Twin-roll Casting Research Articles

Development of microstructure of asymmetrically rolled AA3003 aluminium sheets with Zr addition

Asymmetric rolling is a method of severe plastic deformation, which can produce large volumes of deformed material and is easy to implement for industrial use. Asymmetric rolling with different circumferential velocities of rolls gives rise to intense plastic shear strains, resulting in grain refinement and improving mechanical properties. An aluminium alloy from AA3003 series enhanced by 0.16 wt% Zr was produced by twin-roll casting in industrial conditions. A part of specimens was annealed at 450∘C in order to precipitate fine dispersion of Al3Zr particles, which may enhance recrystallization resistance at elevated temperatures. Materials were subjected to cold rolling and to asymmetric rolling in several steps. Both methods result in a substantial elongation and fragmentation of grains. The grain refinement is more distinctive after asymmetric rolling. During isochronal annealing Vickers microhardness of materials decreases. The main drop occurs in the asymmetrically rolled material between 400 and 450∘C. In the annealed material the main drop connected with recrystallization is shifted by 50∘C to higher temperatures due to the presence of Al3Zr particles. Recrystallization occurs in the cold-rolled material between 450 and 500∘C. The imposed deformation by rolling is lower in this specimen and hence the driving force for recrystallization is also lower than in the asymmetrically rolled material. After recrystallization, the grain size of both cold-rolled and asymmetrically rolled materials is comparable. However, grains are still elongated in the rolling direction.

John David Hunt. 12 December 1936—8 December 2012

John Hunt was a scholar of the solidification of metallic alloys, and published seminal works that are of influence today in the design of alloy casting, welding and additive manufacturing processes. John was raised in the West Country of England, and following school did National Service in the Royal Air Force. He entered the University of Cambridge in 1957, being awarded an honours BA in metallurgy in 1960, and a PhD for studies of solidification of eutectic alloys in 1963. John met Ann Carroll during his Cambridge studies; they married in 1961 and later had three children. From 1963 to 1965 John carried out research with Ken Jackson at Bell Laboratories, New Jersey, USA. They published groundbreaking papers on solidification and pioneered the use of transparent organic analogues. John returned to England and, following a year working at the UK Atomic Energy Authority at Harwell, he joined the faculty at the University of Oxford in 1966. During a 36-year academic career at Oxford, John made significant contributions to the scientific understanding of solidification on many topics, including dendritic array growth, eutectic and peritectic solidification, the columnar-to-equiaxed transition and twin roll casting. As well as being a talented experimentalist, he published many groundbreaking theoretical analyses and also pioneered the use of numerical methods for the simulation of solidification phenomena. He was an accomplished educator of undergraduate and postgraduate students, and a fellow of St Edmund Hall, Oxford. John was a family man, and kept geese and pigs, grew apples and made legendary cider at his old farmhouse home north of Oxford.

Effects of annealing on bonding strengthening mechanism of Ti/Al composite sheets by twin-roll casting with roller vibration

Effects of annealing on bonding strengthening mechanism of Ti/Al composite sheets by twin-roll casting with roller vibration

Strength and ductility enhancement of twin-roll cast Al-Zn-Mg-Cu alloys with high solidification intervals through a synergistic segregation-controlling strategy

Center segregation and banded intergranular segregation (B-IGS) should be well controlled to improve the mechanical properties of twin-roll cast (TRC) aluminum alloys, especially for alloys with high solidification intervals. In the present work, a synergistic grain refinement strategy was designed using an Al-5Ti-B grain refiner and Ti, Zr, and Sc microalloying elements to simultaneously control center segregation and B-IGS in TRC Al-Zn-Mg-Cu alloys. As the grain size decreased, center segregation defects were eliminated and transformed into dispersed B-IGS defects; simultaneously, the width and length of the B-IGS were also reduced. Moreover, the macro-distributions of the alloying elements along the thickness direction became more homogeneous due to a weak shear-induced dilation effect. The well-controlled multiscale segregation improved the uniformity of the alloy macrostructure, accelerated the redissolution of the crystalline phase, dispersed the aggregated residual phase, and refined grains under the T6 state. Hence, the strength and ductility of the alloys under the T6 state were simultaneously improved, and the hardness distribution along the thickness direction became more homogenous. Furthermore, the underlying mechanisms of segregation evolution and strength and ductility enhancements were elucidated. This work provides a novel strategy to effectively control multiscale segregation and produce high-performance aluminum alloys with high solidification intervals by TRC.

