Effect Of Ti On Microstructure And Properties Of Mg-Zn Alloy

Magnesium alloys,as green light metal structural materials,have the advantages of high specific strength,high specific stiffness,good shock absorption and easy recovery.Among them,Mg-Zn alloy is one of the most widely used magnesium alloys in commercial applications.Its strength is high and aging hardening effect is remarkable,but its casting performance is poor,and it is easy to produce serious hot cracking tendency and micro-porosity,and the grain and non-equilibrium divorced eutectic structure are coarse.Previous studies have shown that Ti microalloying has a certain potential to improve the as-cast structure of the alloy.Moreover,when Ti is added with Mg in the way of Zn-Mg-Ti master alloy,the as-cast structure of the alloy has fine grains,and the mechanical properties and corrosion properties have been significantly improved.However,due to the difficulty in controlling the composition of Zn-Mg-Ti master alloys and the difficulty in controlling the preparation process parameters,the addition effect of Zn-Mg-Ti master alloys is still unstable.At the same time,the effects of Zn-Mg-Ti master alloys（including alloy composition,morphology,etc.）on as-cast structure,properties and solidification behavior,as well as the mechanism of Ti on Mg-Zn alloy need to be further studied.In view of the above problems,the effects of master alloys and Ti elements on the structure,properties and solidification behavior of Mg-Zn alloys have been studied.Firstly,the intermediate alloys consisting of Zn_82.8Mg_14.1Ti_3.1（A）and Zn_81.9Mg_15.1Ti₃（W）were prepared by varying cooling rate,and the effects of different process parameters on the structure and morphology of Zn-Mg-Ti master alloys were investigated.Secondly,the intermediate alloys with 8%addition of Zn_82.8Mg_14.1Ti_3.1（A）and Zn_81.9Mg_15.1Ti₃（W）were added into pure magnesium respectively to obtain Mg-A-720 and Mg-W-720 as-cast alloys.The effects of two kinds of master alloys on the structure,properties and solidification behavior of as-cast alloys were studied by OM,XRD,SEM,EDS and DSC.Finally,the mechanism of Ti on Mg-Zn alloy was discussed by calculating phase diagrams.The experimental results show that the preparation process significantly changes the microstructure of Zn-Mg-Ti master alloy.Increasing cooling rate will change TiZn3phase from"regular block"to"irregular block"and Zn-Mg-Ti quasicrystalline phase from"four-petal"to"dendrite".With the increase of cooling rate,the size of the second phase in the master alloy decreases obviously,and the volume fraction of the quasicrystalline phase increases.The master alloys with different microstructures affect the microstructures and solidification behavior of the final alloy materials.Although the as-cast structures of Mg-A-720 and Mg-W-720 are composed of a-Mg,Mg₅₁Zn₂₀0 and a few MgZn phases,the second phase in Mg-A-720 is reticulate,while the second phase in Mg-W-720 is granular or rod-like.The solidification process of Mg-6.5Zn alloy under non-equilibrium conditions（Scheil model）is L→a-Mg+Mg₅₁Zn₂₀.The eutectic temperature is about 341℃,the liquidus temperature is 631℃,and the volume fraction of Mg₅₁Zn₂₀0 in the alloy is about 6.3%.It is found that the eutectic temperature of Mg-W-720 is slightly higher than that of the corresponding Mg-Zn binary alloy,the liquidus temperature is higher than that of the binary alloy,and the volume fraction of the second phase is lower than that of the binary alloy.This indicates that Ti can improve the thermal stability of the matrix and the second phase in Mg-Zn alloy and inhibit the solidification and precipitation of Mg-Zn phase.Mg-A-720 and Mg-W-720 alloys exhibit different microstructures and properties under the same heat treatment conditions（400℃/12h+175℃/10h）.Among them,the solution effect of Mg-W-720 is obvious,while the second phase of Mg-A-720 remains at the grain boundary.The hardness of the two alloys after aging treatment is higher than that of the solid solution state,and the aging strengthening effect is significantly higher than that of the corresponding binary system.Among them,the aging strengthening effect of Mg-W-720 alloy is better than that of Mg-A-720 alloy.