Low Solidification Rate Research Articles

Study on the dilution mechanism of laser-cladding AlCoCrFeNi high-entropy alloy coatings

Abstract High-performance AlCoCrFeNi high-entropy alloy (HEA) coating is manufactured by employing laser cladding to enhance the wear-resistant properties of 45# steel. Finite element simulations and experiments are combined to examine the effects of dilution rate on coating properties. The findings demonstrate that the rate of dilution increases as the speed of the laser scan increases, leading to a noticeably greater Fe concentration in the coating. Due to the low solidification rate at 4 mm/s, a moderate convection can be formed, which leaves the Fe content comparable to that of the other elements. Convection of the melt pool is exacerbated by the faster scanning speed, which allows more Fe to flow from the substrate into the coating and raises Fe concentration. The average values of microhardness are 581.54 HV, 601.86 HV, and 276.75 HV at scanning speeds of 4, 6, and 8 mm/s, respectively. Moderate dilution of the substrate can increase the microhardness, while excessive dilution can lead to a higher Fe concentration in the coating, which seriously deteriorates the properties. This study provides a guide to fabricating high-performance AlCoCrFeNi HEA coatings for 45# steel.

Tailored solidification microstructures for innovative use of high-density materials in lightweight products

As more industries move to capitalize on the technological benefits of additive manufacturing, researchers are exploring ways to design new alloys with properties that cannot be achieved through traditional manufacturing methods. One approach is to tailor the solidification microstructures of lightweight components using dense materials. This study examines the microstructures and mechanical properties of near eutectic Al-Cu alloys under different thermal histories, covering both high and low solidification rates found in various additive manufacturing techniques. Slow cooled lattice structures of diamond type unit cell were produced at a relatively low cooling rate by a hybrid investment casting process involving 3D printing of the lattice patterns, and rapid solidified powders of various sizes were generated by Impulse Atomization. Microstructural analysis revealed different eutectic morphologies and spacing depending on the cooling rate and location. The alloys strength was increased by spheroidization of their eutectic phases. The alloys eutectic structures were spheroidized using two spheroidization mechanisms, including (i) Thermo-mechanically by plastic deformation of as solidified samples, followed by heat treatment, and (ii) Chemically by addition of Mg and Si to the near eutectic Al-Cu alloy. Both the thermo-mechanical and the chemical spheroidization mechanism are found to improve the mechanical properties of the alloys. This study demonstrates a potential cost-effective use of heavy alloys in high-performance applications through additive manufacturing (e.g. using lattice structures) by optimizing microstructures and enhancing mechanical properties.

Toward sustainability for upcycling SoG-Si scrap by an immersion rotational segregation purification process

Recycling solar grade silicon (SoG-Si) scrap is critical for the sustainability of solar energy and fulfilling to close material loops in circular economics. A major challenge is to control trace impurities efficiently in short time for upgrading scrap to a level of SoG-Si. Here, a new immersion rotational separation purification (IRSP) process is developed to enhance trace impurities removal and the crystal growth from remelted metal by driving the rotating shear flow to improve the efficiency of solute and heat transfer. In a departure from conventional practice that forces extremely low solidification rate limited by the diffusion layer depth at approximately equilibrium solidification interface, here IRSP achieves the deep removal of impurities and the increase of solidification rate by the non-equilibrium rotating solidification interface. In the non-equilibrium solidification process caused by mold rotation, the rotating shear flow not only reduces the diffusion layer depth to improve impurity removal efficiency, but also increases the temperature gradient of the melt at the solidification front that is conducive to the increase of crystal growth rate. The new method is applicable to molten Si and the production of SoG-Si (99.9998%) has been demonstrated, which is able to improve the quality of recycled SoG-Si requiring a low energy input. It is expected that this process helps close high purity Si material loops, reduce resource consumption and the carbon footprint of the photovoltaic industry.

