Twin-roll Casting Research Articles

Role of Zn on the room temperature formability and strength in Mg–Al–Ca–Mn sheet alloys

We clarified the role of Zn on the mechanical properties and texture evolution in Mg-1.2Al-0.5Ca-0.4Mn-xZn (wt.%, x = 0, 0.8 and 1.6) alloy sheets fabricated by twin-roll casting and hot rolling. The room temperature stretch formability of the solution-treated sheet is improved with increasing Zn content, x. The Mg-1.2Al-0.5Ca-0.4Mn-1.6Zn alloy shows excellent stretch formability with the largest Index Erichsen value of 8.2 mm due to a weak transverse direction split texture. The development of the transverse direction split texture is attributed to the preferential nucleation of recrystallized grains at double twin boundaries promoted by the moderate segregation of Zn and Al. Subsequent artificial aging at 170 °C for 2 h (T6) leads to a substantial increase in strength without loss of ductility. The peak-aged Mg-1.2Al-0.5Ca-0.4Mn-1.6Zn alloy exhibits a high tensile yield stress of 210 MPa with an excellent elongation of 30.1% because of the dense dispersion of Guinier Preston zones within the Mg matrix. The insights gained in this work would establish a base for rational design and fabrication of bake-hardenable magnesium alloy sheet with excellent room temperature formability and high strength.

Improvement of Spatial Inhomogeneity of Solute Elements and Mechanical Properties of Twin-Roll Cast Al-Mg-Si Alloy in Presence of Electromagnetic Fields

The objective of this work is to investigate the effects of electromagnetic external fields on the spatial distribution of alloying elements and the mechanical properties of Al-Mg-Si alloy prepared using twin-roll casting. The formation mechanism of macro/microsegregation and mechanical properties were examined, and the results indicated that the amount of centerline segregation defects was significantly reduced with the application of an external field. When a pulsed electric field and static magnetic field were applied simultaneously, the distribution of alloying elements in the thickness direction of the twin-roll cast sheet was the most uniform and the segregation was obviously suppressed. By virtue of the Lorentz force, the content of alloying elements in the matrix clearly increased, suggesting that such application of external fields can reduce segregation defects and improve the uniformity of the spatial distribution of alloying elements, ultimately improving the overall mechanical properties of AA6022 aluminum alloy.

Microstructure, tensile and fatigue properties of high strength Al 7075 alloy manufactured via twin-roll strip casting

High strength Al 7075 alloy was manufactured via twin-roll strip casting (TRC) process, and its microstructure, tensile and fatigue properties were investigated. Commercial Al 7075 alloy fabricated via direct-chill (DC) casting process was also used for comparison. T6 and T651 heat treatments were implemented for both the materials to precipitate MgZn2(η) phases. The TRC alloy showed globular grain shape with η phases evenly distributed at the grain boundary and matrix interior, while the DC alloy showed elongated grains due to hot forging process and η phases clumped together at grain boundaries. The TRC alloy’s yield strength was 518.5 MPa, tensile strength was 578.2 MPa, and elongation was 6.9%. Comparing both the alloys’ mechanical properties, the TRC alloy’s strength was at least about 40 MPa higher and elongation was about 4% lower than the DC alloy. The TRC alloy showed about 20 MPa higher fatigue (fatigue limit) than the DC alloy. The DC alloy was observed to have coarse cracks on fatigue fractured surface. By contrast, the TRC alloy showed uniform fractured surface without coarse cracks, and the evenly-distributed η phases improved fatigue resistance efficiently. The present study sought to examine the correlation among TRC process-led microstructure, tensile and high-cycle fatigue properties while discussing the strengthening mechanism.

Zr55Cu30Al10Ni5 amorphous alloy sheets with large plasticity fabricated by twin-roll strip casting

Zr55Cu30Al10Ni5 (at.%) amorphous sheets with thickness of ~0.5 mm and width of ~75 mm were successfully fabricated by twin-roll strip casting. The amorphous alloy sheets fabricated using a high rolling force exhibited a high-energy state, a high free-volume content, and structural inhomogeneity, which led to an engineering strain greater than 62% in compression at room temperature. This study provides an effective method for fabricating large amorphous alloy sheets with good plasticity.

