Papers by Keyword: Dendrite Arm Spacing

Abstract: The problem state of computer analysis of shrinkage defects in castings is analysed based on a review of theoretical and experimental studies the filtration permeability of solidifying alloys. It is shown that the use of the dimensionless permeability coefficient and its modification by introducing various corrections to the Karman-Kozeni equation, obtained for the stationary morphology of non-intersecting filtration channels, is inapplicable for the conditions of continuous change of their geometry during formation of dendrite structure. A computer model of the permeability for the mesoscale ensemble of secondary dendritic branches under the conditions of their diffusion-capillary coalescence is developed. Based on the thermodynamic apparatus and solidification modeling the influence of composition of Al-Cu-Mg alloys and heat removal velocity on the permeability coefficient under the evolution of the dendritic structure is analysed.

Abstract: The paper describes influence of the structure type and its parameters on the mechanical properties of cast products (not subjected to plastic forming), made out of Al-Si-X alloys. The special attention has been focused on the dendritic structure parameters: dendritic arms spacing of the first order (DAS 1) and the second order (DAS 2). The results of investigations of mechanical properties of the test castings made using three basic casting technologies: gravity sand casting (GSC), gravity die casting (GDC) and high pressure die casting (HPDC), are presented. All the castings were made out of the same AlSi9Cu3 alloy. The test castings (adherent samples and separately cast samples) were subjected to a static tensile test and their mechanical properties were determined. In the next stage, the samples fractures and zones near the fractures (metallographic specimen) were subjected to visual testing, penetrating testing PT and metallographic microscopic tests. The microstructure, including the size of the DAS 2, was determined. Evaluation of the porosity state was also undertaken. A customer casting was also manufactured and subjected to the metallographic, tomographic (CT) and strength tests. The castings solidification simulation tests were performed for all the three casting technologies (GSC, GDC and HPDC) using the Nova Flow &Solid system. The results were used for estimation of the correlation between the cooling rate of the particular casting solidifying in the above mentioned mould types and for the DAS size. The coupled influence of the structure parameters on the mechanical properties, regarding the occurrence of porosity, was also analyzed.

Abstract: Recently several experiments on directional solidification of Al-6.5wt.Si-0.93wt.%Fe (AlSi7Fe1) alloy were performed under terrestrial conditions and onboard the International Space Station (ISS) in the Materials Science Lab (MSL) with use of electromagnetic stirring and without it. Analysis of the samples showed that stirring with a rotating magnetic field lead to the accumulation of iron-rich intermetallics in the center of the sample and influenced the primary dendrite spacing while the secondary dendrite arm spacing were not affected. In the present paper the accumulation of the intermetallics b-Al₅SiFe in the center of the samples due to RMF stirring is demonstrated numerically and the evolution of primary and secondary dendrite arm spacing is discussed.

Abstract: Electroslag remelting (ESR) is an advanced process to produce high quality steel. During the ESR process, the steel electrode is melted and then solidified directionally in a water-cooled mold. The quality of the ingot is strongly dependent on the shape of melt pool, i.e. the depth and thickness of mushy zone, which is in turn influenced by the bulk and interdendritic flow. Here, we perform a numerical study to investigate the effect of crystal morphological parameter such as primary dendrite arm spacing on the solidification of the ESR ingot ( 750 mm). The crystal morphology is dominantly columnar and dendritic, thus a mixture enthalpy-based solidification model is used. Accordingly the mushy zone is considered as a porous media where the interdendritic flow is calculated based on the permeability. The permeability is determined as function of the liquid fraction and primary dendrite arm spacing according to Heinrich and Poirier [Comptes Rendus Mecanique, 2004, pp. 429-44]. The modeling results were verified against experimental results.

Abstract: Although numerous solidification experiments have been conducted for Al, Cu and Si alloys to investigate microstructural features like primary and secondary dendrite arm spacing, solute distribution with in secondary arms and second phase fraction, no systematic solidification study on Mg alloys has been performed to understand the evolution of microstructural features as a function of cooling rate and solute content. The present study focuses on the experimental microstructural evolution of Mg-3, 6 and 9 wt. % Al alloys in the cooling rate range of 1 K/sec to 1000 K/sec. The results suggest that secondary dendrite arm spacing and amount of second phase formation are strongly dependent on both solute content and cooling rate.

