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Additive manufacturing (AM) of metals, also known as metal 3D printing, typically leads to the formation of columnar grain structures along the build direction in most as-built metals and alloys. These long columnar grains can cause property anisotropy, which is usually detrimental to component qualification or targeted applications. Here, without changing alloy chemistry, we demonstrate an AM solidification-control solution to printing metallic alloys with an equiaxed grain structure and improved mechanical properties. Using the titanium alloy Ti-6Al-4V as a model alloy, we employ high-intensity ultrasound to achieve full transition from columnar grains to fine (~100 µm) equiaxed grains in AM Ti-6Al-4V samples by laser powder deposition. This results in a 12% improvement in both the yield stress and tensile strength compared with the conventional AM columnar Ti-6Al-4V. We further demonstrate the generality of our technique by achieving similar grain structure control results in the nickel-based superalloy Inconel 625, and expect that this method may be applicable to other metallic materials that exhibit columnar grain structures during AM. 3D printing of metals produces elongated columnar grains which are usually detrimental to component performance. Here, the authors combine ultrasound and 3D printing to promote equiaxed and refined microstructures in a titanium alloy and a nickel-based superalloy resulting in improved mechanical properties.

Cet in Solidifying Roll - Thermal Gradient Field Analysis As the first step of simulation, a temperature field for solidifying cast steel and cast iron roll was created. The convection in the liquid is not comprised since in the first approximation, the convection does not influence the analyzed occurrence of the [C -> E] (columnar to equiaxed grains) transition (CET) in the roll. The obtained temperature field allows to study the dynamics of its behaviour observed in the middle of the mould thickness. This midpoint of the mould thickness was treated as an operating point for the [C -> E] transition. A full accumulation of the heat in the mould was postulated for the [C -> E] transition. Thus, a plateau at the [T(t)] curve was observed at the midpoint. The range of the plateau existence [t[C] [Lef-right arrow] t[E]] corresponded to the real period of transition, that occurs in the solidifying roll. At the second step of simulation, the thermal gradients field was studied. Three ranges were distinguished: a/ for the formation of the columnar structure (the C - zone): The columnar structure formation was significantly slowed down during incubation period. It resulted from a competition between columnar growth and equiaxed growth expected at that period of time. The relationship was postulated to correspond well with the critical thermal gradient, [G[crit.]]. A simulation was performed for the cast steel and cast iron rolls solidifying as if in industrial condition. Since the incubation divides the roll into two zones (columnar and equiaxed) some experiments dealing with solidification were made on semi-industrial scale. A macrosegregation equation for both mentioned zones was formulated. It was based on a recent equation for redistribution after back-diffusion. The role of the back-diffusion parameter was emphasized as a factor responsible for the redistribution in columnar structure and equiaxed structure.

In recent research, additions of solute to Ti and some Ti-based alloys have been employed to produce equiaxed microstructures when processing these materials using additive manufacturing. The present study develops a computational scheme for guiding the selection of such alloying additions, and the minimum amounts required, to effect the columnar to equiaxed microstructural transition. We put forward two physical mechanisms that may produce this transition; the first and more commonly discussed is based on growth restriction factors, and the second on the increased freezing range effected by the alloying addition coupled with the imposed rapid cooling rates associated with AM techniques. We show in the research described here, involving a number of model binary as well as complex multi-component Ti alloys, and the use of two different AM approaches, that the latter mechanism is more reliable regarding prediction of the grain morphology resulting from given solute additions. Predictive scheme for Ti alloys with equiaxed microstructures is often limited by the methods based on growth restriction factors, Q. Here, the authors present a predictive solution based on the freezing range of alloys for columnar to equiaxed transition during fusion-based additive manufacturing.

