Study On Microstructure And Mechanical Properties Of High-strength Magnesium Alloy During High Strain-rate Multi-directional Forging

In recent years,magnesium which is regarded as a potential green material has been used widely in many areas in order to save energy,protect environment and increase the use efficiency.Magnesium has the lowest density and the richer resource in earth.The structure of magnesium is close-packed hexagonal under standard atmosphere pressure,and the slip system is less and easily generate strong basal texture in the process of transformation.The use of magnesium is limited by its anisotropy and poor plasticity.Study on microstructure,texture,mechanical properties of high-strength magnesium alloy during high strain-rate multi-directional forging in this paper which provided the necessary theoretical basis and experimental data for the development of large magnesium alloy forging wit h excellent properties.The results show that the as-prepared GW93 magnesium alloys form a large number of twins and dynamic recrystallization grains,the microstructure has been refined rapidly,and the mechanical properties were significantly improved after one heat forging for 30 passes at 450 ?,which presented the ultimate tensile strength(UTS),yield strength(YS)and elongation of 302 MPa,226 MPa and 9.5% at room temperature respectively.When they are compared to the initial solid-solution state,the improved percentages are 33%?72% and 32% respectively.However,it is found that there is a significant static recrystallization,and the recrystallization grains grow up rapidly after medium annealing,meanwhile,the mechanical properties decreased.A homogeneous structure with the average grain size of 6.3?m can be obtained after GW93 alloy subjected to two heat forging for 100 passes at 450? and the complete dynamic recrystallization of the alloy occurred.During the high strain-rate multi-directional forging process,the macro-texture is weakened obviously with the increase of the forging passes,and formed a weak non-basal texture finally.Annealing process can provide a good organizational basis for the next deformation.With the increase of annealing temperature and time,the static recrystallization grains will grow gradually.In the early stage of aging(0-4h),the hardness increases slowly,this stage can be judged as the incubation period;the aging time in the stage of 4-18 h,the volume fractio n of the second phase of GW93 magnesium alloy increases rapidly with the increase of aging time,the hardness reach to its peak of HV129.16 and HV123.28 respectively at the aging time of 18 h,the mechanical properties were increased by 63.12% and 50.94%compared with the initial forging state,showing the age hardening high response ability.A fter peak aging,the strength of the alloy is further increased,the tensile strength could reach 420 MPa,but it shows a poor ductility.Microstructure evolution of AZ80 magnesium alloy during high speed multi-direction forging is studied that the microstructure evolution mainly corresponding to the two kinds of mechanism: 400? in the first stage of multi-directional forging,cumulative strain passes less than 30 passes,the main mechanism is twin recrystallization;the second stage is more than 30 passes the main mechanism for the thermally activated growth,but there are still a lot of fine grains.After forging 50 passes at 330?,the average grain size is about 5.2 ?m of AZ80 magnesium alloy.Increasing the initial forging temperature can improve the recrystallization ratio.With the decrease of the initial forging temperature,the proportion of precipitates of AZ80 magnesium alloy is increased obviously during the proc ess of multi-direction forging.For AZ80 magnesium alloys,solid solution treatment prior to forging is critical.When the beta phase cannot be dissolved into magnesium alloy matrix,non-equilibrium grain boundary coarse phase beta-Mg17Al12 rupture easily form the crack source during the forging process,it will affect the stability of AZ80 magnesium alloy forging specimen.