Rapid Solidification Process Research Articles

A Novel Approach to Investigate the Superheating Grain Refinement Process of Aluminum-Bearing Magnesium Alloys Using Rapid Solidification Process.

The superheating process is a unique grain refining method found only in aluminum-containing magnesium alloys. It is a relatively simple method of controlling the temperature of the melt without adding a nucleating agent or refining agent for grain refinement. Although previous studies have been conducted on this process, the precise mechanism underlying this phenomenon has yet to be elucidated. In this study, a new approach was used to investigate the grain refinement mechanism of aluminum-containing magnesium alloys by the melting superheating process. AZ91 alloy, a representative Mg-Al alloy, was used in the study, and a rapid solidification process was designed to enable precise temperature control. Temperature control was successfully conducted in a unique way by measuring the temperature of the ceramic tube during the rapid solidification process. The presence of Al8Mn5 and Al10Mn3 particles in non-superheated and superheated AZ91 ribbon samples, respectively, manufactured by the rapid solidification process, was revealed. The role of these Al-Mn particles as nucleants in non-superheated and superheated samples was examined by employing STEM equipment. The crystallographic coherence between Al8Mn5 particles and magnesium was very poor, while Al10Mn3 particles showed better coherence than Al8Mn5. We speculated that Al10Mn3 particles generated by the superheating process may act as nucleants for α-Mg grains; this was the main cause of the superheating grain refinement of the AZ91 alloy.

Uniform Droplet Spraying of Magnesium Alloys: Modeling of Apollonian Fractal Structures on Micrograph Sections

A variety of advanced manufacturing processes have been developed based on the concept of rapid solidification processing (RSP), such as uniform droplet spraying (UDS) for the additive manufacturing of metals and alloys. This article introduces a morphological simulation of fractal dendric structures deposited by UDS of magnesium (Mg) alloys on two-dimensional (2D) planar sections. The fractal structure evolution is modeled as Apollonian packs of generalized ellipsoidal domains growing out of nuclei and dendrite arm fragments. The model employs descriptions of the dynamic thermal field based on superposed Green’s/Rosenthal functions with source images for initial/boundary effects, along with alloy phase diagrams and the classical solidification theory for nucleation and fragmentation rates. The initiation of grains is followed by their free and constrained growth by adjacent domains, represented via potential fields of level-set methods, for the effective mapping of the solidified topology and its metrics (grain size and fractal dimension of densely packed domains). The model is validated by comparing modeling results against micrographs of three UDS-deposited Mg–Zn–Y alloys. The further evolution of this real-time computational model and its application as a process observer for feedback control in 3D printing, as well as for off-line material design and optimization, is discussed.

Formation of Twin Boundaries in Rapidly Solidified Metals through Deformation Twinning.

The rapid solidification process is relevant to many emerging metallurgical technologies. Compared with conventional solidification processes, high-density microstructure defects and residual thermal stress are commonly seen in rapidly solidified metals. Among the various defects, potentially beneficial twin boundaries have been observed in the rapidly solidified nanocrystalline microstructures of many alloy systems. In this work, a pathway for forming twin boundaries in rapid solidification processes is proposed. A detailed derivation of strain inhomogeneities upon thermal shrinkage and the deformation twinning phase field method is given. By calculating cooling-induced thermal strain inhomogeneity in nanocrystalline metals and growth thresholds for deformation twinning using the phase field method, it is shown that residual thermal strain hotspots in the microstructure can reach the threshold for deformation twinning when the shear elastic property of grain boundaries is significantly different from the bulk.

