Twin Roll Research Articles

Investigation of the microstructure and the influence of iron on the formation of Al8Mn5 particles in twin roll cast AZ31 magnesium alloy

The influence of iron on the formation of Al8Mn5 particles during twin roll casting of AZ31 magnesium alloy and the alloy microstructure has been investigated. Flower-shaped Al8Mn5 particles with sizes of 50–500nm were observed at the grain centres and the grain boundaries. Additionally, Fe-particles with faceted morphologies and dimensions of less than 100nm were observed at the centres of the Al8Mn5 particles, suggesting that the α-Fe particles acted as effective nucleation sites for the Al8Mn5 particles.

Properties of 15Ag2O∙70V2O5∙ 15P2O5 glass prepared by melt quenching, twin rollers and mechanosynthesis method

Three different methods – melt quenching (MQ), twin rollers (TR) and mechanosynthesis (MS) – were used for the preparation of the 15Ag2O·70V2O5∙15P2O5 glass — a good amorphous semiconductor. It was demonstrated that physical properties of the final material depend to some extent on the preparation method. As-received samples were characterized by: X-ray diffractometry (XRD), differential scanning calorimetry (DSC), Raman scattering, X-PDF method and impedance spectroscopy (IS) in a wide temperature range from −80 to 230°C. Main conclusion of these studies is, that substantially different synthesis routes can be used to produce glasses of almost identical thermal and structural properties but different electronic conductivity. The explanation of the origin of that difference was proposed and discussed.

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.

Structural investigation of amorphous Na2O–P2O5–B2O3 correlated with its ionic conductivity

A series of sodium borophosphate compounds was prepared by twin roller quenching technique. The compounds have the following compositions: (10B2O3–(90−x) P2O5−xNa2O) (BPNax) and (10Na2O–(90−x) P2O5−xB2O3) (NPBox) with x=0, 5, 10, 15, 20mol%. DTA and XRD are used to evidence the crystallization conditions.Change in the local structural environment has been investigated by Raman and by 11B, 31P MAS and QMAS NMR spectroscopies.The large 31P chemical shift of P3 and P2 units with increasing x and the decrease in 11B chemical shift of B4 units with decreasing x indicate that phosphorous is linked to boron through a BO. This feature reveals that the short-range order is amended in the network.Optical absorption was used to determine glassy energy gap Eg, which we used to calculate size polydispersity index and optical index.Electric and dielectric properties were investigated through impedance spectroscopy.For all compositions, variation in the temperature of dc conductivity was found to follow an Arrhenius type relationship. An improvement in the ionic conductivity was reported with the increase of both alkali and borate contents in the glass host.

Effect of melt conditioning on heat treatment and mechanical properties of AZ31 alloy strips produced by twin roll casting

EPSRC – LiME, UK and Towards Affordable, Closed-Loop Recyclable Future Low Carbon Vehicle Structures – TARF-LCV(EP/I038616/1), Department of Mechanical Engineering, Imperial College London, UK, Mr. Steve Cook, Mr. Peter Lloyd, Mr. Graham Mitchell and Mr. Carmelo and BCAST, Brunel University London.

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.

The effect of hot rolling on the microstructure, texture and mechanical properties of twin roll cast AZ31Mg

In this study, a commercially available twin roll cast AZ31 Mg sheet was hot rolled at 400°C to investigate the influence of rolling parameters (speed and percent thickness reduction) on microstructure, texture and mechanical properties. As compared to the twin roll cast condition, the microstructure of the successfully rolled materials was much finer and relatively less bimodal but contained profuse twinning and varying fractions of dynamically recrystallized grains. The extent of dynamic recrystallization (DRX) during rolling was found to be significantly influenced by percent thickness reduction. In the parameters investigated here, the rolling speed had a weak effect on DRX. The results showed that hot rolling strengthened the weak basal texture of the twin roll cast material and in addition to necklace grain formations at grain boundaries, DRX inside the twins was also identified as a key recrystallization mechanism. Mechanical testing revealed that at room temperature hot rolling improved the yield strength with retention of limited tensile ductility (<15%). However at 200°C, the rolled material exhibited more a balanced mechanical properties with large tensile elongations. Microstructural analysis of the failed tensile tested specimens showed that the deformation behavior was dependent on grain size and crystallographic texture.

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.

Physical Simulation of the Flow Field in a Vertical Twin Roll Strip Caster – A Water Model Study

Twin‐roll strip casting is a near‐net shape method for the production of metal strips in the range of several millimeters. The benefit in comparison to other methods is the short length of the facility and the lowered energy consumption in comparison to conventional continuous slab casting. This results in a reduction of costs and large energy savings by up to 85%. In this work the flow inside the pool of a twin roll strip caster is investigated. A modular submerged entry nozzle (SEN) was designed, which allows an adjustment of the flow for the single submerged faces of the SEN in the pool. This makes it possible to control the flow in several ways, to create different flow patterns. The measurements were made for a steady state. For the measurements in the pool the particle image velocimetry method (PIV) was used to determine two‐dimensional velocity fields. The surface motion at the rolls was investigated by video analysis. It was shown that this experimental setup is suitable to create several characteristic flow patterns inside the pool by combining the three main flows (front, long, and bottom). Also, it was obtained that high volume flows through the front chambers create a very uneven meniscus with large differences in height. The best result correlated to the meniscus was achieved for a medium volume flow at the long chambers and a high volume flow through the bottom.

