Early Stages Of Solidification Research Articles

Microstructural refinement in a high elastic modulus Al-18Si-8Ni casting alloy

A new Al-Si-based casting alloy with high elastic modulus in the range of 90‐100 GPa and low density, less than 3 g/cm3, was developed by alloy design and microstructural refinement approaches. The addition of 8 wt.% Ni to a hypereutectic Al-18Si alloy introduces large volume fractions (∼40 vol.%) of hard primary Si and primary Al3Ni phases. Coarse primary Si and primary Al3Ni formed in the early stage of solidification, however, can interfere with the melt flow, reducing the castability, and also were deleterious to elongation. By the assessments of the crystallographic misfit between a nucleant substrate and a nucleated solid, we discovered key inoculants, AlP and TiB2, which can successfully refine such coarse and brittle primary phases, Si and Al3Ni, respectively. More importantly, along with the improved elastic modulus, the combined addition of AlP and TiB2 results in a significant increase in the elongation up to 1% with an advantageous yield strength of over 230 MPa. The important role of an inoculant, particularly TiB2 in nucleating the Al3Ni phase is also discussed based on the theoretical lattice misfit calculations and experimental characterization, further confirming their orientation relationship between the Al3Ni and the TiB2 as (110)[11¯0]Al3Ni∥ (0001)[12¯10]TiB2.

Formation Mechanism of Secondary Inclusions in Fe-36mass%Ni Alloy Using a Novel Combination Analysis Technique

Controlling the size, number, and composition of secondary inclusions is vital in the production of high-quality steels. In this study, experimental and computational investigation of the relationship between secondary inclusion formation in Fe-36mass%Ni alloy and cooling rate was carried out. Assuming the case of large ingots, solidification experiments using various cooling rates (0.17 to 128 K/min) were employed and the size, number, composition, and distribution of inclusions were analyzed by SEM-EDS automatic inclusion analysis. Like previous studies, inclusion number density increased with increasing cooling rate, while inclusion size decreased with increase of cooling rate. On the contrary, oxide inclusion area fraction was found to have little relationship with the cooling rate and was instead found related with oxygen content of the sample. As a new attempt to investigate the relationship between microsegregation and secondary inclusion formation, a combination of SEM-EDS analysis and EPMA mapping analysis was carried out. By superimposing information of microsegregation and inclusions, it was found that high-Al2O3 inclusions formed during the early stage of solidification, whereas low-Al2O3 inclusions formed during the later stage of solidification. These findings suggest that Al2O3 inclusions formed in the early stage of solidification reacted with the remaining Si-enriched liquid steel and changed into low-Al2O3 inclusions. Experimental results were also confirmed by thermodynamic calculations. Present work made it possible to understand deeper the relationship between microsegregation and secondary inclusion formation.

Solidification microstructure variations in additively manufactured Ti-6Al-4V using laser powder bed fusion

Laser powder bed fusion (LPBF) offers unique opportunities to produce metallic components without conventional design and manufacturing constraints. During additive manufacturing process, titanium alloys like Ti-6Al-4V undergo solid-state transformation that conceals initial solidification microstructure from room-temperature observations. Revealing the as-solidified microstructure can be critical to understanding the early stages of solidification. Using orientation relationships between parent (α) and child (β) phases, the as-solidified microstructures across the LPBF build volume has been reconstructed. Based on the as-solidified parent phase information, variations of the thermal and solidification conditions that occur during the LPBF of Ti-6Al-4V are revealed. The results show that how high cooling rates in the initially solidified lower layers contributed to orientation distribution during parent phase solidification, compared to upper layers in the build volume. Furthermore, the approach demonstrates the potential to further explore solidification microstructure and defect formation in titanium alloys during additive manufacturing.

Crystallization of Slag Films of CaO-Al2O3-BaO-CaF2-Li2O-Based Mold Fluxes for High-Aluminum Steels' Continuous Casting.

