Solidification Microstructure Selection Research Articles

Constructing solidification microstructure selection map for AlSi10Mg alloy processed by laser powder bed fusion

The cellular structures, which consist of cells and eutectic networks at the cell boundaries, determine the mechanical properties of the Al–Si alloys fabricated by laser powder bed fusion (L-PBF). Therefore, it is scientifically interesting and technologically important to establish the relationship between cellular structures and processing parameters, which is applicable to all kinds of processing methods involving rapid directional solidification, such as L-PBF. In this work, a rapid solidification microstructure selection map (SMSM) of the Al–10Si system in temperature gradient (G)–solidification velocity (V) space is constructed by a synergistic calculation with dendritic and eutectic growth models compatible to the L-PBF process. Considering that the divorced eutectic reaction should occur as the width of the liquid film entrapped in the α-Al cell boundaries becomes smaller than the Al–Si eutectic spacing, the G–V domain suitable for acquiring the cellular structures decorated with divorced eutectic Si networks is determined on the SMSM. The microstructure evolution trends predicted by the SMSM are well consistent with those in the L-PBFed AlSi10Mg melt pools with different processing parameters. The length scales of actual microstructures observed by the experiments from this work and the literature are quantitatively compared to those estimated by the SMSM, and the experimental results are basically in accord with the SMSM. It is demonstrated that divorced eutectic Si networks prefer to form at the bottom of the melt pool, where G is large (> 5 × 106 K/m) and V is small (3 × 10−4–6 × 10−3 m/s) in the SMSM. The outcomes of this work may provide fundamental guidance for tailoring the microstructures of the L-PBFed Al–Si alloys for achieving optimal mechanical properties.

Understanding cellular structure formation in rapidly solidified Al–Ni alloys by constructing solidification microstructure selection maps

Understanding cellular structure formation in rapidly solidified Al–Ni alloys by constructing solidification microstructure selection maps

Solidification microstructure selection maps for laser powder bed fusion of multicomponent alloys

Solidification Microstructure Selection (SMS) maps provide a simple yet effective approach to predict the non-equilibrium solidification microstructure and grain morphology during Additive Manufacturing. In this study, SMS maps have been created for the Inconel 625 (IN625) alloy processed by Laser Powder Bed Fusion (LPBF). Toward this end, theoretical solid growth models, a model of the Columnar to Equiaxed Transition (CET), interface response theory, thermal simulation results and computational thermodynamics are utilized. The predicted microstructures are compared both qualitatively and quantitatively to experimentally-obtained micrographs. The theoretical analysis was also compared to the earlier analytical calculation for Al-10Si-0.5Mg alloy to show how differences in thermophysical properties affect the microstructural predictions. The theoretical predictions are shown to be in good agreement with the experimental results in terms of the resulting microstructure and dendrite arm spacings. A discussion on the use of SMS maps, formed over a broad range of thermophysical conditions, to help guide industry in improving LPBF microstructure, is provided.

Revisiting solidification microstructure selection maps in the frame of additive manufacturing

Understanding microstructural development in additive manufacturing under highly non-equilibrium cooling conditions and the consequent effects on mechanical properties of the final component is critical for accelerating industrial adoption of these manufacturing techniques. In this study, simple but effective theoretical solidification models are recalled to evaluate their ability to predict of microstructural features in additive manufacturing applications. As a case study, the resulting solidification microstructure selection maps are created to predict the stable growth modality and the columnar to equiaxed transition (CET) of an Al-10Si-0.5Mg alloy processed via Selective Laser Melting. The potential of this method in microstructural predictions for additively manufactured products, as well as outstanding challenges and limitations, are discussed.

Solidification microstructure selection of the peritectic Nd-Fe-B alloys

Bridgman directional solidification and laser remelting experiments were carried out on Nd11.76Fe82.36B5.88 and Nd13.5Fe79.75B6.75 alloys. Microstructure evolutions along with solidification parameters (temperature gradient G, growth velocity V and initial alloy composition C0) were investigated. A solidification microstructure selection map was established, based on the consideration of solidification characteristics of peritectic T1 phase. In Bridgman directional solidification experiments, with the increasing growth velocities, the morphology of T1 phase changed from plane front or faceted plane front to dendrites. In laser remelting experiments, a transition from primary γ-Fe dendrites to T1 dendrites was found. Theoretical predictions are in good agreement with experimental results.

Electron beam surface melting of model 1200 Al alloys

Electron beam surface melting has been used to characterise the phase content formed in a number of model 1200 series Al alloys with increasing solidification velocity in the range 2–50 mm s −1, typical of that experienced during continuous strip casting. Phases were extracted from the Al matrix and analysed by X-ray diffraction. A qualitative solidification microstructure selection map has been produced, showing that, for a given Fe content of 0.55 wt.%: with increasing solidification velocity the metastable aluminides FeAl 6 and FeAl m displace equilibrium Fe 4Al 13 at Si contents <∼0.15 wt.%, and that α-AlFeSi displaces these Fe aluminides at Si contents >0.15 wt.%, and that α-AlFeSi is an equilibrium phase at a Si content ≥0.50 wt.%.

