Study On Microstructure And Properties Of ZA73 Magnesium Alloy

Mg-Zn-Al (ZA) series magnesium is widely studied because of its good characteristics such as satisfied cast ability, good heat resistant property, low cost and so on. Low alloying ZA73 magnesium maintains good over-all properties, further more, the density is low. It is quite valuable to profound study the microstructure and properties of ZA73 series magnesium.Optics microscope, X-ray diffraction analysis, differential analysis, scanning electronic microscope (EDS) and transmission electron microscope was used to study the microstructure of ZA73 and its modified alloy, and associated with universal test machine and thermal simulation (Gleeble-1500) the relationship between microstruc- ture, mechanical property and forming behavior was studied.The result shows that, there is large quantity of eutectic compound in the microstructure of ZA73 magnesium alloy. The microstructure is consist ofα-Mg phase andÏ„phase [Mg32(Al,Zn)49],most of which is semi-continuous distribution at grain boundary. After solution and aging, the many phase of the alloy is alsoÏ„phase, but the shape, number and distribution changed: most ofÏ„phase disappear after 20h solution, but it would precipitate at grain boundary after aging treatment; after 50h solution and aging there is few grain boundary precipitations, whereas, granularÏ„phase homogeneous precipitated intracrystalline. The room-temperature mechanical property of ZA73 magnesium alloy was obviously improved after solution and aging treatment, but the elevated temperature mechanical property deteriorate.With Er addition the quasi-continuous grain boundary networked eutectic compound (Ï„phase) was changed into discontinuous globular particles. In addition that, spherical Al-Er compounds were also identified in the matrix. After extrude forming, the dynamic recrystallization grain is refined. With the increase of Er content, the room-temperature mechanical property could be improved and tensile strength reaches the peak with 0.7% Er addition. The elevated temperature tensile strength doesn't always decrease with the increase of the temperature, but reaches its peak at 150℃.Deformation temperature and strain rate are the key factors influencing the flow stress and plasticity. The flow stress decreases with the increase of temperature, while increase with the increase of strain rate. Moreover, when deformed in the temperature range of 200²50℃, net-worked dendritic microstructural feature disappears, instead, the second phases disperses in the matrix in the form of particles of smaller size. The microstructure goes coarse when deformed at 300℃under lower strain rate. It is concluded that the increase of the amount of small particles and the coarsening of microstructure lead to the deterioration of hot plasticity. Higher strain rate and temperature is considered to favour hot deformation.Bars with sound surface quality were successfully extruded at 350℃under higher strain rate of about 0.1 S^-1. The tensile ultimate strength of the as-extruded bar reaches 355MPa, in the mean time maintaining a high elongation of 19%. In addition to, the grain size was refined to 2～7μm.