Direct Chill Casting Research Articles

Microstructure and mechanical properties of Mg-Nd-Zn-Zr billet prepared by direct chill casting

Abstract High-quality Mg-Nd-Zn-Zr magnesium alloy billets with diameter of 200 mm were successfully prepared by direct chill (DC) casting. The results show that microstructures of the as-cast billet prepared by DC casting are mainly composed of equiaxed a-Mg and Mg12Nd eutectic compound distributing along the grain boundaries. The average grain size decreases along the radius of the billet. And the alloying elements of Nd and Zn distribute homogeneously across the large billet. The optimum process parameters for DC casting of the Mg-Nd-Zn-Zr magnesium alloy billet with diameter of 200 mm have been experimentally determined as follows: casting temperature 710 °C and casting speed 80 mm/min.

Deformation and heat treatment ability of three-layer 6009/7050/6009 Al alloy clad slab prepared by direct-chill casting

Abstract Three-layer 6009/7050/6009 aluminum alloy clad slab was fabricated by an innovative direct-chill casting process. To study the response of the clad slab to plastic deformation and heat treatments, homogenization annealing, hot rolling, solution and aging were successively performed on the as-cast 6009/7050/6009 clad samples. The results revealed that excellent metallurgical bonding between 7050 alloy layer and 6009 alloy layer was achieved under optimal parameters. The clad ratio obviously decreased when the annealed sample was rolled to 55% hot reduction level, and then changed slightly with further rolling. Furthermore, the content of rodlike Zn-rich phases increased significantly in 7050 alloy layer in the homogenized clad samples after rolling at 55%, 65% and 75% hot reduction levels, and the higher level of hot reduction resulted in narrower diffusion layer. Subsequent solution and aging significantly improved the hardness in 7050 alloy layer, interfaces and 6009 alloy layers of the rolled samples except for the thin side for the 75% hot reduction sample.

Mechanical and Fractographical Characterization of Extruded Al-Mg-Si-Zr Alloys

In this study, the chemical composition of Al-0.8Mg-0.8Si alloys was modified with the addition of 0.1 and 0.2 wt.-% Zr. The billets were manufactured by direct chill casting method, homogenized at 560 °C for 6h and then extruded in order to obtain profiles having hollow and circular sections. Recrystallization layer (shell) became narrower due to the addition of Zr. This was attributed to the formation of very fine precipitates (Al3Zr) within the matrix. The mechanical properties showed that both yield and tensile strengths increased as a function of Zr content. Tensile fracture surfaces were examined by scanning electron microscope and the fractographs reflected the effect of grain structure on the fracture behavior of studied alloys. All fracture surfaces indicated typical dimple ruptures, however, the size of dimples were observed as finer structures as a function of Zr content. As seen in cross-sectional graphs, as the Zr content increased the grain structure was refined due to Al3Zr precipitates. These fine precipitates caused the formation of fine and shallow dimples under loading.

Microstructural and Mechanical Characterization of Al-0.80Mg-0.85Si-0.3Zr Alloy

Abstract In this study, Al-0.80Mg-0.85Si alloy was modified with the addition of 0.3 wt.-% zirconium and the variation of microstructural features and mechanical properties were investigated. In order to produce the billets, vertical direct chill casting method was used and billets were homogenized at 580 °C for 6 h. Homogenized billets were subjected to aging practice following three stages: (i) solution annealing at 550 °C for 3 h, (ii) quenching in water, (iii) aging at 180 °C between 0 and 20 h. The hardness measurements were performed for the alloys following the aging process. It was observed that peak hardness value of Al-0.80Mg-0.85Si alloy increased with the addition of zirconium. This finding was very useful to obtain aging parameters for the extruded hollow profiles which are commonly used in automotive industry. Standard tensile tests were applied to aged profiles at room temperature and the results showed that modified alloy had higher mechanical properties compared to the non-modified alloy.

Development of high mechanical properties and moderate thermal conductivity cast Mg alloy with multiple RE via heat treatment

A new cast Mg–2Gd–2Nd–2Y–1Ho–1Er–0.5Zn–0.4Zr (wt%) alloy was prepared by direct-chill semi-continuous casting technology. The microstructure, mechanical properties and thermal conductivity of the alloy in as-cast, solid-solution treated and especially peak-aged conditions were investigated. The as-cast alloy mainly consists of α-Mg matrix, (Mg, Zn)3 RE phase and basal plane stacking faults. After proper solid-solution treatment, the microstructure becomes almost Mg-based single phase solid solution except just very few RE-riched particles. The as-cast and solid-solution treated alloys exhibit moderate tensile properties and thermal conductivity. It is noteworthy that the Mg alloy with 8wt% multiple RE exhibits remarkable age-hardening response (ΔHV=35.7), which demonstrates that the multiple RE (RE=Gd, Nd, Y, Ho, Er) alloying instead of single Gd can effectively improve the age-hardening response. The peak-aged alloy has a relatively good combination of high strength/hardness (UTS (ultimate tensile strength)>300MPa; TYS (tensile yield strength)>210MPa; 115.3 HV), proper ductility (ε≈6%) and moderate thermal conductivity (52.5W/(mK)). The relative mechanisms mainly involving aging precipitation of β¢ and β′′ phases were discussed. The results provide a basis for development of high performance cast Mg alloys.

