Solidification Front Velocity Research Articles

Effect of Solid/Liquid and Eutectic Front Velocities on Microstructure Evolution in Al-20%Cu Alloys

During the solidification process, microstructures are affected by the experimental conditions, the thermophysical characteristics of the alloy, and the type of grain-refining particles. Unidirectional solidification experiments were performed in a vertical Bridgman-type furnace to investigate the effect of the solidification front velocity on the solidified microstructure of a non-refined and refined Al-20%Cu alloy. The samples were solidified by rapidly increasing the sample velocity (v) range from 0.02 mm/s to 0.2 mm/s while maintaining an almost constant temperature gradient (~5 K/mm). As a result, despite changes in the solid/liquid front velocity along the sample, the microstructure of the non-refined alloys remained columnar. In the refined alloy, the columnar structure changed into an equiaxed structure at two different front velocities.

Dendritic deformation modes in additive manufacturing revealed by operando x-ray diffraction

Dynamic solidification behavior during metal additive manufacturing directly influences the as-built microstructure, defects, and mechanical properties of printed parts. How the formation of these features is driven by temperature variation (e.g., thermal gradient magnitude and solidification front velocity) has been studied extensively in metal additive manufacturing, with synchrotron x-ray imaging becoming a critical tool to monitor these processes. Here, we extend these efforts to monitoring full thermomechanical deformation during solidification through the use of operando x-ray diffraction during laser melting. With operando diffraction, we analyze thermomechanical deformation modes such as torsion, bending, fragmentation, assimilation, oscillation, and interdendritic growth. Understanding such phenomena can aid the optimization of printing strategies to obtain specific microstructural features, including localized misorientations, dislocation substructure, and grain boundary character. The interpretation of operando diffraction results is supported by post-mortem electron backscatter diffraction analyses.

Monitoring of thermo-cycles in fibre laser welding of duplex stainless steel 2205 sheets and its correlation with microstructures and mechanical properties

The study reports the influence of change in the heat supplied (43 J mm−1 to 18.5 J mm−1) on the microstructures as well as mechanical properties of weld joints obtained by welding of Duplex stainless steel 2205 using fibre laser. In-process thermal monitoring of the molten weld pool was carried out using IR pyrometer. Cooling rates (i.e. solidification and solid) were calculated from the thermo-profiles of weld pool, and it increases with the decrease in heat input. From the optical images, it is observed that columnar grains originated from the fusion zone walls and merged at the center. Since, solidification front velocity is comparable on both sides’ leads to a central edge. Ferrite phase content observed in fusion zone microstructure, increases with the increase in solid cooling rate. The result suggests that the joints fabricated at lowest heat input displayed highest tensile strength. The maximum tensile strength been reported to be 872.5 ± 10.8 MPa, and failure occurred at parent metal. Tensile strength of weld joints of DSS 2205 was found to have improved with increasing cooling rate. Higher cooling rate results in the formation of fine dendritic grains as well as higher ferrite content in the weld metal.

Direct ageing of LPBF Al-1Fe-1Zr for high conductivity and mechanical performance

The novel Al-1Fe-1Zr alloy leverages the non-equilibrium solidification conditions characteristic of the laser powder bed fusion (LPBF) process, namely a high thermal gradient (G) and solidification front velocity (R). The multiscale characterization of its microstructure in as-built and peak-aged conditions by SEM, TEM with EDS and automated crystal orientation mapping (ACOM), brings new insights on alloy design for LPBF. The solidification conditions permit the solute supersaturation of Fe and Zr, with the latter precipitating as L12-Al3Zr nanoparticles following ageing, bringing a high strengthening contribution. In conventional Al alloy ingots, Fe tends to create coarse particles which are detrimental to ductility. Here the Fe particles show a fine globular morphology in the as-built condition, and fine faceted Fe-rich intermetallics are obtained upon ageing. Nanoindentation and in situ tensile tests in the SEM coupled with microscale strain measurements shed light on the relation between microstructural and strain heterogeneities in the as-built condition. Our characterization results for this new Al-1Fe-1Zr grade are synthesized into a simple model for its solidification upon LPBF. In particular, the origin of melt pool boundaries is revealed. These are associated with Fe depletion caused by an early planar growth front, and the evolution of these zones upon ageing is presented. With a high potential for industrial applications, the novel alloy with the two elements Zr and Fe exhibiting low solubility at equilibrium and diffusivity in the Al matrix permit to achieve high conductivity (27 MS/m) and yield strength (330MPa) after a simple direct ageing step at 400°C/4h.

