Final Stage Of Solidification Research Articles

Automatic Microstructural Characterization and Classification Using Higher-Order Spectra on Ultrasound Signals

During the gas tungsten arc welding of nickel based superalloys, the secondary phases such as Laves and carbides are formed in final stage of solidification. But, other phases such as $$\gamma {''}$$ and $$\delta $$ phases can precipitate in the microstructure, during aging at high temperatures. Nevertheless, choosing the appropriate conditions of welding can minimize the formation of the Nb-rich Laves phases and thus reduce the susceptibility to solidification cracking. This study aims at the automatic microstructurally characterizing the kinetics of phase transformations on an Nb-base alloy, thermally aged at 650 and 950 $$^{\circ }$$ C for 10, 100 and 200 h, through backscattered ultrasound signals at frequency of 4 MHz. For this, an automated processing system was designed using the spectrum representation of higher order statistics. The ultrasound signals are inherently non-linear and thus the conventional linear time and frequency domain methods can not reveal the complexity of these signals clearly. Bispectrum (the spectral representation of third order correlation) is a non-linear method which is highly robust to noise. In the proposed system, the bispectrum coefficients are subjected to linear discriminant analysis (LDA) technique to reduce the statistical redundancy and reveal discriminating features. These dimensionality reduced features are fed to the classification and regression tree, random forest and k-nearest neighbor (k-NN) classifiers to automatic microstructural characterization. Bispectrum coupled with LDA and k-NN yielded the highest average accuracy of 95.0 and 78.0 %, respectively for thermal aging at 650 and 950 $$^{\circ }$$ C. Thus, the proposed processing system provides high reliability to be used for microstructure characterization through ultrasound signals.

Automatic microstructural characterization and classification using dual tree complex wavelet-based features and Bees Algorithm

During the gas tungsten arc welding of nickel-based superalloys, the secondary phases such as Laves and carbides are formed in final stage of solidification. But, other phases such as γ″ and δ phases can precipitate in the microstructure, during aging at high temperatures. However, it is possible to minimize the formation of the Nb-rich Laves phases and therefore reduce the possibility of solidification cracking by adopting the appropriate welding conditions. This paper aims at the automatic microstructurally characterizing the kinetics of phase transformations on an Nb-base alloy, thermally aged at 650 and 950 °C for 10, 100 and 200 h, through backscattered ultrasound signals at frequency of 4 MHz and dual tree complex wavelet transform (DTCWT)-based feature extraction technique. The feature set comprises of statistical attributes (such as variance, skewness and kurtosis) extracted from the complex wavelet coefficients which are obtained using the DTCWT decomposition of a backscattered ultrasound signal. Also, the performance of the proposed feature extraction technique is compared with the conventional discrete wavelet transform. Finally, these features are fed to the probabilistic neural network (PNN) and radial basis function classifiers to automatic microstructural classification. The training process of these networks depends on the selection of the smoothing parameter of the networks’ activation function in a hidden layer. In this article, we introduce the application of the Bees Algorithm to the automatic adaptation of smoothing parameters. The proposed feature extraction technique coupled with the optimized PNN yielded the highest average accuracy of 96 and 83 %, respectively, for thermal aging at 650 and 950 °C. Thus, the proposed processing system provides high reliability to be used for microstructure characterization through ultrasound signals.

A study on quantitative evaluation of soft reduction amount for CC bloom by thermo-mechanical FEM model

Soft reduction has been proved to be the most promising way to minimize center segregation together with the strand center porosity elimination. The negative pressure driven residual melt flow could be balanced by a reasonable reduction amount to compensate the volume shrinkage in the crater mushy zone. The optimum soft reduction amount to bloom casting for given heavy railway steel has been studied in the paper from the point of view of the volume shrinkage compensation. A solidification analysis model was developed to determine the reduction amount based on the local shrinkage in the final solidification stage. The volume shrinkage in the center inter-dendritic region has been quantitatively calculated through the evaluation to the changing melt density. Another thermo-mechanical coupled FEM model was developed to investigate the bloom deformation behavior upon soft reduction operation. The squeezing effect of the soft reduction on the center mushy zone at various solid fractions was revealed quantitatively as well. The results shown that the theoretical soft reduction amount should decrease from 12.2 mm to 4.0 mm corresponding to the solid fraction from 0.3 to 0.9. A new definition for the calculation of the internal soft reduction efficiency was proposed and compared with the conventional one. The nomogram for the determination of optimum soft reduction has been presented for the bloom casting at various center solid fraction. Accordingly, an optimized soft reduction practice was determined for quality high speed railway steel production with clearly improved strand soundness.

