Solidification Velocity Research Articles

Lattice Structures Manufactured by SLM: On the Effect of Geometrical Dimensions on Microstructure Evolution During Processing

Employing selective laser melting direct microstructure manipulation is feasible through adjustment of thermal gradients and solidification velocity. Currently, the exposure strategy and laser energy have to be adapted in order to meet a processing window suited for introducing highly anisotropic microstructures. As selective laser melting allows for production of filigree complex structures, the impact of geometry on the microstructure evolution is investigated in the current study and it is shown that miniaturization of structures as well leads to the evolution of anisotropic microstructure.

Formation of Intermetallic Phases in AlSi7Fe1 Alloy Processed Onboard the ISS

This paper provides an analysis of the formation of intermetallic phases in AlSi7Fe1 alloy in samples processed onboard the ISS. Based on axial 2D cross-sections obtained from regions of pure diffusive growth and also solidified with forced melt flow, the sizes and distribution of intermetallic β-Al5FeSi phases were determined for different solidification velocities. In diffusive case the phases are larger and more homogeneously distributed than in case of induced melt flow. Additionally, especially for lower solidification velocity, the enrichment of Si and Fe in the centre part of the sample results in a few but rather large β-Al5FeSi particles.

Three Dimensional Morphology of Mn Rich Intermetallics in AlSi Alloys Investigated with X-Ray Tomography

Elementary Mn has a great importance as neutralizer of Fe intermetallics like β-Al5FeSi, which have detrimental effect on mechanical characteristics of AlSi alloys. Presence of Mn in AlSi alloys causes the formation of other intermetallic phases. To understand the effect of solidification conditions and fluid flow on the microstructure of AlSi-based alloys and the addition of Mn leading to Mn-based intermetallics, Al-5 wt pct Si 0.2/0.4/1.0 wt pct Mn alloys have been directionally solidified under defined thermal (gradient 3 K/mm, solidification velocity 0.02-0.12 mm/s) and fluid flow (rotating magnetic field 6 mT) conditions. The primary Al-phase and Mn-based intermetallic phases were studied using 3D X-ray tomography. The spatial morphology of primary phase and intermetallics were characterized with respect to different fluid flow and solidification conditions. The tomography has showed the 3D complicated structure of Mn phases developed.

Columnar-to-Equiaxed Transition in Solidification Processing of AlSi7 Alloys in Microgravity the CETSOL Project

This paper gives an overview of the experiments on-board the International Space Station (ISS) performed so far by the CETSOL team. Al-7 wt% Si alloys with and without grain refiners were solidified in microgravity. Detailed grain structure analysis showed columnar growth in case of non-refined alloy, but the existence of a columnar to equiaxed transition (CET) in refined alloy. One main result is a sharp CET when increasing the solidification velocity and a progressive CET for lowering the temperature gradient. Applying a front tracking model this behavior was confirmed numerically for sharp CET. Using a CAFE model both segregation and grain structures were numerically modeled and show a fair agreement with the experimental findings.

Fabrication of Porous Copper with Directional Pores by Continuous Casting Technique Through Thermal Decomposition of Hydride

Lotus-type porous copper with aligned long cylindrical pores was fabricated by continuous casting technique through thermal decomposition method (TDM) in an argon atmosphere of 0.1 MPa. A pellet of titanium hydride was supplied into molten copper with adjusting the time interval to maintain the constant concentration of hydrogen to be dissolved in the melt, when the transfer velocity of the unidirectional solidification is changed. Long lotus-type porous copper slabs were fabricated with constant solidification velocity. The effect of the transfer velocity on the porosity and pore size was investigated. The average pore diameter was independent of the transfer velocity, but the porosity is slightly dependent on the velocity. It is apparent that the continuous casting technique can be applicable for production of lotus metals through TDM.

일정한 응고속도를 갖는 2성분 응고에서 조성 대류의 특성 및 안정성

2성분 응고계에서 다공성 mush 층에서의 조성 대류는 생성되는 제품의 질에 영향을 준다. 본 연구에서는 일정한 속도로 응고되는 mush 층을 고려하였다. 선형 안정성 이론을 사용하여 mush 층에 대한 교란방정식을 유도하였고, 기본상태 온도장과 mush 층에서 기공률의 분포를 수치해법으로 조사하였다. 과열량이 클 때 mush 층의 두께는 열경계층의 두께에 비해 상대적으로 작았다. 과열량이 감소함에 따라 mush 층의 두께를 기준으로 한 Rayleigh 수는 증가하였고, mush 층은 조성 대류에 대해 안정해졌다. mush 층의 윗면에 등온조건을 적용한 경우보다 온도 및 열속의 연속조건을 액체-mush 계면에 적용한 경우에 임계 Rayleigh 수가 더 작게 얻어졌다. In binary solidification compositional convection in a porous mushy layer influences the quality of the final products. We consider the mushy layer solidifying from below with a constant solidification velocity. The disturbance equations for the mushy layer are derived using linear stability theory. The basic-state temperature fields and the distribution of the porosity in the mushy layer are investigated numerically. When the superheat is large, the thickness of the mushy layer is relatively small compared to the thickness of the thermal boundary layer. With decreasing the superheat the critical Rayleigh number based on the thickness of the mushy layer increases and the mushy layer becomes stable to the compositional convection. The critical Rayleigh number obtained from the continuity conditions of temperature and heat flux at the mush-liquid interface is smaller than that from the isothermal condition at the upper boundary of the mushy layer.

