During Direct Chill Research Articles

Predicting the effect of different magnetic fields on the solute segregation during direct chill casting of large-size magnesium alloy slab

PurposeThe purpose of this paper is to reveal the solute segregation behavior in the molten and solidified regions during direct chill (DC) casting of a large-size magnesium alloy slab under no magnetic field (NMF), harmonic magnetic field (HMF), pulsed magnetic field (PMF) and two types of out-of-phase pulsed magnetic field (OPMF).Design/methodology/approachA 3-D multiphysical coupling mathematical model is used to evaluate the corresponding physical fields. The coupling issue is addressed using the method of separating step and result inheritance.FindingsThe results suggest that the solute deficiency tends to occur in the central part, while the solute-enriched area appears near the fillet in the molten and solidified regions. Applying magnetic field could greatly homogenize the solute field in the two-phase region. The variance of relative segregation level in the solidified cross-section under NMF is 38.1%, while it is 21.9%, 18.6%, 16.4% and 12.4% under OPMF2 (the current phase in the upper coil is ahead of the lower coil), HMF, PMF and OPMF1 (the current phase in the upper coil lags behind the lower coil), respectively, indicating that OPMF1 is more effective to reduce the macrosegregation level.Originality/valueThere are few reports on the solute segregation degree in rectangle slab under magnetic field, especially for magnesium alloy slab. This paper can act a reference to make clear the solute transport behavior and help reduce the macrosegregation level during DC casting.

Modelling of solidification structure and macro-segregation in an industrial DC cast slab

A cellular automaton (CA)-finite element (FE) model was implemented for multi-scale modelling of micro-segregation, mesoscopic grain structure and macroscopic segregation during direct chill (DC) casting of industrial billets or slabs. The macroscopic transport of mass, momentum, energy and solutes was solved on an FE grid, while the mesoscopic grain structure governed by nucleation, growth kinetics and grain evolution was calculated on a CA grid. The solidification path was determined using a modified micro-segregation model for multi-component aluminium alloys. An Euler representation was used for pre-processing and post-processing, and a Lagrangian representation was used for expanding the calculation domain and for resolving the CAFE model. By simulating a DC casting experiment of the 2024 aluminium alloy, a typical grain structure was reproduced, and the composition map showed a reasonable deviation. This model was applied to an industrial-scale DC cast slab of an Al-3.5Cu-1.5 Mg (wt. %) alloy, and three simulations with different nucleation undercoolings were performed for a grain-unrefined slab, a grain-refined slab and an equilibrium solidified slab, respectively. The slabs tended to solidify at equilibrium with the decreasing nucleation undercooling. The earlier release of latent heat yielded a smaller liquid undercooling region ahead of the solidification front, and a finer grain structure. A typical grain structure with coarse equiaxial grains at the centre and fine columnar grains near the bottom surface as well as sidewall was observed for the grain-unrefined slab. By contrast, the grain structure of the grain-refined slab was fully equiaxial. Furthermore, the grain structure, temperature field, melt flow and macro-segregation were quantitatively investigated.

Modelling the fluid flow, solidification and segregation behavior in electromagnetic DC casting of magnesium alloy

Making clear the metallurgical transport behaviors during direct chill (DC) casting is of great significance to control the solidified structure and reduce the casting defects, but it still remains difficult because the high temperature and the opaque of melt make them hard to observe and detect directly. Numerical modelling provides a way to reveal the physical fields involved during DC casting. The present research used a two-dimensional axisymmetric model which coupled the heat transfer, fluid flow, solidification and species transfer in order to study the effect of different magnetic fields (harmonic magnetic field (HMF), pulsed magnetic field (PMF) and out-of-phase pulsed magnetic field (OPMF)) on the flow pattern, solidification characteristic and solute distribution in the electromagnetic DC casting of magnesium alloy billet having the diameter of 500 mm. The results show that when the magnetic field is on, there are the desirable variations including the homogenous temperature profile and faster solidification rate, as well as the alleviated central negative segregation and the reduced whole segregation level on the cross section of the billet. For the different magnetic fields, OPMF shows the strongest stirring effect, leading to the highest solidification rate at the initial solidification stage. Furthermore, the central negative segregation and the segregation level along the cross section are alleviated most as compared to other magnetic fields.

