| Research on microstructures and mechanical properties of Mg-Mn-RE alloy for the characteristics that strength and deformability of magnesium alloys are generally low. And then, the specimen was extruded at 400℃which was heat-retaining 7h at 400℃. The extruded specimens were forged. And now, the effect of heat treatment processes and alloying element for alloy phases precipitates are unknown. So research on the effect of different types of cooling (air cooling and water-cooling) and alloying element (Al, Mn, Zn, Y, Ce, and so on) for alloy phases precipitates of Mg-Mn-RE alloy, commercial AZ61, AM50 and AM50+RE magnesium alloys. The research work of the dissertation includes:(1) In order to obtain suitable microstructures for extrusion, influences of different heat treatment processes on microstructure and mechanical properties of Mg-Mn-RE alloy were studied. Test results show that alloy elements, Y and Ce, weren't solution at all, but dendritic crystal was disappeared at 400℃and 500℃heat treatment. Low melting point alloy phases precipitates smelt, solute and refine at 620℃heat treatment.(2) The specimen was extruded at 400℃and 5 mm/s rate which was heat-retaining 7h at 400℃. Test results show that the grain was refined and the dynamic re-crystallization occurred during extrusion. As cast grain size is about 100μm, and extruded grain size is about 5μm. The results of mechanical properties of the extruded alloy exhibit the hardness, tensile strength, yield strength and elongation are 68 HV,255 MPa,191 MPa and 26%, respectively.(3) Mg-Mn-RE alloy was forged at room temperature,360℃and 450℃, for grain size is big, formability is low and strength is low of magnesium alloy as cast and heat treatment. Test results show that the grain was distorted and the rare earth compounds separated, refined and uniformly distributed in the matrix during forging, at room-temperature forging deformed process, forging 3 times molding specimens. And hardness value of the forged alloy is higher than that of extruded alloy, the value of that is 101 HV when the forging deformation is 25%. When the relative deformation is 25% for different forging times (1 times,2 times and 3 times) molding specimens at 360℃, alloy specimens have happened to the dynamic recrystallization, grain was coarse, dislocation and twin crystal density is larger of forging 2 times molding sample than 1 times and 3 times and it's hardness is HV 85. Grain size increased and hardness reduced as forging temperature rising, but the level of rare earth compounds separated, refined and uniformly distributed in the matrix was reduced, when the relative deformation is 25% for forging 3 times molding specimens at different temperature (room temperature,360℃and 450℃). (4) For Mg-Mn-RE alloy, alloy phase particles precipitate in water-cooling process at 520℃. For AZ61 and AM50+RE magnesium alloys, alloy phase particles precipitate in air-cooling process at 520℃and heat-retaining 6 h. For AM50 magnesium alloy, alloy phase particles precipitate in heat-retaining process at 520℃and heat-retaining 6 h.(5) Alloy phase particles precipitate in heat treatment process, due to Al and Mn elements addition to magnesium. Alloy phase particles precipitate in air-cooling process, due to Zn element addition to magnesium. Alloy phase particles precipitate in water-cooling process, due to rare-earth element, Y and Ce, addition to magnesium.Research on extrution and forging provides theoretical basis for the wide application of magnesium alloy. Analysis of alloy phase particles precipitation in heat treatment processes is very important for magnesium alloy basic theory study. |
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