Primary Dendrite Arm Research Articles

Evaluation of Permeability of Interdendritic Channels for Al-Cu and Sn-Pb Alloys

A particularly important defect related with solute segregation during solidification is the microporosity, which influences mechanical properties of castings. Considering only the formation of porosity due to shrinkage, during directional solidification, it is necessary a flow of liquid metal into the interdendritic channels to compensate for metal contraction, and pores are formed when the pressure drop in the liquid flow at a point within the mushy zone exceeds the pressure acting at this point. The increase in roughness of the interdendritic channels, caused by the successive ramification of the dendrite arms promotes a pressure drop inside the channel. In this paper a model developed by the authors is used to predict the permeability in directional solidification of Al-Cu and Sn-Pb alloys. From comparisons with experimental results models were choose to estimate primary and secondary dendrite arms spacing and applied to calculate the variation of permeability in directional solidification. From these results, applying a numerical method the pressure drop in the interdendritic channels was determined for the different alloys to analyze the influence of the composition of the alloy in the possibility of pore formation.

Solidification of AlSi Alloys in the ARTEMIS and ARTEX Facilities Including Rotating Magnetic Fields – A Combined Experimental and Numerical Analysis

The solidification microstructure is the consequence of a wide range of process parameters, like the growth velocity, the temperature gradient and the composition. Although the influence of these parameters is nowadays considerably well understood, an overall theory of the influence of convection on microstructural features is still lacking. The application of time dependent magnetic fields during directional solidification offers the possibility to create defined solidification and flow conditions. In this work, we report about solidification experiments in the ARTEMIS and ARTEX facilities including rotating magnetic fields (RMF). The effect of the forced melt flow on microstructural parameters like the primary and secondary dendrite arm spacing is analyzed for a wide range of magnetic field parameters. The experimental analysis is supported by a rigorous application of numerical modeling. An important issue is hereby the prediction of the resulting macrosegregation, i.e., differences in the composition on the scale of the sample (macroscale) due to the RMF. For the considered configuration and parameters an axial enrichment of Si is found beyond a certain magnetic field strength. The results are compared to available theories and their applicability is discussed.

The influence of melt convection on dendritic spacing of downward unsteady-state directionally solidified Sn-Pb alloys

Microstructures are the strategic link between materials processing and materials behavior. A dendritic structure is the most frequently observed pattern of solidified alloys. The microstructural scales of dendrites, such as primary and secondary arm spacings, control the segregation profiles and the formation of secondary phases within interdendritic regions, determine the properties of cast structures. In this work, the influence of thermosolutal convection on dendrite arm spacings is experimentally examined in the downward vertical unsteady-state directional solidification of Sn-Pb hypoeutectic alloys. The experimental observations are compared not only with the main predictive theoretical models for dendritic spacings but also with experimental results obtained for Sn-Pb alloys solidified vertically upwards. Primary dendritic arm spacings have been affected by the direction of growth, decreasing in conditions of downward vertical solidification when compared with those grown vertically upwards. Further, the unsteady-state l1 predictive models did not generate the experimental observations.

Prediction of Prior Austenite Grain Size of High-phosphorous Steels through Phase Transformation Simulation

The effects of phosphorous content and cooling rate on the phase evolution of low carbon steels were analyzed by DICTRA simulation focusing on the micro-segregation behavior of phosphorous during solidification and subsequent cooling process. Through the phase transformation simulation, the effects of metallurgical parameters such as P content, cooling rate, and primary dendrite arm spacing on the starting temperature of austenite grain growth were clarified. Using the values of austenite grain growth starting temperature calculated by the phase transformation simulation, prior austenite grain size of low carbon steels were estimated based on a classical grain growth modeling in a P content range from 0.01 to 0.20 mass% and in a wide cooling rate range from 1.7 to 800 K s−1. The prior austenite grain size predicted with the present approach showed good agreement with a group of selected experimental data of 100 mm thick slabs and 2 mm thick strips.