Study on the influences of different process temperatures on microstructural evolution of Mg-11%Al-2%Zn-1%Si alloy through isothermal semi-solid treatment

Due to the eutectic reaction, the Mg2Si phase is inclined to creating an unfavorable fishbone shape at low solidification rates. This can significantly degrade the mechanical characteristics and become the main barrier to the practical application of Mg-Si based alloy. It is regarded that isothermal semi-solid treatment is an efficient technique to modulate the fishbone-shape eutectic Mg2Si phases in Mg-Si based alloys. In this work, a new type of Mg-11%Al-2%Zn-1%Si alloy is successfully fabricated through isothermal semi-solid treatment, and the influences of different process temperatures on microstructural evolution of Mg-11%Al-2%Zn-1%Si alloy are studied. The findings illustrate that the mean size of the α-Mg grains increases and their trendency of spheroidization becomes more evidently after held for 30 min at different process temperatures from 550 °C to 580 °C. Additionally, through isothermal semi-solid treatment, the eutectic Mg2Si phases in experimental alloy are totally changed from their original fishbone shape into polygon or granule shapes. With increasing the process temperatures, the mean size and morphologies of eutectic Mg2Si phases have no significantly changes. The morphological change theory of eutectic Mg2Si phases through isothermal semi-solid treatment is also studied.

Effect of Mg and Si contents and TiC nanoparticles on the center segregation susceptibility of twin-roll cast Al–Mg–Si alloys

Center segregation during the twin-roll casting (TRC) has always been a barrier to the fabrication of high-performance Al–Mg–Si strips. In this work, Al–Mg–Si alloys with different Mg and Si contents were fabricated by vertical-type twin-roll caster to investigate the effect of Mg and Si contents on central segregation susceptibility. Alloys that undergo a eutectic reaction to form Si would lead to the enrichment of eutectic Si in the central region due to the low reaction temperature of eutectic Si, resulting the severe central segregation. For alloys with the same eutectic reaction during the solidification, the alloy with higher |ⅆT/ⅆ(fs1/2)| showed agglomeration of the eutectic phases due to its lower solidification rate. For the alloys with the same Mg/Si ratio, the width of central segregation increased with the increase in Mg and Si contents due to the increase in the proportion of near-equiaxed grains in the central region. The introduction of TiC nanoparticles could mitigate the central segregation band due to the control of solute diffusion by the fine near-equiaxed grain structure.

Unveiling the dynamic instability mechanism of microstructure transformation in faceted oxide eutectic composite ceramics

Microstructure control is a great challenge in the high-temperature gradient directional solidification of eutectic composite ceramics due to the complex solidification behavior. Herein, the microstructure transformation of faceted Al2O3/Er3Al5O12 thermal emission eutectic composite ceramics is explored over wide ranges of compositions (13.5 mol%–22.5 mol% Er2O3) and solidification rates (2–200 μm/s). Entirely coupled eutectics with primary phases suppressed are fabricated and the coupled zone is broadened in a wide range of 15.5 mol%–22.5 mol% Er2O3 at low solidification rates. The competitive growth between eutectic and dendrite is evaluated on the basis of the maximum interface temperature criterion. In addition, the mechanisms of irregular eutectic spacing selection and adjustment under different solidification rates are revealed based on Magnin–Kurz model. A successful prediction of lamellar to rod-like eutectics is achieved associated with the dynamic instability of lamellar eutectic and the corresponding enlarged coexistence region is mapped based on the interface undercooling. According to the well microstructure tailoring, the flexural strength of Al2O3/Er3Al5O12 eutectic composite ceramics has improved from 508 MPa up to 1800 MPa due to the refined eutectic spacing with low fluctuation. The eutectic composite ceramics show strong selective optical absorption and the intensity increases with the refining microstructure. The as-designed Al2O3/Er3Al5O12 composites with microstructural tailoring have great potential as integrations of structural and functional materials.

