Showing papers on "Directional solidification published in 2019"

Microstructure and Mechanical Properties of Mg–RE–TM Cast Alloys Containing Long Period Stacking Ordered Phases: A Review

Abstract: Casting magnesium alloys hold the greatest share of magnesium application products due to their short processing period, low cost and near net shape forming. Compared with conventional commercial magnesium alloys or other Mg–RE-based alloys, the novel Mg–RE–TM cast alloys with long period stacking ordered (LPSO) phases usually possess a higher strength and are promising candidates for aluminum alloy applications. Up to now, two ways: alloying design and casting process control (including subsequent heat treatments), have been predominantly employed to further improve the mechanical properties of these alloys. Alloying with other elements or ceramic particles could alter the solidification pattern of alloys, change the morphology of LPSO phases and refine the microstructures. Different casting techniques (conventional casting, rapidly solidification, directional solidification, etc.) introduce various microstructure characteristics, such as dendritic structure, nanocrystalline, metastable phase, anisotropy. Further heat treatments could activate the transformation of various LPSO structures and precipitation of diverse precipitates. All these evolutions exert great impacts on the mechanical properties of the LPSO-containing alloys. However, the underlying mechanisms still remain a subject of debate. Therefore, this review mainly provides the state of the art of the casting magnesium alloys research and the accompanying challenges and summarizes some topics that merit future investigation for developing high-performance Mg–RE–TM cast alloys.

Strengthening of Mg-based long-period stacking ordered (LPSO) phase with deformation kink bands

Abstract: The mechanical properties of the Mg-based LPSO-phase are expected to be strongly affected by the microstructure due to its anisotropic crystal structure. However, the fine details have not been sufficiently understood yet. This study first clarified the detailed microstructural factors that govern the strength of the LPSO-phase by examining alloys with microstructures that were significantly varied via directional solidification and extrusion processes. Refining the microstructure is significantly effective for strengthening LPSO-phase alloys. The yield stress of LPSO-phase alloys with random texture was previously reported to be increased by reducing the “length” of plate-like LPSO-phase grains. In addition, it was found in this study that the formation stress in the deformation kink band, which is a unique deformation mode in an LPSO-phase alloy, can be increased by decreasing the “thickness” of the grains. Furthermore, the study used directionally solidified crystals provided direct evidence that the introduction of the deformation kink band effectively increases the yield stress and work-hardening rate of alloys by hindering the motion of basal dislocations. This “kink-band strengthening” was found to have considerable temperature dependence. The strengthening is significant at or below 200 °C, but the effect gradually decreases above 300 °C and is accompanied by the operation of non-basal slip. The results quantitatively clarified that kink-band strengthening is one predominant reason why the LPSO-phase extruded alloy exhibits an unusually high yield stress at any loading orientation.

External Field Assisted Freeze Casting

Abstract: Freeze casting under external fields (magnetic, electric, or acoustic) produces porous materials having local, regional, and global microstructural order in specific directions. In freeze casting, porosity is typically formed by the directional solidification of a liquid colloidal suspension. Adding external fields to the process allows for structured nucleation of ice and manipulation of particles during solidification. External control over the distribution of particles is governed by a competition of forces between constitutional supercooling and electromagnetism or acoustic radiation. Here, we review studies that apply external fields to create porous ceramics with different microstructural patterns, gradients, and anisotropic alignments. The resulting materials possess distinct gradient, core–shell, ring, helical, or long-range alignment and enhanced anisotropic mechanical properties.

Microstructure Evolution of Binary and Multicomponent Manganese Steels During Selective Laser Melting: Phase-Field Modeling and Experimental Validation

Abstract: In additive manufacturing processes, solidification velocities are extremely high in comparison to ordinary directional solidification. Therefore, the dependencies of the primary dendrite arm spacing (PDAS) on the process parameters deviate from the dependencies predicted by standard analytical methods. In this work, we investigate the microstructure evolution and element distribution in Fe-18.9Mn and Fe-18.5Mn-Al-C alloys solidified during the selective laser melting process. A quantitative multicomponent phase-field model verified by Green-function calculations (Karma, Rappel: Phys. Rev. E, 1998, 57, 4323) and the convergence analysis is used. The resulting non-standard dependencies of the PDAS on the process parameters in a wide range of solidification velocities are compared with analytical calculations. It is shown that the numerical values of the PDAS are similar to the values predicted by the Kurz–Fisher method for the low and intermediate solidification velocities and are smaller for the solidification velocities higher than 0.03 m/s. The PDAS and the Mn distribution in a Fe-18.5Mn-Al-C alloy are compared to the experimental results and a very good agreement is found.

A Parallel Cellular Automata Lattice Boltzmann Method for Convection-Driven Solidification

Abstract: This article presents a novel coupling of numerical techniques that enable three-dimensional convection-driven microstructure simulations to be conducted on practical time scales appropriate for small-size components or experiments. On the microstructure side, the cellular automata method is efficient for relatively large-scale simulations, while the lattice Boltzmann method provides one of the fastest transient computational fluid dynamics solvers. Both of these methods have been parallelized and coupled in a single code, allowing resolution of large-scale convection-driven solidification problems. The numerical model is validated against benchmark cases, extended to capture solute plumes in directional solidification and finally used to model alloy solidification of an entire differentially heated cavity capturing both microstructural and meso-/macroscale phenomena.

