Mushy Zone Research Articles

Differential scanning calorimetry of aluminium EN AB-42000 alloy rheocasting semi-solid in different stage heating rates

Differential scanning calorimetry (DSC) is used to identify the thermal histories of samples to analyse and diagnose production and quality concerns connected to industrial rheocasting semi-solid alloy, that had undergone different tempers of aluminium alloy EN AB-42000 alloy. In this study, the solidus temperatures of several alloy samples are investigated using thermodynamic calculations and DSC observations in this work. The balance of important characteristics, including pseudo-eutectic, thermal sensitivity, heat flow, and enthalpies behaviour, of Al alloys has been investigated using experimental data from DSC and solid fractions. In addition, the choice of heating rates is critical as high rates can blur the two peaks in the mushy zone, while low rates lead to slower measurements. Using smaller sample weights and slower rates is preferable to obtain more accurate results. Analysing the shape of the fs curve, exact composition, and a reference composition without contaminants is essential for understanding complex behaviours, including pseudo-eutectic phenomena. The thermal sensitivity of compositions also plays a crucial role in the analysis. Despite heat flow decreasing with decreased sample weight, the measurement limit can still be exceeded at high heating or cooling rates (20 °C/min) during the eutectic reaction. The eutectic reaction exhibits higher peaks with enthalpies ranging from 360 to 430 mJ/g. However, drawing conclusions regarding trends in heating versus cooling or comparing low-mass and higher-mass samples can be challenging. The non-equilibrium transformation of the eutectic occurs within a more confined temperature range. Increasing rates lead to overlapping reactions, resulting in complex thermal behaviour.

Small-scale layered structures at the inner core boundary

The fine-scale seismic features near the inner core boundary (ICB) provide critical insights into the thermal, chemical, and geodynamical interactions between liquid and solid cores, and may shed light on the evolution mechanism of the Earth’s core. Here, we utilize a dataset of pre-critical PKiKP waveforms to constrain the fine structure at the ICB, considering the influence of various factors such as source complexity, structural anomalies in the mantle, and properties at the ICB. Our modeling suggests a sharp ICB beneath Mongolia and most of Northeast Asia, but a locally laminated ICB structure beneath Central Asia, Siberia, and part of Northeast Asia. The complex ICB structure might be explained by either the existence of a kilometer-scale thickness of mushy zone, or the localized coexistence of bcc and hcp iron phase at the ICB. We infer that there may be considerable lateral variations in the dendrites growing process at ICB, probably due to the complicated thermochemical and geodynamical interaction between the outer and inner core.

Close-contact melting and natural convection in unconstrained melting: A parametric study

Close-contact melting (CCM) is a phenomenon that can be achieved in latent thermal energy storage (LTES) units where the falling of solid PCM is realized. On the other hand, natural convection (NC) plays an important role in LTES units due to circulations based on the buoyant flow of liquid phase change material (PCM). However, the relative contribution of CCM and NC on the melting heat transfer remains unclear when they coexist. This study considers a set of parametric cases that numerically investigate the effects of CCM and NC on an LTES unit with annular fins where CCM is enabled with a shell made of aluminum heated by the heat transfer fluid (HTF). The enthalpy-porosity method is used to model the melting in the LTES unit, and it is established that CCM can be realized using this method with a small value of mushy zone constant. However, with this small value, the mushy zone turns into a highly viscous liquid. The parametric study considers the number of fins and their clearance and concludes that with a decrease in fin clearance and an increase in fins, the effects of CCM are boosted, resulting in faster melting. The behavior of CCM and NC are characterized using the transient behavior of heat transfer rate for different surfaces and defining a factor that calculates the contribution of each surface on the melting. The role of CCM in the melting is up to 50%, while NC plays a less significant role. This study helps with the design of LTES units where CCM is enabled.