Study on the microstructure evolution mechanism of copper single phase alloys under deep undercooling conditions

This article mainly used the deep undercooling technology to treat Cu60Ni40, Cu60Ni38Co2, and Cu60Ni35Co5 alloys to obtain different undercoolings. By using high speed cameras, the rapid solidification process was recorded to analyze the relationship between its evolution law and the magnitude of undercooling. When observing its microstructure, it was also found that there was a critical undercooling degree throughout its entire solidification process. The influence of Co element addition on solidification rate, recalescence effect, and critical undercooling of copper based single-phase alloys under undercooling conditions was explored. EBSD analysis was conducted on the refined microstructure of the alloy that achieved the maximum undercooling, and the significance of grain orientation and orientation difference distribution was found. TEM tests were conducted on a Cu60Ni38Co2 alloy with a maximum undercooling of 259 K, and it was found that there are high-density dislocation networks in some areas within them. Finally, the Vickers hardness of the above alloys was measured and recorded using a microhardness tester (HVS-1000Z), and the evolution between microhardness and undercooling, as well as the changes in microhardness under critical undercooling, were analyzed. Based on the analysis of EBSD, TEM, and microhardness data above, it is found that grain refinement at high undercooling is dominated by recrystallization.

Effect of Cu and Cr addition on the structure, anticorrosion and nanomechanical properties of new Al-Ni-Fe-(Cr,Cu) alloys

This work used a thermodynamic approach to design and investigate new complex compositional Al-Ni-Fe-(Cr,Cu) alloys based on aluminum. This work tends to understand the change in microstructure during the rapid solidification process and to evaluate the anticorrosion and nanomechanical properties of the developed alloys. Optimizing thermodynamic parameters such as configurational entropy and Gibbs free energy was used to predict the chemical composition of the studied alloys. The samples were cooled in two ways. The ingots were slowly cooled, while the plates were cast using fast cooling by the high-pressure die-casting method. The presence of a quasicrystalline decagonal phase D-Al71Ni24Fe5 was identified together with crystalline Al3Ni, Al3Ni2, and Al9Ni1.3Fe0.7 phases for the rapidly solidified Al72Ni24Fe4 alloy. The best electrochemical parameters were observed for the Al72Ni24Fe2.5Cr1.5 alloy. The local galvanic microcells were formed in the studied alloys due to large potential differences (>50 mV) between the Al-Ni and Al-Ni-Fe phases. The highest average indentation hardness values were observed for Al72Ni24Fe4 (9.98 ± 1.75 GPa) after normal and rapid solidification. The higher ductility of the Al72Ni24Cr1.5Fe2.5 alloy compared to Al72Ni24Fe4 and Al72Ni24Cu1.5Fe2.5 could be confirmed by the lowest average hardness and Young's modulus values.

Nanoparticles induced intragranular and dislocation substructures in powder bed fusion for strengthening of high-entropy-alloy

In recent years, high entropy alloys (HEAs) have attracted considerable attention because of their excellent mechanical and functional properties. Despite this, as-cast high entropy alloys cannot meet the requirements for structural parts with high strength. Powder bed fusion (PBF) technology and nanoparticle additives are expected to enhance the overall properties of HEAs. We present here an AlN nanoparticle-reinforced high-entropy alloy matrix nanocomposite fabricated by powder bed fusion with excellent mechanical properties. Experimental results demonstrate that the addition of 1 wt% AlN nanoparticles results in an increase in the density of dislocations and subgrain boundaries within the HEA, in turn increasing its yield strength, ultimate tensile strength by 27.7%, 23.9% respectively, while elongation decreases slightly. In addition, the fracture toughness of the AlN/FeNiCrCo composite is greater than that of FeNiCrCo HEA because of the well-balanced combination of strength and ductility. The formation of the substructure of HEA grains was investigated using molecular dynamics (MD) by investigating the effects of rapid melting and solidification during SLM of AlN/FeNiCrCo composite. It is found that during the rapid solidification process, a large number of subgrain boundaries and dense dislocation network structures are formed in the grains due to the heat transfer and the difference in lattice structure between nanoparticles and HEA. These findings agree with the experimental results. The substructures contribute significantly to the composite's strength while maintaining its ductility, which allows it to be more suitable for practical applications.

