formed at grain boundaries can delay crack propagation to a certain extent. MC carbide debonds from γ matrix subsequently due to the dislocation accumulation and higher interface energy, which provides a pathway for crack initiation and propagation. Moreover, the fragmentation and degeneration inside MC carbide promote crack initiation.

Regulation of microstructure, phase transformation behavior, and enhanced high superelastic cycling stability in laser direct energy deposition NiTi shape memory alloys via aging treatment

The remarkable mechanical properties, fatigue resistance, wear and corrosion resistance, biocompatibility, shape memory effect, and superelasticity of NiTi shape memory alloys make them applied in diverse engineering areas, including satellite antennas, oil pipelines, and vascular stents. This paper focuses on the preparation of Ni50.8Ti49.2 alloy via laser directed energy deposition (LDED) utilizing pre-alloyed powder and the heat treatment of solid solution at 950 °C for 8 h followed by subsequent aging at 350 °C, 450 °C and 550 °C for 2 h. The influence of varying aging temperature on the microstructures, phase transformation behavior and superelastic cyclic stability of them was comprehensively summarized and discussed. The martensitic transformation temperatures of NiTi alloys after aging decrease as the aging temperature increases. Specifically, NiTi alloys aged at 450 °C for 2 h exhibit excellent superelasticity and high cyclic stability. After an initial loading-unloading cycle with an 8.079 % pre-strain, the alloy achieves 100 % complete recovery. Even after 10 cycles, the recoverable strain remains at approximately 7.374 %, with a recovery rate exceeding 90 %. These outstanding properties of high superelasticity and superior cyclic stability are significantly better than most other LDED NiTi alloys. This exceptional performance is primarily attributed to the synergistic effects of the R-phase and Ti3Ni4 precipitates on the shape memory alloy.

Effect of Strontium Modifier on Machinability Characteristics of Heat Treated A357 Alloy Using Statistical Techniques

The current study sought to determine the influence of strontium addition on the microstructure and machinability of the A357 alloy. Furthermore, the influence of cutting parameters and aging temperatures on the machining performance of the modified alloy was investigated using a statistical technique. The machinability characteristics were investigated by milling experiments with a carbide tool. Experimental trials were carried out using Taguchi's L27 orthogonal array. Process parameters studied were strontium (Sr) percentage, aging temperature, cutting speed, and feed rate. Cutting force, surface roughness, and tool wear were investigated as responses. The microstructure of the specimens revealed that the addition of Sr to the A357 alloy helped to achieve refined grain structure. Furthermore, increasing the Sr concentration from 4 to 8% resulted in the enhanced refined microstructure. ANOVA analysis of responses revealed that Sr%, aging temperature, and feed rate have a significant effect on all the responses considered. However, cutting speed has exhibited the least influence. Further, the increase in Sr% resulted in an increase in cutting force and tool wear. Whereas, a decrease in surface roughness was observed due to increased Sr%. Whereas the increase of aging temperature, cutting temperature, and feed rate has resulted in the increase of response values. The aging temperature has shown significant influence on the variation of cutting force and surface roughness..

The aging-hardening behavior of short-time in high pressure die casting AlSi9MgMnZn alloy

Short-time aging is an effective industrial technique for enhancing the mechanical properties of high pressure die cast (HPDC) aluminum alloy components. However, the evolution of precipitation behavior during short-time aging of HPDC alloys remains unclear. In this study, the short-time aging behavior of HPDC AlSi9MgMnZn alloy was investigated. The results show that the AlSi9MgMnZn alloy exhibits effective aging-hardening, with an increment in hardness of approximately 30 HV through the optimal short-time aging treatment (aged at 180 °C for 120 minutes). Higher aging temperatures led to reduced peak hardness and rapid over-aging above 220 °C. The microstructure observation indicates that the peak short-time aging promotes the simultaneous precipitation of β″ precipitates, GP zones and nano Si phases. The contribution of different strengthening mechanisms to hardness was estimated, implying that the coexistence of β″ precipitates and GP zones can provide a positive contribution to aging-hardening during short-time aging. Moreover, as the aging temperature increases, the appearance of β′ precipitates reduces the strengthening effect, leading to a decrease in hardness.

The Influence of Oil and Thermal Aging on the Sealing Characteristics of NBR Seals.

