Rapid Solidification Process Research Articles

In situ study on fracture behaviors of SiC/2A14Al composite joint： Co-construction of microstructure and mechanical properties via laser welding

SiC p /Al matrix composite (SiC p /AMCs) has been widely used in aerospace field of structural frames, but it is difficult to realize the lightweight connection of complex structural parts. Understanding the fracture mechanism from the source will provide a decisive insight into the preparation (e.g. additive manufacturing) and processing (e.g. welding) of new high strength lightweight aluminum matrix composite. In this work, the fracture characteristics and the correlative damage mechanisms near fracture crack tip in a notorious SiC/2A14Al joint was conducted by in-situ scanning electron microscopy tensile test and transmission electron microscopy . The sandwich eutectic microstructure Mg 2 Si–Al–AlCu of clean interface and micron scale heterogeneous heterostructure inhibited the crack propagation . The sandwich eutectic microstructure disperses the extra stress caused by deformation, and the dislocation multiplies and moves at the interface of Mg 2 Si–Al and Al–AlCu phase, which slows down the stress concentration at the interface and avoids catastrophic crack propagation. Based on the fracture crack initiation source characterization of the joint, a rapid solidification process strategy was designed to further improve the mechanical properties of the joint, and the joint with tensile strength of 255 MPa was successfully prepared. Under the premise of ensuring full penetration and the same heat input, it is easier to obtain satisfactory tensile strength with high laser power - high velocity welding parameters, which is due to the suppression of the Al 4 C 3 . This work can be regarded as a breakthrough in the research of high strength aluminum matrix composites guided by reverse failure analysis strategy.

Effect of topologically close-packed clusters on glass-forming ability of rapidly supercooled MgCuY alloy

The glass-forming ability (GFA) of metallic glasses can be enhanced by adding trace elements, but it is difficult to explain the reasons from microcosmic viewpoint due to the limitation of experimental conditions. Therefore, in order to reveal the effects of trace elements on GFA, the rapid solidification process of Mg65Cu(35-x)Yx (x = 5,10,15,20,25) ternary alloys are simulated by molecular dynamics (MD) in this paper, then the short- and medium-range order, thermodynamics and dynamics are investigated. The average atomic potential energy shows that the GFA becomes stronger monotonically with the increase of Y element. S433 and S544 can evolve into S555 with higher fivefold symmetry and packing density during solidification, which the topologically close-packed (TCP) clusters are mainly composed of S555. And the higher TCP clusters system has stronger GFA, because TCP clusters are easy to connect with each other to form large-sized nanoclusters, and the larger size nanoclusters have a smaller average displacement, which hinders the crystallization. As TCP atoms with slower dynamic characteristics than other atoms, the dynamics with higher Y element becomes slower with the increase of TCP clusters, and thus adding Y element plays a promoting role for GFA.

Effect of Process Parameters on Laser Powder Bed Fusion of Al-Sn Miscibility Gap Alloy

Al-Sn binary system is a miscibility gap alloy consisting of an Al-rich phase and a Sn-rich phase. This system is traditionally applied in bearings and more recently found application as form-stable phase change material (PCM) exploiting solid-liquid phase transition of Sn. A careful choice of production process is required to avoid macro-segregation of the two phases, which have different densities and melting temperatures. In the present study, the additive manufacturing process known as laser powder bed fusion (LPBF) was applied to an Al-Sn alloy with 20% volume of Sn, as a rapid solidification process. The effect of process parameters on microstructure and hardness was evaluated. Moreover, feasibility and stability with thermal cycles of a lattice structure of the same alloy were experimentally investigated. An Al-Sn lattice structure could be used as container for a lower melting organic PCM (e.g., a paraffin or a fatty acid), providing high thermal diffusivity thanks to the metallic network and a “safety system” reducing thermal diffusivity if the system temperature overcomes Sn melting temperature. Even if focused on Al-Sn to be applied in thermal management systems, the study offers a contribution in view of the optimization of manufacturing processes locally involving high solidification rates and reheat cycles in other miscibility gap alloys (e.g., Fe-Cu) with similar thermal or structural applications.