Numerical investigation of solidification behavior and inclusion transport with M-EMS in continuous casting mold

The present study deals with the development of a multiphase numerical model of the continuous casting process of steel bloom. Two SEN configurations that differ in port angles are considered for the study of flow pattern, temperature distribution, solidification thickness, interface fluctuation and inclusion behavior during the process. The influence of mold electromagnetic stirring (M-EMS) on the various output parameters is examined. The results show that the upper circulation loop formed in the 15° port angle case is responsible for higher inclusion removal, high interface level, and low solidification thickness in the meniscus region compared to that of the 30° case. The presence of M-EMS completely changes the flow pattern below SEN. Strong rotational flow coupled with oppositely directed swirl flow provides high tangential velocity near the solidification front and high axial velocity in the core region. It results in fluctuation in shell thickness along the casting direction and decreasing the superheated steam penetration. However, flow patterns remain unaffected above the SEN as upwards swirl flow is not able to reach this region. On applying the M-EMS, inclusion entrapment in solidifying shell increases for both 15° and 30° cases. In contrast, inclusion removal at the interface decreases for 15° case and increases for 30° case.

Solidification of an annular finned tube ice storage unit

Increasing building energy consumption rates is leading to significant surcharges for both the peak grid loads and the peak building energy consumption costs. The power load mismatches between user demands and energy supplies can be effectively reduced by ice storage systems. In order to solve the problem of heat transfer attenuation in the cold storage process. This study used a two-dimensional axisymmetric transient model of an annular finned tube ice storage unit to investigate the solidification characteristics and heat transfer rates with water as the storage medium and ethylene glycol as the heat transfer medium. The model was validated against experimental data. The calculations then investigated the effect of the annular fin height and fin spacing on the phase change and the solidification front velocity. The results show that increasing the fin height and decreasing the fin spacing both increase the ice storage solidification rate and improve the cold storage capacity. At a fin height of 50 mm, its solid phase fraction was 4.96 times larger than that of the bare tube. The cold storage capacity with the 50 mm high fins is more than 3.68 times that without fins at 480 min. The 4 mm fin spacing has a 26.3% greater cold storage capacity than the 12 mm fin spacing.

Phase field assisted analysis of a solidification based metal refinement process

Ultra pure metals have various applications, e. g. as electrical conductors. Crystallization from the melt, e. g. via zone melting, using the segregation of impurities at the solidification front is the basic mechanism behind different technical processes for the refining of metals and semi-metals. In this paper, we focus on a crystallization methodology with a gas cooled tube (“cooled finger”) dipped into a metallic melt in a rotating crucible. The necessary requirement for purification in a solidification process is a morphologically stable solidification front. This is the only way to enable macroscopic separation of the impurities, e. g. by convection. For cellular or dendritic solidification morphologies, the segregated impurities are trapped into the interdendritic melt and remain as microsegregations in the solidified metal. Morphological stability depends on the temperature gradient G at the solidification front, the solidification front velocity V front and thermodynamic alloy properties like the segregation coefficients of the impurity elements. To quantify the impact of these parameters on the morphological evolution, especially on the planar/cellular transition and thus on microsegregation profiles, phase field simulations coupled to a thermodynamic database are performed for an aluminium melt with three impurities, Si, Mn and Fe. In particular, we have investigated the morphology evolution from the start of solidification at the cooled finger towards a stationary growth regime, because in the technical process a significant fraction of the melt solidifies along the initial transient. To solve the transient long range temperature evolution on an experimental length scale, the temperature field has been calculated using the homoenthalpic approach together with a 1D temperature field approximation. The simulations provide the process window for an energy efficient purification process, i. e. low thermal gradients, and elucidate the benefit of melt convection.