Ice-templating technology holds great potential to construct industrial porous materials from nanometers to the macroscopic scale for tailoring thermal, electronic, or acoustic transport. Herein, we describe a general ice-templating technology through freezing the material on a rotating cryogenic drum surface, crushing it, and then re-casting the nanofiber slurry. Through decoupling the ice nucleation and growth processes, we achieved the columnar-equiaxed crystal transition in the freezing procedure. The highly random stacking and integrating of equiaxed ice crystals can organize nanofibers into thousands of repeating microscale units with a tortuous channel topology. Owing to the spatially well-defined isotropic structure, the obtained Al 2 O 3 ·SiO 2 nanofiber aerogels exhibit ultralow thermal conductivity, superelasticity, good damage tolerance, and fatigue resistance. These features, together with their natural stability up to 1200 °C, make them highly robust for thermal insulation under extreme thermomechanical environments. Cascading thermal runaway propagation in a high-capacity lithium-ion battery module consisting of LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode, with ultrahigh thermal shock power of 215 kW, can be completely prevented by a thin nanofiber aerogel layer. These findings not only establish a general production route for nanomaterial assemblies that is conventionally challenging, but also demonstrate a high-energy-density battery module configuration with a high safety standard that is critical for practical applications. In this work, the authors present an ice-templating strategy to produce large-scale isotropic nanofiber aerogels using a unique process of freezing the material on a rotating cryogenic drum surface, crushing it, and then re-casting the nanofiber slurry enabling high-throughput and design flexibility.

Al–Nb–B master alloy has been regarded as a promising grain refiner that can reduce grain size of hypoeutectic Al–Si casting alloys. However, its grain refinement performance remains to be improved. In this work, the grain refinement efficacy of Al–Nb–B master alloy is significantly enhanced by modifying the Nb/B ratio through thermodynamic calculation. An Al–Nb–B master alloy with optimum Nb/B ratio of ~ 10:1 provides a fully equiaxed structure across the sections of the Al–10Si and commercial Al–9Si–0.08Ti alloys with an average grain size below 220 μm. The phenomenon is attributed to the existence of NbAl3 and the higher number density of NbB2 at the Nb/B ratio of ~ 10:1, which offers sufficient active nucleating sites to promote the formation of smaller grains. Moreover, the segregation behavior of Si atoms and interfacial energies after doping Si are investigated by first-principles calculations, and the results reveal that Si tends to segregate to the NbAl3/α-Al interface, whereas grain refining potency of NbAl3 for Al remains unchanged. This study has implications for strategic design of Al–Si cast alloy with fine and equiaxed grain structure inoculated by grain refiner.

In large-ingot castings, the settling of equiaxed dendrites often results in distinct cone-shaped negative segregation in the lower region of the ingot. To accurately predict and control such macrosegregation, it is important to understand the kinetic behavior of equiaxed dendrites in the melt. The phase-field lattice Boltzmann (PF-LB) model is powerful for simulating dendrite growth with melt convection and solid motion. However, it is computationally expensive and represents only the short-distance motion of dendrites in three-dimensional (3D) simulations. For an efficient 3D evaluation of the effect of dendrite motion and rotation on growth behavior, we introduce the moving frame algorithm to PF-LB simulations. Here, the computational domain tracks the settling dendrite to express long-distance settling without restricting the domain size. The PF-LB simulations were accelerated by parallel computing using a combination of multiple GPUs and adaptive mesh refinement (AMR), also referred to as parallel GPU-AMR. The moving-frame algorithm was modified to adapt to AMR. From the simulation results, we demonstrate that the proposed method helps evaluate the effect of dendrite rotation on the settling and growth velocities of equiaxed dendrites in 3D.

This study investigated the evolution of solidification microstructure and dynamic recrystallisation (DRX) during the laser solid forming of the Ni-based Inconel 625 superalloy. The as-deposited microstructure mainly showed epitaxially grown columnar grains with fine equiaxed grains between them. These fine equiaxed grains were formed by the discontinuous DRX (DDRX) and continuous DRX (CDRX) processes, which were induced by the cyclic thermal stress resulting from the repeated laser deposition. The bulging of pre-existing grains and sub-grain rotation were the main mechanisms of the DDRX and CDRX phenomena, respectively. Additionally, after the occurrence of DRX, the dislocations were released and there was no distortion in the recrystallised grains. Coarse equiaxed grains were present in the top zone of the deposit; these grains were formed by the columnar-to-equiaxed transition during the solidification of the molten pool after the end of the laser re-melting and deposition process.

