Equiaxed Zone Research Articles

Center Segregation with Final Electromagnetic Stirring in Billet Continuous Casting Process

With a multiphase solidification model built, the effect of F-EMS parameters on center segregation was investigated in 160 mm × 160 mm billet continuous casting process. In the model, the initial growth of equiaxed grains which could move freely with liquid was treated as slurry, while the coherent equiaxed zone was regarded as porous media. The results show that the stirring velocity is not the main factor influencing center segregation improvement, which is more affected by current intensity and stirring pool width. Because solute transport is controlled by solidification rate as stirring pool width is 73 mm, center segregation declines continuously with current intensity increasing. As liquid pool width decreases to 61 mm and less latent heat needs to dissipate in the later solidification, the center segregation could be improved more obviously by F-EMS. Due to center liquid solute enrichment and liquid phase accumulation in the stirring zone, center segregation turns to rise reversely with higher current intensity and becomes more serious with stirring pool width further decreasing to 43 mm. As the stirring pool width is 25 mm, the positive segregation has already formed and solute could still concentrate with weak stirring, leading to center segregation deterioration. With the optimized current intensity (400 A) and stirring pool width (61 mm) set for continuous mode, center segregation improvement is better than that of alternative mode.

Crystallographic Investigation of the Initial Solidification Grain Structure in Al-Si Alloy

As-solidified structure of an ingot is composed of the chill, columnar and equiaxed zones. The whole solidified structure is strongly affected by the chill crystals. Some initial solidification grains have been observed on the ingot surface and thought to be traces of the nucleation point. The aim of this study is, therefore, to develop the experiment technique to make one ‘grain’ and to crystallographically investigate the initial solidification grain using EBSD analysis. In order to start solidification at a very specified position, a small metallic protrusion was installed on an insulating plate. Al-6 wt%Si alloy was melted at 800 °C and was poured on the metallic protrusion. In this study, the amount of protrusion was varied to investigate the growth mechanism of the initial solidification grain. The longitudinal cross section of the specimen was observed by an optical microscope, a scanning electron microscope. The starting position of solidification was the area that was on the metallic protrusion. In this initial solidification grain, it was difficult to observe the dendritic structure. The shape of this grain was about hemispherical. The grain area seemed to increase with increasing the amount of protrusion. The results of EBSD analysis showed that almost all initial solidification grains were composed by several crystals. The reason of this is that the nucleation frequency may increase with the amount of protrusion. The dendrite grew radially from the initial solidification grain continuously. The crystallographic structure was also continuous on the boundary of the initial solidification grain.

Macro-micro modeling and simulation for the morphological evolution of the solidification structures in the entire weld

The heat and mass transfer and the morphological evolution of the various weld solidification structures for the entire weld were simulated through the macro-micro modeling which combined the welding process and the solidification structures in the weld pool. The weld profiles were calculated for different welding conditions, and the effects of the welding parameters on the solidification conditions were analyzed along the trailing pool boundary. The formation mechanism of the axial columnar structure and the equiaxed grains in the weld were studied by the developed model. The results indicate that the temperature gradient G, solidification rate Rw, and the ratio G/Rw along the trailing pool boundary are related with the location in the weld and the welding parameters. If there does not exist heterogeneous nuclei in the weld pool, the axial columnar structure forms in the central region of the weld at low welding speeds owing to the anisotropic kinetics of the dendritic structure. When the heterogeneous nucleation occurs in the weld pool, the weld structure is directly determined by the competitive growth between the columnar and new nucleated grains. At low welding speeds, the preferential orientations of the survival grains get closer to the welding direction with the decrease of the distance to the weld centerline. With the increase of the welding speed and welding current, more grains form in the weld pool, and the new nucleated grains also become more competitive. The columnar grains from the fusion line are blocked by the equiaxed grains. The continuous equiaxed grain zone forms in the weld, and the columnar to equiaxed transition (CET) occurs.

