Initial Stage Of Solidification Research Articles

From crack-prone to crack-free: Eliminating cracks in additively manufacturing of high-strength Mg2Si-modified Al-Mg-Si alloys

Large solidification ranges and coarse columnar grains in the additively manufacturing of Al-Mg-Si alloys are normally involved in hot cracks during solidification. In this work, we develop novel crack-free Al-Mg2Si alloys fabricated by laser powder-bed fusion (L-PBF). The results indicate that the eutectic Mg2Si phase possesses a strong ability to reduce crack susceptibility. It can enhance the grain growth restriction factor in the initial stage of solidification and promote eutectic filling in the terminal stage of solidification. The crack-free L-PBFed Al-xMg2Si alloys (x = 6 wt.%, 9 wt.%, and 12 wt.%) exhibit the combination of low crack susceptibility index (CSI), superior ability for liquid filling, and grain refinement. Particularly, the L-PBFed Al-9Mg2Si alloy shows improved mechanical properties (e.g. yield strength of 397 MPa and elongation of 7.3 %). However, the cracks are more likely to occur in the region near the columnar grain boundaries of the L-PBFed Al-3Mg2Si alloy with a large solidification range and low eutectic content for liquid filling. Correspondingly, the L-PBFed Al-3Mg2Si alloy shows poor bearing capacity of mechanical properties. The precise tuning of Mg2Si eutectic content can offer an innovative strategy for eliminating cracks in additively manufactured Al-Mg-Si alloy.

Gel‐Derived Amorphous Precursor Enables Homogeneous Pure‐Phase α‐FAPbI3 Films for Efficient and Stable Perovskite Solar Cells

AbstractPure‐phase α‐formamidinium lead iodide (α‐FAPbI3) perovskite solar cells (PSCs) possess potential high efficiency because of suitable bandgap. However, achieving high‐quality α‐FAPbI3 films during the spin‐coating process is challenging without anti‐solvent extraction, due to the spontaneous formation of the more stable δ‐FAPbI3 phase and the complexity of multi‐component perovskite precursor solutions. Here a gel precursor with an amorphous structure to eliminate the need for anti‐solvent extraction in achieving the homogeneous and highly oriented pure‐phase α‐FAPbI3 is devised films. The incorporation of N,N′‐dimethylpropyleneurea, and alkylammonium chloride results in extended iodoplumbate clusters in the perovskite precursor solution. In situ, spectroscopic analysis reveals that the emergence of the stable amorphous gel precursor during the initial solidification stage ensures a uniform solute distribution. This effectively prevents preferentially oriented growth in the crystal phase precursor, leading to a significant enhancement in film homogeneity. As a result, power conversion efficiencies of 24.17% for the PSC, and 19.3% for the module are achieved, along with superior operational stability, retaining 96.1% of their initial efficiency after 850 h at the maximum power point tracking. These demonstrate the best performance for the reported pure‐phase α‐FAPbI3 PSCs without anti‐solvent extraction, showing great promise for scalable manufacture.

Experimental analyses of solidification phenomena in an ice-based thermal energy storage system

Latent thermal energy storage devices can efficiently store surplus thermal energy during off-peak hours. The system's phase change material (PCM) improves energy storage capacity and isothermal properties. Most PCMs have weak thermal conductivity, which hampers heat transfer. Thus, the present analysis involves an insight into the heat transfer characteristics and thermal performance of a vertical tube-in-tank cold storage system. The objective of this investigation is to gain a better understanding of the significance of buoyancy-driven convection within the side bulk region during the discharging of water as PCM in the heat exchanger. A series of experiments are conducted to investigate the effect of varying the initial bulk temperature on the rate of PCM solidification. The effects of three distinct initial bulk temperatures which are 20 °C, 15 °C, and 5 °C on the solidified mass fraction, thermal performance, and heat transfer rate at various radial and axial locations in PCM are examined. It is seen that PCM experienced a varying cooling rate with varying axial height and is found to be the highest in the bottom region. PCM temperature decreases from top to bottom under the cases of 20 °C and 15 °C, respectively. In contrast, a narrow-ranged thermocline layer is observed under 5 °C bulk temperature, making uniform temperature distribution within the bulk. The influence of bulk natural convection prevails in the later stages of the discharging process under 5 °C bulk, whereas it predominantly exists during the initial stages of solidification under bulk temperatures of 20 °C and 15 °C.