Effect of processing parameters on the properties of freeze-cast Ni wick with gradient porosity

Wicks are the main component of Loop Heat Pipe systems, whereby coolant liquid flow through their porous structure. They are usually formed by a primary wick to produce liquid transportation by capillary force, and a secondary wick that is continuously wetted by the liquid coolant. Traditionally, the two wicks are manufactured separately and subsequently joined, thereby creating an interface that reduces the liquid transfer efficiency. In order to overcome this situation, a gradient porous wick is proposed and successfully manufactured through the freeze-casting method in a single operation. The influence of two different dispersant agents, KD4® and stearic acid was studied on the processing parameters, final pore size and morphology, and capillarity performances. A variate of gradient porosity was obtained by applying a diverse thermal gradient and solidification front velocity during directional solidification. The rheological characterisation of the camphene-based NiO suspensions was performed using a rotational viscometer. The final pore size and morphology were characterised by Optical Microscopy, Field Emission Scanning Electron Microscopy, and X-ray computed tomography. The use of stearic acid improves the particle stabilisation and generates pore enlargement with an equiaxed pore structure, while commercial dispersant KD4® shows a dendritic pore morphology at lower thermal gradient.

Effects of local nonequilibrium in rapid eutectic solidification—Part 2: Analysis of effects and comparison to experiment

The developed model of diffusion‐limited and diffusionless solidification of a eutectic alloy describes the relation “undercooling (ΔT)‐velocity (V)‐interlamellar spacing (λ)” for two cases. Namely, when the solidification front velocity V is smaller than the solute diffusion speed in bulk liquid VD, V < VD, the model predicts a regime of eutectic solidification similarly to known classical models. If the solidification front velocity V is higher than the diffusion speed, V > VD, the solidification is mainly controlled by kinetic and thermal undercoolings. New expressions for the solute distribution coefficient and slope of the liquidus lines are supplied. The influence of the model parameters on the growth kinetics during eutectic solidification is discussed. Model predictions are compared with experimental data for the solidification of an Fe–B alloy with eutectic composition. Computational results show that the model agrees well with experimental data especially for low and high undercoolings, extending the undercooling range that can be covered by sharp interface modeling.

Doped iron oxide scaffolds with gradient porosity fabricated by freeze casting: pore morphology prediction and processing parameters

Iron oxide scaffolds with gradient porosity were fabricated using the freeze-casting technique. Doped iron oxide particles were employed to generate stable particle suspensions in camphene using the ball-milling technique. Viscosity measurements were performed to determine the effect of the milling time and the additive incorporation. The freeze-casting process implemented in this work created a wide diversity of solidification front velocities, (2–80 µm s−1), and thermal gradients, (0.2–20°C cm−1) during sample fabrication. The solidification problem of liquid/solid systems has often been reviewed, but usually only under steady-state conditions. This study presents a much simpler approach for the correlation of the average dendrite arm spacing and pore morphology with different processing parameters.

Modification of the microstructure by rotating magnetic field during the solidification of Al-7 wt.% Si alloy under microgravity

Abstract Directional solidifications of Al-7 wt.% Si alloy were carried out under microgravity on board the International Space Station to investigate the impact of a rotating magnetic field (RMF) on the solidified microstructure. It has been found that the RMF significantly influences the solidified microstructure in the conditions corresponding to the lowest growth rate, whereas it has negligible influence on the microstructure under the highest growth rate, indicating that the RMF intensity and the forced liquid flow are too weak, compared to the solidification front velocity, to impact the microstructure. For the lowest growth rate applied, the RMF application results in a more uniform eutectic phase distribution and a smaller dendrite arm spacing. This is ascribed to the RMF induced forced liquid flow that makes more uniform Si concentration in the bulk liquid above the solid-liquid interface. Additionally, the RMF application possibly modifies the columnar dendritic network by changing dendrite growth directions. This observation can be attributed to the combined effect of the RMF induced forced liquid flow and of the thermoelectric magnetic force on the dendrites resulting from the application of the RMF as well.

Segregation and activation of Sb implanted in Si by UV nanosecond-laser-anneal-induced non-equilibrium solidification

In advanced logic devices, access resistance to transistors is dominated by metal–semiconductor contact resistivity. Recent studies report values below 1 × 10−9 ohm cm2, realizing metastable incorporation of dopants into epitaxially grown semiconductor materials. In this study, we have investigated segregation and activation of antimony (Sb) implanted in silicon (Si) epilayers by using UV nanosecond pulsed laser annealing (LA). The Sb-implanted Si epilayers were partially or fully molten by LA, followed by the analysis of atomic and electrically active dopant concentrations as well as the observation of surface morphology evolution. To discuss the impact of the solute trapping phenomenon on substitutional incorporation of the Sb atoms, we also simulated the evolution of solidification front velocity in the LA-induced non-equilibrium solidification. It is noteworthy that the active level of the Sb atoms largely surpasses their reported equilibrium solubility limit (∼2.6 × 1020 at./cm3 compared to ∼6.8 × 1019 at./cm3) when the non-equilibrium solidification approaches a near-complete solute trapping regime.

