Secondary Dendrite Arm Spacing Research Articles

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.

EFFECTS OF AIR-COOLING-HOLE GEOMETRIES ON A LOW-PRESSURE DIE-CASTING PROCESS

A significant precondition for the production of high-quality castings is keeping an optimum temperature of the respective parts of the die cavity surface. This temperature depends on the temperature of the material, the quantity of metal, the method of cooling the casting die, the thermal conductivity of the die material, and the time during which the casting remains in the die. In addition, the cooling characteristics of alloy steel dies, used in the production of aluminum-alloy wheels with the low pressure die casting (LPDC) method, have critical effects on the mechanical and metallurgical properties of the product. Ducted air coolers are widely used for the cooling of these alloy steel dies. However, the geometrical designs of the air-cooling holes are limited. In this study, we define the effects of the geometry of the cooling holes on the cooling power of the die, the efficiency of the air consumption with the Full Factorial Experimental Design method and to determine the optimum values for LPDC. Pilot production has been carried out on an industrial scale to verify the data obtained by experimental design. The experimental and real data were compared based on the values of the yield strength and the secondary dendrite arm spacing in the microstructure.

Effect of Pressure on Second Dendrite Arm Spacing and Columnar to Equiaxed Transition of 30Cr15Mo1N Ingot

In order to clarify the effects of pressure on the second dendrite arm spacing (SDAS) and columnar to equiaxed transition (CET) of 30Cr15Mo1N ingot, changes in solidification parameters, cooling rate and columnar dendrite tip growth rate with pressure have been investigated. A formula is proposed to obtain the quantitative correlation between pressure (≤ 2 MPa) and interfacial heat transfer coefficient: . And then, A formula is proposed to calculate SDAS with cooling rate: , which is applicable for low pressure (≤2 MPa) and low cooling rate (0.4–1.4 K · s−1). There are tiny changes in solidification mode and equilibrium partition coefficient with increasing pressure from 0.5 to 2 MPa. Therefore, increasing pressure refines SDAS mainly by enlarging cooling rate. Pressurization can increase columnar dendrite tip growth rate by enhancing undercooling, and inhibit nucleation and growth of equiaxed dendrite by decreasing constitutional undercooling. It results that the distance between the center of ingot and CET position decreased with increasing pressure from 0.5 to 2 MPa. And a formula is proposed to calculate columnar dendrite tip growth rate with undercooling: , which is suitable for low pressure (≤2 MPa).

Effect of pulse frequency on the morphology, microstructure, and corrosion resistance of high‑nitrogen steel prepared by laser directed energy deposition

Quasi-Continuous-Wave laser direct energy deposition offers a number of advantages, such as a lower heat accumulate, a higher cooling rate and improved mechanical properties over Continuous-Wave counterpart. However, how pulse frequency of Quasi-Continuous-Wave affects sample's morphology, microstructure and corrosion resistance is not clear. In this work, high‑nitrogen steel samples were fabricated by Quasi-Continuous-Wave laser directed energy deposition with optimized process parameters to investigate the effect of pulse frequency on coating's surface morphology, secondary dendrite arm spacing, micro-hardness, and corrosion resistance based on experiments and temperature field simulation by Finite Element Method. The results indicated that as the pulse frequency increased (5–20 Hz), the surface profile curve of the sample tended to be stable and the surface roughness decreased. Meanwhile, when the pulse frequency increased from 20 Hz to 80 Hz, the maximum peak temperature in the melt pool decreased from 2860 °C to 2690 °C. Correspondingly, the minimum peak temperature increased from 1210 °C to 1480 °C. Once the pulse frequency exceeded 60 Hz, the melt pool can keep continuous during the laser-off period. Furthermore, a lower pulse frequency resulted in a refined equiaxed dendrite microstructure in samples and a more even distribution of nitrogen in the melt pool due to an improved cooling rate and reduced shape control factor. As a consequence, the samples fabricated at a lower pulse frequency exhibited higher micro-hardness and corrosion resistance.

CA Modeling of Microsegregation and Growth of Equiaxed Dendrites in the Binary Al-Mg Alloy.

