In this study, the three-dimensional (3D) finite element model of COMSOL Multiphysics was used to simulate the thermodynamic behavior of the AlSi10Mg molten pool (MP) during selective laser melting (SLM), and the results were verified by multiple groups of single-track SLM experiments. The simulations take into account numerous thermophysical phenomena such as fluid flow, heat conduction, heat radiation, and fluid mass transfer. The influence of laser power, scanning speed, and other process parameters on temperature, shape, and size of MP was investigated. The simulated results and the experimental validation show that the model predictions are highly consistent with the measured data for most sets laser parameters, especially the MP width of 142 mu m vs. 145 mu m, depth of 53 mu m vs. 56 mu m at a laser power of 250 W and a scanning speed of 1000 mm/s, which confirms the reliability and accuracy of the model. Furthermore, the model innovatively forecasts the critical parameters for solidification in MP and delves into the intricate relationship between the position in MP and key parameters such as temperature gradient, cooling rate, and solidification rate. The model's versatility ensures its applicability to SLM simulation processes across diverse alloys and varying parameters. This research not only enhances our comprehension of SLM but also establishes a scientific foundation for optimizing processes, controlling microstructures, and enhancing material performance.

SrFe(12-x)PrxO(19)(x = 0-0.5) powders were synthesized and the effects of praseodymium doping amount, citric acid dosage, and calcination temperature on its structure, magnetic properties, and microwave absorbing properties were studied. The results of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy show that praseodymium doping leads to the lattice expansion and makes strontium ferrite easier to form spherical polyhedron in microscopic morphology. The Fe2O3 impurity phase disappears and FePr1-ySryO3-delta impurity phase appears with the increase of the calcination temperature. It was inferred from Raman spectra and X-ray photoelectron spectroscopy that some Pr3+ ions form Pr4+ ions and the latter may occupy iron or strontium sites, and the transition from the Fe3+ to Fe2+ may occur at 4f(2) site. The vibrating sample magnetometer results show that the saturation magnetization decreases first and then increases; the coercivity increases and the anisotropy field decreases after doping praseodymium. Praseodymium-doped strontium ferrite has strong absorption in low frequency band (C-band) or medium frequency band (X-band), while the matching thickness corresponding to high frequency band (Ku-band) is small. The maximum reflection loss reaches -50.95 dB and the effective absorption bandwidth (RL < -10 dB) is 4.76 GHz. Increasing the calcination temperature or doping amount will reduce the matching thickness, while increasing the amount of citric acid will disperse the grains and reduce the dielectric loss. It can be concluded that praseodymium valence change leads to the increase of Fe2+ ions and oxygen defects and there is also interface polarization between M-phase and heterogeneous phase, which together increases the dielectric loss of the material. Meanwhile, praseodymium doping can significantly increase the magnetic loss caused by natural resonance. These show that praseodymium doping improves the impedance matching of the material, increases the attenuation constant, and significantly improves the wave absorption performance. (c) 2023 Elsevier Ltd. All rights reserved.

In this study, we carried out multiscale simulations that integrate a macro-scale finite element method simulation for mass and heat transfer of the transient molten pool and a micro-scale quantitative phase-field model for dendritic growth and solute distribution based on selective laser melting (SLM) experiments. Macroscopic simulations reveal an approximately inverse coupling of solidification velocity Vs and thermal gradient G near the solid-liquid interface at the tail end of the molten pool. It was found that the maximum flow velocity of the Inconel 718 alloy melt in the molten pool exceeds 2 x 10-2 m/s and the maximum negative pressure reaches -5.28 x 10-4 Pa. Through quantitative phase-field simulations, the map about G=Vs for each solidification morphology was obtained, and it was found that solidification velocity dependent solute partition coefficients k(Vs) can strongly influence the estimated stability regions. The power law relationship of the primary dendritic arm spacing (PDAS) A1 versus solidification velocity A1ecVsa agrees with the previous experimental results for lower G, and the exponent a decreases with increasing G. The PDAS A1 versus cooling rate R following A1ecR-0.512 is consistent with previous investigations. We found that A1 is a nonlinear function with G-0.5V-0.25 A1ec(G-0.5V-0.25 s )a, with a ranging from 0.887 to 2.466 for various G. Comparison of the predicted A1 with Trivedi's model reveals that A1 is still nonlinear with G-0.25V-0.25 (c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Tin (IV) oxide is a highly promising electron transport layer (ETL) for lead halide perovskite solar cells due to its high conductivity, transparency, wide band gap, and the possibility of low-temperature processing. Nonetheless, charge carrier recombination processes at the SnO2/perovskite interface diminish the device performance. Here, we demonstrate that SnO2 doping with guanidine hydrochloride (G-SnO2) leads to efficient surface passivation and a larger band offset between the ETL and the perovskite layer, resulting in reduced voltage losses and faster electron transfer. Moreover, G-SnO2 facilitates the growth of highly crystalline perovskite layers. Consequently, a power conversion efficiency of up to 23.48% and a high open-circuit voltage of 1.18 V are obtained in solar cells incorporating the G-SnO2 ETL. These devices also exhibited negligible hysteresis and maintained more than 96% of their initial power conversion efficiency after 1,250 h exposure to the air without encapsulation.