Influences of Different Isothermal Temperatures on Microstructural Evolution of Mg2Si/AZ91D Composite Fabricated by Semi-solid Treatment Process

Due to the eutectic reaction, the Mg2Si phase is inclined to creating an unfavorable fishbone shape at low solidification rates. This can significantly degrade the mechanical characteristics and become the main barrier to the practical application of Mg2Si/AZ91D composites. It is regarded that semi-solid treatment process is an efficient technique to modulate the fishbone-shaped Mg2Si phases in Mg2Si/AZ91D composite. In this work, a new type of Mg2Si/AZ91D composite is successfully fabricated by semi-solid treatment process, and the influences of different isothermal temperatures on microstructural evolution of Mg2Si/AZ91D composite are studied. The findings illustrate that the mean size of the α-Mg grains increases and their trendency of spherification becomes more evidently after held for 30 min at different isothermal temperatures from 550° C to 580°C. Additionally, through semi-solid treatment process, the Mg2Si phases in Mg2Si/AZ91D composite are totally changed from their original fishbone shape into polygon or granule shapes. With increasing of the isothermal temperatures, the mean size and morphologies of Mg2Si phases have no significantly changes. The morphological change theory of Mg2Si phases in Mg2Si/AZ91D composite by semi-solid treatment process is also studied.

Process and Heat Resources for Wire Arc Additive Manufacturing of Aluminium Alloy ER4043: A Review

The application of wire arc additive manufacturing (WAAM) in manufacturing has raised interest among researchers. In this paper, the introduction of additive manufacturing and wire arc additive manufacturing, various heat resources for WAAM, aluminium alloys, aluminium alloys ER4043, and performance evaluation of WAAM of ER4043 have been discussed in detail based on bead geometry, microstructure, microhardness, and tensile properties as well as the building path strategies, problems, and future directions. Based on this review, aluminium alloy 4043 (ER4043) is an Al-Si alloy frequently employed as a filler wire because it has superior fluidity and significantly fewer flaws in additively built structures. Next, dwell time and cooling efficiency during the WAAM process significantly affect bead geometry. Besides, a finer microstructure can be obtained with a better cooling rate. However, a coarser microstructure is obtained along with the increased deposition height due to heat accumulation and low solidification rate. Heat input is identified as the main cause of porosity, and CMT with a lower heat input is preferable and outperformed GTAW and GMAW in terms of mechanical properties.

Enhancing the elastocaloric effect and thermal cycling stability in dendritic-like Ni50Mn31.6Ti18.4 single crystal

Solid-state cooling based on elastocaloric effect possesses potential for refrigeration. The improvement of elastocaloric effect and thermal cycle stability is beneficial to enhance the refrigeration efficiency and service life of refrigeration equipment. Here, single-crystal Ni50Mn31.6Ti18.4 shape-memory alloys were successfully prepared using directional solidification technique. The microstructure, martensitic transformation, isothermal stress-strain responses and elastocaloric effect of the single crystals with different solidification rates have been systematically studied. The single crystal solidified at 10 µm s−1 exhibits lower stress hysteresis Δσhy and much more superior adiabatic temperature |ΔTad| than those of alloys solidified at 100 µm s−1 and 500 µm s−1, with the values of 34 MPa and 24.8 K. In addition, the single crystal at low solidification rate also performs excellent cyclic stability. All these results show that the single crystal with larger austenite primary dendrite arm spacing (PDAS) can effectively enhance the elastocaloric effect and thermal cycling stability in Ni50Mn31.6Ti18.4 alloys. This work sheds a bright light on the relationship between the elastocaloric performance and microstructure for shape-memory alloys.