Modeliranje omejevanja neposrednega hlajenja z vodo med vertikalnim konti litjem gredic iz Al-zlitin

Modeliranje omejevanja neposrednega hlajenja z vodo med vertikalnim konti litjem gredic iz Al-zlitin

Effect of flow, heat transfer and magnetic energy on the grain refinement of 7A04 alloy under electromagnetic pulse

Abstract In this paper, a transient numerical simulation method was used to analyze the direct chill casting process of 7A04 alloy under electromagnetic pulse treatment. The distribution and evolution of the electromagnetic force, fluid and thermal fields were studied to determine their effect on the solidification characteristics of alloy ingots. A modified nucleation theory for the refinement of grain size was presented considering the effect of electromagnetic energy on the critical Gibbs free energy Δ G * $\Delta {G^*}$ of the unity, which was verified by the corresponding experiments. The results showed that the swirl rings of the axial section in the melt were restrained and a lower temperature gradient was formed in the radial section when the magnetic flux intensity was increased. Under these conditions for nucleation, the grain refinement could be explained by the electromagnetic pulse energy that reduced the energy barrier and improved the nucleation rate, either heterogeneous nucleation or homogeneous nucleation.

Transient thermal stress development in direct chill cast ingots with application of a wiper

ABSTRACTA full 3D transient model of direct chill casting is developed to predict the thermal stress during the startup phase for AA5182. Simulations show that tensile stress starts to develop at ingot surface and gradually migrates to ingot center. By varying the position of wiper below the mold, the maximum residual stress in the ingot center is reduced by up to 35% compared to that without the use of a wiper. The closer the wiper is inserted below the mold, the larger is the stress reduction effect, but at a greater risk of remelting surfaces below the wiper. In order to prevent this, forced-convection air cooling is recommended on ingot surface below the wiper. The current study recommends application of a moving wiper, from a lower to the bottom of the sump, to combine the benefit of reducing thermal stress and centerline segregation without surface bleeding-out.

Rate and temperature dependent deformation behavior of as-cast WE43 magnesium-rare earth alloy manufactured by direct-chill casting

In this work, we study the deformation behavior of a direct chill cast WE43 Mg alloy. This material initially has equiaxed grains approximately 40µm in diameter and a random texture. The room temperature, quasi-static response exhibits little plastic anisotropy when evaluated parallel to and normal to the solidification direction and no initial yield tension-compression asymmetry. The deformation at room temperature is accompanied by significant basal texture development and formation of three types of deformation twins: {101̅2}〈1̅011〉, {101̅1}〈1012̅〉, and {112̅1}〈1̅1̅26〉 as well as double twins {101̅1}〈1012̅〉-{101̅2}〈1̅011〉, although each in small amounts < 10% up to failure. We find that in the elevated 250°C ± 20°C regime, the material exhibits a negative strain rate sensitivity, with a decreasing flow stress as the strain rate increases. In most of the high-temperature, 250°C to 350°C, and high-strain-rate, 0.01/s to 10/s, tests, the material failed at moderate compression strains, 0.35–0.45. Subsequent fracture analyses find that the material fractures transgranularly by a typical shear fracture, often with the presence of additional arrested cracks. At elevated temperatures and under strain rates sufficiently low (i.e., 275°C and 0.01/s, 300°C and 0.1/s, 350°C and 1/s, 375°C and 10/s) or at a temperature of 400°C deformation conditions, the material exhibited pseudo-super plastic behavior, experiencing relatively high deformation strains (> 1.0 true strain) without fracturing.