Dynamic Recrystallization Behaviour of Twin Roll Cast AZ31 Strips during Hot Deformation

The recrystallization behaviour of a Twin Roll Cast AZ31 alloy, under deformation conditions corresponding to the initial stands of a magnesium hot rolling process, has been investigated. A set of strain levels, beginning at the critical strain for the onset of dynamic recrystallization has been studied. The dynamic recrystallization, taking place during hot deformation of magnesium alloy AZ31 is analyzed with help of plane strain compression tests conducted together with microscopic examination. Dynamically recrystallized volume fraction and size of dynamically recrystallized grains increase with increasing strain. When the strain is constant, the dynamically recrystallized volume fraction is higher at higher deformation temperatures or lower strain rates. Furthermore, it has been found out that deformation temperature plays a major role, rather than strain rates, in activating the dynamic recrystallization process during deformation. The results were used to identify the coefficients for the JMAK-model approach on the dynamic recrystallization of an AZ31 TRC-sheet.

Asymmetric cast-rolling of 1050 aluminum alloy strip under multi-energy field

Technological parameters of asymmetric cast-rolling under multi-energy field were investigated on horizontal twin roll caster (d400 mm×500 mm), and their effects on structures and properties of 1050 strips were analyzed by comparing with traditional cast-rolling. Results show that when length of cast-rolling area is 70 mm, melt temperature of head box is 670 °C, cast rolling speed is 1.3 m/min, exciting current is 10 A, center frequency is (13±1) Hz, ultrasonic power is 200 W and ultrasonic frequency is (20±0.2) kHz, the 1050 strip with the best microstructure can be prepared successfully; its center segregated layer disappears; the average grain size is reduced by about 40%; the crystal grains are distributed evenly; micro segregation decreases obviously; the precipitated phases are distributed along the grain boundaries evenly; and the tensile strength, yield strength, elongation and micro-hardness of cast-rolled strip are improved by 22.6%, 23.66%, 38.75% and 9.90%, respectively.

Effect of Inter-Metallic Phases on the Bonding Strength and Forming Properties of Al/Mg Sandwiched Composite

Composite sheet of Twin Roll Cast (TRC) AZ31-Mg alloy and industrial pure Al-1050 was fabricated as Al/Mg/Al with a hot roll bonding process. As the diffusion zone at the interface between the layers plays a crucial role in the formation of bonding strength and formability of clad sheets. It is important to describe the processes of inter-diffusion layer generation in order to assess the abilities of the laminate composites for further processing. Thus, the aim of this study was to investigate the development of the bonding strength, microstructure and mechanical properties of the bonding interface directly after roll bonding and additionally post-annealing processes. Microstructure characterization techniques such as optical and scanning electron microscopy including energy dispersive X-ray analysis (EDX) were applied to investigate the bonding area. No inter-metallic phases were present directly after the roll bonding process. The creation and growth of Al2Mg3 and Al12Mg17 phases were apparent after annealing at temperatures of 250 to 400 °C at different times ranging from five to 120 minutes. The results prove that the growth rate of inter-metallic phases increases considerably with an increase in the annealing temperature. The micro-hardness of the interface-area was also investigated. It was observed that the two inter-metallic phases were significantly harder than the substrate Mg and Al. In order to examine the influence of the inter-metallic phases on the resulting bonding strength after the annealing process, the shear bonding strength test has been conducted for different samples that were annealed at different heat treatment conditions. The results indicate that the optimum annealing temperature is 200 °C leading to a maximum bonding strength. Moreover, the mechanical properties of the composite after roll bonding and post annealing were determined by means of room temperature tensile test. The fracture mechanism after tensile test was also discussed.Keywords:Rollbonding,Al/Mghotrollbonding,TwinrollcastAZ31,bondingstrength

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.

Solidification of Aluminum Copper Alloy during Twin Roll Strip Casting

Twin roll casting is an innovative route to produce thin metal strip directly from the melt. In this paper, solidification characteristics in roll strip cast aluminum alloys is described. Al-Mg-Si alloy was melted in a crucible and poured to stainless steel roller. The roll was connected to a driving motor which the rotation speed can vary from 10 rpm to 100 rpm. During this process, the melt was solidified to form a strip. A specimen of 50mm was cut from the cast strips. The strip was characterized for the thickness, hardness and microstructure. Variation in hardness, microstructure of the produced trip is explained as a comparison to normal casting.