YAG laser surface densification of a zircon refractory by adding AlN nanoparticles

A zircon refractory zirconium silicate was surface melted with a pulsed YAG laser to introduce AlN nanoparticles, with the aim of improving its surface density and modifying the corresponding microstructure. The microstructure and phase structure features of the refractory induced by the laser beam were studied by scanning transmission microscopy, incorporating energy dispersive X-ray and X-ray diffraction analysis. Results show that the addition of AlN nanoparticles into the refractory surface significantly reduced the laser beam energy required to melt the surface of the refractory. The laser-melted depth was increased with an increase of laser power from 133 to 200 W. The microstructure of the laser-treated zone with the addition of AlN nanoparticles has a much finer dendritic structure with undeveloped primary dendrite arms while without the addition of AlN nanoparticles, a coarser dendrite with much long primary dendrite arms was observed. The phase structure of the laser-treated layer with the addition of AlN nanoparticles was found to be composed of m-ZrO 2, c-ZrO 2, ZrSiO 4 and H–AlN.

Effects of Cooling Rate on Solidification Microstructure of ZK60 Magnesium Alloy

In this paper, the influence of the cooling rate on the solidified structure of ZK60 Mg alloy has been studied by means of Gleeble-1500D thermal simulation instrument. The result showed that the grain size and grain shape depended on the cooling rate, Primary Dendrite Arm Space ( λI ) and Secondary Dendrite Arm Space ( λII) sharply decreased with the increasing of solidifying cooling rate (v) in the range of experimental cooling rate (0.2~100K/s). When superheat was constant, the empirical formulas of the relation between λI λII and v was obtained. In addition, the dependence between micro-hardness (HV) and Secondary Dendrite Arm Space ( λII) for ZK60 casting alloy was proved to be similar with Hall-Petch formula. The empirical formula for λII-HV has been proposed.

Effect of cooling rate on microstructure evolution of rapidly cooled high-impurity steels

The effect of cooling rate on secondary dendrite arm spacing (SDAS) and prior austenite grain size (PAGS) has been investigated in high-phosphorous steels (0.1% P) subjected to a rapid cooling process of electron-beam (E-beam) surface remelting. A heat flow calculation was carried out to estimate the cooling rates of surface remelting experiments. The measured SDAS values depended on the cooling rate in good accordance with the Suzuki equation [A. Suzuki, T. Suzuki, Y. Nagaoka, Y. Iwata, J. Jpn. Inst. Met. 32 (1968) 1301–1304] up to the cooling rate of 10 5 K s −1. The classical grain growth modeling (CGM) [N. Yoshida, O. Umezawa, K. Nagai, ISIJ Int. 44 (2004) 547–555] was employed to clarify the effect of the cooling rates on the PAGS. The growth during the continuous cooling process was calculated for each cooling rate by CGM. With increasing the cooling rate, the growth of the prior austenite grains decreased and the PAGS converged to primary dendrite arm spacing (PDAS) when the cooling rates exceeded 10 3 K s −1. An analytic model that can be applied to predict the PAGS for cooling rates from 10 0 to 2 × 10 4 K s −1 was presented based on the present results.

Predicting dendrite arm spacing and their effect on microporosity formation in directionally solidified Al-Cu alloy

In the case of metallic alloys, which present dendritic structure, the mechanical properties of foundry products depend on primary and secondary arm spacing. For the prediction of microporosities it is necessary to characterize precisely the dendritic structure formed during solidification, to calculate the permeability and also to estimate the radius of the gas bubble to determine the pressure due to gas/metal surface tension. Therefore, it is important in a computational simulation of the solidification processes to use reliable equations to correlate the calculated thermal parameters with primary and secondary dendrite arm spacing. This study presents a numerical and experimental analysis of some models to predict the primary and secondary arm spacing as a function of thermal parameters. Comparison between the numerical and experimental results for Al 4.5 wt% Cu alloy allowed the selection of adequate equations to predict the dendritic spacing during unidirectional solidification.

