Investigation On Microstructure Evolution And Formability Of Ultrafine-grained 1060 Aluminum Alloy Sheets By Cryorolling

It was difficult to refine the grain size of aluminum alloy,and during severe plastic deformation,its strength even weakens when the strain exceeds a specific amout.In order to solve the above problem,AA1060 was selected as the research object in this thesis,and ultrafinegrained AA1060 sheet was prepared by accumulative roll bonding technology,and researched its microstructure and property response law in the process of cryorolling.The mechanism for improving the mechanical properties and formability of ultrafine-grained AA1060 sheet was illustrated,as well as the effect of cryogenic temperature,rolling differential speed ratio,and reduction rate on the microstructure and properties of ultrafine-grained aluminum alloy sheet.The main research work and innovation achievements are as follows:(1)Ultrafine-grained AA1060 sheet was prepared by accumulative roll bonding process.The results showed that the first five cycles,the grain refinement were obvious,the dislocation density of the grains increased continuously,the grain shape changes from equiaxed grain in annealing state to slender grain,and it’s strength increased with the increase of rolling passes.The tensile strength of the material obtained in the first cycle was126 MPa,which was 51 MPa higher than the annealing sheet of 75 MPa.The tensile strength of the material in the third cycle was 137 MPa,which reached 143 MPa in the fifth cycle.Prior rolling,the mean grain size of the material was 32 μm,after the third pass,the mean grain size was 0.78 μm,and after the fifth pass,the grain size was refined to 0.67 μm.It was found that the strength of the sheet increased obviously in the first three cycles during the accumulative roll bonding deformation process,but the strength of the sheet increased slighly after the fourth and fifth cycles.In the accumulative roll bonding process,work-hardening promoted the increase of dislocation density and grain refinement,and improved the mechanical properties of materials.When the fourth cycle of the accumulative roll bonding cycle was increased,the mean grain size of AA1060 sheet would not be greatly reduced with an increase in rolling cycles.(2)The evolution law of microstructure and properties of ultrafinegrained AA1060 sheet during cryorolling was studied.The results showed that cryorolling can effectively strengthen the grain refinement effect of ultrafine-grained aluminum alloy sheets and further improve the mechanical properties of the material.When the reduction was 50% the mean grain size,tensile strength and hardness of the sheets were 0.58 μm,147 MPa and 43 HV at room temperature.When the reduction increased to 80%,they were 0.35 μm,161 MPa and 49 HV respectively.When the rolling temperature was reduced to 83 K with the reduction increasing from50% to 80%,the tensile strength of the material was increased from 163 MPa to 185 MPa,the mean grain size was reduced from 0.36 μm to 0.27μm,and the hardness of the plate was increased from 51 HV to 55 HV.Compared with the tensile strength of 75 MPa and the hardness of 28 HV prior rolling.Cryorolling can improve the tensile strength and hardness by221% and 196% respectively.Cryorolling produced ultrafine-grained AA1060 sheets with a closer interface bonding degree than accumulative roll bonding,and the corresponding relationship between rolling temperature and grain size was established.When the rolling temperature reduced from room temperature to cryogenic temperature,dynamic recovery of crystals was inhibited and higher dislocation density was accumulated,which was beneficial to refining grain size and improving the mechanical properties of the sheet.(3)Studing the influence of asymmetric cryorolling on the mechanical properties and microstructure evolution of ultrafine-grained AA1060 sheets.The results showed that the tensile strength and hardness of ultrafine-grained AA1060 sheet were increasing with the increase of differential speed ratios(1.0,1.2 and 1.4)during asymmetric rolling at room temperature,the tensile strength were 142 MPa,149 MPa and 152 MPa respectively,and the hardness of the material were increased from 53 HV to 57 HV.Asymmetric cryorolling inhibited the dynamic recovery and recrystallization of grains at cryogenic temperature,and further strengthened the fine grain strengthening,with tensile strength of 163 MPa,171 MPa and 179 MPa respectively,and the hardness increased from 59 HV to 67 HV.When the differential speed ratio increased to 1.4 under the annealing conditions of 473 K and 523 K,the tensile strength and elongation of asymmetric cryorolling had better than room temperature rolling,which showed that the thermal stability of the asymmetric cryorolling sheet was better.It was found after annealing at 473 K,the different speed ratios mainly affected the volume fraction of Copper texture.After annealing at 523 K,the volume fraction of S texture was mainly affected by the different speed ratios.The contents of the Copper texture and S texture on the β orientation line decrease as the annealing temperature rises,reducing the shear strain of asymmetric cryorolling and the anisotropic of the material.(4)By analyzing the in-plane anisotropy value,cupping forming value and earing rate of AA1060 sheet under various processing conditions,the mechanism of cryorolling and asymmetric cryorolling in restraining material anisotropy and improving its formability compared with room temperature rolling was studied.The results showed that the in-plane anisotropy values of tensile strength and elongation in 0°,45° and 90°directions were 2.7% and 42.8%,respectively,and the cupping value and earing rate were 5.4 mm and 16.3%,respectively.The tensile strength and plane anisotropy of the cryorolling at 83 K specimen were 1.9% and 16.2%,as well,and the cupping value and earing rate are 6.8 mm and 14.2%,correspondingly.In the asymmetric cryorolling sample(83 K,differential speed ratio 1.4),the plane anisotropy values of tensile strength and elongation were 1.5% and 4.4% respectively,and the cupping value was7.5 mm.In contrast,it was found that the ultimate tensile strength and elongation of the sample with differential speed ratio of 1.4 had the smallest in-plane anisotropy value and the largest in cupping value during asymmetric cryorolling at 83 K.The texture of aluminum alloy sheets’ plane anisotropy was directly connected to it.Reducing rolling temperature and increasing differential speed ratio can improve the composition and distribution of Copper texture and S texture of AA1060 sheet,reduced its plane anisotropy and improved the formability of sheet.