Influence of size and distribution of W phase on strength and ductility of high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca alloy processed by indirect extrusion

A high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca (wt%) alloy containing W phase (Mg3Y2Zn3) prepared by permanent mold direct-chill casting is indirectly extruded at 350°C and 400°C, respectively. The extruded alloys show bimodal grain structure consisting of fine dynamic recrystallized (DRXed) grains and unrecrystallized coarse regions containing fine W phase and β2ʹ precipitates. The fragmented W phase particles induced by extrusion stimulate nucleation of DRXed grains, leading to the formation of fine DRXed grains, which are mainly distributed near the W particle bands along the extrusion direction. The alloy extruded at 350°C exhibits yield strength of 373MPa, ultimate tensile strength of 403MPa and elongation to failure of 5.1%. While the alloy extruded at 400°C shows lower yield strength of 332MPa, ultimate tensile strength of 352MPa and higher elongation to failure of 12%. The mechanical properties of the as-extruded alloys vary with the distribution and size of W phase. A higher fraction of DRXed grains is obtained due to the homogeneous distribution of micron-scale broken W phase particles in the alloy extruded at 400°C, which can lead to higher ductility. In addition, the nano-scale dynamic W phase precipitates distributed in the unDRXed regions are refined at lower extrusion temperature. The smaller size of nano-scale W phase precipitates leads to a higher fraction of unDRXed regions which contributes to higher strength of the alloy extruded at 350°C.

Mechanism and Morphology of Formation of Micropores in the Structure of DC Cast AlMgSi Alloy

The phenomena defects in the triple junction grain boundaries affect the properties of the microstructure of aluminum alloys. For that purpose the microstructure of the alloy EN AW 6060 on the billet sample cast by DC (Direct Chill) technology was analysed in order to gain insight into the origin and form pores in the final stages of solidification. Mechanisms of solidification shrinkage, thermal contraction and low permeability of interdendritic channels networks were discussed in moment when melt becomes isolated in separate locations forming new structure. Under such conditions, the tensile stress caused by anisotropic thermal contraction of a coherent dendrites network, causing the formation of pores in the corners of the grain boundaries whose morphology is determined by SEM / EDS analysis.

Influence of Fe and Si Addition on the Properties and Structure Conductivity Aluminium

AbstractThe effect of iron and silicon addition on the structure and properties of aluminium wire rod obtained in the laboratory horizontal direct chill casting process has been analysed. In addition, the impact of laboratory wire drawing process has been examined. The addition of iron and velocity of casting increase the strength of aluminium wire rod in as-cast condition while the electrical conductivity drop acceptable. Moreover, the laboratory wire drawing process causes work-hardening wires and increase drawing tension as a result of fragmentation of structure and growth of grain boundaries. It has been shown that iron is beneficial for mechanical and technological properties of aluminium.

Simulation of direct chill casting under the influence of a low-frequency electromagnetic field

A comprehensive, multiphysics, meshless, numerical model is developed for the simulation of direct chill casting under the influence of a low-frequency electromagnetic field. The model uses mixture-continuum-mass, momentum and energy-conservation equations to simulate the solidification of axisymmetric aluminium-alloy billets. The electromagnetic-induction equation is coupled with the fluid flow and used to calculate the Lorentz force. The involved partial-differential equations are solved with the meshless-diffuse-approximate method by employing second-order polynomial shape functions and a 13-noded local support. An explicit time-stepping scheme is used. The boundary conditions for the heat transfer involve the effects of hot-top, mould chill and direct chill. The use of a meshless method and the automatic node-arrangement generation made it possible to investigate the complicated flow structures in geometrically complex inflow conditions, including sharp and curved edges, in a straightforward way. A time-dependent adaptive computational node arrangement is used to decrease the calculation time. The model is demonstrated by casting an Al-5.25wt%Cu aluminium alloy billet with a radius of 120 mm. Results on simplified and realistic inflow geometry are considered and compared. The effect of the low-frequency electromagnetic force on the temperature, liquid fraction and fluid flow are investigated under different current densities and frequencies.

Industrial Direct Chill Slab Caster of Tin Bronze (C903) Using a Porous Filter in the Hot-Top

A 3D computational fluid dynamics (CFD) modeling study has been carried out for the tin bronze (C903) slab of industrial size in a vertical direct chill caster. The melt is delivered from the top across the entire cross section of the caster. An insulated hot-top is considered above the 80-mm mold to control the melt level in the mold. A porous filter is considered in the hot-top region of the mold to arrest the incoming inclusions and homogenize the flow into the mold. The melt flow through the porous filter is modeled on the basis of the Brinkmann–Forchheimer-extended non-Darcy model. Results are obtained for four casting speeds varying from 40 to 100 mm/min. The metal–mold contact region, as well as the convective heat transfer coefficient at the mold wall, is also varied. In addition to the above, the Darcy number for the porous media is also changed. All parametric studies are performed for a fixed inlet melt superheat of 62 °C. The results are presented pictorially in the form of temperature and velocity fields. The sump depth, mushy region thickness, solid shell thickness (ST) at the exit of the mold, and axial temperature profiles are also presented and correlated with the casting speed through regression analysis.