Two-phase viscoplastic model for the simulation of twin roll casting

Abstract A two-phase Eulerian-Eulerian volume-averaged model was used to predict the outcome of the twin-roll casting process for inoculated Al-4 wt.% Cu alloys. The model is able to address the deformation of the mush during hot rolling by treating the solid/liquid mixture as a viscoplastic skeleton saturated with liquid. A parameter measuring the mean absolute deviation around the nominal composition was applied to the normalized macrosegregation distribution and the results were then used to determine an optimal process window where minimal compositional variation is achieved in the final strip. In the case of casting an 8 mm thick Al-4%Cu strip with rolls of 800 mm diameter, casting speeds between 40 and 42 mm/s and heat transfer rates given by HTCs between 5000 and 5250 W/m2/K minimize the macrosegregation deviations in the strip and thus predict an optimal strip quality.

Microstructure and Texture Evolution during Twin-Roll Casting and Annealing of a Mg–6.8Y2.5Zn–0.4Zr Alloy (WZ73)

In the present work, the microstructure and texture of a Mg–6.8Y–2.5Zn–0.4Zr sheet manufactured by twin-roll casting were investigated. The twin-roll cast state consisted of two apparent phases: the α-Mg matrix, which was made up of dobulites with an average grain size of approximately 50 µm and the LPSO (long-period stacking ordered) phase, which formed network-like precipitates along the grain boundaries. After twin-roll casting, annealing was carried out under conditions of different temperatures ranging from 450 °C to 525 °C and holding times between 2 h and 24 h. It was found that heat treatment led to the formation of a microstructure in which grains were apparent. Furthermore, it could be observed that high temperatures > 500 °C led to changes in the morphology of the LPSO structures. On one hand, the network-like structure dissolved while, on the other hand, both rodlike and blocky LPSO phases precipitated predominantly at the grain boundaries of the α-Mg matrix. This process was fostered by high temperatures and long holding times.

Effects of annealing temperature on the interfacial microstructure and bonding strength of Cu/Al clad sheets produced by twin-roll casting and rolling

Abstract In this paper, Cu/Al clad sheets were prepared by twin-roll casting and multi-pass cold rolling. Annealing treatment was performed at temperatures of 250 °C, 300 °C, 350 °C and 400 °C, separately. The influences of the annealing temperature on the interface layer structure, peel strength and peeled surface microstructure of the clad sheets were studied. The crack propagation behavior in the peeling process of the clad sheets was revealed, and the strengthening mechanism of the interface was explained. The results showed that the Cu/Al interface metallurgical bonding rate increased and the intermetallic compound layer thickness could be controlled within 550 nm after annealing at 250 °C, which encouraged more cracks to propagate along the Al matrix, and the average peel strength could reach approximately 39 N/mm. However, after high-temperature annealing treatment (350 °C and 400 °C), the clad sheet broke completely in the intermetallic compound layer, and a large number of cleavage fractures appeared, which caused the bonding strength to decrease sharply.

Size effects of Y-enriched intermetallic precipitates in Fe-20Cr-5Al-0.25Y alloy on the spallation resistance of thermally grown α-Al2O3 scales

Standard ingot casting and twin-roll strip casting (TRSC) were used to process Fe-20Cr-5Al-0.25Y (wt.%) alloy, in which Y-enriched intermetallic precipitates with different sizes had been formed. Cyclic oxidation tests were carried out at 1200℃ in air, and the size effects of precipitates in alloy on the spallation resistance of thermally grown α-Al2O3 scales were investigated. The results clearly demonstrated that the spallation resistance of α-Al2O3 scales could be improved by refining the Y-enriched precipitates, which was attributed to the fact that the small precipitates can generate high density of interfacial oxides to effectively key the alumina scale to the substrate.

Burr Formation in Casting Using Vertical-Type High-Speed Twin-Roll Caster

The mechanism and region of burr formation in casting using a vertical-type high-speed twin-roll caster were investigated using a side-dam plate with a window. A383, an aluminum alloy typically used for casting due to its excellent fluidity at semisolid condition was used. The solid fraction of the semisolid metal ejected from the window was low. When solid metal was ejected from the window, no burr formed. The results suggest that a burr formed when the semisolid metal was ejected in gap between the side-dam plate and the side surface of a roll. The semisolid metal was ejected near the point where the semisolid metal on the two rolls met.