Application of Linear Discriminant Analysis to Ultrasound Signals for Automatic Microstructural Characterization and Classification

During the gas tungsten arc welding of nickel based superalloys, the secondary phases such as Laves and carbides are formed in final stage of solidification. But, other phases such as ?^" and ? phases can precipitate in the microstructure, during aging at high temperatures. Nevertheless, choosing the appropriate conditions of welding can minimize the formation of the Nb-rich Laves phases and thus reduce the susceptibility to solidification cracking. This study proposed and evaluated the performance of an automated processing system to microstructurally characterizing the kinetics of phase transformations on a Nb-base alloy, thermally aged at 650 and 950 °C for 10, 100 and 200 h, using Linear Discriminant Analysis (LDA) on Background echo and Backscattered ultrasound signals at frequencies of 4 and 5 MHz. The main goal of this work is to design a more practical processing system in terms of the accuracy and the speed of processing. This system is composed of three methodologies: the first methodology uses LDA coefficients of normalized ultrasound signals, the second methodology uses LDA coefficients of error signals of the third-order linear prediction model of normalized ultrasound signals and the third methodology uses LDA coefficients of Discrete Cosine Transform. In all three methods, the Probabilistic Neural Network was used as a classifier. The highest accuracies were provided by the third methodology with average classification accuracies of 94.50 and 75.50 %, respectively for thermal aging at 650 and 950 °C. Indeed, LDA proved to be an efficient processing step for microstructural classification tasks.

Influence of Supercooling on the Formation of Primary Phase during Solidification

It is reported actual volume fraction of the primary phase in alloys is larger than the equilibrium value. Larger volume fraction of the primary phase may cause shrinkage cavities and surface or internal cracks. Although control of the solidified structure is important for the quality of cast products, this problem has not been fully elucidated. Taking these results into account, this study has been carried out in order to comprehend the phenomenon of larger volume fraction of primary phase. The Sn-Pb alloy has been used as a test alloy to examine the relation between supercooling for nucleation and the volume fraction of primary phase. Actually, the volume fraction of the primary phase in Sn-Pb alloy is larger than that of the lever rule. It was also observed that the volume fraction of β-Sn decreases with decreasing the supercooling in the early stage of solidification. In the final stage of solidification, however, the effect of supercooling on volume fraction of primary phase is small. Futhermore, when the supercooling was low, the volume fraction of primary phase slowly increased.

Influence of Supercooling on Formation of Primary Phase

It is reported actual volume fraction of primary phase in alloys is larger than the equilibrium value. Larger volume fraction of the primary phase may cause shrinkage cavities and surface or internal cracks. Although control of solidified structure is important for the quality of cast products, this problem has not been elucidated. Taking these results into account, this study has been carried out in order to comprehend a phenomenon of larger volume fraction of primary phase. Sn-Pballoy has been used as a test alloy and to examine the relation between supercooling for nucleation and the volume fraction of primary phase has been mainly characterized. Actually, volume fraction of primary phase in Sn-Pballoy is larger than that of lever rule. It was also observed that the volume fraction of β-Sn decreases with decreasing the supercooling in early stage of solidification. In the final stage of solidification, however, the effect of supercooling on volume fraction of primary phase is small. Furthermore, when the supercooling was low, volume fraction of primary phase was slowly increased.

Reduction in macrosegregation on 380 mm×490 mm bloom caster equipped combination M+F-EMS by optimising casting speed

Based on the plant trials and the developed coupled mathematical model of electromagnetism, turbulent fluid flow, heat and solute transport, the effect of F-EMS position on the degree of bloom macrosegregation cast with combination stirring (M+F-EMS) has been evaluated and compared for medium carbon steel, 40Cr, and high carbon steel, GCr15. The optimum F-EMS position is mainly related to the solidification characteristic of the molten steel in the liquid pool that depends on the average natural convection velocity and solidification time during the final solidification stage. For a given steel grade and a given F-EMS position, the trend of centre porosity and shrinkage cavity variations is to first decrease and then increase with increasing casting speed. The optimal solidification ratio at the F-EMS centre is increased as the carbon content increases due to the increasing solidification time, where the optimal solidification ratios for GCr15 and 40Cr are 72 and 68% respectively.

