Deformation Behavior Of Large Strain Rolling Of Mg Alloys Sheets And Its Effect On Microstructures And Properties

Because of the advantage in reserves of resource and its low density,high strength-weight ratio,good damping and electromagnetic shielding performance,Mg and its alloys show a great potentiality to achieve light-weight goal for industry.Mg alloys become more attractive to military,aerospace,3C industries and even achieved a large-scale industrial production and application.Rolling is one of the most important ways to produce Mg alloys with variable dimentions with high surface quality.There are numbers of studies on rolling of Mg alloys,even some practical rolling methods,coming out in the past,but the commercial rolling method are still high cost and with long manufacture period,which limited the widely used of Mg alloy sheets.Base on the previous work,this paper focus on large strain rolling(LSR)of Mg alloys sheets.By increasing rolling speed and control rolling temperature,the Mg alloys sheets could be producd by large strain rolling due to enhancement of dynamic recrystallization.The sheets rolled by LSR have good surface quality with litte edge cracks and excellent mechanical properties.Further more,this method is high effective and lower cost compared to commercial one.Large deformation during rolling also help to refine microstructure,control the size and distribution of secondary phases,leads to the high performance of rolled sheets.In this paper,the possibility of large thickness reduction rolling of AZ31 Mg alloys sheets are tested and mechanisum of deformation behavior during LSR is also investigated.Rolling temperature and speed are selected as two main factors for rolling to investigated.Effect of two factors on flow behavior,microstructure and texture evolution are analysed to find the optimal condition of LSR.Further more,AZ31,AZ61 and AZ91 are rolled by LSR to observe its suitability on AZ series Mg alloys.High performance Mg alloys sheets were produced by LSR and the mechanisums,including dynamic recrystallization and precipitation,were also discussed.Sn and Ca were added into AZ31 to modifiy texture and deformation behavior.Twinning behavior and recrystallization are dicussed detailly.Conclusion of this paper are listed below:(1)Large rolling reduction as more than 70% per pass could be achieved by large strain rolling and rolled sheets obtained good surface quality without edge crack.The relationship between rolling parameters and deformation mechasium during hot rolling is fully investigated.Increasing rolling speed and temperature could improve rolling ability,which leads to high surface quality and few edge cracks.High rolling temperature is benefit for activation of non-basal slip systems and enhance of recrystallization.Increasing rolling speed also greatly promoted dynamic recrystallization(DRX).The softer of material caused by DRX prevent cracking during hot rolling,so it is the key requirement for LSR.(2)The mechanisum of recrystallization during middle/high speed rolling of AZ31 Mg alloys sheet is fully discussed.Recrystallization behavior of Mg alloys during LSP including continuous DRX,shear band nucleation(SBN)and twinning induce DRX(TDRX).Continuous DRX is the dominant recrystallized behavior during rolling and refined grains exhibits similar type of orientation as deformed grains and has relatively weak texture.Grains produced by SBN distributed along shear bands and their size is commonly samller than 1?m.Nucleation of TDRX are commonly in compresstion twin structure and {101(?)1}-{101(?)2} twinning structure.The refined grains exhibit smilar orientation as the twins.(3)Characteristics of the inhomogeneous evolution of the microstructure of the magnesium alloy sheet under large strain rolling conditions are revealed.Microstruture in LSR samples are inhomogeneous as: a)microstrcutrue at surface and in the mid-layer,b)the formation of shear bands.Surface of the sheet exhibit strong basal texture and high hardness compared to the mid-layer as the surface is full of refined grains.This difference comes from the strat of rolling when the sheet was bitting by the rollers.The surface of the sheet largelt deformed and the difference remained.The formation of shear bands is regarded as flow localization.It helps to coordinate the local strain and prevent the formation of cracks.(4)Deformation behavior and microstructure evolution of AZ-based magnesium alloy with variable Al content during large strain rolling were obtained.Addition of Al limited the formation of {101(?)1}-{101(?)2} twins during LSR,while some grain like extension twinnings in rolled AZ91 Mg alloys.In AZ31 and AZ61,the dominant recrystallized behavior is continuous DRX,while is particle stimulated nucleation and disconitous DRX in AZ91.Some Nano size of Mg17Al12 phases were observed in AZ91 and they precipitated during LSR.Recrystallized grains in AZ91 exhabits weak texture which lead to texture weakness.(5)LSR is an effective way to produce grain refined Mg sheets with excellent mechanical properties.With increasing the thickness reduction,both strength and ductility of AZ31 Mg alloys are increase.When thickness reduction reached 60% in one pass,yield strength of AZ31 rolled sheet increase ~51 MPa to 221 MPa compared to as-extruded one and elongation is ~18%.Strenth of the sheet largely increase without sacrificing ductility due to LSR.For AZ91,its yield strength increased to 322 MPa,ultimate strength reached ~422MPa and elongation to ~8%.LSR is not only an effective way to produce Mg alloys sheets,but also a method to enhance their mechanical properties.(6)Mechasium of texture modification by addtion of Ca in AZ31 Mg alloys is revealed.Addtion of Ca into AZ31 could weak basal texture by increasing fraction of non-basal texture component reffectively.The reason is appearace of Al-Ca particles which distributed randomly with an average size larger than 2?m.These phase stimulated the nucleation of recrystallized grains which exhabits random orientations.These refined grains produce non-basal texture component,leads to the weakness of rolled texture.(7)Effect of Sn addtion on deformation mechanisum during hot rolling of AZ31 alloys is fully analyzed.Addtion of Sn largely affects twinning behavior during deformation.Both number and area fraction of twins largely increase and the types and variant of twins were various.The change of twinning behavior should affect the recrystallization behavior in the further deformation.