Large Secondary Dendrite Arm Spacing Research Articles

Study on MnS Inclusion Aggregation along Continuous Casting Slab Thickness of Medium Carbon Structural Steel

Precipitation of MnS inclusions in steel affects the mechanical properties of the material significantly. The evolution of MnS inclusions along the continuous casting slab thickness and its influencing factors has not been clearly established and comprehensively studied. In this paper, solidification macrostructure, sulfur segregation and MnS inclusions in the continuous casting slab of medium carbon structural steel 45# were studied by various methods, including the metallographic observations, elemental analysis, scanning electron microscope (SEM) with Energy Dispersive Spectrometer (EDS) observation, automatic particle analysis, and thermodynamic calculations. The 2D/3D morphologies of MnS inclusions suggest that the sulfides turn from globular to rodlike, and further to dendritic shape along the slab thickness progressively. Furthermore, it was found that MnS inclusions are remarkably aggregated in the columnar crystals and the equiaxed crystals mixed zone, where the sulfides have the largest average diameter of 6.35 μm and the second maximum area fraction of 0.025% along the slab thickness. In order to reveal the mechanism of this phenomenon, the precipitation temperature of MnS inclusion in the 45# steel was clarified by thermodynamic calculation and experimental observation, and the quantitative relationships among the distribution of sulfur content, secondary dendrite arm spacing (SDAS), and precipitation area fraction of MnS inclusions were discussed. Moreover, the inclusion size was numerically predicted to compare with the measured value. The results indicate that the large SDAS, high sulfur content and low cooling rate accounting for the large-size aggregated MnS inclusions in the mixed zone. Unfortunately, the dendritic MnS inclusions, even if the average diameter exceeds 52 μm, can act as the nucleation sites for ferrites, and the distribution of the sulfides promotes uneven microstructure in the steel.

Effect of SiO2 and TiO2 on the Crystal Morphology of CaF2Al2O3CaO‐Based Electroslag Remelting Slag

The effect of SiO2 and TiO2 on the crystal morphology of 60%CaF220%Al2O320%CaO‐type electroslag remelting (ESR) slags are investigated by thermodynamic simulations, differential scanning calorimetry, X‐ray diffraction, scanning electron microscope, and energy dispersive X‐ray spectroscopy. The results suggest that the addition of TiO2 and SiO2 changes the type, amount, and sequence of crystal precipitation. CaF2 is confirmed to be the main crystalline phase in the slag, the morphology of which depends on the slag composition and cooling rate: As SiO2 content increases, the crystal morphology of CaF2 in the slag changes from dendrites with smooth surface and relatively large secondary dendrite arm spacing to dendrites with sharp tips and relatively small secondary and tertiary dendritic arm spacings. A classical crystallographic model, Jackson α factor, is proved to correctly predict the crystal morphology as observed in the experiments. It is inferred that the addition of 5 wt% TiO2 and 4 wt% SiO2 and a proper cooling rate are favorable to improve the lubrication property of 60%CaF220%Al2O320%CaO‐type ESR slags.

Microstructure, Texture and Mechanical Properties of Al Alloy A380 Prepared by Directional Solidification Method

The mechanical properties of Al-Si alloys are affected by several microstructural features such as secondary dendrite arm spacing (SDAS), size and shape of eutectic Si-particles, presence of intermetallics as well as by porosity. In the current study, Al-Si-Cu alloy A380 was prepared by a unique directional solidification method to produce samples with two different SDAS of 9 μm and 27 μm. The lower solidification rate resulted in larger SDAS, larger grain size, larger eutectic Si and larger intermetallics including Fe-rich β phase. The microstructure with higher solidification rate was found to be finer and more homogeneous with smaller eutectic Si and intermetallics. The specimen with larger SDAS exhibited stronger texture than the one with smaller SDAS. The specimen with smaller SDAS showed improved mechanical properties including YS, UTS and ductility.