Solidification of a liquid drop enclosing a bubble on an inclined surface

In the present paper, we report the numerical results of the solidification process of a liquid drop enclosing an air bubble, i.e., a hollow drop, on an inclined surface. The surface is tilted from 0° (corresponding to a horizontal surface) to 90° (corresponding to a vertical surface), and the contact angle of the drop ranges between 60° and 130°. The drop is deformed while solidification happens in the presence of volume expansion. This volume change results in the formation of a tip at the drop top. The tip becomes more asymmetric as the tilting angle, i.e., the surface angle, increases. However, the solidification time is not affected by the surface angle. During the initial stage of solidification, the drop liquid accumulates to the bottom, by gravity, and thus results in lowering the bubble position. The bubble then moves upwards due to buoyancy force. It stops moving as it is trapped by solidification. A larger bubble reduces the tip shift. We also consider the effect of the Bond number and the Stefan number on solidification.

Analysis of a Single Crystal Solidification Process of an Ni-based Superalloy using a CAFE Model

The efficiency of gas turbines depends on the gas turbine working temperature. Single crystal blades are being applied more often than equiaxed blades in gas turbine engines, to increase the turbine inlet temperature, resulting in enhanced turbine engine efficiency. Single crystal blades endure creep conditions at high temperature better than polycrystal blades because the single crystals do not include grain boundaries. The single crystal process is a breakthrough technology, however, production yield is relatively low compared with polycrystal, and their mechanical properties depend on the crystallographic orientation of the single crystals. In this study, a thermal simulation model, the 3D cellular automation-finite element (CA-FE), was used on the single crystal process with the Bridgman method. The simulation model was well expected, by analysis of the microstructure and EBSD, on the grain selection in the single crystal process. The evolution of single crystal grains was analyzed on process of grain selection in start block and spiral selector. Single crystal orientation was also investigated to determine the effect of nucleation density, forming in the initial stage of solidification.

Wall impact of mushy frozen water drops

The initial stage of solidification of a free supercooled water drop consists of the formation of a cloud of dendrites surrounded by liquid water. Although the impact of such mushy drops onto solid surfaces can significantly influence the process of ice accretion, neither the dynamics of impact of such mushy drops, nor their material properties have ever been investigated in detail. In this study, the kinematics of spreading and the geometry of the residual splat have been characterized for the impact of dendritically frozen water drops prepared under well-defined conditions. The impact process is then modelled theoretically, assuming plastic flow in the mushy drop. The model provides a description of the flow development near the impact axis and allows estimation of the values of the apparent yield strength of the solid-liquid mixture for different initial temperatures of the water drop, i.e. for varying solid-liquid ratios.

Numerical simulation approach to investigate the effect of gas tube design on the impurities distribution and thermal properties of multi-crystalline silicon ingot grown by directional solidification process

Multi-crystalline Silicon (mc-Si) grown by directional solidification (DS) system is considered as a crucial photovoltaic material due to its relatively low cost and high yield. The numerical modelling is very useful for understanding the impurities segregation during mc-Si growth process. In the present paper, numerical simulation is carried out to investigate the impact of Argon gas tube design as shower head. In this Argon gas tube modification act on the non-metallic impurities, melt-crystal interface, temperature distribution and stress levels in the silicon ingot during the different stages of solidification process. The proposed design resulted in remarkable reduction in the back transfer of CO from heater component of the DS furnace. The generated von misses stress is also discussed as the function of different stage of solidification. The results concluded that the stress level at the initial stage of solidification is lower but it is large at above 75 % solidification rate. Also, we found that the modified gas tube design gives better temperature distribution with melt-crystal interface (m-c) shape. The modified argon tube design will be helpful for enhancing the quality of silicon ingot by controlling the chemical reaction between silicon melt and impurity transport.