Microstructure and hardness of laser remelted surfaces of Al-5%Cu and Al-4%Cu-1%Ni alloys

Al-based alloys are potential candidates for advanced rapid solidification processes, such as Laser Surface Remelting (LSR) and Additive Manufacturing (AM). However, currently few alloy candidates have emerged for use such as AlSi12 eutectic and AlSi10Mg. This occurs because other alloys are more susceptible to the formation of defects such as porosity, cracks and distortions, which impair high-performance applications. The addition of Ni in Al-Cu alloys is expected to improve the mechanical properties at high temperatures and favors the reduction of the solidification interval, which may decrease the fraction of solidification cracking and porosity. Given the context, the present work investigates the microstructural changes and hardness of Al-5 wt%Cu and Al-4 wt%Cu-1 wt%Ni alloys processed either by centrifugal casting or LSR. By varying laser beam speed and laser average power, a total of 4 combinations for each alloy was accomplished. The CALPHAD computations with 1%Ni alloying revealed a solidification interval reduction of approximately 30%, without reducing the total intermetallics fraction if compared to the binary Al-5%Cu alloy. The microstructure of the rapidly solidified Al-Cu-Ni samples was characterized by a dendritic α-Al matrix, surrounded by a α-Al+Al2Cu+Al7Cu4Ni constituent. A growth transition from epitaxial (bottom) to elliptical/globular (center) was observed in the molten pools with a significant cell spacing reduction from 7.5 µm (as-cast) to 0.6 µm (LSR), which favors an increase in Vickers hardness of approximately 85% if compared to the as-cast samples. The molten pool of the ternary alloy reached a higher hardness of 120 HV as a result of both AlCu and AlCuNi intermetallics acting mutually to strengthen the α-Al matrix.

Effect of Cooling Rate on the Crystalline Morphology of Bi2O3‐Fe2O3 Pseudo‐Binary System

Herein, the crystallographic morphology of the 80%Bi2O3‐20%Fe2O3 pseudo‐binary system under different cooling rates (100, 6.3, 0.12 K s−1) is investigated using an optical microscopy system. The interfacial morphology evolution and interfacial kinetics in two dimensions during crystal growth are systematically studied. At the low cooling rate (ΔT < 10 K), blade‐like crystals appear slowly in the melt. In turn, the emergence of numerous irregularly shaped grains in the melt at the high cooling rate (ΔT > 100 K) indicates a rapid solidification process. Tetragonal crystals appear typically during melt solidification accompanied with triangular crystals simultaneously (15 K < ΔT < 35 K). The X‐ray diffraction and EDS data reveal that both kinds of polygonal crystals are attributed to Bi25FeO40. In addition, the crystal growth kinetics are presented as the dependence of the growth rate on the subcooling, and thus the growth mechanism of crystals is discussed. According to the results, Bi25FeO40 crystals grow through a two‐dimensional nucleation mechanism.

An additively manufactured heat-resistant Al-Ce-Sc-Zr alloy: Microstructure, mechanical properties and thermal stability

In this study, a high-strength and heat-resistant Al–Ce–Sc–Zr alloy was designed and fabricated by additive manufacturing. The as-built alloy shows an excellent combination of strength and ductility compared to other Al–Ce-based alloys, including an ultimate tensile strength of 445 MPa, a yield strength of 344 MPa and an elongation of 10%. Even at 300 °C, the alloy still retains a superb tensile yield strength of 233 MPa, which is the best among the high-temperature aluminum alloys reported so far. The excellent performance is mainly attributed to a significant amount of fine, homogeneous and coarsening-resistant Al11Ce3 intermetallic nanoparticles formed during the rapid solidification process of laser additive manufacturing. The formation of coherent Al3(Sc, Zr) nano-precipitates and interfacial segregation at the interface of intermetallic nanoparticles during high temperature exposure further improve the high temperature properties of the alloy. Our work has the potential to provide design concepts for the development of novel high temperature aluminum alloys.

Effects of pressure on microstructure evolution of liquid Fe–S–Bi alloy during rapid solidification: A molecular dynamics study

To understand the effects of pressure on microstructural evolution, a molecular dynamics simulation study has been performed under pressures of 0–20 GPa for liquid Fe–S–Bi alloy during the solidification process. The variations in the radial distribution function, average atomic energy, and H-A bond index of the cooling system are analyzed. The rapid solidification process of liquid Fe–S–Bi alloy into crystalline and amorphous alloys is investigated from different perspective. The results show that the glass transition temperature Tg, the sizes of the MnS atomic groups, and major bond-types increase almost linearly with increasing pressure. In addition, the recovery rate of Bi increased first and then decreased with increasing pressure, reaching a peak of 68.97% under 5 GPa. The manganese sulfide compound is embedded in the alloy with a spindle-shape under 20 GPa, which is a better clusters structure.

