Twin-roll Casting Process Research Articles

Effect of Oscillating Field on Microstructure of Twin-roll Cast 7075 Aluminum Alloy

Abstract The morphology and the hardness of 7075 Al alloy during oscillating twin-roll casting (TRC) process at 670 °C were investigated. It was observed that compared with that by traditional process, the dendritic grains of the strips by oscillation field TRC process were severely broken, refined and equiaxed. Segregation bands at the surface and the center of 7075 strips were dramatically reduced in electromagnetic oscillatory field conditions, even completely disappeared in alternating oscillatory condition. The transverse and the rolling direction HV hardness of 7075 strips manufactured by traditional process were 1104 and 1356 MPa, respectively. Upon applying the electromagnetic field, the maximum hardness of 1615 MPa was obtained in alternating oscillating TRC process. Besides, the properties of strips will be further improved with increasing of magnetic field. No matter in what conditions, the refinement of electromagnetic field to strips will increase at elevated temperature.

Process parameters and roll separation force in horizontal twin roll casting of aluminum alloys

A two-dimensional finite element analysis was carried out to predict roll separation force, temperature distribution, and the shape of the mushy zone during horizontal twin roll casting of A7075 aluminum strips. Various effect of roll speeds and initial temperatures of the melt on casting was considered. Modeling twin roll casting focused on roll separation force had merits of predicting mechanical interaction between the rolls and solid strips, in addition to temperature distribution. The roll speed has a great effect on roll separation force, compared with heat transfer coefficient and melt temperature. The predicted roll separation force was compared with the measured one during twin roll casting. The results can be used effectively to reduce the experimental trial-and-error procedures of twin roll casting process.

Casting speed optimal design of 304 stainless steel during twin roll strip casting process by 3D modelling

The exit temperature distribution of 304 stainless steel during twin roll strip casting process was measured. The crack morphology of the actual steel strip was observed by scanning electron microscope. Then, by establishing a three-dimensional model including casting rollers, the flow and temperature fields of 304 stainless steel during the twin roll strip casting process were modelled. The modelled temperature result is in accordance with the measured one, so the flow and temperature fields of 304 stainless steel during the twin roll casting process with different casting speeds can be modelled. The results of the flow field show that a large area of vortex forms in the pool. Those of the temperature field show that the temperature in the vortex zone is lower 30–50°C than that in the refluxing zone, and a thin shell forms on the interface between the molten pool and the casting roller. The whole temperature of the molten pool is increased with the increase in the casting speed, and an optimal casting speed is ∼0·7 m s−1.

Superplastic forming ability of as‐cast AZ91 Mg alloy prepared by twin roll casting

In this study, the twin roll casting process has been utilized to prepare initial as‐cast strip of AZ91 alloy that has been further tested in uniaxial tension at 325, 350 and 375 ºC, and at strain rates from 10–2 to 10–4 s–1. The ability of AZ91 strip to undergo superplastic or superplastic‐like deformation in longitudinal direction, in transverse direction, and in 45 degrees to longitudinal direction was investigated with no further thermal or thermomechanical processing applied prior to the testing. At temperature 350 ºC and medium strain rate the maximum elongation reached almost 200% in direction parallel to the strip casting, however in the transverse and 45 degrees direction, different temperature, and at higher strain rates the maximum elongation was lower. Based on microstructural investigation by means of optical microscopy and scanning electron microscopy (SEM) it could be seen that continuous dynamic recrystallization of initial coarse dendritic as‐cast microstructure during elevated temperature deformation is active and responsible for high elongations. Additional analysis is provided by means of a strain rate sensitivity characterization and its evolution with increased level of strain. Strain rate sensitivity of AZ91alloy increased with increasing level of strain from 0.20 to 0.33.