The impact of employing carbon nanotube and Fe3O4 nanoparticles along with intermediate boiling fluid to improve the discharge rate of phase change material

Despite the fact that solid–liquid phase change materials (PCMs) have various applications in thermal energy storage systems, the low solidification rate of PCMs, which is due to the low thermal conductivity has limited the range of applications of PCMs. One of the methods of increasing the solidification rate of PCMs is using a boiling fluid as an intermediary between the solid–liquid PCM and the condenser to prevent the direct contact between the phase change material and the condenser tubes, this method is also known as the intermediate boiling fluid (IBF) method. The IBF method has been shown to significantly increase the solidification rate (2 orders of magnitude). In this study, the effect of nanoparticles addition on the solidification rate of PCM in the IBF method is investigated. Paraffin and acetone are used as the solid–liquid PCM and the IBF, respectively. Hence, Carbon nanotubes (CNT) and Fe3O4 as carbon-based and metallic nanoparticles, respectively, are dispersed in the acetone at three different concentrations of 1 wt%, 2 wt%, and 4 wt%. The results revealed that, compared to the case without using nanoparticles, the addition of Fe3O4 nanoparticles at concentrations of 1 wt%, 2 wt%, and 4 wt% decreases the total solidification time by 4%, 16%, and 20%, respectively. Likewise, the addition of CNT nanoparticles at the same concentrations decreased the total solidification time by 16%, 28%, and 36%, respectively. Furthermore, the addition of Fe3O4 and CNT at 4 wt% reduces the solidification start time by 20% and 30%, respectively.

Microstructure evolution, mechanical properties of FeCrNiMnAl high entropy alloy coatings fabricated by laser cladding

In this work, FeCrNiMnAl high entropy alloy (HEA) coatings under different laser energy density (Led, 50–92 J/mm2) were fabricated on 17-4PH stainless steel by laser cladding (LC). The influence of different laser energy density on the geometric morphology, microstructure, microhardness and tribological properties of HEA coating was investigated. To obtain the specific solidification parameters (temperature gradient and solidification rate), the finite element model was established to express the transient change of molten pool temperature field. The columnar-to-equiaxed transition (CET) corresponding to different zones of HEA coating was revealed. The result showed that the geometric parameters increased with the rise of Led because of the Marangoni convection. Equiaxed crystals and columnar crystals were distributed on the top and bottom zones of HEA coatings respectively, and the CET behavior was observed. Higher temperature gradient and lower solidification rate were conducive to the growth of columnar-crystals. Furthermore, at the Led of 92 J/mm2, HEA coatings showed relatively high crack sensitivity. HEA coatings significantly improved the tribological properties owing to metal oxide particles generated by oxidation. In the mechanical properties test, the maximum microhardness of prepared HEA coating (1800 W) was 587.83 HV0.2, which was 1.63 times that of 17–4PH substrate. Metal oxide particles generated a flat wear-resistant enamel layer which protected the coating surface. The tribological properties was optimal at the Led of 64 J/mm2, the average friction coefficient and wear rate were 0.4654 and 7.026 × 10−8 mm2/N, respectively.

Sensitivity analysis of design parameters for melting process of lauric acid in the vertically and horizontally oriented rectangular thermal storage units

Widespread commercialization of PCM-based latent heat storage systems is limited by the low melting and solidification rates during the phase transition process. In this study, fins are used to enhance the phase change process. A parametric study is conducted to understand the effect of fins on the thermal performance of vertically- and horizontally-oriented rectangular storage tanks. Throughout simulations, the fin volume, and thereby the mass of PCM in the tank, were kept constant. It was observed that the horizontal enclosures can take advantage of the development of strong natural convection flow until near the end of the melting process, whereas with vertical counterparts the strength of the convection currents was diminished during the shrinkage stage. The results suggest that longer and thinner fins are more beneficial for enhancing the melting rate than shorter and thicker fins. It was concluded that for horizontal enclosures with fin lengths of 25 and 35 mm, increasing the number of fins does not necessarily shorten the melting time. The maximum melting time reduction compared to the 3-fin vertical enclosure with a fin length of 25 mm (chosen as our benchmark case) was 75.1% when nine 45-mm-long fins are used in a horizontal enclosure.

