Directional Solidification Microstructure Research Articles

Effect of Ce and solidification cooling rate on the microstructure and mechanical properties of AA2017 aluminum alloy

Controlling dendritic growth and grain size in new Ce-containing Al-based alloys becomes crucial due to new envisioned applications and Ce rising demand in casting processes. In the present work Ce effect on the 2xxx Al series microstructures at various industrial-scale solidification cooling rates was investigated. The directional solidification technique is crucial in this endeavor since can generate several solidified samples related to several cooling rates. The AA2017 alloy and a modified version containing 3 wt % of Ce were both produced under directional solidification and various microstructure aspects were characterized. The mechanical behavior was analyzed by microhardness and compression tests. The Ce addition reduced primary and secondary dendritic spacing without impacting grain size. Cells were only observed to form on the Ce-containing alloy at cooling rates of approximately 19 °C/s; while dendritic configurations dominated all other conditions. While round pockets composed of Al+Al2Cu+Mg2Si ternary eutectic formed the interdendritic zones in the AA2017 alloy, the AA2017-Ce alloy microstructure was mainly constituted by elongated AlCeSi + Al8CeCu4 interdendritic phases. Finally, the addition of Ce favored an increase in the microhardness and compressive strength values of the AA2017 alloy, which is attributed to the refinement of the solidification structure, as well as the formation of a greater fraction of secondary reinforcement phases.

Microstructural characteristics and material removal mechanisms in nano-scale finished surface of mirror-like injection mould fabricated by laser metal deposition and post treatments

The manufacture of injection moulds with mirror-like surfaces by laser metal deposition is a state-of-the-artdevelopment trend in manufacturing industries. The high surface quality and long-term service performance are two critical demands for laser-metal-deposited injection moulds. In this paper, the microstructure and material removal mechanisms of laser-metal-deposited moulds after nano-scale surface finishing and post treatment were analyzed. Our results confirmed that the differences in temperature gradient and melting composition during solidification would result in both columnar and equiaxial dendritic αʹ-Fe grains distributed in the mould surfaces, and the surface defects of the moulds after nano-scale finishing were directly related to the aggregation of brittle M23(C,B)6 networks stripped out from inhomogeneous αʹ-Fe matrices. Both laser-surface-heat and vacuum-furnace-heat post treatments were beneficial to the microstructural homogenization. The former could achieve directional solidification microstructures with columnar αʹ-Fe dendrites and well-distributed M23(C,B)6 networks, effectively relieve stress concentration during finishing and successfully enhance surface quality and performance. The latter would produce the formation of retained γ-Fe and M2(C,B) particles, weakening surface quality of moulds. Therefore, the laser-surface-heat post treatment seemed to be one of the most promising ways to address microstructural homogenization of laser metal deposited moulds.

Phase-field model simulations of alloy directional solidification and seaweed-like microstructure evolution based on adaptive finite element method

Based on the multi-grain growth phase field model of multi-element single-phase system, the adaptive finite element method using non-uniform grid is studied to solve the problem. The Al-4wt.%Cu binary alloy is taken as an example to study the planar solidification process and morphological evolution during the initial transition phase of solidification were studied under the condition of large computational domain and thin interfacial layer thickness. The effects of disturbance and anisotropy intensity on the microstructure of the solidification, the influence of physical parameters such as temperature gradient and cooling rate on the interface morphology growth and evolution were quantitatively analyzed. The growth mechanism of the interface morphology during directional solidification and the transformation mechanism of seaweed tissue were discussed. The results show that under the condition of low temperature gradient and high cooling rate, the flat interface is unstable and forms cell or dendrites, and then the tip is continuously split to form seaweed structure morphology. The adaptive finite element method is one order of magnitude lower in computing time and storage space comparing to the finite difference method. When the system size is larger, the superiority of the adaptive finite element method can be better reflected, which facilitates the simulation of the large-scale multi-field coupled phase field model.