Prediction of Thermal Distortion during Steel Solidification

Thermal distortion during the initial stages of solidification is an important cause of surface quality problems in cast products. In this work, a finite element model including non-linear temperature-, phase-, and carbon-content-dependent elastic–viscoplastic constitutive equations is applied to study the effect of steel grade and interfacial heat flux on thermal distortion of a solidifying steel droplet. Due to thermal contraction, the bottom surface of the droplet bends away from the chill plate and a gap forms. It is shown that, regardless of the nature of the heat flux, the gap forms and grows the most very early during solidification (~0.1 s) and remains almost unchanged afterward. Increasing the heat flux decreases the time for evolution of the gap and increases its depth. When the carbon content is less than 0.10%C, the gap depth is very sensitive to the heat flux, but for higher carbon contents, this sensitivity is much weaker. The highest gap depths are predicted in ultra-low carbon (0.003%C) and peritectic steels (0.12%C), and agree both qualitatively and quantitatively with the experimental measurements. Thus, the current thermal-mechanical model, including its phase-dependent properties, captures the mechanism responsible for nonuniform solidification, depression sensitivity and surface defects affecting these steels.

Macro-Micro Coupled Simulation of SLM Process with Inconel 718 Superalloy

As an increasingly mature additive manufacturing technology for metal materials, Selective Laser Melting (SLM) technology has become a hot topic in many application fields. However, due to the fast-moving velocity and small scale of the laser beam in the SLM process, it is very difficult to directly observe the microstructural changes in the SLM additive manufacturing process. In this study, a macro-micro coupled simulation model of Inconel 718 SLM process was established to study the solidification behavior of the molten pool. The macro temperature field is obtained by the finite difference method based on the birth and death grid algorithm. The local temperature intercepted from the macro temperature field is employed as the input condition for phase field microstructure calculation. Dendrite morphology, intercellular spacing, and microsegregation are simulated under the coupled model. The result shows that the solidification structure of IN718 alloy in the micro molten pool formed by SLM grows in the form of a non-flat interface. The primary dendrite spacing predicted by the simulation is in good agreement with Hunt model at the initial stage of solidification. The solute trapping caused by non-equilibrium solidification makes dendrites dissolve more Nb, resulting in microsegregation.

Solidification and segregation characteristics of Ni-based superalloy C700R-1 for ultra-supercritical steam turbine rotor

The solidification characteristics of C700R-1 alloy was investigated by DSC analysis, in-situ observation, quenching experiment and Thermo-Calc calculation. The segregation behavior during solidification was analyzed by Scanning Electron Microscope and Electron Probe Microanalysis. At the initial stage of solidification, the volume fraction of liquid experienced a steep decrease, accompanied by a rapid formation of dendrites. The residual liquid solidified slowly due to segregation at the final stage. Al, Ni, W and Co elements segregated in the dendrite stems, while Ti, Nb and Mo were enriched in the interdendritic, in which MC carbides also precipitated. At the end of solidification, the segregation of Mo and Cr is further aggravated, resulting in extremely low solidus and formation of eutectic region. The study on the solidification characteristics of C700R-1 alloy demonstrates that aggravation of segregation leads to deterioration of the solidification conditions. • DSC, in situ, quenching experiment and Thermo-Calc calculation were applied. • L→γ at 1370 ℃; L→γ + MC at 1298 ℃; L→γ + μ at 1217 ℃. • The solidification interval is widened due to the addition of Ti, Nb, Mo and W. • The segregation of Mo and Cr leads to the formation of multiple eutectic phases.

