Transient Thermal Cycles Research Articles

Numerical simulation and properties analysis of Ta10W alloy joints prepared by electron beam welding

Vacuum electron beam welding has the characteristics of concentrated energy, controllable heat input, almost no pollution, and a small heat-affected zone. In this study, the Ta10W alloy was successfully connected by electron beam welding (EBW), and numerical simulations were employed to examine the distribution of temperature and stress fields throughout the welding process. Additionally, the influence of EBW on the microstructure and corrosion resistance of the welded joints was scrutinized. Findings indicate that the welding heat process led to a reduction in grain boundary energy, thereby facilitating thermal activation processes that contributed to grain growth. Grain boundary migration and possible dynamic crystallization led to the appearance of the coarse columnar crystal structure in the welded joint. A three-dimensional, nonlinear, transient, thermo-mechanically coupled finite element model was developed for the electron beam welding of Ta10W alloy. The shape characteristics and size of the weld and transient thermal cycles calculated by numerical simulations were in reasonable agreement with experimental results. Electron beam welding reduced the corrosion resistance of the Ta10W alloy, and the corrosion product was mainly composed of Ta, O and Na.

Development of a Novel DEMO divertor target: spiral plate module

In DEMO, the divertor must endure steady state loads of 10 MW m−2 and transient thermal cycling up to 20 MW m−2. A novel divertor target, termed the Spiral Plate Module (SPM) and designed to meet these loads, was initially optimized by a one-dimensional, steady-state model. The best design was a trade-off between the wall overheat (τ s )—a figure of merit for cooling performanceand the pumping ratio (η P )—a comparison of the pressure drop and the incident heat flux on a target surface. A three-dimensional, steady-state, conjugate heat transfer study showed significant correlation to the one-dimensional model. Compared to other divertor target concepts, the SPM achieved the lowest wall overheat of any target design. The hydraulic performance was also on par with comparable designs, demonstrating a very low pumping ratio. This novel, modular divertor target could be used in future fusion power plants to effectively cool the plasma facing components with low pressure drop.

Predictive Model for Thermal and Stress Field in Selective Laser Melting Process—Part II

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.

AnT²COK, an analytical model for cyclic oxidation kinetics of hot-work tool steels in transient thermal cycling conditions

In hot forming processes, steel tools are subjected to rapid temperature transients and cyclic oxidation. AnT²COK, a new analytical model based on diffusion-controlled oxide growth in cyclic conditions, is proposed to predict the parabolic oxidation kinetics in spallation-free conditions. Compared to cyclic oxidation models of the literature (COREST, COSP, DICOSM, etc.), it is applicable for any shape of thermal cycle, fully transient or with dwell time. The model is applied to thermal fatigue tests on X38CrMoV5 steel, for Tmax between 500 and 685 °C. Its validity is demonstrated above 550 °C, using frequency factor and activation energy calculated from isothermal tests.

Design and Performance Evaluation of Al2O3-SiC Composite for Direct-Bonded Copper Substrate

A computational material design approach is applied to propose a novel ceramic material for direct-bonded copper (DBC) substrate with enhanced thermal and structural performance. The material design inherently consists of many competing requirements that require careful decisions regarding key trade-offs in terms of material composition, inclusion size, shape, and distribution to achieve the target properties. The alumina-silicon (Al2O3-SiC) composite, as compared to commercial alumina, used in DBC is found to be the most suitable design among other candidates with improved thermal and structural properties. In order to study the performance characteristics and the effects of the new ceramic composite with improved properties in terms of structural behavior and fatigue life of the DBC substrate, the normal working and extreme thermal cycling conditions were simulated and analyzed using finite element method. The temperature, strain, and localized stress distribution within the substrate at a steady-state condition were analyzed, and the improved Coffin–Manson law was used to calculate the fatigue life of the substrate under extreme thermal cycling conditions. The proposed Al2O3-SiC composite is found to be more robust than the commercial alumina as DBC substrates considering the thermal–mechanical performance. The fatigue life cycle of the DBC substrate with the proposed material is predicted to be about two times longer than the commercial alumina DBC ceramic under transient thermal cycling test.

Equivalent heat source approach in a 3D transient heat transfer simulation of full-penetration high power laser beam welding of thick metal plates

