Progress in twin roll casting of magnesium alloys: A review

Twin roll casting was commercialized for a strip production from ferrous and non-ferrous alloys in the 1950 s;however,its application to magnesium has proven difficult and still creates major challenges.This report describes global efforts in expanding manufacturing capabilities of magnesium sheet through twin roll casting path,offering many benefits,including a reduction in number of processing steps and energy savings.In addition to hardware design,alloy transformation during processing,product microstructure and properties,examples of successful solutions along with present technology and knowledge limitations are discussed.A particular attention is paid to developments at Canmet MATERIALS,having the only in North America pilot scale twin roll casting facility,devoted to magnesium.Efforts are described that aim at design of new magnesium alloys,which could take advantage of unique processing conditions during twin roll casting and contribute to the overall progress in magnesium sheet manufacturing.

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 ℃ 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.

Effect of asymmetric rolling process on the microstructure,mechanical properties and texture of AZ31 magnesium alloys sheets produced by twin roll casting technique

Symmetric rolling(SR)and asymmetric rolling(ASR)processes were carried out on 6 mm thick AZ31 magnesium alloy sheets that were produced by twin roll casting(TRC)technique.Before rolling processes,sheets were heat treated in order to obtain a homogenized microstructure.In this study,for the ASR process the rolling speed ratio between upper roller and lower was selected as 1.25.Both SR and ASR processes were utilized with 40%reduction per passes using 2 pass schedule for a total reduction ratio of 0.67.Symmetric and asymmetric rolled sheets were characterized using optical microscopy(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM)techniques.Texture measurements were performed by using X-ray diffraction(XRD)technique and mechanical properties were investigated by tensile tests and also hardness measurements.

Mathematical modeling of thermo-mechanical behavior of strip during twin roll casting of an AZ31 magnesium alloy

The effect of set-back distance on the thermo-mechanical behavior of the strip during twin roll casting(TRC)of an AZ31 magnesium alloy was modeled using finite element method(FEM).Model validation was done by comparing the predicted and measured exit strip surface temperature as well as the secondary dendrite arm spacing(SDAS)through the thickness of the sheet to those measured during experiments.Model results showed as the set-back distance increases,the strip exit temperature decreases and the solidification front moves toward the entry of the roll gap.The cast strip also experiences more plastic deformation and consequently,the normal stress on the strip surface and effective strain at the strip center-line increase.Moreover,higher separating forces were predicted for longer set-back distances.Model predictions showed that changing the set-back distance by varying the final thickness has a more significant effect on the temperature and stress-strain fields than altering the nozzle opening height.

3-D Numerical Simulation of the Electromagnetic Dam of Twin Roll Casting using Edge Element Method

The 3-dimension numerical simulation study on the electromagnetic dam used in the twin roll caster has been performed by using the edge element method. It was found that the materials and structures of the roll collars have great influence on the distribution of the magnetic flux density, eddy current density and the electromagnetic force in the molten pool. The conductive collars make the magnetic flux density decreased in the molten pool, but it also makes the magnetic force more uniformly, and the force in the low part of the molten pool where needs greater force have increased some what. The conductive collars make the EMD device more effective than the nonconductive collars.

NUMERICAL STUDY ON 3-D FLUID FLOW WITHIN THE MOLTEN POOL DURING UNEQUAL DIAMETER TWIN ROLL STRIP CAST-ROLLING

In this paper three-dimensional flow field in the molten pool between unequal diameter twin rolls has been studied by BFC technology and SIMPLEC numerical method. From the computed flow pattern,the characteristic of molten steel flow has been discussed. The dynamics generated by the feeding flow and the motivation of counter-rotating twin rolls produces a recirculating flow field nearby the small roll due to the larger room and less resistant here and fluid flows along the roll surface tangential nearby large roll and extends to two side dams. There is faintly flow around the dams of molten pool

