Solidification Process Research Articles

In-Situ Observation Combined with Numerical Simulation of the Solidification and Subsequent Cooling Process for Hot Stamping Steel

In-Situ Observation Combined with Numerical Simulation of the Solidification and Subsequent Cooling Process for Hot Stamping Steel

Understanding fatigue crack propagation pathways in Additively Manufactured AlSi10Mg

Abstract Alloys produced through additive manufacturing (AM) offer substantial advantages, particularly in controlling material utilisation and precisely manipulating processing parameters, resulting in finely tuned material properties. However, the grain structure of AM material is typically complex, influenced by factors such as solidification dynamics, processing parameters, thermal gradients, and residual stress. Fatigue analysis shows considerable scatter due to entrained defects which limits their use as structural components. In this study, fatigue-failed samples from selective laser melted (SLM) AlSi10Mg alloy, oriented horizontal and vertical to the build direction were analysed to understand crack propagation paths. Here X-ray Computed Tomography (CT) was used to examine internal porosity from which fatigue cracks initiate, complemented by electron backscattered diffraction (EBSD) mapping. This enabled us to recognize the crucial role of the complex grain microstructure in controlling fatigue crack propagation.

Grain size assessment using EBSD on heterogeneous additively manufactured microstructures

Abstract Grain microstructures are important for a whole range of materials properties. With the widespread use of additive manufacturing (AM), current standards for grain boundary evaluation, such as ASTM E2627-13, may not be fit for purpose due to their microstructural complexity. They are notoriously anisotropic across a range of length scales due to the non-equilibrium nature of the solidification process and the tracking of the heat source. In this work we examine the grain microstructure of a nickel superalloy produced by laser powder bed AM by electron back scatter diffraction. We find that the guidelines provided by the ASTM E2627-13 are not suitable for such heterogeneous and anisotropic materials and some modified guidelines are provided on the field of view and EBSD step size that should be used to characterise such materials.

Effect of Ni addition on the crystallization behaviors of Cr-Co alloy

Optimizing the crystallization kinetics of Cr-Co alloys through doping is of practical significance to their industrial applications. By employing molecular dynamics (MD) simulations, this study investigates the effect of Ni concentration on the solidification process of Cr30Co70-xNix (x=0, 10, 20, 30, 40) alloys, focusing on crystallization behavior during quenching and microstructural characterization. The evolution of potential energy indicates that higher Ni concentrations promote crystallization, resulting in a lower liquid-solid transition temperature and enhanced relaxation. However, Ni doping does not alter the preference for chemical ordering, with Cr-Co bonding predominant across all the compositions. Microstructural analysis reveals a shift towards fcc-dominant ternary alloys with increased Ni content. Furthermore, Ni addition impacts defect distribution, notably reducing "other" types of defects in the Cr-Co binary alloy. These findings provide a quantitative reference for adjusting the crystallization behavior of Cr-Co alloys through doping, with implications for their design, manufacturing, and utilization.

Die steel design for additive manufacturing

A novel printable die steel was computationally designed and experiemtnally validated for selective laser melting (SLM), utilizing the advantages of the rapid solidification processes. During gas atomization, nanoscale TiN particles are intended to be insitu precipitated at 1790°C, nucleating δ-ferritic grains. Additionally, the chemical composition is adjusted to stabilize a complete δ-ferritic solidification via Scheil modeling to enhance the printability of the die steel. This work further introduces the concept of the matrix die steels aiming to dissolve solidification and primary carbides during solutionizing at a targeted temperature of 1100°C. Thus, the C-content is reduced to 1.4 mol.-% (0.3 wt.-%) compared to the benchmark H13 die steel with which contains 1.85 mol.-% (0.4 wt.-%). Even though a lower C-content is used, optimizing M2C driving force during tempering enables the die steel to achieve a peak hardness of 536 HV is achieved. Lastly, a superior thermal conductivity of 40 W m-1 K-1 is predicted at 450°C for the BCC matrix of the printable matrix die steel. The material design is based on thermo-chemical models interfaced with thermodynamic calculations as implemented in the Calculated Phase Diagram (CALPHAD) method.