Unique strength-ductility balance of AlCoCrFeNi2.1 eutectic high entropy alloy with ultra-fine duplex microstructure prepared by selective laser melting

• A highly densed AlCoCrFeNi 2.1 eutectic high entropy alloy with ultra-fine structure and remarkable strength-ductility balance is fabricated by selective laser melting (SLM). • The ultra-fine grains and heterogeneous eutectic structures are introduced into the AlCoCrFeNi 2.1 due to the rapid solidification. • The SLM-ed AlCoCrFeNi 2.1 presents an excellent match of high tensile strength (1271 MPa), yield strength (966 MPa), and good ductility (22.5%) at room temperature. • A fan blade with smooth and bright surface is successfully produced with gas-atomized powders. As a typical dual-phase eutectic high entropy alloy (EHEA), AlCoCrFeNi 2.1 can achieve the fair matching of strength and ductility, which has attracted wide attention. However, the engineering applications of as-cast AlCoCrFeNi 2.1 EHEAs still face challenges, such as coarse grain and low yield strength resulting from low solidification rate and temperature gradient. In this study, selective laser melting (SLM) was introduced into the preparation of AlCoCrFeNi 2.1 EHEA to realize unique strength-ductility balance, with emphasis on investigating the effects of processing parameters on its eutectic microstructure and properties. The results show that the SLM-ed samples exhibit a completely eutectic structure consisting of ultra-fine face-centered cubic (FCC) and ordered body-centered cubic (B2) phases, and the duplex microstructure undergoes a morphological evolution from lamellar structure to cellular structure as laser energy input reducing. The SLM-ed AlCoCrFeNi 2.1 EHEA presents an excellent match of high tensile strength (1271 MPa), yield strength (966 MPa), and good ductility (22.5%) at room temperature, which are significantly enhanced by the ultra-fine grains and heterogeneous structure due to rapid solidification rate and high temperature gradient during SLM. Especially, the yield strength increment of ∼50% is realized with no loss in ductility as compared with the as-cast samples with the same composition. On this basis, the precise complex component with excellent mechanical properties is well achieved. This work paves the way for the performance improvement and complex parts preparation of EHEA by microstructural design using laser additive manufacturing.

Effect of laser remelting on the microstructure and mechanical properties of AerMet100 steel fabricated by laser cladding

AerMet100 ultrahigh-strength steel is used in safety-critical applications with limited repair methods. A repair technology using laser cladding (LC) has recently been developed, but it also has limitations due to mechanical anisotropy of the columnar crystal in the cladding layer. Herein, laser remelting (LR) is a process in which an LR layer with lower power density but without powder delivery has been applied after each LC layer is deposited to inhibit the epitaxial growth of columnar crystals and relieve the mechanical anisotropy. It was found that the LR process melted columnar crystal tips and rod-shaped carbides of the LC layer of AerMet100 steel into fragments and smaller sized granular carbides, respectively. In the LR melt pool, the fragments blocked the columnar solidification front to hinder its growth, or nucleated at the bottom of the melt pool to form short-thick columnar and equiaxed crystals to inhibit the epitaxial nucleation of the columnar crystals. The undissolved granular carbides clustered at the tips of the columnar dendrites and stopped the growth of columnar crystals. Moreover, the undissolved carbides also served as heterogeneous nucleation sites in the LR melt pool, extending the equiaxed crystal solidification zone to higher thermal gradient and lower solidification rate, which resulted in the formation of the LR layer whose microstructure morphology was mainly equiaxed grains. The AerMet100 forged steel repaired by the LC plus LR process minimised the impact of the crystal texture of the LC layers on the mechanical anisotropy due to the slowdown of crack propagation in the equiaxed crystal zone. It exhibited superior mechanical properties to those of AerMet100 steel repaired by LC alone.

