Primary Aluminum Dendrites Research Articles

Microstructure and mechanical properties of hypoeutectic Al–6Ce–3Ni-0.7Fe (wt.%) alloy

A hypoeutectic, Fe-modified Al–Ce–Ni alloy (Al–6Ce–3Ni-0.7Fe, wt.%) is studied in terms of microstructure, thermal stability, ambient temperature strengthening, and creep resistance. The as-cast microstructure consists of primary Al dendrites and interdendritic binary eutectic regions (Al–Al11Ce3 and/or Al–Al9(Fe,Ni)2), with micron/submicron lamellar spacing, depending on the location along the height of the ingot. The cast alloy exhibits excellent coarsening resistance at 400 °C, with mostly unchanged microstructure and microhardness after 6 weeks of aging, indicating good thermal stability of Al11Ce3 and Al9(Fe,Ni)2. Orowan strengthening and load transfer are identified as strengthening mechanisms at ambient and elevated temperature. A high volume fraction of the intermetallic phases (providing load transfer) and relatively coarse eutectic spacing (for modest Orowan strengthening) result in a moderate as-cast microhardness of 566 ± 32 MPa. Creep resistance at 300 and 350 °C is similar to a binary Al-12.5Ce eutectic alloy (with twice the Ce content) because of two countering effects: Al–6Ce–3Ni-0.7Fe shows a higher volume fraction of strengthening intermetallic phases, but it also exhibits a large fraction of primary Al dendrites which weaken the alloy. By contrast, the alloy, when laser-remelted at the surface, has a fully eutectic microstructure without primary aluminum dendrites achieved by high undercooling on solidification, with a refined network of eutectic phases that doubles the microhardness as compared to the cast alloy. Whereas coarsening is faster due to the shorter diffusion distances between the eutectic phases, hardness remains ∼30% higher than the as-cast alloy after ∼6 weeks aging at 400 °C.

Microstructure, deformation and fracture mechanisms in Al-4043 alloy produced by laser hot-wire additive manufacturing

High deposition rate additive manufacturing (AM) processes with minimal feedstock costs are desirable to reduce part fabrication times and minimize the cost of implementation. Wire-based AM processes meet these demands but suffer from higher heat inputs that can lead to thermal distortion and large heat-affected zones neighboring the melt pools. Here, the tensile deformation response of an Al 4043 (Al-Si) alloy deposited on an Al 6061-T6 substrate using a laser hot-wire AM process was investigated. Digital image correlation during tensile testing was combined with optical microscopy and electron backscatter diffraction data to understand the deformation mechanisms that occurred in samples from a deposited wall in the as-built condition. During tensile testing, localized deformation was observed at the melt pool boundary regions that correlated with serrations on the stress-strain curve. The primary aluminum dendrites in regions adjacent to the melt pool boundary regions were coarser than in bulk deposited material, leading to localized deformation of the coarser microstructure. EBSD post-test analysis confirmed that the accumulated strains had occurred in the melt pool boundary regions. Analysis of the thermal and solidification conditions in the melt pool indicated that the coarsened region was associated with partial remelting of the eutectic phase. This understanding of deformation mechanisms associated with the inhomogeneities at melt pool boundaries can enable AM-specific considerations for improved process and alloy design, particularly for eutectic alloys. • Al 4043 parts were deposited using laser hot-wire additive manufacturing. • The deposited Al 4043 had similar mechanical properties to arc-based AM. • DIC during tensile testing revealed serrated flow resulting from localized deformation. • Partial remelting of the eutectic near the melt pool boundary resulted in coarser dendrites than bulk regions. • Localized deformation was correlated with the coarser microstructure near the melt pool boundary.