Domination and effect of multi-parameters in direct chill casting based on establishment of thermo model by numerical simulation and experiment

In this paper, a mathematical model validated by in-situ temperature measurement data of the direct chill (DC) casting process has been established in order to study the domination and effect of multi-parameters including secondary cooling water flow rate (Q), casting speed (V), condition of primary cooling, and spraying water temperature (Tw) on DC casting index of sump depth (dsump), maximum and minimum temperature gradient (Tg,max and Tg,min) at mushy zone and shape of solidus line. Further, the height of impingement zone (Hip) is systematic quantitative analysis by a novel designed test (Hip=1.08865Q`+0.06272V−4.75597). The calculated temperature using the numerical simulation exhibits a good match with the actual temperature measurement data. The results of multi-factors on index by the designed orthogonal test indicate that V dominates the dsump (dsump=0.033+27.74V) and Tg,min (Tg,min=2569.15−766342.20V) at mushy zone, Q dominates the Tg,max (Tg,max=9195.12+2344770Q−27.83Tw) by analysing the range (R), variance (S) and significance (F). Likewise, by discussing the combination of R, S, F and microstructure observation, the priority influencing the shape of solidus line at center part was V. The Q dominates the shape of solidus line at the position between the center and edge part.

A volumetric heat source model for the approximation of the melting pool in laser beam welding

AbstractLaser beam welding is a metal fusion technique that, with its high advance speed and low thermal distortion, is used more frequently nowadays in industry. During welding, the solidification front is located in the so‐called mushy zone, which forms the transition region between the completely solidified and mixture zone of solid and fluid material behind the laser beam. In this zone, various process parameters can lead to the formation of included melt areas that may cause cracking when they solidify and shrink. The properties of the solidification region are influenced by several parameters. Factors such as welding speed, temperature gradient, and chemical composition affect the likelihood of solidification cracking. To study the heat affected zone more accurately, different models describing the melting pool in finite element approaches using a heat source are discussed. The goal is to develop a volumetric heat source model based on the conical model, which closely represents the temperature distribution illustrated by the Lamé curves. Boundary value problems are presented to showcase the results.

Influence of recoil pressure, mushy zone flow resistance and reflectivity on melt pool shape in laser powder bed fusion simulation

Melt pool dimensions (depth: D and width: W) are strongly correlated with defects generated in laser powder bed fusion. The melt pool dimensions have been evaluated by CFD simulation including solid–liquid–gas phase change and laser multiple reflection. Such a multi-physics simulation requires a large number of parameters. Preliminary simulations for parameter identification require a lot of time and effort. The parameter sensitivity to the melt pool dimensions is important; however, the systematic evaluations have hardly been found in the literature. In this study, we performed systematic parametric evaluation for three parameters related to recoil pressure, flow resistance in solid–liquid mushy zone and Fresnel laser reflection. As a result, the D values increased with increasing recoil pressure, but the W values did not. The flow resistance force influenced the velocities in the mushy zone but not the melt pool dimensions. The D increased with increasing reflectivity, but the W did not. The melt pool dimensions varying recoil pressure, flow resistance force and reflectivity were all inconsistent with the experimental melt pool dimensions. In order to agree with the experimental melt pool dimensions, the problems to be solved were discussed by comparing the previous studies.

Numerical investigation of the effect of thermal expansion coefficient and mushy zone constant on modelling of the phase change process to provide reliable selection criteria of modelling parameters

Phase change materials (PCMs) have been involved in many critical applications due to their ability to store thermal energy and stabilize the temperature. The accuracy of PCMs numerical modelling is affected by the software input assumptions including the thermal expansion coefficient and the mushy zone parameter. The thermal expansion coefficient is used to calculate the density value, to be applied in the modelling equations, but volume expansion is generally not considered by PCM modelling softwares. Due to this fact, the assumed value for the thermal expansion coefficient has to be selected properly, before conducting numerical tests. Another influential input is the mushy zone constant that has a significant effect on the output results, and it has to be selected from a wide range of values between 103 and 1010.In this study a numerical investigation for the influence of both parameters was conducted to provide valid selection criteria. It was found that changing the expansion coefficient controlled the melting process time without affecting the melting profiles, where increasing the expansion coefficient accelerated the melting process, reducing the melting time and vice versa. The melting interface profiles were found directly related to the mushy zone parameter. Increasing the mushy zone parameter value decreased heat transfer rate through the mushy region, which significantly changed the melting profiles. These tests were repeated using different orientations to confirm these findings. Finally, reliable selection criteria were introduced to select the proper values for numerical modelling of phase change process. This selection process was tested using Lauric acid material at different orientations.