Dependency of the dendritic arm spacings and tip radius on the growth rate and composition in the directionally solidified succinonitrile–carbon tetrabromide alloys

Succinonitrile (SCN)–carbon tetrabromide (CTB) alloys for different concentrations of CTB ( C 0) were unidirectionally solidified with a constant temperature gradient ( G=7.5 K/mm) at different growth rates ( V=6.5–103.5 μm/s). The primary dendrite arm spacings, secondary dendrite arm spacings and dendrite tip radius were measured as a function of growth rates and compositions. Theoretical models for the dendrite arm spacings and tip radius have been compared with the experimental observations, and a comparison of our results with the current theoretical models and previous experimental results have also been made.

Interface Morphologies and Microstructure of a Single Crystal Superalloy under High Thermal Gradient Directional Solidification

The interface morphologies single crystal superalloys CMSX-2 has been studied over a range of cooling rate with large variations in withdrawal speed in directional solidification. The superfine cellular structure was obtained under both high thermal gradient up to 1000K/cm and fast withdrawal rate up to 1mm/sec. The high rate directional solidification results in reduction in primary and secondary dendrite arm spacing, refinement of λ’ phase, reduced microsegregation of alloying elements and smaller size of γ-γ’ eutectics.

Phase Field Simulation on Directional Solidification of Succinonitrile (SCN)–Acetone Organic Model Alloy

Phase field simulation on directional solidification of succinonitine (SCN)-acetone organic model alloy was performed. The developiuent of concentration and phase field profile was calculated in order to clarify the uependences of growth velectiy V and temperature gradient G on the morphology of growing crystal. Concerning the primary dendrite arm spacing λ 1 , the relationship between λ 1 4 G 2 /kΔT 0 and V was examined, where k and ΔT 0 are partition coefficient and temperature difference between liquid andsolid phases. respectively. The calculated values of λ 1 4 G 2 V/kΔT 0 did not show a universal relationship but the friends of the present calculation can be understood by extrapolation from experimental data. The power law index of V for the λ 1 4 G 2 V/kΔT 0 -V plot took similar values between the experiments and the present calculation for some cases. The λ 1 -V relationship did not show depcndences on temperature of the low temperature side and nor acetone concentration. For the curvature radius R, the R--V relationship can be fitted by a simple function regardless the value of G. In addition, the calculated values of R and the power low indexes of V for R were compared to the dendrite growth theory and good accordance was confirmed.

Interface morphology evolvement and microstructure characteristics of hypoeutectic Cu–1.0 wt%Cr alloy during unidirectional solidification

Effect of unidirectional solidification rate on microstructure of hypoeutectic Cu–1.0%Cr alloy was investigated. The microstructure evolution of Cu–1.0%Cr alloy was noticed especially during the unidirectional solidification with the different solidification rates. It is shown that eutectic (α+β) and primary α(Cu) phase grew up equably in parallel to direction of solidification. A kind of fibriform microstructure will appear when unidirectional solidification rate is up to some enough high certain values. When temperature gradient was changeless, the interface morphology evolution of the primary α(Cu) phase underwent to a series of changes from plane to cell, coarse dendrite, and fine dendrite grains with increasing the solidification rates. Primary dendrite arm spacing λ1 of α(Cu) phase increases with increasing the solidification rate where the morphology of the solid/liquid (S/L) interface is cellular. However, λ1 decreases with further increasing the solidification rate where the S/L interface morphology is changed from cell to dendrite-type. Its rule might accord with Jackson–Hunt theory model. An experience equation obtained is as follows: . On the other hand, secondary dendrite spacing λ2 of primary α(Cu) phase will thin gradually with increasing the solidification rate. Moreover, secondary dendrite will become coarse in further solidification. Another experience equation about relationship among secondary dendrite arm spacing (λ2), temperature gradient GL and the velocity of the S/L interface (V) is that: . In addition, the volume fraction of eutectic will decrease with the increase of solidification rate.