Modelling the fluid flow, solidification and segregation behavior in electromagnetic DC casting of magnesium alloy

Making clear the metallurgical transport behaviors during direct chill (DC) casting is of great significance to control the solidified structure and reduce the casting defects, but it still remains difficult because the high temperature and the opaque of melt make them hard to observe and detect directly. Numerical modelling provides a way to reveal the physical fields involved during DC casting. The present research used a two-dimensional axisymmetric model which coupled the heat transfer, fluid flow, solidification and species transfer in order to study the effect of different magnetic fields (harmonic magnetic field (HMF), pulsed magnetic field (PMF) and out-of-phase pulsed magnetic field (OPMF)) on the flow pattern, solidification characteristic and solute distribution in the electromagnetic DC casting of magnesium alloy billet having the diameter of 500 mm. The results show that when the magnetic field is on, there are the desirable variations including the homogenous temperature profile and faster solidification rate, as well as the alleviated central negative segregation and the reduced whole segregation level on the cross section of the billet. For the different magnetic fields, OPMF shows the strongest stirring effect, leading to the highest solidification rate at the initial solidification stage. Furthermore, the central negative segregation and the segregation level along the cross section are alleviated most as compared to other magnetic fields.

Thermodynamic Research on the Precipitation of Ti2O3, TiN and TiC in Continuous Casting of Titanium Microalloyed Steel

The composite precipitation of Ti2O3 + TiN during continuous casting has an important influence on the microstructure and properties of the slab. In order to study the precipitation conditions of Ti2O3, TiN and TiC second phase in titanium microalloyed steel, the solid-liquid phase line temperature, the initial precipitation temperature of different second phase, the equilibrium and actual solubility product of Ti2O3, TiN and TiC at different temperatures are calculated, and the precipitation rules of titanium microalloyed steel in liquid steel and two-phase region are analyzed. The results show that: Ti2O3 and TiN can precipitate in molten steel, and the precipitation order of Ti2O3 is prior to that of TiN, while TiC does not precipitate. Due to the enrichment of Ti, O, C and N in the liquid phase during solidification, the equilibrium precipitation conditions of Ti2O3 and TiN are reached when the temperature is lower than 1469°C of the liquidus, and the precipitation begins at the initial stage of solidification. When the temperature in the two-phase region is lower than 1332°C, the precipitation of TiC begins.

Formation mechanisms and control method for stray grains at melt-back region of Ni-based single crystal seed

An experimental study together with numerical simulation was conducted to investigate the formation mechanisms and control methods for stray grains at melt-back of seed in this paper. The numerical simulation results showed that the convex solid/liquid (S/L) interface changed very slowly into a concave interface during the initial solidification stage and the undercooling was slightly affected by the S/L curvature change. The formation of stray grains below melt-back interface at seed perimeter could be suppressed with the decrease of clearance between the seed and mold. However when the clearance was further decreased to 0, an orientation deviation layer formed originated from the deformation of un-melted seed caused by different expend coefficient between the seed and mold. The nucleation of stray grains above the melt-back interface at the seed perimeter could be suppressed using the mold with small surface roughness. The isolated un-melted dendrite stem at semi-solid mushy of the seed with the large original primary dendrite spacing transfer to complex network structure, suppressing the formation of the stray grains, when the primary dendrite spacing of seed decreased or the seed underwent solution heat treatment. Then, the formation mechanisms of the stray grains at the melt-back region are discussed based on above results, and a method is proposed to suppress the formation of the stray grains for preparing single crystal components by re-used seed.

Hierarchical refinement of primary phases in a multicomponent Al-14Si-CuNiMg casting alloy by ultrasonic melt treatment