3D Simulation for Melt Laser Anneal Integration in FinFET’s Contact

Process integration feasibility of UV nanosecond melt laser annealing (MLA) in 14 nm node generation FinFET's contact for dopant surface segregation and activation is assessed by using a 3D TCAD simulation tool. In a n-type source/drain (S/D) in-situ phosphorous doped epilayer, Sb ion implantation is performed, considering the advantage of its surface segregation in lowering of the contact resistivity. The simulation results show that the heat sources created by the laser irradiation are confined mainly in the replacement metal gate (RMG) part, suggesting a potential interest of controlling the polarization of laser light to enlarge the process window by reducing the laser absorption in the RMG part. Also, the estimated solidification front velocity (V) in the MLA-induced epilayer regrowth (~4 m/s) satisfies the requirements (~1 m/s <; V <; ~15 m/s) to enable the surface segregation and metastable activation of the dopants. The surface segregation is also experimentally confirmed in the FinFET contact module.

Surface segregated Ga, In, and Al activation in high Ge content SiGe during UV melt laser induced non-equilibrium solidification

The impact of solidification front velocity (SFV) on dopant segregation and activation surpassing the equilibrium solid solubility limit was investigated in high Ge content SiGe using UV nanosecond melt laser annealing (MLA). We first simulated the SFV evolution in the MLA-induced solidification of a SiGe binary system. It was then combined with the near-surface atomic and electrically active dopant concentrations measured in the Ga, In, and Al-implanted SiGe after MLA. Solute trapping phenomenon and self-compensation effect of the dopants were discussed to capture a part of their segregation and activation dynamics in the presented SiGeX ternary systems.

Engulfment and distribution of second-phase nanoparticle during dendrite solidification of an Al–Si binary alloy: a simulation study

To achieve optimum strengthening effects of external nanoparticles (NPs), uniform dispersion of NPs in the melt is necessary for the casting manufacturing of metal matrix nanocomposites in which dislocation-based strengthening mechanisms play a significant role. However, the engulfment of nanoparticles within the solidifying grains and the avoidance of pushing them outside the solidification front are always a major challenge. Therefore, the understanding of local interface velocity and interface/particle dynamics during alloy solidification is of significant importance. Existing numerical studies on particle engulfment/pushing do not take into consideration the anisotropy of crystal growth and assume planar solidification interface, and thus they are unable to obtain the nanoparticle distribution in realistic alloy solidification. In this research, we investigate the engulfment/push behavior and the overall distribution of SiO2 nanoparticles in the dendrite solidification of an Al–Si binary alloy. Phase-field method is used to simulate the dendrite growth and to predict local solidification front velocity. In combination with the critical engulfment velocity obtained from a non-steady-state particle/front interaction model, the engulfment/push behavior of the entire solidification domain as well as the final distribution of nanoparticles can be analyzed. It is found that the distribution pattern of NPs obtained from simulation is overall consistent with the limited experimental results in the literature. In addition, the two main dislocation-based strengthening effects, e.g., Orowan bowing and CTE (coefficient of thermal expansion) mismatch strengthening, brought by external nanoparticles are quantitatively predicted. The degree of undercooling (60, 80, and 100 K) and nanoparticle size (10, 20, 30, 40, and 50 nm) are varied to investigate their influence on the NP engulfment behavior and the resulted strengthening effects.

Microstructure characterization and grain morphology of alloy 625 with 0.4 wt% boron modification manufactured by laser wire deposition

Laser wire deposits using Alloy 625 modified with 0.4 wt% B were manufactured on stainless steel 304 substrates. A layer boundary with a thickness of around 250 μm was formed between the layer cores during deposition. Results show that the solidification features in the layer boundary were coarser than the layer core due to the recalescence mechanism. Continuous eutectics were observed segregating the inter-dendritic regions in both the layer boundary and the layer core. The eutectics consisted of mainly Laves phase with a small amount of NbC precipitates. Solidification front velocities (SFV) were calculated from the Kurz-Giovanola-Trivedi (KGT) model. Results showed that they developed in the layer boundary and in the layer core at 0.06 m/s and 0.1 m/s respectively. Electron backscattered diffraction (EBSD) mapping revealed that small equiaxed grains nucleated in the layer boundary, while large columnar grains were prevalent in the layer core. Pole figures showed a strong oriented texture was present along the (100) plane. The columnar to equiaxed transition (CET) model developed by Hunts was considered and the results were in good agreement with the observed grain morphologies.