On the Deformation of Dendrites During Directional Solidification of a Nickel-Based Superalloy

Abstract: Synchrotron X-ray imaging has been used to examine in situ the deformation of dendrites that takes place during the solidification of a nickel-based superalloy. By combining absorption and diffraction contrast imaging, deformation events could be classified by their localization and permanence. In particular, a deformation mechanism arising from thermal contraction in a temperature gradient was elucidated through digital image correlation. It was concluded that this mechanism may explain the small misorientations typically observed in single crystal castings.

Porous polysilazane-derived ceramic structures generated through photopolymerization-assisted solidification templating

Abstract: A novel approach to the preparation of porous polysilazane-derived ceramics is presented, combining non-aqueous solidification templating (freeze casting) of a liquid preceramic polymer with a photo-induced thiol-ene “click” reaction at temperatures below −10 °C. Upon directional solidification of the reaction mixture consisting of the preceramic polymer, a quaternary thiol, a photoinitiator, and camphene as pore-structuring agent, low-temperature photopolymerization of the preceramic polymer was achieved by irradiation with visible light, its feasibility demonstrated by photorheology. Effects of the polymer/structuring agent ratio and the solidification rate on porosity and pore morphology were evaluated using porosimetry and tomography techniques. The structural features were set in relation to mechanical properties, showing compressive strengths up to 74 MPa at porosities between 58% and 76%. This new processing technique facilitates the generation of polysilazane-derived ceramics with highly tailorable pore structures, with prospective uses in processes ranging from membrane-based separation to catalysis.

Computational and experimental modeling of new forging ingots with a directional solidification: the relative heights of 1.1

Abstract: The work’s purpose is to increase the forging ingot’s quality. A crystallization mechanism of shortened ingots has been investigated. The shortened ingots allow changing a direction of the metal solidification. The rational geometry of the ingot (a ratio of height to diameter and a value of taper) needs to be determined. The special technique of the theoretical and experimental investigations has been developed. The theoretical investigations have been carried out by finite element method. An adequacy test of the theoretical results has been carried out by experimental investigations on transparent and metallic models (pure aluminum) of the ingots. The macrostructure investigations of the shortened ingots with directional solidification have been produced. A hardness distribution on the longitudinal section has been investigated. It was found that for the ingot with the ratio of H/D = 1.1 the most part of the ingot’s body (60–70%) has a dense, homogeneous and small-grained structure. The small grain, which provides a dense structure, forms at the bottom and in the corners from the bottom side. The distribution of the grain dimensions along the ingot radius at different height levels allows to conclude that the bottom and middle parts along the height have the same small-grained structure. The high and uniform hardness values are characteristic for the ingot with H/D = 1.1. The middle and hot-top layers of the ingot have commensurate hardness, which have a difference from the bottom part maximum on the 5%. The ingot with the reverse taper of 7% allows to make a most dense and homogeneous structure. Thus, the improvement in the large forged quality ingots is possible by providing the directional solidification and reverse taper.

Microstructure and mechanical properties of Ti–Re alloys manufactured by selective laser melting

Abstract: In the present study, a commercially pure (CP) Ti and Ti–Re alloys containing 2 and 4 wt% of Re were manufactured by selective laser melting (SLM) and characterized in terms of microstructure, strength and fatigue crack propagation resistance. On the contrary to a homogenous lath-type martensitic α′ microstructure with no signs of directional solidification observed for CP Ti, the Re addition led to development of columnar prior β grains oriented along building direction with a much finer acicular α′ martensite in Ti–Re alloys. The width of martensitic α′ needles decreased with increasing Re content. Re affected also the formation of different phase constituents. The presence of ω phase precipitates as well as residual undissolved Re particles was noticed in Ti–Re alloys. Ti–Re alloys exhibited also the substantially increased ultimate tensile strength and drastically reduced ductility in comparison to CP Ti. These findings have been discussed in the paper considering the highly refined acicular α′ martensitic structure, the increased oxygen content as well as the presence of strengthening ω phase precipitates in Ti–Re alloys. Finally, the brittleness of Ti–Re alloys caused the deterioration of their fatigue crack propagation resistance.

Thermal dependence of large-scale freckle defect formation

Abstract: The fundamental mechanisms governing macroscopic freckle defect formation during directional solidification are studied experimentally in a Hele-Shaw cell for a low melting point Ga-25wt.%In alloy, and modelled numerically in 3D using a microscopic parallelised Cellular Automata lattice Boltzmann method. The size and distribution of freckles (long solute channels, or chimneys) is shown to be strongly dependent on the thermal profile of the casting, with flat, concave and convex isotherms being considered. For the flat isotherm case, no large-scale freckles form, while for concave or convex isotherms large freckles appear but in different locations. The freckle formation mechanism is as expected buoyancy-driven, but the chimney stability, its long-term endurance and its location, are shown to depend critically on the detailed convective transport through the inter-dendritic region. Flow is generated by curved isopleths of solute concentration. As solute density is different from that of the bulk fluid, gravity causes ‘uphill´ or ‘downhill’ lateral flow from the sample centre to the edges through the mush, feeding the freckle. An excellent agreement is obtained between the numerical model and real-time x-ray observations of a solidifying sample under strictly controlled temperature conditions.

Formation of Accumulated Misorientation During Directional Solidification of Ni-Based Single-Crystal Superalloys

Abstract: The formation of accumulated misorientation during directional solidification of Ni-based single-crystal superalloys with 〈001〉 direction deviating from cylinder axis by 20 deg was investigated at different withdrawal rates. The accumulated misorientation increased from the converging side of the dendrite with respect to the mold wall to the diverging side in the radial direction. With increased solidification distance, the accumulated misorientation in the solidification direction reached 7.6 deg, and was not significantly affected by withdrawal rates.