Direct Chill Casting Research Articles

Comparative Analysis of Structure and Properties of Nb-B Inoculated Direct Chill Cast AA4032 Alloy Extruded from As-Cast and Homogenized Conditions

Al-Si wrought piston alloys can lack properties due to inefficient grain refining. A novel Al-Nb-B grain refiner was introduced some time ago, but has still not been assessed in industry for wrought alloys. This paper describes the first trial of Al-Ni-B addition and its impact on the full-scale manufacturing, structure, and properties of the AA4032 products extruded with and without billet homogenization. It is shown that Nb-B inoculation gives opportunities not only to have a refined as-cast structure but also a more homogenous distribution of the solute. In contrast, homogenization drives nucleation and coarsening of the Mg2Si phase that is retained during further extrusion and heat treatment also affecting the precipitation and properties. It was observed that non-homogenized specimens perform better during machining and tensile testing compared to homogenized specimens. The results are supported by electron microscopy investigations of microstructure formation during different steps in downstream processing.

Effect of a low frequency electromagnetic field on the direct-chill (DC) casting of AZ80 magnesium alloy ingots

AbstractThe effects of a low frequency electromagnetic field applied during semi-continuous direct-chill casting on AZ80 ingots were investigated. The experiments were performed both in the absence and in the presence of the electromagnetic field. In contrast to the ingot cast in the conventional process, the microstructure of the ingot was significantly refined by the electromagnetic field and a high degree of grain refinement was realized when the operating frequency was at 22 Hz. The ingot exhibited a relatively increased and uniform distribution of Vickers hardness and decreased segregation of Al when the operating frequency of the electromagnetic field was in the range of 15– 30 Hz.

Structures and macrosegregation of a 2024 aluminum alloy fabricated by direct chill casting with double cooling field

Central region coarse grains and centerline segregation are common defects in aluminum ingots fabricated by direct chill (DC) casting. A double cooling field was introduced into the DC casting process to reduce these defects, whereby the external cooling was supplied by the mold and water jets, and intercooling was achieved by inserting a rod of the same alloy into the molten pool along the central axis of the ingot. Rather than forming a good metallurgical interface during solid-liquid compound casting, in the present work, the purpose of inserting the rod is to enforce internal cooling and consequently decrease the sump depth. Moreover, the insertion provides more nucleation sites with the unmolten á-Al particles. The structure and the macrosegregation of 2024 Al alloy ingots prepared by DC casting with and without the inserts were investigated. Results show that when the inserting position is 50 mm above the upper edge of the graphite ring, significant grain refinement in the central region of the ingot and a reduced centerline segregation are achieved.

Significant strengthening effect in ultra-fine grained Al alloy made by fast solidification and hot extrusion processes

Grain refinement with a facile process without additional alloying is quite desired for strengthening the Al alloys. Using the melt spinning process, we obtained the ultra-fine grained foils of Al–Mg–Si–Cu alloy (AA6061), then the foils followed by hot extrusion to produce the rods for industrial applications. The yield strength of the extruded melt-spun rod is 102% higher than that of the extruded rods of the alloy by a conventional direct chill casting. The electron backscatter diffraction results reveal that the average grain diameter in the extruded melt-spun sample is only 1.35 μm, indicating that the sample is an ultra-fine grained alloy. Through the Hall–Petch relationship, the finer grains contribute to a 66.9% increase in yield strength of the extruded rod. Moreover, the <111> fiber texture occupied 68% of the volume fraction after hot extrusion, contributing to 30.5% improvement of the yield strength in the extruded melt-spun sample. This study shows that the ultra-fine grained Al–Mg–Si–Cu alloy rods have excellent mechanical properties, which provides a strong potential for industrial application.

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.

