Investigation On Mn Containing Magnesium Alloy Sheets Prepared By On-line Heating Rolling

As a green material and strategic material in the 21 st century,magnesium alloy has the advantages of environmental friendliness,light weight,high specific strength,excellent electromagnetic shielding,and excellent damping and shock absorption properties.In addition,magnesium is rich in resources,and our country is a big country with a abundant magnesium resource.The large-scale application of magnesium has important strategic significance to alleviate the shortage of iron and aluminum ore resources in our country.Magnesium alloy sheets have shown huge market potential in the automotive field,3C products,precision instrument parts,audio diaphragms,and etching panels.However,due to the poor room temperature deformability,the processing cost of the magnesium alloy sheets is high,which limits the application of magnesium alloy sheets.How to efficiently prepare magnesium alloy sheets with good shape and excellent mechanical properties at low cost has become the key to promoting the market application of magnesium alloy sheets.In order to further reduce the production cost of preparing high-performance magnesium alloy sheets with fewer defects and excellent mechanical properties,on the one hand,it is necessary to develop magnesium alloys suitable for low-temperature deformation;on the other hand,it is very important to develop new rolling technologies for reducing energy consumption and saving costs.The previous research of our group reveals that adding a small amount of alloying elements,such as Al or Zn,to the low-cost Mg-Mn alloy can further improve the mechanical properties of the extruded bar;Furthermore,on-line heating rolling is expected to further improve the forming properties of magnesium sheets.Therefore,based on this research,Mg-2.0Zn-1.5Mn alloy,Mg-1.0Mn-0.5Al alloy with excellent mechanical properties and AZ31 magnesium alloy were selected to deforme by on-line heating rolling.In this paper,we focus on the effect of rolling temperature and deformation amount on the microstructure and mechanical properties of single-pass and multi-pass rolling sheets.The main conclusions of the paper are as follows:(1)On-line heating rolling can significantly increase the maximum deformation without edge cracks and improve formability.The comparative study on AZ31 sheets proves that online heating rolling can significantly reduce the rolling edge cracks of magnesium alloys and improve the rolling forming ability;The maximum deformation without edge cracks of the three kinds of Mn-containing magnesium alloy sheets deformed by online heating rolling in single pass and multiple passes increases with increasing the rolling temperature.While,the MA10 alloy shows the smallest value of maximum deformation without cracks and the maximum deformation without cracks of ZM21 alloy is slightly lower than AZ31 when the rolling temperature remain unchanged.The specific performance is as follows: When rolling temperature is 150?,the maximum deformations without cracks of AZ31,ZM21 and MA10 magnesium alloys deformed in single pass rolling are 53%,51% and 38% respectively.When rolling temperature is 300?,the maximum deformations without cracks of AZ31,ZM21 and MA10 magnesium alloys deformed in single-pass rolling are 71%,67%,and 66%respectively.While there is no big difference in the maximum deformation without cracks of MA10 alloy,ZM21 alloy and AZ31 alloy when the rolling temperature is same during the multiple passes defomation.(2)The microstructure of AZ31 and ZM21 magnesium alloys deformed by multi-pass online heating rolling becomes more uniform.In single-pass and multi-pass on-line heating rolling process,microstructure of AZ31 and ZM21 magnesium alloy shows similar variation tendency with temperature or pass reduction.The degree of recrystallization increases with increasing the pass deformation or rolling temperature,result in more uniform microctructure.Compared with ZM21 alloy,AZ31 alloy shows a higher degree of recrystallization in a single pass rolling.In addition,different from the single-pass rolling structure,shear bands are observed only in the multi-pass rolling structure,and the shear band area obviously increases gradually with the increase of rolling temperature.Especially,shear band disappears when rolling at 300? with 45%per pass reduction,showing a very uniform structure.Different from AZ31 and ZM21 alloys,the structure of MA10 alloy rolled in single-pass and multi-pass shpws a large number of large grains and partial twins at all rolling temperatures and deformations.The degree of recrystallization in MA10 alloy is very low,the maximum value of which is not more than 30%.The possible reason is that the dislocations are pinned strongly by a large number of evenly distributed nano-scale second in the alloy,which hinders the movement of the dislocations during the deformation process and inhibits the continuous recrystallization process.The recrystallization changes of AZ31 and ZM21 alloys are specifically manifested as follows: As the single pass reduction increases from 30% to 75%,the degree of recrystallization of AZ31 at 150°C increases from39%to 79%,and it increases from 74% to 84% at 300°C.At the same variation condition of deformation,the degree of recrystallization of ZM21 at 150? increased from 6% to 59%,and it increased from 64% to 73% at 300?.In the multi-pass rolling with per pass reduction of 33%,the recrystallization degrees of AZ31 magnesium alloy rolled at 150?,225? and 300? are 3%,40% and 84%,respectively,and the degree of recrystallization of ZM21 magnesium alloy rolled at 150?,225? and 300? are 19%,26% and 96%,respectively.