Secondary Recrystallization Behavior in Fe-3%Si Grain-oriented Silicon Steel Produced by Twin-roll Casting and Simplified Secondary Annealing

Grain-oriented silicon steels were produced by the shortest processing route involving twin-roll strip casting, two-stage cold rolling with intermediate annealing, and simulated continuous annealing. The secondary recrystallization behavior of grain-oriented silicon steels under different inhibition conditions was in-situ observed by combining the confocal laser scanning microscopy (CLSM) and electron backscattered diffraction (EBSD) techniques. The results revealed that the optimal temperature of secondary recrystallization showed a proportional relationship with the Zenner pinning force. In the case of weak pinning force, the abnormal grain growth occurred quickly at ~1050 °C. The corresponding growth rates were in the range of 60–1400 μm/min and decreased gradually as the secondary recrystallization proceeded. In the case of strong pinning force, the incubation time and onset temperature of the secondary recrystallization was significantly increased, but the total time of the secondary recrystallization was obviously shortened from 685 s to 479 s, and the final magnetic induction of B8 was increased from 1.7 T to 1.85 T. After the secondary annealing, some island grains and coarse primary grains were retained. The formation of island grain was related to the low migration of grain boundaries. The findings of coarse γ- grains indicated that the primary grain size also played a crucial role during secondary recrystallization, apart from the primary recrystallized texture, which attracted more attention previously.

Numerical and Experimental Research on Fluid Flow, Solidification, and Bonding Strength During the Twin-Roll Casting of Cu/Invar/Cu Clad Strips

A new twin-roll casting (TRC) process to produce Cu/Invar/Cu clad strips, which have greater application potential, is proposed. The symmetrical cast-rolling zone is divided into two asymmetrical parts by introducing the substrate strip in the middle. The thermal-flow coupled simulation was conducted, and it realized the coupling analysis of casting roll, molten pool, and substrate strip. The results indicate that the kissing point (KP) shape in each part is asymmetric and vortexes are more likely to occur near the KP. Besides, univariate analyses show that the influence of the cast-rolling velocity, substrate preheat temperature and casting temperature on the KP length and average outlet temperature is linear, and the influence of the substrate thickness is nonlinear. Process window prediction models were obtained, which laid the foundation of setting process parameter combinations for providing the required KP length. Furthermore, numerical simulation results indicate that the fluidity of the liquid metal ensures the continuity of production. The macro-structures evolution in the cast-rolling zone indicates that deformation below the KP ensures the quality of the product. Finally, the Cu/Invar/Cu clad strips with metallurgical bonding were fabricated. The ultimate tensile strength and maximum peeling strength are 250 MPa and 126.5 N/mm, respectively. Hence, the TRC process for trimetallic clad strips is developed successfully and stability is basically achieved through equipment design, process window prediction, experimental validation, and bonding strength characterization. These methods can be conducive to the development of other new TRC processes for multiply clad strips.

Casting of High Aluminum Content AM Series Magnesium Alloys by Using a Horizontal Twin Roll Caster

In this paper, twin roll casting of magnesium alloys with high aluminum content such as, Mg-11 mass%Al-0.2 mass%Mn, Mg-12 mass%Al-0.2 mass%Mn, Mg-13 mass%Al-0.2 mass% have been performed for the purpose of use as an original material for hot forging. Also the mechanical properties of the cast materials were examined. A 10 miri-meters thick strip was cast by the use of a horizontal twin roll caster. The microscopic observation was conducted to investigate into the precipitation of the metal compounds such as Mg17Al12, and the Vickers hardness of the cast strips test were performed. From the result of the roll casting experiments, a 10 mm thick strip was continuously cast at a roll speed of 3.1 m/min. The average grain size of the casting strips was about 46 micron meters. When the aluminum content was 13%, the hardness of the twin roll cast (TRC) strips became 1.7 times higher than that of extruded AZ 31. Also, a uniaxial compression test at elevated temperature was conducted to obtain a true strain-true stress curve for examining possibility of direct hot forging (DHF) of TRC magnesium alloys with high aluminum content.