A three-dimensional multi-physics numerical model was developed for the calculation of an appropriate equivalent volumetric heat source and the prediction of the transient thermal cycle during and after fusion welding. Thus the modelling process was separated into two studies. First, the stationary process simulation of full-penetration keyhole laser beam welding of a 15mm low-alloyed steel thick plate in flat position at a welding speed of 2mmin-1 and a laser power of 18kW was performed. A fixed keyhole with a right circular cone shape was used to consider the energy absorbed by the workpiece and to calibrate the model. In the calculation of the weld pool geometry and the local temperature field, the effects of phase transition, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature were taken into account. The obtained local temperature field was then used in a subsequent study as an equivalent heat source for the computation of the transient thermal field during the laser welding process and the cooling stage of the part. The system of partial differential equations, describing the stationary heat transfer and the fluid dynamics, were strongly coupled and solved with the commercial finite element software COMSOL Multiphysics 5.0. The energy input in the transient heat transfer simulation was realised by prescription of the nodes temperature. The prescribed nodes reproduced the calculated local temperature field defining the equivalent volumetric heat source. Their translational motion through the part was modelled by a moving mesh approach. An additional remeshing condition and helper lines were used to avoid highly distorted elements. The positions of the elements of the polygonal mesh were calculated with the Laplace’s smoothing approach. Good correlation between the numerically calculated and the experimentally observed weld bead shapes and transient temperature distributions was found.

Microstructural evolution and mechanical properties of maraging steel produced by wire + arc additive manufacture process

Wire+arc additive manufacture is developed for producing large-scale metallic components. In this paper, maraging steel parts were produced, and the microstructure and mechanical properties were investigated. The microhardness and tensile strength of the as deposited alloy reduced from the bottom to the top due to the transient thermal cycling, which resulted in partial aging and non-uniform formation of intermetallic compounds along the building direction. Solutionizing, followed by 3h aging, significantly reduced the microstructural heterogeneity and increased the mechanical properties by 24.7% through the formation of large amounts of finely distributed precipitates. The as deposited alloy possessed superior strength to the wrought alloy in solutionized condition but inferior to the later in aged condition, which was attributed to the less pronounced aging response of the low-angle columnar grains characterized microstructure and the presence of retained and reverted austenite.

Laser welding of steel to aluminium: Thermal modelling and joint strength analysis

The integrity of steel-aluminium dissimilar alloy joints is dependent on the thermal cycle applied during the joining process. The thermal field has a direct influence on the growth of the intermetallic compounds (IMC), which result from the reaction between iron (Fe) and aluminium (Al), but it also determines the size of the bonding area of the joint. A finite element (FE) thermal model was developed to predict the transient thermal cycle at the Fe-Al interface for different levels of applied energy by changing the power density and interaction time. The time-temperature profiles were correlated to the weld geometry, IMC layer thickness and mechanical strength. The experimental results showed that having a small bonding area is equally detrimental to the mechanical strength of the joint as having a thick IMC layer. The FE model suggested that comparing to time, the temperature is more important in laser welding of steel to aluminium as this is the factor which most contributes to the growth of the IMC layer and the formation of the bonding area. However, it was not possible to identify a thermal field able to produce simultaneously a large bonding area and a thin IMC layer to optimize the joint strength.

Transient cooling of electronic components by flat heat pipes

Abstract This paper presents a theoretical investigation of a Flat Heat Pipe (spreader) designed for the cooling of multiple electronic components in transient state. This model is a transient model, coupling 3D thermal model with a 2D hydrodynamic one through the mass flux of evaporation–condensation, which occurs in a mass conservation equation. The model makes it possible to obtain the FHP wall transient temperatures, the transient pressures, velocities and temperatures in both liquid and vapor phases. A comparison of the behaviour of the FHP and an equivalent solid plate submitted to a transient thermal cycle shows that the FHP enhanced the electronic components cooling for the long thermal cycle duration when the solid plate is more efficient for the very short transient thermal cycles. The FHP provides also a very low thermal resistance, which helps to minimise the temperature gradient and then the hot spots and overheating.

Effect of transient thermal cycles in a supercritical water-cooled reactor on the microstructure and properties of ferritic–martensitic steels

Microstructural and mechanical property changes in modified 9Cr–1Mo and HCM12A ferritic–martensitic steels resulting from short-duration thermal transients that occur during loss of feedwater flow events in a supercritical water reactor (SCWR) were studied. Specimen blanks were exposed to reference transients with 810 and 840°C maximum temperatures using a thermal cycle simulator, and the subsequent microstructure, hardness, and creep-rupture strength were evaluated. Exposure to five consecutive cycles at either temperature resulted in no significant changes – only very slight indications of overtempering. Subsequent study of a wider variety of transient conditions showed that significant ferrite-to-austenite transformation occurred during thermal transients whose maximum temperature exceeded 860°C, or during transients with holds exceeding 10s at 840°C maximum temperature. The subsequent presence of untempered martensite in the microstructure, coupled with severe overtempering, resulted in an order of magnitude decrease in creep-rupture strength at 600°C. The findings were consistent with measured Ac1 temperatures for the two steels and the dependence of Ac1 on heating rate.

