Conventional Direct Chill Casting Research Articles

Synthesis of Al–Sn alloys by direct chill casting under the effect of mechanical stirring: an experimental and simulation optimization study

Abstract Aluminium–tin (Al–Sn) immiscible alloys for bearing materials are difficult to cast by the conventional direct chill casting method. At higher concentrations, Sn accumulates at the bottom of the as-cast solid due to a significant density difference in Al and Sn. In the present investigation, Al–Sn alloys were fabricated by the direct chill (DC) casting method under the influence of mechanical forced convection (MFC). A computer-based simulation approach has been implemented to study the effect of MFC device position and impeller speed on melt flow and solidification. Optimized parameters were experimentally validated and the quality of as-cast ingots was evaluated. Hence, this optimization approach can be used to produce better quality Al–Sn bearing materials with uniformly distributed Sn phase in Al-matrix by the direct chill casting route.

Effect of temperature field on the porosity and mechanical properties of 2024 aluminum alloy prepared by direct chill casting with melt shearing

Porosity severely deteriorates the mechanical properties of aluminum alloys. Large-sized and aggregated porosity often occurs in the center of Direct-Chill (DC) cast ingot. The variation of the temperature field in the sump affects the formation of porosity. In the present work, high shear technology was introduced into the DC casting process and the rotation speed and position of the high shear device were precisely controlled based on the measured temperature fields of conventional DC casting to realize the control of the temperature field and study its effects on the microstructure and mechanical properties of DC cast ingot. The results showed that when the temperature of the melt near the shear site (TS) was close to the temperature of the melt near the center of ingot and at starting position of the slow cooling zone of conventional DC casting (TG), forced convection can affect the cooling zone which is below the graphite ring in the sump, resulting in uniform microstructure of the ingot. If TS was above the liquidus and close to TG (temperature field II), the microstructure shows rod grains with fine secondary dendrite arms, refined porosity and second phase particles. For the lower TS which is below the liquidus and close to TG, (temperature field III), the mixed microstructure of spherical grains and small equiaxed was formed. And the increase of size and content of porosity can decrease of mechanical properties of the ingot. The concomitant relationship between floating/spherical grains and porosity was also discovered and discussed.

Processing map of 2219 Al alloy prepared by internal electromagnetic stirring direct chill casting

Hot compression tests are carried out for the 2219 Al alloys, which are prepared by conventional direct chill casting and internal electromagnetic stirring direct chill casting respectively. The processing maps are constructed and the as-deformed microstructures are observed to investigate the effects of internal electromagnetic stirring on the hot deformation of 2219 Al alloy. The results show that the internal electromagnetic stirring expends of stability domain, especially increasing the corresponding range of strain temperature. The suitable deformation parameters of internal electromagnetic stirring direct chill casting 2219 Al alloy includes a relatively lower strain temperature and a relatively higher strain rate. The internal electromagnetic stirring promotes the formation of more homogeneous as-deformed microstructures and more dynamic recrystallization grains.

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.

Numerical modelling and experimental validation of the effect of ultrasonic melt treatment in a direct-chill cast AA6008 alloy billet

In this work, we study how ultrasonic cavitation melt treatment (UST) affects the temperature distribution, sump profile, and resulting microstructure in the direct-chill (DC) casting of an AA6008 aluminum alloy. Two 152 mm diameter billets were cast; one was treated with UST (UST-DC casting) in the hot top while the other was not (conventional DC casting). To investigate the temperature distribution, temperature was measured at multiple points in both billets. The sump profile was visualized by pouring Zn into the sump during casting. The microstructure was analyzed by measuring the grain size of as-cast billets. A numerical model of DC casting and UST-DC casting has been validated with the temperature measurements across the billets, and the experimental results agrees well with the numerical model. It is found that the sump profile quantification with thermocouple measurements is more accurate and less prone to interpretation than with Zn tracing. Numerical simulation results show that UST application in the hot top with sonotrode position at 20 mm above the graphite ring level depresses the liquidus isotherm but does not affect the solidus isotherm, resulting in a thinner transition region compared with conventional DC casting. Grain structure analysis verifies that structure refinement with UST has been achieved at the given sonotrode position. The strongest grain refinement was at the center of the billet with the average grain size 50% smaller than that without UST. The results are discussed in terms of the known mechanisms of UST, i.e. dendrite fragmentation and deagglomeration of nucleating substrates.