A two-dimensional model based on the Cellular Automaton (CA) technique for simulating free dendritic growth in the binary Al + 5 wt.% alloy was presented. In the model, the local increment of the solid fraction was calculated using a methodology that takes into account changes in the concentration of the liquid and solid phase component in the interface cells during the solidification transition. The procedure of discarding the alloy component to the cells in the immediate vicinity was used to describe the initial, unstable dendrite growth phase under transient diffusion conditions. Numerical simulations of solidification were performed for a single dendrite using cooling rates of 5 K/s, 25 K/s and 45 K/s and for many crystals assuming the boundary condition of the third kind (Newton). The formation and growth of primary and secondary branches as well as the development of component microsegregation in the liquid and solid phase during solidification of the investigated alloy were analysed. It was found that with an increase in the cooling rate, the dendrite morphology changes, its cross-section and the distance between the secondary arms decrease, while the degree of component microsegregation and temperature recalescence in the initial stage of solidification increase. In order to determine the potential of the numerical model, the simulation results were compared with the predictions of the Lipton-Glicksman-Kurz (LGK) analytical model and the experimental solidification tests. It was demonstrated that the variability of the dendrite tip diameter and the growth rate determined in the Cellular Automaton (CA) model are similar to the values obtained in the LGK model. As part of the solidification tests carried out using the Derivative Differential Thermal Analysis (DDTA) method, a good fit of the CA model was established in terms of the shape of the solidification curves as well as the location of the characteristic phase transition temperatures and transformation time. Comparative tests of the real structure of the Al + 5 wt.% Mg alloy with the simulated structure were also carried out, and the compliance of the Secondary Dendrite Arm Spacing (SDAS) parameter and magnesium concentration profiles on the cross-section of the secondary dendrites arms was assessed.

High-cycle fatigue failure analysis of cast Al-Si alloy engine cylinder head

Due to the premature fracture of several cast Al-Si alloy cylinder heads at the same position during high-cycle fatigue (HCF) bench test, the HCF failure analysis including finite element simulation, microscopic observation and fatigue tests were conducted to determine the root cause of the failure. The results show that the external load was not the main cause of the fracture. The large defects with complex shapes and low compactness near the surface, and the poor surface finish of the water jacket surface are the main causes for the quick initiation of cracks. Furthermore, the alloy at the failure location exhibits relatively high porosity and secondary dendrite arm spacing, accelerating the crack propagation rate. Compared with the alloy at the non-failure locations, the yield strength, ultimate tensile strength and elongation of the alloy at the failure location are reduced by 9.3%, 25.8% and 65.3%, respectively. The fatigue life of the alloy at the non-failure location is several times the alloy at the failure location, verifying that the local large defects and poor microstructure characteristics are the main causes for the low HCF strength of the cylinder head.

Automatic Determination of Secondary Dendrite Arm Spacing in AlSi-Cast Microstructures

A new procedure for the automatic measurement of the secondary dendrite arm spacing (SDAS) from microscopic images is presented. The individual primary and secondary dendrite arms are identified through suitable segmentation techniques and clustered in such a way that dendritic structures are obtained suitable for SDAS measurement. The algorithms are applied to two different hypoeutectic aluminum cast alloys, and the quality of the measurements obtained is assessed through a comparison to manually measured SDAS values. A good agreement between the automated measurements and the distribution of manual measurements is found for both cast structures considered. In addition, a decrease in computation time is observed which allows for an increase in measurement density that is used to characterize the microstructures.

A new microstructure‐based multiaxial fatigue life prediction model for A319 casting alloys

AbstractThis paper describes a microstructure‐based multiaxial nonproportional fatigue life prediction model applied to A319 alloy. The materials made with different casting cooling rates and Sr modification are characterized and quantified in terms of secondary dendrite arm spacing, size, and aspect ratio of eutectic Si particles. Multiaxial nonproportional fatigue tests have been performed on six groups of A319 alloys to systematically analyze the effect of microstructure and loading path on the fatigue properties of Al–Si cast alloy. The first part of the paper is focused on microstructure quantitative characterization to determine the influence of different casting conditions, followed by stress response behavior and fatigue fracture analysis. Finally, a new multiaxial fatigue life prediction model for Al–Si alloy is proposed, for which 56% of the data points fall in the bound lines of factor of 2 and 83% of the data points fall in the bound lines of factor of 3.