Enhanced thermoelectric properties of 3D printed Bi2Te2.7Se0.3 by atomic interface engineering

The performance of n-type Bi2Te3 materials prepared by selective laser melting (SLM) 3D printing technology tends to be lower than p-type materials. Herein, a bottom-up interface engineering strategy based on Atomic Layer Deposition (ALD) is presented to improve the properties of n-type Bi2Te2.7Se0.3 (BTS) materials prepared by SLM. To validate this strategy, a ZnO ultrathin layer was applied to the grain boundaries of BTS, synergistically optimizing carrier/phonon transport. In terms of electrical transport, the energy filtering effect triggered by the constructed ZnO/BTS interface effectively enhances the Seebeck coefficient of the Bi2Te3 material printed by SLM, substantially increasing the power factor. Additionally, the high thermal stability of the ALD-deposited ZnO ultrathin layer effectively suppresses the volatilization of Te in BTS, regulating the carrier concentration. Regarding thermal transport properties, the multiscale defects introduced by the non-equilibrium solidification process during SLM printing cause effective phonon scattering across a wide frequency range, which in turn reduces the thermal conductivity of the material. Atomic-level interface modification in SLM-printed BTS enhanced the ZT value to 1.25. Exploiting this superior thermoelectric performance, a thermoelectric module was constructed. It demonstrated a conversion efficiency of 5.7 % with a temperature difference of 200 K, comparable to the advanced Bi2Te3-based thermoelectric modules. The integration of an ALD interface modification strategy with SLM technology has not only demonstrated potential applications in Bi2Te3-based thermoelectric material development, but also bears significant importance for the accelerated advancement of high-performance thermoelectric materials in other systems.

Numerical simulation study on solidification proceoss of titanium slab ingot by electron beam cold hearth melting

Abstract Titanium and titanium alloys are key basic support materials in the field of engineering technology and high technology, and are widely used in the fields of natural gas transportation, chemical corrosion, and marine development. Titanium alloy ingots are often prepared with more solidification defects such as surface cracks and cold shuts, resulting in lower utilization of titanium metal and higher cost of titanium products. The root of this is the lack of in-depth knowledge of the ingot melting and casting process, and the failure to control the thermal conditions of the billet in the molding process within a reasonable range. In this study, based on the Lagrange Euler algorithm, combined with ProCAST finite element software to establish a numerical model, revealing the solid-liquid interface morphology, the length of the transition region, and the change rule of thermal stress under the influence of different process parameters in the solidification process of titanium slab ingot. The results show that with the increase in pulling speed, the depth of the solid-liquid phase line and the width of the mushy zone of slab ingot increase, and the length of the transition region grows. With the increase in casting temperature, the depth of the solid-liquid phase line of the slab ingot decreases, and the mushy zone gradually becomes narrower. The casting temperature and pulling speed are positively correlated with the value of the thermal stress equivalent stress in slab ingots, and the probability of cracks in the corners and ingot surface is higher. This study provides effective theoretical guidance for the realization of stable mass production of high-quality titanium slab ingot.

Micro-encapsulated phase change material suspensions for heat and mass transfer: A thermo-physical characterization

