Numerical Simulation Of Asymmetric Cast-Rolling And Research On Aluminum/Magnesium Rolling Composite Technology

As lightweight metal materials,aluminum alloys and magnesium alloys have been widely used in the field of automotive lightweighting.The twin-roll casting process is widely used in the production of aluminum alloy plates due to its simple process,short process and high efficiency.However,the aluminum alloy plate produced by the traditional symmetrical twin-roll casting and rolling has a large grain size and serious center segregation,which leads to poor mechanical properties of the cast-rolled plate.By changing the method of molten metal pouring,the molten metal is first poured onto a roll.After the cooling effect of a single roll,it enters the double-roll rolling zone with the rolls.When the cast slab leaves the roll gap,it is subjected to rolling production to form asymmetric cast-rolling.The use of asymmetric casting-rolling technology can improve the shortcomings of cast-rolled billets such as coarse grains and serious center segregation.However,there is little research on this process at present.Magnesium alloy has strong development potential due to its advantages of light weight,low density,high specific strength and high specific rigidity.However,its development is limited due to its poor room temperature formability,weak corrosion resistance,and easy oxidation.The advantages of aluminum alloys,such as good plasticity at room temperature and strong corrosion resistance,just make up for the shortcomings of magnesium alloys.Therefore,combining aluminum alloys and magnesium alloys to prepare aluminum/magnesium composite panels has very broad application prospects.In the research of this paper,the finite element software Pro CAST was used to simulate the variation of temperature field and solidification microstructure of 6061 aluminum alloy in the process of asymmetric casting-rolling.According to the simulation results,the appropriate casting and rolling process parameters were selected to guide the asymmetric casting-rolling experiments.In addition,the 6061 aluminum alloy plate obtained from the asymmetric casting-rolling experiment and AZ31 extruded plate were used as the substrate to successfully fabricate the Al/Mg composite plate through the hot rolling composite experiment.The microstructure and morphology of the composite plate,interface element diffusion and mechanical properties were analyzed,and the conclusions were drawn as follows:（1）The finite element simulation of thermal-fluid-microstructure coupling during the asymmetric casting and rolling of aluminum alloy was carried out.The simulation results show that the temperature field and the shape of the liquid hole in the casting and rolling zone are asymmetric due to the change of the casting mode of molten metal.For the same cross section,the temperature of the billet in contact with the two sides of the rolls is quite different,and the shape of the liquid hole is also shown as an asymmetrical arc.According to the results of microstructure simulation,due to the change of solidification process of the molten metal,the side of the casting and rolling billet contacting the roll first（solidification side first）shows a large proportion of columnar crystalline structure,and the side of the casting and rolling billet contacting the roll later（solidification side later）shows more equiaxed crystalline structure.（2）Analysis of different cast-rolling process parameters（pouring temperature of roll speed）on aluminum alloy non-symmetric roll casting temperature field and solidification microstructure and the proportion of equiaxial crystal of the columnar crystal grain size and the effect of crystal orientation Angle,choose reasonable technological parameters for:pouring temperature is 690℃,roll casting speed is 8 m/min,with the parameters for the asymmetric rolling experiment of aluminum alloy,successful preparation of the aluminium alloy cast-rolling plate thickness of 2.25 mm.The microstructure of asymmetric casting-rolling aluminum alloy plate obtained by experiment is in good agreement with the simulation results.（3）The microstructure of the 6061 aluminum alloy plate obtained by asymmetric casting and rolling experiments is different from that obtained first the later solidification,and the microstructure and properties of the Al/Mg composite plates prepared by multi-pass hot rolling composite experiment with different solidification side aluminum alloy plates are also different.The bonding interface of the first solidification side Al/Mg composite plate is discontinuously distributed,and there are a lot of defects such as voids and cracks.The bonding interface of the later solidification side Al/Mg composite plate is relatively straight and tightly bonded.The first solidification side Al/Mg composite plate has a tensile strength of 218MPa and an elongation of13.7%,and the later solidification side Al/Mg composite plate has a tensile strength of 237MPa and an elongation of 15.3%.The comprehensive mechanical properties of the later solidification side Al/Mg composite plate are better than the first solidification side Al/Mg composite plate.The reason can be analyzed according to the element diffusion at the bonding interface of the composite plates.Because there is a wide diffusion intermediate layer composed of brittle and hard intermetallic phases Mg₂Al₃ and Mg₁₇Al₁₂ at the bonding interface of the first solidification side Al/Mg composite plate,thickness of 25μm,the brittle phase affect the deformation capacity of composite plate and become the source of the fracture of the composite plate.The diffusion interlayer at the bonding interface of the later solidification side Al/Mg composite plate is relatively narrow,with a thickness of 7μm,and no brittle and hard metal intermediate phase is formed.The composite plate has better mechanical properties.（4）Further analysis of the effect of different rolling temperatures on the structure and properties of the Al/Mg composite plate,the results are as follows:When the total reduction is constant,the aluminum plates later solidified side are combined with the magnesium plates.The results show that as the rolling temperature increases,the curve shape at the bonding interface of the aluminum/magnesium composite plate increases,and the defects increase,and the thickness of the diffusion layer at the bonding interface of the aluminum/magnesium composite plate also increases.In addition,the increase in rolling temperature will cause the mechanical properties of the aluminum/magnesium composite plate to continue to decline.When the rolling temperature is 350℃,the tensile strength of the aluminum/magnesium composite plate is253MPa and the elongation is 18.5%.When the rolling temperature is 400℃,the tensile strength of the aluminum/magnesium composite plate is 237MPa and the elongation is 15.3%,when the rolling temperature is 450℃,the tensile strength of the aluminum/magnesium composite plate is 148MPa,and the elongation is 5.8%.The reason is that the temperature rises and the atomic diffusion intensifies to form a large number of brittle and hard metal mesophases.Moreover,aluminum alloys and magnesium alloys are easily oxidized at high temperatures.The formation of oxide film hinders the composite of aluminum and magnesium,thereby affecting the mechanical properties of the composite plate.