Equiaxed dendritic growth in nearly isothermal conditions: A study combining in situ and real-time experiment with large-scale phase-field simulation

The equiaxed dendritic growth of Al-Cu alloys in nearly isothermal temperature field under continuous cooling condition is studied using in situ and real-time observation of experiments by synchrotron X-ray radiography and large-scale quantitative two-dimensional (2D) phase-field (PF) simulations. It is revealed that the equiaxed dendritic morphology and the secondary dendritic arm spacing (SDAS) in the 2D PF simulations are in a reasonable agreement with the experimental data. Increasing the cooling rates results in a smaller SDAS, as predicted by the analytical Kattamis-Flemings model. The transformation kinetics of solid fraction can be described by the Johnson-Mehl-Avrami-Kologoromov (JMAK) theory, but quantitative differences between the experiments and 2D PF simulations are significant. The maximum solute concentration Cmax in liquid is approximately equal to the equilibrium concentration, which depends on the undercooling rather than the cooling rate. But the minimum solute concentration Cmin in solid decreases with the cooling rate, thus leading to a larger segregation ratio SR = Cmax/Cmin. Moreover, the liquid gravity-driven natural convection is considered in simulations. The liquid flow slightly increases the SDAS but has no apparent effect on solid fraction, and the segregation ratio is slightly reduced by the liquid convection, which could be attributed to the almost same Cmax and enlarged Cmin.

Casting defects and microstructure distribution characteristics of aluminum alloy cylinder head with complex structure

Due to the complex structure of the large cylinder head, it is prone to produce uncontrolled casting defects and uneven microstructure distribution. In order to predict the porosity defects and secondary dendrite arm spacing (SDAS) in the as-cast cylinder head, the low pressure casting process of aluminum alloy cylinder head was simulated. Moreover, to verify the accuracy of the simulation results, microscopic observations were performed, in which the simulation results are reasonably matching with that of experiment results. Both simulation and experiment results demonstrate that porosity defects mainly exist in thick walls as well as thick-thin wall junctions. In addition, the top plate presents the finest microstructure, while the microstructure of the force wall is the coarsest. Additionally, the correlation of casting parameters with porosity defects and microstructure was discussed, suggesting that increasing the cooling rate could partially reduce porosity defects. Furthermore, the finding that decreasing the cooling rate can reduce the SDAS of the casting, resulting in a dense microstructure, was drawn. Besides, it has been concluded that regular shapes and thin walls are advantageous for both reducing porosity defects and obtaining uniform microstructures. This provides an effective approach for predicting the casting defects and microstructure of large complex structural castings through numerical simulation. In addition, a quantitative understanding of the casting parameters, complex structure-microstructure relationship for cast aluminum alloys will allow better determination of process parameters based on microstructure and properties requirements of complex components.

Rapid solidification and metastable phase formation during surface modifications of composite Al-Cr cathodes exposed to cathodic arc plasma

A combination of both conventional and advanced high-resolution characterization techniques was applied to study the modified layers on the surface of three composite Al-Cr arc cathodes with identical nominal composition of Al-50 at.% Cr but varying powder grain sizes. The results revealed that the modified layers consist mainly of metastable phases such as Cr solid solution, high temperature cubic Al8Cr5, supersaturated Al solid solution, and icosahedral quasicrystal. The metastable phase formation indicates that high cooling rates were involved during the solidification of molten material produced in the arc craters during cathode spot events. The average cooling rate was estimated to be 106 K/s based on secondary dendrite arm spacing measurements and supporting phase-field based simulations. The formation mechanisms of the modified layers are discussed based on the obtained results and the current literature.

Prediction of Secondary Dendrite Arm Spacing in Al Alloys Using Machine Learning

Prediction of Secondary Dendrite Arm Spacing in Al Alloys Using Machine Learning

Casting Microstructure Inspection Using Computer Vision: Dendrite Spacing in Aluminum Alloys

This paper investigates the determination of secondary dendrite arm spacing (SDAS) using convolutional neural networks (CNNs). The aim was to build a Deep Learning (DL) model for SDAS prediction that has industrially acceptable prediction accuracy. The model was trained on images of polished samples of high-pressure die-cast alloy EN AC 46000 AlSi9Cu3(Fe), the gravity die cast alloy EN AC 51400 AlMg5(Si) and the alloy cast as ingots EN AC 42000 AlSi7Mg. Color images were converted to grayscale to reduce the number of training parameters. It is shown that a relatively simple CNN structure can predict various SDAS values with very high accuracy, with a R2 value of 91.5%. Additionally, the performance of the model is tested with materials not used during training; gravity die-cast EN AC 42200 AlSi7Mg0.6 alloy and EN AC 43400 AlSi10Mg(Fe) and EN AC 47100 Si12Cu1(Fe) high-pressure die-cast alloys. In this task, CNN performed slightly worse, but still within industrially acceptable standards. Consequently, CNN models can be used to determine SDAS values with industrially acceptable predictive accuracy.

