Effect Of Rolling On Microstructure And Mechanical Properties Of LZ91 Alloy

Compared with traditional alloys,magnesium-lithium alloys have lighter density,higher specific strength,specific stiffness,and easier plastic deformation,and have been applied in aerospace and 3C fields.Mg-9Li-1Zn(LZ91)alloy,as a typical commercial grade duplex magnesium-lithium alloy,has good comprehensive mechanical properties and has attracted wide attention.At present,there are many shortcomings in the rolling process of LZ91 alloy,such as the edge of the alloy after cold working large deformation is easy to crack,the formation mechanism of Zn-containing precipitation phase and magnesium-rich phase is not clear,the alloy strengthening mechanism is unknown,and it is difficult to design the rolling process based on the law of microstructure evolution.Therefore,it is particularly critical to grasp the microstructure evolution of LZ91 alloy in the rolling process and its influence on the mechanical properties of the alloy.Based on the cast LZ91 alloy,this paper explores the influence of the rolling process on the microstructure and mechanical properties of LZ91 alloy,and conducts microstructure characterization and mechanical properties testing of the alloy through optical microscope,scanning electron microscope,electron probe,transmission electron microscope,X-ray diffraction,Vickers hardness tester and universal tensile machine,revealing the microstructure evolution law of LZ91 alloy in the rolling process,and providing a theoretical basis for the rolling process design of LZ91 alloy.In this paper,the microstructure of the cast LZ91 alloy,which contains two phases:an irregular blocky a phase and a β phase.Subsequently,the effects of 250℃ high temperature rolling,25℃ room temperature rolling and-196℃ low temperature rolling on the microstructure and mechanical properties of the alloy were studied in turn.The study found that in the samples of the three processes α phases will be flattened and elongated,and the magnesium-rich phases will be precipitated in the β phases.The difference between the three is that dynamic recrystallization occurs in the β phase of the sample rolled at high temperature,and the size is between 5-20 μm,which plays the role of fine crystal strengthening;in addition,the magnesium-rich phase in the β phase is mostly long strips with a width between 3-5 μm,and the strengthening mechanism of high temperature rolling is fine crystal strengthening and precipitation strengthening.There is no dynamic recrystallization in the samples rolled at room temperature and low temperature,and the magnesium-rich phases in the β phase are needle-like and punctate.The strength of the alloy prepared by the two processes was greatly improved,taking the deformation amount of 70%as an example,the tensile strength increased by 53%and 55%respectively,but the plastic shape decreased more,and both decreased by about 66%.The strengthening mechanisms of room temperature rolling and low temperature rolling are process hardening and precipitation strengthening.In this paper,the effects of 250℃ high temperature rolling 25℃ room temperature rolling process on the microstructure and mechanical properties of alloys were studied.The study found that this process has the advantages of both high-temperature rolling and room temperature rolling,that is,the α phase is flattened and elongated during the high-temperature rolling process,dynamic recrystallization occurs in the β phase,the structure is refined,the long strip magnesium-rich phase is precipitated in the β phase,and the MgLi2Zn and MgLiZn phases are also precipitated around the α phase and in the β phase.Subsequent room temperature rolling makes more reinforced phase MgLi2Zn phases retained,and is not completely converted into a softened phase MgLiZn phase,and a large number of small ellipsoidal secondary magnesium-rich phases are precipitated in the β phase,these magnesium-rich phases and MgLi2Zn phases play a role in precipitation strengthening,and the mechanical properties of the alloy are greatly improved in the room temperature rolling process.The strengthening mechanism of the alloy is the second phase precipitation strengthening,fine crystal strengthening and process strengthening.When the deformation of high-temperature rolling is 80%and the deformation of room temperature rolling is 10%,the comprehensive mechanical properties of the alloy are the best,and the yield strength,tensile strength and elongation are 233 MPa,251 MPa and 20%,respectively.After this paper,the influence of 250℃ high temperature rolling in the cross direction+25℃ room temperature rolling in the unidirectional direction on the microstructure and mechanical properties of LZ91 alloy was studied.It is found that cross-rolling can change the distribution of α phases and magnesium-rich phases,especially the distribution of magnesium-rich phases becomes disordered with the change of rolling direction.A large number of magnesium-rich phases with cross-distribution are precipitated in the β phase,which divides the β phase into countless small meshes,which destroy the integrity and continuity of the β phase and form a microscopic organization similar to "small grains".Compared with the one-way high-temperature rolling+unidirectional room temperature rolling process,the tensile strength of the cross-rolled alloy is 20 MPa higher,which benefits from the small grid formed by the magnesium-rich phase during the cross-rolling process,which plays a role in refining the microstructure.