Solidification crack elimination in TIG welding of Al7075: Experimental investigation and modified RDG modelling

Although solidification crack susceptibility modelling has been a point of interest for many years in fusion welding of aluminum alloys, the underlying mechanism of solidification crack elimination is not yet fully understood. To contribute in closing this knowledge gap, the present study investigates and proposes a new modified Rappaz-Drezet-Gremaud (RDG) physical model in autogenous and heterogeneous TIG welding of Al7075 sheets. The core novelty of the new model is that it includes the shrinkage strain rate, which allows to better capture the effects of capillary pressure, coherency point, as well as final dendrite or grain size in the fusion zone. In agreement with the experimental results and previous models, the new model shows that the defining mechanism of eliminating solidification cracks in heterogeneous joints is the capacity of nanoparticles to initiate heterogeneous grain nucleation, altering the fusion zone grain morphology from dendritic to fine equiaxed, with only little impact of the resulting grain size. This leads to two main effects causing reduced solidification crack susceptibility: (1) a reduction in coherency point by about 15 % from 902 K to 879 K, which reduces strain accumulation on the coherent solid network; (2) a considerable increase in capillary pressure across the entire mushy zone, allowing to maintain the total pressure at void/liquid interfaces in the positive range, which prevents the growth of potentially formed voids between grains at any temperature during solidification. In contrast, dendrite widths below 68 μm are required to prevent solidification cracks in autogenous joints without TiC nanoparticles, indicating a greater role of grain refinement. Overall, the proposed model highlights the more determinant role of the temperature-dependent solidification shrinkage strain rate in solidification crack susceptibility as it is an order of magnitude greater compared to the temperature-independent thermal contraction strain rate that has been the main or sole focus of previous models.

Weldability study of 304HCu stainless steel using varestraint and “Gleeble” based hot ductility tests

The susceptibilities of 304HCu SS to solidification and liquation cracking are assessed in the current work and the underlying mechanisms are clarified. The development of γ + Nb4C3 eutectic along the interdendritic areas in the fusion zone is attributed for solidification cracking in this stainless steel. To forecast and rank the susceptibility of an alloy to solidification cracking, a novel technique is put forth and experimentally confirmed. Using the measured temperature gradient, welding speed and Scheil's solidification module, a relationship between the liquid fraction and mushy zone width is derived. Liquation cracking susceptibility evaluated using “Gleeble™” based hot ductility test is observed to be high which is attributed to the constitutional liquation of primary Nb(C,N) and secondary M23C6 phases present in the base metal. Further, the influence of phosphorus on the local solidus temperature in the partially melted zone of the alloy is discussed in the context of liquation cracking.

The Stray Grains from Fragments in the Rejoined Platforms of Ni-Based Single-Crystal Superalloy

Nickel-based single crystal superalloy is the most important material for blade preparation. However, some solidification defects inevitably occur during the process of preparing single-crystal blades through directional solidification. In this study, in order to study the origin of misorientation defects during solidification, a model with rejoined platforms was designed according to the geometry of single-crystal guide vanes. Electron Back-Scattering Diffraction (EBSD) was used to quantify the orientation deviation of the dendrites and identify the solidification defects in the rejoined platforms. The results showed that stray grain defects appeared in the platforms and their misorientation changed gradually, not abruptly. Combined with the simulation results, it was proposed that the stray grains formed as the result of the dendrites fragment, which was induced by solute enrichment in the mushy zone during solidification. Meanwhile, it was accompanied by a obvious dendritic deformation, which was caused by solidification shrinkage stress. This suggested that the fragmentation was induced by multiple factors, among which, the concave interface shape provided favorable conditions for solute enrichment, and the dynamic variability in the local thermal gradient and fluctuations of the solidification rate might play catalytic roles.