Mn对Mg-6.5Zn合金热裂倾向性的影响

Mg-Zn-Mn alloy has high hot cracking tendency (HCT), but few researches focus on its hot crack- ing behavior and mechanism. The effect of Mn on the HCT of Mg-6.5Zn-xMn alloys was studied by the designed equipment which can measure and record the subtle changes of temperature, shrinkage displacement and shrinkage stress during solidification in this study. The results indicate that the larger the maximum contract rate (vmax) and the stress accumulating coefficient (k), which are put forward to evaluate HCT, the higher the HCT is, and there is high- er HTC when vmax or k presents at high fraction of solid. The vmax of Mg-6.5Zn-xMn alloy increases with the in- crease of Mn content, however its position move towards to lower fraction of solid, and the k reaches the maximum value and presents at high fraction of solid at 0.35%Mn, which means the greatest HCT in this composition. The hot cracks of these alloys initiated and propagated at final stage of solidification (with higher fraction of solid), and the intergranular feeding channels could be observed. The thicker the liquid film around grains formed by the low melting point phases and the finer the grains, the less the HCT of the alloy is. After dendritic separation, interden- dritic bridging formed by the jointing of dendrite arms could enhance the adhesive force between grains at final stage of solidification. However, the break of interdendritic bridging due to the hindrance to grain contraction would result in the hot cracks. KEYWORDS Mg-6.5Zn alloy, Mn, magnesium alloy, hot cracking

Microstructural evolution and solidification cracking susceptibility of Fe–18Mn–0.6C–xAl steel welds

In this work, the solidification behavior and solidification cracking of Fe–18Mn–0.6C–xAl (x = 1.49, 2.37, 4.79, 6.04 wt%) alloys were investigated. A longitudinal Varestraint test was applied to evaluate the solidification cracking tendency of Al-added high-Mn steel welds. In terms of total crack length and maximum crack length at 4 % applied strain, the solidification cracking susceptibility of high-Mn steel decreased with increasing Al content. Addition of Al suppressed the formation of low melting point eutectics (γ + (Fe,Mn)3C) along the grain boundaries during the final stage of solidification, which resulted in the decrease of solidification cracking tendency. The Al segregated extensively to the dendrite core opposite to Mn and C during solidification, which promoted the formation of δ ferrite. Further, the transition of the solidification sequence from the primary austenitic to primary ferritic mode provided a noticeable improvement in solidification cracking resistance in high-Mn steel welds similar to austenitic stainless steel welds.

Distribution and propagation of dislocation defects in quasi-single crystalline silicon ingots cast by the directional solidification method

We investigated the distribution and propagation of dislocation defects in quasi-single crystalline (QSC) silicon ingots. The dislocation distributions in both the central single crystalline region and the surrounding polycrystalline region of the ingot are measured and compared. Although the dislocation density in the central single crystalline region is much lower than that in the surrounding polycrystalline region, dislocations in this region propagate and spread with the growth of the silicon crystal, especially at the final stage of solidification. The performances of solar cells made of wafers from different parts of the QSC silicon ingot are also compared. The results show that solar cells made of wafers from the middle part of the ingot have the best performance and those made of wafers from the top part of the ingot have the worst performance.

Solidification characteristics and hot tearing susceptibility of Ni-based superalloys for turbocharger turbine wheel

The solidification characteristics and the hot tearing susceptibility were investigated on two Ni-based superalloys for turbocharger turbine wheel, K418 and K419. The segregation behaviors of the alloying elements and the precipitation phases were also studied. The results show that the solidification behavior of K419 alloy is complicated when compared with K418 due to the interdendritic segregation of many kinds of strong interdendritic partitioning elements in the remaining liquid at the final stage of solidification. The segregation of multiple elements in interdendritic liquid results in an extremely low solidus in K419. A long residual liquid stage is found during the solidification of K419, giving rise to reduced cohesion strength of dendrites and increased sensitivity to hot tearing. A hot tearing susceptibility coefficient (HTS) criterion is proposed based on a hot tearing sensitive model. The HTS value of K419 alloy is larger than that of K418 alloy.