Microstructure, Mechanical Properties, and Crack Propagation Behavior in High-Nb TiAl Alloys by Directional Solidification

Titanium aluminide (Ti-47Al-6Nb-0.1C) alloys were prepared using a cold crucible directional solidification technique with an input power range of 35 to 55 kW under a withdrawing velocity of 0.4 mm/min. The macro/microstructure was characterized, and the mechanical properties were evaluated. The results show that the directional solidification (DS) ingots exhibit β-solidification characteristics at an input power range of 35 to 55 kW, and a well-developed DS microstructure was acquired at an input power range of 40 and 45 kW. With the increasing input power, the lamellar spacing decreases, resulting in the increasing tendency in the average room-temperature yield strength. The steady-state creep rate strongly depends on the lamellar spacing, and the creep life depends on the DS microstructure. The well-developed DS alloy can significantly improve the creep properties but has little influence on the room-temperature tensile properties. Moreover, after testing the fracture toughness, the crack propagation showed interlamellar cracks, translamellar cracks, and intercolony boundary cracks that primarily propagated along the colony boundary after creep testing.

选区激光熔化SRR99高温合金的定向凝固组织及偏析

选区激光熔化SRR99高温合金的定向凝固组织及偏析

In-situ fabrication of amorphous/eutectic Al2O3–YAG ceramic composite coating via atmospheric plasma spraying

The microstructure and property of plasma-sprayed Al2O3–Y3Al5O12 (YAG) composite coatings were investigated. The sprayable feedstock was composed of Al2O3/Y2O3 granules obtained from the spray drying. As-sprayed Al2O3–YAG composite coating showed obvious amorphous characteristics. Post-heat treatment facilitated the complete crystallization of the Al2O3–YAG composite coating. The heat-treated Al2O3–YAG ceramic coating possessed the deposition microstructure comprising three-dimensional interpenetrating network of α-Al2O3/YAG eutectic phases, which is very similar to the directional solidification microstructure of Al2O3/YAG eutectic bulk ceramics. This eutectic coating microstructure does not correspond to a traditional eutectic microstructure. Al2O3–YAG ceramic coating with eutectic composition is obtained from the crystallization of an amorphous solid ceramic instead of the solidification of the liquid. Furthermore, the plasma-sprayed Al2O3–YAG eutectic ceramic coating with the heat treatment exhibits excellent microstructure and performance stabilities under high temperature.

Simulation of liquid channel of Fe-C alloy directional solidification by phase-field method

In directional solidification, two characteristic parameters determine the dendritic growth: the thermal gradient and the pulling velocity. To achieve the suitable microstructure and improve the performance of casting, they are usually used to resize the pulling velocity or temperature gradient in directional solidification process. The structures obtained under different directional solidification conditions, and their associated properties both have been hot research points. It is difficult to observe the microstructure, which is usually on a micrometer scale, directly in experiment, and the phase-field method becomes a strong tool to understand the dendrite growth pattern. We mainly study the liquid channel formed after Fe-C alloy dendrite tip splitting under the specific condition of directional solidification and analyze the influence on liquid channel of pulling velocity in this paper. We choose the fixed thermal gradient G =20 K/mm which is on the order of the experimental value, and pulling velocity VP no more than 10 mm/s to keep the cooling rate in the range of low speed in dendrite growth, so that the interface kinetic effect can be neglected. Recent experimental results show the different interfacial energies in various compositions of Al-Zn alloy and Fe-C alloy, then we can investigate a series of directional solidification microstructures with fixed alloy Fe-0.5 wt.%C composition at different interfacial energies in our simulations. We find that the liquid channel is formed as a result of anisotropy competition between system and materials, the length and C concentration of liquid channel increase with the pulling velocity increasing, while the diameter of liquid channel is constant. It is interesting to find that there is a minimum of pulling velocity almost equal to 1 mm/s, the tip will not split and no liquid channel forms in the following steps either when the velocity is smaller than the minimum. We also compare the segregation caused by solute enrichment in liquid channel and solute segregation between dendrite arms in a series of simulations: the former is more serious than the latter. Then we point out the way to reduce the segregation caused by liquid phase channel by reducing the pulling velocity properly. It will be more practical to couple the flow field with other external field, such as magnetic field, in the simulation.