Experimental investigation on melting heat transfer of paraffin embedded with periodic structure metal framework

Experiments on the melting of pure paraffin and paraffin embedded with a periodic structure metal framework (PEPSMF) in a rectangular cavity are conducted to investigate the effect of the metal framework on phase change. The test section is a closed cavity composed of plexiglass and heating copper plates that is filled with pure paraffin, or PEPSMF. During experiments, constant wall temperatures (Twall = 50, 55, and 60 °C) are applied to the left sides of the test section. Visible and infrared imaging technologies are used to capture the phase fields and temperature fields. High-precision thermocouples are employed to measure the temperature at different locations inside the PEPSMF and on the surface of the heating wall. In addition, the local thermal non-equilibrium effect between the metal framework and the paraffin is analyzed thoroughly. The results reveal that the evolution of the PEPSMF's mushy zone shows unique features due to the heat conduction of the metal framework. PEPSMF has a reduced melting time and a more uniform temperature distribution than pure paraffin. The significant local thermal non-equilibrium effect is found in PEPSMF, which becomes more prominent closer to the heating wall horizontally. The local thermal non-equilibrium effect shows complex fluctuations, with a maximum value of 1.25 °C.

Apple-like Shape of Freezing Paraffin Wax Droplets and Its Origin.

Paraffin wax stores energy in the form of latent heat at a nearly constant temperature during melting and releases this energy during solidification. This effect is used in industrial energy storage. At the same time, the possible deformation of even small volumes of material as a result of phase change is insufficiently studied. In this paper, the physical nature of such deformation, probably for the first time, is studied on the example of a droplet of paraffin wax. An unusual change in the shape of a melted droplet of paraffin wax placed on a relatively cold glass plate was observed in the laboratory experiments. As the droplet solidifies, its upper surface becomes nearly flat, and a dimple is formed in the center of this surface, making the droplet look like a fruit (pumpkins are more commonly shaped like this, but the authors prefer apples). A series of experiments, as well as physical and numerical modeling of the droplet's thermal state, taking into account the formation of a mushy zone between liquidus and solidus, made it possible to understand the role of gravity and gradual increase in viscosity and density of paraffin wax on changing the droplet shape and, in particular, to clarify the mechanism of formation of the dimple on its upper. It was shown that the mushy zone between the liquidus and solidus of the paraffin wax is responsible for the dimple formation.

Computational fluid dynamics model for a C-shaped sodium hydroxide freeze valve

Freeze valves are safety components implemented in molten salt reactor (MSR) designs that utilize a salt’s phase change to isolate two parts of a system from each other. The correct prediction of the behavior of a freeze valve is crucial to the safe operation of MSRs. A freeze valve made from a nickel (Ni-201) tube bent into a C-shape and filled with sodium hydroxide (NaOH) is studied experimentally at the MS1 loop facility at Seaborg Technologies. The molten salt initially fills the freeze valve and is cooled by means of an external air blower and solidifies to close the valve. A computational fluid dynamics code STAR-CCM+ is applied to model the thermodynamic and flow properties of the freeze valve. The model accounts for both conjugate and radiative heat transfer, utilizing a volume-of-fluid method with an enthalpy-porosity-based phase change numerical method. The influence of impurity content on the salt’s melting point is modelled. Experimental temperature measurements are used to determine the model parameters, and the simulation results show a good agreement with the experimental data. Correlation coefficients between the experimental and simulation data are above 0.996, with the average absolute value of their difference being approximately 5 °C. Our sensitivity analysis shows that the performance of the freeze valve is strongly dependent on the latent heat and the flowability threshold of the salt used by the software to model salt dynamics in the mushy zone.