Abstract: 7075 aluminum alloy ingot with the diameters of 100 mm was produced by Horizontal Direct-Chill (HDC). The temperature in the melt was measured and the ingots were examined in detail with the aim to reveal the floating grains in the HDC ingot. Experimental results show that very large floating grains with coarse dendrite arm spacing (DAS) mainly concentrate in the half-moon area near bottom surface of the HDC casting ingot, under the function of gravity. The floating grains in the HDC casting process is much bigger than that founded in Vertical Direct-Chill (VDC) casting process.

Abstract: The use of wrought magnesium for automobile structural components is an important component of the mass reduction strategy for automobiles to improve their fuel efficiency. Compared to Direct chill casting, Twin Roll Casting (TRC) allows major reduction of hot rolling steps in the production of Mg sheet due to the thin thickness of the as-cast strip. This TRC route can substantially reduce the time and cost to produce Mg alloy sheet product. In this work, AZ31 magnesium alloy was casted to 5 and 6 mm thick strips under different process conditions. Microstructure of these strips was analyzed using optical microscopy, SEM and EPMA. TRC strip was annealed under two different conditions: 2 hours at 330 and 1 hour at 400°C. It has been found that heat treatment at 400°C for 1 hour reduces centerline segregation significantly. TRC strips were rolled down to 2 mm and annealed at 450°C for 2 minutes. The average grain size was 4-6 µm and mechanical properties were comparable with commercial AZ31 sheet.

Abstract: A stochastic mathematic model contained the effects of dendrite morphology, solidification shrinkage and dissolved gases was formed to simulate microporosity formation and growth. Microporosities appear in the interspaces of primary dendrites as well as secondary dendrites from microscopic view of A356 aluminum alloy experimental ingot with a metal mold. In the past literatures it took the volumetric fraction of microporosities as a function of the local density. In the present work a single pore size and distribution were predicted concerning the combination of shrinkage and dissolved gases and dendritic spacing. The dendritic spacing is a main parameter to decide the pore pattern. For shrinkage and dissolved gases causes, the favorable one is determined by dendritic spacing, also the local cool rate and tip growth rate. The dense degree of the experimental ingots in different casting conditions was discussed. The variations of dens degree from the measured values in different casting conditions are similar to that of porosity volume fraction from the predicted results.

Abstract: A single crystal superalloy including 6 wt.% Re, CMSX10 was studied by directional solidification at various thermal gradients, 13 ~ 23oC/mm and solidification rates, 1~100 μm/s. A high thermal gradient could be obtained by applying the liquid metal, such as Ga-In, as a cooling media in directional solidification apparatus, and also by adjusting the cold chamber in the Bridgman system. As increasing the solidification rate, the planar interface changed to cellular and dendritic interfaces. The higher thermal gradient contributed to reducing the dendrite arm spacing effectively, which results in reducing the size of eutectic, as well as higher solidification rate. The length of the mushy zone decreased with increasing the thermal gradient and increased with increasing the solidification rate.

Abstract: A quantitative study of the interactions between microstructural features such as secondary dendrite arm spacing (DAS), eutectic structure and fatigue behavior of two Al-Si-Mg casting alloys with silicon contents of 7% and 10% respectively, has been conducted. In the condition of minimizing casting defects, the influence of microstructural features on the mechanical performance becomes more pronounced. Depending on the magnesium content affecting the strength of the matrix, the tensile properties were changed upon experimental conditions; the tensile strength was increased with magnesium content, whereas the elongation was increased in the reverse case. The increase of both of high cycle fatigue and low cycle fatigue lives with decreasing the secondary dendritic arm spacing is observed, mainly due to homogeneous deformation owing to the fine size of eutectic silicon and Fe intermetallic particles. The fatigue dynamometer of a diesel cylinder head shows the same tendency with the results of specimen fatigue tests with microstructures.