Aluminum integral foams with tailored density profile by adapted blowing agents

The goal of the present work is the variation of the structure of aluminum integral foams regarding the thickness of the integral solid skin as well as the density profile. A modified die casting process, namely integral foam molding, is used in which an aluminum melt and blowing agent particles (magnesium hydride MgH2) are injected in a permanent steel mold. The high solidification rates at the cooled walls of the mold lead to the formation of a solid skin. In the inner region, hydrogen is released by thermal decomposition of MgH2 particles. Thus, the pore formation takes place parallel to the continuing solidification of the melt. The thickness of the solid skin and the density profile of the core strongly depend on the interplay between solidification velocity and kinetics of hydrogen release. By varying the melt and blowing agent properties, the structure of integral foams can be systematically changed to meet the requirements of the desired field of application of the produced component.

Variations of Microsegregation and Second Phase Fraction of Binary Mg-Al Alloys with Solidification Parameters

A systematic experimental investigation on microsegregation and second phase fraction of Mg-Al binary alloys (3, 6, and 9 wt pct Al) has been carried out over a wide range of cooling rates (0.05 to 700 K/s) by employing various casting techniques. In order to explain the experimental results, a solidification model that takes into account dendrite tip undercooling, eutectic undercooling, solute back diffusion, and secondary dendrite arm coarsening was also developed in dynamic linkage with an accurate thermodynamic database. From the experimental data and solidification model, it was found that the second phase fraction in the solidified microstructure is not determined only by cooling rate but varied independently with thermal gradient and solidification velocity. Lastly, the second phase fraction maps for Mg-Al alloys were calculated from the solidification model.

Directional Solidification of AlSi Alloys with Fe Intermetallic Phases

Abstract Directional solidification technique is an important research instrument to study solidification of metals and alloys. In the paper the model [6,7,8] of directional solidification in special Artemis-3 facility was presented. The current work aimed to propose the ease and efficient way in calibrating the facility. The introduced M coefficient allowed effective calibration and implementation of defined thermal conditions. The specimens of AlSi alloys with Fe-rich intermetallics and especially deleterious β-Al5FeSi were processed by controlled solidification velocity, temperature gradient and cooling rate.

Aligned porous barium titanate/hydroxyapatite composites with high piezoelectric coefficients for bone tissue engineering

It was proposed that the piezoelectric effect played an important physiological role in bone growth, remodelling and fracture healing. An aligned porous piezoelectric composite scaffold was fabricated by freeze casting hydroxyapatite/barium titanate (HA/BT) suspensions. The highest compressive strength and lowest porosity of 14.5MPa and 57.4% with the best parallelism of the pore channels were achieved in the HA10/BT90 composite. HA30/BT70 and HA10/BT90 composites exhibited piezoelectric coefficient d33 of 1.2 and 2.8pC/N, respectively, both of which were higher than the piezoelectric coefficient of natural bone. Increase of the solid loading of the suspension and solidification velocity led to the improvement of piezoelectric coefficient d33. Meanwhile, double-templates resulted in the coexistence of lamellar pores and aligned macro-pores, exhibiting the ability to produce an oriented long-range ordered architecture. The manipulation flexibility of this method indicated the potential for customized needs in the application of bone substitute. An MTT assay indicated that the obtained scaffolds had no cytotoxic effects on L929 cells.

Microstructural evolution in Mg–Zn alloys during solidification: An experimental and simulation study

A comprehensive microstructural evolution of Mg–1.5, 4.0 and 5.5wt% Zn alloys with respect to the solidification parameters such as thermal gradient (G), solidification velocity (V), cooling rate (GV) and solute (Zn) content were investigated in the present study. Solidification techniques such as directional solidification and wedge casting were employed in order to obtain cooling rates between 0.05 and 250K/s. Microstructural features such as secondary dendrite arm spacing (SDAS), primary dendrite arm spacing (PDAS), microsegregration along the secondary dendrites and secondary phase fractions were experimentally determined. A solidification model that incorporates solute back diffusion, secondary arm coarsening, dendrite tip undercooling and dynamically linked with accurate thermodynamic databases is used to explain the experimental results.