The Effect of Melt Conditioning on Segregation of Solute Elements and Nucleation of Aluminum Grains in a Twin Roll Cast Aluminum Alloy

An aluminum alloy was cast by a laboratory scale horizontal twin roll caster with or without melt conditioning by the intensive shearing prior to solidification and then examined by high-resolution electron microscopy. The combined twin roll casting process with solidification formed channels and induced centerline segregation without the conditioning. In comparison, the melt conditioning minimized the severe segregation on the surface as well as at the centerline. Furthermore, large amounts of solute elements were uniformly distributed along grain boundaries or interdendritic regions. Analytical electron microscopy detected a fine oxide particle or a fragmented aluminum particle particularly at the center region of one nucleated aluminum grain. In addition, large oxide particles of about 1 to 5 μm nucleated aluminum grains easily due to low undercooling necessary for the heterogeneous nucleation, whereas small oxides with the size of about 100 to 200 nm requiring large undercooling were pushed along the grain boundaries instead of contributing to the nucleation. The enhanced nucleation of aluminum grains and well-distributed solute atoms in the melt by the melt conditioning resulted in the minimization of macro- and micro-segregations and the formation of a uniform microstructure.

Structural and mechanical properties of 7075 alloy strips fabricated by roll-casting in a static magnetic field

The influences of a 0.2 T static magnetic field on the microstructure of 7075 aluminum alloys sheets produced with a twin-roll continuous caster at 675°C were investigated in this paper. Under a uniform magnetic field, the primary dendrites were refined and tended to be equiaxed. The microstructure consisted of an intermediate case between dendritic and equiaxed grains. Moreover, the use of an external static field in the twin-roll casting process can reduce heat discharge, resulting in a decrease in undercooling, and may also account for the abatement of segregation bands. In addition, the static magnetic field effectively improved the solute mixing capacity, and the added atoms more easily diffused from precipitates to the α-Al matrix, which resulted in an increase in the mechanical properties of the rolled sheets. Specimens prepared both in the presence of a static magnetic field and in the absence of a static magnetic field exhibited brittle-fracture characteristics.

3D stress simulation and parameter design during twin-roll casting of 304 stainless steel based on the Anand model

This study first investigated cracks on the surface of an actual steel strip. Formulating the Anand model in ANSYS software, we then simulated the stress field in the molten pool of type 304 stainless steel during the twin-roll casting process. Parameters affecting the stress distribution in the molten pool were analyzed in detail and optimized. After twin-roll casting, a large number of transgranular and intergranular cracks resided on the surface of the thin steel strip, and followed a tortuous path. In the molten pool, stress was enhanced at the exit and at the roller contact positions. The stress at the exit decreased with increasing casting speed and pouring temperature. To ensure high quality of the fabricated strips, the casting speed and pouring temperature should be controlled above 0.7 m/s and 1520°C, respectively.

Melt Conditioned Twin Roll Casting (MC-TRC) for Aluminium Alloys

Twin roll casting (TRC) has been demonstrated to be an effective route for producing Al sheet at a reduced cost. However, the quality of the Al alloys sheets produced by the TRC process is limited by the formation of centreline segregation, which hampers the downstream processing, degrade the mechanical performance of the final rolled products, limit TRC to dilute alloys with a narrow freezing range, and hence prevent its application in a wide range of engineering sectors. To improve the quality of the TRC strips, a new technology, melt conditioning twin roll casting (MC-TRC) has been developed. It has been demonstrated that the MC-TRC process is capable of producing Al-alloy strips with minimal centerline segregation which is attributed to enhanced nucleation by well dispersed naturally occurring oxides in liquid Al alloys. This work aims to investigate the upper composition limit allowed for the MC-TRC process using the binary Al-Mg alloys as model alloys. The results showed that melt conditioning increases the range of alloys (solute content up to 7%) that can be twin roll cast without severe segregation.