In this study, solidified films of CaO-Al2O3-BaO-CaF2-Li2O-based mold fluxes with different contents of Al2O3 addition were acquired by immersing an improved water-cooled copper probe in bulk molten slags. This probe can obtain films with representative structures. Different slag temperatures and probe immersion times were employed to investigate the crystallization process. The crystals in the solidified films were identified using X-ray diffraction, the morphologies of the crystals were observed using optical microscopy and scanning electron microscopy, and the kinetic conditions, especially the activation energy of devitrified crystallization in glassy slags, were calculated and discussed based on the differential scanning calorimetry. The results indicated that after adding extra Al2O3, the growing speed and thickness of the solidified films increased, and more time was required for the film thickness to reach a steady state. In addition, fine spinel (MgAl2O4) precipitated in the films at the early stage of solidification after adding 10 wt% of extra Al2O3. Together with LiAlO2, spinel (MgAl2O4) acted as nuclei for the precipitation of BaAl2O4. The apparent activation energy of initial devitrified crystallization decreased from 314.16 KJ/mol (original slag) to 297.32 KJ/mol (5 wt% Al2O3 added) and 269.46 KJ/mol (10 wt% Al2O3 added). The crystallization ratio of the films also increased after adding extra Al2O3.

Effect of Ni and Mo on microstructure and mechanical properties of grey cast iron

ABSTRACT Alloying elements (Ni, Mo) were added to grey cast iron, and the effect of Ni, Mo on microstructure and mechanical properties were analysed in this study. When Ni was added alone, the precipitation of graphite was inhibited at the early stage of solidification, and part of graphite was formed between the dendrites, resulted in an uneven distribution of graphite. Ni played a role of solution strengthening and reduced the pearlite interlammer spacing, improved the mechanical properties. The addition of Mo refined the austenite dendrites and made the carbon unevenly distributed, which led to the abnormal growth of graphite. Mo increased the eutectoid supercooling degree, reduced the pearlite interlammer spacing and improved tensile strength. When Ni and Mo were added together, the elements beneficial to strength (Mo, Sn) were not uniformly distributed, and the influence on graphite size and pearlite interlammer spacing was not obvious, resulted in poor mechanical properties.

Application of a new grain boundary technology for quasi-single crystalline silicon ingots

A new functional grain boundary group (FGBG) technology was employed to generate quasi-single crystalline silicon ingots with a high single crystalline area ratio. Fabrication of the FGBG structure was achieved and special grain boundaries were formed between adjacent seed crystal strips with 〈100〉 crystal orientation, with a 30° crystal orientation angle between the special grain boundaries. The influence of FGBG on the evolution of grains, dislocation, and surface morphology was analyzed by photoluminescence and electron back-scattered diffraction. FGBG effectively suppressed the side polycrystalline grains (spontaneous nucleation on the crucible wall) intrusion at the early stage of solidification. Additionally, when the crystal growth height was increased, FGBG acted as a barrier layer for the external polycrystalline grains under the control of an appropriate horizontal temperature gradient and slightly convex solid–liquid interface shape, preventing most of the polycrystalline grains around the ingot from growing into the crystal, and blocking the dislocations generated by polycrystalline grains near the grain boundary. The application of this FGBG technology will greatly improve the quality of quasi-single crystalline silicon ingots with a weight of 1400 kg (G7).

