Study On The Preparation Process And The Related Mechanisms Of Al-Mg Alloy Sheets With High Strength And High Plasticty

The 5xxx series（Al-Mg series）alloys have been widely used in marine,aviation,aerospace and automotive fields.However,the strength of Al-Mg series alloys with medium and low Mg content is low,while the corrosion resistance and plasticity of Al-Mg series alloys with high magnesium content are poor,which is difficult to meet the application requirements.Grain refinement can simultaneously improve the strength and the plasticity of these alloys,which is the most effective means to prepare Al-Mg based alloy with high strength and high plasticity.If the high Mg alloyed Al-Mg alloy with low stacking fault energy experiences mudium-high strain rate deformation,DRX process will be further enhanced and fine DRX grains can be obtained.If the fine DRX grain structure is subjected to subsequent cold deformation,warm deformation and annealing treatment,the finer subgrains can be obtained,which would improve the mechanical properties of the alloy.Out of this consideration,in this paper,the grain refinement effect of medium-high strain rate rolling（MHSRR）on the Al-6.5Mg alloy is firstly investigated,the strengthening mechanism and high plasticity mechanism are also revealed.On this basis,the effects of rolling parametes（strain rate,temperature and thickness reduction）on the DRX behavior of the 5A12 alloy are studied by the wedge-shape samples rolling experiment,the DRX mechanism is revealed.The effects of rolling temperature and thickness reduction on the microstructure and mechanical properties of 5A12 alloy are verified based on the slab samples MHSRR experiment,the mechanism of high Mg content and microalloying elements influencing recrystallization microstructure characteristics of the alloy is also discussed.On the basis of the effects of cold（warm）rolling and annealing treatment on the microstructure and mechanical properties of as-MHSRRed 5A12 alloy,the relationship between mechanical properties and microstructure characteristics of the alloy is clarified.This work provides theoretical foundation for the preparation of Al-Mg alloy sheets with high strength and large plasticity.The main conclusions are as follows:（1）The evolution regularities of microstructure and mechanical properties of the Al-6.5Mg alloy during the MHSRR with single pass rolling and large thickness reduction(=8.6 s^-1,with the thickness reduction of 72%)are revealed.The higher deformation temperature can promote the DRX and weaken the texture of the alloy with the deformation temperature from 360℃to 440℃.The DRX of the alloy MHSRRed at 400℃is complete and the grain orientation is nearly random.The fraction of high angle grain boundaries（HAGBs）,the average DRX grain size,dislocation density and the maximum texture density are 91.8%,3.1μm,1.6×10¹²m^-2and 2.27,respectively.The strength of the as-MHSRRed alloy sheet gradually decreases with the increasing deformation temperature,while the elongation gradually increases.The as-MHSRRed sheet rolled at 400℃shows better comprehensive performance with the yield strength（YS）,ultimate tensile strength（UTS）and elongation（El）of 197MPa,354 MPa and 34.0%,respectively.The main strengthening mechanism of the alloy is solid solution strengthening and grain refinement strengthening.The high plasticity can be ascribed to the uniform fine-grained structure and the random grain orientation distribution.（2）The DRX behavior and the related mechanism of the 5A12 alloy with high Mg content during MHSRR are revealed based on the wedge-shape samples rolling experiment.When the rolling thickness reduction is 80%,the DRX volume fraction of the alloy increases at first and then decreases with the higher deformation temperature（360-440℃）,and it also increases at first and then decreases with the higher deformation strain rate(5-20 s^-1).Based on the better rolling parameters optimized by the wedge-shape samples rolling experiment,the alloy rolled at 400℃/10 s^-1 with the thickness reduction of 80%gains the largest DRX volume fraction（95.4%）and the smallest average grain size（2.6μm）.The DRX volume fraction of the alloy increases at first and then decreases with the higher Z parameter（Z=exp（Q/RT））.The highest DRX volume fraction（95.4%）is achieved with the Z parameter of 3.6×10¹⁴ s^-1.The discontinuous DRX related to grain boundary and the continuous DRX connected with deformation bands（DBs）occur simultaneously in the alloy during MHSRR and the latter is dominant.（3）The effects of cold rolling（CR）and low temperature annealing treatment on the precipitates,substructres and mechanical properties of the as-MHSRRed 5A12alloy sheets are studied based on the slab samples MHSRR sheets rolled by single pass rolling and large thickness reduction.The high Mg content and the microalloying elements（Mn,Zr and Ti）can make the alloy obtain finer DRX grains.The fraction of HAGBs,the average DRX grain size,the dislocation density and the maximum texture density of the alloy rolled by 1 pass MHSRR(=8.6 s^-1,with the thickness reduction of 72%)at 400℃ are 95.6%,1.9μm,2.8×10¹²m^-2and 1.63,respectively.The YS,UTS and El of the alloy(MHSRR（400℃,1P）are 288 MPa、462 MPa and27.2%,the YS,UTS and El of the alloy(MHSRR（360℃,1P）rolled by 1 pass MHSRR(=8.6 s^-1,with the thickness reduction of 72%)at 360℃ are 294 MPa、463MPa and 27.2%,respectively.The as-MHSRRed sheets with fine-grained structure subjected to 50%cold rolling gain high strength and large plasticity.The YS,UTS and El of the sheet（MHSRR（400℃,1P）+CR）are 523 MPa,609 MPa and 14.0%,respectively.The YS,UTS and El of the sheet（MHSRR（360℃,1P）+CR）are 534 MPa,617 MPa and10.0%,respectively.When the as-CRed sheets（MHSRR（360℃,1P））+CR)with fine-grained structure is subjected to the low temperature annealing at 70-250℃,the static precipitation behavior ofβ-Al₃Mg₂ phases increases and the dislocation density decreases with the higher annealing temperature.The strength of cold rolled sheet decreases with the increasing annealing temperature,while the elongation increases.The as-CRed sheet subjected to the annealing treatment at 90℃for 1h gains the better comprehensive properties,with the YS,UTS and El of 517 MPa,614 MPa and 16.6%,respectively.（4）The effects of warm rolling on the microstructure and mechanical properties of the as-MHSRRed 5A12 alloy sheets are studied based on the slab samples MHSRR sheets with multi-passes rolling and small thickness reduction.When the alloy deformed at same rolling temperature,strain rate(=8.6 s^-1)and total thickness reduction（72%）,the strength of the sheet rolled by 4 passes of MHSRR is stronger than the sheets rolled by single pass and 2 passes of MHSRR,while the plasticity is lower.The YS,UTS and El of the sheet（MHSRR（360℃,4P））rolled by 4 passes MHSRR at 360℃are 468 MPa、567 MPa and 9.7%,respectivly.When the sheet（MHSRR（360℃,4P））is deformed by warm rolling at 90-250℃,the El of the sheet increases with the higher warm rolling temperature,while the YS increases at first and then decreases.The subgrains with average size of 154±19 nm are developed in the sheet subjected to warming rolling at 250℃.The YS,UTS and El are 570 MPa,646 MPa and 6.2%,respectively,the contribution values of dislocation strengthening,solution strengthening and subgrain strengthening to YS are 260 MPa,84 MPa and 186 MPa,respectively.