The Nature of Plasma Spraying

By its nature, plasma spraying is a rapid solidification process in which finely powdered material injected into a plasma jet is almost instantly melted and propelled with high velocity, created by a strong magnetohydrodynamic force against a suitable surface [...]

Microstructure refinement mechanisms in undercooled solidification of binary and ternary nickel based alloys

Molten glass purification and cycle superheating technologies were used to make Ni65Cu35, Ni65Cu33Co2 and Ni65Cu31Co4 alloys obtain maximum undercoolings of 320 K, 292 K and 300 K respectively. In order to analyze the relationship between morphological characteristics of solidification front and undercooling change during migration of solid–liquid interface, a high-speed camera was used to capture pictures of the recalescence process. Observing the microstructure of the undercooled alloys using metallographic microscope, the characteristics and evolution of microstructure during rapid solidification process of undercooled liquids were analyzed. It was found that grain refinement mechanisms of highly undercooled Ni–Cu–Co alloys was the same as those of the Ni–Cu alloys. Dendrite remelting leads to the grain refinement at low undercooling, while the dominant factor of grain refinement at high undercooling is recrystallization process induced by stress. The internal driving force can be divided into two parts: one is the thermal stress generated by the releasing of solidification latent heat during recalescence process, and the other is the stress and strain accumulated by interaction of liquid flow and primary dendrite during rapid solidification. We also found that addition of third element Co not only played an important role in solidification rate and recalescence effect, but also significantly improved the average hardness of grain refined microstructure, which was about 80% higher than that of as cast alloy. The addition of trace Co was also conducive to the formation of non-segregation solidification microstructure.

Strategies to enhance hydrogen storage performances in bulk Mg-based hydrides

Bulk Mg-based hydrogen storage materials have the potential to provide a low-cost solution to diversify energy storage and transportation. Compared to nano powders which require handling and processing under hydrogen or an inert gas atmosphere, bulk Mg-based alloys are safer and are more oxidation resistant. Conventional methods and existing infrastructures can be used to process and handle these materials. However, bulk Mg alloys have smaller specific surface areas, resulting in slower hydrogen sorption kinetics, higher equilibrium temperatures, and enthalpies of hydride formation. This work reviews the effects of the additions of a list of alloying elements and the use of innovative processing methods, e.g., rapid solidification and severe plastic deformation processes, to overcome these drawbacks. The challenges, advantages, and weaknesses of each method and future perspectives for the development of Mg-based hydrogen storage materials are discussed.

Improved passivation ability via tuning dislocation cell substructures for FeCoCrNiMn high-entropy alloy fabricated by laser powder bed fusion

Elemental-decorated dislocation cell is a specific substructure in additively manufactured metals and alloys due to the intrinsic layer-by-layer rapid melting and solidification process, and the novel cellular structures would have significant influences on the service performances of the printed components. In this work, sub-micro Mn-decorated dislocation cells are verified in FeCoCrNiMn high-entropy alloys (HEAs) fabricated via laser powder bed fusion (L-PBF), and the related passivation capabilities are comparably investigated with the as-casted and heat-treated L-PBF counterparts (HEA-800, 800 °C for 2 h). The passive current density of L-PBF HEA-800 is the lowest in borate buffer solutions, while the as-casted HEAs show the highest corrosion rate therein. The high-density dislocation and the alleviation of Mn segregation at the cellular boundaries are conducive to the superior protectiveness (stable and thick passive film) for L-PBF HEA-800 and the underlying mechanisms for different passivation behaviors are discussed.