Structural relaxation in metastable magnetic submicronic wires

Changes triggered by low temperature metastable relaxation annealing in the peculiar magnetic behavior of rapidly solidified submicronic amorphous wires are reported. The overall aim was to understand the effects of the associated stress relief process on magnetization switching and magnetic domain wall propagation in these low dimensional amorphous wires with cylindrical symmetry. The results have shown that, even though the employed relatively low annealing temperatures did not initiate any crystallization, i.e., the amorphous state has been preserved, the consequences in terms of magnetic behavior are significant, with important variations in both magnetic switching field and domain wall velocity values. The observed modifications have been interpreted based on the interplay between the magnetoelastic and magnetostatic anisotropies within the submicronic amorphous wires, as well as on the complex changes in the underlying nonlinear stress distribution generated by the rapid solidification process. The outcomes of this investigation, alongside the general framework for their explanation, have unraveled novel ways of accurately tailoring and controlling the magnetic characteristics of the amorphous submicronic wires for developing new applications in sensors and domain wall logic devices, in which rapid switching and fast domain walls can make an important impact.

Microstructural evolution and hardness of rapidly solidified hypereutectic Al-Si surface layers by laser remelting

ABSTRACT Selective laser melting technique is used to manufacture Al-25 wt.%Si and Al-30 wt.%Si surface layers with a thickness of 0.5–0.6 mm followed by a laser rapid solidification process to refine the silicon to a nano-size and increase the hardness. The as-produced alloyed layers were heterogeneous and composed of clustered aggregates of primary Si crystals of size 5–7 µm, decorated with α-Al cells/dendrites, and embedded in a matrix consisting of α-Al dendritic crystals and Al-Si eutectic. However, after several remelting scanning treatments at a fast speed and low power, significant refinement of all phases was achieved, and a dramatic increase in the proportion of the interwoven nano-sized Al-Si fibrous eutectic. The primary Si sizes were reduced to ≤1 µm, while the eutectic spacing decreased and approached as low as 10–15 nm at the highest cooling rate. The high cooling rate significantly reduced the size and the amount of the α-Al cells/dendrites especially those surrounding the primary Si, thereby stimulating ultrafine Al-Si fibrous eutectic to grow adjacent to the primary Si crystals and spread massively, which in turn contributed to the increase in hardness. Transmission electron microscopy revealed block-like primary Si, worm-like Si, and nanoscale fibrous eutectic Si, which was internally nano-twinned.

Resistance Spot Welding of Additively Manufactured Maraging Steels—Part I: Nugget Formation

Abstract Application of additively manufactured steels is unavoidably involved in the resistance spot welding with conventionally manufactured steels. However, the microstructural evolution of an additive manufactured steel at high temperatures is still unknown, especially for the rapid solidification process. This paper investigated the microstructural evolution of a selective laser melted maraging steel during the rapid solidification process via resistance spot welding. Asymmetrical fusion zone with boat shape was found in the spot weld due to the rougher surface and larger electrical resistance of maraging steel via selective laser melting (SLM) process. The rapid expansion of fusion zone at end of welding process was caused by the carbide formation at the heat-affected zone of maraging steel via selective laser melting process. Besides, printing orientation affected the surface roughness of a selective laser melted maraging steel and subsequently significantly influenced the early stage of formation of fusion zone of additively manufactured maraging steel. We expect that our findings will pave the way to the future application of additively manufactured steels in the industries.

Atypical pathways for lamellar and twinning transformations in rapidly solidified TiAl alloy

Additively manufactured alloys undergo a rapid solidification process followed by thermal cycles, which promotes the microstructures to return to equilibrium. The microstructural behavior therein is especially complicated and inconceivable at times, thereby greatly restricting the tailorability of microstructures for additive manufacturing. To improve the understanding of microstructure formation in rapidly solidified TiAl alloy, Ti–47Al–2Cr–2Nb powders were aging-treated to reveal the microstructural evolution behavior on multiple scales, and the microstructural transformation mechanisms were comprehensively analyzed from the viewpoints of thermodynamics and kinetics. The results show that the rapidly solidified TiAl alloy almost retains the α2 phase, which then transforms into nanolamellar structures or equiaxed γ microstructures depending on the temperature. The completely different microstructure selections were determined by the competitive growth of lamellar structures and equiaxed γ grains. More importantly, atypical pathways for lamellar and twinning transformations were first discovered in these microstructural transformation processes. A 6H-type long periodic stacking ordered (LPSO) phase was observed to be in the form of straight laths in the retained α2 phase, and 9R- and 18R-type LPSO phases were discovered in the equiaxed γ grains. The 6H-type LPSO phase transformed into nano γ laths, whose formation driving force was deduced to result from the extremely high elastic energy stored in the retained α2 phase. The 9R- and 18R-type LPSO phases transformed into a more stable γtwin phase to reduce the entire energy of the system. This research may potentially enhance the understanding of the microstructure of additively manufactured TiAl alloy and consequently, the tunability of such microstructure as required.