Despite the growing popularity of phase change materials (PCM) and suspensions of micro-encapsulated phase change materials (mPCM) in both industrial and scientific applications, their properties characterization remains partial and mainly limited to thermal properties. The characterization of such suspension is a crucial aspect for comprehending their behavior and optimizing their performance. This paper is dedicated to a wide characterization of two suspensions containing micro-encapsulated phase change material. In pursuit of a thorough understanding, we also extend our characterization efforts to the bulk PCM encapsulated within the particles.Our primary focus is on determining the materials thermal properties such as latent heat and specific heat capacity using differential scanning calorimetry. Different cooling and heating rates have been employed in our measurements. Regarding our experiments, the bulk phase change material is predominantly identified as octadecane. Results obtained from calorimetry are then compared between the bulk PCM and the mPCM suspensions. By comparing the magnitudes of latent heat obtained for suspensions with the bulk material, we can accurately determine the mass fraction of PCM within each suspension. Noteworthy, during the solidification process an additional latent heat peak is observed only for the encapsulated PCM.The density of the materials is also measured. The phase change of PCM included in the capsules can be observed in densimetry: within the studied temperature range (283.15–306.15 K), the most significant density variations occur during intervals associated with phase change transitions. In ranges where the PCM, included in the capsules, is in a single-phase state (solid/liquid), we provide linear laws that account for density variations with temperature.Finally, our characterization work focuses on the rheological properties of the materials. While the bulk PCM in liquid phase exhibits a Newtonian viscosity, mPCM suspensions present a non-Newtonian viscosity. Both shear-thinning and shear-thickening behaviors are observed depending on the suspension particle volume fraction ϕ of mPCM. This paper concludes with a full comparison of our characterization results with models and correlations proposed in the literature.

Towards optimizing cooling rate, microstructure, and compressive strength in rapidly solidified Cu–20Fe alloy

There is currently interest in the development new Cu–Fe alloys with higher Fe content (>5 wt%), a low-cost element. In order to improve electrical, thermal and mechanical properties, there is a need to decrease the size of the Fe-rich phase and improve its distribution, whose possibilities rely on adding alloying third elements and heat treatments. However, the prospect of controlling the size of the phase through dendritic spacing control by means of rapid solidification processes may be feasible and deserves to be explored, as is the case in the present study. The following methods were employed to evaluate the round stepped cast samples of the Cu-20 wt% Fe alloy: centrifugal casting in a Cu mold, optical microscopy, CALPHAD, Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS), X-ray Diffraction (XRD), Vickers hardness, and compressive tests, in samples corresponding to different coarsening microstructures. Finally, dendritic growth is modelled using the Kirkwood equation. The results demonstrate a promising processing route with control of the secondary dendrite arm spacing (SDAS), whose methodology and modeling can serve as a basis for other alternative processes with the Cu–Fe alloys, allowing suitable size and distribution of Fe phase to be attained. It is further demonstrated that the Kirkwood model is able to match experimental SDAS in both slowly and rapidly solidified Cu–Fe samples. While Fe (BCC) is expected to increase strength, this is only slightly observed in the hardness results for smaller SDAS, as the loads are more superficial. In the case of compressive yield strength, SDAS in the observed range (1.4 μm–2.2 μm) is found to have no effect.

Carboxymethylcellulose and rare earth metal cation self-assembly: Tunable encapsulation of carbon dots in a multifunctional transparent film

Due to their unique properties, carbon dots have attracted significant interest across diverse fields. However, obstacles such as uneven size distribution in the course of synthesis and excessive aggregation lead to fluorescence quenching during solidification process. Moreover, conventional doping procedures frequently yield carbon dot films with constrained functionality, inferior durability, and harsh environments, thereby restricting their practical applications. To address these issues, this study fabricates N-doped carbon dots via a straightforward hydrothermal approach and incorporates them into CMC films by metathesis with CeCl3·7H2O, resulting in a unique carbon dot-encapsulated film. This film overcomes the problem of fluorescence quenching caused by excessive aggregation of carbon dots and allows for size control by adjusting the concentration of Ce(III). The resulting film achieves extreme transparency and superior performance compared to existing UV-blocking films. In addition, the film exhibits exceptional resistance to acids and bases, bleaching, excellent mechanical properties, and good thermal and water stability. Based on these properties, the film achieves size-controlled in-situ photoluminescence, full UV shielding, Cu2+ ion detection, anti-counterfeiting, and information encryption. The water-processable and water-adhesive characteristics of CMC enable its use in real-life situations, opening the door to wider use of solid-state carbon dots for multifunctional and intelligent applications.