In-situ X-ray investigation of growth behavior of primary silicon particles and aluminum dendrites during solidification of a hypereutectic Al-Si-Cu alloy

The solidification behavior of hypereutectic Al-18 wt%Si-10 wt%Cu alloy was investigated using in-situ X-ray radiography. Sheet-like samples were solidified both under isothermal cooling conditions and in a moderate temperature gradient with different process parameters. The real-time observation method allows for the investigation of the growth of faceted primary Si particles and non-faceted Al dendrites. Cooling down below the liquidus temperature, firstly primary silicon particles nucleate and grow. At low cooling rates just a few Si particles nucleate and grow continuously to large sizes, whereas at higher cooling rates a larger number of smaller Si particles appear. As cooling continues below the nucleation temperature of aluminium, simultaneous growth of both columnar and equiaxed Al dendrites in the melt surrounding the primary Si particles was observed. In-situ observation as well as post-mortem analysis of the solidification morphology demonstrated that nucleation and growth of Al dendrites in the melt was nearly independent of existing Si particles and no indication was found that the Al phase starts growing from the surface of Si particles. The growth of the equiaxed Al dendrites can be explained by the thermal undercooling controlled by the application of a constant and continuous cooling, but the reason for the fast growing columnar Al dendrites in an isothermal melt is still not clear.

Synthesis and Dry Sliding behaviour of LM24-TiB₂ Reinforced Particulate Composite

The existing effort deals with synthesis of (LM24) composite developed by reinforcing 3wt % and 5wt % TiB2 particles in the size range of 20-60 μm in the LM24 matrix. Hardness of the alloy and the composites were evaluated to see the changes in the hardness of the materials. The sliding wear tests were performed using pin on disc machine to study the wear properties. Effect of various experimental parameters like sliding distance, and applied pressure were studied on sliding wear response of the alloy and the composite. Micro structural examination of the alloy and composite was done to see the structural changes and distribution of the particles in the matrix. Worn surfaces were examined to see the mechanism of material removal. Results show that reinforcement of TiB2 particles in LM24 alloy increases the hardness and wear resistance. Microstructural study of the base alloy LM24 and composites were performed having special attention to the dispersion of TiB2 particles in LM24 base alloy matrix and interface closeness between TiB2 particles and LM24 alloy matrix. The microstructure of aluminium alloy shows primary Al dendrites and intermetallic phases around in the inter-dendrites region. The TiB2 particles were noted to be more or less uniformly distributed within the metallic matrix and these particles were trapped within the primary aluminium dendrites instead of inter-dendritic region.The importance of this work is to study the importance of TiB2 particles in enhancing the properties of the base alloy. In general, the properties of unreinforced materials show poor mechanical as well as wear properties and there is a need to improve the properties to a certain level so that the material can be extended to a certain level for its usefulness. In the present work, it has been observed that after reinforcing TiB2 particles to the base alloy considerable amount of enhancement in the properties can be achieved. It is quite sure that this project work will certainly help in designing of the various parts.

Combination of experimental and numerical methods for mechanical characterization of Al-Si alloys

In general, mechanical properties of Al-Si alloys strongly depend on the morphology and arrangement of microconstituents, such as primary aluminium dendrites, silicon particles, etc. Therefore, a detailed characterization of morphological and mechanical properties of the alloys is necessary to better understand the relations between the underlined properties and to tailor the material’s microstructure to the specific application needs. The mechanical characterization usually implies numerical simulations and mechanical tests, which allow to investigate the influence of different microstructural aspects on different scales. In this study, the uniaxial tension and compression tests have been carried out on Al-Si alloys having different microstructures. The mechanical behavior of the alloys has been interpreted with respect to the morphology of the microconstituents and has been correlated with the results of numerical simulations. The advantages and limitations of the experimental and numerical methods have been disclosed and the importance of combining both techniques for the interpretation of the mechanical behavior of Al-Si alloys has been shown. Thereby, it has been suggested that the density of Si particles and the size of Al dendrites are more important for the strengthening of the alloys than the size-shape features of the eutectic Si induced by the modification.