Strong cube recrystallization texture in silicon steel by twin-roll casting process

Through-process texture evolution was investigated in Fe–2.8wt.% Si sheets cold-rolled directly from twin-roll cast strip. A strong cube texture ({001}〈100〉) is successfully developed after high-temperature annealing. According to micro-texture analysis, the enhanced cube texture is primarily the result of the frequency advantage of cube recrystallized grains, especially in large grain size ranges, suggesting the dominant role of the oriented nucleation mechanism. Two main types of nucleation sites for cube grains are identified: cube deformation bands in the interior and near the grain boundary of λ (from {001}〈210〉 to {114}〈841〉-{113}〈631〉) deformed regions, and shear bands in {111}〈110〉 deformed matrices. The initial casting texture tends to produce more cube deformation bands and cube shear bands compared with conventional processes. Moreover, the selected processing parameters are favorable for making various nucleation sites work effectively at a very early stage of recrystallization. The results obtained in the present study can provide an efficient way for cube texture control in body-centered cubic alloys.

Scale-up modeling of the twin roll casting process for AZ31 magnesium alloy

In the present study, a 2-D finite-element method (FEM) thermal-fluid-stress model has been developed and validated for the twin roll casting (TRC) of AZ31 magnesium alloy. The model was then used to quantify how the thermo-mechanical history experienced by the strip during TRC would change as the equipment was scaled up from a laboratory size (roll diameter=355mm) to a pilot scale (roll diameter=600mm) and to an industrial scale (roll diameter=1150mm) machine. The model predictions showed that the thermal history and solidification cooling rate experienced by the strip are not affected significantly by caster scale-up. However, the mechanical history experienced by the strip did change remarkably depending on the roll diameters. Casting with bigger rolls led to the development of higher stress levels at the strip surface. The roll separating force/mm width of strip was also predicted to increase significantly when the TRC was scaled to larger sizes. Using the model predicted results, the effect of both casting speed and roll diameter was integrated into an empirical equation to predict the exit temperature and the roll separating force for AZ31. Using this approach, a TRC process map was generated for AZ31 which included roll diameter and casting speed.

Twin Roll Casting (TRC) of Magnesium Alloys – Opportunities and Challenges

Twin Roll Casting (TRC) has been successfully employed for the past sixty years to produce aluminum, steel and, in the past ten years, magnesium sheet. Although the TRC process is relatively simple, its application for commercial-scale magnesium strip production has proven difficult. This is primarily due to inherent characteristics of magnesium alloys, such as their high reactivity to oxygen, low specific heat and latent heat of fusion, and large freezing ranges, which can induce formation of casting defects if various TRC processing parameters, such as metal delivery design, heat transfer in the roll gap, and casting speed, aren’t tightly controlled. Research is underway worldwide to concurrently gain a better understanding of TRC processing variables in order to provide optimum casting conditions which will reduce defects, and develop new magnesium alloys with properties tailored to the TRC process. The opportunities and challenges associated with magnesium TRC will be outlined and include: 1) defect formation during TRC of magnesium alloy AZ31, 2) the feasibility of producing clad magnesium strip via TRC and 3) the effect of scale-up (moving from a laboratory unit to commercial production) will have on the TRC process for magnesium.

Solidification Mechanism of the Melt Conditioned Twin Roll Cast Magnesium Alloy

Twin roll casting (TRC) has been demonstrated to be a process capable of producing Mg sheets at a significantly reduced cost. However, the quality of the Mg sheets produced by the TRC process is limited due to the formation of coarse columnar grains and severe centreline segregation, which reduces both the strength and ductility of Mg-alloys. In this paper, melt conditioned (MC) has been employed prior to twin roll casting (TRC) of AZ31 alloy to understand its effect on the solidification mechanism. For comparison, AZ31 alloy strips were also produced through conventional TRC process. The results showed that the solidification mechanism can be completely altered by introducing MC in TRC process.