Nitrile Butadiene Rubber (NBR) is widely used as a sealing material due to its excellent mechanical properties and good oil resistance. However, when using NBR material, the seal structure is unable to avoid the negative effects of rubber aging. Hence, the influence of oil and thermal aging on the characteristics of NBR seals was studied by coupling the mechanical behavioral changes with the tribological behavioral changes of NBR in oil and the thermal environment. For this paper, aging testing and compression testing of NBR were carried out. Additionally, friction testing between friction pairs under different aging times was carried out. The surface morphology of the NBR working surface under different aging conditions was also observed. Finally, coefficients of different test conditions were introduced into the finite element model of NBR seals. It can be seen from the results that the elastic modulus increased with the increase in aging time in the thermal oxidative aging testing. The elastic modulus after 7 days of thermal oxidative aging increased by 135.45% compared to the unaged case, and the elastic modulus after 7 days of oil aging increased by 15.03% compared to the unaged case. The compression set rate of NBR increased significantly with the increase in aging time and temperature. The coefficient of friction (COF) between friction pairs increased first and then decreased with the increase in aging time. The maximum contact pressure decreased by 2.43% between the shaft and sealing ring and decreased by 4.01% between the O-ring and groove. The proportion of the effective sealing area decreased by 3.05% between the shaft and sealing ring and decreased by 6.11% between the O-ring and groove. Furthermore, the sealing characteristics between the O-ring and groove were better than those between the shaft and sealing ring.

Thermal Treatment Effects on Structure and Mechanical Properties of Polybutylene Terephthalate and Epoxy Resin Composites Reinforced with Glass Fiber.

In this study, two types of composites, polybutylene terephthalate (PBT) and epoxy resin (ER), reinforced with 20% of glass fiber (GF) are used as the comparative research objects. Their mechanical properties after thermal aging at 85~145 °C are evaluated by tensile strength and fracture morphology analysis. The results show that the composites have similar aging laws. The tensile strength of GF/PBT and GF/ER decrease gradually with the increase of aging temperature, while their elastic moduli are independent of the thermal treatment temperature. Scanning electron microscopy study of the fracture surface shows that separation of glass fiber from PBT and ER matrix becomes more obvious at higher aging temperature. The fibers on the matrix surface appear clear and smooth, and the whole pulled out GFs can be observed. As a main mechanical strength degradation mechanism, the deterioration of interface adhesion between the matrix and GF is discussed. A large difference in coefficients of thermal expansion of the matrix and GF is a main factor of the mechanical degradation.

The space of secondary α phase significantly affects the mechanical properties of metastable β-type Ti-4V-2Mo-2Fe alloy

In this study, the evolution of the α phase precipitated from metastable β-type Ti-4V-2Mo-2Fe alloy under various aging conditions and its impact on mechanical properties were investigated. With the increase of aging temperature and time, the yield strength of the alloy decreases from the highest 1435 MPa to 995 MPa, and the elongation increases from 3 % to 14 %. Comparing with the volume fraction of αs, the space of αs was identified as the principal factor affecting the alloy’s mechanical properties. The contribution of αs to strength increases as the αs spacing decrease, while elongation improves with the αs spacing increases.

Tensile Behavior and Microstructure of the 6082 Alloy Sheet with High-Temperature Aging Treatments

The present study investigates the tensile behavior and microstructure evolution of the 6082 aluminum alloy aged with high temperature. A universal testing machine was applied to explore the tensile behavior, while features of the fracture surface were characterized via scanning electron microscopy (SEM). The microstructural evolution was assessed through optical microscopy (OM) and transmission electron microscopy (TEM). The findings illustrate that the 6082 alloy sheet achieves peak strength following treatment at 180 °C for 8 h for the 0° orientation specimen, with the yield strength and tensile strength reaching 345 MPa and 373 MPa, respectively. An increase in aging temperature results in a decline in strength, accompanied by an improvement in elongation. After the treatment at 330 °C for 0.5 h, the corresponding yield strength falls below 150 MPa, with elongation exceeding 12%. The alloy sheet consistently exhibits ductile fracture characteristics with various aging treatments. The aging processes have no obvious influence on grain morphology. The fibrous grain structure is responsible for the anisotropic mechanical properties. The alloy aged at 180 °C for 8 h demonstrates the greatest precipitate density with the smallest precipitate size. As the aging temperature increases, the precipitate distribution becomes less uniform, and the precipitates grow coarser, leading to a decline in the precipitate density and corresponding strength of the alloy. Furthermore, it is noted that smaller precipitates are more effective in suppressing the mechanical anisotropy of the alloy.

Effect of hygrothermal aging on mechanical properties of continuous flax fiber composites

AbstractThe influence of hygrothermal aging on the mechanical properties of flax fiber‐reinforced polymer (FFRP) composites was studied by examining the mechanical properties and failure mechanism of the composite laminates exposed in distilled water at different temperatures. The resin transfer molding (RTM) process was used to manufacture FFRP laminates, which were then subjected to accelerated aging tests in distilled water at different temperatures. The moisture absorption characteristics of FFRP were analyzed by a water absorption test. Tensile, bending, and low‐velocity impact tests of laminates after different aging conditions were performed. Combined with DSC and FTIR tests, the failure mechanism of FFRP exposure to hygrothermal aging was analyzed. The result indicated that the increase in temperature will accelerate the moisture absorption process of FFRP, and the hygroscopic behaviors at different temperatures conform to Fick's law. In the early stage of hygrothermal aging, the composites undergo post‐curing, and the tensile and bending strengths increase. After that, the tensile and bending strengths are decreased because of the thermo‐oxygen aging of materials and fiber/matrix interface damage. After hygrothermal aging, the impact peak force of FFRP decreases obviously while the absorbed energy increases due to the increase of internal defects and plasticization of composites. The increase of hygrothermal aging temperature aggravates the aging of materials, resulting in a more obvious decline in the tensile, bending, and impact properties of the composites.Highlights The effect of hygrothermal aging on the mechanical properties of FFRP is investigated. With the increase of hygrothermal aging time, the tensile and bending strength of FFRP increased at the early stage of aging and then decreased. After hygrothermal aging, the impact peak force of FFRP decreases and the absorbed energy increases due to the increase of internal defects and plasticization of composites.