High Speed Twin Roll Casting of Aluminum Alloy Strips

The twin roll caster for aluminum alloys has many a dv antages. For example, rapid solidification, low equipment cost, and low running cost, etc. However, the twin roll caster has some disadvantages. They are slow c asting speed and limitation of alloys. In the conventional twin roll caster, the casting speed is slower than 10 m/min. Aluminum alloys, which freezing range is wide, can not be cast. In the present study, break through of these problems wa s tried, and a new type twin roll caster was devised. A vertical type twin roll caster equipped with a nozzle was devised in order to cast aluminum strips at the speeds higher than 60 m/min. Characteristic features of this caster were vertical type, use of copper rolls, low separating force (n o operation of rolling), no use of lubricant, and equipment of the nozzle. In th e high speed roll casting, feed of molten metal of the vertical type twin roll c aster was easier than that of the conventional horizontal type caster. The use o f the copper rolls made cooling rate higher. The low separating force and the us e of the copper rolls prevented sticking of the strip to the roll. The lubricant was not needed, as the sticking of the strip to the roll did not occur. No use of the lubricant was useful to increase the cooling rate and casting speed of th e strip. The nozzle was used in order to improve contact condition (heat transfe r) between the melt and the rolls by hydrostatic pressure. In the twin roll cast er of the present study, many devices were done to improve the cooling condition of the strip in order to increase the casting speed. It was said that the roll casting of A5182 was very difficult, as freezing zone of A5182 was very wide. However, A5182 strip could be cast at high speeds up to 120 m/min using the twin roll caster of the present study. The microstructure of the strip cast using the conventional type twin roll caster is columnar structu re. The microstructure of the strip cast using the twin roll caster of the prese nt study was not columnar but equiaxed structure. It is said that mechanical pro perties of the strip cast using the twin roll caster is cheaper than that of the strip made from DC casting. However, the mechanical properties of the strip cas t using the twin roll caster in the present study were almost same as the proper ties of the strip made from DC casting. The thickness of the strip cast using th e twin roll caster of the present study was from 1.5 mm to 2.5 mm, and this thic kness was thinner than that of the strip cast using the conventional twin roll c aster. Semisolid roll casting was tried in order to increase the roll speed more and mo re. The twin roll caster of the present study was equipped with a cooling slope in order to make semisolid slurry. The cooling slope was the simplest process to make the semisolid slurry. Solid rate of the semisolid slurry was about 10%. Th e casting speed increased up to 180 m/min by the effect of semisolid casting. Me chanical properties were improved by the semisolid casting, too.

Numerical Simulation of Two Types of Electromagnetic Edge Dam Used in Twin Roll Strip Casting

Two types of electromagnetic edge dams were analyzed by using finite element method in present paper. The magnetic vector potential method and edge element method were used. The distributions of the magnetic field, the eddy current intensity and the magnetic force were obtained from the computing. The differences in these fields were explained according to the two types of electromagnetic dam, and characters of their application in twin roll casting were also discussed.

Influence of Feeding Mode on Flow and Temperature Field for Twin Roll Strip Casting Process

A three-dimensional finite element method was developed to simulate the fluid flow,heat transfer and solidification for twin roll strip casting.An improved two-equationκ-εmodel was used to incorporate the turbulence in fluid flow.The influence of vertical feeding and submerged entry nozzle feeding on the flow and temperature field was discussed.The optimum submersion depth and entry angle of submerged nozzle were obtained through comparison of the simulation results.

Microstructure and Eutectic Carbide Morphology of the High Speed Steel Strips Produced by Twin Roll Strip Casting Process

The M2 high-speed steel strip was produced by using the laboratory scale twin roll strip caster. The microstructure and eutectic carbide morphology of thus produced products were observed and analyzed, and the comparison of those with conventional products was carried out. The effects of the processing parameters such as the melting temperature, the pouring temperature, rolling speed and separating force on the microstructure and eutectic carbide morphology and their distribution were analyzed. The spheroidizing process of the strips in the annealing process was investigated. The relations between the growth and spheroidizing of the eutectic carbide and the annealing technology were obtained, and the mechanism of the twin roll strip casting process improving the eutectic carbide spheroidizing was discussed. The theoretical instruction for determining the subsequent treatment process was provided.