Directional Solidification Microstructure of a Ni-Based Superalloy: Influence of a Weak Transverse Magnetic Field

A Ni-based superalloy CMSX-6 was directionally solidified at various drawing speeds (5–20 μm·s−1) and diameters (4 mm, 12 mm) under a 0.5 T weak transverse magnetic field. The results show that the application of a weak transverse magnetic field significantly modified the solidification microstructure. It was found that if the drawing speed was lower than 10 μm·s−1, the magnetic field caused extensive macro-segregation in the mushy zone, and a change in the mushy zone length. The magnetic field significantly decreases the size of γ’ and the content of γ-γ’ eutectic. The formation of macro-segregation under a weak magnetic field was attributed to the interdendritic solute transport driven by the thermoelectric magnetic convection (TEMC). The γ’ phase refinement could be attributed to a decrease in nucleation activation energy owing to the magnetic field during solid phase transformation. The change of element segregation is responsible for the content decrease of γ-γ’ eutectic.

Influence of thermal stabilization treatment on the subsequent microstructure development during directional solidification of a Ti–46Al–5Nb alloy

Directional solidification (DS) experiments with thermal stabilization (TS) treatments were performed on Ti–46Al–5Nb (at.%) alloys in a Bridgman-type furnace using a quenching technology. Influence of the TS treatment on mushy zone and directional growth afterwards were investigated. The results show that the length of the mushy zone decreases but the β dendrite spacing in directional growth significantly increases with increasing TS time. During the DS process, β dendrite spacing is more homogeneous and its growth direction is more inclined to parallel to the axial direction with increase of the TS time. Al solute concentration in the mushy zone in a steady-state is always lower than that in original as-cast alloys. The mushy zone with the columnar β and α grains is easily produced after TS treatment on the alloys with microstructures of the directional dendrite segregation morphology before DS starting. TS treatment results in the redistribute of solute Al thus changes the phase constituent in the mushy zone. An appropriate TS is necessary to produce the L + β + α region in the mushy zone, which is of great benefit to control DS microstructures of TiAl peritectic alloys.

Effect of power on microstructure and mechanical properties of Ti44Al6Nb1.0Cr2.0V0.15Y0.1B alloy prepared by cold crucible directional solidification

Abstract A newly designed alloy Ti 44 Al 6 Nb 1.0 Cr 2.0 V 0.15 Y 0.1 B ingot was prepared by the cold crucible directional solidification (CCDS) technique under a range of input power with the constant drawing speed of 8.33 μm/s. Experimental results show that the as-cast microcrack can be eliminated completely after CCDS, and the most desirable microstructure, a high percentage of small angle lamellas microstructure is acquired at input power of 45 kW. It also indicates that the interlamellar space gradually decreases with the increasing of power. The measurement of nanoindentation hardness shows that the hardness of B 2 and γ phase in all cast routes is consistent, but in lamella, the hardness increase linearly with the interlamellar space decrease after logarithm. The relationship between the interlamellar space and the nanoindentation hardness was given. The room temperature tensile results confirmed with the desirable directional solidification microstructure, the highest ultimate tensile strength (UTS) value and the elongation are achieved.

Microstructures and mechanical properties of Nbss/Nb5Si3 in-situ composite prepared by electromagnetic cold crucible directional solidification

The Nbss/Nb5Si3 in-situ composite with the nominal composition of Nb–24Ti–12Si–4Cr–4Al–2Hf (at%) has been successfully fabricated by electromagnetic cold crucible directional solidification. Microstructures of the initial transition (IT) zone and the steady-state growth (SSG) region were observed by scanning electron microscope, and their phase compositions were analyzed with X-ray diffraction (XRD). The room temperature fracture toughness and the tensile strength at 1250°C were measured. The results indicated that from IT zone to SSG zone, the major silicide transferred from γ-(Nb,Ti)5Si3 to β-(Nb,Ti)5Si3. The directional solidification microstructure with coupled Nbss and Nb5Si3 was created in SSG region. Compared SSG region with IT zone, the room temperature fracture toughness and the high temperature tensile strength increased about 50% and 24%, respectively, which was contributed to the directional solidification microstructure and the type transformation of silicide.

Directional Solidification Microstructure Control in CM247LC Superalloy

Directional solidification of the superalloy CM247LC has been carried out to investigate the microstructural evolution under various solidification conditions. It can be clearly seen that the solid/liquid interface morphology depends primarily on the growth rate and temperature gradient. The planar interface was stable at low velocities and the increased velocity encouraged the development of dendritic interface. As a result, the conditions for the planar to cellular transition and the cellular to dendritic transition were obtained and therefore the solidification microstructure map was constructed based on these results. The experimental results were in good agreement with theoretical predictions for this alloy. However, the condition for the dendritic to equiaxed transition was discussed in the light of the heat balance at the solid/liquid interface.