Effect of solidification direction on microstructure and mechanical property of single crystal superalloy CMSX-4

The microstructure and mechanical property of single crystal superalloy CMSX-4 fabricated by downward direction solidification (DWS) and upward direction solidification (UWS) were systematically investigated by optical microscope (OM), scanning electron microscope (SEM), electron probe microanalyzer (EPMA), nano-indentation instrument and finite element simulation method. Meanwhile, the effect of remelting heat treatment on microstructure evolution and mechanical property of the superalloy was also explored. Compared to the traditional UWS process, the DWS process was superior in inhibiting the formation of freckle defects and alleviating the segregation of difficult-diffusion elements. Moreover, the expected application of thin shell casting in the DWS process was also more beneficial to the control of freckles and the refinement of microstructure. The change of solidification direction would alter the solute distribution in the mushy zone. Different convection patterns were then formed under the action of gravity, thereby affecting the microstructure of the casting. In the following process of remelting heat treatment, the generation and elimination of incipient melting structures occurred in both UWS and DWS samples. However, more incipient melting structures in the DWS samples made longer elimination time required. After remelting heat treatment, the mechanical property of the DWS sample was comparable with that of the UWS one, since the element segregation in the final microstructure was reduced, and the size, morphology and uniformity of γ' phase were also adjusted.

X-radiography front tracking gradient furnace for directional solidification of bulk Al-alloys

A unique gradient furnace for directional solidification experiments with bulk Al-alloy samples developed at German Aerospace Center is presented. It allows for in situ process control in solidifying samples by using x-radiography, and further insight into the solidification process is gained in combination with x-ray computational tomography on the solidified samples. Tracking of interfaces during directional solidification of bulk samples via in situ x-radiography (TIREX) enables the investigation of the melting process and observation of the movement of the entire mushy zone through the sample, tracing the solid–liquid interface during directional solidification and correlating the observations with the microstructure of the samples. Monitoring the temperature profile inside the sample by in situ observation of the length of the mushy zone is particularly important because the temperature gradient G and the rate of interfacial growth v determine the microstructure of solidification. The x-radiography setup offers temporal and spatial resolutions of 0.5 s and 70 μm, respectively, with a field of view of 10 × 50 mm2. Constant solidification velocities of up to 0.15 mm s−1 at a temperature gradient of up to 8 K mm−1 can be achieved in a temperature range of 537–1373 K. A flat solid–liquid interface inside a rod-like sample with 5 mm diameter is achieved by surrounding the sample by thermal isolating graphite foam. Performance tests with hypoeutectic Al–10 wt. % Cu alloy samples show the functionality of the furnace facility.

Combine effect of feeding rate and modeling parameter on the extrusion pressure in material extrusion additive manufacturing

Numerical modeling of the complex flow through the hot-end in the material extrusion (ME) additive manufacturing (AM) system is presented. A modeling parameter known as the mushy zone parameter and feeding rate was varied to predict extrusion pressure and other flow variables during ME. The simulation shows that the mushy zone parameter (Amush) influences the extrusion pressure for tough-polylactic acid (TPLA) filament. At all the feeding rates (40, 60, and 80 mm/min) considered, the extrusion pressure and dampening of the flow increase with an increase in the mushy zone parameter. However, the pressure drop is minimum for the lowest feeding rate of 40 mm/min and increases with an increase in the feeding rate. Generally, the temperature distribution in the hot-end suggests that the impact of Amush on the thermal field is lower when compared to the feeding rate. This indicates that the additional thermal energy needed to raise the temperature of semicrystalline materials to melting temperature due to the melting enthalpy being small. The current model suggests that improving the understanding of the complex flow physics in the hot-end can increase the printing parts production rate, redesigning the hot-end, inline process parameter monitoring, and control.© 2023 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved.This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/)Peer-review under responsibility of the Scientific Committee of the NAMRI/SME.

Towards supportless laser powder bed fusion: fundamental understanding of the formation of 1st layer overhang through parameter optimisation and in-situ high-speed thermal imaging

ABSTRACT One of the most revolutionary aspects of Laser Powder Bed Fusion (LPBF) is to be able to lift the design constraints from conventional manufacturing. However, as a rule of thumb, any surfaces lower than 45° with respect to the horizontal plane would still require sacrificial supports in order to complete the printing. Fundamentally, it is unclear whether it is feasible to print a 1st layer 0° overhang nor how the print parameters for the 1st layer can be optimised. This research demonstrates that large 1st layer 0° overhangs can be printed with a coverage above 90%. For the first time, the parameter space of laser power, scan speed and hatch spacing for the 1st layer has been simultaneously explored efficiently. The use of the rate of change of the mushy zone is proposed for the parameter selection instead of the average melt pool temperature. Adaptive Parameter 28 (AP28: 250W_4000 mm/s_50 µm) is the best choice. Several surface morphological phenomena are discussed. The core parameter (285W_960 mm/s_110 µm) causes severe balling and its mechanism is revealed. The average melt pool temperature of the 1st layer by the core parameter is lower than that of the bulk. The ramifications beyond the 1st layer are also illustrated.