Microstructure evolution and precipitation behaviour of the Cu-Ni-Si-Cr alloy based on twin-roll strip casting

ABSTRACT The twin-roll strip casting (TRSC) process was introduced because of its sub-rapid solidification characteristics, and the element segregation and ageing precipitation of the Cu-6Ni-1.4Si-0.1Cr alloy were studied. The results are as follows: TRSC can effectively refine grain size and inhibit the segregation of the Cu-6Ni-1.4Si-0.1Cr alloy. Two types of precipitates are formed mainly during the ageing process: disk-shaped δ-Ni2Si and rod-shaped β-Ni3Si. The comprehensive properties of the alloy are improved, and the precipitation strengthening mechanism is either from the dislocation shearing mechanism or the Orowan mechanism in different stages of ageing. The Cu-6Ni-1.4Si-0.1Cr alloy achieves optimum performance after ageing at 350°C for 4 h, where the tensile strength and conductivity are 1112 MPa and 27.35% IACS, respectively. The Avrami equations were established for the alloy to describe the kinetics of phase transformation and electrical conductivity during ageing, and the calculated results are consistent with the experimental data.

Enhanced long-term thermal stability and mechanical properties of twin-roll cast Al–Mg–Si alloys with Mn and Cu additions

The long-term thermal stability of Al–Mg–Si alloy is of great significance for its engineering application. In this work, the effects of Mn and Cu on the microstructure and mechanical properties of the twin-roll cast (TRC) AA6005 alloy after peak aging and long-term thermal exposure were studied. The results show that the addition of Mn introduces Al(FeMn)Si dispersoids, which improves the mechanical properties of the alloy under the peak aging condition by dispersion strengthening. After thermal exposure at 150 °C for 1000 h, the Al(FeMn)Si dispersoids have good thermal stability while do not significantly improve the mechanical properties. The reason may be related to the decreased number density of the β″ phase after alloying with Mn. The addition of Cu changes the aging precipitation sequence, and the needle-like pre-Q′ phase is formed during artificial aging. Moreover, due to the low diffusion rate of Cu in the Al matrix, Cu atoms can better stabilize the structure and then improve the coarsening resistance of the precipitates, and therefore the Cu-containing precipitates exhibit better thermal stability than the β″ phase. After thermal exposure at 150 °C for 1000 h, the yield strength of the 0.5Cu alloy decreases by only ∼12 MPa (from ∼295 to ∼283 MPa), which is significantly lower than that of the AA6005 alloy (∼41 MPa, from ∼274 to ∼233 MPa). The yield strength of the 0.25Mn0.25Cu alloy decreases by ∼27 MPa (from ∼294 to ∼267 MPa) and is more suitable for industrial production as its composition is within the range of commercial 6xxx aluminum alloys. The results would provide a strategy for the preparation of Al–Mg–Si alloys with high thermal stability.