Microstructure and Wear Properties of Al 7075 Alloy Manufactured by Twin-Roll Strip Casting Process

Al 7075 alloy was manufactured using the twin-roll strip casting (TRC) process, and the mechanical and wear properties of the fabricated TRC process were investigated. To compare the properties of the alloy manufactured by TRC, another Al 7075 alloy was fabricated by conventional direct chill (DC) casting as a comparative material. Based on initial microstructure observations, the Al 7075 alloy manufactured by the DC process showed relatively elongated grains compared to the Al 7075 alloy by TRC process. In both alloys, η(MgZn2) phases were present at the grain and grain boundaries. In the Al 7075 alloy manufactured by the DC process, the η(MgZn2) phases were coarse with a size of ~86 nm and were mainly concentrated in the local area. However, the Al 7075 alloy manufactured by TRC had relatively fine η(MgZn2) phases size of ~40 nm, and they were evenly distributed throughout the matrix. When the mechanical properties of the two alloys were compared, the TRC process showed higher hardness and strength properties than the DC process. In room temperature wear test results, the TRC process exhibited lower weight loss and wear rates compared to the DC process at all wear loads. In other words, the TRC process resulted in relatively superior wear resistance properties compared to the conventional DC process. The wear behavior of both alloys changed from abrasive wear to adhesive wear as the wear load increased. However, the TRC process maintained abrasive wear up to higher loads. Based on the above results, a correlation between the microstructure and wear mechanism of the Al 7075 alloy manufactured by TRC is also suggested.

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.

Low frequency electromagnetic casting of large size ZA21 magnesium alloy slab ingot

Surface quality and microstructure of ZA21 magnesium slab ingots by the direct-chill (DC) casting and low frequency electromagnetic direct chill casting (LFEC)were Deeply and systematically investigated. The surface quality of alloy ingot by LFEC casting is better than that without electromagnetic casting. The crack of ingot was avoided by applying electromagnetic field. The microstructure of ingot without electromagnetic field is coarse columnar grains, on the contrary, the microstructure of ingot by applying electromagnetic field is uniform equiaxed grains. The electromagnetic field can accelerate the flow of the melt and make the dendrite arm break to form equiaxed grains. In addition, the flow of the melt reduces the stress concentration in the solidification process, so that the crack can be avoided.

In-house design of forced convection direct chill casting simulator for casting immiscible Al-Sn alloys

ABSTRACT Al-Sn immiscible alloys possess enhanced tribological properties with increased fatigue strength, which makes them competent among self-lubricating bearing materials. In the present work, a direct chill (DC) casting simulator has been designed and manufactured to cast Al-Sn alloy by forced convection technology. The study embodies two different mechanisms for mechanical forced convection (MFC) by: (i) high shear (HS) rotor-stator device, (ii) conventional stirrer (CS) without stator. Shearing rate of 800 rpm for 60 seconds were kept constant for both the cases. Severe segregation of Sn has been observed in CS-DC cast billet due to prevailed phase separation and gravity sedimentation of Sn-rich phase from Al matrix. However, in the case of HS-DC cast billet uniform distribution of Sn and fine equiaxed grains throughout the volume of the billet has been achieved.

In-situ studies on deformation and fracture characteristics of AZ91 Mg alloy

In this work, the influence of the second phases on the tensile properties of a direct chill cast AZ91 alloy in as-cast and solutionized conditions has been evaluated. Solutionization heat-treatment resulted in a decrease in the volume fraction of the eutectic β-Mg17Al12 phase, and an increase in the grain size of the α-Mg was observed. Increasing the solutionizing time resulted in lowering the yield strength and increased overall ductility of the alloy. The process of crack initiation and propagation were elucidated using in-situ tensile testing in the scanning electron microscope (SEM) in assistance with electron back-scattered diffraction (EBSD) and fractography. The crack initiation in the as-cast and partially solutionized conditions occurred predominantly in the β-Mg17Al12 eutectic, whereas Al-Mn inclusions did not contribute significantly to the damage process. While, in the fully solutionized condition, the absence of the β-Mg17Al12 eutectic, the crack was observed to initiate by the pile-up of dislocation at the grain boundary in the α-Mg matrix and by the fracturing of Al-Mn inclusions. In all the conditions, different types of twins formed during deformation; however, these twins did not contribute to fracture initiation.