The microstructure prediction of magnesium alloy crystal growth in directional solidification

Directional solidification process can better control the grain orientation and eliminate the horizontal grain boundary, so excellent performance of alloy can be achieved by it. In the paper, the macro and micro analysis were coupled to predict the microstructure growth of magnesium alloys in directional solidification process. FEM (finite element method) was used to calculate undercooling temperature, by which the macro analysis results were coupled into the micro analysis. Phase-field method (PFM) offers the prospect of carrying out realistic numerical calculation on dendrite growth in metallic systems, and it was employed to calculate real dimension of magnesium alloy microstructure in directional solidification. Different magnitude values of anisotropy that predicted the columnar dendrite evolution were calculated. Quantitative comparison of numerical simulation results to the experiment analysis were carried out. The microstructure obtained by PFM is in good agreement with the actual pattern by the optical microscope observation.

Flow field and its effect on microstructure in cold crucible directional solidification of Nb containing TiAl alloy

A 3D finite element model was established to investigate the flow field in TiAl melt under different process parameters that include the position of the solidification interface, the meniscus height, the heating power and the current frequency. A square cold crucible with an inner cross section of 36mm×36mm was employed to directionally solidify Ti–46Al–6Nb–B (at.%) ingots. The flow pattern in the melt is highly parameter-related. Generally, there are two flow swirls with opposite directions in the melt. The flow near the solidification interface can be minimized with an optimal meniscus height. The flow velocity is decreased with a decrease of heating power and an increase of current frequency. A weaker lateral flow near the solidification interface is beneficial for the continuous growth of columnar grains. The microsegregation of a directionally-solidified ingot can be reduced by controlling the flow in the melt, which is one of the advantages of cold crucible directional solidification.

Simulation of Bridgman Directional Solidification Microstructure Using Phase Field Method

In this paper, the directional solidification microstructure of Bridgman system was simulated using phase-field method, and different calculated results were obtained with four pulling velocities. When the pulling velocity is 0.06 cm/s, the columnar crystals competitively grow in the initial stage, and have a necking phenomenon in the last stage. When the pulling velocity is 0.04 cm/s, the columnar crystals become thinner and competitively grow all the time, and the microsegregation is bigger. When the pulling velocity is 1.00 cm/s, planar interface comes back, and solute trapping takes place. The columnar crystals become much thinner, and microsegregation decreases. When the pulling velocity is 3.00 cm/s, the grain boundary of columnar crystals becomes unconspicuous, and the degree of microsegregation approaches 1.

Effect of rotating magnetic field on microstructure formation of directionally solidified Sn–1.6Cd peritectic alloy

In order to investigate the effect of rotating magnetic field on the microstructure formation of peritectic alloys, directional solidification experiments of Sn–1.6Cd peritectic alloy have been conducted under different rotating magnetic field conditions. The directional solidification microstructure of Sn–1.6Cd peritectic alloy changes from banded structure to axisymmetric isolated banded structure to axisymmetric oscillatory tree-like banded structure and to single primary phase structure when the magnetic Taylor number of forced-melt flow generated by a rotating magnetic field increases from 0 to 91 to 364 and to 1456. The second and third banded structures are observed in a peritectic alloy for the first time. The results indicate that it is possible to control solidification microstructure and prepare a single primary phase structure by using a rotating magnetic field during directional solidification of peritectic alloys. The experiments show that the effect of forced-melt flow on solute distribution transforms from solute buildup to homogenization with an increase in the magnetic Taylor number. The formation mechanisms of those structures are also discussed.