Correlation of microstructure, texture and intermetallic particles during multi-step cross-rolling and annealing in a strip-cast AA8011 alloy

In multi-step cross rolling (MSCR), twin roll cast (TRC)/strip-cast AA8011 aluminum sheet was cold-rolled up to 72 % thickness reduction by rotating +/- 90º with respect to normal direction (ND) after each rolling pass. The intermetallics particle size after MSCR got reduced to ∼ 0.5–3 µm compared to unidirectional cold rolling (UCR) ∼ 1.5 – 4.5 µm (Kumar et al. 2018) after the same thickness reduction of 72 %. Subsequently, it was annealed at 375 ºC for varying times (1–240 min). Both investigations (UCR/MSCR) showed no removal of centerline segregation (CLS) of Al-Fe-Si intermetallics. After 72 % cold rolling (UCR/MSCR), deformation texture (i.e., Brass (Bs) {110} < 112 > , copper (Cu) {112} < 111 > , Goss {110} < 011 > and S {123} < 634 > ) strengthened at the expense of Cube {100} < 001 > . However, cube improves during annealing at 375 ºC after MSCR than UCR. Moreover, it was observed that particle-stimulated nucleation (PSN) (intermetallics particle size ≥ 2 µm) played an important role in promoting random texture. For instance, the % volume fraction of Brass, Copper, Goss, S, Cube were 6.1 %, 5.3 %, 3.7 %, 9.0 %, 4.8 % (Kumar et al. 2018) and 6.5 %, 3.6 %, 1.9 %, 7.7 %, 5.3 % for UCR and MSCR, respectively. Hence, MSCR effectively reduces intermetallic particles size, decreases the overall anisotropy and improves the recrystallization texture (especially Cube component of texture) of TRC AA8011 aluminum alloy.

The effect of the TiC-TiB2 nanoparticles on the microstructure and mechanical properties of Al-Mn-Si strips fabricated by twin-roll casting

In this work, Al-Mn-Si and TiC-TiB2/Al-Mn-Si strips were prepared by vertical-type twin-roll casting (TRC), and the effect of TiC-TiB2 nanoparticles (NPs) on the microstructure and mechanical properties of TRC Al-Mn-Si strips were studied. The TRC strips had the characteristics of coarse grains near the surface and fine grains in the center. The introduction of NPs resulted in the formation of uniform and fine grain structures due to the grain refinement effect of NPs. The refining effect on the surface grains was much greater than that on the central grains because the larger thermal undercooling of the melt at the surface led to the improvement of the nucleation efficiency of NPs. TiC-TiB2 particles could eliminate the central segregation of TRC Al-Mn-Si strips due to the control of solute diffusion by the uniform and fine grain structures, thereby reducing α-Al(Mn, Fe)Si formation in the central region. The mechanical properties of the TRC Al-Mn-Si strips were improved with the introduction of NPs. The yield strength at room temperature was increased from 79.0 MPa to 92.9 MPa, and the uniform elongation was increased from 7.2% to 10.1%, due to the fewer eutectic α-Al(Mn, Fe)Si, refined precipitates and grain structures caused by the addition of the TiC-TiB2 particles.

In vivo degradation and bone reaction of long-term fixation with a magnesium alloy made by twin-roll casting in a rat femur model.

The effect of casting parameters on the microstructure and corrosion resistance of Mg alloys is still limited, especially in clinical animal experiments. We prepared a new magnesium rare earth alloy (Mg-Re, where Re is Ce or La) by vertical two-roll casting and Mg-A by further rolling. The microstructure characteristics, degradation behavior, and bone reaction of the two alloys were studied. Ti, Mg-Re, and Mg-A alloy plates were implanted in a rat femur model, and their degradation behavior was observed 48 weeks later. In vivo experiments showed no significant changes around the femur in the Ti group, excluding external factors that may cause bone remodeling and lead to new bone formation. Mg-A induces more new bone formation than Mg-Re, which meets the necessary conditions to prevent pathological fracture. The specimen staining and sectioning showed that the liver and heart of rats implanted with magnesium alloys had no pathological changes and the cell structure was normal, similar to that of rats without a magnesium alloy. Mg-A alloy has good healing potential as a biodegradable implant material.

