Direct Chill Research Articles

Effect of casting speed on floating grains and macrosegregation of direct-chill cast 2024 alloy with intensive melt shearing

The ingot was prepared by direct-chill (DC) casting technology with different casting speeds under the influence of intensive melt shearing to explore the effect of casting speed and intensive melt shearing on the floating grains and negative centerline segregation. The results indicate that the application of intensive melt shearing in DC casting process can distribute the floating grains uniformly, reduce the area fraction of the floating grains, alleviate the negative centerline segregation, and improve the uniformity of temperature field in the sump. It is also suggested that under the influence of intensive melt shearing, the casting speed plays a crucial role in the amounts and distribution of floating grains. At low casting speed, the intensive melt shearing can significantly reduce the area fraction of the floating grains and distribute them uniformly throughout the ingot. However, this effect gradually disappears with the increase of casting speed.

Effect of harmonic magnetic field and pulse magnetic field on microstructure of high purity Cu during electromagnetic direct chill casting

The effects of two types of magnetic fields, namely harmonic magnetic field (HMF) and pulse magnetic field (PMF) on magnetic flux density, Lorentz force, temperature field, and microstructure of high purity Cu were studied by numerical simulation and experiment during electromagnetic direct chill casting. The magnetic field is induced by a magnetic generation system including an electromagnetic control system and a cylindrical crystallizer of 300 mm in diameter equipped with excitation coils. A comprehensive mathematical model for high purity Cu electromagnetic casting was established in finite element method. The distributions of magnetic flux density and Lorentz force generated by the two magnetic fields were acquired by simulation and experimental measurement. The microstructure of billets produced by HMF and PMF casting was compared. Results show that the magnetic flux density and penetrability of PMF are significantly higher than those of HMF, due to its faster variation in transient current and higher peak value of magnetic flux density. In addition, PMF drives a stronger Lorentz force and deeper penetration depth than HMF does, because HMF creates higher eddy current and reverse electromagnetic field which weakens the original electromagnetic field. The microstructure of a billet by HMF is composed of columnar structure regions and central fine grain regions. By contrast, the billet by PMF has a uniform microstructure which is characterized by ultra-refined and uniform grains because PMF drives a strong dual convection, which increases the uniformity of the temperature field, enhances the impact of the liquid flow on the edge of the liquid pool and reduces the curvature radius of liquid pool. Eventually, PMF shows a good prospect for industrialization.

Hot Compression Behavior of New Al-6Mg and Al-8Mg Alloy with Improved Hot Workability Fabricated by Direct Chill Casting Method

A hot compression test of new Al-6Mg and Al-8Mg alloys was conducted to understand the dynamic recrystallization (DRX) behavior by Mg contents. To investigate the hot workability of Al-Mg with high Mg contents, the hot deformation behavior of Al-6Mg and Al-8Mg alloys was analyzed by a hot compression test in the temperature range of 300–450 °C, and the strain rate range of 10−3–100/s. Subsequently, high-temperature deformation behavior was investigated through the processing map and microstructure observation. In this study, the results have shown that, as the Mg contents increase, the maximum and yield strength increase while rapid flow softening after the peak strain has been observed due to accelerated dynamic recrystallization (DRX). Finally, the increase of Mg contents affects an increase of heat dissipation efficiency to be an indicator of regular deformation.

Microstructures and Macrosegregation of Al-Zn-Mg-Cu Alloy Billet Prepared by Uniform Direct Chill Casting.

In this study, large-sized Al–Zn–Mg–Cu alloy billets were prepared by direct chill casting imposed with annular electromagnetic stirring and intercooling; a process named uniform direct chill casting. The effects of uniform direct chill casting on grain size and the alloying element distribution of the billets were investigated and compared with those of the normal direct chill casting method. The results show that the microstructures were refined and the homogeneity of the alloying elements distribution was greatly improved by imposing the annular electromagnetic stirring and intercooling. In uniform direct chill casting, explosive nucleation can be triggered, originating from the mold wall and dendrite fragments for grain refinement. The effects of electromagnetic stirring on macrosegregation are discussed with consideration of the centrifugal force that drives the movement of melt from the central part towards the upper-periphery part, which could suppress the macrosegregation of alloying elements. The refined grain can reduce the permeability of the melt in the mushy zone that can restrain macrosegregation.

Direct Chill Casting and Extrusion of AA6111 Aluminum Alloy Formulated from Taint Tabor Scrap.