Fe-bearing phase formation, microstructure evolution, and mechanical properties of Al-Mg-Si-Fe alloy fabricated by the twin-roll casting process

The composition and morphology of the Fe-bearing phase play a significant role in the mechanical properties of aluminum (Al) alloy sheets. In this work, we have studied the formation of Fe-bearing phases in an Al-0.53Mg-0.69Si-0.14Fe (wt%) alloy under the twin-roll casting (TRC) and the direct chill casting (DC) processes, respectively. We found that the π-AlFeMgSi phase was formed in the as-cast TRC sample, while the β-AlFeSi phase was formed in the as-cast DC sample. The differences in the formation of Fe-bearing phases are attributed to the promotion of the peritectic reaction to form the π-AlFeMgSi phase via the TRC process. Furthermore, after homogenization treatment at 550 °C and 2 h, the π-AlFeMgSi phase in the as-cast TRC sample completely decomposed into the fine β-AlFeSi phase, while it was insufficient to dissolve the β-AlFeSi phase in the as-cast DC sample. This is owing to the lower dissolution temperature of the π-AlFeMgSi phase compared to the β-AlFeSi phase. The decomposition of the π-AlFeMgSi phase expands the solid solubility and promotes the aging precipitation, resulting in higher yield strength of the TRC sample than that of the DC sample during the peak-aging treatment. Our work provides new insight into the development of high-performance AA6xxx series alloys based on the TRC process.

Influence of various aspects of low-Reynolds number turbulence models on predicting flow characteristics and transport variables in a horizontal direct-chill casting

Computational Fluid Dynamics (CFD) was used to simulate the solidification in a horizontal direct-chill (HDC) casting using five low-Re k-ɛ turbulence model: including the v2¯−f model of Durbin (V2F), Yang-Shih (YS), Chang-Hsieh-Chen (CHC), Abid (AB), and Abe-Kondoh-Nagano (AKN). The model incorporated turbulence energy production due to buoyancy in the k and ɛ equations to predict flow pattern, eddy viscosity ratio, turbulence intensity, turbulence velocity, turbulence energy production due to buoyancy, solid fraction, and temperature. Three sets of grids were systematically chosen in such a way that the grid refinement factor was above 1.3. The results obtained indicate that the solution was independent of grid size. The flow pattern predicted using the turbulence models are generally similar, but the V2F indicated additional minor counter-clockwise recirculating cells toward the bottom mold. Further, the solutions indicate that turbulence energy production due to buoyancy contributes to the strong turbulence levels predicted towards the top mold region. The predicted velocity of the mean flow for the V2F model in the flow domain is higher, followed by the YS model and least for the CHC, AB, and AKN models. The eddy viscosity ratio is highest with a value of 144 at y+<100 for the CHC model, followed by AKN and AB with eddy viscosity ratios of 144 and 137 respectively. Then, the YS has a maximum eddy viscosity ratio of 114 at y+<50, V2F has the least maximum eddy viscosity ratio of 78 at y+<150. The turbulence intensity, the turbulence velocity, and the turbulence energy production due to buoyancy were highest towards the top mold for the CHC, AB, and AKN than the YS and V2F models. The V2F and YS predicted lower temperature curves and higher solid fraction curves towards the mid-section of the profiles than the CHC, AB, and AKN models.

Characterization of AA6111 aluminum alloy thin strips produced via the horizontal single belt casting process

ABSTRACT In this research, AA6111 aluminum alloy strips, 250 mm wide and ~6 mm thick, were produced via the horizontal single belt casting process. The strip microstructure was analyzed using optical and scanning electron microscopy, and the measured average grain size was 90 μm. This is significantly smaller than the average grain size of the products produced via direct chill (DC) casting route (i.e., 231 μm). An analysis of intermetallic compounds was also conducted using energy dispersive spectroscopy. These were observed to be distributed within and along the grain boundaries and are enriched in Mg, Si, and Cu. The shear punch test was performed to determine the ultimate tensile strength of the cast strip, which was observed to be 280 MPa. This value is significantly higher than the strength documented in the literature for DC cast products. A 3D profilometer was used to determine the top and bottom surface roughness of the as-cast AA6111 strips. The strip top surface was found to lie within 120 μm range, whilst the bottom surface roughness was less than 30 μm.