Development of a mathematical model to study the feasibility of creating a clad AZ31 magnesium sheet via twin roll casting

A previously developed and validated thermal-fluid mathematical model of the twin roll casting (TRC) process for magnesium alloy AZ31 was used to quantitatively study the feasibility of producing a clad magnesium strip via the TRC process. The clad material was varied to identify the effect of material composition on the feasibility of producing a clad strip. The clad alloys chosen included pure Zn, pure Al, AA3003, and AA5182 aluminum alloys. In the analysis, the effect of casting speed and clad sheet thickness (100 and 500 μm) on the thermal history in the magnesium strip and clad layer was analyzed. Assessment of the process feasibility was determined based on the exit temperature of the clad strip at the centerline, temperature of the clad sheet prior to the roll bite entry, and fraction solid of both the core (magnesium sheet) and clad along the core/clad interface. The results indicated that using pure Zn as a clad material is not feasible due to premelting of the clad strip prior to introduction into the TRC apparatus. All three aluminum alloys studied proved to be feasible in terms of a clad material, and it was found that the effect of clad thickness and clad material chemical composition on the thermal history (temperature distribution) of the clad strip was negligible. It was also predicted using the thermodynamics package FactSage™ that the intermetallic phase at the core/clad interface will be primarily γ-Mg (Mg17Al12). For AA5182 clad material, formation of β-Mg (Al3Mg2) is also possible.

Physical and mechanical properties of 7075 sheets produced by EP electro- and electromagnetic cast rolling

In this study the effect of twin-roll casting (TRC) producing conditions on microstructure and mechanical properties of Al-7075 alloys has been investigated in detail. When introducing an electric pulse (EP) of 400I peak current and 30Hz into the conventional TRC process, the dendrites are refined and equiaxed accompanied by thick segregation bands beingabated. The most uniform structure of 7075 alloy is obtained after applying a static magnetic field of 0.2T at the same time. EDS spectrum analysis shows that the netlike T (AlZnMgCu) phase is resolved while the mixture of η (MgZn2) and CuMg2 phases improves. Moreover the increase in solute diffusivity is due to the modification of heat and mass convections by the incoming EP energy, facilitating the realization of excellent mechanical properties. The tensile tests reveal that the optimum process, in which the highest strength and ductile manner were obtained, is considered to be EP electromagnetic TRC modification.

Twin Roll Casting of Al-Mg Alloy with High Added Impurity Content

The microstructural evolution during twin roll casting (TRC) and downstream processing of AA5754 Al alloy with high added impurity content have been investigated. Strip casts with a high impurity content resulted in coarse α-Al grains and complex secondary phases. The grain size and centerline segregation reduced significantly on the addition of Al-Ti-B grain refiner (GR). Coarse-dendrite arm spacing (DAS) “floating” grains are observed in the impure alloy (IA) with higher volume in the GR strips. Two-dimensional (2D) metallographic analysis of the as-cast strip suggests that secondary phases (Fe-bearing intermetallics and Mg2Si) are discrete and located at the α-Al cell/grain boundaries, while three-dimensional (3D) analysis of extracted particles revealed that they were intact, well interconnected, and located in interdendritic regions. Homogenizing heat treatment of the cast strip breaks the interconnective networks and modifies the secondary phases to a more equiaxed morphology. During rolling, the equiaxed secondary phases align along the rolling direction. X-ray diffraction (XRD) analysis suggests that α-Al(FeMn)Si and Mg2Si are the predominant secondary phases that are formed during casting and remain throughout the downstream processing of the GR-IA. The high-impurity sheet processed from TRC resulted in superior strength and ductility over the sheet processed from small book mold ingot casting. The current study has shown that the TRC process can tolerate higher impurity levels and produce formable sheets from the recycled aluminum for structural applications.