Effect of melting rate on microsegregation and primary MC carbides in M2 high-speed steel during electroslag remelting

Large-size primary MC carbides can significantly reduce the performance of M2 high-speed steel. To better control the morphology and size of primary MC carbides, the effect of melting rate on microsegregation and primary MC carbides of M2 steel during electroslag remelting was investigated. When the melting rate is decreased from 2 kg·min−1 to 0.8 kg·min−1, the columnar dendrites are gradually coarsened, and the extent of segregation of Mo and V is alleviated, while the segregation of Cr becomes severe. At 2 kg·min−1, the number of primary MC carbides per unit area with the sizes in the range of 2 µm to 6 µm accounts for about 75% of all MC carbides, while the carbides are mainly concentrated on the size larger than 8 µm at 0.8 kg·min−1. Thermodynamic calculations based on the Clyne-Kurz (simplified to C-K) model shows that MC carbide can be precipitated in the final solidification stage and a smaller secondary dendrite arm spacing caused by higher melting rate (2 kg·min−1 in this experiment) facilitates the refinement of primary MC carbides.

Modeling Ablation Casting

Research published in open literature on ablation casting has been mainly experimental. Modeling of this innovative technology is scarce. This article describes an initial attempt in modeling ablation casting process. Material properties and boundary conditions were defined for calculating cooling curves and microstructural length scales in A356 alloy castings made using sand casting and ablation casting processes. Model predictions were compared with published data on cooling curves and solidification times in an A356 alloy casting. The validated model was then used to predict unique solidification features associated with the ablation casting process, including the eutectic solidification time, shrinkage porosity formation, eutectic front morphology, the advancing rates of the eutectic front and the associated eutectic silicon morphology. Simulation results indicate that the progressive advancing ablation cooling is capable of reducing secondary dendrite arm spacing, modifying the eutectic silicon phase owing to the water quench effect, and eliminating shrinkage porosity using much smaller risers than usual if the ablation is correctly driven towards the risers.

Microstructure characteristics and mechanical properties improvement of gravity cast Al-7Si-0.4Mg alloys with Zr additions

Al-7Si-0.4Mg alloys are one of the most commonly applied aluminum alloys in various casting processes. In this study, 0.06%, 0.14% and 0.20% Zr are added to gravity cast Al-7Si-0.4Mg alloys. The microstructural analysis of the as-cast alloys with Zr shows the formation of (Al,Si)3(Zr,Ti) and π-Fe phases. Compared with alloys without Zr, the alloys with Zr have finer grains; additionally, a small secondary dendrite arm spacing (SDAS) is obtained with a proper Zr content. After T6 heat treatment, the Mg in Fe-rich intermetallics diffuse into the matrix, decreasing the size of the intermetallic phases. Nanoscale precipitates of the (Al,Si)3(Zr,Ti) phase are found in α-Al grains. Due to the finer grains and smaller SDAS as well as the presence of (Al,Si)3(Zr,Ti) precipitates, the tensile strength and elongation of the alloys with 0.06% Zr are 332 MPa and 8.7%, respectively, whereas those of the Al-7Si-0.4Mg alloys without Zr are 302 MPa and 4.6%, respectively.

In-situ microstructural control of A6082 alloy to modify second phase particles by melt conditioned direct chill (MC-DC) casting process – A novel approach

• High shear melt conditioning in the sump during direct chill casting of A6082 billet. • Novelty of maintaining 2–7 °C above alloy liquidus as the sump thermal condition. • In-situ control of solidification during casting by intensive melt shearing. • Precise tailoring of as-cast microstructure to modify second phase particles. • Potential for industries to eliminate homogenisation and use recycled Al alloys. Controlling the formation of noncompact second phase particles during direct chill (DC) casting of aluminium alloys with grain refiner addition remains challenging, as it results in energy intensive homogenisation and deformation problems. In this work, we employed a novel strategy in the DC casting of A6082 alloy to produce billets with a fine-scale dispersion of second phase particles. The strategy involves maintaining 2–7 °C above alloy liquidus as a thermal condition in the sump by in-situ melt conditioning (MC) using a rotor-stator high-shear device operated at a critical rotation speed. As a result, in-situ control of solidification behaviour is achieved to precisely tailor the as-cast microstructure. The billet grain refinement is attained by MC-DC casting without the deliberate addition of chemical grain refiners. The microstructure of the MC-DC cast billet at the critical rotation speed showed a fine-scale dendritic structure with refined secondary dendrite arm spacing (SDAS). The solidification front proceeded with a shallow sump and a corresponding shorter solidification time, higher cooling rate, higher temperature gradient, and smooth solidification rate profile. The ideal fine-scale dendrites with low SDAS divided the remaining eutectic liquid into fine-scale and isolated liquid pockets, resulting in fine-scale, compact morphology, and uniform distribution of second phase particles in the as-cast microstructure. The MC-DC casting process showed the ability to increase the cast house production rate by increasing the casting speed without bleeding the billet. The present approach could be beneficial for eliminating or reducing the homogenisation practice and may also introduce significant flexibility in using recycled Al alloys in the industry.