Scale up design study on process vessel dimensions for ultrasonic processing of water and liquid aluminium

Scaling up ultrasonic cavitation melt treatment (UST) requires effective flow management with minimised energy requirements. To this end, container dimensions leading to the resonance play a crucial role in amplifying pressure amplitude for cavitation. To quantify the importance of resonance length during the treatment of liquid aluminium, we used calibrated high-temperature cavitometers (in the range of 8–400 kHz), to measure and record the acoustic pressure profiles inside the cavitation-induced environment of liquid Al and deionized water (used as an analogue to Al) excited at 19.5 kHz. To achieve a comprehensive map of the acoustic pressure field, measurements were conducted at three different cavitometer positions relative to the vibrating sonotrode probe and for a number of resonant and non-resonant container lengths based on the speed of sound in the treated medium. The results showed that the resonance length affected the pressure magnitude in liquid Al in all cavitometer positions, while water showed no sensitivity to resonance length. An important practical application of UST in aluminium processing concerns grain refinement. For this reason, grain size analysis of UST-treated Al-Cu-Zr-Ti alloy was used as an indicator of the melt treatment efficiency. The result showed that the treatment in a resonance tank of L=λAl (the wavelength of sound in Al) gave the best structure refinement as compared to other tested lengths. The data given here contribute to the optimisation of the ultrasonic process in continuous casting, by providing an optimum value for the critical compartment (e.g. in a launder of direct-chill casting) dimension.

On the intermetallic constituents in the sodium-induced edge cracking of hot-rolled AA5182 aluminum alloys

ABSTRACT The productivity and recovery of hot-rolled Al-Mg-Mn sheets often suffer from sodium-induced edge cracking. In this study, AA5182 ingots with different levels of Na were prepared by direct-chill (DC) casting, and were then subsequently subjected to homogenisation and hot-rolling processes. The edge crack occurrence during hot rolling in the pass #11 (84% reduction) and pass #18 (96% reduction) was investigated, with special attention paid to the impact of intermetallic constituents in the ingot subsurface region, where all edge cracks were initiated. Correlations between the susceptibility to edge cracking, enriched intermetallic constituents in the inverse segregation zone, Mg2Si cavitation, meniscus bands, and Na contamination were observed. The results revealed that Fe-rich intermetallic particles were not involved in crack formation, while Mg2Si particles had a strong tendency to debond from the Al matrix and form cavities at the edges of the rolled strip. The higher the Na content, the more severe the Mg2Si cavitation, and consequently, the higher the susceptibility to edge cracking. Numerous debonded Mg2Si particles contaminated with Na in the subsurface region provided favourable conditions for crack initiation and propagation. The role of the meniscus bands on the large and profound edge cracks was also elaborated.

Prediction of grain structure in direct-chill cast Al–Zn–Mg–Cu billets using cellular automaton-finite element method

A cellular automaton-finite element (CA-FE) model was used to predict the solidification grain structure of permanent mold cast A7086 alloy. In this model, a Gaussian distribution of nucleation sites was adopted, and the Kurz-Giovanola-Trivedi model was extended to a multicomponent Al–Zn–Mg–Cu alloy system to determine the growth kinetics of the dendrite tip. For describing the Gaussian distribution of nucleation sites on the mold surface, an empirical relationship between the initial cooling rate of the melt and the nucleation density was proposed. Under various casting conditions, the calculated grain structures agreed well with the experimental results. Subsequently, the model was applied to the direct-chill (DC) cast billets, and the simulated grain structures reproduced well the experimental results. This confirmed that the current CA-FE model can be used practically and effectively in DC casting processes.