Modeling of the Coupling of Microstructure and Macrosegregation in a Direct Chill Cast Al-Cu Billet

The macroscopic multiphase flow and the growth of the solidification microstructures in the mushy zone of a direct chill (DC) casting are closely coupled. These couplings are the key to the understanding of the formation of the macrosegregation and of the non-uniform microstructure of the casting. In the present paper we use a multiphase and multiscale model to provide a fully coupled picture of the links between macrosegregation and microstructure in a DC cast billet. The model describes nucleation from inoculant particles and growth of dendritic and globular equiaxed crystal grains, fully coupled with macroscopic transport phenomena: fluid flow induced by natural convection and solidification shrinkage, heat, mass, and solute mass transport, motion of free-floating equiaxed grains, and of grain refiner particles. We compare our simulations to experiments on grain-refined and non-grain-refined industrial size billets from literature. We show that a transition between dendritic and globular grain morphology triggered by the grain refinement is the key to the explanation of the differences between the macrosegregation patterns in the two billets. We further show that the grain size and morphology are strongly affected by the macroscopic transport of free-floating equiaxed grains and of grain refiner particles.

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.

Grain Refinement and Improvement of Solidification Defects in Direct-Chill Cast Billets of A4032 Alloy by Melt Conditioning

Melt-conditioned, direct-chill (MC-DC) casting is an emerging technology to manipulate the solidification process by melt conditioning via intensive shearing in the sump during DC casting to tailor the solidification microstructure and defect formation. When using MC-DC casting technology in an industrial scale DC cast billet of an A4032 aluminum alloy, significant grain refinement and uniform microstructure can be achieved in the primary α-Al phase with fine secondary dendritic arm spacing (SDAS). Improved macrosegregation is quantitatively characterized and correlated with the suppression of channel segregation. The mechanisms for the prevention of channel segregation are attributed to the increased local cooling rate in the liquid–solid phase region in the sump and the formation of fine equiaxed dendritic grains under intensive melt shearing during MC-DC casting. A critical cooling rate has been identified to be around 0.5 to 1 K/s (°C/s) for the channel segregation to happen in the investigated alloy based on quantitative metallographic results of SDAS. Reduction and refinement of microporosity is attributed to the improved permeability in the liquid–solid phase region estimated by the Kozeny–Carman relationship. The potential improvement in the mechanical properties achievable in MC-DC cast billets is indicated by the finer and more uniform forging streamline in the forgings of MC-DC cast billet.

Mechanical properties and cold cracking evaluations of four 7××× series aluminum alloys using a newly developed index

Cold cracking is a severe challenge during the direct-chill casting of high-strength 7××× series aluminum alloys. A finite element method (FEM) simulation combined with a cold cracking criterion has been demonstrated to possess obvious technical advantages in cold cracking prediction. However, the current absence of mechanical properties and effective criteria for 7××× series aluminum alloys inhibits the progress of the technique. In this study, the corresponding mechanical properties of four typical 7××× series aluminum alloys are investigated. The cold cracking tendencies of the alloys are evaluated by a new cold cracking index (CCI) developed by the authors. It is shown that AA7055 has the highest cold cracking propensity among the four alloys, followed by AA7050, AA7085, and AA7022, respectively. The cold cracking tendency is basically consistent with the amount of nonequilibrium eutectics of the alloys under the same casting process. It is also shown that the application of water wiper can effectively decrease the occurrence of cold cracking.

Jet Mixing in Direct-Chill Casting of Aluminum: Crater Effects and its Consequence on Centerline Segregation

Recent reports have demonstrated the possibility of mitigating macrosegregation during the Direct-Chill casting of rolling slab ingots using an impinging jet. Herein, an analytical model is presented to predict the shape of the crater formed due to the impact of the jet on the slurry region. The model takes into account alloy composition, physical dimension, and casting speed on the distribution of forces and crater shape. The calculated shape of the crater profile is used to explain the centerline depletion in the impingement region previously reported.

Effects of Process Parameters on Solidification Structure of A390 Aluminum Alloy Hollow Billet

The effects of process parameters on the solidification structure of A390 aluminum alloy hollow billets prepared by direct-chill casting were investigated. The decrease of casting temperature deteriorated the homogeneity and increased the size of primary Si particles in the hollow billet. Although the average size of primary Si particles was not obviously affected by the increase of casting speed, the thickness of Si-depleted layer at the inner wall increased with the higher casting speed. The tensile strength of A390 alloy is a function of the percentage of coarse Si particles (larger than 35 μm) and the average size of primary Si particles. Higher and more stable tensile strength can be received in the hollow billet with the casting temperature of 1050 K (777 °C), because the fine and uniformly distributed primary Si particles were obtained in the hollow billet.