High-temperature tensile properties of a NiTi–Al-based alloy prepared by directional solidification and homogenizing treatment

To develop a new type of light-weight, high strength, high-temperature structural material, cast bars of a NiTi–Al-based alloy with low Nb and Hf contents were prepared by DS (directional solidification). The cast bars prepared by optimal DS were homogenizing treatment at 950°C for different times. The tensile strength of the homogenized test samples at elevated temperatures was tested. The results reveal that the DS microstructure is a cellular–dendrite or acicular cellular structure that preferentially grows along the [001] orientation. With an increase in the drawing velocity, the cellular crystals and cellular arm spacing gradually decrease, but the directionality of the structure weakens. With an increase in the homogenization time, the cellular structure becomes increasingly coarse, and the precipitated phases are more dispersed and distributed more homogeneously. At 800°C and 850°C, the highest tensile strengths occur with 12h of homogenizing treatment, resulting in strengths of up to 340MPa and 263.6MPa, respectively; at 900°C, the tensile strength is 171.5MPa. This alloy is expected to become a high-temperature structural material for applications at above 800°C. The highest elongation at 800°C is 42.4%, and at 850°C and 900°C, the highest elongation is 47.2% and 70.8%, respectively. The alloy exhibits good plasticity at high temperatures.

Evolution of directional solidification microstructure of Pb-80wt%Sn alloy under medium-density direct current

The effect of a medium-density DC-current and its direction on the directional solidification microstructure and cooling behavior of Pb-80wt%Sn alloy were studied. Experimental results indicated that the microstructure was refined with DC compared to the one without. With the increase of DC density, the primary (λ1) and secondary (λ2) dendrite arm spacing of the primary phase decrease clearly. It is also found that not only the intensity of the DC current but also its direction affects the microstructure. The size of λ1 and λ2, in samples treated with positive DC, decreases more clearly than in those with negative DC. The interface morphology in samples processed without and with DC is different when the current direction is changed. Finally, the mechanism of microstructure evolution is discussed.

New evidence for the dual origin of anomalous eutectic structures in undercooled Ni–Sn alloys: In situ observations and EBSD characterization

Rapid solidification processes of glass-fluxed Ni 81.3Sn 18.7 eutectic alloys at various undercoolings were observed in situ using a high-speed camera. The microstructure of the samples was characterized using optical microscopy, scanning electron microscopy and electron back-scattering diffraction analysis. It was found that the recalescence front sweeps the sample surface from one side to the other, which coincides with the formation of a directional solidification microstructure. Such a finding suggests that it is feasible to measure crystal growth velocities in the undercooled liquid by tracking the loci of the recalescence front. The measured data show sudden acceleration of crystal growth kinetics at a medium undercooling, giving evidence for a transition from coupled growth to uncoupled growth. Microscopic studies revealed that the sample with an undercooling of 202 K comprises two types of anomalous eutectic, which are distinguished by the size and orientations of Ni-rich grains. The results verify a recent hypothesis that anomalous eutectic in undercooled Ni–Sn alloys has a dual origin: it results from eutectic dendrites growing at low undercoolings and from single-phase Ni-rich dendrites growing at high undercoolings.

Laser Direct Metal Deposition Technology and Microstructure and Composition Segregation of Inconel 718 Superalloy

Multilayer of laser direct metal deposition (DMD) was prepared by depositing a gas atomized pre-alloyed powder with a composition close to Inconel 718 alloy on Inconel 718 high temperature alloy substrate. The effects of the DMD parameters on the build-up rate and the structure of the deposited layer were studied. The laser DMD sample was further processed by a solution treatment. The microstructure and property of the laser DMD zone before and after heat treatment were investigated as well. The results show that the laser parameters of actual laser power of 650 W, scanning speed of 5.8 mm/s, beam diameter of 1 mm, powder feed rate of 6.45 g/min, with a corresponding specific energy of 90–130 J/mm 2, can be recommended as optimum parameters for high build-up rate of Inconel 718 alloy. Under the condition of optimized parameters, a directional solidification microstructure was obtained and the average distance between the columnar crystals was 5–10 μm. The microcomposition segregation was found between the columnar crystal trunk and columnar crystal. The elements of Nb, Mo, Ti concentrated in the columnar crystal trunk. After the heat treatment, the segregation was greatly minimized, and the segregation ratios were close to 1. The hardness of the laser deposited layer did not show obvious difference along the height of the layer either for the as-deposited layer or for the heat treated layer. However, the microhardness of the laser DMD zone after heat treatment was obviously higher than that after the as-deposited treatment. During the heat treatment process, some Nb- and Mo-rich phases precipitated and strengthened DMD layer.