The combined effects of ultrasonic melt treatment (UST) with the cooling rate on the microstructural evolution in a multicomponent Al-14Si-CuNiMg casting alloy was investigated. UST was applied to a melt at 800°C, followed by casting into a steel step mold at cooling rates in the range of 4-32 K/s. UST in the fully liquid state significantly reduced the sizes of the primary phases (the phases formed before Al) crystallized in the initial stage of solidification, and the structural refinement by UST became more pronounced for a faster cooling. With trace additions of Ti, V and Zr, the primary phase is most likely (Al,Si)3(Zr,Ni,Fe), and primary Si and then Al3Ni are sequentially formed upon solidification. AlP particles, well-known nucleants for primary Si, were also found to exist inside the (Al,Si)3(Zr,Ni,Fe) and Al3Ni which exhibited a very low lattice misfit with AlP (~0.7 and 4.7%, respectively). This importantly suggests that AlP can nucleate all the primary phases while it has the highest efficiency of structural refinement by UST for (Al,Si)3(Zr,Ni,Fe) phase that first nucleated in the molten metal. The nucleation of the primary phases was quantified using an analytical model based on exponential statistics. The hierarchical refinement operated by UST is further discussed.

Precipitation of TiS in Al–Ti Simultaneously Deoxidized Steel

Herein, the precipitation of TiS in an Al–Ti simultaneously deoxidized steel is observed, and the results confirm the precipitation of single TiS, two‐layered oxide (Al2O3 or TiOx)–TiS composite, and three‐layered Al2O3–TiOx–TiS composite inclusions in the steel. Thermodynamic calculations reveal that TiS precipitated in the experimental steel during the solidification process when the solid fraction (xs) is 0.880. In addition, the precipitation of TiOx during solidification depends on the equilibrium partition of O. When the O content in steel is very low, TiS rather than TiOx is precipitated. In addition, the enrichment of O in the residual liquid steel results in the precipitation of TiOx during the solidification process. Oxide–TiS is formed by TiS precipitation on the surface of Al2O3 and TiOx precipitation in the liquid steel or at the initial stage of solidification. However, Al2O3–TiS is formed only in preformed solid steel with low oxygen content. The lattice misfit results suggest that it is easier to match TiS to TiO and TiO2, while the misfit of TiS(001)/bcc Fe(100) is 6.06.

A numerical study of a liquid compound drop solidifying on a horizontal surface

In this study, we present the three-phase numerical simulation results of an axisymmetric compound drop solidifying on a cold plate by the front-tracking method. The compound drop initially has a hemispherical shape with a concentric inner gas core (i.e. gas bubble). The temperature of the plate is lower than the fusion temperature of the shell liquid and thus causes the shell liquid to solidify from the plate surface. It is found that the solid-liquid interface propagates parallelly to the plate during the initial stage of solidification. Then, the region near the outer interface moves faster than that near the inner gas core does. The solidification process completes around the inner core first, and the drop top is the last region to be solidified. Accordingly, a break point is available on the temporal variation of the average height of the solid-liquid interface. Besides, in the presence of volume expansion, the solidified compound drop also has an apex at the top. The numerical results show that the presence of the inner gas bubble prolongs the solidification time. We also find that varying the Stefan number St (in the range of 0.032 – 1.0) or the Bond number Bo (in the range of 0.18 – 3.2) has a minor effect on the inner gas core after complete solidification even though the solidification time decreases with an increase in St or Bo. The effects of the Prandtl number and volume change on the final product of the solidification process are also considered.

Effect of Magnesium Treatment on the Hot Ductility of Ti-Bearing Peritectic Steel