Two-phase growth in peritectic Fe–Ni alloys

Bridgman crystal growth experiments were carried out to investigate the solidification behavior of Fe–Ni alloys containing nominally between 4 and 4.5 at.% Ni. Due to macrosegregation, a radial concentration gradient was established across the cylindrical specimens. Due to this gradient, a series of solid/liquid interface morphologies was observed. Oriented two-phase microstructures, which formed either lamellar or fibrous δ-ferrite in an austenite ( γ) matrix, were found in the central region of specimens with a composition of some 4.2 at.% Ni and a G/ V ratio close to the critical ratio for solid/liquid interface breakdown. At slightly smaller concentrations, oscillatory two-phase structures formed which were similar to the 2- λ instabilities of off-eutectic alloys. The observations confirm that at low solidification rates the stable growth morphology in peritectic alloys cannot be selected by the highest growth temperature criterion. A recently developed nucleation and constitutional undercooling criterion (NCU) was applied to establish a solidification microstructure selection map. Reasonable agreement was obtained between calculated and experimental results. Based on eutectic growth theory the possibility of simultaneous two-phase growth in peritectic alloys is discussed.

Unidirectional solidification of Zn-rich Zn–Cu peritectic alloys—I. Microstructure selection

A solidification microstructure selection diagram has been determined for Zn-rich Zn–Cu peritectic alloys containing up to 7.37 wt% Cu over the growth velocity range 0.02–4.82 mm/s and at a temperature gradient of 15 K/mm by means of the Bridgman technique. Regular and plate-like cellular growth was observed in a range of composition near the peritectic point at growth velocities above 0.5 mm/s. The minimum growth velocity for the formation of plate-like cellular growth was about 2.64 mm/s in the compositional range from 2.17 to 4.94 wt% Cu. Two transitions, namely, arrayed to nonaligned growth of primary dendrites, and peritectic to non-peritectic reaction, were also characterized and analyzed. The transition velocity from arrayed to nonaligned growth of primary dendrites increased with increasing alloy concentration, which is consistent with the prediction of a modified Hunt model. The transition growth velocity from peritectic to non-peritectic reaction exhibited a maximum near the peritectic composition.

Solidification microstructure selection in the Al-rich Al–La, Al–Ce and Al–Nd systems

The effect on solidification and microstructure of solidification conditions in wedge chill casting and Bridgman growth have been investigated for Al 5.7 to 19 wt% X (X=La, Ce, or Nd) alloys. The systems are shown to exhibit an asymmetrical zone of coupled eutectic growth on a plot of front velocity versus alloy composition. Only the equilibrium phases αAl and Al 11X 3 form under the conditions studied and the results are consistent with a eutectic composition of ∼12 wt% X for all three systems, as proposed in 1990 by Kononenko and Golubev. Both primary αAl and primary Al 11X 3 underwent significant morphological changes as the boundaries of the coupled zone were approached. The positions of these boundaries at high velocities yielded values of the growth parameters for primary Al 11X 3 and the αAl–Al 11X 3 eutectic similar in magnitude to recent measurements for corresponding primary phases and eutectics in the related Al–Fe and Al–Ni systems.

Solidification Phase and Microstructure Selection Maps for Fe-Cr-Ni Alloys.

Directional solidification and laser experiments have been performed in Fe-18.0%Cr-Ni alloys to evaluate the effects of alloy composition and growth velocity on microstructure and phase selection. Solidification microstructure selection maps are described by microstructure (dendrite/cell, plane front and eutectic) models and compared with experimental results. Theoretical predictions are in good agreement with experimental results. It demonstrates the potential of this approach to analyze the microstructure and phase formation.

Growth Temperature Measurements and Solidification Microstructure Selection of Primary Al 3Ni and Eutectic in the αAl-Al 3Ni System

Competition between growth of contending primary phases and their eutectics is a key consideration in determining the solidification microstructure of a material that results from particular conditions. Application of the principle that solidification is led by the constituent which grows at the highest temperature under the prevailing conditions gives predictions that are in good accord with experimental fact. The necessary growth temperatures and their dependence on materials composition and solidification conditions have been measured, however, for rather few constituents in relatively few systems as yet. The present purpose is to extend to primary Al{sub 3}Ni and the Al-Al{sub 3}Ni eutectic in the Al-Ni system the measurements made earlier during Bridgman growth, for primary Al{sub 3}Fe(Al{sub 13}Fe{sub 4}) and for the Al-Al{sub 3}Fe and Al-Al{sub 6}Fe eutectics in the Al-Fe system. These measurements will be compared with predictions of dendrite and eutectic growth theory and related to the relevant boundary of the reported Al-Al{sub 3}Ni solidification microstructure selection diagram. Some preliminary results together with other findings were reported earlier in a cited reference.