In situ observation and phase-field simulation on the influence of pressure rate on dendritic growth kinetics in the solidification of succinonitrile

The influence of pressure rate on dendritic growth kinetics in solidification of succinonitrile was studied via in situ observation based on a novel apparatus established in our previous work and using phase-field modeling where a pressure term associated with pressure rate was introduced. The experimental and simulation results revealed that dendrites grew much faster at higher pressure rate, resulting in dendrites characterized by more developed secondary arms and larger secondary dendrite arm spacing (SDAS), while dendrites growing at lower pressure rate was more cellular-like with small secondary arms. Higher pressure rate facilitated the competitive growth of dendrites, which led to fewer but larger dominate primary dendrites and larger primary dendrite arm spacing (PDAS) in the final microstructure. The cellular-to-dendrite transition (CDT) was more advanced at higher pressure rate, and it was demonstrated that it was the higher pressure rate not the high value of pressure that motivated CDT, via elevating effective undercooling and thus growth velocity at CDT moment. Furthermore, the growth kinetics was analyzed quantitatively, and the variation of tip velocity at different pressure rates was consistent with that of the corresponding undercooling induced by pressure and thermal condition. Moreover, the slope of growth and re-melting velocity—the tip acceleration—increased with pressure-rising rate and pressure-declining rate, respectively, even in a complicated periodic pattern, which was qualitatively consistent with the theoretical relationship of tip acceleration and pressure rate.

Quantitative microstructure and fatigue life of B319 casting alloys

In this study, the microstructure of B319 casting alloys and effects of five different casting conditions on microstructure were studied. Multi-scale microstructure was quantified in terms of secondary dendrite arm spacing (SDAS), and Si particle size and aspect ratio. The effects of SDAS, Si aspect ratio and size on fatigue life were analyzed. The results indicate that the size and aspect ratio of Si particles are a function of SDAS which is dependent on cooling rate during solidification. The fatigue life decreases with SDAS increasing as SDAS is smaller than 30 μm while it increases with SDAS increasing as SDAS is larger than 60 μm. In addition, the fatigue life decreases with Si aspect ratio and size increasing at the same SDAS. Moreover, SDAS and Si particles have also influence on fatigue fracture, such as the area of cracks propagation region and the roughness of fatigue fracture. The cracks propagation area is smaller, and the fatigue fracture is similar to tensile fracture with larger SDAS. Besides, the longitudinal section of fatigue fracture is rougher with large SDAS and elongated Si particles.

Mechanism and Control of Sulfide Inclusion Accumulation in CET Zone of 37Mn5 Round Billet

In the current study, the R/2 accumulation was found from the results of sulfide inclusion distribution across the diameter of 37Mn5 round billet using ASPEX. In order to reveal the mechanism and furthermore control this phenomenon, the distribution of sulfur content and the macrostructure of billet after etching were analyzed. The results showed that there was obvious segregation in the same position as sulfide inclusion accumulation, where the columnar-to-equiaxed transition (CET) took place. Therefore, the mechanism of sulfide inclusion accumulation in the CET zone was discussed on the basis of the mechanism of R/2 sulfur segregation. The interlaced and coarse dendrites in the CET zone blocked solute concentrated from select crystallization, thus caused sulfur segregation near R/2. Higher sulfur content and larger secondary dendrite arm spacing (SDAS) in the CET zone benefited the precipitation and growth of sulfide inclusions. The effect factors of CET position were discussed on the purpose that by proper cooling conditions, the morphology of solidification structure can be controlled, by which the element segregation can be modified; thereafter, the control of sulfide inclusion distribution in the billet can be realized.

Adaptive Hierarchical-Concurrent Multiscale Modeling of Ductile Failure in Heterogeneous Metallic Materials

