The Research On Microstructure And Mechanical Properties Of Mg-6Zn-1Y-Zr Magnesium Alloy

Mg-6Zn-1Y,Zr alloy is one of Mg-Zn-RE series important representative alloys of magnesium alloy,its mechanical properties is very superior due to its strengthening and toughening quasicrystal I phase and relatively low rare earth content and is applied in automotive,aerospace,national defense and other fields.But the alloy's performance stability is poor because its thermal mechanical processing performance is poor and the alloy is easy to break during the process of rolling and forging induced by the relatively high zinc content.For this case,the Mg-6Zn-1Y-Zr alloy is chosen as experimental material in this paper,and its microstructure and mechanical properties of the casting alloy are analyzed and researched in detail,the heat treatment process of the alloy is further optimized,and influence of the forging and rolling on ZW61 alloy's microstructure and mechanical properties is systematically researched.The second phase of as-cast ZW61 magnesium alloys is I phase and a small amount of W phase.I phase is located at the grain fork boundary,net-like distribution,and a small amount of W phase mainly exists in the grain boundary in the form of rods and granular.By multilevel solid solution treatment,the thick part of the second phase will be dissolved in the matrix,and the higher the solid solution temperature solid solution,the more obvious effect.Because more second phases are dissolved in the magnesium substrate,the ZW61 alloy samples with 500 ? of ultimate solution temperature showed the best solid solution strengthening effect and age hardening effect,the hardness of solid solution state and subsequent age hardening behavior of the sampe is better than low temperature solid solution of the other two kinds of samples(The ultimate solution temperature of 460 ? and 420?respectively).However,a small amount of low melting point phase in the alloy melt that is induced by the high temperature solid solution,So the ZW61 alloy with 500? of solid solution temperature has poor mechanical properties.Relatively,the ZW61 alloy samples with 460? of ultimate solution temperature,a follow-up after peak aging,room temperature show the best mechanical properties,tensile strength,yield strength and elongation can reach 229.86 MPa,122.89 MPa and 9.84%,respectively.After the hydraulic forging,no obvious dynamic recrystallization is observed on the ZW61 alloy,and the original solution state bulky grain direction along the finish forging plate plane gets squashed,grain deformation degree increases with the decrease of the thickness of finish forging,the second phase not being solid soluted into magnesium matrix was broken after forging,and along the forging forming streamline plate plane direction.Because of the addition of a small amount of rare earth elements Y,forging process will stimulate more basal slip,and cause ZW61 alloy not to form typical base texture after forging,but the texture intensity increases with the decrease of the thickness of final forging.Although there is no obvious dynamic recrystallization,elaboration is limited,the big strain after forging which results in the organization formed a large number of dislocation,shows obvious strain strengthening effect.Therefore,compared with the casting alloy,the room temperature mechanical properties of ZW61 alloy forgings rise significantly.Final forging thickness is 70 mm,after peak aging,along the surface of the length of the alloy,the room temperature tensile strength,yield strength and elongation can reach 266.13 MPa,163.99 MPa and 11.9%,respectively.In addition,ZW61 alloy forgings shows obvious mechanical anisotropy,mechanical properties,especially the yield strength of the plate thickness direction are much smaller than the direction of the plate,the anisotropy increases with the decrease of the thickness of final forging.In order to study the microstructure and mechanical properties evolution of ZW61 alloy under the condition of high strain rate deformation,the high-speed hammer forging and rolling two hot machining processes for plastic processing was selected.Through multi-channel time,direction,small deformation hammer forging processing,it is found that the ZW61 alloy microstructure shows no obvious change,and no obvious recrystallization and twin appear.The original solution state bulky grains is not like hydraulic forging grains which occur obvious deformation.But,after the hammer forging processing treatment,the ZW61 alloy mechanical properties at room temperature,especially the yield strength have been improved due to fine grain strengthening and deformation strengthening.The solid solution and hydraulic forging ZW61 alloy samples after the hot rolling process,solid solution state samples after rolling have been found severe crackle,so it can be concluded that ZW61 alloy ingot is not suitable for treatment using the hot rolling process,but forging ZW61 alloy samples remain relatively intact after hot rolling and only the pass reduction increases to 30%,an obvious edge crack appears,and extends to the surface.The microstructure of rolled plate and hydraulic forging ZW61 alloy is very similar,there are the large elongated deformation along rolling direction mainly grain,few recrystallization organization.Texture also is a base surface texture,compared with the hydraulic forging state,texture intensity increased obviouly,it is likely to be result of increase of rolling direction elongation sharply.