Solidification microstructure of Ti-43Al alloy by twin-roll strip casting

Solidification microstructure of Ti-43Al alloy by twin-roll strip casting

Phase Transformation Behaviors of Medium Carbon Steels Produced by Twin Roll Casting and Compact Strip Production Processes

Medium carbon steels have been widely used in the fields of tool and die manufacturing due to their outstanding hardness and wear resistance. In this study, microstructures of 50# steel strips fabricated by twin roll casting (TRC) and compact strip production (CSP) processes were analyzed to investigate the influences of solidification cooling rate, rolling reduction, and coiling temperature on composition segregation, decarburization, and pearlitic phase transformation. The results show that a partial decarburization layer with a thickness of 13.3 μm and banded C-Mn segregation were observed in the 50# steel produced by CSP, leading to the banded distributions of ferrite and pearlite in the C-Mn poor regions and C-Mn rich regions, respectively. For the steel fabricated by TRC, owing to the sub-rapid solidification cooling rate and short processing time at high temperatures, neither apparent C-Mn segregation nor decarburization was observed. In addition, the steel strip fabricated by TRC has higher pearlite volume fractions, larger pearlite nodule sizes, smaller pearlite colony sizes and interlamellar spacings due to the co-influence of larger prior austenite grain size and lower coiling temperatures. The alleviated segregation, eliminated decarburization and large volume fraction of pearlite render TRC a promising process for medium carbon steel production.

Effect of cooling rate on the order degree, residual stress, and room temperature mechanical properties of Fe-6.5wt.%Si alloy

The top side-pouring twin-roll casting (TSTRC) process was used to prepare a 100 × 1.7 mm non-oriented Fe-6.5 wt% Si alloy as-cast strip. The as-cast strip is directly rolled at 700 °C into 0.4 mm without the intermediate hot rolling step. This paper discusses the effects of different cooling rates in furnace cooling, air cooling, oil quenching, and water quenching on the order degree, residual stress, room temperature mechanical properties, and cold-rolled processing properties of Fe-6.5wt.%Si alloy after annealing of warm-rolled sheets are systematically investigated. Meanwhile, the effects of order degree, residual stress, and grain boundary element segregation on the mechanical properties were analyzed. The results show that as the cooling rate increases, the order degree of the Fe-6.5 wt% Si alloy decreases and the maximum residual tensile stress increases. The fracture modes of the furnace cooling, air cooling, and oil quenching samples are through-crystal disintegration fracture, and the fracture mode of the water quenching samples is an along-crystal fracture. The oil-quenched samples exhibit good room temperature plasticity due to the relatively low order degree and residual tensile stresses in the Fe-6.5wt.%Si alloy. Oil quenching improves the room temperature plasticity and cold rolling properties of Fe-6.5wt.%Si alloy warm rolled sheets, reduces edge cracks, and offers greater room temperature cold rolling processibility of the Fe-6.5wt.%Si alloy warm rolled sheets.

Effect of Cu content on central-segregation composition and mechanical properties of Al–Mg–Si alloys produced by twin-roll casting

Dissolving the inherent central segregation is critical to improve the performance of twin-roll casting (TRC) aluminum strips. In this study, we investigated the effects of Cu content (0.8, 1.2, and 1.6 wt%) on the central-segregation composition and mechanical properties of TRC Al–0.8Mg–1.0Si alloys. Cu promoted the segregation-phase transformation from the higher-melting-temperature Mg2Si phase to the lower-melting-temperature Cu-containing phases (Q-AlCuMgSi and θ-Al2Cu), resulting in an easier dissolution of central segregation. Therefore, the segregation of 0.8 and 1.2 wt% Cu alloys was almost dissolved following cold rolling and solution treatment compared to Cu-free alloys. However, after a higher content of Cu (1.6 wt%) was added, the segregation became too severe to dissolve. After modifying with 1.2 wt% Cu, the yield strength (YS) of the TRC alloy increased from ∼306 to 335 MPa, and the elongation was increased from ∼13 to 17%. The contribution of solution strengthening and increasing volume fraction of precipitates results in the strengthening effect of Cu. Meanwhile, the dissolution of segregated phases is also advantageous for solution and precipitation strengthening. This study could lead to the development of a simple method to dissolve TRC central segregation and produce TRC Al–Mg–Si alloys with excellent performance.