Fabrication, microstructural modifications, elastic properties and radiation attenuation performance of ZnO nano-sized particles-reinforced Pb-based alloys for radiation shielding applications

We intended to obtain new appropriate materials suitable for radiation protection applications. Therefore, the choice of appropriate materials in industrial and medical applications has become one of the major research topics. An attempt has been made to conceive the possibility of using different compositions Sn-40Pb, Sn-39.5Pb-ZnO, Zn-40Pb and Zn-39.5Pb-ZnO were prepared and synthesized by the melt-spun process. This work aims to investigate the effects of rapid solidification processing and loading 0.5 wt.% ZnO NPs into the Sn- and Zn-based binary systems and the stability of the structure, mechanical and shielding properties. The structure revealed that the plain and composite alloys contain the crystalline β-Sn (body-centred tetragonal), α-Pb (cubic) Fm-3 m for the Sn-Pb system in addition to α-Pb(fcc), Zn (Hexagonal) P63/mmc and revealed ZnO (Zincite) P63mc for the Zn-Pb system. Doping ZnO nano-sized cause refines the eutectic structure as revealed in microstructural morphology. The experimentally measured density values for the plain and composite alloys have been used in evaluating the linear attenuation coefficient, which is further used to calculate mass, attenuation coefficient, mean free path, half value layer and tenth value layer. Furthermore, the variation of a mean free path, HVL and TVL with incident photon energy for the binary and composite alloys has been investigated. It was found that nano-ZnO-doped Sn-Pb and Zn-Pb enhance the amount of radiation absorbed in the material inside and as a result of which affected the radiation attenuation properties of the material. It was concluded and recommended that the composite alloys containing ZnO NPs can be used effectively as a shielding material for gamma rays with a comparable attenuation percentage compared with plain alloys.

Selection of solidification pathway in rapid solidification processes

Rapid solidification processing of alloys enables the formation of exotic nonequilibrium microstructures. However, the interrelationship between the processing parameters and the resulting microstructure is yet to be fully understood. In melt spinning (MS) and additive manufacturing (AM) of rapidly solidified alloys, opposite microstructure development sequences were observed. A fine-to-coarse microstructural transition is typically observed in melt-spun ribbons, whereas melt pools in AM exhibit a coarse-to-fine transition. In this paper, the microstructural evolutions during these two processes are investigated using phase-field modeling. The variation of all key variables of the solid-liquid interface (temperature, composition, and velocity) throughout the entire rapid solidification of AM and MS processes was acquired with high accuracy. It is found that the onset of nucleation determines the selection of the solidification pathway and, consequently, the evolution of temperature and velocity of the interface during the rapid solidification. The switching of control mechanisms of the solid-liquid interface, which happens in both processes but in opposite directions, is found to cause the velocity jump and disrupt the microstructure development.

Phase-Field Simulation of Microstructure Formation in Gas-Atomized Al–Cu–Li–Mg Powders

Al-Cu-Li (2xxx series) powders for additive manufacturing processes are often produced by gas atomization, a rapid solidification process. The microstructural evolution of gas-atomized powder particles during solidification was investigated by phase-field simulations using the software tool MICRESS. The following topics were investigated: (1) the microsegregation of copper and lithium in the particle, and the impact of lithium addition on the formation of secondary phases in Al-2.63Cu and Al-2.63Cu-1.56Li systems, (2) the effect of magnesium on the nucleation and final mass fraction of T1 (Al2CuLi) growing from the melt, and (3) the effect of increased magnesium content on the T1 and S' (AlCu2Mg) phase fractions. It is observed that the addition of lithium into the Al-Cu system leads to a decrease in the solid solubility of copper in the primary matrix; consequently, more copper atoms segregate in the interdendritic regions resulting in a greater mass fraction of secondary precipitates. Our result agrees with findings on the beneficial impact of magnesium on the nucleation and precipitation kinetics of T1 precipitates in the conventional casting process with further thermomechanical heat treatments. Moreover, it is observed that the increase in magnesium from 0.28 wt.% to 0.35 wt.% does not significantly affect the nucleation and the amount of the T1 phase, whereas a decrease in T1 phase fraction and a delay of T1 formation are encountered when magnesium content is further raised to 0.49 wt.%.