Prediction of macrosegregation during the solidification involving a peritectic transformation for multicomponent steels

Micro-macro segregation models are based on the multi-scale approach of the segregation, i.e., on the coupling of a macrosegregation model, which describes transport phenomena at the ingot scale, with a microsegregation model, which describes solidification at the scale of the primary or secondary dendrite arm spacing. The goal of this work is to illustrate the influence of the microsegregation phenomena on the macrosegregation. A microsegregation model for multicomponent steel alloys has been developed which takes into account solutal diffusion in solid and liquid phases, a correct description of the multicomponent phase diagram, and the variation of the local average solute concentration (open system). The peritectic transformation has been tackled and specific attention has been drawn on the accurate determination of the δ/γ and γ/liquid interface movements during solidification. Examples of micro-macro calculations obtained with this microsegregation model on ingot cases are presented and original results which illustrate specific effects of the peritectic transformation on macrosegregation are discussed.

Permeability of interdendritic channels

During solidification of metallic alloys that freeze over a range of temperatures a variation of solute concentration ahead of the solid/liquid interface is observed leading to a dendritic growth of the solid phase. The shrinkage of the alloy during solidification must be compensated by a flow of liquid metal in the fine mesh of interdendritic channels formed, in order to avoid pore formation. For typical dendrite arm spacing this flow can be treated as a flow through a finely porous media and the Darcy's law can be applied if the permeability of the interdendritic channels is known. In this paper, a heuristic method to estimate variation of permeability of interdendritic channels during solidification of metallic alloys as function of primary and secondary dendrite arms spacings and liquid fraction, applied to liquid flow parallel and normal to primary dendrite arm, is proposed and compared with experimental results obtained by different authors.

Dissimilar welding of nickel-based Alloy 690 to SUS 304L with Ti addition

This study investigates the effects of Ti addition on the weldability, microstructure and mechanical properties of a dissimilar weldment of Alloy 690 and SUS 304L. Shielding metal arc welding (SMAW) is employed to butt-weld two plates with three welding layers, where each layer is deposited in a single pass. To investigate the effects of Ti addition, the flux coatings of the electrodes used in the welding process are modified by varying additions of either a Ti–Fe compound or a Ti powder. The results indicate that the microstructure of the fusion zone (FZ) is primarily dendritic. With increasing Ti content, it is noted that the microstructure changes from a columnar dendritic to an equiaxed dendritic, in which the primary dendrite arm spacing (PDAS) becomes shorter. Furthermore, it is observed that the amount of Al–Ti oxide phase increases in the inter-dendritic region, while the amount of Nb-rich phase decreases. Moreover, the average hardness of the FZ increases slightly. The results indicate that Ti addition prompts a significant increase in the elongation of the weldment (i.e. 36.5%, Ti: 0.41 wt%), although the tensile strength remains relatively unchanged. However, at an increased Ti content of 0.91 wt%, an obvious reduction in the tensile strength is noted, which can be attributed to a general reduction in the weldability of the joint.

Influence of melt convection on the columnar to equiaxed transition and microstructure of downward unsteady-state directionally solidified Sn–Pb alloys

A combined theoretical and experimental approach is developed to quantitatively determine the solidification thermal parameters: transient heat transfer coefficients, tip growth rates and cooling rates during downward unsteady state solidification of hypoeutectic Sn–Pb alloys. For the growth conditions examined, solid in the top and melt below, with gravity pointing down, the rejection of solute into the melt during solidification results in increased melt density. The resulting thermosolutal convection can start in the melt both within the interdendritic region and ahead of the dendrite array. The experimental results have shown that melt convection may be causing pileup of fractioned dendritic arms, which must stimulate the CET occurrence. The results have supported a criterion recently proposed based on a critical cooling rate. For upward unidirectional condition, this critical value was found to about 0.014K/s for hypoeutectic Sn–Pb alloys. In the present study, in conditions of downward solidification, melt convection seems to favor the structural transition, which is anticipated and occurs for a critical cooling rate of about 0.03K/s, for any of three hypoeutectic alloys experimentally examined. Primary dendritic arm spacings have been affected by the direction of growth, decreasing in conditions of downward vertical solidification when compared with those grown vertically upwards. A tendency of reduction of secondary dendritic arms has also been observed for the Sn 5wt.% Pb alloy solidified downwards when compared with those grown vertically upwards.

