Study On Microstructure And Properties Of Mgnesium Alloys In Casting-Forging Complex Forming

To study the evolution rules of microstructures and mechanical properties of magnesium during cast preformed forging process, the typical magnesium AZ81 alloy is selected as the researching object. By the analysis of deformation behavior of as-cast microstructure during hot press deformation, a constitutive mold of the isothermal deformation was established. The effects of rare-earth addition and compression ratio on microstructure and properties are also investigated. All these lay a sound base to the application of the cast preformed forging process.By means of DTA analysis, thermal simulating test(TST),XRD,OM,SEM and EBSD, the squeeze casting and homogenizing annealing process, the evolution of microstructures and properties during forging deformation and post heat treatment and the deformation behavior during hot press of AZ81 and AZ81E is investigated. And the evolution rules of microstructures and properties, the relationship between flow strain and stress, the constitutive mold and the processing mapare characterized. The main conclusions are as follows:1. AZ81E alloy is prepared by adding rare-earth into AZ81 alloy, and both squeeze casting specimens are selected for the investigation of microstructures and mechanical properties. The results show that: mechanical properties with rare-earth addition are significantly improved as a result of the refinement of grain and distribution of rare earth compounds on the grain boundaries.2. To improve the plastic deformation properties, the effects of homogenizing annealing process on the deformability of magnesium are investigated, and the best homogenizing annealing regularities of the two alloys are both annealing at 400℃for 8h±2h.3. The evolution of flow strain under hot deformation in the range of 340～430℃at 0.003～3.0s-1 strain rate is investigated, and the statistic mold to determine flow stress by introducing the Zener-Hollomon parameter are proposed:a) Relationship between the peak stress and the Z parameter of AZ81 alloy Relationship between the DRX peak strain and the Z parameter of AZ81 alloyb) Relationship between the peak stress and the Z parameter of AZ81E alloy Relationship between the DRX peak strain and the Z parameter of AZ81 alloy4. The plastic processing map of AZ81 and AZ81E alloys at compression ratio of 0.2～0.8 are established according to the curves of flow stress-strain. And the best processing regularities of the two alloys are deformed at 370℃for 0.03 S-1 strain rate and at 400℃for 0.03 S-1. And the critical strain of DRX, peak strain, saturated stress and starting point of recrystallization under the propose processing are obtained by analyzing the hardening curves.5. The effects of temperature and strain rate on microstructure of magnesium under hot deformation are analyzed by introducing the Zener-Hollomon parameter and the relationship between the average sizes of grain and deform processing.6. The evolution of microstructures and orientation texture,and the changes of mechanical properties of the specimens after forging deformation are investigated, and the changes of properties and distribution mechanisms ofβ-phase are also studied.All above lays a sound base for the low-cost production of high performance Mg parts.