Simulation of solidification microstructure of Fe-6.5%Si alloy using cellular automaton-finite element method

3D microstructures of Fe–6.5%Si (mass fraction) alloys prepared under different cooling conditions were simulated via finite element-cellular automaton (CAFE) method. The simulated results were compared to experimental results and found to be in accordance. Variations in the temperature field and solid-liquid region, which plays important roles in determining solidification structures, were also examined under various cooling conditions. The proposed model was utilized to determine the effects of Gaussian distribution parameters to find that the lower the mean undercooling, the higher the equiaxed crystal zone ratio; also, the larger the maximum nucleation density, the smaller the grain size. The influence of superheat on solidification structure and columnar to equiaxed transition (CET) in the cast ingot was also investigated to find that decrease in superheat from 52 K to 20 K causes the equiaxed crystal zone ratio to increase from 58.13% to 65.6%, the mean gain radius to decrease from 2.102 mm to 1.871 mm, and the CET to occur ahead of schedule. To this effect, low superheat casting is beneficial to obtain finer equiaxed gains and higher equiaxed dendrite zone ratio in Fe–6.5%Si alloy cast ingots.

The accuracy of a 2D and 3D dendritic tip scaling parameter in predicting the columnar to equiaxed transition (CET)

The dendrite tip kinetics model accuracy relies on the reliability of the stability constant used, which is usually experimentally determined for 3D situations and applied to 2D models. The paper reports authors' attempts to cure the situation by deriving 2D dendritic tip scaling parameter for aluminium-based alloy: Al-4wt%Cu. The obtained parameter is then incorporated into the KGT dendritic growth model in order to compare it with the original 3D KGT counterpart and to derive two-dimensional and three-dimensional versions of the modified Hunt's analytical model for the columnar-to-equiaxed transition (CET). The conclusions drawn from the above analysis are further confirmed through numerical calculations of the two cases of Al-4wt%Cu metallic alloy solidification using the front tracking technique. Results, including the porous zone-under-cooled liquid front position, the calculated solutal under-cooling and a new predictor of the relative tendency to form an equiaxed zone, are shown, compared and discussed two numerical cases. The necessity to calculate sufficiently precise values of the tip scaling parameter in 2D and 3D is stressed.

Simulation of macrosegregation in a large vertical continuous casting of steel

A three-phase mixed columnar-equiaxed solidification model considering fluid flow, heat and solute transport is applied to simulate the solidification in a vertical continuous casting. The key features of solidification phenomena in this process, such as evolution of columnar phase, evolution and floatation/sedimentation of equi- axed crystals, thermal solutal convection of the melt and the flow caused by crystal sedimentation, development of as-cast structure, the columnar-to-equiaxed transition (CET), and formation of macrosegregation, are simulated. It is predicted that there is an equiaxed zone in the central part of the strand, and the rest section is filled with columnar phase (or dominant with columnar phase). A relatively strong negative segregation in the equiaxed zone and a mostly neutral concentration in the columnar region are found. Near the CET, there is a so-called middle radius positive segregation band. Formation mechanisms of this segregation pattern are discussed.

Solidification Structure of Continuous Casting Large Round Billets under Mold Electromagnetic Stirring

The solidification structure of a continuous casting large round billet was analyzed by a cellular-automaton-finite-element coupling model using the ProCAST software. The actual and simulated solidification structures were compared under mold electromagnetic stirring (MEMS) conditions (current of 300 A and frequency of 3 Hz). Thereafter, the solidification structures of the large round billet were investigated under different superheats, casting speeds, and secondary cooling intensities. Finally, the effect of the MEMS current on the solidification structures was obtained under fixed superheat, casting speed, secondary cooling intensity, and MEMS frequency. The model accurately simulated the actual solidification structures of any steel, regardless of its size and the parameters used in the continuous casting process. The ratio of the central equiaxed grain zone was found to increase with decreasing superheat, increasing casting speed, decreasing secondary cooling intensity, and increasing MEMS current. The grain size obviously decreased with decreasing superheat and increasing MEMS current but was less sensitive to the casting speed and secondary cooling intensity.