Effects of sinusoidal vibration of casting-roller on interface microstructure of Ti/Al laminated composites by twin-roll casting

A new vibration cast-rolling technology of “electromagnetic stirring + dendrite breaking + asynchronous rolling” was proposed with the adoption of sinusoidal vibration of the casting-roller to prepare Ti/Al laminated composites, and the effects of sinusoidal vibration of casting-roller on the interfacial microstructure were investigated in detail. In the interface of traditional cast-rolling materials, the ridge-shaped aluminum materials extend in a straight line or grow transversely into some tiny ridge-shaped branches which look like a quasi-cleavage fracture. With the enhancement of the casting-roller vibration, a large area of hilly aluminum materials in the interface is bonded or present equiaxed dimples gathering to form honeycomb network morphology, which presents ductile fracture characteristics. The vein patterns and morphology differentiation in the interface show that the density and uniformity of composite intensity distribution can be enhanced by increasing the vibration frequency, but the level of enhancement is not that much obvious. The hilly morphology and equiaxed dimples in the interface show that the unit composite strength can be enhanced by increasing the vibration amplitude, while the density of strength distribution is sparse. Under the combined effect of “amplitude + frequency,” the toughness and composite quality of the interface are enhanced by the uniform superposition of the discrete strengths. The area ratio of aluminum bonded to titanium side interface (81%) is 3 times higher than that of traditional cast-rolling material (22%) on the condition of ductile fracture of cast-rolling material with strong vibration (amplitude 0.87 mm, vibration frequency 25 Hz) of casting-roller, and the bonding strength (27.0 N/mm) is twice as high as that of traditional cast-rolling materials (14.9 N/mm), which indicates that the ductile connection between the interface layers can be greatly enhanced by vibration roll-casting.

Effect of Homogenization Heat Treatment on Elongation Anisotropy in Cold-Rolled and Annealed Al–Si Alloy Sheets Fabricated from Vertical-Type High-Speed Twin-Roll Cast Strips

Effect of Homogenization Heat Treatment on Elongation Anisotropy in Cold-Rolled and Annealed Al–Si Alloy Sheets Fabricated from Vertical-Type High-Speed Twin-Roll Cast Strips

Towards relieving center segregation in twin-roll cast Al-Mg-Si-Cu strips by controlling the thermal-mechanical process

Serious center segregation greatly limits the application of twin-roll casting (TRC) technology for producing 6xxx alloy strips. Herein, Al-0.9Mg-0.6Si-0.2Cu-0.1Fe (wt.%, 6061) strips with different thicknesses were fabricated by TRC, and we found that the center segregation was well relieved with the thickness increased from 3 mm to 4 mm. To reveal the mechanisms of mitigation of center segregation in the 4 mm strip, various techniques including solidification simulation, crystallographic calculation, electron backscatter diffraction (EBSD), and electron probe micro-analyzer (EPMA) were utilized. The results disclosed that the Fe-containing phase in the 3 mm strip was π-AlFeMgSi, while the counterpart in the 4 mm strip was α-AlFeSi. The α-AlFeSi could serve as nucleation substrates for Mg2Si and Q-AlCuMgSi phases, thus promoting the uniform distribution of elements and preventing the accumulation of phases in the center region. Three matching planes between the α-AlFeSi and Q/Mg2Si were examined as: (112¯0)α-AlFeSi // (0001)Q, (0001)α-AlFeSi // (110)Mg2Si, and (112¯0)α-AlFeSi // (11¯0)Mg2Si. Meanwhile, the smaller roll separating force during the TRC process in the 4 mm strip could weaken the force-induced liquid flow behavior in the semi-solid region, which is the other reason for the alleviation of center segregation. Owing to the elimination of the center segregation, a more excellent fracture elongation was achieved in the as-homogenized 4 mm strip (∼29%) compared with the counterpart of the 3 mm strip (∼20%). This work may provide a strategy to eliminate the center segregation, thus further promoting the application of TRC process and producing high-performance Al alloy strips efficiently.