This article addresses two topics on multiscale modeling of heterogeneous metals and alloys. The first topic focuses on developing an adaptive hierarchical-concurrent multilevel modeling framework for ductile fracture. The microstructure of aluminum alloys, for example, is characterized by a dispersion of heterogeneities such as silicon and intermetallics in a ductile aluminum matrix. The multilevel model invokes two-way coupling, viz. hierarchical models for homogenized constitutive modeling and concurrent models with scale transition in regions of localization and damage. Adaptivity is necessary for evolving microstructural deformation and damage. A macroscopic analysis in regions homogeneity incorporates homogenization-based continuum plasticity-damage models. A microscopic analysis using locally enhanced-Voronoi cell finite-element method is required for regions of high macroscopic gradients caused by underlying localized plasticity and damage. Coupled macroscopic and microscopic analysis is conducted concurrently. Physics-based level change criteria are developed to improve accuracy and efficiency. The second topic discusses a nested dual-stage homogenization method for microstructure-based homogenized continuum plasticity models for cast aluminum alloys with large secondary dendrite arm spacing. Two distinct statistically equivalent representative volume elements are identified and used in the asymptotic expansion-based homogenization and self-consistent homogenization processes, respectively. The two-stage homogenization enables an evaluation of the overall homogenized model of a cast alloy from limited experimental data, as well as material properties of constituents like interdendritic phase and pure aluminum matrix.

Study on Pressurized Solidification Behavior and Microstructure Characteristics of Squeeze Casting Magnesium Alloy AZ91D

Squeeze casting technology for magnesium alloys has a great application potential in automobile manufacturing and has received increasing attention from both academic and industrial communities. In this study, the pressurized solidification behavior of magnesium alloy AZ91D in squeeze casting process was investigated using computer-aided cooling curve analysis (CA-CCA). It was found that the applied pressure increased both the start and end temperatures of primary α-Mg formation but had little effect on the sizes of temperature ranges. Moreover, the applied pressure increased the start temperature and decreased the end temperature of eutectic reaction during the solidification, resulting in a larger temperature range of eutectic reaction compared with solidification under atmospheric pressure. The grains were remarkably refined, and the eutectic fraction increased with increasing applied pressure. The dendritic microstructure with a larger secondary dendrite arm spacing (SDAS) was observed under a higher applied pressure at the central part of the experimental casting. By correlating the CA-CCA and SDAS data, it was found that SDAS and the cooling rate at the maximum α-Mg growth could be fit into the power law equation in classic solidification theories.

Dual-stage nested homogenization for rate-dependent anisotropic elasto-plasticity model of dendritic cast aluminum alloys

This paper proposes a nested dual-stage homogenization method for developing microstructure based continuum elasto-viscoplastic models for large secondary dendrite arm spacing or SDAS cast aluminum alloys. Microstructures of these alloys are characterized by extremely inhomogeneous distribution of inclusions along the dendrite cell boundaries. Traditional single-step homogenization methods are not suitable for this type of microstructure due to the size of the representative volume element (RVE) and the associated computations required for micromechanical analyses. To circumvent this limitation, two distinct RVE’s or statistically equivalent RVE’s are identified, corresponding to the inherent scales of inhomogeneity in the microstructure. The homogenization is performed in multiple stages for each of the RVE’s identified. The macroscopic behavior is described by a rate-dependent, anisotropic homogenization based continuum plasticity (HCP) model. Anisotropy and viscoplastic parameters in the HCP model are calibrated from homogenization of micro-variables for the different RVE’s. These parameters are dependent on microstructural features such as morphology and distribution of different phases. The uniqueness of the nested two-stage homogenization is that it enables evaluation of the overall homogenized model parameters of the cast alloy from limited experimental data, but also material parameters of constituents like inter-dendritic phase and pure aluminum matrix. The capabilities of the HCP model are demonstrated for a cast aluminum alloy AS7GU having a SDAS of 30 μm.

Effects of Secondary Dendrite Arm Spacing on Microporosity of A357 Alloy

Microporosity in cast aluminum alloys formed during solidification of castings can be due to the evolution of dissolved hydrogen gas from the liquid metal, the inability of liquid metal to feed through interdendritic channels, or a combination of both. The secondary dendrite arm spacing (SDAS) of the casting may affect either the evolution of hydrogen gas or the ability of feeding. In this investigation, a method based on the Nearest-Neighbor-Distance (NND for short) cluster analysis of image analysis data was used for distinguishing the gas and shrinkage pores in A357 alloy, in order to study the effects of SDAS on the microporosity formation. It shows that the shrinkage pores are prone to form in the specimens with small SDAS. The discrete, isolated gas microporosity is prone to form in the specimens with large SDAS.