Comparative Study on Erosive Wear Response of SiC Reinforced and Fly Ash Reinforced Aluminium Based Metal Matrix Composite

In this study an attempt has been made to compare the microstructure, mechanical property and erosive wear response of ADC 12 alloy reinforced with 10 wt% of SiC and Fly ash. Both the composites and base alloy were fabricated through liquid metallurgy route. The wear behaviour was examined by the sample rotation technique using slurry pot erosion tester. The wear tests were conducted in two different environments namely mining and basic atmosphere. Sand concentrations in the slurry are varied from 0-60% in the step of 20 while speed of the rotation was fixed at 1100 RPM for all the samples. Wear rates were computed using weight loss technique. To examine the influence of the SiC and Fly ash particle dispersion, matrix alloy was also characterized under similar conditions.The microstructure of as cast aluminum alloy shows primary Al dendrites and secondary intermetallic phases around the dendrites. The SiC and Fly Ash particles are noted to be uniformly distributed within the metallic matrix and these particles are trapped within the primary aluminium dendrites instead of interdendritic region. Both the composites exhibited better wear resistance than the matrix alloy in the mining environment while in basic environment reverse trend of behavior is obtained. ADC12-SiC composite exhibited better wear resistance than the ADC12-Fly ash composite in both media irrespective of the sand content.

Microstructure analysis of Al–Si–Cu alloys prepared by gradient solidification technique

Al – Si – Cu alloys were cast with the unique gradient solidification technique to produce alloys with two cooling rates corresponding to secondary dendrite arm spacing (SDAS) of ~9 and ~27 μm covering the microstructural fineness of common die cast components. The microstructure was studied with optical microscopy and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and electron backscattered diffraction (EBSD). The alloy with higher cooling rate, lower SDAS, has a more homogeneous microstructure with well distributed network of eutectic and intermetallic phases. The results indicate the presence of Al – Fe – Si phases, Al – Cu phases and eutectic Si particles but their type, distribution and amount varies in the two alloys with different SDAS. EBSD analysis was also performed to study the crystallographic orientation relationships in the microstructure. One of the major highlights of this study is the understanding of the eutectic formation mechanism achieved by studying the orientation relationships of the aluminum in the eutectic to the surrounding primary aluminum dendrites.

Preparation of Globular Microstructure Aluminum Alloy Using Cooling Slope Casting as Feed Material for Equal Channel Pressing Process

Equal channel angular pressing (ECAP) could be used to achieve ultra fine grains in bulk aluminum alloy through severe plastic deformation. Typically a feed material of as-cast aluminum alloys is used with a typical hypoeutectic solidification structure, consisting of primary aluminum dendrites and interdendritic network of lamellar eutectic silicon. On the other hand, semi-solid metal casting provides non-dendritic and globular microstructure which is one of a considerable factor in obtaining homogenous microstructure after ECAP. This work is an attempt to produce aluminum alloy with globular microstructure using cooling slope semi-solid casting process which is believed suitable as a feedstock for ECAP. The aim of this work described in this paper was to understand of microstructural evolution of aluminum structure during cooling slope casting process. Two experiments were carried out. A sample was casted via a cooling slope into a vertical cold mild steel mould at pouring temperature of 640°C. Cooling slope length of 250 mm, slope temperature of room temperature and tilt angle of 60o was applied. Another sample was casted directly into a vertical cold mild steel mould at pouring temperature of 640°C. The primary α-Al phases in the sample that casted without cooling slope was mostly in dendritic throughout the section of sample whilst the primary α-Al phases transformed completely into non-dendritic in the sample that was casted via the cooling slope. Therefore, the transformation is believed resulted from the effect of cooling slope

In situ quantification of the nucleation and growth of Fe-rich intermetallics during Al alloy solidification

Real-time in situ synchrotron X-ray tomographic microscopy was used to gain new insights into and quantify the nucleation mechanisms and growth kinetics of β-Al5FeSi intermetallics during solidification of an aluminium Al–7.5Si–3.5Cu–0.6Fe (wt.%) alloy. Three new insights were obtained. First, the plate-like β-intermetallics appeared to nucleate mainly on or near the primary aluminium dendrites and to a lesser extent off the oxide skin on the surface of the specimen. Second, for this alloy composition, β-intermetallic formation was largely complete before the formation of Al–Si eutectic. Third, the β-intermetallics formed via fast lateral growth, wrapping around and in between the primary dendrite arms. Further, the nucleation and growth dynamics of β-intermetallics were quantified as a function of undercooling in a functional form that could be easily used in microstructural simulations. The frequency of intermetallic interaction mechanisms, such as plate nucleation vs. impingement and branching, were also quantified.