A comparable study of Mg98.15Y1Zn0.85 sheets fabricated by twin-roll casting and direct-chill casting and related annealing behavior

The effects of twin-roll casting (TRC) and direct-chill casting (DC) on the microstructure and mechanical properties of Mg98.15Y1Zn0.85 alloys were investigated during homogenization, finish rolling and annealing. Special attention paid on the evolution of central segregation as well as second phase transformation. The central segregation produced by TRC disappeared after homogenization and reappeared through intermediate annealing with final rolling. In the subsequent annealing process, as the annealing temperature increased, the segregation gradually grew, disappeared and finally reappeared. In addition, the disappearance and reappearance of the intermediate segregation simultaneously changed the phase compositions and morphology: the discontinuous W phase in the as-roll state disappeared through homogenization and then reappeared after the intermediate annealing. Increasing the final annealing temperature gradually transformed the long-period stacking-ordered LPSO- and W-mixed network structure into LPSO clusters, dispersed LPSO structures, mixed granular and network like W phase and continuous network-like W phase. Particularly, the chain network of W phase exhibited excellent strain hardening and significantly increased the yield strength of the alloy.

Research on causes of hot-tearing of AA3003 aluminum billet during DC casting process

AA3003 aluminum alloy made from raw scrap materials that have the advantage of economical use, but hottearing often occurs in the product billets of the direct chill casting process. This study used ANOVA analysis method for determination of chemical composition of AA3003 aluminum billet products to show influence of chemical composition on hot-tearing ability. The evaluation of the microstructure and chemical composition distribution of the elements by optical and scanning electron microscopes combined with energy dispersive spectroscopy showed the existence of impurities such as Cu, Zn, Fe, Pb exceeding the allowable limit in aluminum billets, especially at grain boundary, which can be the main reason for the hot-tearing of cast aluminum billets.

Multiphysics Modelling of Ultrasonic Melt Treatment in the Hot-Top and Launder during Direct-Chill Casting: Path to Indirect Microstructure Simulation

This study concerns the numerical simulation of two competing ultrasonic treatment (UST) strategies for microstructure refinement in the direct-chill (DC) casting of aluminium alloys. In the first, more conventional, case, the sonotrode vibrating at 17.3 kHz is immersed in the hop-top to treat the sump melt pool, in the second case, the sonotrode is inserted between baffles in the launder. It is known that microstructure refinement depends on the intensity of acoustic cavitation and the residence time of the treated fluid in the cavitation zone. The geometry, acoustic field intensity, induced flow velocities, and local temperature are factors which affect this treatment. The mathematical model developed in this work couples flow velocity, acoustics modified by cavitation, heat transfer, and solidification at the macroscale, with Lagrangian refiner particles, used to determine: (a) their residence time in the active zones, and (b) their eventual distribution in the sump as a function of the velocity field. This is the first attempt at using particle models as an efficient, though indirect, alternative to microstructure simulation, and the results indicate that UST in the launder, assisted with baffle separators, yields a more uniform distribution of refining particles, avoiding the strong acoustic streaming jet that, otherwise, accompanies hot-top treatment, and may lead to the strong segregation of refining particles. Experiments conducted in parallel to the numerical studies in this work appeared to support the results obtained in the simulation.

Scalable ultrasonic casting of large-scale 2219AA Al alloys: Experiment and simulation

An ultra-large 2219 Al alloy ingot (i.e., 1.38 m in diameter and 4.6 m in length) was manufactured using a direct-chill casting technique assisted by four ultrasonic sonotrodes. Then, the effects of ultrasonication on the matrix grains, the α-Al/θ-Al2Cu eutectic structures and the macrosegregation of Cu were experimentally determined. Using numerical simulation, the distribution of physical fields (i.e., the liquid fraction, the temperature gradients and the velocity field) in the sump was simulated and compared between the ingots with and without ultrasonication. After introducing ultrasonication, grain refinement occurred due to the increased cooling rate generated by the accelerated heat transfer in the sump. Meanwhile, the macrosegregation of the major element (Cu) was relieved, which was mainly attributed to the effect of ultrasound-forced convection. Besides, eutectic growth was restricted in the narrowed mushy zone under ultrasound. As a result, eutectic agglomerations were accordingly reduced. The relevant mechanisms were discussed in terms of the experimental results and simulation.