AA6111 aluminum automotive body-sheet alloy has been formulated from 100% Taint Tabor scrap aluminum. Direct chill casting with and without high shear melt conditioning (HSMC) was used to produce the AA6111 alloy billets. Both homogenized and non-homogenized billets were extruded into sheets. The optical micrographs of the melt conditioned direct chill (MC-DC) samples showed refined equiaxed grains in comparison to direct chill (DC) cast and direct chill grain refined (DC-GR) samples. Optical metallography showed extensive peripheral coarse grain (PCG) for the DC, DC-GR and MC-DC planks extruded from the homogenized standard AA6111 billets while planks extruded from modified AA6111 billets (with recrystallization inhibitors) showed thin PCG band. The co-addition of recrystallization inhibitors Mn, Zr, and Cr with elimination of the billet homogenization step had a favorable impact on the microstructure of the AA6111 alloy following the extrusion process where a fibrous grain structure was retained across the whole section of the planks. The mechanical properties of as-cast planks extruded from non-homogenized billets were similar to those extruded from homogenized billets. Eliminating the homogenization heat treatment step prior to extrusion has important ramifications in terms of processing cost reduction.

Study of Al-Si alloys grain refinement by inoculation

Abstract The grain size and morphology of any cast metal have significant influence on its mechanical properties. The subsequent processing required to achieve the desired mechanical properties can be presumed based on the grain microstructure morphology. Solidified grains of Aluminium Silicon (Al-Si) alloy contain columnar structures along the periphery, more predominantly in the cases of direct chill casting. These columnar shaped grains in mold casting propagate from the periphery to the center of the ingot. The fine equiaxed grains attributes to better strength for any metal and hence fine grain sizes are preferred to coarse columnar grains. Though there are various techniques available to achieve the columnar to equiaxed transition, this paper intends to provide an overview of refining the Al- Si alloy grain structures at the expense of master alloys inoculated with refiner chemical elements and a lateral emphasis is also given to the elements that perform as grain modifiers which modify the irregular hard primary Silicon and eutectic Silicon flakes prevailing in Al-Si alloys.

Prediction of hot tearing susceptibility of direct chill casting of AA6111 alloys via finite element simulations

Abstract To predict hot tearing susceptibility (HTS) during solidification and improve the quality of Al alloy castings, constitutive equations for AA6111 alloys were developed using a direct finite element (FE) method. A hot tearing model was established for direct chill (DC) casting of industrial AA6111 alloys via coupling FE model and hot tearing criterion. By applying this model to real manufacture processes, the effects of casting speed, bottom cooling, secondary cooling, and geometric variations on the HTS were revealed. The results show that the HTS of the billet increases as the speed and billet radius increase, while it reduces as the interfacial heat transfer coefficient at the bottom or secondary water-cooling rate increases. This model shows the capabilities of incorporating maximum pore fraction in simulating hot tearing initiation, which will have a significant impact on optimizing casting conditions and chemistry for minimizing HTS and thus controlling the casting quality.

Floating Grain Characterization and Its Effects on Centerline Segregation of Direct-Chill Cast Al–Mg–Si Alloy Billets

Microstructure and microsegregation analysis were performed on a direct-chill (DC) cast Al–Mg–Si alloy billet, aiming to quantitatively investigate the microscopic features of floating grain and its contribution to the negative centerline segregation. Microstructure results show some irregular grains, whose grain size and secondary dendrite arm spacing (DAS) are two times larger than that of the regular fine grains, appear only in the billet central region. The duplex grain structure, a mixture of internal coarse and peripheral finer dendrites within one grain, was clearly shown by the EPMA results, exhibiting special microsegregation features. The coarse dendrites are more solute-depleted than the nearby finer-DAS structure, in which the average concentration of solute Mg nearly equals to the nominal content. Based on the microscale features and solidification information of the alloy, the irregular grains exhibiting duplex dendrite feature are confirmed to be floating grain, which internal coarse dendrites are believed to mainly contribute to the negative centerline segregation. And the resulting segregation degree is quantitatively evaluated to be ∼0.041 for solute Mg considering the floating grains in the billet center as against with the assumed 100% fine-DAS microstructure.

Enhancement of chip breakability of aluminium alloys by controlling the solidification during direct chill casting

Refinement and uniform dispersion of the free machining elements within the matrix is always desirable for aluminium billets produced by direct chill (DC) casting. The conventional grain refiner addition cannot ensure uniformity of microstructure and homogeneous distribution of insoluble or intermetallic phases. Melt conditioned direct-chill (MC-DC) casting is a key technology for producing uniform microstructure and refinement of secondary phases. This physical approach combines conventional vertical direct chill (DC) casting with a high shear device directly immersed in the sump for in situ microstructural control. Increased heat extraction rate due to forced convection, a larger temperature gradient at the solidification front and a shallower sump contribute to fine equiaxed dendritic grains with fine dendritic arm spacing that favor the fine and uniform distribution of second phase particles in the as-cast billet. This trend of the microstructural features was preserved even after thermomechanical downstream processing, giving rise to much improved machinability for the extruded solid bars.