Selective laser sintering and spark plasma sintering of (Zr,Nb,Ta,Ti,W)C compositionally complex carbide ceramics

AbstractTwo advanced manufacturing processes, spark plasma sintering (SPS) and selective laser sintering (SLS), have been developed for synthesis of (Zr,Nb,Ta,Ti,W)C compositionally complex carbide (CCC) via reactive sintering of a powder mixture of constitute monocarbides. X‐ray diffraction analysis confirmed that the single‐phase CCC can be formed by both SPS and SLS. While a homogenous microstructure with uniform metal element distributions was developed during SPS, three‐layer microstructures with a thin TiC‐rich layer and two TaC‐rich layers along with a TiO2‐rich surface layer containing W nanoparticles were formed during SLS. In addition, cellular structures with W, Zr, and Ti element segregation and dislocations on cell boundaries were observed in the SLS‐CCC sample, indicating the effect of nonequilibrium conditions on microstructure formation during laser melting followed by rapid cooling and solidification process. Compared to the SPS‐CCC sample, the SLS‐CCC showed enhanced hardness and reduced thermal conductivity, which may be related to their unique cellular structures.

Effect of hot isostatic pressing on microstructure and properties of cooled hot dip aluminum coating in magnetic field

The effect of hot isostatic pressing (HIP) on the microstructure and properties of hot dip aluminum coating cooled in a magnetic field was investigated in this study. In order to improve the microstructure and properties of magnetic dip aluminum coating, hot isostatic pressing technology was used for post-treatment. Initially, a traditional aluminum-impregnated coating was prepared on the surface of titanium alloy TA15, an alternating electromagnetic field was applied during the forming and solidification process of the coating. Finally, the coating was treated with hot isostatic pressing technology. Analyzed three different coatings of the microstructure and element distribution, and tested the microhardness of the coatings at various positions. The test results revealed that the TA15 titanium alloy hot-dip aluminum coatings obtained through the three different processes exhibited a gradient structure. Compared with the traditional hot-dipped aluminum air-cooled coating, when an appropriate intensity of alternating electromagnetic field was applied during the coating solidification process, the outer coating structure was enhanced, the number of holes was reduced, the microstructure density increased, and the number of cracks significantly decreased. The defects of the 800 °C hot isostatic magnetic cold and hot dip aluminum coating were repaired under high temperature and pressure, resulting in a uniform and fine microstructure. The comprehensive properties of the magnetic cold and hot dip aluminum coating on the surface of the titanium alloy were effectively enhanced through hot isostatic pressing.

Misorientation increase of small-angle grain boundaries during directional solidification of silicon

In situ observation of the solidification process of Si combined with ex situ characterization revealed that small-angle grain boundaries (SAGBs) increase misorientation through two paths. First, coalescence with other SAGBs leads to a boost in misorientation and SAGBs tend to coalesce during solidification. Second, the absorption of individual lattice dislocations also increases the misorientation of an SAGB. Three possible mechanisms behind dislocation absorption by SAGBs have been proposed and are discussed in this paper. Among these mechanisms, the incorporation of same-sign lattice dislocations is regarded as the prominent mechanism when misorientation increases during solidification.

Effect of current intensity on molten pool and solidification in vacuum arc remelting process of Ti60 alloy

Current intensity is a critical parameter in the vacuum arc remelting (VAR) process, significantly impacting the molten pool, solidification structure, and ultimately, the quality of the ingot. This study establishes a transient electromagnetic-fluid-thermal-solidified coupled model to examine the effects of current intensity on the shape of molten pool and the solidification process of Ti60 alloy ingots produced by VAR process. Findings indicate that with the increase in current intensity, there is a corresponding rise in the magnetic induction intensity, current density, and Lorentz force. This enhancement leads to more vigorous flow in the liquid phase, creating a vortex flow opposing to the ingot's center at the boundary, influenced by the strong current. As a consequence, the molten pool's shape tends to shift from a “V” shape to a “U” shape, and the depth of molten pool increases while the mushy zone thickness diminishes. For instance, at 17000 A, the depth of molten pool surges by 57.43%, and the mushy zone thickness contracts by 62.01% compared to 5000 A. Over time, the depth of molten pool gradually decreases, with its reduction range ( Δh) within 400 to 6000 s inversely related to current intensity. At 17000 A, Δh is 23.71% less than at 5000 A. Furthermore, elevating the current intensity results in a higher temperature gradient within the ingot and a marked reduction in local solidification time, particularly in the ingot's central area.