Effect of Rapid Solidification Processing on the Microstructure and Corrosion of 316L Austenitic Stainless Steel

Austenitic stainless steels processed by rolling are widespread in technological applications, since they have excellent mechanical and corrosion properties. This study investigated the effect of the cooling rate, microstructure and properties of 316L austenitic stainless steel under cold rolled conditions and by rapid solidification. The microstructure of the cold rolling processing steel was composed of austenite and a low percentage of delta ferrite. For the rapid solidification condition, the microstructure evolved from columnar and acicular dendrites to equiaxed dendrites with decreasing cooling rates, without the presence of delta ferrite due to the high cooling rate. Furthermore, thermal analyses in both routes revealed that oxidation kinetics was slower after rapid solidification in synthetic air. The microhardness in the cold rolling condition was lower than in the rapid solidification condition since the microstructure under the solidification condition is more refined. The sample in the rapid solidification condition region RS1 presented the highest corrosion resistance considering the pit potential. The passivation current density in the cold rolled condition was 5.72x10-5A/cm2, while under the rapid solidification condition, regions RS1 and RS2 were 2.24x10-5 A/cm2 and 3.72x10-6 A/cm2, respectively, and region RS3, did not present a passivation region in a broad range of potentials.

Zn22Al4Ag ALLOY MICROWIRES CASTING TO RAPID SOLIDIFICATION PROCESS BY ROTATING DRUM

In this work, the Zn22Al4Ag alloy was synthesized by melting in a muffle furnace.The alloy obtained was characterized by Scanning Electron Microscopy Energy Dispersive Spectroscopy and was analyzed by the X-Ray Diffraction technique, where the crystallinity of the material was verified before and after being processed. Likewise, the Differential Scanning Calorimetry technique was used to obtain the temperatures where phase transformations occurin the alloy. These results were fed to the Termocalc®, software to numerically obtain the phase diagram of the alloy. Subsequently, a section of the ingot was taken to the rapid solidification process by rotating drum. The process variables were manipulated: jet stability, nozzle diameter, distance from the nozzle surface to the cooling medium, the delay time of the molten material in the crucible, speed of the rotating drum and jet angle, until obtaining a microwire with a diameter of ~ 160µm. Finally, it was determined that inadequate control of these parameters can result in powders, flakes or blockage of the crucible outlet. Potentially uses within the micro and nanoworld as an analogy to structural elements and electrical conductors, in addition to its current use as a coating anti-corrosive. Key Words: ZnAlAg alloy, Melt spinning process, Microwire, DSC analysis, Thermodynamic simulation

Analysis of grain size and five-fold twins during rapid solidification processes inTi3Al alloy.

The wide application of titanium aluminum (Ti–Al) intermetallic compounds for aerospace and automotive fields has accelerated the research and development of Ti3Al alloy. In this study, simulation is adopted to investigate the crystallization behavior during rapid solidification of Ti3Al alloys using molecular dynamics at different cooling rates of 1010 K s−1, 1011 K s−1, 1012 K s−1, and 1013 K s−1. The evolution of microstructures is characterized by taking advantage of the average potential energy, the pair distribution function and visualization. The results show that the system has formed a microstructural configuration with the face-centered cubic structure as the main structure and the hexagonal close-packed structure as the supplement. An increase in the cooling rate will reduce the grain size of the alloy, which in turn will increase the number of grains. At the cooling rate at which the alloy can crystallize, the system forms five-fold twin structures. Meanwhile, we obtain a deeper insight into the properties of five-fold twins in terms of atoms on different sites, and establish a standard model of the same specification for comparison to get the commonality and differences of the five-fold twins between the standard and the solidified. In addition, the evolution of dislocation densities and distribution of dislocation lines in the system under different conditions are analyzed. This study further explores crystallization behavior on the atomic scale and it is hoped that this research will contribute to expanding the understanding of Ti3Al alloy during the growth process.

