Study On Electromagnetic Twin-roll Casting Process For 6181 Aluminum Alloy

Under the country's increasingly stringent energy-saving policies,the lightweight of automobile has become the focus of attention.It is an effective means to replace some steel structures with aluminum alloy.The 6181 aluminum alloy,one of the heat treatable construction materials,has a series of excellent performances,low density,high strength and superior processing property,fine corrosion resistance and recyclability.Thus,These indicate that,6181 aluminum alloy sheet will be the preferred materials on automobile for people.The twin-roll casting(TRC)technology is short process,low cost,good flexibility and high efficiency to process the 6181 aluminum alloys.However,one of the main challenges in the technology is very easy to make macro-segregation in the center of the sheets,which reduces the quality of the sheets.Therefore,It is significance to study the method of improving the central segregation of cast rolling sheets.In this paper,Al-Mg-Si alloy was used as the main research object.The effects of pouring temperature and rolling rate on the temperature field,flow field and the quality of slab are studied by combining simulation and experiment,which determined the parameters of casting rolling.Under the condition of reasonable casting,rolling parameters and duty cycle,frequency of pulse current,the influence rule and mechanism of different intensity pulse current,static magnetic field and compound field on central segregation of casting rolling sheets were discussed.(1)The finite element software was used to simulate temperature and flow fields of cast-rolling 6181 aluminum alloys,and the technological parameters was determined preliminary according to the length of the cast-rolling zone and the depth of the liquid cave.The results show that casting rolling rate of 0.6 m/min and pouring temperature of 700?,casting rolling rate of 0.8 m/min and pouring temperature of 690? and 700?,and casting rolling rate of 1.0 m/min and pouring temperature of 670? are suitable for TRC process.It is also found that,with the increase of casting rolling rate,the kiss point moves gradually towards the exit of molten pool,the depth of liquid cavity increases gradually,the convection during casting rolling zone is enhanced,and the temperature of molten pool is also improved.(2)The defects of the sheets,such as surface stripes,flims and edges,gas pits,cracks,heavy skins and sickle bending,as well as their solving methods,which often occur during the process of cast rolling were studied.According to the simulation results and the slab quality,the ideal casting rolling parameters are determined.The roll casting speed is 0.8 m/min and the pouring temperature is 690?.(3)It is found that,the morphology of grain structure in the cross-section of cast rolled slab is mainly composed of small and equiaxial crystals,which have a certain gradient column and three parts of the mixed crystal region.Severe segregation was formed at the center of the plate.(4)The effect of pulsed electric field with different duty ratio and frequency on the solidification structure is studied.The final results show that when pulse current frequency is 20%,the duty ratio.of 20%to improve the organization of the effect is prominent;(5)The effect of combined field of pulse electric current and static magnetic field on the solidification structure is discussed.Durng the compound field,when the pulse current frequency is 20%,and the static magnetic induction intensity is 24 mT,the inhibition of the segregation of the alloy solidification and the refining effect of grain are especially significant.(6)The effects of a single pulsed electric current on the macro segregation of TRC 6181 aluminum alloy were systematically studied.The results show that:with the increase of pulse current intensity,the central segregation of cast rolled slab is suppressed more obviously.The pulse current promotes the growth of primary dendrites and inhibits the two dendrite growth.In the pulsed electromagnetic field,the secondary dendrites are fused and break away from the primary dendrites.(7)The effects of combined fields of pulse electric current and static magnetic field on the macro segregation of TRC 6181 aluminum alloy were systematically studied.The combined field with 500 A pulsed current and 24 mT steady magnetic field has the best inhibitory effect on the central segregation.According to the simulation results of the flow field and temperature field during the cast rolling sheets with the combined fields,it was found that the electromagnetic force increased with the increase of the static magnetic field strength.Kiss point gradually moved to the direction of casting rolling exit.At the same time,the dendrite was fused,the initial dendrite spacing was reduced and hinders the solute moving to the solid liquid interface.(8)After homogenization,cold rolling,solid solution,pre-aging and baking paint treatment,it was found that uneven distribution of elements existed near the center line of casting rolled sheets.The element distribution was uniform after the subsequent treatment of the cast rolled sheets with the compound field.The mechanical properties of the cast rolling process sheets have reached the standard of the traditional process...