Recrystallization behavior of twin roll cast low carbon steel strip

The dynamic and static recrystallization behaviors of twin roll cast low carbon steel strip were investigated with an attempt to provide guiding deformation parameters for the on line hot rolling.In order to investigate dynamic recrystallization behavior,as cast strip was reheated and soaked with austenite grain size similar to the width level of the as cast columnar structure.Tensile test was used and the deformation temperature is in the range of 900℃to 1 100℃and strain rates are 0.01 s^(-1),0.1 s^(-1),1 s^(-1).The activation energy and stress exponent were determined as 306kJ/mol and 4.69 respectively.The ratio of critical strain to the peak strain is 0.65,and that of critical stress to the peak stress is 0.92.The dependence of the peak strain on the initial grain size and Zener - Hollomon parameters Z isε_p =9.1×10^(-4)×D_0^(0.48)Z^(0.13).The kinetics of the dynamic recrystallization and recrystallized grain size was predicted using models published.The as cast coarse austenite were dramatically refined after complete dynamic recrystallization.For static recrystallization,the tensile test was carried out on Gleeble -3500 thermo - mechanical simulator.The deformation temperature is in the range of 800℃to 1 200℃with strain rate 0.01 s^(-1) to 1s^(-1).The pre strain is fixed at 0.04 to 0.12 and the inter-hit delay time varies from 1 s to 3 000 s.The activation energy and Avrami exponent of static recrystallization were determined as 241 kJ/mol and 0.54 respectively.A kinetics model was proposed to describe the static recrystallization kinetics.The predicted results were in good agreement with the experimental results.

Research and development of transnormal twin roll-casting technology

Focusing on the research and development of transnormal twin roll casting technology and modeling and simulation of twin roll casting process, the progresses made in China to date in the following aspects were introduced, the analysis and calculation of heat conductance both in contact zone between the strip and the outside of the roller and the inside of the roller; the 3 D numerical simulation of molten aluminum flow in feed tip nozzle; the research of the effects of electromagnetic fields on textures and mechanical properties of casting rolling strip, and the modeling and simulation of twin roll casting process including the methods of modeling and some significant results.

Production of 150 cm wide AZ31 magnesium sheet by twin roll casting

150 cm wide AZ31 magnesium alloy sheet was produced by twin roll casting.The sheets of 6 mm thickness were cast. The sheet material was characterized using various techniques including metallography and XRD.The mechanical properties were investigated by tensile tests and hardness measurements.The procedures were developed for thermomechanical treatment of cast sheet material.The products of these procedures were characterized and the results were discussed for possible potential applications.

Application of Anand's constitutive model on twin roll casting process of AZ31 magnesium alloy

Twin-roll thin strip casting process combines casting and hot rolling into a single process, in which thermal stress and thermal mechanical stress were involved. Considering the high temperature gradient, the existing of liquid and solid regions and rolling deformation, suitable constitutive model is the key to describe the process. Anand's model is a temperature-dependent, rate-dependent and unified of creep and plasticity model and the Jaumann derivative was employed in Anand's model which makes the constitutive model frame-indifferent or objective, therefore the highly nonlinearities behavior in the twin-roll casting process can be simulated. The parameters of the Anand's model were regressed based on the compression tests of AZ31 magnesium alloy. The simulation results reveal that the Anand's model can well describe the deformation characteristics of twin-roll casting process. Based on the simulation results, the form of evolution equations in Anand's model was discussed.