Mechanism of transient thermal cycle failure in ZrO2 /NiCrAl functionally graded thermal barrier coatings

In this paper, the failure mechanism under transient thermal cycle loading is studied for a functionally graded thermal barrier coating (FGTBC) system consisting of ZrO2 and NiCrAl plasma sprayed on a steel substrate. Controlled experiments and numerical analysis (thermoelastic finite element method) were performed and the results indicate that the surface tensile stress is responsible for the initiation of surface cracks and the tensile stress normal to the interface is large enough to break the interfacial bond (the magnitude of the interfacial bond strength of the ZrO2 layer in the FGTBC is below the strength of the ZrO2 , i.e. the interface is weaker than the ZrO2 material). By comparing the mode I stress intensity factor and the fracture toughness, the surface cracks are considered to be arrested, this characteristic is confirmed by SEM studies. The interfacial crack propagation causes a segment of the ZrO2 layer in the FGTBC to spall.

Process Uniformity and Electrical Characteristics of Thin Gate Dielectrics Grown by Ramped-Temperature Transient Rapid Thermal Oxidation of Silicon

ABSTRACTRapid thermal oxidation (RTO) of Si using transient linearly-ramped-temperature saw-toothed (LRT-ST) and triangular (LRT-TA) thermal cycles has been examined through evaluations of the process uniformity, slip dislocation patterns, and electrical characteristics of MOS devices. The strong effects of the thermal cycle parameters on process uniformity and slips indicate that the overall performance of an RTP tool must be specified both under the steady-state and transient thermal cycles. The electrical characteristics of MOS devices with LRT-grown gate oxides are comparable to those for devices with oxides grown by the trapezoidal thermal cycles.

Process uniformity and slip dislocation patterns in linearly ramped-temperature transient rapid thermal processing of silicon

Rapid thermal processing (RTP) of silicon using transient linearly ramped-temperature saw-toothed and triangular thermal cycles has been evaluated by characterization of the process uniformity and slip dislocation line patterns for a wide range of process parameters. Rapid thermal oxidation was chosen as the process vehicle for these studies. The process uniformity and slip dislocation line patterns are strongly affected by both the transient and steady-state segments of the thermal cycles. The strong dependencies of the process uniformity and slip dislocation lines on the thermal cycle parameters suggest that the overall performance of a RTP reactor must be specified not only under steady-state thermal conditions, but also for controlled transient thermal cycles. Transient ramped-temperature RTP cycles with medium-to-high peak process temperatures (i.e. T/sub max/=1100 degrees -1150 degrees C) were found to be the optimal process conditions for growing thin gate oxides in the range of 60-120 AA with superior process uniformity and minimum slip dislocation line generation. The results of this work provide insight and useful methodology for process optimization in order to improve process uniformity, minimize generation of slip dislocation lines, and obtain good device electrical characteristics. >

Tensile properties of welded helium charged 304L stainless steel

The room-temperature tensile properties of helium-containing 304L stainless steel subjected to thermal cycles simulating fusion welding conditions have been measured. Helium was introduced into tensile specimens by tritium charging, and aged to generate 526 appm 3He by radioactive decay. The specimens were then vacuum annealed to remove the tritium. The helium-charged samples were subjected to transient thermal cycles simulating those occurring in the heat affected zone of a gas tungsten arc weld. Peak temperatures above 1073 K caused severe ductility losses, fracture mode changes (from ductile transgranular rupture to ductile intergranular fracture), and losses in ultimate tensile strength. These effects are attributed to helium redistribution and void growth on the grain boundary. Subsequently, grain boundary failure occurs during tensile testing because of the large quantity of trapped helium on the boundary.

Transient Annealing of Ion-Implanted Silicon Using A Scanning IR Line Source

ABSTRACTRecent interest in finding an efficient method for transient annealing of ion-implanted silicon has led to studies of various rapid annealing schemes such as graphite heaters and high intensity incoherent light sources as alternative methods to laser annealing. In this paper, we describe a recent study of transient annealing of ion-implanted silicon using a scanning IR line source created by a single tungsten filament enclosed in a quartz envelope. Various dopants (B+, P+ and As+) with fluences of 1014 to 1016 ions/cm2 were implanted and annealed under both transient and steady-state thermal conditions. Dopant depth distributions were analyzed using the SIMS technique. Sheet resistance measurements indicated that almost 100% activations of the implanted dopants were achieved. Sensitivities of dopant activation to transient annealing conditions were studied as a function of dopant concentrations, and high-dose As- and B-implanted samples were found to be sensitive to transient thermal cycle, particularly to the peak temperature. Recrystallization was studied with Rutherford backscattering spectroscopy using 2 Mev He+ ions.

Thermal-Shocking Austenitic Stainless Steels With Molten Metals

Abstract Specimens of AISI 347 pipe and pipe welds have been subjected to repeated thermal shocks by quenching the inner surface with cool sodium-potassium alloy. The equipment used and a method of measuring the transient fluid and wall temperatures are described. Temperature changes encountered during tests and resulting stresses are shown. The effects of 2500 transient thermal cycles on one specimen are discussed.