Numerical model to predict the position, amount and size of microporosity formation in Al–Cu alloys by dissolved gas and solidification shrinkage

This paper presents a numerical and experimental analysis of the formation of interdendritic microporosities during unidirectional solidification of Al 4.5 wt.% Cu alloy. A model, based on the method of finite difference is applied to simulate the solidifying process, permitting the determination of structural parameters, as primary and secondary dendrite arm spacing, permeability of the interdendritic channels and the formation of microporosity between the dendritic arms. The model permits the prediction of microporosity due to metal shrinkage during solidification and due to the evolution of hydrogen resulting from the difference in its solubility in the liquid and solid phases, including determination of their dimensions and locations. The model allows establishing the minimum gas content and the ideal thermal conditions to minimize microporosity formation. To test and optimize the model developed, experimental testing was performed, using a specially built unidirectional solidifying oven and in situ measurements of the hydrogen content in the liquid metal. Using optical microscopy, the primary and secondary interdendritic spacing were obtained, also including determination of microporosity dimensions and locations. Results obtained are presented with the application of the model for Al 4.5 wt.% Cu alloy solidification under different conditions.

Simulation of constrained dendrite growth of multicomponent alloys using a Calphad method

The conventional theory of constrained dendrite growth for binary alloys has been extended to multicomponent alloys based on the Calphad method by consideration of solute interactions in each phase. The variable solute partition coefficients and liquidus slopes under different tip undercooling were calculated in detail for a series of Al-Si-Mg alloys. Their influence on the kinetics of dendrite growth was demonstrated quantitatively. The most important microstructure scale, the primary dendrite spacing, was predicted in several Al-Si-Mg alloys. By comparing with the experimental results of former researchers, it is found that the present method coupled with Calphad is a satisfactory way for the prediction of primary dendrite arm spacing.

Effect of growth rate and composition on the primary spacing, the dendrite tip radius and mushy zone depth in the directionally solidified succinonitrile–Salol alloys

The succinonitrile (SCN)–Salol alloys for four different concentrations Salol were unidirectionally solidified for five different growth rates in a constant temperature gradient. The microstructure parameters, viz., the primary dendrite arm spacing, λ 1, dendrite tip radius, R and mushy zone depth, d, were measured. The dependence of the microstructure parameters on the solidification parameters for SCN–Salol alloys were determined by linear regression analysis. The results are compared with theoretical models and published data.

Rapid solidification of undercooled monotectic alloy melts

Structural evolution of bulk undercooled Ni–11.56 at.% Pb monotectic alloy was investigated systematically by using molten glass denucleating combined with superheating cycling. Within the achieved undercooling range 10–286 K, the solidification structures were classified into three categories. When the undercooling was less than 50 K, the structures consisted of coarse dendrites and interdendritic lead phase. With the undercooling increasing into the range of 70–232 K, the dendrite clusters were refined and fine lead particles separated out from the supersaturated primary dendrite arms because of solute trapping. When the undercooling exceeded 242 K, the granular grains formed and fine lead particles homogeneously distributed in the whole sample. The phase selection of high temperature melts was analyzed by adopting steady state nucleation theory. The calculated results shown that undercooled Ni–11.56 at.% Pb monotectic alloy melts solidified in the form of α(Ni) dendrites essentially during the stage of rapid solidification and after recalescence, the interdendritic residual melts solidified in the equilibrium mode. Based on the observation of the solidification structures and the calculated results with BCT dendritic growth model, it was confirmed that the granulation mechanism of the granular grains was owing to the primary dendritic disintegration and subsequent recrystallization.