Understanding the influence of ultrasonic streaming on solidification of AZ80 magnesium alloy during direct-chill casting using a transient coupling modeling

In this modeling, the influence of ultrasonic streaming on physical fields during direct-chill (DC) casting of AZ80 magnesium alloy is investigated by a transient 2D axisymmetric mathematical model, which couples the ultrasonic field, fluid flow and temperature field using COMSOL Multiphysics software. The influences of input acoustic pressure, inserted depth and shape of ultrasonic horn on physical fields are discussed systematically. Compared with the DC casting, the ultrasonic casting (UC) has more turbulent melt flow, uniform temperature gradient, and shallow liquid sump. The acoustic streaming driving force dramatically changes the melt flow and temperature distribution. The increase of input acoustic pressure from 0.2 MPa to 2.2 MPa enlarges the affected area of ultrasonic streaming. However, a further increase of input acoustic pressure to 4.2 MPa has sight effects on physical fields. With the increase of inserted depth of ultrasonic horn from 10 mm to 50 mm, the circulation caused by ultrasonic streaming is enlarged. The convex surface of the ultrasonic horn promotes the transverse flow of melt compared with the flat horn. Therefore, the transverse temperature gradient and depth of liquid sump are decreased.

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

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

Simulation on DC casting of magnesium alloy under out-of-phase pulsed magnetic field with different coil connection strategies

Four annular coils arranged vertically that have been loaded with two pulsed currents with different initial phases were used to create out-of-phase pulsed magnetic field (PMF) during direct-chill (DC) casting of magnesium alloy. A 2D axisymmetric mathematical model that couples electromagnetic field with fluid field and thermal field was established based on the finite element method by using COMSOL Multiphysics software. A two-step approach was adopted to handle the coupling problems, which the varying Lorentz force and Joule heat were treated as the averaged values during the steady calculation of DC casting process. Effects of coil connection strategies on the resulting Lorentz force, melt flow and solidification characteristics during DC casting of magnesium alloy under out-of-phase PMF were investigated and discussed. Results show that the application of magnetic field greatly strengthens the melt convection when compared to no magnetic field. Using out-of-phase PMF could control the range and intensity of melt convection by changing coil connection strategies, achieving lower overheating in the central part and higher temperature uniformity from the center to the edge or reducing the sump depth than a single PMF in which four coils are connected in series and loaded with the same pulsed current.

Numerical study of flow and heat transfer behaviors during direct-chill casting of large-size magnesium alloy billet under pulsed magnetic field

Purpose The purpose of this paper is to investigate the effect of pulsed magnetic field (PMF) with different duty cycles on the melt flow and heat transfer behaviors during direct-chill (DC) casting of large-size magnesium alloy billet and find the appropriate range of duty cycle. Design/methodology/approach A transient two-dimensional mathematical model coupled electromagnetic field, flow field and thermal field, is conducted to study the melt flow and temperature field under PMF and compared with that under the harmonic magnetic field. Findings The results reveal that melt vibration and fluctuation are generated due to the instantaneous impact of repeated thrust and pull effects of Lorentz force under PMF. The peak of Lorentz force decreases greatly with the increasing duty cycle, but the melt fluctuation region is expanded with higher duty cycle, which accelerates the interior melt velocity and reduces the temperature gradient at the liquid-solid interface. However, PMF with overly high duty cycle has adverse effect on the melt convection and limited influence on the interior melt. A duty cycle of 20% to 50% is a reasonable range. Practical implications This paper can provide guiding significance for the setting of duty cycle parameters on DC casting under PMF. Originality/value There are few reports on the effect of PMF parameters during DC casting with applying PMF, especially for duty cycle, a parameter unique to PMF. The findings will be helpful for applying the external field of PMF on DC casting.

Influence of liquid metal feeding on the flow and macrosegregation in DC casting

ABSTRACT The influence of liquid metal feeding on the flow and macrosegregation during direct chill (DC) casting of Al-4.5wt-%Cu billets is investigated. An analytical study for constricted inlets predicts that the inflow only reaches the bottom of the sump for narrow inlets, a result confirmed by steady-state axisymmetric simulations of equiaxed solidification. For wide inlets, and inlets using a combo bag, the inflow is swiftly entrained towards the chill and the final macrosegregation predictions are all similar. For narrow inlets without a combo bag, penetration of the thermal jet to the bottom of the sump locally delays solidification producing an inclination in the packed solid interface and the possible growth of columnar dendrites in this region.