Advanced microfluidic techniques for in situ PLGA microparticle formation: A dynamic solvent extraction approach

Microencapsulation processes often use the "solvent extraction/evaporation" technique, which involves (i) dissolving a polymer in an organic solvent (optionally with a bioactive component), (ii) forming microdroplets, (iii) removing the solvent, and finally (iv) harvesting and drying the particles ex situ. These last two steps lack control over the solidification processes, which adversely affects the morphology and size distribution of the final microparticles. In this study, we present a continuous production process for solid PLGA microparticles within a microfluidic chip, covering droplet formation, solvent extraction, and solidification. This method offers the advantage of enhanced control over the solidification process within a closed system, leading to morphologically uniform particles. The setup includes a co-axial junction integrated into a chip with an 80 cm long serpentine channel. We generated droplets with various PLGA/ethyl acetate concentrations and tracked their shrinkage until solidification. Results showed that all droplets underwent identical shrinkage processes with consistent rates, velocity profiles, and sizes. The shrinkage rate was linear and unaffected by PLGA concentration, which only influenced final particle size.To demonstrate our setup's advantages, we compared it with the conventional off-chip method, where droplets shrink in a beaker. Off-chip shrinkage was non-linear and inconsistent, influenced by factors such as the number of surrounding droplets and production time. Droplets produced within a 10-s difference had solidification delays up to 250 s. In contrast, on-chip production was uniform, predictable, and 4–10 times faster. SEM imaging confirmed that on-chip particles were consistently spherical with a rough, wrinkled surface, unlike the irregular, bumpy off-chip particles.In summary, on-chip production and shrinkage of PLGA particles is faster and produces more uniformly shaped particles compared to off-chip methods. This study suggests that on-chip production could significantly enhance the efficiency and quality of drug-loaded microparticles, particularly for drug delivery applications.

Effects of Heat Treatment and Processing Conditions on the Microstructure and Mechanical Properties of a Novel Ti–6.3Cu–2.2Fe–2.1Al Alloy

Adding transition alloying elements to achieve a columnar to equiaxed transition (CET) in Ti alloys gains attention in additive manufacturing (AM). AM, which may be categorized as an advanced solidification process, is commonly a near‐net‐shaped process. This thus does not allow traditional thermomechanical processing to reduce grain size, which drives research toward novel alloys able to establish a primary grain structure with equiaxed grains upon solidification. The Ti–Cu alloy system catches attention through inducing the CET, but so far, no focus is placed on its secondary, solid‐state, and phase transformations during the heat treatment, therefore, requiring modified heat‐treatment strategies to accommodate for the inability of mechanical preforming prior to heat treating and thus achieving desired mechanical properties. In this work, insights are provided on microstructural evolution and tensile properties of a novel Ti–6.3Cu–2.2Fe–2.1Al alloy gained in an extensive heat‐treatment study. In the results, a lamellar α+β Widmannstätten microstructure with Ti2Cu precipitation and other features including grain boundary α, precipitate‐free zones, and very fine secondary α precipitates are shown. Utilizing micrographs and fractographic imaging, the fracture mode is identified as quasi‐cleavage mode with preferential crack initiation at grain boundary α layers. Tensile tests on heat‐treated samples show high strengths with simultaneously limited ductility.

Effect of grain structure on the magnetic properties of AlNiCo 8 alloys

AlNiCo alloy has gained popularity again in recent years due to its good corrosion resistance, affordability, and superior performance at high temperature. However, the lower magnetic properties of AlNiCo limit its wide variety of applications. Directional solidification process is a key process that affects the properties of AlNiCo alloy. During the solidification process, grains with various structures are formed due to the difference of cooling rate at different positions of the ingot. As the cooling rate slows down, the average grain size gradually increases, and orientation degree of the grains becomes better, resulting in the increase of coercivity (Hc) and remanence (Br). Therefore, the top sample has the best Hc and Br, the bottom is the worst, and the middle is between the two. It is found that when the average grain size increases, the percentage of transverse grain boundaries and high angle grain boundaries (HAGBs) decrease, the truncated α1 phase decreases, and the Hc rises, whereas the magnetic moment along the easy-axis direction is not affected by the external magnetic field when the grain orientation coincides, and the Br value increases, and the Br value increases. Large and highly oriented grains are associated with excellent performance, providing a new method to enhancing magnetic properties by regulating the grain structure.