Laser additive manufacturing of nano-TiC particles reinforced CoCrFeMnNi high-entropy alloy matrix composites with high strength and ductility

CoCrFeMnNi high-entropy alloy (HEA) matrix composites reinforced with nano-sized TiC particles were successfully fabricated by laser powder bed fusion (LPBF), which is widely used laser additive manufacturing technology. The microstructural evolution and mechanical properties of the HEA composites fabricated at various processing conditions, were investigated. The LPBF-fabricated HEA composites showed typical hierarchical microstructure characteristics including epitaxially grown columnar grains, submicron-sized cellular substructures, nano-sized precipitates, and a high density of dislocations. The rapid solidification process inherent to LPBF resulted in micro segregations of Mn and Ni along the cell walls, which contributed to a pronounced impurity-drag effect toward the present dislocations. TiC1-x with a size of 35–100 nm uniformly precipitated along the cell boundaries as well as inside the cells. Entangled dislocations easily formed around the cell walls and the nano-TiC1-x precipitates leading to a high dislocation density in the LPBF-fabricated HEA composites. The HEA composites with 1 wt% nano-TiC addition exhibited a promising mechanical performance readily characterized by a yield strength of 779 MPa, tensile strength of 940 MPa and an elongation of 30%. The advantageous combination between strength and ductility can be attributed to the prevailing subgrain boundaries, solid solution strengthening, grain refinement, nano-precipitation hardening, dislocation hardening, plasticity and storage. This work demonstrates the importance of hierarchical microstructures of HEA materials via incorporation of nano-sized TiC particles for imparting exceptional mechanical properties.

Reimann Brake Ramp for planar flow casting processes and analysis of ribbon gluing

Planar flow casting is a rapid solidification process used to manufacture thin, metallic ribbons, and foil. Liquid metal is forced through a nozzle against a heat-sink wheel, and it rapidly solidifies into thin ribbons. A puddle of molten metal, held by surface tension, forms between the nozzle and wheel. This study examines a well-defined periodic surface defect called herringbone (HB), which is commonly produced when casting zirconium based alloys. The presence of this defect is related to processing conditions and puddle dynamics. Its formation has been correlated with the pinning of the liquid puddle at the nozzle edge. Here, the uniformity of thickness along a ribbon was successfully controlled (over a length of 50 m) using the Reimann Brake Ramp, which reduces the wheel speed at the start of the cast. For the alloy used in this study, the variation in the dimensionless thickness parameter, τ/G, with the Euler number (Eu=ΔP/ρU2) at assigned values of B/G followed an allometric scaling, with an exponent value close to the theoretical value of 1/3. Furthermore, the nozzle inclination was related to the ribbon thickness, τ, and the ribbon quality. Moreover, a newly developed automatized melt spinner permitted monitoring and controlling of the process parameters, elucidating the gluing phenomenon of the ribbon observed during the starting phase. The ribbon gluing was influenced by the puddle geometry, the recirculation in the puddle, and later, to the ribbon cooling rate. Within these results, high-quality ribbons with control thickness over a considerable length are achieved.

Study on Microstructure Refinement Mechanism in Deeply Supercooled Solidification of Ni-25at.%Cu Alloys

Maximum undercooling degrees were obtained by glass purification and superheating. Rapid solidification in an undercooled alloy was systematically studied. As the undercooling increases, the solidification front appears randomly. The microstructure undergoes a transformation from coarse dendrite grains to granular grains to oriented dendrite to equiaxed crystal. The results show two grain refinement under high and low undercooling. No high strength textures are found under high undercooling conditions, and many grain boundaries of high angle nature are observed, which confirms recrystallization. In the TEM characterization of high undercooling, high density dislocation strain indicates that accumulated stress in the rapid solidification process induces microstructure plastic strain and promotes recrystallization. The results show that the grain boundary of refined grain is coarser at low undercooling, but narrower at high undercooling. Twins and subgrains observed at large undercooling indicate that the grain refinement mechanisms are very different.

