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The influence of heat treatment on the microstructure of products manufactured by direct laser deposition using titanium alloy Ti-6Al-4V
摘要： The titanium alloy Ti-6Al-4V is an important alloy in the aerospace industry. The Direct Laser Deposition (DLD) allows manufacturing products of complex geometric shapes from this alloy, which are not inferior in quality to traditional methods. As a result of previous studies, it was found that the product structure does not match the specified requirements for the Ti-6Al-4V alloy and heat treatment is required. In this research, annealing of samples was carried out in different modes, where the time and annealing temperature varied. The microstructure of the products was studied. As a result of the work, it was found that at an annealing temperature of 600–650 °C, the microstructure is heterogeneous: individual grains of the b-phase are preserved. With an increase of the annealing temperature (750 °C–900 °C), the prior b-grains become less distinguishable. It was found that the length decreases with increasing plate thickness of the metastable a0-phase with increasing temperature. With an increase in the propagation time of grain boundaries, the structure becomes homogeneous. The microhardness of the samples after heat treatment decreases slightly, remains in the range of 300–400 HV0,5.
关键词： Direct laser deposition,Direct metal deposition,Microstructure,Additive manufacturing,Ti-6Al-4V,Heat treatment
更新于2025-09-23 15:19:57
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Predictive Model for Thermal and Stress Field in Selective Laser Melting Processa??Part II
摘要： Finite Element Analysis (FEA) is used to predict the transient thermal cycle and optimize process parameters to analyze these effects on deformation and residual stresses. However, the process of predicting the thermal history in this process with the FEA method is usually time-consuming, especially for large-scale parts. In this paper, an effective predictive model of part deformation and residual stress was developed for accurately predicting deformation and residual stresses in large-scale parts. An equivalent body heat flux proposed from the single layer laser scan model was imported as the thermal load to the layer by layer model. The hatched layer is then heated up by the equivalent body heat flux and used as a basic unit element to build up the macroscale part. The thermal history and residual stress fields of two solid parts with different support structures during the SLM process were simulated. Layer heat source method has the capability for fast temperature prediction in the SLM process, while sacrificing modeling details for the computational time-saving purpose. Thus numerical modeling in this work can be a very useful tool for the parametric study of process parameters, residual stresses and deformations.
关键词： distortion,finite element analysis,residual stress,layer heat source model,SLM
更新于2025-09-23 15:19:57
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Finite-element inverse analysis of residual stress for laser welding based on a contour method
摘要： The thermal effect of laser welding degrades the local material properties, and this inevitably leads to thermal deformation and thermal residual stress in welded joints. In this study, the residual stress distribution of laser-welded Al–Li alloy parts was measured by a combination of the contour method and finite-element simulation. First, the contour deformation of the cutting surfaces of welded parts resulting from the release of residual stress was measured by a coordinator. Then, the reverse contour deformation was applied to the finite-element model as the displacement boundary condition to invert the full-field residual stress of the cutting surface. Furthermore, a thermal/structural sequential coupling analysis method was used to establish a complete three-dimensional finite-element model of a laser-welded plate and calculate the residual stress field, taking the actual weld morphology as the characteristic parameter of the heat source, using the improved conical heat source model of laser welding. The result is consistent with the results of the contour method.
关键词： Laser welding,Residual stress,Contour method,Thermal/structural finite-element analysis,Improved conical heat source model
更新于2025-09-23 15:19:57
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Evaluation of performance of near-field thermophotovoltaic systems based on entropy analysis
摘要： Near-field radiative heat transfer can significantly improve the output power of thermophotovoltaic (TPV) systems. Therefore, it is crucial to explore how to increase the energy conversion efficiencies of near-field TPV systems. In this study, based on the fluctuation–dissipation theorem with the effective medium theory, we evaluate the performance of a near-field TPV system from the formulation of thermodynamics. It is found that a near-field TPV system consisting of InAs or GaSb cell can achieve higher heat flux or efficiency limit, respectively. Moreover, the TPV system with a hyperbolic metamaterial (HMM) emitter composed of nanowire or nanohole arrays can achieve higher heat flux and efficiency limit compared to that of the bulk reference. This is attributable to the HMMs being able to support hyperbolic modes, and the radiation with the HMM emitter exhibits a favorable entropy content for the energy conversion efficiency. This work provides an approach to determine the efficiency limit and establish a target for efficiency of the near-field TPV system without considering how the system works. The results of this study will facilitate the design and application of the HMM emitter and material of the TPV cell to improve the efficiency of near-field TPV systems.
关键词： Thermophotovoltaic systems,Near-field radiative heat transfer,Energy conversion efficiency,Hyperbolic metamaterial,Entropy analysis
更新于2025-09-23 15:19:57
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An optomechanical heat engine with feedback-controlled in-loop light
摘要： The dissipative properties of an optical cavity can be effectively controlled by placing it in a feedback loop where the light at the cavity output is detected and the corresponding signal is used to modulate the amplitude of a laser ?eld which drives the cavity itself. Here we show that this effect can be exploited to improve the performance of an optomechanical heat engine which makes use of polariton excitations as working ?uid. In particular we demonstrate that, by employing a positive feedback close to the instability threshold, it is possible to operate this engine also under parameters regimes which are not usable without feedback, and which may signi?cantly ease the practical implementation of this device.