Structure Modification upon Ultrasonic Processing of an AA4032 Piston Alloy: Comparison of Permanent Mold and Direct-Chill Casting

Piston Al-Si alloys have very complex compositions and multi-phase heterogeneous structure, so it is necessary to control the formation of primary and eutectic compounds. In this study, the ultrasonic melt processing (USP) of a eutectic Al-Si piston alloy (AA4032-type) was performed in a permanent mold and during direct-chill (DC) casting to study its effects on the structure refinement and modification. The principal difference between these two ways of casting is that in the permanent mold the solidification front progressively moves towards the ultrasound source, while in the DC casting the position of the solidification front is fixed in space. The results showed that the USP can successfully refine primary Si, Fe-containing intermetallics and aluminum grains. Refinement of primary Si was accompanied by the increase in its amount, which was attributed to both enhanced heterogeneous nucleation and fragmentation. The refinement of Fe-containing intermetallics and Al grains resulted from the fragmentation mechanism and were more pronounced when USP was applied below the liquidus temperature in the permanent mold. However, the eutectic phases coarsened upon USP, and this effect was most pronounced when USP was applied to the semi-solid material. This was related to the strong attenuation of acoustic waves, which effectively heats the semi-solid material and induces corresponding coarsening of the phases. Acoustic streaming induced by an oscillating sonotrode affected the depth of the sump while simultaneously decreasing the macrosegregation, which reflects the dominant role of the melt flow directed against natural convection. The results demonstrated the importance of the solidification stage at which the USP was applied and the specifics of the USP mechanisms acting at the different stages of solidification.

Effect of the Intensity of Melt Shearing on the As Cast Structure of Direct Chill Cast 2024 Aluminum Alloy

Intensive melt shearing was achieved by a stator–rotor high-shear unit and was used in the sump during direct chill (DC) casting process to investigate the influence of the rotation speed on the as cast structure of an aluminum alloy. It has been demonstrated that intensive melt shearing has a significant effect on achieving a uniform temperature distribution and obtaining a fine and uniform as cast structure. We also found that the intensity of the melt shearing plays an important role in the effect. As the rotation speed increases, the melt around the high-shear unit transformed from a liquid to a semisolid state. At a rotation speed of 3000 rpm, intensive melt shearing can only increase the nuclei through fragmentation in the region close to the solidification front by forced convection. When the rotation speed increases to 6000 rpm, intensive melt shearing can also evidently increase the number of effective nuclei, mainly by increased fragmentation in a large volume around the high-shear unit.

Effect of Main Elements (Zn, Mg, and Cu) on Hot Tearing Susceptibility During Direct-Chill Casting of 7xxx Aluminum Alloys

New 7xxx aluminum alloys with high alloying contents are being designed, which could induce serious hot tearing defects during direct-chill (DC) casting. Among all factors affecting hot tearing of 7xxx alloys, undoubtedly alloying elements play a significant role. In this study, the effect of main alloying elements (Zn, Mg, and Cu) on hot tearing of grain-refined Al-xZn-yMg-zCu alloys was investigated by a dedicated hot tearing rating apparatus simulating the DC-casting process. It was found that the minimum and maximum hot tearing susceptibilities occur for 4 to 6 and 9 wt pct Zn, respectively, indicating the complicated effect of Zn content. The hot tearing resistance of grain-refined Al-9Zn-yMg-zCu alloys is enhanced with increasing Mg content but is deteriorated with increasing Cu content. This can be attributed to the interaction of the thermal stresses, melt feeding, and final eutectics. The observed tendencies of the main alloying elements on hot tearing were also confirmed for four commercial 7xxx alloys. In addition, both the load value at non-equilibrium solidus and the SKK criterion proposed by Suyitno et al. using measured load developments were found to be good indicators in predicting hot tearing susceptibility. This study can provide a beneficial guide in designing 7xxx alloys considering the potential occurrence of hot cracks beforehand.