A Three Dimensional Model to Characterize the Centerline Segregation in CC Slabs

In this investigation the formation of centerline segregation and shrinkage holes in the final stages of solidification during the continuous casting process of slabs is described. Therefore experimental and theoretical investigations of the melt behavior during the solidification are carried out. Based on these results, a model is presented to explain the formation of V‐shaped segregation and core porosities caused by the restricted flow of melt through the mushy zone in slabs. The theoretical explanation is manifested by various observation, calculations and analyses by electron microprobe mappings. The main results are that V‐segregation forms in quasi periodical frequency in longitudinal direction within continuous cast slabs. The areas of formation seem to be independent, like little billets lying side by side in one slab. Consequential a three dimensional model about the behavior of the liquid melt in the final stages of solidification is designed.

Effects of Y on hot tearing susceptibility of Mg–Zn–Y–Zr alloys

The hot tearing susceptibility of MgZn2.5YxZr0.5 (x=0.5, 1, 2, 4, 6) alloys was evaluated by thermodynamic calculations based on Clyne–Davies model. The microstructure and morphology of hot tearing regions of the alloys were observed by X-ray diffraction and scanning electron microscopy. The solidification temperature and shrinkage stress during the solidification of MgZn2.5YxZr0.5 alloys in the “T” type hot tearing permanent-mold were acquired with the attached computer. The effect factors of hot tearing susceptibility of MgZn2.5YxZr0.5 alloys, such as the solidification temperature interval, the variation of solid fraction in vulnerable region, the residual liquid fraction in the final stage, the type of the second phase of the alloys were discussed based on the above calculation and observation. The results demonstrated that the hot tearing susceptibility in the investigated alloys was found as follows–MgZn2.5Y2Zr0.5>MgZn2.5Y0.5Zr0.5>MgZn2.5Y4Zr0.5>MgZn2.5Y6Zr0.5>MgZn2.5Y1Zr0.5. The highest hot tearing susceptibility of MgZn2.5Y2Zr0.5 alloy related to the following reasons: the largest freezing range, the biggest changing of the variation of solid fraction in vulnerable region, the least liquid film in the final stage of solidification, the formation of the second phase which worsens the liquid flow and interdendritic feeding after dendrite coherency.

Study on the Macrosegregation Behavior for the Bloom Continuous Casting: Model Development and Validation

With the aid of a coupling electromagnetic-thermal-solute transportation model validated by the industrial investigation, a three-dimensional (3-D) plus two-dimensional (2-D) hybrid modeling method has been presented for the exploration of the macrosegregation and macroscale transport phenomena in the bloom continuous casting (CC) processes of high-carbon GCr15-bearing steel. The evolution and characteristics of solute distribution and its influence on the porosity formation in the strand during CC process have been revealed. Solute segregation degree changes from a positive to a negative value with distance from strand surface in the region of initial solidification shell within thickness of 25 mm, which can be attributed to the circulation flow ahead the solidification front and the floatation of solute-rich molten steel at the upper part of the mold. The discontinuous, nonfrozen band induced by the zigzag solute distribution is proven to be the main reason that leads to the porosity formation in the final solidification stage of the CC strand. As the solidification proceeds, the segregation degree of C at the strand center is increased from 1.0 to 1.2, while the melt liquidus temperature is reduced from 1726 K (1453 °C) to 1706.91 K (1433.91 °C) during the CC process. Moreover, with the action of gravity and thermosolutal convection, a negative segregation region in the concave shape and an irregular positive segregation zone are produced in the fixed and loosened side of shell, respectively.

Origin of the lathy ferrite in AISI 304 stainless steel during directional solidification

Abstract The solidification path has been revealed and the mechanism of three-phase reaction in 18-8 austenitic stainless steel has been clarified using directional solidification and liquid quenching. It is found that primary delta-ferrite precipitates from the melt at the initial stage of solidification. With the decrease in the melt temperature, it is confirmed that austenite is formed through peritectic transformation rather than eutectic reaction. After peritectic reaction, partial delta-ferrite is retained since peritectic reaction is incomplete and delta-ferrite and peritectic austenite is enveloped by the surrounded liquid. At the final stage of solidification, the rest of liquid transforms into austenite directly. Experimental evidences have been presented to account for the solidification details.