<i>In Situ</i> and Real-Time Observation of the Solidification Process of Al–20 mass%Cu Alloy by Synchrotron X-ray Radiography

High brilliance synchrotron X-ray radiography has been used to in situ study the solidification process of Al­20mass%Cu alloy with nominal spatial and temporal resolutions of 7.4µm and 300ms, respectively. The block of favorably oriented dendrites by the unfavorably oriented ones was first observed in metal solidification, which related to solute interaction of neighbor dendrites. Solute interaction can also retard the growth of dendrites at the grain boundary, which makes the unusual overgrowth phenomenon possible. Fragmentation occurred when dendrite branches were detached from mother dendrite by local re-melting during the dendrite ripening. Fragmentations together with nucleated dendrites contributed to the columnar to equiaxed transition (CET) and the gravity played a significant role in the final microstructure due to the density difference between primary aluminum dendrites and the melt. In addition, the microporosity formation was observed in the inter-dendrite at the last stage of solidification. [doi:10.2320/matertrans.M2014034]

Modification mechanism of eutectic silicon in Al–6Si–0.3Mg alloy with scandium

Abstract The modification mechanism of eutectic silicon in Al–6Si–0.3Mg alloy with scandium was studied. The crystallographic orientation relationships between primary dendrites and the eutectic phase of unmodified and modified Al–6Si–0.3 Mg alloys were determined using electron backscatter diffraction (EBSD). The orientation of aluminum modified with scandium in the eutectic phase was different from that of the neighboring primary dendrites. This result implies that eutectic aluminum grows epitaxially from the surrounding primary aluminum dendrites in the unmodified alloy and that eutectic aluminum grows competitively from the surrounding primary aluminum dendrites in the modified alloy. The pole figure maps of eutectic Si in the [1 0 0], [1 1 0] and [1 1 1] axes of the unmodified and Sc-modified alloys were different, suggesting that the eutectic Al and Si crystals in modified alloy growth are more isotropic and cover a larger set of directions. The lattice fringes of Si of the alloys with and without Sc modification were different in the TEM results. The lattice fringes of Si in modified alloy were found to be multiple twins. However, this was not observed in the unmodified alloy. The growth characteristic of eutectic Si crystal in modified alloy suggests the occurrence of multiple twinning reactions and the formation of a high density of twins. This modification mechanism by Sc is explained by the results of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) analysis, which provide strong evidence of the occurrence of the impurity-induced twinning (IIT) mechanism.

Grain refinement mechanism in an Al–Si–Mg alloy with scandium

Grain refinement of the primary aluminum (α-Al) phase in a hypoeutectic Al–Si alloy using scandium (Sc) was studied to identify the grain refinement mechanism. Optical microscopy (OM), Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) techniques were extensively used in this study. We found that Sc refined grains of primary aluminum. However, the grain refinement efficiency of Sc was considerably lower than that of titanium (Ti) in the Al–Si–Mg foundry alloy. It was evident that the precipitated Sc-containing phases acted as heterogeneous nucleation sites for the primary aluminum phase. The Sc-containing heterogeneous sites are irregular in shape with sizes between 3 and 5μm. At least three groups of nuclei based on their chemical composition were found, i.e., (i) Al and Sc, (ii) Al, Si, Mg, and Sc, and (iii) Al, Si, Mg, Sc, and Fe. Crystal orientation mapping showed primary aluminum dendrites with one orientation in each grain near Al3Sc particles. The grain refinement mechanism of Sc for aluminum relies on heterogeneous nucleation of Al3Sc particles, with less responsibly for grain growth restriction. Many intermetallic phases with Al, Si, Fe, Mg and Sc as their major components were found, and these phases could not effectively act as heterogeneous nuclei.