Optimization of ultrasonic cavitation processing in the liquid melt flow

Ultrasonic processing (USP) during direct-chill (DC) casting of light metal alloys is typically applied in the sump of a billet. This approach, though successful for structure refinement and modification, has two main drawbacks: (a) mixture of mechanisms that rely heavily on dendrite fragmentation and (b) a limited volume that can be processed by a single ultrasonic source. We suggest moving the location of USP from the sump to the launder and applying it to the melt flow for continuous treatment. The apparent benefits include: (a) degassing of the melt volume, (b) grain refinement through activation of non-metallic inclusions, fragmentation of primary crystals, and deagglomeration of grain refining substrates, and (c) a possibility to use a single ultrasonic source for processing large melt volumes. To optimize this process with regard to the acoustic intensity and melt residence time in the active cavitation zone, flow modification with baffles as well as informed location of the ultrasonic source are required. In this paper, we demonstrate the results of experimental trials where the degassing degree and grain refinement have been the indicators of the USP efficiency for two aluminium alloys, i.e. LM25 and AA7050. The results are supported by acoustic measurements and computer simulations.

Microstructure and properties of Al-Si alloys under intermediate frequency electromagnetic field

The intermediate frequency electromagnetic (IFEM) field was applied to treat Al-1%Si (high-purity) and Al-8%Si alloys during direct chill (DC) casting process. The ingot was studied by means of metallographic examination and mechanical tests. The results showed that compared to the conventional casting, both Al-1%Si and Al-8%Si alloys processed under IFEM field exhibited superior refinement as well as an enhancement in the mechanical properties, especially the elongation. Si element segregation of high purity Al-1%Si alloy had effectively improved under IFEM field. Numerical simulation was used to better understand the role of IFEM field, that showed the inducing forced convection and the subsequent achievement of a uniform temperature field of IFEM field during the solidification could be accounted for the experimental observations.

Effect of pulsed magnetic field on the grain refinement and mechanical properties of 6063 aluminum alloy by direct chill casting

Pulsed magnetic field (PMF) was successfully applied during direct chill (DC) casting of 6063 Al alloy. The effect of PMF on the solidification structure and mechanical properties was investigated. The results indicated that the coarse equiaxed grain turns into homogeneous and refined equiaxed grain under the effect of PMF, and the grain refinement effect increases with increasing the input current in the study. This can be attributed to the reduced temperature gradient in the melt and enhanced heterogeneous nucleation caused by forced convection, based on the simulation results of Lorenz force, flow field, and temperature field during DC casting under PMF. The application of PMF can also efficiently reduce the macro- and microsegregations of Mg and Si elements, which are beneficial to the uniformly dispersed precipitation of Mg2Si phase after T6 treatment, resulting in a significant enhancement of strength and hardness. The best mechanical properties of the 6063 Al alloy are achieved under PMF with an input current of 500 A, exhibiting the increments of 35.6% in hardness, 42.2% in ultimate tensile strength, and 30.6% in elongation, respectively. This study may offer a promising method for industrial preparation of high-quality alloys by DC casting.

Effect of electromagnetic vibration on the microstructure of direct chill cast Al-Zn-Mg-Cu alloy

An electromagnetic vibration was achieved by the combined application of an alternating magnetic field and a stationary magnetic field during direct chill (DC) casting process. The ingots with 200 mm in diameter were prepared under the influence of electromagnetic vibration. The effect of electromagnetic vibration on the microstructure of an Al-Zn-Mg-Cu alloy was studied. The results showed that electromagnetic vibration has a significant effect on the solidification behaviour, under the influence of electromagnetic vibration during DC casting process, the microstructure is significantly refined and the uniformity of microstructure is evidently improved. This paper introduces the DC casting technology with the application of electromagnetic vibration, presents these results and gives corresponding discussions.

Mechanical Properties and Constitutive Behavior of as-Cast High Strength AA7xxx Alloys below Solidus Temperature

High strength AA7xxx alloys have been extensively used in aerospace industry. However, experience in cast house demonstrates that such alloys are particularly prone to cold cracking and ingot distortion during direct chill (DC) casting, which leads to big amount of scraping or even total rejection of ingot. Those stress induced defects are greatly affected by casting process, and fine tuning of casting parameters is critical to improve the quality and productivity. Recently, numerical modeling has been widely utilized in direct-chill casting for the purpose of thermal mechanical analysis and cracking evaluation. Parameter optimization has become convenient. The model needs the input of constitutive properties of the AA7xxx alloys, of which the microstructure should resemble those formed during DC casting. Unfortunately, these constitutive data are not yet available in literatures. In this study, the mechanical properties of two high strength AA7xxx alloys were measured at temperature range from solidus down to room temperature through on-cooling compressive tests and the effect of strain rate on stress-strain behavior was also studied. The results were fitted to extended Ludwik equation which has been proved to be able to properly describe the stress-strain response of aluminum alloys. The results of the current work shed some light on the evolution of mechanical behavior of high strength AA7xxx alloys during cooling from high temperature.