Development and validation of a one-dimensional solidification model — Part I: Analytical model

The utilization of high-freezing point coolants in Gen IV reactors offers advantages in terms of high thermodynamic efficiency and inherent safety. However, it also introduces the risk of coolant solidification, which has been identified as a design basis accident for these reactor types. Accurate one-dimensional (1D) solidification models play a crucial role in reactor licensing and operation. Nevertheless, existing 1D models primarily focus on the growth of the solidified layer thickness, neglecting the important mushy zone effects that occur at the boundary layer overlap. This paper presents a novel general solidification model that incorporates the mushy zone effects and is applicable to high-freezing point coolants. The model has demonstrated favorable accuracy, with a maximum relative error of 10% when compared to experimental data. The findings highlight that the omission of mushy zone effects can lead to significant deviations from the actual scenario.

Microstructure evolution and phase transformation of the mushy zone in a quenched β-solidifying TiAl alloy

This study investigates the phase constitutions and transformations that occur in the mushy zone and in the adjacent phase fields of a directionally solidified Ti-44Al-8Nb-1Cr alloy via quenching technique. The results indicate that the mushy zone consists of unmelted β dendrites and interdendritic liquid, whose formation can be attributed to the difference in melting point aroused by local heterogeneity in solutecontent. The β dendrite is composed of numerous subgrains with various orientations. During quenching, the β dendrite transforms into Widmanstätten α via a precipitation reaction, owing to the decreasing cooling rate caused by heat transfer from the surrounding liquid. Additionally, after quenching, the interdendritic liquid is transformed into γ plates. Within the single β phase field and the lower part of the mushy zone, a massive transformation of β to γ occurs . Conversely, in the β+α phase field, both β and α phases are retained to ambient temperature. During the heating process, the transformation of α → β gives rise to the formation of β variants, which affects the orientation of β dendrites in the mushy zone. The growth kinematics of the α → β transformation was elucidated, revealing the preferential growth directions of 〈111〉 and 〈112〉 for β variants. Furthermore, this study presents an illustration of the formation process of the mushy zone and the microstructural evolution during the heating and quenching process.

Electroslag Hollow Ingots for Nuclear and Petrochemical Pressure Vessels and Pipes

The paper presents ground reasoning and results of experiments and modeling of heavy hollow ingot manufacturing using advanced electroslag technology. The requirements for ingots for huge diameter reactor pressure vessels include high density, homogeneity, and minimal segregation, which are very difficult to achieve by traditional casting. In the electroslag remelting process (ESR), hollow ingots form in between two copper water-cooled molds under effective heat removal. This improves the solidification pattern due to the shortening of a solidifying volume thickness more than twice compared with a solid ingot of the same diameter. The shallow liquid metal pool and narrow mushy zone at the ESR hollow ingot solidification assure their high metallurgical quality. Due to the dense and low segregation structure, ESR hollow ingots proved to be used for as-cast pipes and heavy wall billets for further forging. The results of a mathematical simulation within the range of simulated dimensions (the outer diameter up to 2900 mm, wall thickness up to 750 mm) also predict the favorable solidification pattern for thick-wall hollow ingots of big diameters. The analysis made and the modeling results provide a framework for scaling up the sizes of hollow ingots produced by ESR and widening their application for manufacturing heavy wall large diameter shells for nuclear and petrochemical industries. The higher reachable productivity of hollow ingot formation and lower capacity of power supply source than that for solid ingots of the same diameter and weight are also preconditions of their energy saving and cost-effective manufacturing.