Controlling the crystal texture and microstructure of NiTi alloy by adjusting the thermal gradient of laser powder bed melting

The development of a favorable texture can enhance the recoverable strain of polycrystalline NiTi shape memory alloy. Nevertheless, texture tuning of NiTi alloy prepared through additive manufacturing presents more intricate challenges compared to alloys produced using traditional thermomechanical methods, as the solidification process is influenced by various factors. In this study, dense NiTi alloys were fabricated in laser powder bed melting (LPBF) technology. The optimal processing parameters were obtained including a hatching spacing of 24 μm, a forming angle of 90°, and a volume energy density of 139 J/mm3. A robust [001] texture has been developed in NiTi alloys by adjusting the thermal gradient through the precise control of hatching space and energy density. Alloys with this texture manifest a stable and fully recoverable strain of 4.82 %. A novel microstructure evolution mechanism has been proposed to elucidate the texture development during the LPBF manufacturing process under varying thermal gradients. Our findings provide insights into adjusting crystal texture in additive-manufactured NiTi parts through thermal gradient manipulation to achieve superior superelasticity.

Yttrium induced formation of new pores in an as-cast nickel-based single crystal superalloy

Pores are the most fatal defects in single crystal superalloys, which severely deteriorates the mechanical properties. Yttrium (Y) has been applied in commercial superalloys, but its effect on the morphology change of casting pores is not clear. This work focuses on the effect of Y on the evolution of pores formed after directional solidification in a second-generation superalloy. SEM, TEM and XRT were used to characterize the pores, and Thermo-Calc was used to calculate the surface tension change during the solidification process. Results show that, besides the formation of two types of conventional shrinkage pores during solidification, new types of pores, i.e., Type III and Type IV, due to the addition of Y element to 0.5 wt% in superalloy, were also observed. The formation of Type III pore could be ascribed to the surface tension drop in the liquid phase, which enhances the wettability at the solid/liquid interface, and promotes the mobility of liquid phase, due to the incorporation of Y element. The formation of Type IV pore is due to the simultaneous formation and growth of M23C6 and Ni3Al in the Ni5Y phase, triggering the formation of closed zone with a small but irregular shape at the multi-phase interface.

Formation of orthopyroxene-magnetite symplectites by reactive melt flow: Insights into the Shangzhuang layered intrusion in Beijing, China

Interstitial microstructures in layered intrusions can provide crucial insights into the formation, transport, evolution, and solidification processes of magma. The symplectites commonly observed in the Shangzhuang layered intrusions in Beijing, China, can be classified into three types based on their occurrence and related primocryst minerals. Regardless of variation in types, the vermicular symplectites consistently exhibit a volume ratio of magnetite to orthopyroxene at approximately 1:4. Orthopyroxene in the symplectites shows no geochemical difference from those primocrysts. Inter-cumulus hornblende formed during the late stage after the formation of symplectites so that the symplectites are always enclosed by hornblende. By utilizing the hornblende geothermobarometer, we have also constrained that the symplectites have been formed within a relatively wide temperature range of 1040–915 °C through a reaction between the interstitial immiscible Fe-rich melt and the primocryst olivine and orthopyroxene. The reaction can be simplified as Ol/Opx (primocryst) + Fe-rich melt → Opx (symplectite) + Mt (symplectite). Besides, based on the mass balance and reaction results, the Si/O ratio of Fe-rich melt is estimated to be 1:3. The study of these symplectites contributes to refining the processes of reactive melt flow in mafic layered intrusions during late-stage magmatic crystallization.