Surface cracking is a major defect in the production of continuous casting slabs of peritectic steel. The difference in crystal structure between δ phase (before peritectic transformation of steel) and γ phase (after peritectic transformation) results in volume contraction, which leads to uneven cooling of mold and thus forming slab shells with different thicknesses. Then, coupled with the concentration of local stress, surface cracking occurs on slabs. In this paper, the effect of magnesium treatment on the hot ductility of Ti-bearing peritectic steel was studied, and the characteristics of solidification structure and TiN particles were analyzed. Magnesium treatment for Ti-bearing peritectic steel could significantly improve the hot ductility of continuous casting slabs by refining the original austenite structure. After the magnesium treatment, the average grain size of the original austenite of peritectic steel decreased by about 18.7%, and the size of Mg-rich TiN particles decreased by about 41%. In addition, the minimum reduction of area at the third brittle zone after the magnesium treatment was higher than 60%, and the fracture appearance changed from intergranular fracture to ductile fracture after the treatment. The contents of Mg, Ti, O, and N in peritectic steel and the cooling conditions were adjusted reasonably to promote the formation of highly dispersed Mg-rich TiN particles with a sufficient number density and a proper size in the initial solidification stage of peritectic steel, so as to induce the high-temperature δ-ferrite nucleation. Based on the fine δ structure formed by peritectic transformation, through the use of structure heredity and the pinning effect of secondary-precipitated nano TiN particles on the austenite grain boundary, a fine and dense original austenite structure could be obtained to improve the hot ductility of peritectic steel. Industrial tests showed that through the magnesium treatment, the surface cracks of Ti-bearing peritectic steel were effectively restrained, and the corner cracks of slabs were basically eliminated.

Mathematical modeling of the supercooled steel ingot formation. Problem statement

The article considers hydrodynamic and heat exchange processes taking place in a supercooled melt, which is located in a stationary chill mold. We share the developed physical and mathematical model of a supercooled ingot solidification on the basis of the solidification process theory. A method for numerical realization of the obtained model is described. The article assesses the effect of circulating flows caused by forced convection on the solidification of a melt which is in a fixed chill. Studies show that only at the initial stage of solidification, certain mixing effect can be observed. The further influence of mixing practically does not affect the temperature distribution in the ingot body.

Microstructural characterization of laser-cladded NiCrAlY coatings on Inconel 625 Ni-based superalloy and 316L stainless steel

A detailed characterization of the primary microstructure of laser-cladded NiCrAlY coatings deposited on two different substrate materials is presented in this article. Amdry 962 powder was used to deposit coatings by LENS (Laser Engineered Net Shaping) on Inconel 625 Ni-based superalloy and 316L stainless steel. An overall characterization of coatings was performed from a qualitative point of view with particular attention paid to cracks and pores. A description of microstructural aspects of the internal morphology of coatings, such as the microstructure, chemical, and phase composition were also investigated. The distribution of alloying elements, such as Fe, Mo, Nb, and Y, was investigated in cladded coatings on two different substrates. It was revealed that the cladded coatings contained cracks and gas voids typical for laser-cladding processes. The dendritic microstructure was the result of rapid solidification due to differences in the chemical composition of dendritic and interdendritic areas, which was related mainly to the yttrium concentration. Additionally, epitaxial growth during the initial stages of solidification was determined to be related to the diffusion of substrate alloying elements into the dilution zone. The strong influence of substrate materials, especially iron, in the final microstructure of coatings was analyzed. The oxidation of yttrium and the formation of Y-Al-O oxides were also analyzed.

Viscosity, undercoolability and short-range order in quasicrystal-forming Al-Cu-Fe melts

Understanding the mechanisms which relate properties of liquid and solid phases is crucial for fabricating new advanced solid materials, such as glasses, quasicrystals and high-entropy alloys. Here we address this issue for Al-Cu-Fe alloys which can serve as a model for studying microscopic mechanisms of quasicrystal formation. We study experimentally two structural-sensitive properties of the liquid – viscosity and undercoolability – and compare results with ab initio investigations of short-range order (SRO). It is shown that the main features of interatomic interaction in Al-Cu-Fe system, extracted from radial distribution function and bong-angle distribution function, are the same for both liquid and solid states. In particular, the system demonstrates pronounced effective repulsion between Fe and Cu as well as strong chemical interaction between Fe and Al. Analysis of orientational short-range order reveals existence in the supercooled liquid Fe-centered icosahedral clusters which are discussed in the literature as structural motifs of the quasicrystal phase. In the concentration region corresponding to the composition of the icosahedral phase, minima on the viscosity and undercoolability isotherms are observed which affect noticeably the initial stage of solidification. Thus, we argue that SRO and structural-sensitive properties of a melt may serve as useful indicators of solid phase formation.