Solidification Behaviour of Al–SiC Composite Foam

This paper describes the solidification behaviour of Al composite melt in the narrow cell wall region in Al foam. Aluminium alloy foams prepared through melt route essentially involve entrapment of gaseous phase in a high viscous liquid followed by solidification. During solidification, the composite melt present in the narrow channel amongst the gas bubbles largely restricts the flow of heat and composite melt solidifies in a restricted region covered by almost zero conducting phases. The solidification front velocity has been computed using different models and their results have been compared. The distribution of SiC particles and matrix microstructure has been evaluated and compared with the experimental observation. It is noted that the solidification front velocity is slowed down due to the presence of gas bubbles and solidification is expected to initiate within the melt away from the melt/bubble interface which facilitates pushing SiC particles towards the gas bubble/melt interface.

Effect of solidification conditions and surface pores on the microstructure and columnar-to-equiaxed transition in solidification under microgravity

Microgravity solidification experiments were carried out in the Material Science Laboratory on board the International Space Station. The influence of grain refinement, rotating magnetic field (RMF) and surface pores on the microstructure and columnar-to-equiaxed transition (CET) were investigated in two selected Al-based samples solidified under microgravity conditions. The increase of the furnace pulling velocity leads to a finer dendrite structure, a smaller eutectic percentage and a more uniform eutectic distribution in the interdendritic regions. On the one hand, grain refinement ensures the occurrence of CET, which is progressive in the studied experiment because of the high temperature gradient. On the other hand, in the non-refined alloy a RMF applied during solidification fails to trigger the CET, because the forced liquid flow is too weak compared to the solidification front velocity to transport fragments from the mushy zone above the solidification front. The presence of the pores at the sample surface leads to a peculiarity in the eutectic percentage and weakens the decrease of the dendrite arm spacing for both samples. These effects are ascribed to a forced extra liquid flow into the mushy zone due to the pore that promotes the growth of the dendrites along the liquid flow direction, resulting in elongated grains and postponing the CET in the refined alloy.

Effect of Cast Geometries on Particle Dispersion in Metal Matrix Composites

Metal composite materials are not being extensively used because of their complex processing, agglomeration and dispersion of particles in case of discontinuous reinforcement. Significant amount of research work has been carried to understand and improve primary processing of metal composites using liquid processing route. Most of the research revealed the fact that composites were cast either in plate geometry or geometry of crucible. Present work mainly focuses to examine effect of mold cavity geometries on dispersion of particles in solidifying composite slurries. Aluminium alloy composites using silicon carbide (SiC) particles were manufactured using stir casting technique. ‘T’ shape and plus shape geometries were cast in the present work. Critical velocity and solidification front velocity was analyzed to investigate effect of cast geometries on particle dispersion. Microstructural examination revealed that cast geometries have significant effect on dispersion of particles.

In situ investigation of phase transformations in Ti-6Al-4V under additive manufacturing conditions combining laser melting and high-speed micro-X-ray diffraction

We present combined in situ X-ray diffraction and high-speed imaging to monitor the phase evolution upon cyclic rapid laser heating and cooling mimicking the direct energy deposition of Ti-6Al-4V in real time. Additive manufacturing of the industrially relevant alloy Ti-6Al-4V is known to create a multitude of phases and microstructures depending on processing technology and parameters. Current setups are limited by an averaged measurement through the solid and liquid parts. In this work the combination of a micro-focused intense X-ray beam, a fast detector and unidirectional cooling provide the spatial and temporal resolution to separate contributions from solid and liquid phases in limited volumes. Upon rapid heating and cooling, the β ↔ α′ phase transformation is observed repeatedly. At room temperature, single phase α′ is observed. Secondary β-formation upon formation of α′ is attributed to V partitioning to the β-phase leading to temporary stabilization. Lattice strains in the α′-phase are found to be sensitive to the α′ → β phase transformation. Based on lattice strain of the β-phase, the martensite start temperature is estimated at 923 K in these experiments. Off-axis high speed imaging confirms a technically relevant solidification front velocity and cooling rate of 10.3 mm/s and 4500 K/s, respectively.