During ground-based solidification, buoyancy flow can develop by the density difference in the hypoeutectic type of the alloys, such as Al-7 wt% Si alloy. Buoyancy flow can affect the thermal field, solute distribution in the melt, and the position and amount of the new grains. As solidification is a very complex process, it is not very easy to separate the different effects. Under microgravity conditions, natural convection does not exist or is strongly damped due to the absence of the buoyancy force. Therefore, experiments in microgravity conditions provide unique benchmark data for pure diffusive solidification conditions. Compared to the results of the ground-based and microgravity experiments, it is possible to get information on the effect of gravity (buoyancy force). In the framework of the CETSOL project, four microgravity solidification experiments were performed on grain refined (GF) and non-grain refined Al-7 wt% Si alloy onboard the International Space Station in the Materials Science Laboratory. These experiments aimed to study the effect of the solidification parameters (solid/liquid front velocity vSL, temperature gradient GSL) on the grain structure and dendritic microstructures. The microgravity environment eliminates the melt flow, which develops on Earth due to gravity. Four ground-based (GB) experiments were performed under Earth-like conditions with the same (similar) solidification parameters in a vertical Bridgman-type furnace having four heating zones. The detailed analysis of the grain structure, amount of eutectic, and secondary dendrite arm spacing (SDAS) for different process conditions is reported and compared with the results of the microgravity experiments. GB experiments showed that the microstructure was columnar in the samples that do not contain GF material or in case the solid/liquid (vSL front velocity was slow (0.02 mm/s)). In contrast, in the sample which contained GF material, progressive columnar/equiaxed transition (PCET) was observed at vSL = 0.077 mm/s and GSL = 3.9 K/mm. The secondary (SDAS) dendrite arm spacing follows the well-known power law, SDAS=K[t0]13, where K is a constant, and t0 is the local solidification time for both GB and µg experiments.

In this work, the grain orientation and mechanical properties of Inconel 625 are tailored by varying the process parameters during directed energy deposition. Under the same deposition speed, increasing the current is effective in promoting the columnar-to-equiaxed transition due to modifications on the thermal cycle. The solidification conditions (temperature gradient and cooling rate) were characterized during the process. A comparison with an existing solidification map for Inconel 625 indicates that the temperature gradients in the melt pool of the sample fabricated with larger current decrease sufficiently to permit the nucleation and growth of equiaxed grains. Uniaxial tensile testing showed that the sample with equiaxed grain microstructure exhibits a higher yield strength (increase by 36 %) when compared to the sample with columnar grains. Contributions of various strengthening mechanisms to the yield strength are quantified in terms of grain boundary strengthening, dislocation strengthening, and solid-solution strengthening. It is found that the higher yield strength of samples that possess equiaxed grains can be attributed to the enhanced dislocation strengthening arising from the large average Taylor factor.

The influence of structure and composition on precipitation phenomena in Al-bearing BCC/HCP Mg alloys are studied via diffusion couple technique. Interdiffusion induced by the resultant composition gradient results in a change in crystal structure from HCP to BCC in the diffusion zone. The Vickers hardness in the diffusion zone is much higher than that in the Mg–5.5at.%Al and Mg–38at.%Li, which is attributed to the chemical ordering by nano-sized secondary ordered D03–Mg3Al precipitation in the BCC Mg–Li–Al diffusion zone. The reasons for different precipitation in Al-bearing Mg alloys with various matrices are discussed. Generating ordered precipitates can be an effective approach to improve both strength and ductility in HCP Mg alloys.