Effect of casting thickness on microstructure and mechanical properties of the high-W superalloy K416B

The high-W superalloy K416B is often used in aero-engine turbine blades. These have a varied casting thickness, and this may cause significant differences in the macrostructure, microstructure and mechanical properties of the blade. In this study, five castings of different thicknesses were produced by a specimen module which shared the same “cast-mold-environment” in order to simulate the varied thicknesses across a turbine blade. The macro and microstructure of specimens with different cast thicknesses were analyzed by optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), electron probe microanalyzer (EPMA) and transmission electron microscopy (TEM). Microhardness and 975 °C/ 235 MPa rupture tests were performed to investigate the differences in mechanical properties. The results indicated that a larger cast thickness caused a lower solidification rate and increased element microsegregation. The relationship between the cast thickness H and secondary dendrite arm spacing λ2 was λ2 = 11.39H0.55. The average area of eutectic γ′ and MC carbide phases was proportional to cast thickness. In addition, abnormal fine grains were observed at the bottom of the 40 mm specimen and the top of the 20 mm specimen. Large size α-W phases could be observed in these regions and are suggested to be the cause of the grain refinement. The hardness test results showed that the hardness of the specimen decreased as the cast thickness increased. The 975 °C/235 MPa rupture test results showed that the rupture property decreased in the 40 mm specimen, but that there was no significant alteration in the 3–20 mm specimens. These results provide a guidance for turbine blades manufacturing made from this high W cast polycrystalline superalloy.

Effect of discrete deposition and segregated heat-treatment on the macro- and microstructure, and tensile properties of large-scale titanium alloy components by laser melting deposition

The challenge of fabrication large-scale laser melting deposited titanium alloy (LLMDT) components was high residual stress, warpage deformation, and even fracture during long-time continuous deposition. To address this challenge, a method named discrete deposition and segregated heat-treatment (DDSH) was used to fabricate the LLMDT components and its effect on the macro- and microstructure, and tensile properties have been systematically studied. The results showed the macrostructure of the DDSH interface region showed columnar grains due to its higher thermal gradient and lower solidification rate than them in other LLMDT parts with columnar grains and equiaxed grains. The microstructure in the whole LLMDT components consisted of ultrafine basket-weave microstructure. The average width of the α lamellae in the DDSH interface region and other LLMDT parts was 0.38–0.424 μm and 0.477–0.54 μm, respectively. The average width of α lamellae was finer in the DDSH interface region than that in the other LLMDT parts owing to its higher thermal gradient and higher cooling rate compared with the other LLMDT parts. For the LLMDT components in different directions with the same region, the UTS and the YS in the H-sample were slightly higher than those in the V-sample, while the EL in the H-sample was lower than those in the V-sample. The anisotropic ductility was owing to the continuous grain boundary exhibited different fracture mechanisms under different directions with loading stress. For the LLMD components in different regions with the same direction, the highest fluctuation ratio of the UTS, the highest fluctuation ratio of the YS, and the highest fluctuation ratio of the EL was 0.012%, 0.08%, and 0.145%, respectively. The reason was the average with of the α lamellae in the whole LLMDT components showed a little difference.

Element Distribution and Its Induced Peritectic Reaction during Solidification of Ti-Al-Nb Alloys

The element distribution and the microstructures of directionally solidified ingots of Ti-45Al-8Nb and Ti-46Al-8Nb alloys were studied by scanning electron microscope (SEM) and electron probe microanalyzer (EPMA) equipped with wavelength-dispersive X-ray spectroscope (WDS). At high solidification rates, e.g., more than 50 μm/s, the ingot solidified in columnar β dendrites, while at low solidification rates, e.g., less than 30 μm/s, the solidification path changed from initial β solidification to L + β→α peritectic solidification, forming cellular dendrites with the β phase matrix surrounded by the α phase. The difference of Ti content in dendritic arms and interdendritic regions was not pronounced. The composition segregation was mainly caused by the mutual conversion of Al and Nb contents. Therefore, it was difficult to distinguish the variation of Ti in microstructure by EPMA-WDS map and line profiles. The composition of the peritectic α phase was different from that of the α phase transformed directly from the β phase. The Al content of the former was about 1 at% higher than that of the latter, while the Nb content was about 1 at% lower. The change of solidification path in the final solidified part resulted from the more severe segregation caused by slow solidification.