Nanoparticle-mediated ultra grain refinement and reinforcement in additively manufactured titanium alloys

Additive manufacturing, also known as 3D printing, overcomes many design and manufacturing constraints to allow almost direct production of metals into complicated geometries. However, coarse columnar grain structures, up to the millimeter-scale, are commonly produced in titanium and its alloys through the layer-by-layer process and this causes significant anisotropy in mechanical properties. Here we report an innovative approach for microstructure refinement of an additively manufactured Ti-6Al-4V alloy via directed energy deposition of boron nitride nanotube (BNNT) decorated powders. With only 0.4 wt% BNNT, this process results in unprecedented grain refinement down to a few micrometers and over 50% strength enhancement. A unique texture-weakened structure comprising fine equiaxed grains is achieved via a novel nanoparticle-mediated nucleation mechanism enabled by local hypereutectic precipitation in the rapid solidification process. This mechanism is highly suited to the metallurgical environment of metal additive manufacturing and creates a pathway for screening effective grain refiners in titanium and other alloy systems. • An innovative approach for microstructure refinement of Ti-6Al-4V alloy was developed via direct laser deposition of BNNT decorated powders. • With only 0.4 wt% BNNT, this process results in unprecedented β grain refinement down to ~ 9 µm and over 50% strength enhancement. • The uncommon strength enhancement is facilitated by a combined effect of nano-whisker reinforcement and solid solution hardening. • A unique texture-weakened structure comprising fine equiaxed grains is achieved via a novel nanoparticle-mediated nucleation mechanism enabled by local hypereutectic precipitation in the rapid solidification process.

On the phase-field modeling of rapid solidification

The phase-field method has been widely used to study the rapid solidification processes such as additive manufacturing. To lower the computational cost, many simplifications and modifications were made to the original set of phase-field equations for alloy solidification. However, how those simplifications affect the modeling outcome is not well studied. In this work, we developed a phase-field model incorporated with both the coupled thermal-solutal diffusion and solute trapping for the rapid solidification of dilute binary alloys. We focused on the quantification of the influences of the simplifications of interface kinetics, solute trapping, and thermal diffusion on the modeling outcomes. By turning on and off those three effects individually, we explore how the temperature, solute concentration and interfacial velocity are affected. Our investigation shows that the interface kinetics and thermal diffusion cannot be ignored for quantitative modeling of a rapid solidification process for alloys. In the end, we used the rapid solidification of a melt-spun ribbon as an example to show that the phase-field model with all the three aspects considered is capable of capturing the complex interplay among temperature, solute concentration, and interfacial velocity and predicting the resultant solidification structures.

Effect of crystallite size on the low-temperature solid-solid phase transformations in the WO3 system

Ferroelectric nanostructured ε phase tungsten trioxide (ε-WO3) is emerging as an important functional material with unique gas sensing and optoelectronic properties. However, ε-WO3 phase is typically unstable in the bulk at room temperature (RT) and it has only been produced at ambient conditions by rapid solidification processes, such as Flame Spray Pyrolysis (FSP). This work aims to assess the overall stability of the ε phase at the nanoscale by studying its transformation from the stable γ phase to upon cooling. In-situ Raman spectroscopy has explored the effect of crystallite size on the solid-solid phase transformation from the symmetric γ (monoclinic) phase into the acentric ε (monoclinic) phase. The phase transformation onset temperature (Tonset) ranges from −43 °C for bulk state to −150 °C for crystallites of 68 nm in size and varies linearly with the inverse of the crystallite size radius (1r).