New Twin-Roll Cast Al-Li Based Alloys for High-Strength Applications

Al-Li based alloys are attractive materials for the aerospace industry. The twin-roll casting of such materials could provide properties not achievable by conventional direct-chill casting and downstream processing methods due to significantly higher solidification rates. An Al-Li-Cu-Mg-Zr alloy was twin-roll cast with the same alloy containing a small addition of Sc. The microstructure of as-cast materials and the influence of Sc on the behavior of the alloy at elevated temperatures were studied by means of light and electron microscopy and by resistivity measurements. A fine-grained structure was formed during twin-roll casting, but several surface and internal defects were found on the strips, which should be suppressed by a further adjustment of the casting conditions. The addition of Sc had a positive effect on grain size uniformity and microstructure stabilization at elevated temperatures, as shown by the precipitation of a fine dispersion of coherent Sc- and Zr-containing precipitates.

Effect of Low-Frequency Electromagnetic Casting on Micro-Structure and Macro-Segregation of 5A90 Alloy Ingots.

The effect of low-frequency electromagnetic fields on the micro-structure and macro-segregation of 5A90 alloy ingots during the semi-continuous casting process were quantitatively investigated. The ingots of a 5A90 alloy with a diameter 170 mm were produced by the conventional direct chill casting (DCC) process and low-frequency electromagnetic casting (LFEC) with 10 Hz/100 A. The results showed that LFEC can substantially refine the micro-structure and shorten the width of the columnar grain area of an ingot. The refinement effect came with the relieving of grain boundary segregation and an improvement in the macro-segregation of the ingot. Compared with the traditional DCC process, the tensile properties of the aged alloy prepared by the LFEC process were improved due to the effects of the increase in solid solubility and the strengthening of the grain refinement, so that the stability of the tensile properties was also improved. Meanwhile, the rate of yield increased by 2.3% with a decrease in the peeling thickness of the ingot.

Effect of wiper on thermal stresses during direct chill casting of magnesium alloy

Understanding the thermomechanical behaviour of an alloy during the Direct Chill (DC) casting process is essential to alleviate the defects in the casting. In this work, conventional direct chill casting process is numerically modelled for evaluating the thermal stress profiles in the casting of AZ31 magnesium alloy. A cooling strategy is used where a wiper is positioned below the mold to reduce the cooling intensity and magnitude of thermal stresses. In this work, the effect of the wiper on the reduction of thermal stress is analysed in detail during the DC casting of AZ31 magnesium alloy. A 2-D computational domain growing along with the casting speed is used to replicate the DC casting process. The temperature and stress profiles are calculated by sequentially solving the energy and momentum balance using the Finite Element Method. The effect of a cooling strategy, on the variation of radial, axial and circumferential stresses is analysed. It is observed that there is a decrease in the circumferential and radial stresses developed in the billet below the mold. Implementation of wiper resulted in higher temperature billet surface which led to reduction of the stresses both in the circumferential and radial direction.

Study on micro-structure, solid solubility and tensile properties of 5A90 Al–Li alloy cast by low-frequency electromagnetic casting processing

In this paper, the low-frequency electromagnetic casting (LFEC) processing was introduced as an efficient pollution-free casting technique, and how it influenced the micro-structure and mechanical properties of 5A90 Al–Li alloy was investigated. The mechanical properties of the alloy had a close connection with the casting metallurgical quality and the precipitation phases. Compared to the conventional direct chill casting (DCC), the LFEC could promote the precipitate of the δ′ (Al3Li)/β′ (Al3Zr) and reduce the width of PFZ, which enhanced the effect of the precipitation strengthen and alleviated the strain localization. Meanwhile, the micro-structure of the ingot was refined significantly and the metallurgical quality was improved after LFEC treatment due to the modification of the flow and temperature field of the sump that caused by the application of low-frequency electromagnetic field. Maximum yield strength and ultimate tensile strength of as-aged alloy cast by LFEC with 150 A/15 Hz could reach 371.85 MPa and 495.5 MPa, while elongation remained at 9.82%.

C-DC and MC–DC casting of Al-alloys – A comsol approach

Present work endeavours to understand the solidification behaviour in conventional direct chill casting (C-DC) of aluminium alloy billets. The simulations were generated for C-DC and MC–DC (melt-conditioned) processes. For the first time, COMSOL Multiphysics has been successfully used to simulate the heat flow in C-DC casting of Al-alloy billet. The results show that the temperature gradient in the sump is minimised in the case of MC–DC when compared with C-DC. The simulation results also revealed the effect of casting speed on the sump profile in both the cases.

