The Effect Of La On Microstructure And Properties Of AZ31 Magnesium Alloy

As the lightest light metal structure material,Magnesium alloy has extensive application prospect in the business areas and is considered as eco-structural material in future.However,magnesium alloy,which is hexagonal crystal structure,is seriously restricted in developing further,because of its poor plasticity,poor forming ability at room temperature,and low absolute strength.In this paper,the effects of rare earth of La,cold-rolling process and recrystallization annealing process on the microstructure,mechanical property,corrosion resistance were investigated by using room temperature tensile test,Vickers-hardness and the Tafel plot measurement,optical microscopy(OM),X-ray diffraction(XRD),energy spectrum analysis(EDS)and the Scanning electron microscopy(SEM).Several research contents and conclusions can be summarized as follows:(1)To study the effects of La on microstructure and properties of AZ31 magnesium alloy,we designed some experiments that the contents of La are 0.2%,0.3%,0.4%,0.6% and 0.9%.It shows that the grain size of AZ31 can be refined obviously by adding a small amount of La result of receiving more fine and equiaxial grain structure.What’s more,the original coarse second phase Mg17Al12 is also refined resulting to the producing of the Al11La3 phase with good thermal stability,whose size is very fine like needle and its number is increasing more and more.When the content of La is 0.6%,the grain size can be refined to 40μm and the mechanical properties of AZ31 will be much better.In addition,the adding of La can improve the corrosion resistance of AZ31.When the La content is 0.4%,the corrosion resistance of the alloy after aging treatment with 75% total processing ratio is much better,and its static corrosion rate is about 10mm/a.What’s more,When the content of La is 0.6%,the static corrosion rate of the alloy is 17.9mm/a,which is still lower than that of the commercial AZ31 magnesium alloy whose static corrosion rate is about 60mm/a.(2)In order to study the effect of the cold rolling parameters on microstructure and properties of AZ31-0.6La alloy,three kinds of rolling process are designed with the total reductions of 10%,20% and 30%,respectively.It shows that the critical cold-rolling reduction is 30%,and if the reduction is larger than that,the microstructure of the alloy will be cracked.When the cold-rolling reduction is 10%,there will be a large amount of dislocations and twins in the microstructure of the alloy.And after the treatment of the 240℃×1h annealing and 200℃×24h peak-aging,the mechanical properties and corrosion resistance of the AZ31-0.6La alloy will be much better,whose strength,yield strength,elongation and static corrosion rate in the aging condition with 75% total processing ratio is 340 MPa,240MPa,25%,1.02mm/a,respectively.(3)To study the influence of the recrystallization annealing processing on the microstructure and mechanical properties of the 10%-cold-rolled AZ31-0.6La alloy plates,we designed some experiments whose recrystallization annealing temperature is 240℃,280℃ and 320℃ with its time from 5min to 180 min.It shows that after annealing treatment of 240℃×2.5h,the static recrystallization of the microstructure will complete,getting much more fine recrystallized structure.(4)Combined with Vickers-hardness(total cold-rolled reduction include 5%,10% and 15%,whose recrystallization annealing temperature is 240℃,280℃ and 320℃ with its time from 5min to 180min)and Arrhenius equation,the static recrystallization kinetics equation of AZ31 magnesium alloy by determining the activation energy of static recrystallization of AZ31 magnesium alloy and a model on the changing law of grain size will be established.