Superior thermoelectric cooling performance by suppressing bipolar diffusion effect and enhancing anisotropic texture in p-/n-type Bi2Te3 based compounds

Thermoelectric technologies can be used for waste heat power generation, and for solid-state cooling without environmentally hazardous refrigerants or moving parts. Thermoelectric cooling materials like the Bismuth Tellurides have been intensively investigated for several decades, but to date, thermoelectric cooling device applications have been limited to niche markets, because of their low cooling efficiency and high power consumption. Here, we demonstrate a thermoelectric module with significantly enhanced cooling performance and low power consumption, with fast cooling, using state-of-the-art p-/n-type Bi2Te3 based thermoelectric materials. Small grain size and anisotropic texture were achieved an exceptional high performance in the p-type Ag0.0006Bi0.46Sb1.5Te3.07 and n-type (CuI)0.002Bi2Te2.7Se0.3 compounds through rapid solidification and hot extrusion processes, resulting in significantly improved thermoelectric performance over a wide temperature range (zTmax = 1.45 for the p-type and 1.40 for the n-type compounds). The observed temperature-dependent thermoelectric properties in the n-type (CuI)0.002Bi2Te2.7Se0.3 compound could be explained in terms of Rashba band splitting rather than the multiband Boltzmann transport equations. We suggest that the enhancement of thermoelectric performance is due to the increase in chemical potential by Ag- (p-type) and CuI-doping (n-type) which suppresses bipolar diffusion, while the anisotropic texture and micro grains decreases lattice thermal conductivity. The thermoelectric module using these state-of-the-art thermoelectric materials demonstrated a reduction in power consumption by 38% and an increase of cooling speed by 170% compared with commercial single crystal-based TE devices, which is promising for practical applications.

Rapid solidification synthesis of novel (La,Y)2(Zr,Ti)2O7 pyrochlore-based glass-ceramics for the immobilization of high-level wastes

Pyrochlore-based glass-ceramics are attracting attention as a promising host material for immobilizing minor actinide in high-level wastes. However, the loading capacity of pyrochlore-based materials for actinide elements is to be improved, and only conventional synthetic approaches of powder sintering and glassy bulk crystallization were reported. Here, new (La,Y)2(Zr,Ti)2O7 pyrochlore-based glass-ceramics were subtly designed and successfully prepared through a rapid melting and solidification process. The solidification behavior of samples with different cooling rates was investigated, the dense and crack-free (La,Y)2(Zr,Ti)2O7 pyrochlore-based glass-ceramics were prepared at the cooling rate of 500 °C/s (within 1.5 s). The present amorphous La2O3-TiO2-Al2O3 phase demonstrates the outstanding thermal stability compared with the currently used borosilicate glass for high-level waste immobilization due to the lack of conventional glass-forming components. The results show the great feasibility of this novel synthesis route for glass-ceramics by direct solidification and its good potential for immobilizing high-level wastes.

Toxic elements-free low-cost Ti-Fe-Si metallic glass biomaterial developed by mechanical alloying

Ti-Fe-Si systems that are free of toxic elements and have high glass-forming abilities are considered to be theoretically ideal biomaterials. Currently, Ti-Fe-Si amorphous alloy production by rapid solidification processing (RSP) is limited to strips, which restricts its size. In order to eliminate this size restriction, this study developed Ti-Fe-Si amorphous alloy powder for the first time using mechanical alloying (MA), thereby laying a foundation for its subsequent bulk preparation by the consolidation process. Furthermore, the effects of the big-to-small grinding balls weight ratio (BSR), the ball-to-powder weight ratio (BPR), and the milling time during MA on the amorphous transition of samples were systematically investigated. This work provides a new avenue to use Ti-Fe-Si metallic glass in medical devices.

The effect of HCP on the formation of twin boundaries and dislocations in Ni–Co alloys

In this study, molecular dynamics (MD) was used to simulate the rapid solidification process of Ni47Co53 and Ni48Co52 alloys at a cooling rate of 1012 K/s. The effects of HCP on the formation of twin boundaries and dislocations in two Ni–Co alloys are studied. It is found that the difference of HCP clusters is the main effect that producing discrepancies on microstructure of two alloys. The number of HCP clusters accounted for 9.23% in Ni47Co53 alloy. They are regularly arranged to form the number of single-layer twin boundaries, and each twin boundary ends in a dislocation. The FCC and HCP structures coexist in the same atomic layers, which is easy to create dislocations. The relatively standard FCC crystal and only 0.32% HCP clusters are formed in Ni48Co52 alloy at 300 K. That small amount of HCP clusters are dispersed on the surface, and cause the formation of dislocation in the border with FCC clusters.