Modelling viscoplastic behavior of solidifying shell under applied electromagnetic breaking during continuous casting

Since several decades the continuous casting (CC) process became one of the dominant technologies for the metal production. The quality optimization and the production rate growth are the primary targets. Nowadays the numerical modelling is a valuable tool to assist in these aims. Moreover, it efficiently competes with the physical experiment and the industrial trials. One of the key issues with the high casting speeds especially for the thin slab products are the strong turbulent flow of the fresh melt being feed from the submerged entry nozzle (SEN) and the non-uniformity of the solidifying shell thickness. Thereby the electromagnetic braking (EMBr) is typically applied to damp the hot jets and to evenly redistribute the superheat. In the previous work it was shown by the authors that the presence of the highly conductive solid shell plays crucial role for the melt flow under the applied magnetic field. Excluding this interaction does not allow predicting the EMBr effects on the melt flow correctly during the solidification. Recently a viscoplastic deformation model was implemented to model a withdrawal of the solidified shell in a funnel type CC mold for a full 3D engineering geometry. An extended model was used to predict the macro-segregation during twin-roll casting of an Al-alloy using 2D assumption due to the large width / thickness ratio of the casted sheet. In the current study an effort is done to combine the viscoplastic deformation model of the solidified shell and the magnetohydrodynamics effects of the EMBr. The flow field alternation as well as the thickness of the solidified shell during CC are presented and analyzed with and without the magnetic field been applied.

Evolution of shear texture during the asymmetric rolling and its annealing behavior in a twin-roll casting AA6016 sheet: an ex-situ electron backscatter diffraction study

To better understand the enhanced formability of Al–Mg–Si alloy automotive outer panels by introducing shear texture F{111}<112> orientations, we investigate the shear texture evolution of a twin-roll casting AA6016 sheet with an initial R-cube and E texture during asymmetric rolling (ASR). The macro-textures of the sheet with a deformation of 30%, 50%, and 85% are studied using X-ray diffraction. Results show that the initial R-cube evolved to Dillamore/copper with the increase in strain, whereas the initial E texture is weakened accompanying the enhancement of the F texture during ASR. Moreover, the local recrystallization textures are analyzed using the sheet that has been annealed at 330°C at different times via the ex-situ electron backscatter diffraction. The F texture is dominant for the ASR sheet, which could be attributed to the rapid nucleation and grain growth of grains with F orientation (F-grains). Furthermore, the spatial relationship at 19.7° [1¯10] between the grain with copper orientation and F-grains is another important reason to promote the F orientation development. Finally, the formability of the ASR sheet is found to significantly improve.

Evolution of microstructure, texture, and mechanical properties in a twin-roll cast AA6016 sheet after asymmetric rolling with various velocity ratios between top and bottom rolls

In this paper, we report the evolution of microstructure, texture, and mechanical properties of an AA6016 sheet prepared by twin-roll casting and subsequent asymmetric rolling (ASR). By controlling the velocity ratio (VR) of upper and lower rollers, it is determined that a strong shear strain is induced along the sheet thickness during the ASR process, which results in the formation of shear texture with a {111}//normal direction (ND). When the deformation sheets are annealed at 330 °C, the number and size of grain nuclei are increased with an increase in VR after partial recrystallization. When the sheets are completely recrystallized, different recrystallization mechanisms in symmetrically rolled (SR) and ASR sheets can be determined. The main recrystallization mechanism of the SR sheet is characterized as discontinuous recrystallization (DRX), while continuous recrystallization (CRX) contributes to the development of recrystallization in the ASR sheet, which is accompanied by the formation of finer-grain-band/ultrafine grains. Moreover, compared to that of annealed SR sheets, the shear texture ({111}//ND) of the annealed ASR sheet is enhanced during the nucleation and grain growth of recrystallization while the cube texture weakens, which results in a distinct increase in the Lankford coefficient for the annealed ASR sheet.

Fatigue modeling for wrought magnesium structures with various fatigue parameters and the concept of highly strained volume

Mechanical fatigue tests of unnotched, notched, and bending twin-roll cast AZ31B magnesium alloy specimens are performed in which strain fields are analyzed with digital image correlation. Clearly, delimited macroscopic bands of twinned grains (BTGs) in which the compressive strain is significantly higher compared to the adjacent regions are observed. Conventional fatigue parameters, e.g., the strain amplitude, exhibited higher values within the BTGs. This findings are confirmed by the fact that for all investigated specimens the initial macroscopic cracks are observed within the BTGs. Consequently, for the presented concept of highly strained volume, fatigue parameters are determined from the highly strained regions with high strain amplitudes. This paper focuses on the application of the effective strain amplitude fatigue parameter decomposed in an elastic and plastic portion, the Smith-Watson-Topper fatigue parameter and energy-based fatigue parameters within the concept of highly strained volume. An extended stress–strain hysteresis model is presented to compute stress–strain hystereses for arbitrary load ratios, required to determine the mentioned fatigue parameters. The application and evaluation of five different fatigue parameters within the concept of highly strained volume demonstrates the accurate description of the fatigue life until failure.