2D and 3D characteristics of intermetallic particles and their role in fracture response of AZ91 magnesium alloy

The presence of intermetallic constituent particles strongly influences the deformation and fracture characteristics of as-cast magnesium alloys. The present study investigates the two-dimensional (2D) and three-dimensional (3D) microstructure of these intermetallic particles in a direct chill as-cast AZ91 alloy and their effect on the tensile deformation of the alloy. Electron backscattered diffraction (EBSD) was employed to characterize the non-equilibrium eutectic β-Mg17Al12 phase, which further assisted in hypothesizing the solidification process of the alloy. Transmission Kikuchi diffraction (TKD) indicated the small-sized spherical Mg17Al12 precipitates, formed adjacent to the non-equilibrium eutectic precipitate, did not exhibit any orientation relationship reported in the literature. Further, micron-sized AlMn inclusions were observed to be single phase γ2-Al8Mn5 particles exhibiting a cyclically twinned structure with 202¯1 habit plane. Additionally, optical microscopy (OM) in the dark-field (DF) and differential interference contrast (DIC) mode, in assistance with high resolution – transmission electron microscopy (HR-TEM) confirmed that the nano-sized γ2-Al8Mn5 particles resulted in a bimodal grain size distribution of the alloy. The three-dimensional (3D) spatial distribution of these intermetallic constituent particles along with fractography facilitated in understanding the damage process of the alloy in uniaxial tension.

Influence of secondary cooling strategies on thermal gradients in the direct chill casting of magnesium alloys

Direct chill (DC) casting is the widely used technology for the production of ingots or billets of nonferrous alloys. Non-uniform thermal gradients generated during the process play a predominant role in generation of cracks within the billet. In the present work, a temperature-based finite element technique is used to estimate thermal gradients developed in different regions of billet during DC casting of magnesium alloys. Shrinkage-dependent realistic heat transfer coefficient and temperature-dependent nonlinear boiling curves are used as boundary conditions in the mould and secondary cooling zones, respectively. The simulated temperature profiles during DC casting of AZ31 alloy are validated with real-time plant measurements. Secondary cooling strategies such as pulsed water flow and water removal with wiper are widely used in industry for reducing thermal gradients in DC casting. In this work, the influence of these two techniques on the reduction of thermal gradients is investigated for the DC casting of wrought AZ31 and cast AZ91 alloys. Usage of wiper reduces thermal gradients by 30% in case of AZ31 alloy casting. Surface reheating is observed with the use of wiper in AZ91 alloy. The use of pulsed water is found to reduce thermal gradients at the bottom of the sump in AZ31 casting, whereas no thermal gradient reduction is observed in AZ91 casting for the investigated conditions.

Contactless Ultrasonic Treatment in Direct Chill Casting

Uniformity of composition and grain refinement are desirable traits in the direct chill (DC) casting of non-ferrous alloy ingots. Ultrasonic treatment is a proven method for achieving grain refinement, with uniformity of composition achieved by additional melt stirring. The immersed sonotrode technique has been employed for this purpose to treat alloys both within the launder prior to DC casting and directly in the sump. In both cases, mixing is weak, relying on buoyancy-driven flow or in the latter case on acoustic streaming. In this work, we consider an alternative electromagnetic technique used directly in the caster, inducing ultrasonic vibrations coupled to strong melt stirring. This ‘contactless sonotrode’ technique relies on a kilohertz-frequency induction coil lowered towards the melt, with the frequency tuned to reach acoustic resonance within the melt pool. The technique developed with a combination of numerical models and physical experiments has been successfully used in batch to refine the microstructure and to degas aluminum in a crucible. In this work, we extend the numerical model, coupling electromagnetics, fluid flow, gas cavitation, heat transfer, and solidification to examine the feasibility of use in the DC process. Simulations show that a consistent resonant mode is obtainable within a vigorously mixed melt pool, with high-pressure regions at the Blake threshold required for cavitation localized to the liquidus temperature. It is assumed that extreme conditions in the mushy zone due to cavitation would promote dendrite fragmentation and coupled with strong stirring, would lead to fine equiaxed grains.