Grain refinement of Mg-alloys by native MgO particles: An overview

A fine and equiaxed solidification process delivers multidimensional benefits to Mg-alloys, such as improved castability, reduced casting defects, enhanced mechanical properties, increased corrosion resistance and potential for increased recycled contents. Despite extensive research on grain refinement of Mg-alloys in the last few decades, currently, there is no effective grain refiner available for refining Mg-Al alloys, and our current understanding of grain refining mechanisms is not adequate to facilitate the development of effective grain refiners. Under the EPSRC (UK) LiME Hub's research program, substantial advances have been made in understanding the early stages of solidification covering prenucleation, heterogeneous nucleation, grain initiation and grain refinement. In this paper, we provide a comprehensive overview of grain refinement of Mg-alloys by native MgO particles. We show that native MgO particles can be made available for effective grain refinement of Mg-alloys by intensive melt shearing regardless of the alloy compositions. More importantly, we demonstrate that (1) the addition of more potent exogenous particles will not be more effective than native MgO; and (2) MgO particles are difficult to be made more impotent for grain refinement through promoting explosive grain initiation. We suggest that the most effective approach to grain refinement of Mg-alloys is to make more native MgO particles available for grain refinement through dispersion, such as by intensive melt shearing.

On the elimination of solidification cracks in fusion welding of Al7075 by TiC-nanoparticle enhanced filler metal

Solidification cracks in fusion welding of high strength and precipitation hardened aluminum alloys have been a major challenge for many years. In this research, the weldability of 3-mm thick Al7075 sheets in fusion arc welding was investigated. Welding was conducted with (heterogeneously) and without (autogenously) TiC-nanoparticle enhanced Al7075 filler metal. Results show that the nanoparticles were effective in eliminating welding solidification cracks completely. Two mechanisms are proposed: (1) TiC nanoparticles act as nucleants in the early stage of solidification to alter the microstructure from dendritic to equiaxed and to refine the grains; (2) TiC-nanoparticles act as nucleants in the late stage of solidification to change the shape of eutectic precipitates from continuous to discontinuous and to promote the formation of the Al-Cu-Mg phase at the expense of the deleterious Mg(Cu,Zn)2 (η) phase. Tensile mechanical properties of autogenous weld could not be obtained due to large solidification cracks. In contrast, the heterogenous joint showed relatively good tensile strength and ductility that, however, were slightly lower compared to unwelded Al7075. This was directly linked to the high amount of porosity in the fusion zone due to the use of non-optimal welding parameters adopted in this study to promote solidification cracking for better comparative study and investigation of effects of nanoparticles.

SLM printing of cermet powders: Inhomogeneity from atomic scale to microstructure

It is very challenging for 3D printing based on the selective laser melting (SLM) technology to obtain cermet bulk materials with high density and homogeneous microstructures. In this work, the SLM process of the cermet powders was studied by both simulations and experiments using the WC-Co cemented carbides as an example. The results indicated that the evolution of the ceramic and metallic phases in the cermet particle during the heating, melting and solidification processes were all significantly inhomogeneous from atomic scale to mesoscale microstructures. As a consequence, the microstructural defects were caused intrinsically in the printed bulk material. The formation and growth of the bonding necks between the particles were mainly completed at the later stage of laser heating and the early stage of solidification. Both simulations and experiments demonstrated that thin amorphous layers formed at the ceramics/metal interfaces. This work disclosed the mechanisms for the evolution from the atomic scale to microstructure during the SLM printing of cermet powders, and discovered the origin of the defects in the printed cermet bulk materials.

Observing the Effect of Grain Refinement on Crystal Growth of Al and Mg Alloys during Solidification Using In-Situ Neutron Diffraction