Fracture criterion for predicting edge-cracking in Hot rolling of twin-roll casted AZ31 Mg alloy

Edge-cracking as one of main defects for rolled Mg alloy sheets is inevitable due to the combined characteristics of the material and process. This study aims to quantitatively analyze the damage distributions induced by rolling deformation in transverse direction (TD) for twin-roll casted (TRCed) AZ31 Mg alloy, and optimize the Freudenthal fracture criterion to predict the crack initiation and propagation during the hot rolling. FEM simulations and laboratory experiments of edge-cracking behaviors during the rolling process were carried out at temperatures of 250∼400 °C and reduction rates of 30%∼45%. Overall cracking indexes, including temperature- and reduction-dependent transverse depth of edge-cracking, distributions of damage based on the Freudenthal criterion, and critical damage values to fracture were discussed. Results show that the fracture mode of edge cracks is dominated by 45° crossed shear cracking in the longitudinal (RD–ND) section of rolled sheets, and the cracking depth along the TD increases as the rolling temperature decreases and reduction rate increases. The transverse damage decreases linearly from the edge to the interior during the rolling when using the Freudenthal criterion, which can be attributed to the combined effects of temperature difference and effective stress difference along the TD. The critical damage value of Freudenthal criterion was successfully obtained for establishing the criterion for edge-cracking prediction in AZ31 Mg alloy rolling, which is within the range of 0.59∼0.88 and has a clear linear relationship with Ln(Zener-Hollomon parameter, Z).

A Novel Approach to Improve the Microstructure and Mechanical Properties of Al-Mg-Si Aluminum Alloys during Twin-Roll Casting.

The main purpose of this present study was to investigate the different processing conditions on the microstructure, segregation behavior of alloying elements, and mechanical properties of Al−Mg−Si alloy twin-roll cast slab prepared using a novel twin-roll casting technology. The simulation of temperature field, distribution of alloying elements, tensile properties, hardness, and conductivity were examined by a Leica optical microscope, scanning electron microscopy, energy dispersion spectroscopy, electron probe microanalysis, and tensile tests. The results indicated that when the traditional twin-roll casting method was used to produce aluminum alloy strip, there are obvious centerline segregation defects due to the deep crystallization front depth and symmetrical solidification characteristics. When the forced-cooling technology was applied in the twin-roll casting process, by virtue of the changing of crystallization front depth and crystallization front shape, the segregation defects are obviously suppressed. Suggesting that this method can significantly improve the uniformity of alloying elements in the thickness direction of the twin-roll cast slab, ultimately improve the mechanical properties of AA6022 aluminum alloy.

Research on cast-rolling force in twin-roll continuous strip casting based on uncertainty analysis

The uncertainties of various parameters, including casting temperature, rolling velocity and roll gap are investigated in twin-roll continuous strip casting process for S304 stainless steel. A sample-based stochastic model based on Latin hypercube sampling is established to evaluate the influence of various uncertain parameters on the height of Kiss Point and the cast-rolling force. A polynomial mathematical model combined with numerical simulation is built with full factorial design approach. The convergence of variance is used to represent the variability of the input parameters, and Inter Quartile Range (IQR) is adopted to qualify the uncertainty of output parameters. The IQR analysis shows that casting temperature is the most dominant factor among all output parameters including roll gap and rolling velocity.

Revealing the recrystallization behavior of an excellent strip casting 4.5 wt% Si non-oriented electrical steel

An excellent 4.5 wt%Si non-oriented electrical steel was fabricated by warm rolling and isothermal annealing of twin-roll cast strip. The detailed evolution of microstructure and texture was studied using the combination of electron back-scattering diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscope (TEM) for revealing the recrystallization behavior. The favorable {100}〈013〉 nuclei originate in the shear bands within deformed {111}⟨11-2⟩ and {110}⟨1-10⟩ grains. Based on the EBSD analysis, oriented nucleation is the predominant mechanism for the formation of strong {100}⟨013⟩ texture. The rearrangement of dislocations and cell transformation to sub-grains were characterized. The mechanisms of (sub-)grain coalescence, as well as strain induced grain boundary migration, take effect simultaneously, during the nucleation.