In the colloidal-salt system of sodium silicate and sodium chloride, threefold and fourfold symmetric dendrites and diffusion-limited aggregation (DLA) of sodium chloride crystals have been synthesized by a simple one-step gelation process without changing the solution composition. The morphology variation is realized by changing the sol pretreatment and gelation process's heating mode. And the gelation degree of sodium silicate and the anisotropy of crystal growth is the morphology formation mechanism. Fourfold symmetric dendrites are related to preferential growth in the (1 1 0) direction, while threefold symmetric dendrites are due to preferential orientation changes from (1 1 1) to (100) and then to (1 1 1). EDS composition analysis showed that the high gelling degree is more accessible to precipitate regular fractal dendrites with preferred orientation.

Columnar and equiaxed structures, which occur during solidification of metallic alloys, influence the texture and properties of castings, welded joints and additively manufactured components. During transient solidification, where grain refiner particles provide the predominant nucleation mechanism, a Columnar to Equiaxed Transition (CET) occurs when conditions that had originally favoured directional columnar growth change to those favouring equiaxed. Constitutional undercooling ahead of the columnar front can permit equiaxed nucleation and growth. By carrying out experiments in microgravity conditions, liquid flows due to thermal and solutal buoyancy effects are suppressed. In these diffusion-controlled conditions, we have observed examples of both sharp (clear) and progressive (gradual) CET. The experimental outcomes, especially the observation of a progressive CET, has highlighted the need for a continuum model that allows for competitive columnar and equiaxed structure development; hence, the Concurrent Columnar to Equiaxed Transition (C2ET) model is proposed. The C2ET is thermally transient and relies on the well-known concept of extended growth for impingement mechanics; thereby, greatly reducing numerical complexity. Importantly, the proposed approach removes the need for a specific equiaxed-blocking criterion, which is often proposed as an essential requirement in other CET models. The C2ET model is validated by four experimental solidification scenarios: two velocity jumps and two thermal-gradient decreases. The velocity jumps induced sharp CETs; whereas, thermal-gradient decreases gave progressive CETs. The C2ET model gave good agreement for the columnar and equiaxed transition zones for both sharp and progressive CET. Results are compared with the classic Hunt model. Unlike Hunt's model, the C2ET model predicted all macrostructure transitions faithfully using a single (or consistent) set of nucleation input parameters across all four scenarios. Since, the same level of grain refinement was used in each experiment, a consistent set of nucleation parameters was expected. The validated approach can enable effective simulation at lower computational cost for industrial processes that rely on a solidification processing step.

During the solidification of hypoeutectic Al–7% Si alloy, density differences develop in the melt due to variations in concentration and temperature. On Earth, melt flow can occur due to gravity, which then affects the solidification process. The microgravity environment strongly eliminates convection in the melt and allows investigation of the solidification process in purely diffusive circumstances. In this study, four solidification experiments were performed on grain-refined and non-grain-refined Al–7 wt% Si alloy on-board the International Space Station (ISS) in the Materials Science Lab (MSL) to study the effect of solidification parameters (solid/liquid front velocity (v) and temperature gradient (G)) on the grain structure and dendritic microstructure. The grain structure has been analyzed in detail in some earlier studies. The aim of this work was to carry out detailed analysis of the macrosegregation caused by the diffusion of Si from the initial mushy zone during the homogenization step and the subsequent solidification phase of the experiments as well as the correlated distribution of eutectic along the solidification direction. The secondary dendrite arm spacing (SDAS) for different process conditions was also studied. For these two issues, microgravity experimental results were compared to simulation results. The macrosegregation was calculated by the finite difference method. Because the steady-state solidification conditions were never reached, the solidification process was characterized by the average front velocity and temperature gradient. Considering the actual liquidus temperature (TL) caused by macrosegregation, the SDAS was calculated as a function of the average processing parameters and the actual liquidus temperature with the classical Kirkwood’s equation. As a result, good agreement was obtained between the calculated and measured SDAS.

Directional solidifications of Al-7 wt.% Si alloy were carried out under microgravity on board the International Space Station to investigate the impact of a rotating magnetic field (RMF) on the solidified microstructure. It has been found that the RMF significantly influences the solidified microstructure in the conditions corresponding to the lowest growth rate, whereas it has negligible influence on the microstructure under the highest growth rate, indicating that the RMF intensity and the forced liquid flow are too weak, compared to the solidification front velocity, to impact the microstructure. For the lowest growth rate applied, the RMF application results in a more uniform eutectic phase distribution and a smaller dendrite arm spacing. This is ascribed to the RMF induced forced liquid flow that makes more uniform Si concentration in the bulk liquid above the solid-liquid interface. Additionally, the RMF application possibly modifies the columnar dendritic network by changing dendrite growth directions. This observation can be attributed to the combined effect of the RMF induced forced liquid flow and of the thermoelectric magnetic force on the dendrites resulting from the application of the RMF as well.