Solidification microstructure maps in Ni [sbnd]Al alloys

Laser surface resolidification experiments have been performed on Ni Al alloys with 70–89 at. % Ni in the velocity range of 5·10 −4–5·10 −1 m/s. Microstructures have been studied by optical and transmission electron microscopy. Growth of primary γ-Ni, β-NiAl and metastable β-γ eutectic occurred whereas no primary γ′-Ni 3Al was observed. A solidification microstructure selection map has been drawn, showing the coupled zone of metastable eutectic and the limit of plane front growth of γ-Ni. The microstructure map has also been calculated on the basis of plane front, cellular/dendritic and eutectic growth models. There was a good agreement between experimental and calculated results, showing the potential of this approach for prediction of solidification microstructure.

Composition and solidification microstructure selection in the interdendritic matrix between primary Al3Fe dendrites in hypereutectic Al[sbnd]Fe alloys

The composition and constitution of matrix microstructure between plate-like Al3Fe dendrites in Bridgman-grown hypereutectic AlFe alloys has been determined as a function of alloy concentration C0 and growth velocity V in the ranges 2.5 < C0 < 28.1 wt% Fe and 0.01 <V < 5.0 mm/s. The transition at V = 0.1 mm/s from a fully eutectic matrix at C0 = 3.5 wt% Fe to one containing αAl dendrites at C0 ≥ 4.7 wt% Fe is attributed to growth temperatures of αAl dendrites that are higher than those of eutectic in a matrix of lower iron-content, which results from these conditions. The matrix eutectic changes from irregular αAlAl3Fe to regular αAlAlxFe with increasing V, the transition velocity increasing from 0.1 to 0.2 mm/s for C0 values of 9.5 and 14 wt%Fe up to 0.34–1.0 mm/s for C0 values of 18.7–28.1 wt%Fe. This increased transition velocity, compared with that for αAlAl3Fe to αAlAl6Fe at lower concentration, is indicative of a lower formation temperature for the αAlAlxFe than the αAlAl6Fe eutectic.

Metastable Phase Diagrams and Rapid Solidification Processing.

Efficient metallurgical and processing development requires ability to predict the solidification microstructure of products. Models were developed to calculate possible growth morphologies and phases in order to select the most efficient one for a given solidification condition. It is then possible to predict solidification microstructure selection maps (SMSM) indicating the microstucture as a function of the composition and the growth rate. A summary of the methodology of solidification microstructure prediction is given. Results obtained in a large velocity range for Al-Fe and Al-Cu systems are reviewed with experimental comparison and emphasis on the role played by metastable phase equilibria in the modelling. Finally, an outline of the actual work extending this approach to multicomponent alloys is given. Provisional calculated solidification microstructure maps for the Al-Cu-Si system are presented.

Solidification microstructure selection and characteristics in the Zn-based ZnAl system

A solidification microstructure selection diagram has been determined for ZnAl alloys containing 2–10 wt% Al by Bridgman and TIG weld traversing over the growth velocity range 0.01–21 mm/s. Secondary dendrite arm spacings λ 2 of αZn and βAl decreased with increasing V such that λ 2V 1 3 is constant at fixed alloy concentration. Eutectic cell size Λ decreased with increasing V such that ΛV 1 2 is constant at 690 ± 130 μ m 3 2 s −1 2 . Eutectic interphase spacing λ decreased with increasing V such that λV 1 2 is constant at 7.1 ± 0.8 μ m 3 2 s −1 2 over the growth velocity range 0.01–21 mm/s in excellent agreement with predictions of the Jackson-Hunt model. Hardness of the eutectic exhibited a Hall-Petch relationship with eutectic interphase spacing λ and eutectic cell size Λ. The value of k y from the eutectic interphase spacing is intermediate in value between those for grain size hardening of the pure constituents.

Solidification microstructure selection and characteristics in the zinc-based Zn system

A solidification microstructure selection diagram has been determined for ZnMg alloys containing up to 4.5 wt% Mg over the growth velocity range 0.001–8 mm/s at temperature grandient 15 K/mm by means of the Bridgman technique. Results are compared with the cmicrostructures of chill cast wedges made from the same alloys. The observed transition from equilibrium eutectic E αβ ( αZn- β Mg 2Zn 11) to nonequilibrium E αγ ( αZn- γ MgZn 2) to E αβ and back to E αγ with increasing V is explained on the basis of competitive growth. Secondary dendrite arm spacing λ 2 of αZn decreases with increasing V such that λ 2V 1 3 is constant at fixed alloy concentration in agreement with predictions of arm coarsening theory. Eutectic interphase spacing λ decreases with increasing V such that λV 1 2 is constant at 7.3 ± 1.3 μ m 3 2 s −1 2 for rod-like E αβ compared with 4.6±1.6 μ m 3 2 s −1 2 for lamellar/spiral E αβ , both values being in reasonable agreement with predictions of the Jackson-Hunt model. Higher hardness levels for E αβ than E αβ at the same growth velocity are attributed to the larger volume fraction of hard intermetallic phase at the same Mg-content in E αβ .