Electron Back Scattering Diffraction Analysis of A357 Alloy Modified by Sr

Microstructure of A357 alloy modified by Sr has been investigated by the Electron Back Scattering Diffraction (EBSD) mapping technique using a Field Emission Gun Scanning Electron Microscopy (FEG-SEM). An appropriate sample preparation technique by ion milling was applied to obtain a sufficiently smooth surface for EBSD mapping. Results show that the eutectic morphology in microstructure of A357 alloy modified by Sr was changed to fine fibrous, and the grain size was refined. By comparing the orientation of the aluminum in the eutectic to that of the primary aluminum dendrites, the nucleation and growth mechanism of the eutectic solidification in A357 cast alloy was determined. The eutectic Si phase of the modified sample nucleates on the heterogeneous nuclei located in the region between primary α-Al dendrites and grows up, while the eutectic Si phase of the sample without modification nucleates on the primary α-Al dendrites and grows up.

The influence of surrounding environment on the fatigue properties for a high pressure die cast AlSi9Cu3 alloy

ABSTRACTThe influence of mechanical strength for aluminium castings is often correlated in the literature to the occurrence of cast defects like porosity. However, because aluminium castings in the automotive industry are often used without surface treatment such as painting etc., the influence of corrosion and its effect on fatigue properties are required. Basically a surrounding environment can affect fatigue strength either by enhancing initiation or by increasing fatigue crack propagation properties. In this study, the influence of pre‐exposure prior to fatigue testing has been evaluated. This implies that environmental enhancement of fatigue initiation due to corrosion has been studied.Two different environments [seawater acidified test (SWAAT) and Volvo Indoor Corrosion Test (VICT)] often employed for assessment of corrosive properties in the automotive industry have been used for pre‐exposure of specimens. Based on experimental results, it is shown that fatigue strength is reduced by approximately 50% for specimens pre‐exposed to SWAAT environment, while insignificant influence was found for specimens pre‐exposed to VICT environment. The degradation of fatigue strength was found to be due to localized corrosive attacks. Propagation of these corrosive attacks takes place preferably in the eutectic phase and especially at the borderline between primary aluminium dendrites and the eutectic phase.

New Application of Electromagnetic Vibration Process for Creation of High Potential Metallic Materials

We have developed the new process for refinement of metallic materials during solidification without addition of refiners or without rapid cooling. This process uses electromagnetic body force based on the vibrations caused by simultaneous imposition of direct magnetic field and alternative electric current on the alloy melt during solidification. The vibrations create cavitation in the melt and it breaks out during growth of it. Then explosive force is released toward the surroundings such as the primary solid particles and they are fractured finely. Finally fractured solid particles solidified as very fine grains. This process was applied to hyper- and hypo-eutectic Al-Si alloys, and AZ91D Mg alloys. Primary silicon crystals in Al-17mass%Si alloy decreased the size remarkably by the optimum electromagnetic vibration condition. Primary aluminum dendrites particles in Al-7mass%Si and primary magnesium dendrite particles in AZ91D are also decreased its size markedly. This process has also been applied to create non-equilibrium state metallic materials such as metallic glasses. We have succeeded to obtain metallic glasses in Mg-Y-Cu alloy system and Fe-Co-Si-B-Nb alloy system. These alloys solidified as fully crystals in this cooling rate without electromagnetic vibrations. These glasses are bulky and are used as structural material.

Crystal fragmentation and columnar-to-equiaxed transitions in Al-Cu studied by synchrotron X-ray video microscopy

Recent improvements in detectors combined with the eminent brightness and collimation offered with modern synchrotron sources open the way forin situ X-radiographic investigations of solidification fundamentals and phenomena in real alloys at resolutions approaching regular video microscopy. Here, the authors present observations on dendrite fragmentation from columnar fronts in Al-Cu and subsequent transport phenomena. From directional solidification experiments it has been found that the tendency for crystal fragments to detach by remelting of branch roots in the mush dendrite network strongly depends upon the relative buoyant and settling motions of crystal fragments and mush liquid, respectively. At the copper concentrations studied (20 to 30 wt pct), primary aluminum dendrites are lighter than the melt and solidification experiments parallel and anti-parallel to gravity show significant differences in detachment tendency. The experimental results compare well with three different models proposed for fragmentation at different mush locations; however, the results also demonstrate that these models differ substantially both with respect to detachment frequency and the ability for detached fragments to cause eventual columnar to equiaxed transitions. Under particular conditions it has been found that crystal fragmentation could lead to an alternating mesoscale segregation.