Tin oxide (SnO2 ) is an emerging electron transport layer (ETL) material in halide perovskite solar cells (PSCs). Among current limitations, open-circuit voltage (VOC ) loss is one of the major factors to be addressed for further improvement. Here a bilayer ETL consisting of two SnO2 nanoparticle layers doped with different amounts of ammonium chloride is proposed. As demonstrated by photoelectron spectroscopy and photophysical studies, the main effect of the novel ETL is to modify the energy level alignment at the SnO2 /perovskite interface, which leads to decreased carrier recombination, enhanced electron transfer, and reduced voltage loss. Moreover, X-ray diffraction reveals reduced strain in perovskite layers grown on bilayer ETLs with respect to single-layer ETLs, further contributing to a decrease of carrier recombination processes. Finally, the bilayer approach enables the more reproducible preparation of smooth and pinhole-free ETLs as compared to single-step deposition ETLs. PSCs with the doped bilayer SnO2 ETL demonstrate strongly increased VOC values of up to 1.21 V with a power conversion efficiency of 21.75% while showing negligible hysteresis and enhanced stability. Moreover, the SnO2 bilayer can be processed at low temperature (70 °C), and has therefore a high potential for use in tandem devices or flexible PSCs.

2.9 mm thick 20 wt% B4Cp/6061A1-T6 composite sheets were friction stir welded using a threaded cermet tool at welding speeds of 50-400 mm/min and a tool rotating rate of 1000 rpm and then subjected to post-weld artificial aging. Sound joints were achieved under all welding speeds. The threaded cermet pin only exhibited slight abrasion at the top thread of the screw after all of the welding operations with a total length of 1200 mm. The B4C particles were blunted and fragmented, and uniformly distributed in the nugget zone (NZ), due to the severe material deformation combined with the particle/pin interaction. The fine precipitates in the matrix alloy were mostly dissolved in the NZ but were significantly coarsened in the heat affected zone (HAZ). The joints exhibited significantly decreased tensile strength compared with the base material. The joint strength increased slightly with increasing welding speed, with maximum joint efficiency of similar to 70% at 400 mm/min. Artificial aging resulted in significant improvement in the NZ hardness and joint strength due to the re-precipitation of the precipitates, with a larger increment for the joint under higher welding speed. The joint at 400 mm/min exhibited the highest joint efficiency of similar to 85% after artificial aging. Artificial aging changed the hardness profiles of the NZ from flat to "M" shapes due to the change in quantities of solutes after FSW with increasing distance from the NZ centerline. The fracture of both as-welded and aged joints occurred in the HAZ.

Directional solidification experiments of grain refined Al -7 wt% Si alloy were carried out on Earth under normal gravity conditions (1 g) and in the Material Science Laboratory on board the International Space Station in microgravity environment (mu g), to investigate the impact of the gravity on the solidification microstructure and the columnar-to-equiaxed transition (CET). The increase of the dendrite growth velocity imposed by the processing conditions during the experiments leads to a size decrease of the dendrite microstructure and to a more homogeneous eutectic distribution under both 1 g and mu g conditions. A progressive CET is obtained in both samples implying the existence of an intermediate region after the inception position of CET defined as the end of growth of the columnar dendrites. However, a more progressive CET and longer dendrites aligned with the applied temperature gradient are observed in presence of gravity. This difference is attributed to the convective flow on Earth. On the one hand, it carries the grains that nucleate ahead of the columnar front away into the bulk liquid phase. On the other hand, it sweeps the solute away from the dendrite tip zone. Consequently, the blocking effect is diminished, allowing extended continuous growth of the elongated dendrites.