Manipulation of the microstructure and properties of La(Fe,Si)13 alloys via solidification kinetics

The microstructural features of as-cast alloys are predominantly influenced by the fabrication process, which eventually determine the properties. In the present work, we demonstrate that the microstructure (e.g., composition, phase constitution and distribution) and the magnetocaloric properties of the La(Fe,Si)13 alloys can be effectively tailored by tuning the solidification kinetics. La(Fe,Si)13 alloys with different dimensions were prepared by suction casting and jet casting. Finite element (FE) simulations reveal a difference in the temperature gradient along the solidification path of the rapid solidification process. The microstructure difference between the edge and center regions in the as-cast ingots along the solidification direction is increased for the samples with larger diameter. This can be attributed to the different solidification kinetics as predicted by the FE simulations. More homogeneous microstructure can be observed in the samples with a smaller size. The largest magnetic entropy change value of 16 J/kg·K can be obtained in thin plates, which is comparable to that of master bulk alloy. Our thorough study of solidification mechanisms during rapid solidification provides research basis for manipulating the microstructure homogeneity and the related properties, which also paves the way for the study of other rapid-solidificated materials.

Supersaturated solid solution enhanced biodegradable Zn-Mn alloys prepared by mechanical alloying and selective laser melting

Zn-Mn alloys have been recognized as promising biodegradable metals due to moderate degradation and good osteogenesis. However, the alloying content of Mn has been severely limited by the low solid solubility (0.8 wt%) in conventional preparation process to avoid the excessive formation of brittle intermetallics. In this work, supersaturated solid solution was achieved in Zn-Mn alloys via mechanical alloying (MA) and selective laser melting (SLM). During MA process, the mixed Zn and Mn powders experienced repeated welding and fracturing, which resulted in refined grains and abundant crystal defects that stored extra energy. This could reduce the energy barrier of interatomic diffusion and induce the rapid diffusion of Mn atoms, leading to supersaturated solid solution with ∼3.6 wt% Mn homogeneously dissolved in Zn lattices. Moreover, the rapid solidification process of SLM effectively maintained the metastable supersaturated structure. In addition, the Zn-15Mn alloys prepared by MA and SLM were composed of fine equiaxed grains (∼4.34 µm) and high content of high-angle grain boundaries (54.8%). Owing to solution strengthening and grain refinement, the Zn-15Mn alloy prepared by MA and SLM exhibited a compressive yield strength of 178 MPa which was 2.8 times and 1.6 times those of pure Zn and Zn-15Mn alloy without MA process, respectively, as well as a more ductile fracture. Meanwhile, the Zn-Mn alloys exhibited accelerated degradation and comparable cytocompatibility to pure Zn. These findings indicated that the combined process of MA and SLM may be an efficient strategy for the preparation of alloys characterized by supersaturated solid solution and refined grain structure.

Effect of surface profiling on the mechanical properties and bond behaviour of mineral-impregnated, carbon-fibre (MCF) reinforcement based on geopolymer

Mineral-impregnated, carbon-fibre composites (MCF) are a new and promising reinforcement type to become substitutes for conventional steel reinforcements or fibre-reinforced polymers. To enhance this reinforcement’s shape stability and load transfer capability to concrete matrices, a surface profiling of this novel material needs to be developed. To this end the automatic, auxiliary helical winding of a thread was implemented to produce semi-finished MCF based on geopolymer (GP) with defined geometrical features. Subsequently, a rapid solidification process was conducted by means of thermally activated geopolymerization of various durations.The results showed that applied surface profiling densified the matrix microstructure of the MCFs and improved shape stability during processing. However, flexural and tensile properties were slightly negatively impaired due to the stress concentrations induced. Furthermore, curing prolonged from 2 to 8 h enhanced the geopolymerization of the matrix gradually and therewith the mechanical performance of the MCFs in their entirety, as confirmed by morphological investigation. Uniaxial tension tests demonstrated that the strengths of all rod variants were in the same range as that of conventional CFRP. Enhanced bond properties were found for MCF with the profiling technology as developed, enabling defined load-bearing behaviour for subsequent application.