Corrosion behaviour of magnesium AZ31 sheet produced by twin roll casting

In this study, corrosion behaviour of magnesium AZ31 alloy sheets produced by different casting methods was investigated. Magnesium AZ31 sheets of 2 mm in thickness produced by conventional direct chill casting+rolling and AZ31 sheets of 2 mm in thickness and 1500 mm in width produced by twin roll strip casting+rolling were used as samples. For the determination of processing method dependent corrosion behaviour, potentiodynamic electrochemical corrosion and also immersion tests were performed in 0.5M Na2SO4 and 0.01M NaCl solutions. Corrosion damage occurred during the tests was extensively investigated by scanning electron microscopy. Results of the work revealed the clear role of processing method dependent size and distribution of intermetallics on the corrosion behaviour of AZ31 sheets.

Recrystallization Behavior of Twin-Roll Cast 3xxx Series Aluminum Alloy

Twin roll casting (TRC) is an efficient process used to produce thin plates directly from molten metal. However, TRC has a much higher cooling rate during solidification compared with conventional direct chill casting. Accordingly, the constituent phases are refined and the added elements are supersaturated by this casting method. Supersaturation of the added elements hinders both recovery and recrystallization because fine dispersoids precipitate from the supersaturated solid solution during annealing [ and coarsen the grain size of the final plate, which leads to degradation of formability. Therefore, it is important to understand the influence of the solute element and dispersoids on recrystallization behavior to obtain an appropriate grain structure for forming. In this study, the effects of the homogenization conditions and cold rolling reduction on the recrystallization behavior of twin-roll cast 3XXX series aluminum alloy were investigated.

Simulation of Low Frequency Electromagnetic DC Casting

A comprehensive multiphysics model has been developed to describe the effect of the low frequency electromagnetic field (LFEM) [1, on solidification in the hot-top Direct-Chill (DC) casting [ of round aluminium alloy billets. The volume averaged equations and the rigid solid phase assumption are assumed for fluid flow and heat transfer [. The electromagnetic induction equation for the field imposed by the coil is solved using the diffuse approximate method (DAM), structured in axial symmetry with Gaussian weight function, 6 polynomial basis and 9 nodded domains. The heat, mass, and momentum transfer equations are solved in primitive variables by meshless [ method using 5 nodded domains of influence and 5 scaled multiquadrics radial basis functions, using collocation. Explicit time stepping is used. Pressure-velocity coupling is performed by the fractional step method. The effects of intensity and frequency of the LFEM [ on the velocity and temperature fields is investigated. A comparison of the calculated results with different LFEM field process variables with that of the conventional hot-top DC casting process indicates that the velocity patterns, the temperature profiles, and the shape of the sump could be modified remarkably.

Effect of DC Casting Processes on Microstructure of AlCu4.5Mn0.8 Alloy

The semi-solid billets of AlCu4.5Mn0.8 alloy were cast respectively by low frequency electromagnetic, low-superheated, and conventional direct chill casting. The effect of casting processes on microstructures was investigated. The results show that due to the effect of the low frequency electromagnetic field, the coarse dentritic microstructure is gradually broken up and turned into the homogeneous, fine rosette-shaped non-dentritic microstructures with the increase of electromagnetic frequency from 10 Hz to 30 Hz. When electromagnetic frequency is 30 Hz, the homogeneous, fine non-dentritic microstructure what is suitable for semi-solid reheating and thixo-forming could be obtained successfully.

Mechanism of grain refinement of an Al–Zn–Mg–Cu alloy prepared by low-frequency electromagnetic casting

Low-frequency electromagnetic casting (LFEC) process had been developed and is being used for the past several years with the application of an induction coil placed outside the conventional direct chill (DC) casting mould. It has been demonstrated that the LFEC process has a significant grain refining effect on aluminium alloys. In the present study, temperature measurement and direct quenching from liquid and/or semi-solid were carried out to study the temperature field during casting process and to understand the mechanism of the grain-refining effect of the LFEC process. The experimental results showed that in contrast to the conventional DC casting process, the liquid melt from the launder, during the LFEC process, is cooled with very high cooling rate directly to 3–6 °C below the liquidus, and the temperature field of the entire melt in the mould, and the hot top is quite uniform, which results in the enhanced heterogeneous nucleation and improved survival rate of the nuclei. This is believed to be the main reason why the LFEC process can significantly refine the grain size of aluminium alloys.