As-Cast Structure and Temperature Field of Direct-Chill Cast 2024 Alloy Ingot at Different Casting Speeds

The ingot was prepared at different casting speeds, and temperature measurements were taken during the direct-chill (DC) casting process. The results showed that the grain size of the equiaxed grains decreases, the volume fraction of the floating grains increases, and the negative centerline segregation becomes more serious as the casting speed increases. Moreover, the vertical widths and the cooling rates of different regions during the solidification process were analyzed through temperature measurements. At the casting speed of 50 mm/min, the vertical widths of the transition, slurry and mushy regions in the ingot center are relatively small (51, 15, and 36 mm, respectively). As the casting speed increases to 65 mm/min, the vertical widths of the transition, slurry, and mushy regions in the ingot center increase to 70, 29, and 41 mm, respectively. The increase in the vertical width of the slurry region is considered to be the main reason for the increase in the volume fraction of the floating grains. Besides, as the casting speed increases, the cooling rate in the mushy region and the region above the liquidus increases, so the grain size of equiaxed grain decreases.

Optimization of cooling conditions to avoid surface cracks in Direct Chill casting of Cu-Fe-P alloy

The industrial production of Cu-Fe-P billet Direct Chill (DC) casting exhibits often difficulties related to the generation of surface cracks on the cast piece. The demand for always higher productivity leads to increased casting speed with significant cooling requirements. This in turn affects the thermal gradient within the billet leading often to stress concentration responsible for surface cracking. In order to deal with this issue while keeping productivity at high levels lowering the industrial cost, a 2D thermo-mechanical Finite Element (FE) model was developed, which was applied and validated based on industrial conditions. The model targets to predict the material failure during DC casting. Based on the proposed approach the billet’s temperature profile and its stress state during secondary cooling are calculated. The simulation results revealed that strong cooling using water jets cannot be applied immediately at the exit of the mold at any casting speed. However, as the distance of the secondary cooling unit from the mold exit increases, the stress concentration drops significantly, so that the jet nozzles can be applied from 750 to 2500 mm away from the mold, depending on the casting speed, without material failure. On the contrary, milder water spraying applying a lower local heat transfer coefficient, but on the same time covering broader impact surface, can result in a more efficient billet cooling. Thus, due to milder cooling the cast-piece material is less prone to failure and cooling can be applied immediately at the mold exit. Furthermore, a sequence of numerous spray arrays can further increase the cooling capacity. Concluding, it is shown that a validated thermo-mechanical 2D FE model can provide deeper insights used industrially as a setting tool, which optimizes the casting conditions in order to further improve productivity and the quality characteristics of the as-cast billet.

Effect of Electromagnetic Stirring Position on Uniform Direct Chill Casting of Large-Sized 7005 Alloy Billet

A numerical simulation is carried out to investigate the effect of the stirring position on large-sized 7005 alloy billet prepared by direct chill casting imposed with A-EMS and intercooling named uniform direct chill (UDC) casting. Moreover, the effect of the stirring position on the Lorentz force, fluid flow, heat transfer, and solidification occurring in UDC casting is investigated. It is found that when the stirring position moves upwards, the Lorentz force imparts the forced flow to the melt more effectively. Moreover, when the stirring position increases from 0 mm to 100 mm with respect to the graphite ring level, the maximum convection velocity of the melt increases from 150 mm/s to 210 mm/s. Furthermore, the temperature field of the melt in the sump becomes more homogeneous as the stirring position elevates. The microstructure of the 7005 billets becomes finer and uniform when the stirring position moves up to 50 mm.

Microstructural Refinement and α‐Dispersoid Evolution in Direct‐Chill Cast Al–Mg–Si–Fe Alloy

High Fe levels, coarse grains, microstructural inhomogeneities, and coarse dispersoids for recrystallization suppression remain problems for industrial application of Al–Mg–Si (6xxx) alloys. Herein, Al–0.81%Mg–0.81%Si–0.7%Fe alloy billets are fabricated by direct chill (DC) casting at different casting speeds. The results indicate that high‐speed (300 mm min−1) DC casting can provide fine grains, refined Fe‐containing phases, relatively high solid–solute content, and improved distribution of Mn and Cr. After homogenization at 500 °C for 6 h, the α‐AlFeMnCrSi dispersoids in the high‐speed cast billet decrease in size and increase in number density, compared with the conventional cast billet (100 mm min−1). The low homogenization temperature (440 °C) also contributes to the size reduction and increases the number density of dispersoids. Air cooling after homogenization (holding temperature ≥500 °C) results in Q‐AlCuMgSi precipitation, which depends on the precipitated substrate of the α‐AlFeMnCrSi dispersoids. Moreover, the precipitation processes of α‐dispersoids and Q phases are discussed according to transmission electron microscopy (TEM) and high‐angle annular dark‐field scanning transmission election microscope (HAADF‐STEM) results. These results mean that high‐speed DC casting can effectively improve the as‐cast microstructure by increasing the cooling rate, and the as‐cast microstructural evolution promotes the precipitation of fine and dense α‐dispersoids from the α‐Al matrix.