The present research uses in-situ neutron diffraction to examine the effect of grain refinement on grain growth during solidification of Al-5 wt.% Cu and Mg-5 wt.% Zn alloys. The alloys were grain refined through additions of Al-5Ti-1B and Zr, respectively. The in-situ neutron diffraction experiments were carried out by heating the alloys to temperatures above the liquidus and subsequently cooling in 5 or 10 °C temperature steps to temperatures below solidus, while being irradiated by thermal neutrons. With the addition of grain refiners, grain size reductions of 92% were observed for both the Al-5 wt.% Cu and Mg-5 wt.% Zn alloys. The refined and unrefined Al-5 wt.% Cu alloys contained α-Al with Al2Cu along the grain boundary regions. Differences in Al2Cu morphology were observed in the grain refined alloys. The Mg-5 wt.% Zn alloy contained MgZn intermetallic phases with primary Mg. The refined Mg-5 wt.% Zn-0.7 wt.% Zr alloy contained Mg, MgZn and Zn2Zr phases. In-situ neutron diffraction enabled quantification of individual plane solid fraction growth for the α-Al and Al2Cu phases in the Al-Cu alloys, and for α-Mg in the Mg alloys. For the unrefined Al-5 wt.% Cu, the coarse microstructure resulted in a rapid solid fraction rise at temperatures just below liquidus followed by a gradual increase in solid fraction until the sample was fully solid. The grain-refined Al-5 wt.% Cu alloys showed a columnar to equiaxed microstructure transition and a more gradual growth in fraction solid throughout solidification. For the Mg-5 wt.% Zn alloy, the more packed (0002) and (101¯1) α-Mg plane intensities grew at a slower rate than the (101¯0) plane intensity, resulting in an irregular grain structure. With the addition of the Zr grain refiner, the Mg-5 wt.% Zn-0.7 wt.% Zr alloy had (101¯0), (0002) and (101¯1) planes intensities all increasing at similar rates, especially at the early stages of solidification. FactSage™ (version 6.4, Montréal, QC, Canada) equilibrium solidification models followed the fraction solid curves developed by tracking the fastest growing planes of the Mg alloys.

Comparative investigation on ingot evolution and product quality under different arc distributions during vacuum arc remelting process

The arc characteristics especially the arc distribution of the vacuum arc remelting (VAR) process are closely related to the ingot evolution and consequently the final product quality. Taking segregation defects and solidification quality as the indexes to evaluate the final product quality under different arc distributions, a three-dimensional full-scale transient model for predicting segregation defects is developed in this paper. Local solidification time (LST) and secondary dendrite arm spacing (SDAS) are used to characterize the solidification quality. This model is adopted to predict channel segregation (freckle) and macrosegregation defects during VAR process of Ni-5.8 wt.%Al-15.2 wt.%Ta alloy under several typical arc distributions. The results show that the diffuse arc has the shallowest molten pool among several arc distributions. At the early stage of solidification, the solute enrichment channels formed under the Gaussian and diffuse arc are slender and concentrated, while they are relatively sparse and uneven in thickness under the rotating arc. The value of volume-averaged segregation extent is always the smallest under the diffuse arc and the largest under the Gaussian arc, with final values of 67.53% and 69.66%, respectively. The volume-averaged values of LST and SDAS are relatively the largest under the Gaussian arc, about 1599 s and 209 μm, respectively; and they are the smallest under the diffuse arc, about 1520 s and 206 μm, respectively. In the same process conditions, the product quality of VAR ingots prepared under the diffuse arc is the best, followed by the rotating arc, and the worst under the Gaussian arc.

Performance evaluation of DRACS system of molten salt reactors using a transient solidification model

A transient solidification model was developed and coupled to ASYST code to simulate the transient behavior of the direct reactor auxiliary cooling system (DRACS) of the fluoride-salt-cooled high-temperature reactor (FHR) during the loss of heat sink (LOHS) accident. The model was firstly validated by comparing the experimental data with the simulation results that deriving from available heat transfer and friction factor correlations. The simulation results are found to be well-fitted with the experimental data. The transient solidification model successfully predicted the occurrence of solidification and the complete blockage of the heat transfer pipes in DRACSs. The solidified layer is found to be beneficial for the decay heat removal at the early stage of solidification, however, a sudden clog is found at the end stage of the solidification. Based on the simulation results, an optimization attempt was made to extend the failure time of DRACS by reducing the length of heat transfer tubes. The model is applicable to all the reactors where solidification may occur, such as other molten salt reactors (MSRs) and lead-cooled fast reactors (LFRs).

