| Recently,the problems of energy and environment are serious,such as energy shortage and global warming,thus the lightweight vehicle becomes inevitable and important for energy conservation and emission reduction.Nowadays Al alloys are still important in automotive lightweight materials.The high strength Al-Zn-Mg-Cu alloys are applied for automotive structure materials,which can replace the steel parts to reduce the automobile weight.Meanwhile,the manufacture of automotive lightweight materials should be green manufacture,rapid manufacture and quality manufacture.The A1 alloy strips can be fabricated by Twin roll casting(TRC)with short process,low energy consumption and low manufacturing costs.However there are still some difficulties in the preparation of Al-Zn-Mg-Cu alloy strips by TRC process due to high content of alloying elements and high strength.Based on the TRC process,the high Zn-containing Al-7.9Zn-2.7Mg-2.0Cu(wt.%)alloy was designed to increase the strength,and the process of TRC,annealing treatment and subsequent rolling was designed.The microstructure of TRC and permanent mold casting(PMC)alloys,and the microstructure and mechanical properties of TRC and PMC alloys sheets after rolling process were investigated comparatively,and the feasibility for high Zn-containing Al-Zn-Mg-Cu alloy fabricated by TRC was discussed.Based on the concept of rapid manufacture,the second-phase particles formed during solidification were controlled by annealing treatment,the effects of second-phase particles on the recrystallization mechanism during subsequent rolling process were investigated.With the different recrystallization mechanism,the microstructure and texture of sheets after rolling process were controlled,and the tensile properties anisotropy of sheets was reduced.In this study,the Al-7.9Zn-2.7Mg-2.0Cu strip with a width of 130mm and a thickness of 5.5mm was successfully fabricated by TRC process,and the TRC process parameters were determined:the initial casting temperature was 690 ?,the rolling speed was 2.36?2.54 m/min,the roller gap was 4.0mm.The equiaxed dendrites were presented,and the fine secondary dendrite ann spacing were obtained in the as-cast TRC strip.The fine Mg(Zn,Al,Cu)2 phases were formed along interdendritic interfaces.The short-time annealing treatment(480 ?/1h)can effectively eliminate the secondary phases in the TRC strip.However,the most of secondary phases were remained after the annealing treatment of 400 ?/1h in the PMC alloy ingot due to the large secondary phases.The secondary phases of the PMC alloy ingot can be removed after long-time homogenization(470 ?/24h).The activation energy maps and processing maps were established from the hot compression test of the annealed and homogenized PMC alloys,and the optimized hot deformation parameters were obtained.The high deformation temperature of 380 ?-450? can be considered as the optimized hot deformation parameters for the annealed and homogenized PMC alloys.The dominant dynamic softening mechanisms was continuous dynamic recrystallization(CDRX)under these hot deformation parameters.Meanwhile,the deformation temperature of 350 ?-380 ? and the strain rate of 0.005 s-1-0.17 s-1 can also be considered as the optimized hot deformation parameters for the annealed PMC alloys.The dominant dynamic softening mechanisms was particle stimulated nucleation(PSN),and CDRX was also occurred under these hot deformation parameters.The dominant dynamic recrystallization(DRX)mechanisms in the TRC and PMC alloys during hot rolling was CDRX by progressive subgrains rotation near initial grain boundaries,and the orientations of the CDRX grains were similar with their matrix.Thus the strong rolling texture was presented in the TRC and PMC sheets after hot rolling.The DRX by PSN was occurred in the TRC sheet annealed at 450 ?.for 1h before rolling and in the PMC sheet annealed at 400 ? for 1h before rolling,and these PSN grains with random orientations were formed around the coarse particles.A number of fine equiaxed grains were formed in the two TRC sheets fabricated by two different annealing treatment(450 ?/1h and 480 ?/1h).hot rolling.cold roling and heat treatment(solution treatment + pre-aging + natural aging.ST+PA+NA),the textures of the two TRC sheets were rolling texture and recrystallization texture.The similar grain structure morphology and texture were presented in the two TRC sheets.More second-phase particles were remained in the TRC strip after the annealing treatment of 450 ?/lh.However,the effects of PSN on the microstructure and texture of TRC sheets were weak during rolling process.PSN recrystallization played a significant role on the microstructure and texture of PMC sheets during rolling process.The recrystallization was improved and a number of fine equiaxed grains were formed in the PMC sheet fabricated by annealing treatment(400 ?/lh),hot rolling,cold rolling and ST+PA+NA treatment,and the texture was weak in this PMC sheet.Thus the tensile properties along the directions of 45° and 90° with respect to rolling direction(RD)were improved.The r value was increased and the ?r value was decreased,and the tensile properties anisotropy of the PMC sheet was improved.Compared between the two TRC sheets,the TRC sheet with the annealing treatment of 480 ?/lh before rolling process obtained the best tensile properties,and the yield strength,tensile strength and elongation along RD were 443MPa,600MPa and 17.8%,respectively.Compared with the PMC sheet,the rolling process of TRC sheets was shorter,and the similar tensile properties were presented in the TRC sheet and PMC sheet.In summary,the Al-Zn-Mg-Cu alloy sheets were fabricated by TRC and subsequent rolling process,and the fabrication process was simplified.The particles were controlled before deformation,and the tensile properties anisotropy of the Al-Zn-Mg-Cu alloy sheet fabricated by conventional casting was improved.This thesis gives well understanding to the preparation and microstructure control of Al-Zn-Mg-Cu alloy sheets with low costs for automotive materials. |
PDF Full Text Request