Microstructural and mechanical characterization of aluminum-lithium alloy 2060 welded by fiber laser

In this study, with constant welding parameters, butt joints consisting of 2.0 mm thick 2060-T8 Al-Li alloy have been fabricated by laser beam welding. A fiber laser using eutectic alloy AA4047 filler wire was employed. The joints' microstructural evolution and the effects of strain rates on the joints' mechanical behaviors were researched. Standard tensile specimens extracted from the welded plates were tested at room temperature with strain rates ranging from 10−4 to 10−1 s−1. Extensive microhardness measurements were also conducted to investigate the relationships between strain rates and microhardness distributions after fractured. It was found that solute segregations of Si and Cu occurred in the grain boundaries in the heat affected zone and fusion zone. Mechanical properties and fracture morphology showed close relationships with the tensile strain rate. The nondendritic equiaxed zone was the minimum hardness and weakest region on the whole joint.

Creep Deformation and Rupture Behavior of Single- and Dual-Pass 316LN Stainless-Steel-Activated TIG Weld Joints

Creep deformation and rupture behavior of single-pass and dual-pass 316LN stainless steel (SS) weld joints fabricated by an autogenous activated tungsten inert gas welding process have been assessed by performing metallography, hardness, and conventional and impression creep tests. The fusion zone of the single-pass joint consisted of columnar zones adjacent to base metals with a central equiaxed zone, which have been modified extensively by the thermal cycle of the second pass in the dual-pass joint. The equiaxed zone in the single-pass joint, as well as in the second pass of the dual-pass joint, displayed the lowest hardness in the joints. In the dual-pass joint, the equiaxed zone of the first pass had hardness comparable to the columnar zone. The hardness variations in the joints influenced the creep deformation. The equiaxed and columnar zone in the first pass of the dual-pass joint was more creep resistant than that of the second pass. Both joints possessed lower creep rupture life than the base metal. However, the creep rupture life of the dual-pass joint was about twofolds more than that of the single-pass joint. Creep failure in the single-pass joint occurred in the central equiaxed fusion zone, whereas creep cavitation that originated in the second pass was blocked at the weld pass interface. The additional interface and strength variation between two passes in the dual-pass joint provides more restraint to creep deformation and crack propagation in the fusion zone, resulting in an increase in the creep rupture life of the dual-pass joint over the single-pass joint. Furthermore, the differences in content, morphology, and distribution of delta ferrite in the fusion zone of the joints favors more creep cavitation resistance in the dual-pass joint over the single-pass joint with the enhancement of creep rupture life.

Crystallographic investigation of grain selection during initial solidification

Normally, macroscopic solidified structure consists of chill, columnar and equiaxed zones. In a chill zone, many fine grains nucleate on the mold surface and grow their own preferred growth direction. Only a few of them continue to grow because of grain selection. In order to understand the grain selection process, crystallographic investigation has been carried out in the zone of initial solidification in this study. 10 g of Al-6 wt%Si alloy was melted at 850 °C and poured on the thick copper plate. Longitudinal cross section of the solidified shell was observed by a SEM and analyzed by EBSD. The result of EBSD mapping reveals that crystallographic orientation was random in the range of initial solidification. Further, some grains are elongated along their <100> direction. Columnar grains, whose growth directions are almost parallel to the heat flow direction, develop via grain selection. Here, a dendrite whose growth direction is close to the heat flow direction overgrows the other dendrite whose growth direction is far from the heat flow direction. However, sometimes we observed that dendrite, whose zenith angle is large, overgrew the other dendrite. It can be deduced that the time of nucleation on the mold surface is not constant.