Investigation of macrosegregation for different dendritic arm spacing, casting temperature, and thermal boundary conditions in a direct-chill casting

The study of macrosegregation for different dendritic arm spacing, casting temperature, and cooling conditions in a Direct-Chill (DC) casting was investigated in the present study. The velocity variance-elliptic relaxation ( $$ \overline{{v^{2} }} - f $$ ) turbulence model was used to account for the turbulence effect in the bulk liquid and slurry region. The thermosolutal effect was implemented in the bulk liquid and slurry flow. The Scheil’s solute redistribution model was used to account for solute partitioning at the solidification interfaces. The pressure-based segregated time-dependent solver was used in the solution method to solve the flow equations. The model validation indicates loawer transition thickness due to: (1) efficient damping of the flow in the slurry zone by the turbulence viscous forces, (2) effective handling of the kinematic wall blocking effects in the vicinity of the slurry zone, and (3) strong mixing of the turbulent fluid motion that tends to decrease the distance between the solidus and liquidus isotherm. In all the cases investigated, it was evident that the thermal boundary condition at the secondary cooling zone, casting inlet temperature and secondary Dendritic Arm Spacing (DAS) affect the segregation ratio both at the centerline and the billet surface.

Study on Solidification Structure Evolution of Direct-Chill Casting High Purity Copper Billet Using Cellular Automaton-Finite Element Method

A heat transfer model and a cellular Automation-Finite Element (CAFE) coupling model were established to analyze the solid/liquid (S/L) interface and solidification structure evolution of high purity copper Direct-chill (DC) casting billet under different casting conditions. The simulation and actual experimental results of liquid sump shape and solidification structure were first compared to verify the accuracy of the model. It is proved that the model is effective for simulating the solidification structure of the actual DC casting high purity copper billet. After that, the model was used to predict the solidification structure under different casting temperatures, casting speeds, and heat transfer coefficients. It is shown that, with the increase of casting temperature, the grain size decreases first and then increases. There is a compromise between grain size and its uniformity, and the grain size is more uniform at higher casting temperature. With the increase of casting speed, the depth of liquid sump and the height of the S/L interface increase, but the total grain number of the billet cross-section decreases gradually. As the heat transfer coefficient increases, the depth of the casting liquid sump becomes shallow, but the height of the solid-liquid interface increases and the grain size increases gradually. For the preparation of high purity copper billets with large cross-sectional dimensions by DC casting, a fine solidified structure could be obtained by appropriately reducing the casting speed and cooling intensity.

Study on the transitional structures of 7075 aluminum alloy ingot after switching off a low-frequency electromagnetic field in the horizontal direct chill casting

The transitional structures of 7075 aluminum alloy ingot after switching off a low-frequency electromagnetic field (LFEF) in the horizontal direct chill casting was investigated. The results showed that the equiaxed grains with the LFEF are firstly transformed to columnar grains and then to feathery grains without the LFEF. Also, there exists a transitional region consisting of relatively fine equiaxed grains between the equiaxed grains and the columnar grains. Crystallographic analysis indicated that only the relatively fine equiaxed grains with ideal orientations can grow into columnar grains. Moreover, there exist multiple twin relationships between some adjacent feathery grains, which have a near-common or common 〈110〉 direction. The equiaxed grains with the LFEF result from the forced convection. The relatively fine equiaxed grains and the columnar grains are attributed to the weakening of the forced convection when the LFEF is switched off. For the feathery grains, it is of interest to find that the quasicrystal-enhanced nucleation mechanism is responsible for their formation under high thermal gradient although it has been reported for twinned equiaxed grains under nearly nil thermal gradient in the literature.

Structure Refinement Upon Ultrasonic Melt Treatment in a DC Casting Launder

This work focuses on ultrasonic melt treatment (UST) in a launder upon pilot-scale direct chill (DC) casting of 152-mm-diameter billets from an AA6XXX alloy with Zr addition. Two casting temperatures (650°C and 665°C) were used to assess their effect on the resulting microstructure (grain size, particle size, and number density).Structure refinement results show the feasibility of UST in the DC casting launder. This is quantified through the corresponding reduction of grain size by around 50% in the billet center, or more towards the billet surface, reduction of the average Al3Zr particle size, and increase in the particle number density. A higher Al3Zr particle density was obtained when the alloy was cast at 665°C. Numerical simulation results and suggestions on how to improve the treatment quality of UST in DC casting launder are also provided.