Predicting the columnar to equiaxed transition (CET) and grain refinement for additively manufactured alloys from thermodynamic databases has been a long-standing challenge and an ongoing source of discussion. Efforts are focused on designing alloy compositions to achieve fully equiaxed microstructures, thereby eliminating the mechanical anisotropy commonly associated with the large columnar grains in additively manufactured alloys. Here, three compositional parameters proposed in the literature are evaluated across a range of Ti alloys: the non-equilibrium solidification range (Δ T s ), the growth restriction factor ( Q ) and constitutional supercooling parameter ( P ). Ti-Fe, Ti-Cu, Ti-Cu-Fe, and Ti-Mo alloys produced via direct energy deposition experimentally verified that P is the most reliable parameter to guide the selection of alloying elements for additively manufactured (AM) alloys. Verification was found by reconsidering results from additional alloy systems and AM methods. The numerical CET models also predict that P is closely related to dendrite tip undercooling at high growth velocities, as found in AM. This work provides a clearer framework for predicting the grain morphology of metallic alloys in AM. The supercooling parameter, P , was found to be the best indicator of grain morphology of metallic alloys produced with high solidification velocities. This provides guidance for the design of new alloys for use with additive manufacturing.

In the present study, Inconel 686 thick-wall part manufactured utilizing gas metal arc welding-based wire arc directed energy deposition (WA-DED) was examined. The microstructure and mechanical properties of the fabricated Inconel 686 component across different sections, such as bottom, middle, and top, were explored, and the influence of cryogenic treatments, such as shallow and deep, on the properties of the fabricated specimens was examined. The optical and scanning electron microscopy revealed differences in microstructure across various regions of the deposited metal. The bottom region showed a columnar structure, the intermediate region displayed a combination of cellular structures, and the top layer featured an equiaxed structure. These variations contribute to heterogeneity and anisotropy in the mechanical characteristics. Moreover, the microstructure of the deep cryogenic treatment (DCT)-treated samples exhibited a finer grain structure in contrast to both the as-built and shallow cryogenic treatment (SCT)-treated WA-DED samples attributed to grain refinement. X-ray diffraction analysis observed that applying DCT decreased grain size, with the average grain size of the DCT-treated sample measuring 22.81 nm, while concurrently increasing the dislocation density to 19.22 × 10 –4  nm –2 . Energy-dispersive X-ray spectroscopy point analysis, elemental mapping, and line mapping were conducted to study the microsegregation and spatial distribution of alloying elements in the grain boundaries and interdendritic regions. Results indicated intensified segregation tendencies of alloying elements molybdenum (Mo) and tungsten (W) with increasing deposited height, peaking in the lowermost region. However, DCT samples exhibit reduced elemental segregation compared to as-built and SCT samples. The tensile strength and microhardness showed substantial differences across various areas. Cryogenic treatments considerably improved the mechanical properties of WA-DED specimens compared to their as-built state. As a result, the tensile strength improved by 7.23%, and the hardness strength increased by 8.98%. Graphical abstract

This paper addresses the use of alloying additions to titanium alloys for additive manufacturing (AM) with the specific objective of producing equiaxed microstructures. The additions are among those that increase freezing ranges such that significant solutal undercooling results when combined with the rapid cooling rates associated with AM, and so be effective in inducing a columnar-to-equiaxed transition (CET). Firstly, computational thermodynamics has been used to provide a simple graphical means of predicting these additions; this method has been used to explore additions of Ni and Fe to the alloy Ti–6Al–4V (Ti64). Secondly, an experimental means of determining the minimum concentration of these alloying elements required to effect the CET has been developed involving gradient builds. Thirdly, it has been found that additions of Fe to Ti64 cause the alloy to change from an α/β Ti alloy to being a metastable β-Ti alloy, whereas additions of Ni do not produce the same result. This change in type of Ti alloy results in a marked difference in the development of microstructures of these compositionally modified alloys using heat treatments. Finally, hardness measurements have been used to provide a preliminary assessment of the mechanical response of these modified alloys.