Effect of superheat on macrostructure and macrosegregation in continuous cast low-alloy steel slabs

The effect of superheat on grain sedimentation and macrosegregation has been investigated using experimental castings of a low-alloyed steel grade. With a high superheat of ∼ 40 °C, the central equiaxed parts of the slabs consisted of randomly oriented fine dendrites but with a low superheat of ∼ 10 °C, coarse globular structures formed. The mean carbon content measured with optical emission spectroscopy was of the order of 15% smaller with coarse globular structures than in fine equiaxed dendritic structures. Electron probe microanalysis of other alloying elements indicates that the negative segregation in the slab central zones is caused by sedimentation of globulites. With superheat in the range ∼ 20 - 40 °C, the equiaxed zone is bordered by a columnar to equiaxed transition (CET) zone. In this region a positive macrosegregation of carbon and other alloying elements was observed. These phenomena are important when considering the through-thickness properties of the slabs and final products.

Practical considerations of the use of cross-weld and compact tension specimens’ creep data

This article gives an overview of the use of cross-weld and compact tension specimen modelling and analyses data to characterise creep behaviour of the high-temperature components. Cross-weld and compact tension specimens are used to describe creep crack growth in heterogeneous material structures, such as welds, and a number of factors that affect the creep behaviour of the structure, associated with this heterogeneity, have been identified. Creep data obtained from cross-weld specimen modelling are substantially affected by the material model used (e.g. Norton power law, Liu-Murakami), stress singularities that arise at the material interfaces and in between the columnar and equiaxed zones of the weld material, residual stresses which arise through the thickness of a multi-pass weld and the extraction orientation of the specimen relative to the welding direction. Creep crack growth data obtained from compact tension specimen testing and analyses are strongly dependent on the material models used (isotropic hardening models, Norton creep law, Liu/Murakami model, etc.), the path dependence of the C*-contour integral fracture parameter for certain heterogeneous material configurations and the accurate computation of material constants for damage mechanics models and the agreement between loading state to the actual stress state of the component to which the compact tension specimen creep data are applied to. This study examines typical results and observations from cross-weld specimen and compact tension specimen creep analyses, identifying the advantages, disadvantages and limitations of each specimen procedure.

The relevance of forced melt flow to grain refinement in pure aluminum under a low-frequency alternating current pulse

Abstract

Modelling of Columnar-to-Equiaxed Transition and Inclusion Distribution in Continuously Cast Steel Billets

Solidification castings can exhibit a columnar or an equiaxed morphology or a combination of both. Since the relative proportions of these two components strongly influence the internal quality of cast product, the study of morphological transition from columnar to equiaxed structure (CET) becomes important. The transition also affects quality parameters like inclusion distribution in castings which has a significant bearing on the properties of cast products. In this work, a combined model for CET and inclusion distribution in continuously cast steel billets is presented. A conduction based transient thermal solidification model is employed in conjunction with Hunt’s criterion for CET to predict the evolution of melt temperature, the location of transition and area-fractions of columnar and equiaxed zones across the billet cross-section. A correlation between melt temperature and equiaxed nuclei density is proposed and incorporated in the model to account for a more realistic variation of CET with melt superheat. The model is compared with available experimental data and is used to explore the effect of process parameters on CET and determine the spatial distribution of non-metallic inclusions in the solidified billet.

Microstructure and mechanical properties of a new Al–Zn–Mg–Cu alloy joints welded by laser beam