Heat transfer in an industrial directional solidification furnace with multi-heaters for silicon ingots

We carried out transient global simulations of melting, growing and annealing processes in an industrial directional solidification furnace with a top and side heater. The power distribution between the two heaters was set to 3:7, 5:5 and 7:3 to investigate its effect on global heat transfer and silicon crystal growth. Quantitative comparisons of temperature distributions are presented. Increasing the top heating power slightly increases the melting time and changes the melting sequence. In the crystal growth process, adjusting the power distribution from 3:7 to 7:3 can lead to an 8–12K change in axial temperature difference in the silicon domain. Both the intensity and pattern of the silicon melt flow are influenced by the change in power distribution. The melt-crystal interface is less convex to the melt with an increase in top heating power, especially at the final stage of solidification. In the annealing process, the ingot temperature decreases to approximately 1600K to reduce the thermal stress and dislocation density. Isotherm shapes are different at this stage and may lead to different thermal stress and dislocation density distributions in the silicon ingot. The numerical results provide a basic understanding of the heat transfer characteristics due to power distribution in an industrial multi-heater directional solidification furnace.

Solidification characteristics and segregation behavior of Ni-based superalloy K418 for auto turbocharger turbine

The solidification characteristics and segregation behavior of K418 alloy were investigated by the DSC measurement and the isothermal solidification quenching process. Phase transformation temperatures, together with the solid fraction with temperature were measured by experiments and comparatively calculated by Thermo-Calc. It is demonstrated that the solidification begins with the formation of primary γ dendrite and terminates with the γ/γ′ eutectic reaction, being accompanied by the formation of MC carbide and M(C, B) carboboride. During the solidification, interdendritic region is significantly enriched with Nb, Ti, Zr, B, C and Mo, which results in the formation of carbide, carboboride and γ/γ′ eutectic. The slow increase of solid fraction at the final stage of solidification is also caused by the strong segregation of the positive segregation elements in interdendritic area. In the case of K418, equilibrium model is shown to be a valuable tool to predict the solidification sequence and phase transition temperatures. Scheil calculation can be used to investigate the segregation behavior during solidification.

Modeling of the Formation of AlN Precipitates During Solidification of Steel

Abstract The study was carried out computer simulations of the formation process of AlN precipitates in the solidification of steel. The chemical composition of steel and non-metallic inclusions formed was determined using the commercial software FactSage. Calculated amount of precipitates formed during cooling of steel between the liquidus and solidus temperatures under conditions of thermodynamic equilibrium. In parallel, the computations were performed using your own computer program. It was found that aluminum nitride is formed at the final stage of solidification, and the condition of its formation is low oxygen content in steel.

Al-Mg 계 다이캐스팅 합금의 미세조직 및 기계적 성질에 미치는 Mg 및 Si의 영향

The effects of Mg and Si contents on the microstructure and mechanical properties in Al-Mg alloy (ALDC6) were investigated. The results showed that phase fraction and size of <TEX>$Mg_2Si$</TEX> and <TEX>$Al_{15}(Fe,Mn)_3Si_2$</TEX> phase in the microstructure of Al-Mg alloy were increased as the Mg and Si contents were raised from 2.5 to 3.5 wt%. With Si content of 1.5 wt%, freezing range of the alloy was significantly reduced and solidification became more complex during the final stage of solidification. While there was no significant influence of Mg contents on mechanical properties, Si contents up to 1.5 wt%, strongly affected the mechanical properties. Especially elongation was reduced by about a half with more than 1.0 wt%Si in the alloy. The bending and impact strength were decreased with increased amount of Si in the alloy, as well. The lowered mechanical properties are because of the growth of particle shaped coarse <TEX>$Mg_2Si$</TEX> phase and precipitation of the needle like <TEX>$\beta$</TEX>-AlFeSi in the microstructure at the last region to solidify due to presence of excess amount of Si in the alloy.

Heat transfer in rapidly solidifying supercooled pure melt during final transient

The heat transfer model for a one-dimensional supercooled melt during the final stage of solidification is considered. The Stefan problem for the determination of the temperature distribution is solved under the condition that (i) the interface approaches the specimen surface with a constant velocity V; (ii) the latent heat of solidification linearly depends on the interface temperature; (iii) all the physical quantities given at the phase boundary are presented by linear combinations of the exponential functions of the interface position. First we find the solution of the corresponding hyperbolic Stefan problem within the framework of which the heat transfer is described by the telegraph equation. The solution of the initial parabolic Stefan problem is then found as a result of the limiting transition V/VH→0(VH→∞), where VH is the velocity of the propagation of the heat disturbances, in which the hyperbolic heat model tends to the parabolic one.