Effect of Physical Fields on Solidification Structures of DC Casting AZ80 Magnesium Alloy Billets

AZ80 magnesium alloy was semi-continuously cast under different physical fields which were conventional direct chill (DC) casting, low frequency electromagnetic casting (LFEC), ultrasonic casting (USC) and electromagnetic-ultrasonic combined casting (ECUC), respectively. The effect of different physical fields on solidification structures of AZ80 alloys was investigated. The results show that compared with the conventional DC casting, structures of AZ80 alloys billets cast with LFEC and USC have been greatly refined. The effective refinement takes place in the edge of billets when LFEC is applied. However, the effective refinement takes place in the center of billets when USC is applied. When combination of low frequency electromagnetic and ultrasonic fields is applied during semi-continuous casting AZ80 magnesium alloy billet, structures of AZ80 alloys are refined significantly in the whole billets everywhere and more uniform.

Structure and mechanical properties of AZ31 magnesium alloy billets by different hot-top semi-continuous casting technology

AZ31 alloy billets of 200 mm in diameter were produced by three different processes of conventional direct chill (DC) casting, low-frequency electromagnetic casting (LFEC) and low-frequency electromagnetic vibration casting (LFEVC), respectively. The effect of LFEC and LFEVC on the microstructures, macrosegregation and mechanical properties of AZ31 alloy billets was investigated. In conventional DC casting, the AZ31 alloy billets exhibited coarse grains (about 370 μm) and severe segregation of Al and Zn. In the presence of a solo low-frequency alternating magnetic field or a low-frequency electromagnetic vibration field applied during DC casting of ϕ200 mm AZ31 billets, grains in the AZ31 alloy billets were effectively refined (about 210 μm) and the macrosegregation of Al and Zn in the billets was greatly decreased. Furthermore, the tensile strength, fracture elongation and hardness of the as-cast AZ31 alloy billets were improved by the processes of LFEC and LFEVC relative to that cast by the process of conventional DC casting.

Effects of the Low Frequency Electromagnetic Field on the Centerline Macrosegregation of 7075 Aluminum Ingots

The effects of the low frequency electromagnetic field on the macrosegregation of the 7075 aluminum ingots were investigated. The 7075 aluminum ingots with the diameter of 200 mm were prepared by the conventional direct chill casting and the low frequency electromagnetic field casting (LFEC) processes, respectively. The temperature during casting at steady state was measured, and the mushy region was observed from the temperature contour. The concentrations of the alloying elements were measured by the spectrograph. It was found that the transition region was broadened, but the mushy zone became narrower with presence of the low frequency electromagnetic field. The centerline macrosegregation of the ingots was alleviated by the low frequency electromagnetic casting process.

Direct Chill Casting of Φ500mm ZK60 Magnesium Alloy Billets under Low Frequency Electromagnetic Field

Effects of low frequency electromagnetic field on surface quality, microstructure and hot-tearing tendency of direct chill casting of Φ500mm ZK60 magnesium alloy billets were investigated. The results show that with the application of the low frequency electromagnetic field, the surface quality of Φ500mm ZK60 magnesium alloy billets has been markedly improved, and the depth of cold fold is decreased. In the conventional direct chill casting, the microstructures of the billet, especially at the center, are coarse. The distribution of the grain size is non-uniform throughout the billet. From the edge to the center, the microstructure gradually changes from fine to coarse in all billets. However, under the low frequency electromagnetic casting, the microstructures of the billet is significantly refined, the distribution of the grains size is relatively uniform from the billets edge to the billets center. And it also shows that the hot-tearing tendency of direct chill casting Φ500mm ZK60 magnesium alloy billets under low frequency electromagnetic field is significantly reduced.

On The Formation of Extruded Surface Segregation Layer in Aluminum Direct Chill Casting Process

The aim of this research work was to develop a mathematical model to study the formation of extruded surface segregation phenomenon during the early stages of solidification in an aluminum direct chill casting process. The mathematical analyses contain a one dimensional mathematical model of heat flow, solidification, interdendritic strain and macrosegregation. Some typical cases in conventional direct chill casting processes related to increase surface segregation and fluctuation in macrosegregation with distance from the ingot surface were simulated. Good agreement was found by previous measurements and simulated results. Model results were discussed to explain the effects of different thermal solidification factors and interdendritic strain on the extruded surface segregation layer phenomenon. The results show that this phenomenon is sensitive to alloy composition and forms in the gap between the ingot surface and mould wall. Also, the model results point out that this phenomenon appears as a direct effect of the extrusion mechanism of interdendritic solute liquid associated with the interdendritic strain distribution in the coherent region in the mushy zone.