Abstract A new type of Al–Zn–Mg–Cu alloy sheets with T6 temper were welded by laser beam welding (LBW). Microstructure characteristics and mechanical properties of the joints were evaluated. Results show that grains in the heat affected zone (HAZ) exhibit an elongated shape which is almost same as the base metal (BM). A non-dendritic equiaxed grain zone (EQZ) appears along the fusion line in the fusion zone (FZ), and grains here do not appear to nucleate epitaxially from the HAZ substrate. The FZ is mainly made up of dendritic equiaxed grains whose boundaries are decorated with continuous particles, identified as the T (AlZnMgCu) phase. Obvious softening occurs in FZ and HAZ, which mainly due to the changes of nanometric precipitates. The precipitates in BM are mainly η′, while plenty of GPI zones exist in FZ and HAZ adjacent to FZ, in the HAZ farther away from FZ, η phase appears. The minimum microhardness of the joint is always obtained in FZ at different times after welding. The ultimate tensile strength of the joint is 471.1 MPa which is 69.7% of that of the BM. Samples of the tensile tests always fracture at the FZ.

The Contribution of Constitutional Supercooling to Nucleation and Grain Formation

The concept of constitutional supercooling (CS) including the term itself was first described and discussed qualitatively by Rutter and Chalmers in order to understand the formation of cellular structures during the solidification of tin, and then quantified by Tiller et al. On that basis, Winegard and Chalmers further considered ‘supercooling and dendritic freezing of alloys’ where they described how CS promotes the heterogeneous nucleation of new crystals and the formation of an equiaxed zone. Since then the importance of CS in promoting the formation of equiaxed microstructures in both grain refined and unrefined alloys has been clearly revealed and quantified. This paper describes our current understanding of the role of CS in promoting nucleation and grain formation. It also highlights that CS, on the one hand, develops a nucleation-free zone surrounding each nucleated and growing grain and, on the other hand, protects this grain from readily remelting when temperature fluctuations occur due to convection. Further, due to the importance of the diffusion field that generates CS, recent analytical models are evaluated and compared with a numerical model. A comprehensive description of the mechanisms affecting nucleation and grain formation and the prediction of grain size is presented with reference to the influence of the casting conditions applied during the practical casting of an alloy.

Rapid Solidification and Magnetic Properties of (Fe,Co)-(Fe,Co)17Gd2 Pseudo-Binary Eutectic Alloys.

The (Fe,Co)-(Fe,Co)17Gd2 pseudo-binary eutectic system has been determined by investigating phase compositions on the analysis of DSC, EDS and XRD. The rapid solidification of Gd7.3Fe30.56Co62.14, Gd9Fe30Co61 and Gd9.63Fe29.79Co60.58 ternary alloys is realized by single roller techniques. With a rising cooling rate, the equiaxed zone near the roller surface expands inwards. For Gd7.3Fe30.56Co62.14 hypoeutectic alloy ribbons, the feathery irregular eutectic zone shrinks and the grains (Fe,Co) near the free surface are refined. (Fe,Co) equiaxed dendrites plus the radial, irregular eutectic ultimately fill in the ribbon at the maximum cooling rate. For Gd9Fe30Co61 near-eutectic alloy ribbons, the growth direction of irregular eutectics near the free surface becomes more and more perpendicular to the surface, and finally the whole ribbons are occupied by the fine, irregular eutectic. The dendritic spacing of the (Fe,Co)17Gd2 phase which grows from the roller surface to the free surface in Gd9.63Fe29.79Co60.58 alloy ribbons becomes smaller. The grain size of the (Fe,Co)17Gd2 dendrite and the fraction of (Fe,Co) soft phase in alloy ribbons, which determine coercivity in the pseudo-binary eutectic system, vary with the increase of the cooling rate. Of the three alloys, the Gd9Fe30Co61 alloy has the best hard magnetic properties at Vr = 20 m/s; the maximum coercivity Hc is 431.34 Oe.

Development and application of an off-line soft reduction model during continuous casting of high-carbon rectangular billet

An off-line soft reduction model was developed in order to improve the inner quality of high-carbon rectangular billet, which includes a heat transfer model, cooling water control model and roll-gap calculation model. The distribution of the total reduction amount in each pinch roll is calculated by using a segmented slope method in the reduction zone. The model application results showed that the inner quality of the high-carbon 82B steel with a section size of 180 mm × 240 mm had clearly been improved. It is evident that the formation of V-segregation in a rectangular billet subjected to soft reduction is suppressed and the centerline shrinkage cavity is clearly decreased. The mean center carbon segregation index was reduced from 1.15 to 1.08 with soft reduction. Besides, the soft reduction process improved the compactness and homogeneity of the central equiaxed dendrite zone and decreased the secondary dendrite arm spacing.