Dendritic microstructure formation in a directionally solidified Al–11.6Cu–0.85Mg alloy

Directional solidifications have been done with Al–11.6Cu–0.85Mg (in weight percent) alloy by using a Bridgman apparatus. The maximum, minimum, and average primary dendrite spacing are measured. The results show that the primary dendrite spacing depends on solidification velocity exponentially. The microstructure evolution in the directionally solidified Al–Cu–Mg alloys is simulated by using three-dimensional (3-D) cellular automaton (CA) method. The simulation is carried out by fully coupling the kinetic calculations with the thermodynamic calculations. The numerical results are in favorable agreement with the experimental ones. They demonstrate that the formation and growth of tertiary dendrite is determined by the interdendritic solid/liquid (S/L) interface stability as well as the overlap of the concentration fields of the relevant dendrites. The evolution of the dendritic structure during solidification is analyzed in detail.

DIRECTIONAL SOLIDIFICATION OF MONOTECTIC COMPOSITION Al-Bi ALLOY

Monotectic systems are a kind of extensive alloys.Many of them have great potentials for practical industry application.A lot of work has been carried out to study the solidification of monotectic alloys.But most of them focused on the microstructure formation during cooling a hyper- monotectic alloy through the miscibility gap in the liquid.Little work was done on the solidification behaviors of an alloy of monotectic composition.Directional solidification experiments were carried out with Al- Bi alloy of the monotectic composition(Al-3.4%Bi,mass fraction).The influences of the solidification velocity on the microstructure were investigated.The microstructure evolution during solidification was analyzed.The results indicate that a solute-rich layer forms in front of the solidification interface and the liquid-liquid decomposition occurs there.When the alloy is solidified at such a high velocity so that the minority phase droplets of all sizes in front of the solidification interface are migrating to the solidification interface,the size distribution of the minority phase particles shows only one peak.When the alloy is solidified at a relatively low velocity,the minority phase droplets within a certain size range may move away from the solidification interface under the concurrent actions of the Marangoni migration,Stokes motion of the droplets as well as the movement of the sample.Al-3.4%Bi alloy solidified under such conditions shows a size distribution of the minority phase particles with two peaks.The average radius of the minority phase particles R depends on the solidification velocity V_0 exponentially according toR∝V_0~(-1/2).With the decrease of the solidification velocity,the dependence of the average radius of the minority phase particles varies towards to R∝ V_0~(-1/3).

In situ transmission electron microscopy of crystal growth-mode transitions during rapid solidification of a hypoeutectic Al–Cu alloy

The rapid solidification dynamics of a pulsed-laser-melted hypoeutectic Al–Cu thin-film alloy were monitored using in situ transmission electron microscopy with high spatial and temporal resolutions. Direct observation of the solid–liquid interface during the transformation allowed measurements of the time-evolving solidification front morphology and velocity. Transitions in the growth mode were detected and related to acceleration and instability at the solidification front. A banded structure that forms during instability at high solidification front velocities, previously only observed ex situ in post-mortem analyses, was imaged during formation, providing insight to the growth mechanisms of this banded morphology.

Phase selection and microstructural formation of rapidly directionally solidified peritectic Fe–Ni alloys by laser surface remelting

Rapid directional solidification of three peritectic Fe–Ni alloys with Ni-concentrations of 4.1 at.%, 4.33 at.%, and 4.6 at.% is achieved by laser surface remelting. It is found that phase selection of these alloys is controlled by the maximum growth temperature criterion. For the first two alloys, when the solidification velocity (Vs) is above a critical value (Vscrit=0.031m/s for Fe–4.1 at.%Ni alloy and Vscrit=0.012m/s for Fe–4.33 at.%Ni alloy), the growth temperature of γ phase (Tiγ) is higher than that of δ phase (Tiδ), leading to a transition from the solidification of δ phase to the solidification of γ phase. While for the third one, within the achieved range of Vs, Tiγ is always higher than Tiδ; only the solidification of γ phase is resultant. At Vscrit, an abrupt increase in cellular spacings (λ) of the first two alloys are observed, which can be attributed to the transition from the solidification of δ phase to the solidification of γ phase. For Fe–4.6 at.%Ni alloy, within the achieved range of Vs, only γ phase solidifies, which causes a monotonous decrease of λ with increasing Vs.

