| It is imperative to lighten cars in order to meet the needs of energy saving and environmental protection.Aluminum sheets for auto-body applications is one of the most effective ways to realize automobile lightening.The heat-treatable 6xxx series aluminum alloy sheet which has good formability,corrosion resistance and bake hardenability becomes the ideal automotive aluminum sheet.However,compared with the traditional steel sheet for automotive sheet,6xxx series aluminum alloy sheet formability is still unsatisfactory and it is high cost,which stops the promotion and application.Conventional preparation of 6xxx series aluminum alloy sheet includes homogenization,hot rolling,intermediate annealing and cold rolling.It is a huge energy consumption that 6xxx series aluminum alloy homogenization temperature is above 500? and takes about 24 h.At the same time,the homogenization treatment will make the crystallization phases of 6xxx series aluminum alloy contracted into sphere and coarsening,which affects the formability.In order to reduce costs,research has been made using 6xxx series aluminum alloy automotive casting process.It can not be mass production because of the high costs of equipment and immature technology.The homogenization-free process has been widely considered to shorten the preparation process and reduce the cost of 6xxx series aluminum alloy automotive sheet,and it promotes the crystallization phases particle configuration to improve the formability.However,there is no study on the effect of homogenization-free on the formability and bake-hardening of 6xxx series aluminum alloy sheet at present.In this experiment,optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and tensile tests were used in the study of DC cast 6A11 aluminum alloy.The effects of hot rolling process parameters(including hot rolling temperature and hot rolling deformation),the annealing temperature and time after hot rolling on the microstructure and properties of 6A11 aluminum alloy sheet were investigated in this experiment so as to optimize the preparation process of the 6A11 alloy automotive sheet from homogenization-free and provide the experimental data and theoretical basis for the final optimization of the low-cost and short process preparation of high-performance aluminum alloy automotive sheet technology.The results show that:(1)The effect of hot rolling temperature from 400? to 520? on the microstructure and properties of sheet of direct hot rolled DC cast 6A11 aluminum alloy from homogenized-free ingot was found:The particle size of the excess crystallization phases in the sheet of direct hot rolled DC cast 6A11 aluminum alloy from homogenized-free ingot was fine and distributed uniformly.When the total hot-rolling reduction reached 89%,microstructure of surface layer and central layer of the hot-rolled plate were almost the same.The higher the hot rolling temperature is,the better the plasticity of the hot rolled plate is,and there was a smaller number of crystallization phases that were bigger in size in the cold-rolling sheet.When hot-rolled deformation of sheet of direct hot rolled DC cast 6A11 aluminum alloy from homogenized-free ingot was 89%,the effect of hot-rolled pass deformation on microstructure and properties of 6A11 aluminum alloy sheet was not obvious.The amount of crystallization phases decreased and recrystallization occured in the 6A11 aluminum alloy sheet after solution treated at 545? for 30 min and water quenched then pre-aging treated at 170? for 7 min.The increase of hot rolling temperature from 460? to 520? had no obvious effect on the grain size and shape of the recrystallized alloy.The influence of hot rolling temperature on the strength,elongation and n value of 6A11 aluminum alloy sheet from homogenized-free ingot was not obvious,but the R value obtained by hot rolling at 460? was the highest.(2)The optimum hot rolling temperature of sheet of direct hot rolled DC cast 6A11 aluminum alloy from homogenized-free ingot was 460?.DC cast 6A11 aluminum alloy from homogenized-free ingot was directly hot-rolled at 460? and cold-rolled,which was annealed at 420? for 2 h.The yield strength and tensile strength of the sheet in pre-aging were 164 MPa and 306 MPa,the elongation was 20.8%,the n15 and r15 were 0.245 and 0.702,and the instantaneous strain hardening index ns was 0.212 at 15%deformation.The yield strength and tensile strength of the sheets after 2%deformation and simulated baking at 170? for 30 min were 292 MPa and 375 MPa,and the elongation was 15.0%.(3)The effect of intermediate annealing temperature from 500? to 545?,and holding time from 3 h to 12 h on the microstructure and properties of sheet of direct hot rolled DC cast 6A11 aluminum alloy from homogenized-free ingot was found:There was a large number of black and gray crystallization phases which were both distributed as broken chain-like along the rolling direction in the 6A11 aluminum alloy hot rolled annealed plate from homogenized-free.The sphere and coarsening phenomenon of crystallization phases were more obvious with the higher intermediate annealing temperature and longer holding time,and there was a smaller number of crystallization phases that were bigger in size in the 6A11 aluminum alloy sheet after solution treated at 545? for 30 min and water quenched then pre-aging treated at 170? for 7 min,and the grain was equiaxed.The increase of sheet.The amount of crystallization phases decreased and recrystallization occured in the intermediate annealing temperature and the prolongation of holding time had no obvious effect on the strength and elongation of 6A11 aluminum alloy sheets from homogenized-free ingot.However,the n value and r value of 6A11 aluminum alloy sheet from homogenized-free ingot with intermediate annealing treated at 545? for 3 h were highest.(4)The optimum intermediate annealing system for t sheet of direct hot rolled DC cast 6A11 aluminum alloy from homogenized-free ingot was 545? for 3h.DC cast 6A11 aluminum alloy from homogenized-free ingot was directly hot-rolled at 460? and cold-rolled,which was annealed at 545? for 3 h.The yield strength and tensile strength of the sheet in pre-aging were 154 MPa and 300 MPa,the elongation was 21.9%,the nis and ris were 0.246 and 0.804,and the instantaneous strain hardening index ns was 0.238 at 15%deformation.The yield strength and tensile strength of the sheet after 2%deformation and simulated baking at 170? for 30 min were 260 MPa and 350 MPa,and the elongation was 16.0%.(5)The optimum preparation process of the 6A11 alloy automotive sheet from homogenization-free was that DC cast 6A11 aluminum alloy ingot from homogenized-free was hot rolled at 460? to 4.2 mm by small pass of large deformation,and cold rolled to 1.1 mm by two passes after intermediate annealing at 545? for 3h.The strength and elongation of the sheets obtained by the optimal process were close to the sheet by traditional process,and the formability was improved obviously. |
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