Effect of casting parameters and deformation on microstructure evolution of twin-roll casting magnesium alloy AZ31

Twin roll casting method is a promising route to directly produce magnesium alloy strip. It is a rapid solidification process with high temperature gradient combined with thermal flow and rolling deformation in the casting region. As-cast strip with proper microstructure is requested to serve as next rolling feedstock. However the microstructure of as-cast strip is sensitive for casting conditions during the casting process and the as-cast microstructure greatly affects the mechanical properties. In this work, the effect of casting speed, pouring temperature, deformation as well as anneal process on microstructure and mechanical properties were investigated. The results revels that twin-roll casting process can effectively refine the grain size, improve the morphology and distribution states of Mg17Al12. The homogenization treatment time can be shorted for the fine microstructure and lower the cost dramatically for the next forming process.

Numerical simulation of the temperature fields of stainless steel with different roller parameters during twin-roll strip casting

The temperature field of stainless steel during twin-roll strip casting was simulated by experiment and a finite element （FE） model. By comparing the measured result with the simulated values, it is found that they fit close to each other, which indicates this FE model is effective. Based on this model, the effects of roll gap （t） and roll radius （R） on solidification were simulated. The simulated results give the relationship between t or R and the position of the freezing point. The larger the t is and the smaller the R is, the closer the position of the freezing point is to the exit.

Simulation of microstructures in solidification of aluminum twin-roll casting based on CA

The microstructures in the solidification process of aluminum twin-roll casting was simulated based on CA(Cellular Automation Method),and the nucleation model based on the normal distribution and KUZR-GIOVANOLS-TRIVEDI(KGT) growth model were used in the calculation. FDM(Finite Difference Method) combined with relative motion was used,and dynamic evolution of microstructures in the process of aluminum twin-roll casting was achieved. Visual Fortran programming language was adopted to calculate and realize the image post-processing. Moreover,the effect of different casting process parameters on the formation of the microstructures was simulated. The results are helpful to explaining the dendritic segregation and size segregation as well as shrinkage-porosity defects. Columnar grains mainly distribute near the casting roller while equiaxed grains distributed far away from the casting roller.

Water Modeling of Twin-Roll Strip Casting

Twin-roll strip casting is regarded as a prospective technology of near net shape continuous casting. The fluid flow field and level fluctuation in the pool have a strong influence not only on composition and temperature homogeneity of pool, but also on the strip quality. A 1 ： 1 water model of a twin-roll strip caster was set up based on the criteria of Froude number and Reynold number similarity. The level fluctuation was measured. The influence of pool depth, casting speed and feeding system configuration on level fluctuation in the pool was studied. The experimental results provided a basis for the optimization of feeding system and process parameters.

Microstructure of aluminum twin-roll casting based on Cellular Automation

Nucleation and growth model based on Cellular Automation(CA) incorporated with macro heat transfer calculation was presented to simulate the microstructure of aluminum twin-roll casting. The dynamics model of dendrite tip (KGT model) was amended in view of characteristics of aluminum twin-roll casting. Through the numerical simulation on solidification structure under different casting speeds, it can be seen that when the casting speed is 1.3 m/min, that is, under conditions of conventional roll casting, coarse columnar grains dominate the solidification structure, and equiaxed grains exist in the center of aluminum strip. When the casting speed continuously increases to 8 m/min, that is, under the conditions of thin-gauge high-speed casting, columnar grains in solidification structure all convert into equiaxed grains. Experimental and numerical results agree well.

Numerical and physical simulation of a twin-roll strip caster

The 1：1 water model of a twin-roll strip caster was set up based on the Froude number and the Reynolds number similarity criteria. A new type metal delivery system was designed for the twin-roll strip caster. The level fluctuation and the fluid flow in the pool of the water model were measured using the level detector and the 3D-LDV （laser Doppler velocimetry） technology. It is shown that a wedged delivery system can produce the desirable level fluctuation and even fluid flow distribution in the pool Numerical simulations for the water model were performed. Comparisons between the numerical and physical simulation results show good agreement near the side dams.