Generalized stability criterion for controlling solidification segregation upon twin-roll casting

• Systematical framework integrating dendritic growth model, overall solidification kinetic model and generalized stability was constructed. • Processing parameters of the casting speed and the flow speed of cooling water in twin-roll casting are coupled. • Sub-rapid solidification kinetics, concentration and segregation by different processing parameters couple were established. • New strategy was proposed: high initial driving force suppresses centerline segregation, and high final generalized stability suppresses edge segregation. Macro- and micro-segregation formed upon twin-roll casting (TRC) can be inherited from sub-rapid solidification to solid-state transformation, even to plastic deformation, thus deteriorating drastically mechanical properties of as-produced thin sheets. Although many works focusing mainly on controlling fields of thermal, concentration and convection have been reported, how to control artificially and quantitatively the segregation using a theoretical connection between processing parameters and solidification models, has not been realized, yet. Regarding it, a systematical framework integrating non-equilibrium dendritic growth and overall solidification kinetics with the TRC parameters, was constructed applying a generalized stability (GS) conception deduced from transient thermodynamic driving force Δ G t and transient kinetic energy barrier Q eff t evolving upon solidification. Departing from this framework considering synergy of thermodynamics and kinetics (i.e., thermo-kinetic synergy), a criterion of high Δ G t -high GS guaranteed that the macro (i.e., the centerline) and the micro (i.e., the edge) segregation can be suppressed by increasing Δ G t and GS at the beginning and the ending stage of sub-rapid solidification, respectively. This typical thermo-kinetic combination producing the microstructure can be inherited into the plastic deformation, as reflected by corresponding strength-ductility combinations. This work realized quantitative controlling of TRC by a theoretical connection between processing parameters and solidification models, where, an optimization for sub-rapid solidification segregation using the GS conception including Δ G t and Q eff t has been performed.

Examination of conditions for producing thin sheets of aluminum alloy A6061 by vertical twin roll casting

Examination of conditions for producing thin sheets of aluminum alloy A6061 by vertical twin roll casting

A novel approach to improve the macro-segregation defect and mechanical properties of Al–Mg–Mn aluminum alloys during twin-roll continuous casting

In this paper, the microstructure and properties of Al–Mg–Mn alloy slabs with different thicknesses by twin-roll continuous casting were investigated. Under the twin-roll continuous casting process, the alloy melt conducts directional crystallization due to the near-horizontal heat transfer direction. In this way, the growth direction of the columnar crystal is changed to improve the shape and size of the sump and effectively eliminate the central segregation defect. Additionally, the improvement of segregation defects was more significant with the increase in thickness. As low angle grain boundary content and solution degree increased, the electrical conductivity decreased, the hardness increased, and the phenomenon of hard-soft-hard core-shell was significantly improved.

Numerical simulation and experimental research on cracking mechanism of twin-roll strip casting

Abstract Based on the three-dimensional field of molten pool and twin-roll strip casting experiments, this work verified the cracking mechanism of the strip by establishing mathematical model and rolling experiments. The results showed that due to the instability of the thermophysical field of the molten pool and the inconsistency of kiss curve height, the newly solidified strip will undergo incompatible deformation through the rolling. The stress concentration will appear around the large reduction area and then form slip bands. When plastic strain exceeds the limit of the metal, the oblique cracks will appear in the slip bands periodically or completely penetrate the strip. In addition, tensile cracks could also be produced by incompatible deformation. Therefore, keeping the uniformity and stability of the thermal physical field in molten pool is the key factor to restrain cracks.