Characterization of the Convoluted 3D Intermetallic Phases in a Recycled Al Alloy by Synchrotron X-ray Tomography and Machine Learning

Fe-rich intermetallic phases in recycled Al alloys often exhibit complex and 3D convoluted structures and morphologies. They are the common detrimental intermetallic phases to the mechanical properties of recycled Al alloys. In this study, we used synchrotron X-ray tomography to study the true 3D morphologies of the Fe-rich phases, Al2Cu phases and casting defects in an as-cast Al-5Cu-1.5Fe-1Si alloy. Machine learning-based image processing approach was used to recognize and segment the different phases in the 3D tomography image stacks. In the studied condition, the β-Al9Fe2Si2 and ω-Al7Cu2Fe are found to be the main Fe-rich intermetallic phases. The β-Al9Fe2Si2 phases exhibit a spatially connected 3D network structure and morphology which in turn control the 3D spatial distribution of the Al2Cu phases and the shrinkage cavities. The Al3Fe phases formed at the early stage of solidification affect to a large extent the structure and morphology of the subsequently formed Fe-rich intermetallic phases. The machine learning method has been demonstrated as a powerful tool for processing big datasets in multidimensional imaging-based materials characterization work.

A 3D Analysis of Dendritic Solidification and Mosaicity in Ni-Based Single Crystal Superalloys

Ni-based single crystal superalloys contain microstructural regions that are separated by low-angle grain boundaries. This gives rise to the phenomenon of mosaicity. In the literature, this type of defect has been associated with the deformation of dendrites during Bridgman solidification. The present study introduces a novel serial sectioning method that allows to rationalize mosaicity on the basis of spatial dendrite growth. Optical wide-field micrographs were taken from a series of cross sections and evaluated using quantitative image analysis. This allowed to explore the growth directions of close to 2500 dendrites in a large specimen volume of approximately 450 mm3. The application of tomography in combination with the rotation vector base-line electron back-scatter diffraction method allowed to analyze how small angular differences evolve in the early stages of solidification. It was found that the microstructure consists of dendrites with individual growth directions that deviate up to ≈4° from the average growth direction of all dendrites. Generally, individual dendrite growth directions coincide with crystallographic <001> directions. The quantitative evaluation of the rich data sets obtained with the present method aims at contributing to a better understanding of elementary processes that govern competitive dendrite growth and crystal mosaicity.

The development of grain structure during additive manufacturing

Additive manufacturing of structural alloys results in the formation of complex microstructures, often with long, columnar grains that grow epitaxially from existing grains. A phase-field modeling framework is presented that considers solidification along a single track of 316L stainless steel in a regime where the solid-liquid interface is moving sufficiently fast that there is absolute interfacial stability with negligible composition variations. By coupling to a thermal field of a moving laser source, the model captures the trajectory of grains solidifying around a melt pool. The effect of interfacial kinetic anisotropy on the predicted as-solidified microstructures is examined through three-dimensional simulations. Through a combination of qualitative and quantitative analyses, we find that kinetic anisotropy has the largest impact along the center of the laser track and that the grain morphology in the early stages of solidification is predominantly due to the shape of the melt pool.

Synchrotron X-ray imaging and ultrafast tomography in situ study of the fragmentation and growth dynamics of dendritic microstructures in solidification under ultrasound

High speed synchrotron X-ray imaging and ultrafast tomography were used to study in situ and in real time the fragmentation and growth dynamics of dendritic microstructures of an Al-15%Cu alloy in solidification under ultrasound. An ultrasound of 30 kHz with vibration amplitude of 29 µm was applied into the alloy melt and produced a strong swirling acoustic flow of ~0.3 m/s. Efficient dendrite fragmentation occurred due to the acoustic flow and the dominant mechanism is the thermal perturbation remelting plus mechanical fracture and separation effect. Acoustic flow fatigue impact and phase collision effects were found to play a minor role in causing dendrite fragmentation. Just 10 s of ultrasound application at the early stage of solidification produced ~100% more dendrite fragments compared to the case without ultrasound, resulting in 20~25% reduction in the average grain size in the solidified samples. Furthermore, the dendrite morphology and tip growth velocity were mainly affected by the initial dendrite fragment number density and their distribution. The systematic and real-time datasets obtained in near operando conditions provided valuable 4D information for validation of numerical models and assistance in developing optimisation strategy for ultrasound melt processing in industry.