Influence of Mold Material on the Mold Stability for Foundry Use

In CO2 molding process, sodium silicate is used as a binder to form a chemical bond to have high strength, good dimensional accuracy, etc. as compared to green sand mold. The mold materials are for present investigation wis. silica sand (ss), red mud (rdm), blast furnace slag (bfs), ferrochrome slag (fcs) and olivine sand (olv). This paper investigates the mold properties such as shear strength, compressive strength, hardness, and permeability as per to the American Foundrymen Society (AFS) standard. The mold properties of these mold materials were investigated for the stability as a mold, as compared with silica sand mold for foundry use. And also investigated the effect of mold material on the morphology of phases of the microstructure of Al-Si alloy cast in a different mold. The result shows that the mold properties of blast furnace slag were better as compared to silica sand mold; the mold properties such as shear strength, compressive strength, hardness and permeability of blast furnace slag mold and silica sand mold are 7.2 psi, 17.5 psi, 88, 630; 6.9 psi, 15.4 psi, 92, 480 respectively. The A319 alloy cast in slag mold having less porosity, lower value of secondary dendritic arm spacing (SDAS), and high tensile strength as compared to silica sand mold.

Spatial Variation of Thermokinetics and Associated Microstructural Evolution in Laser Surface Engineered IN718: Precursor to Additive Manufacturing

The spatial variation of thermokinetic parameters has a significant influence on solidification and microstructural aspects such as grain orientation, types and dimensions of the microstructural features, and crystallographic defects. In laser-based additive manufacturing, these factors are mainly dependent on the process parameters and have a wide implication on the microstructural aspects and, in turn, on the mechanical properties. In view of this, the current study focuses on the spatial variation on thermokinetic parameters such as cooling rate, thermal gradient (G), and solidification velocity (R) within the melt pool formed during laser processing of IN718. The continuous-wave Nd-YAG laser was employed at a laser fluence of 14.85, 19.10, and $$23.34\text { J/mm}^2$$ with varying power (700, 900, 1100 W) at a constant scanning speed of 100 mm/s. The finite element method-based multiphysics heat transfer model, coupled with the dynamic fluid flow, was developed to predict these parameters. The model was correlated with microstructural aspects such as melt pool dimensions, orientation of columnar grains, and secondary dendritic arm spacing. The cumulative diffusion length of Nb obtained via thermo-diffusion calculation during multiple heating/cooling cycles was enough to dissolve the fine intragranular plate-shaped $$\delta $$ precipitates in the heat-affected zone.The spatial variation of the G/R ratio recognized the transition of columnar to equiaxed solidification grains which was associated with the G/R ratio lower than $$10 \text {K s/mm}^2$$ in the top region ( $$\sim 25 \mu \text {m}$$ ) of the melt pool. In addition, the coupled solid mechanics model predicted the evolution of thermal stresses during solidification of the melt pool under a high thermal gradient, which marked the generation of high dislocation density in the solidified melt pool.

Powder characteristics of Al0.5CoCrFeMnNi high-entropy alloys fabricated by gas atomisation method

ABSTRACT The fabrication of high-quality metallic powders has been attracted considerable attention in terms of producing advanced materials through additive manufacturing (AM) technology. The efficiency of AM products mainly dependent on the initial powder, therefore, characterisation of pre-alloyed powders become important. In this study, we explored the fabrication of Al0.5CoCrFeMnNi high-entropy alloy (HEA) powders by gas atomisation process and investigated their powder characteristics according to particle size distribution. X-ray diffraction results revealed the formation of major face-centred cubic and minor body-centred cubic phase by the addition of Al content. The detailed characterisations such as scanning electron microscopy, Auger electron spectroscopy and XPS analysis was carried out to examine the chemical composition of HEA powders. The relationship between secondary dendrite arm spacing and cooling rate was evaluated. The results demonstrated that produced alloy powders are spherical in shape with smooth surfaces, compositional homogeneity, indicating the gas-atomised powders are widely acceptable for additive manufacturing applications.