Microstructure amelioration and strength-ductility improvement of WAAM-LDM hybrid additive manufacturing Ti-6Al-4V alloy by solution treatment and aging

WAAM-LDM hybrid additive manufacturing can achieve high efficiency and high precision preparation of Ti-6Al-4V alloy. However, the inhomogeneous microstructure of WAAM-LDMed Ti-6Al-4V samples results in overall poor tensile properties. Therefore, this paper attempts to ameliorate the inhomogeneous microstructure of the hybrid sample by STA, and simultaneously improve the strength and plasticity. The results showed that the WAAM zone and HAZ were composed of α colonies, the RZ and LDM zone were distributed with fine basketweave structure of the AD samples. With the introduction of STA, the microstructure of each zone of the hybrid sample changes and transformed into a ‘dual-phase’ structure with bi-modal αp and βt. With the increase of aging temperature, the volume fraction of βt increased and the volume fraction of αp phase decreased, accompanied by the formation of bulk α phase and the coarsening of αs phase. It is worth noting that the strength of WAAM zone was obviously improved by STA, and the fracture positions of the STA samples were randomly distributed in the WAAM and LDM zones. Additionally, with the aging temperature increased, the strength of WAAM-LDMed samples was weakened, and the ductility was improved because the coarsened bulk α played a role of coordinating deformation in the tensile process. The excellent strength and ductility (UTS = 976 MPa, YS = 876 MPa, EL = 13.57%) of WAAM-LDMed samples was achieved at aging temperature 750 °C, compared with the AD samples (UTS = 860 MPa, YS = 767 MPa, EL = 12.5%). Finally, by optimizing the STA (940 °C/1 h/WQ + 750 °C/4 h/AC), the amelioration of the microstructure and the improvement of the strength-ductility of the hybrid samples were achieved.

Molecular dynamics study on the influence of thermal aging on the mechanical properties of epoxy resins for high voltage bushing.

Thermal aging significantly deteriorates the mechanical properties and service performance of epoxy resins used for the high-voltage bushing. Current studies on the thermal aging behavior of epoxy resins mainly focus on experimental observations. However, an in-depth understanding of the mechanism of thermal aging of epoxy resins requires the monitoring of structural evolution of epoxy resins during thermal aging at the molecular level. To thoroughly analyze the intrinsic factors affecting structural evolution and the effect of thermal aging on the mechanical properties of epoxy resin for high-voltage bushing, epoxy resin models with different crosslinking degrees were established and thermal aging treatments at various temperatures and time periods were carried out by molecular dynamics simulation. It was found that the tensile strength of the epoxy resin was enhanced with the increase of the crosslinking degree, which was related to the elevation of the proportion of C-N and O-H bonds in its structure. With the increase of thermal aging temperature, the tensile strength of the epoxy resin decreased, which was related to the formation of weak bonds. At the early stage of thermal aging and after a period of high-temperature thermal aging, the strength of epoxy resin significantly decreases. The thermal aging of the epoxy resin is accelerated under external loading. In addition, the crosslinking degree and curing agent also affect the thermal aging resistance of epoxy resins. The results of this study can provide guidance for predicting and improving the thermal aging resistance of epoxy resins. Materials Studio was used to construct molecular models and complete crosslinking reactions. DGEBA and 44DDS (or 33DDS) were mixed at a ratio of 2:1, followed by crosslinking reaction. During this process, the Nose method was used to control temperature, the Berendsen method was used to control pressure, and the polymer consistent force field (PCFF) was used to control the motion and force of atoms. Isobaric-isothermal ensemble (NPT ensemble) was used to heat up epoxy resin models to various thermal aging temperatures of 400K, 500K, 600K and 700K. The models were maintained at these temperatures for different thermal aging times of 100ps, 200ps, 300ps, 400ps, 500ps, 600ps, 700ps and 800ps. Afterwards, the models were cooled down to 300K and subjected to uniaxial tensile testing at this temperature with a strain rate of 1 × 109s-1. The structural configurations and stress-strain data during the tensile process were recorded. The flow stress of the material was derived by counting the average stress in the 20-50% strain interval.