Numerical Simulation of Macrosegregation Formation in a 2.45 ton Steel Ingot Using a Three-Phase Equiaxed Solidification Model

In the present work macrosegregation during solidification of a 2.45 ton steel ingot is simulated with a pure equiaxed model, which was tested previously via modeling of a benchmark experiment. While the columnar structure is not taken into account, a packed layer formed over inner walls of the mold at an early stage of solidification reproduces to some extent phenomena generally related to zones of columnar dendrites. Furthermore, it is demonstrated that interaction of free-floating equiaxed grains with ascending convective flow in the bulk liquid results in flow instabilities. This defines the irregular form of the negative segregation zone, the formation of which at the ingot bottom corresponds to experimental observation. Vertical channels reported in experimental measurements are reproduced in simulations. It is confirmed that intensification of ingot cooling may decrease segregation in the ingot.

In-situ studies and simulation of equiaxed solidification in Al–20Cu alloy

The equiaxed solidification of Al–20Cu alloy under different cooling rates was investigated using in-situ synchrotron X-ray radiography. The results showed that mean grain size increases rapidly at the early stage of solidification, then reaches the final grain size gradually. And the change of final mean grain size with cooling rate follows d = 4132.3 × R −0.86. In addition, the evolution of grain size in Al–20Cu alloy was also simulated by ProCAST software. The experimental and simulation results indicated that, in the range of the cooling rate (5.0–7.5 K min−1), the major factor influencing the final grain size is the maximum nucleation density, while the nucleation undercooling has an insignificant effect on the final grain size.

Investigation on solidification and phase transformation of heterogenous interface structure in dissimilar metal weld between high Cr heat-resistant steel and nickel-based alloy 617

The present study characterized heterogenous interface structure in dissimilar metal weld between a high Cr martensitic heat-resistant steel and nickel-based alloy weld metal (WM), and revealed its solidification and phase transformation processes during welding. The heterogenous interface structure consists of a martensitic peninsula structure and a partially mixed zone (PMZ) of austenitic stainless steel. The K–S relationship between peninsula structure and its surroundings indicates: 1. phase transformation of (δ-Fe) to (γ-Fe) of peninsula occurs only after previous austenitic PMZ has solidified; 2. the subsequent WM solidifies epitaxially on (γ-Fe) of peninsula. Due to that the liquidus temperature of PMZ is lower than that of base metal (BM), the peninsula structure can form in the early stage of solidification by quick freezing of liquid BM.

An integrated microporosity model of 3D X-ray micro-tomography and directional solidification simulations for Ni-based single crystal superalloys

The shrinkage microporosity formed during the directional solidification of Ni-based single crystal superalloys can significantly reduce the fatigue life of turbine blades, which may lead to a catastrophic flight accident due to the failure of aeroengines. We developed an integrated mesoscale model, which combined the shrinkage pressure drop from dendritic growth using cellular automaton method and X-ray microtomography characterization for Ni-based single crystal superalloys. The critical shrinkage pressure drop was derived from the simulation and compared to the empirical functions and experimental measurements. It was found that the pressure drop was so critical for predicting the microporosity accurately, and its relationship with the pore size and fraction was established via direct comparison to X-ray tomography. Using this integrated model, the pore size, percentage, and distribution as a function of directional solidification conditions can be successfully simulated. The model can predict small pores at the roots of the secondary dendrite arms in the early stage of solidification, and the large ones in the interdendritic liquid at the late stage of solidification. Therefore, this proposed approach has demonstrated to predict not only the influence of the processing conditions such as withdrawal rate on pore volume fraction, but also the root cause of pore size distributions.