Effect Of Rare-Earth And Strontium On Rolling Capability Of AZ31 Magnesium Alloy

As one of the lightest alloys,magnesium alloys(Mg alloys) are widely used in aerospace engineering,automobile industry,computers,and electronics,etc.However,due to its HCP lattice structure,Mg alloys are hard to deform at room temperature.Therefore,most of the Mg-based components are prepared by casting.One of the striking problems is that because of the coarse grains in the microstructure,in that way,the mechanical properties of the Mg alloys are not desirable.In addition,many defects can form in the microstructure.All of the above problems and many others will induce limitation of the application of Mg alloys.One solution is plastic working,which can change the original microstructure,such as the grain size,and then improve the mechanical properties.Due to it,the working methods are paid a great attention to,in the area of Mg alloys and their application nowadays.However,due to the inferior working capabilities of Mg alloys,reduction value per pass is very limited.Multi-rolling procedure complicates the processing technology of Mg alloys.In the meanwhile,as dendriticα-phase touches attaches to the neighbor grains in the microstructure,the net-likeβ-phase is distributed at the grain boundaries.It restricts the change in the matrix.Therefore,before plastic working and rolling,the alloys have to experience the homogenization for long time.In addition,the alloys have to be heated to a required temperature every time before the rolling process.If the reduction per pass can be increased,and the homogenization can be shortened,both the productivity and efficiency will be greatly enhanced.We have already known that grain refinement can improve the working capabilities of alloys, and the additions of RE into the alloys refine the grain.In this work,we use the widely used AZ31 Mg alloy as the subjects.In the meanwhile,we studied the effect of RE(Ce,Gd,Dy) and Sr on the reduction per pass and total reduction of AZ31 alloy.The effect of homogenization on the alloy has been investigated.We also discussed reason for the improvement of rolling properties of AZ31 alloy. One of the results is that RE-AZ31 alloy is nonsensitive to the homogenization.It is also noticeable that the enhancement in rolling properties may benefit the direct processing of AZ31 alloy,with exemption of homogenization.The designed thicknesses of the alloy samples during solidification are 28 mm,21 mm,14 mm and 7 mm respectively.It is proved that the as the thickness changes,the cooling rate also changes,which could lead to change in the grain size and rolling properties.The novelty to this work is as followed:1.Both the AZ31 samples with and without homogenization were studied,to investigate the effect of homogenization on the rolling properties.In addition,the dendriticα-phase,net-likeβ-phase and the grain size affected the rolling properties.After long homogenization,rolling properties are greatly improved,which resulted from change in the shape ofα-phase and diffusion ofβ-phase into the solid solution.Due to it,a process with exemption of homogenization is introduced for the first time.Through adding RE and Sr,isometricα-phase is formed during solidification.In addition,the amount ofβ-phase is decreased and the net is broken.Under this situation,after being heated for 5 min and without homogenization,the reduction per pass of the alloy outnumbers original AZ31 alloy with processed by homogenization for 12 h.When the RE-AZ31 is homogenized,the rolling properties are only slightly improved.The change is much less than that of the original AZ31 alloy.In this way,the rolling process without homogenization is achieved.2.We studied the effect of grain size on rolling properties of AZ31 and RE-AZ31 alloy.As the grain size is decreased,the rolling properties are improved.through Unary Linear Regression Model, the relationship between reduction per pass and grain size is achieved for the first time.The result is as followed:y=68.2066-0.6275x(without homogenization),y=86.178-0.857x(with homogenization)x is the grain size.y is the reduction per pass of the alloy.The maximum practical reduction can be calculated in the formula.It can give valuable contributions to the mass production,as the very theoretical basis.3.The relationship between the rolling capability and microstructure has been investigated.It is found that during rolling,the factors order as the shape ofα-phase＞the shape and amount ofβ-phase＞the grain size.A deformation model is introduced in this work.The entire process is divided into three periods.In the first period,the grains move in the microstructure,driven by the rolling force.As the experiment progresses,the grains begin to deform.In this period the movement of grains is not finished.In the last period,the movement of grains is already finished.The grain deformation is dominant in the microstructure.When theα-phase is dendrite,the branches of the phase may extrude into the neighbor grains.As a result,the grains attach to each other tightly,which severely restricts the rotation and other movements of individual grains.When net-likeβ-phase is distributed at the grain boundaries.The neighbor grains may be contained in it.The movement of grains is also restricted.Under this situation,the effect of deformation in the first and second periods on the total reduction is very limited.The main deformation and reduction is supported by the third period.When the grain size is relatively large,the movement of facets covers a long range,which may make dislocation exceed slip limit,and then lead to the formation of cracks.When the isometricα-phase is formed,branches are decreased.The attachment to neighbor grains is also decreased. When the amount ofβ-phase is decreased and independently distributed.The restriction to the movement is decreased.When the grain size is decreased,slip of the small grains is shorter than that of the large ones.With respect to the same reduction,the disloeation of small grains does not exceed the slip limit.However,the dislocation in the large ones reaches and even exceeds the slip limit. Compared to the original alloys,the RE-AZ31 alloy will benefit from the grain refinement.4.The effect of RE and Sr additions on rolling properties of AZ31 alloy has been investigated, when they are independently added into the alloy.As their amounts rise,the grain in the microstructure is decreased.In the meanwhile,theβnetwork is broken.As a result,the rolling properties have been enhanced.When the additions reach their optimum amount in the alloy,the grain refinement is most obvious,andβphase is intermittently distributed.The rolling properties at this point reach the peak.However,when the additions are continuously added,the grain size is increased slightly.Yet the rolling properties are restrained.In this work,the optimum amounts of Ce, Gd,Dy,and Sr are respectively 0.65 wt.%,2 wt.%,1.5 wt.%,and 0.09 wt.%.5.The effect of the RE-Sr composite addition on the rolling properties of the AZ31 alloy has been investigated.As a result,when RE-Sr binary addition is used,the effect of the composite addition on the rolling properties is the sum of the effects of two individual elements.When the optimum amounts of the both elements are added into the alloy simultaneously,the interactive effect is not optimum.When the RE-Sr ternary addition is used,the effect of elements orders as Gd＞Ce＞Sr.The optimum composition of the elements is 0.65 wt.%Ce-2.0 wt.%Gd-0.06 wt.%Sr.It is concluded that with respect to the improvement over the rolling properties of AZ31 alloy,the effect of the ternary addition is stronger than that of the binary one.