Microstructural stability after severe plastic deformation of AZ31 Magnesium

Friction stir processing (FSP) and equal channel angular pressing (ECAP) were used to modify the microstructure of twin roll cast (TRC) AZ31 magnesium. The influence of these processes on the microstructural properties of the material was investigated. It was found that both processes produced microstructures with an average grain size of less than 10 pm, suggesting that they have the potential for superplastic deformation. Heat treatments were performed on the TRC, ECAP and FSP materials to assess their microstructural stability. Both the ECAP and TRC material were found to be fairly stable, showing normal grain growth while the FSP material grew substantially at temperatures above 200°C. The activation energy of grain boundary motion of the TRC material was calculated to be 167 kJ/mol.

Effects of the microstructure of twin roll cast and hot rolled plates on the surface quality of presensitized plates

The effect of the microstructure of plates fabricated both in the traditional process, involving casting, hot rolling and cold rolling (HR), and in the novel twin roll casting + cold rolling (TRC) process on the surface quality of presensitized (PS) plates was analyzed by optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDXS). The formation of pores on the surface of the electrolyzed HR plate could be attributed to the presence of approximately 1-μm-sized large Al-Fe precipitates in the HR plate compared to the smaller precipitates in the TRC plate. Moreover, residual graphite lubricants used during the TRC process were entrapped on the surface of the TRC plate during the subsequent rolling process. The entrapped pollutants tended to further deteriorate the formation of pores on the surface of the TRC plate, and no residual carbon was detected on the surface of the HR plate. Furthermore, the surface quality of the TRC plate can be improved by surface cleaning before the cold rolling process, which could dramatically lower the residual graphite on the surface.

The role of reduced graphene oxide on chemical, mechanical and barrier properties of natural rubber composites

Abstract Natural rubber (NR)-reduced graphene oxide (rGO) composites were produced via latex mixing and co-coagulation approach followed by static hot-press and twin roll mixing process. Due to the process, a fine control of filler dispersion was obtained and the composites exhibited a three-dimensional rGO network or alternatively a homogeneous dispersion of single rGO platelets. The effect of rGO dispersion on chemical crosslink structure, and their influence on mechanical and barrier properties was thoroughly investigated. Small angle X-ray scattering (SAXS) and solid-state 13 C NMR analysis showed that rGO platelets affect the vulcanization process of natural rubber and that the crosslinking sulphur polysulphidic species present in pristine natural rubber decrease with the rGO content. In fact, at rGO content higher than 6 phr, the crosslinking species consist mainly of monosulphidic species which attain a consequent increment of intrinsic crosslinking density. However, the composites with rGO segregated network exhibit both barrier to oxygen and water vapour permeation and mechanical properties improved with respect to pristine rubber and composites with the homogeneous dispersion of single rGO platelets. The results confirm that the morphology of filler has a prominent key role in determining the natural rubber composites properties.

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.

Microstructure and mechanical properties of Al-Si-Mg alloys fabricated by twin roll casting and subsequent symmetric and asymmetric rolling

Al-Si-Mg aluminum alloys were asymmetrically cold-rolled using differential speed rolling mills. The textural and microstructural evolution with the consecutive working sequence was investigated using an electron backscatter diffraction. There were texture and microstructure variations with the speed ratio and a reduction in the area. Strongly-developed recrystallized cube orientations, {100}‹001›, remained during symmetric rolling. The weakened cube orientation mainly changed into random orientations during subsequent annealing, and a small amount of cube orientations remained. However, during asymmetric rolling with a speed ratio of 2, the shear deformation penetrated through the entire thickness direction, and a strong shear texture of rotated cube, {100}‹011›, developed. The distinct shear texture was recrystallized into {356}‹0 17 14› components. The mechanical properties of the symmetrically- and asymmetrically-fabricated sheets were compared with each other. More balanced planar isotropic values of strength and elongation were obtained from the symmetrically-fabricated sheets, which are mainly due to the weakened random texture.

Recrystallization behavior of cold rolled Al-Zn-Mg-Cu fabricated by twin roll casting

The Al-Zn-Mg-Cu alloy strip can be successfully fabricated by twin roll casting. In order to determine the effect of thickness reduction and heat treatment temperature on the recrystallization behavior, the strips were rolled and subsequently heat treated under various conditions. As a result, a fine grained microstructure (average grain size ∼13 μm, average grain aspect ratio ∼1.7) and high mechanical properties (UTS ≥360 MPa, δ ≥20%) were obtained by cold rolling with thickness reduction of 60% and heat treatment at 500 °C for 1 h. Also, the effect of the microstructure on mechanical properties of the Al-Zn-Mg-Cu alloy strip was discussed. Consequently, the low cost and high strength Al-Zn-Mg-Cu alloy sheet can be fabricated by twin roll casting by proper thermo-mechanical processes.