关键词： optomechanics,feedback,quantum heat engine
更新于2025-09-23 15:19:57
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Suppressing Efficiency Roll-Off at High Current Densities for Ultra-Bright Green Perovskite Light-Emitting Diodes
摘要： Perovskite light-emitting diodes (PeLEDs) have undergone rapid development in the last several years with external quantum efficiencies (EQE) reaching over 21%. However, most PeLEDs still suffer from severe efficiency roll-off (droop) at high injection current densities, thus limiting their achievable brightness and presenting a challenge to their use in laser diode applications. In this work, we show that the roll-off characteristics of PeLEDs are affected by a combination of charge injection imbalance, nonradiative Auger recombination, and Joule heating. To realize ultrabright and efficient PeLEDs, several strategies have been applied. First, we designed an energy ladder to balance the electron and hole transport. Second, we optimized perovskite materials to possess reduced Auger recombination rates and improved carrier mobility. Third, we replaced glass substrates with sapphire substrates to better dissipate joule heat. Finally, by applying a current-focusing architecture, we achieved PeLEDs with a record luminance of 7.6 Mcd/m2. The devices can be operated at very high current densities (J) up to ~ 1 kA/cm2. Our work suggests a broad application prospect of perovskite materials for high-brightness LEDs and ultimately a potential for solution-processed electrically pumped laser diodes.
关键词： Joule heat,efficiency roll-off,Auger recombination,charge injection balance,high injection current density,ultrahigh brightness,perovskite light emitting diodes
更新于2025-09-23 15:19:57
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Performance enhancement of photovoltaic panels using two types of nanofluids
摘要： One of the main problems that limit the extensive use of photovoltaic (PV) systems is the increase in the temperature of PV panels. Overheating of a PV module decreases the performance of the output power by 0.4% to 0.5% per 1°C over its rated temperature that in most cases is 25°C. An effective way of improving electrical performance (power output and efficiency) and reducing the rate of thermal degradation of a PV module is to reduce the operating temperature of the PV surface by a cooling medium. To achieve this, nanofluids can be considered as a potentially effective solution for cooling. In this study, two types of nanofluids, namely Al2O3 and TiO2 water‐based mixture of different volume flow rates and concentrations (0.01%, 0.05%, and 0.1%) by weight, were used. Also, three PV panels were cooled simultaneously using nanofluids, water, and natural air, respectively. Results showed that nanofluids for cooling enhanced heat transfer rate much better than water and natural air. Best results were achieved for TiO2 nanofluids at the considered concentration (0.1 wt%). Nanofluid cooling of turbulent flows for such an application has not been investigated before. These results represent the first application of nanofluid cooling in the turbulent flow regimes and in outdoor conditions including real solar irradiation.
关键词： photovoltaic module,Nusselt number,Al2O3,TiO2 nanofluids,heat transfer,electrical performance
更新于2025-09-23 15:19:57
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Laser Heat Treatment of Ferrocarbon Powder Steel
摘要： The laser heat treatment of sintered ZhGr ferrocarbon powder steel with different porosity is considered. The microstructure and microhardness of the steel are determined, and the defects arising in laser treatment are studied.
关键词： microhardness,ferrocarbon steel,porosity,laser heat treatment,powder metallurgy,microstructure
更新于2025-09-23 15:19:57
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New Method for Studying the Ignition Characteristics of Condensed Systems by Laser Radiation
摘要： A method for determining the characteristics of laser ignition of the condensed systems is presented, which ensures uniform distribution of the heat flux density over the surface during ignition. The method consists in measuring the ignition delay time of a rotating cylindrical sample when the continuous laser heat flux is applied to its end surface. The theoretical estimation of the required angular rotation velocity of the sample is carried out, which ensures averaging with a given error of the radiation flux density on the surface of the sample, as well as the degree of surface cooling due to the convective heat transfer. The experimental results on the pyroxylin sample ignition at the radiant heating by a CO2 laser with and without rotation of the sample are presented.
关键词： pyroxylin,laser ignition,ignition delay time,condensed system,convective heat transfer,angular velocity of rotation
更新于2025-09-23 15:19:57
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Photothermal response of plasmonic nanofillers for membrane distillation
摘要： Light-to-heat conversion in plasmonic nanoparticles (NPs) inside polymeric membranes is beneficial for improving the efficiency of membrane distillation for seawater desalination. However, the physical mechanisms ruling photothermal membrane distillation are unclear yet. Here, we model the plasmonic photothermal light-to-heat conversion from Ag, Au, and Cu nanofillers in polymeric membranes for membrane distillation. Photothermal effects in the cases of isolated metallic NPs and their assembly are investigated considering size effects and excitation sources. The increasing content of metallic NPs improves the efficiency of the light-to-heat conversion. For a polymeric membrane, filled with 25% Ag NPs, our model well reproduces the experimental temperature increase of 10 K. Specifically, we find that Ag NPs with a radius of around 30–40 nm are favorite candidates for membrane heating with excitation energy in the visible/near-UV range. The incorporation of a term associated with heat losses into the heat transfer equation well reproduces the cooling effect associated with vaporization at the membrane surface. Compared to Ag NPs, Au and Cu NPs show a broadened absorption cross section and their resonance has a nonlinear behavior with varying the excitation energy, better matching with sunlight radiation spectrum.
关键词： photothermal conversion,membrane distillation,seawater desalination,plasmonic nanoparticles,light-to-heat conversion
更新于2025-09-23 15:19:57