The effect of hot isostatic pressing on the microstructure and tensile properties of an unmodified A356-T6 cast aluminum alloy

In this paper, the effect of HIPping process on the microstructure and tensile properties of an unmodified sand cast A356-T6 aluminum alloy was studied. The microstructure and tensile fracture surfaces of the alloy were examined by transmission electron microscope (TEM), scanning electron microscope (SEM) and optical microscope. The results show that sub-grain boundaries are formed by HIPping process, and some silicon precipitates are formed at the sub-grain boundaries during aging hardening. The needle-shape precipitates are Mg 2Si particles according to SED pattern analysis. The lattice misfit between Mg 2Si and aluminum matrix is about 0.256% for [ 1 1 1 ] Al / / [ 4 1 0 ] M g 2 Si HIPping process significantly reduces porosity volume fraction and pore sizes and thus improves ductility. However, the tensile strength is improved very marginally due to the brittle nature of the unmodified coarse microstructure. The sub-grain boundary formed in the HIPping process has not shown significant influence on the tensile properties. For the studied alloy with large secondary dendrite arm spacing (SDAS) (above 80 μm), the tensile fracture exhibits a transgranular mode (along the cell boundaries) with quasi-cleavage feature.

Plastic deformation behavior of aluminum casting alloys A356/357

The plastic deformation behavior of aluminum casting alloys A356 and A357 has been investigated at various solidification rates with or without Sr modification using monotonic tensile and multi-loop tensile and compression testing. The results indicate that at low plastic strains, the eutectic particle aspect ratio and matrix strength dominate the work hardening, while at large plastic strains, the hardening rate depends on secondary dendrite arm spacing (SDAS). For the alloys studied, the average internal stresses increase very rapidly at small plastic strains and gradually saturate at large plastic strains. Elongated eutectic particles, small SDAS, or high matrix strength result in a high saturation value. The difference in the internal stresses, due to different microstructural features, determines the rate of eutectic particle cracking and, in turn, the tensile instability of the alloys. The higher the internal stresses, the higher the damage rate of particle cracking and then the lower the Young’s modulus. The fracture strain of alloys A356/357 corresponds to the critical amount of damage by particle cracking locally or globally, irrespective of the fineness of the microstructure. In the coarse structure (large SDAS), this critical amount of damage is easily reached, due to the clusters of large and elongated particles, leading to alloy fracture before global necking. However, in the alloy with the small SDAS, the critical amount of damage is postponed until global necking takes place due to the small and round particles. Current models for dispersion hardening can be used to calculate the stresses induced in the particles. The calculations agree well with the results inferred from the experimental results.

Microstructural effects on the tensile and fracture behavior of aluminum casting alloys A356/357

The tensile properties and fracture behavior of cast aluminum alloys A356 and A357 strongly depend on secondary dendrite arm spacing (SDAS), Mg content, and, in particular, the size and shape of eutectic silicon particles and Fe-rich intermetallics. In the unmodified alloys, increasing the cooling rate during solidification refines both the dendrites and eutectic particles and increases ductility. Strontium modification reduces the size and aspect ratio of the eutectic silicon particles, leading to a fairly constant particle size and aspect ratio over the range of SDAS studied. In comparison with the unmodified alloys, the Sr-modified alloys show higher ductility, particularly the A356 alloy, but slightly lower yield strength. In the microstructures with large SDAS (>50 µm), the ductility of the Sr-modified alloys does not continuously decrease with SDAS as it does in the unmodified alloy. Increasing Mg content increases both the matrix strength and eutectic particle size. This decreases ductility in both the Sr-modified and unmodified alloys. The A356/357 alloys with large and elongated particles show higher strain hardening and, thus, have a higher damage accumulation rate by particle cracking. Compared to A356, the increased volume fraction and size of the Fe-rich intermetallics (π phase) in the A357 alloy are responsible for the lower ductility, especially in the Sr-modified alloy. In alloys with large SDAS (>50 µm), final fracture occurs along the cell boundaries, and the fracture mode is transgranular. In the small SDAS (<30 µm) alloys, final fracture tends to concentrate along grain boundaries. The transition from transgranular to intergranular fracture mode is accompanied by an increase in the ductility of the alloys.