Characterization of stainless steel parts by Laser Metal Deposition Shaping

Laser Metal Deposition Shaping (LMDS) is a new rapid manufacturing technology, which can build fully-dense metal components directly from the information transferred from a computer file by depositing metal powders layer by layer with neither mould nor tool. Typically, performed with stainless steel (SS) 316 powder, the orthogonal experiments combining with the ideal overlapping model were applied to ascertain the optimal processing parameters. Then the characteristics of microstructure, composition and phase of as-deposited cladding layers were analyzed through Scanning Electron Microscope (SEM) and X-ray diffraction (XRD), as well as relative model. Furthermore, the cooling rate and the solidification velocity during LMDS were evaluated based on empirical method. With the optimal parameters, some parts were fabricated without obvious defects, and then the mechanical properties of them were tested. Finally, the influencing regularities of critical parameters on microstructure and properties were concluded by comparison. The results prove that the microstructure of SS 316 deposits is composed of the slender dendrites growing epitaxially from the substrate, the mechanical properties are favorable and anisotropic, and the composition is uniform. Besides, the microstructure morphology and the mechanical properties are affected by the varied processing parameters at different degrees. Among them, the scanning speed shows the most remarkable effects on microstructure morphology, characteristic microscale, mechanical properties, as well as geometric shape of as-deposited parts.

Investigation of the Effect of Solidification Velocity on the Quality of Single Crystal Turbine Blades

The occurrence of high temperatures in combustion chambers of jet engines and gas turbines has led to the demand for new technologies and new materials for the manufacture of one of the most critical elements of these systems - the turbine blades. These elements have to withstand extreme temperatures for extended periods without loss of mechanical strength, conditions under which many alloys fail. Such failure is ascribed to the combination of high temperatures and high centrifugal forces, resulting in creep. This is especially prevalent in multi-crystalline structures in which grain boundaries present weaknesses in the structure. High temperature resistant alloys formed as single crystal (SX) structures offer the necessary material properties for safe performance under such extreme conditions. Modelling and simulation techniques were first used to study the directional solidification (DS) of crystal structures during vacuum investment casting. These models allowed the study of the dendritic growth rate, the formation of new grains ahead of the solid/liquid interface and the morphology of the dendritic microstructure. These studies indicated the opportunity to optimise the velocity of the solidification front (solidification rate) for single crystal structures. The aim of this study was therefore to investigate the effect of the solidification rate on the quality of SX castings. The investigations were carried out for nickel-based superalloy CMSX-4 turbine blade casts and rods using the Bridgman process for vacuum investment casting.

Processing of Hierarchical and Anisotropic Porosity LSM‐YSZ Composites

Aqueous dispersions of lanthanum strontium manganite (LSM) and yttria‐stabilized zirconia (YSZ) particles were controllably freeze‐cast and then partially sintered resulting in anisotropic, hierarchically porous ceramics for Solid Oxide Fuel Cell (SOFC) cathodes. The resulting microstructures have aligned pores with a characteristic spacing (λ) between pore centers. The effect of freezing rate, slurry viscosity, and solid loading on solidification velocity and resultant microstructures was explored. Varying these parameters resulted in samples with a range of independently controllable and reproducible microstructures. Homogenous dispersion of LSM and YSZ in the freeze‐cast structures was confirmed through elemental mapping. Freezing rate was found to have a significant effect on λ while solid loading affected overall porosity and ceramic wall‐ to‐pore size ratio but had only a small influence on λ. Viscosity was found to have a complex albeit small impact on λ but a significant effect on particle dispersion and colloid stability.

Microstructural Modeling of Mg Alloys and Experimental Validation

A systematic investigation was carried out in the present study to study the impact of solidification variables like thermal gradient (G), solidification velocity (V) and cooling rate on the second phase fraction and microsegregration in Mg-Al (3, 6 , 9 wt.%) binary alloys. Solidification experiments such as directional solidification and gravity castings were conducted to obtain a cooling rate in the range of 0.05-700 K/s. The experimental results were tested against the microstructural modeling data and a reasonable agreement was obtained between them.

Effect of Travelling Magnetic Fields on the Dendritic Microstructure of AlSi and AlSiMn

The microstructure formation of AlSi alloys is known to be sensitive to specific solidification conditions. In particular, small fractions of heavier alloying atoms can lead to the precipitation of intermetallic phases. Moreover, the mainly dendritic structure is also sensitive to fluid flow in the melt. These two factors and their mutual influence is examined in this paper. The solidification of AlSi7 and AlSi7Mn1 samples was studied while inducing fluid flow by a traveling magnetic field (TMF) of approximately 5 mT strength, traveling up or down the sample axis. All samples were molten and directionally solidified at constant solidification velocities between 0.03 and 0.24 mm/s. The application of two separate heaters allowed the fixation of constant temperature gradients in the solid and liquid parts of the samples, the use of a transparent silica aerogel crucible permitted optical verification of the solidification velocity. Cross sections were cut from the processed samples and the microstructure analyzed using light microscopy and SEM-EDX. From these images, values for the primary, secondary and tertiary dendrite arm spacing were retrieved. Results are presented which show a clear effect of the TMF-induced fluid flow on the binary samples, but almost none for the ternary alloy. Finally, an explanation proposing a process of precipitate particle pushing is given.