Abatement of segregation with the electro and static magnetic field during twin-roll casting of 7075 alloy sheet

This study aims to investigate the influence of electromagnetic field on the distribution and composition of precipitates and on the mechanical properties of 7075 rolled sheets. The non-field and field microstructure and the mechanical properties were studied in detail by optical microscope (OM), electron probe microanalyzer (EPMA), multiple sample tensile as well as hardness tests. The Fine and equiaxed grains were obtained when introducing the alternating oscillating electromagnetic field to the twin-roll casting (TRC) process with 0.13T static magnetic and 386A alternating current (AC) intensities. Due to a damping effect on the convection generated by applying the electro- and static magnetic fields, the undercooling of the melt decreases and the continuous net-like precipitates are refined and broken remarkably. Especially under oscillating electromagnetic field conditions, the best uniform microstructure without mottled segregation was obtained. In addition, the fields can effectively enhance solute mixing capacity and reduce heat discharge, resulting in the increase of mechanical properties of 7075 sheets in both the longitudinal and long transverse directions. The optimum process in the present study, in which the higher solid solubility in Al matrix and the stronger hardness as well as tensile strength was gained as compared to other rolled specimens, is considered as alternating oscillating TRC process.

Heat transfer behavior of top side-pouring twin-roll casting

The interfacial heat transfer between the casting and the substrate from liquid/solid contact to solid/solid contact with pressure was investigated using a set of equipment designed according to the characteristics of the top side-pouring twin-roll casting process. The interfacial heat transfer behavior of this process consists of 4 stages: chilling, solidification shrinkage, compression and cooling. High values of the IHTC ranging from 50,000 to 90,000W/m2°C were detected in the chilling stage, followed by a sharp decrease in solidification shrinkage stage (4000–8000W/m2°C). Due to the pressure, which modeled the effect of rolling in twin-roll casting, the IHTC bounced back to 6000–20000W/m2°C, according to different conditions. The influence of process variables such as pressure magnitude, compress speed, pouring temperature, surface roughness and alloy composition had been discussed. Because of the compress action, the influence of these variables performed in a different way, but it was concluded that the way to improve the contact conditions always accompanied with an increase in the IHTC.

Structure and texture of twin roll cast strips of Zn–Cu–Ti zinc alloy

The structure and crystallographic texture of zinc strips (Zn–Cu–Ti alloy) produced by the continuous horizontal twin roll strip casting method has been characterized. In longitudinal sections normal to the transverse direction, the strips display an approximately symmetrical chevron patterned structure of columnar grains inclined about 30° from the rolling direction. In association with such structure, the macroscopic texture is mainly a ‘normal’ (not cyclic) fibre texture tilted approximately ±30° around the transverse direction plus a similarly tilted weak <0001> fibre texture. A thin layer of small equiaxed grains with a strong (0001) basal texture is present at the free surfaces. The observed structure/texture combination agrees quite well with the expected macrostructure of solidification of the alloy in the twin roll casting process.

Effect of Casting Parameters on Roll Separation Force during Twin Roll Casting

A two-dimensional finite element analysis was adopted to predict temperature distribution and roll separation force during twin roll casting of A7075 aluminum alloy strip. The influence of various parameters such as roll speed, initial temperature of the melt and heat transfer coefficient on temperature distribution and roll separation force was investigated in detail. The results were useful to comprehensively understand the casting process and to effectively reduce the experimental trial-and-error procedures of twin roll casting process.

Study on Molten Zone in Twin-Roll Casting of Magnesium Alloy Strip

Thermal flow finite element simulation was carried out to deeply understand the solidification and deformation phenomena of magnesium alloy in molten zone during twin-roll casting process. The results show that temperature near the nip of the rolls is about 680K based on current conditions, and there are two vortexes in the molten pool during casting, a peak formed at solidus. Due to the metal flows in molten pool and relative rotates of the rolls, temperature field of molten zone is different from normal casting, which will cause the solidification structure changes. An experiment by using a vertical type twin-roll caster was also conducted, and the amount of deformation bands at the joint line is larger than that at the two sides. Substructures can be found due to dynamic recrystallization during casting process.