On Vertical Drag Defects Formation During Direct Chill (DC) Casting of Aluminum Billets

During air-slip direct chill casting of aluminum billets, one of the major defects occurring includes traces along the billet called vertical drags (VDs). If the VDs are too deep or too many, then they cause scraping of the billets. As in the subsequent extrusion process, the surface quality is known to impair both the productivity and quality of the profiles. In cast-house practice, many theories circulate about the causes of VD defects and how to avoid them, but in the literature, no thorough treatments have been made to explain this phenomenon. In the current study, the outer appearance, structure around, and compositions at the defects are analyzed. A theory for the formation of the defects, their cause, and how their appearance is coupled to different alloy types is presented. The segregation in the vicinity of the defects is discussed based on deformation of semisolid materials and coupled to Reynolds dilatancy in granular materials. The theory can explain differences between 6063 and 6005 alloys.

DC Casting of Large Sized Ingot of a High Strength 7xxx Alloy under the Influence of Electromagnetic Field

Hot tearing and cold cracks are major defects during direct chill (DC) casting of large sized ingots of high strength aluminium alloys. In order to solve these problems, based on a low frequency electromagnetic casting (LFEC) process, a new technology, electromagnetic casting with the application of an air blade (EMA) was developed. In the present work, this new technology was used to prepare large sized AA7055 aluminium alloy ingots and the effects of the low frequency electromagnetic field and the air blade on macro-physical fields, microstructure and cracking are studied by numerical and experimental methods. The results show that applying an electromagnetic field can modify the flow direction, increase the velocity of melt flow and homogenize the distribution of temperature in the sump. Applying an air blade can homogenize the distribution of temperature and decrease the stress and strain in the solidified ingot. Furthermore, the microstructure of the ingot is refined remarkably and cracking is eliminated by simultaneously applying the electromagnetic field and the air blade during DC casting.

Modeling the Stress-Strain Behavior and Hot Tearing during Direct Chill Casting of an AZ31 Magnesium Billet

Quantitative knowledge of the thermal mechanical history experienced during direct chill (DC) casting aids the scientific understanding of the process especially in terms of defect formation such as hot tearing. In this work, a thermomechanical finite element (FE) model has been developed to simulate the DC casting of magnesium alloy AZ31 billets. The mathematical model simulates the evolution of temperature, stress, and strain within the billet during an industrial DC casting process. These quantities were then used to calculate the evolution in pressure, and hence hot tearing tendency, within the semisolid regime via the Rappaz–Drezet–Gremaud (RDG) criterion. The temperature predictions were validated against experimental thermocouple data measured during a plant trial at an industrial magnesium DC casting facility. In addition, the residual elastic strains predicted by the model were compared to residual strain measurements made at the Canadian Neutron Beam Centre (CNBC) using a magnesium billet produced during the industrial casting trial. The validated model was then used to quantitatively assess the impact of casting speed on the hot tearing tendency in AZ31 billets.

Cold-Cracking Assessment in AA7050 Billets during Direct-Chill Casting by Thermomechanical Simulation of Residual Thermal Stresses and Application of Fracture Mechanics

Thermally induced strains and stresses developed during direct-chill (DC) semicontinuous casting of high strength aluminum alloys can result in formation of micro-cracks in different locations of the billet. Rapid propagation of such micro-cracks in tensile thermal stress fields can lead to catastrophic failure of ingots in the solid state called cold cracking. Numerical models can simulate the thermomechanical behavior of an ingot during casting and after solidification and reveal the critical cooling conditions that result in catastrophic failure, provided that the constitutive parameters of the material represent genuine as-cast properties. Application of fracture mechanics, on the other hand, can help to derive the critical crack length leading to failure. In the present research work, the state of residual thermal stresses was determined in an AA7050 billet during DC casting by means of ALSIM5. Simulation results showed that in the steady-state conditions, large compressive stresses form near the surface of the billet in the circumferential direction, whereas in the center, the stresses are tensile in all directions. Magnitudes of von Mises effective stresses, the largest component of principal stresses and the fracture mechanics concepts, were then applied to investigate the crack susceptibility of the billet.