It is rather challenging to obtain high-quality Ti joints by conventional friction stir welding because of the problem of over-heating. The welding process and final microstructures and properties of the joints are controlled by both plastic deformation and recrystallization. However, for a long time, studies have only focused on recrystallization mechanisms but ignored deformation modes. In this study, a defect-free ultrafine-grained Ti joint with a joint efficiency of 100% was for the first time produced by submerged friction stirring (SFS) technology. We utilized transmission electron microscopy with a two-beam diffraction technique and electron backscatter diffraction to systematically investigate the deformation mode versus the grain refinement mechanism. The finite element method was utilized to simulate the temperature field throughout the joint for the microstructural explanation. During the whole SFS, prismatic slip occurred, and the other dominant deformation mechanisms changed from {101¯2} twinning and basal slip to pyramidal slip. The variation of slip modes was largely dependent on the twinning and temperature rise. The ultrafine-grained microstructure was attributed to the successive refinement effect of the twin-dislocation interaction, dislocation absorption, dynamic grain boundary migration and texture-induced grain convergence. The effect of the temperature, strain and strain rate on the microstructural evolution mechanisms was discussed. Based on our work, we expect the wide application of SFS in producing ultrafine-grained bulk Ti materials and high-quality joints.

T6-treated 20 wt% B 4 Cp/6061Al sheets were joined under welding speeds of 400–1200 mm/min by friction stir welding (FSW) with a threaded cermet pin. The macro-defect-free FSW joints could be achieved at high welding speeds up to 1200 mm/min, but larger plunge depth was required at the welding speeds of 800 and 1200 mm/min to eliminate the tunnel defect. In the nugget zone (NZ) of the joints, the B 4 C particles were broken up and uniformly redistributed. The NZ exhibited lower hardness than the base metal (BM), and the hardness value almost did not change with increasing welding speed, attributable to the dissolution of precipitates. Compared with the BM, the joints showed lower tensile strength. As the welding speed increased from 400 to 800 mm/min, the joint efficiencies were nearly the same and up to ~ 73%. When the welding speed increased up to 1200 mm/min, the tensile strength significantly decreased, due to the occurrence of kissing bond defect at the bottom of the NZ. With increasing welding speed, the fracture location of the joints transferred gradually from the heat-affected zone to the NZ due to the kissing bond defects.

Commercial 5083Al rolled plates, 2.8 mm thick, were subjected to friction stir welding (FSW) with the aim of achieving low-temperature superplasticity (LTSP) in the nugget zone (NZ). Fine-grained microstructure with average grain sizes of 1.6 and 1.8 µm was obtained in the upper and lower parts of the NZ, respectively. The NZ was subjected to superplastic investigation at 250 and 300 °C. It was indicated that the upper and lower parts of the NZ exhibited similar LTSP values of 550–570% at 300 °C, much higher than those reported previously (< 300%) in friction stir processed 5083Al. This excellent LTSP was attributed to the extremely fine-grained microstructure and predominant high angle grain boundaries (> 84%). Grain boundary sliding was determined to be the dominant deformation mechanism, with grain boundary diffusion as the rate-controlling step.

Microgravity solidification experiments were carried out in the Material Science Laboratory on board the International Space Station. The influence of grain refinement, rotating magnetic field (RMF) and surface pores on the microstructure and columnar-to-equiaxed transition (CET) were investigated in two selected Al-based samples solidified under microgravity conditions. The increase of the furnace pulling velocity leads to a finer dendrite structure, a smaller eutectic percentage and a more uniform eutectic distribution in the interdendritic regions. On the one hand, grain refinement ensures the occurrence of CET, which is progressive in the studied experiment because of the high temperature gradient. On the other hand, in the non-refined alloy a RMF applied during solidification fails to trigger the CET, because the forced liquid flow is too weak compared to the solidification front velocity to transport fragments from the mushy zone above the solidification front. The presence of the pores at the sample surface leads to a peculiarity in the eutectic percentage and weakens the decrease of the dendrite arm spacing for both samples. These effects are ascribed to a forced extra liquid flow into the mushy zone due to the pore that promotes the growth of the dendrites along the liquid flow direction, resulting in elongated grains and postponing the CET in the refined alloy.

Annealed 2.8mm thick B4C/6061Al composite sheets with various B4C particle contents (15, 20, 25, 30wt%) were friction stir welded (FSW) at a tool rotation rate of 1000rpm and traverse speeds of 50 and 150mm/min using a single simple-shaped wear-resistant cermet tool. Sound FSW joints were obtained without severe abrasion of the tool. FSW resulted in obvious homogenization and fragmentation of B4C and the re-distribution of the interfacial products, thereby remarkably increased the hardness of the nugget zone. The hardness profiles of the welded joints were hardly influenced by B4C contents but significantly by the welding speeds. “S” line and a B4C depleted region were formed at the top and the bottom of the nugget zone, respectively. However, they did not deteriorate the mechanical properties of the joints. The tensile strength of all the joints was close or even up to that of the base material with the fracture occurring at the base material.