Influences of platform heating and post-processing stress relief treatment on the mechanical properties and microstructure of selective-laser-melted AlSi10Mg alloys

This study focused on two essential issues in the fabrication strategy utilized in selective laser melting (SLM) technology: 1) the influence of the hot-build platform and 2) the effects of a post-processing stress relief (SR) treatment on the mechanical properties and microstructure of an AlSi10Mg alloy manufactured by SLM. To examine the mechanical properties, surface hardness measurements and split Hopkinson pressure bar (SHPB) experiments were conducted on samples in the as-fabricated condition and following SR treatment. The samples were extracted from the original SLM product at constant distances from the build platform. Considerable variations in the mechanical properties and damage accumulation resulting from the fabrication process and subsequent post-processing SR treatment were successfully correlated to fundamental characteristics in terms of relative porosity, “on-surface” residual stress, microstructure and texture, solubility, and phase composition. It was found that with increasing distance from the heated build platform, there was a graded increase in the surface hardness and dynamic performance, which are attributed to several competing strengthening mechanisms that were activated owing to the fast cooling rates. Conversely, the reduced thermal gradient and lower solidification rate close to the base led to a higher relative density, as indicated by the smaller size of the keyhole pores. The SR treatment resulted in microstructural changes with uniform and low residual stresses, which led to a significant softening in the mechanical properties, regardless of the building height.

A comprehensive study on the complete charging-discharging cycle of a phase change material using intermediate boiling fluid to control energy flow

The low melting and solidification rates of phase change materials (PCM), which traces back to their low thermal conductivity coefficient, has led the application of these materials to face limitations. This paper aims to explore the effectiveness of a novel method called intermediate boiling fluid (IBF) in speeding up the energy storage and transfer processes in PCMs during a complete charging-discharging cycle. Throughout this novel technique, paraffin and acetone are utilized as PCM and IBF, respectively. In the solidification process, there is no direct contact between the cold source and the molten paraffin, while acetone, as an intermediate fluid, is being boiled via absorbing paraffin's heat and ultimately causing paraffin to be cooled down and solidified. The melting and solidification experiments were run in a test cell with and without acetone. The experimental results indicate that utilizing this technique dramatically enhances the solidification rate and improves the melting rate to a moderate level. It is illustrated that by using this method under the optimum condition the solidification time, melting time, and the total melting and solidification time decrease by 77 times, 22 percent, and 80 percent, respectively, compared to the conventional method. It is also concluded that by adjusting the container pressure and using different amounts of intermediate boiling fluid (IBF), the freezing and melting rates of phase change materials can be controlled.

Insights into the impact and solidification of metal droplets in ground-based investigation of droplet deposition 3D printing under microgravity

Droplet deposition 3D printing is an additive manufacturing technique offering a great potential for metal parts fabrication in space, of which some preliminary testing is usually performed on ground in the early research. The previous work (Huang et al., 2020) mimicked the anti-gravity deposition of molten metal droplet through manipulating it into perpendicularly depositing on a vertical substrate. However, the ground-based simulation of droplet deposition 3D printing under microgravity remains an elusive goal, since the spreading and receding processes are still affected by gravity. To address this issue, the prevailing physical mechanisms of gravity effect on droplet impact and solidification are urgent to be defined. Here, we present the studies on the impact dynamics and transient solidification of the molten metal droplet deposited on vertical substrates through numerical modeling and experiments. It is observed that the spreading and retraction of the droplet are asymmetric, besides its solidification shape tilts to gravitational direction. The formation mechanisms of these undesired behaviors are further demonstrated. The results show that the asymmetrical spreading, retraction and solidification shape of the droplet originate from the interaction of gravity and solidification. Moreover, the tilt of the solidified droplet has a correlation with the critical process parameters, i.e. impact velocity, temperatures of droplet and substrate. With a larger impact inertia and a lower solidification rate, the undesired solidification shape can be effectively eliminated. This work provides a foundation for the further investigation of the ground-based physical simulation of outer space droplet deposition 3D printing.