Stray crystal formation in Al–20 wt.% Cu studied by synchrotron X-ray video microscopy

Recent improvements in detectors combined with the eminent brightness and collimation offered at 3rd generation synchrotron sources have opened the field for in situ X-radiographic investigations of solidification fundamentals and phenomena in real alloys at resolutions approaching true video microscopy. Here, observations on dendrite fragmentation from columnar fronts in Al–20 wt.% Cu and subsequent transport phenomena will be presented. From directional solidification experiments, it has been found that the frequency for crystal fragmentation, resulting from remelting of dendrite arm roots, depends strongly upon the buoyant forces. At 20 wt.% Cu concentration, primary aluminium dendrites are lighter than the melt and solidification experiments parallel- and anti parallel to gravity show significant differences in stray crystal formation tendency. Under particular conditions, it has been found that crystal fragmentation could lead to a meso-scale liquid segregation.

Solidification analysis of aluminium engine block

Two V-8 engine blocks with cast-in grey iron liners were cast from a heat-treatable aluminium alloy. These blocks were instrumented by the insertion of 24 type K thermocouples placed at different points in the casting to record the thermal evolution during solidification. One of the blocks was cast after heating up the liners by induction, whereas the second was cast without the use of induction heating. The temperature–time readings from the thermocouples were obtained for thermal analysis. The solidification reactions detected were the formation of primary pre-eutectic aluminium dendrites, and two different eutectics that corresponded to Al–Si and complex Al–Si–Mg–Cu aggregates. It was found that the different solidification reactions were affected by the use of induction to heat up the liners, and by the chilling effect caused by a grey iron inset located at the bottom of the engine block moulds. The results indicate that solidification starts either at the window (bottom) or deck (top) positions in the interliners depending on whether the block was cast with or without the use of induction heating.

Eutectic nucleation and growth in hypoeutectic Al-Si alloys at different strontium levels

The effects of different levels of strontium on nucleation and growth of the eutectic in a commercial hypoeutectic Al-Si foundry alloy have been investigated by optical microscopy and electron backscattering diffraction (EBSD) mapping by scanning electron microscopy (SEM). The microstructural evolution of each specimen during solidification was studied by a quenching technique at different temperatures and Sr contents. By comparing the orientation of the aluminum in the eutectic to that of the surrounding primary aluminum dendrites by EBSD, the eutectic formation mechanism could be determined. The results of these studies show that the eutectic nucleation mode, and subsequent growth mode, is strongly dependent on Sr level. Three distinctly different eutectic growth modes were found, in isolation or sometimes together, but different for each Sr content. At very low Sr contents, the eutectic nucleated and grew from the primary phase. Increasing the Sr level to between 70 and 110 ppm resulted in nucleation of independent eutectic grains with no relation to the primary dendrites. At a Sr level of 500 ppm, the eutectic again nucleated on and grew from the primary phase while a well-modified eutectic structure was still present. A slight dependency of eutectic growth radially from the mold wall opposite the thermal gradient was observed in all specimens in the early stages of eutectic solidification.

Eutectic solidification in hypoeutectic Al–Si alloys: electron backscatter diffraction analysis

Nucleation and growth of the eutectic in hypoeutectic Al–Si foundry alloys has been investigated by the electron backscatter diffraction (EBSD) mapping technique using a scanning electron microscope (SEM). Sample preparation procedures for optimizing mapping have been developed. To obtain a sufficiently smooth surface from a cast Al–Si eutectic microstructure for EBSD mapping, an appropriate preparation technique by ion milling was developed and applied instead of conventional electropolishing. By comparing the orientation of the aluminum in the eutectic to that of the surrounding primary aluminum dendrites, the growth mechanism of the eutectic can be determined. Two different results were found, in isolation or sometimes together, but distinct for different strontium contents: (1) crystallographic orientations of aluminum in eutectic and surrounding primary dendrites are identical, and (2) wide variation in orientations of the aluminum in the eutectic.