(3)Pass reduction and rolling temperature have an important influence on the mechanical properties of magnesium alloy sheets.In the two alloys of AZ31 and ZM21,the effect of pass reduction and rolling temperature on the mechanical properties is different for single-pass rolling sheets and multi-pass rolling sheets.In single-pass rolling sheets,the influence of pass deformation on mechanical properties is much greater than that of rolling temperature,and with the increase of pass deformation,the yield strength of AZ31 and ZM21 alloys increases significantly,but the increase of tensile strength and fracture elongation are not very obvious.In multi-pass rolling sheets,the effect of rolling temperature on mechanical properties is greater than the effect of pass deformation,and the yield and tensile strength of AZ31 and ZM21 alloys decrease significantly with the increase of rolling temperature,accompanied by a decrease in fracture elongation.This may be due to the repeated intermediate heating in the multi-pass rolling,which causes the rolling temperature to have a greater influence on the mechanical properties than the pass deformation.The room temperature tensile properties of MA10 alloy sheets deformed by single-pass and multi-pass on-line heating rolling are relatively poor.The rolling temperature and pass reduction have very little effect on the mechanical properties.The mechanical properties in the 90° direction are significantly better than that in 0° direction,showing strong anisotropy of mechanical properties.This is mainly because the orientation of the deformed large grains elongated in the RD direction in the structure is more biased toward the TD,which makes it easier to produce twins inside the large grains when stretched in the TD direction,and improves the coordinated deformation ability.At the same time,because many large grains are elongated along the RD direction,the grain boundary density along the RD direction in the structure is lower than that in the TD direction.The representative mechanical properties of the above alloys are as follows: In single-pass rolling,the yield and tensile strength in the 0° of AZ31 deformed at at 150°C with pass reduction of 75%are 158 MPa and 262 MPa,respectively,and the fracture elongation is 23.0%.At same rolling condition,the yield and tensile strength of ZM21 in 0° are 150 MPa and 244 MPa,respectively,and the fracture elongation is 16.3%.While,the yield strength value of MA10 sheet in 0° lies between 80 MPa and 120 MPa,and the fracture elongation is less than 8%.In multi-pass rolling,the yield and tensile strength in 0° of AZ31 deformed at150°C with per pass reduction of 45% are 197 MPa and 289 MPa,respectively,and the fracture elongation is 9.7%.When the temperature increase to 300 ? with per pass reduction of 45%,the yield and tensile strength in 0° are 158 MPa and 278 MPa,respectively,and the fracture elongation is 20.8%.Similarly,the yield and tensile strength in 0° of ZM21 deformed at 150°C with per pass reduction of 45% are 194 MPa and 288 MPa,respectively,and the fracture elongation is 5.9%.When the temperature increase to 300 ? with per pass reduction of 45%,the yield and tensile strength in 0°are 168 MPa and 251 MPa,respectively,and the fracture elongation is 19.3%.However,the yield strength of MA10 alloy in 0° lise between 120 MPa and 140 MPa,and the fracture elongation is less than 6%.(4)Cryogenic treatment can improve the mechanical properties of magnesium alloy sheets.After heat treatment and cryogenic treatment,the mechanics of ZM21 alloy and MA10 alloy has improved,but it is not obvious.The best optimization scheme for ZM21 alloy is to only carry out cryogenic treatment,which can further improve the plasticity of the material while maintaining the strength.In the case of cryogenic treatment for 48 hours,the comprehensive mechanical properties of ZM21 are the best,yield and tensile strength are 178 MPa and 280 MPa,respectively,and the fracture elongation reaches 15.0%.For the single-pass rolled MA10 alloy,the mechanical properties and anisotropy can better improved by 12 h cryogenic treatment firstly and then annealing at 400? for 1h.Under this condition,the yield and tensile strength in 0°of MA10 alloy are 109 MPa and 206 MPa,respectively,and the fracture elongation is5.7%.For the MA10 alloy sheets rolled by multiple passes,cryogenic treatment or high temperature annealing is not a good way to improve mechanical properties and anisotropy.