Effect of minor Sc on microstructure and mechanical properties of Al–Zn–Mg–Zr alloy metal–inert gas welds

2.5mm thick cold-rolled Al–Zn–Mg–Zr alloy plates with trace amount of Sc were subjected to metal–inert gas (MIG) welding with Al–Mg–Sc as the filler metal. The influence of Sc additions on microstructure and mechanical properties of the welded joints was investigated by means of optical microscopy (OM), scanning electron microscope (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), Vickers micro-hardness test and room temperature tensile test. Experimental results indicated that trace amount of Sc had significant effect on microstructure and mechanical properties of the joints. In both cases, base metal consisted of a typical fiber structure, different width of the equiaxed zones (EQZs) with fine grains were observed in the weld metal at the fusion boundary, fusion zones exhibited dendritic microstructure and average size of Al–Zn–Mg–0.10%Sc–Zr was 120μm, while in Al–Zn–Mg–0.25%Sc–Zr, grain size decreased drastically to 50μm. The grain refinement is mainly caused by the extremely fine, coherent Al3(Sc, Zr) particles with L12 structure, which act as heterogeneous nucleation. The observed grain refinement resulted in an appreciable increase in fusion zone strength and micro-hardness. Tensile strength of Al–Zn–Mg–0.25Sc–Zr alloy welded joint was 460MPa, approximately 11% higher than Al–Zn–Mg–0.10Sc–Zr alloy welded joint. Micro-hardness in the center of fusion zone rose from 95HV to 110HV. The main strengthening mechanisms of Al–Zn–Mg–Sc–Zr MIG welded joints are precipitation strengthening derived from η′ precipitates, dispersion strengthening and fine grain strengthening caused by coherent secondary Al3(Sc, Zr) particles.

Modelling the creation and destruction of columnar and equiaxed zones during solidification and melting in multi-pass welding of steel

The authors present a novel meso-scale (mm–cm) numerical model of multi-pass welding of stainless steel based on the front tracking formulation. During a typical multi-pass notched butt weld, multiple cycles of melting and solidification occur throughout the weld pool, where previously solidified equiaxed zones are destroyed by melting and subsequently resolidify in columnar form. These thermal cycles result from the thermal effects of the current deposition of liquid filler metal being superimposed onto those of the previous pass. The advancement of the liquidus isotherm is employed to characterize melting, while both columnar and equiaxed dendritic growth in the undercooled melt are considered in the model of solidification. Columnar solidification is simulated using a front tracking method where computational markers are employed to explicitly define the advancing columnar front. Competing equiaxed solidification is modelled using a volume averaging approach. During the first and second welding passes of a case study, the weld pool solidifies as purely columnar. This is due to the high thermal gradient present in the weld pool, which results in a very small region of undercooled liquid ahead of the columnar front and subsequently low levels of equiaxed grain nucleation/growth. Towards the end of the third and final pass the equiaxed grains have sufficient dwell time in the undercooled liquid to form a coherent network and present a physical barrier to the advancing columnar front. A Columnar to Equiaxed Transition (CET) occurs and the remainder of the weld pool solidifies in a fully equiaxed microstructure. The results are compared with Hunt’s analytical model of CET and the agreement is reasonably good. The occurrence of CET throughout the weld pool is directly predicted via this front tracking model without the need to rely on estimated thermal gradients and solidification rates. Further simulations indicate that CET can be promoted, even in early passes, by increasing the grain refiner level and/or by pre-heating the parent metal.