| Magnesium and magnesium alloys are considered as green engineering materials during 21st, while semisolid forming or semisolid processing (SSF/SSP) is called as new metal forming technology during 21st. Thixomolding?, used to manufacture magnesium products, is the unique commercial SSF and is getting more attention, the products of which have low porosity, low shrinkage, high mechanical properties, near net shape, high resistance to corrosion and high dimensional stability. The forming process is a low-cost, environmentally friendly, high-speed injection molding process with simple and safe operating, high efficiency and energy management. With these advantages, this technology has great applied potential and should be the sample of the new SSF during 21st.During the forming process, main processing parameters have great effect on the microstructure and properties of the products, such as barrel temperature, screw rotation speed, shot velocity and mold temperature. However, the basic theory of the technology hasn't been built until now, for example, the effect mechanism of the processing parameters on the microstructure is not clear, the evolution mechanism of the mechanical properties is not deeply researched, heat treatment of thixomolded products hasn't been studied, the processing simulation is not complete, etc. Therefore, the related research to the above problems will have great benefits on the basic theory and practical manufacturing of Thixomolding?.A JSW JLM220-MG prototype Thixomolder?, equipped with a mobile phone housing mold, was used with the raw material of AZ91D for this experiment. In this study, the microstructure characteristics of thixomolded AZ91D were deeply studied; the quantitative analysis system for semisolid magnesium alloys was programmed and used to know the microstructure evolution; the experienced evolution model of unmelted solid particles was summarized combining with the results of isothermal heat treatment of AZ91D chips, of which the thermodynamics and physical characteristics was then discussed. Based on the porosity level, microstructure and the tensile fracture morphology, the effect of processing parameters on the tensile properties and its mechanism were also conducted. Meanwhile, heat treatments of thixomolded AZ91D were made to study the microstructure and hardness evolution. In the end, the viscosity model of semisolid AZ91D for this experiment was built to simulate the filling process, and then optimize the processing parameters and instruct practical production by the simulated results.The results show that typical thixomolded AZ91D microstructure includes unmelted solid phase and liquid phase. The unmelted solid phase can be divided into four kinds, such as spherical solid particle, irregular solid particle, solid particle with liquid pool inside and solid particle with liquid alloy inside. The irregular solid particle is thought as the middle developing morphology between spherical solid particle and solid particle with liquid alloy inside, and the solid particle with liquid pool inside maybe the junior stage of the solid particle with liquid alloy inside. The solid particle with liquid alloy inside could be destroyed into irregular solid particle and finally spherical solid particle. The spherical solid particle is the best and final morphology of the solid particles. Theoretically, the solidified microstructures are different for the semisolid alloy with different solid particle fraction. When the solid volume fraction is lower than 21.8%, the solidified microstructure of liquid alloy in semisolid AZ91D can be primary solid phase and secondaryβphase. When the solid volume fraction is between 21.8% and 76.1%, the solidified microstructure of liquid alloy in semisolid AZ91D can be primary solid phase and secondaryβphase and eutectics. However, actually, the concentration of Al in the liquid phase will be more and more along with the solidification process, and eutectic reaction should happen when the concentration reaches 12.7%. Therefore, the final microstructure of semisolid AZ91D should be unmelted solid phase, primary solid phase, secondaryβand eutectic. The primary solid phase nucleates and grows in the liquid phase of semisolid AZ91D until the unsteady growth appears which is small and round and is mainly influenced by cooling rate. The primary solid particle can grow steadily as large as 6.52-13.04μm under the super cooling degree of 1-2K. Because of the richness of solute in the liquid phase, the content of solute in the primary solid particle is more than unmelted solid particle and increases with the decrease of solidification temperature.In order to preferably analyze microstructure of thixomolded magnesium alloy, a system on quantitative metallography of semisolid microstructure is exploited by using the VC++ program. The system can analyze the feature parameters, such as the solid fraction, the effective solid fraction, sphericity and its distribution, particle size and its distribution, etc. The connected particles can be effectively identified. The statistic results display that the solid volume fraction and size decrease with more regular morphology when barrel temperature increases; the solid volume fraction doesn't have much change but the size decrease with more regular morphology when screw rotation speed increases, while the unmelted solid volume fraction and the size don't have much change but primary solid phase and eutectic are refined with the increase of shot velocity or decrease of mold temperature.The AZ91D chips used for this study have an asymmetrically deformation during mechanical chipping, mainly concentrating on the chip edge and the interface ofα/β. According to the results of isothermal holding with changing holding temperature or holding time of AZ91D chips, the microstructure evolution during the holding is as followed: deformed alloy→recrystallization→eutectic melting and solute diffusion→melting ofα-Mg interface, spheroidization, precipitation of liquid pool→liquid immergence and convergence→solid detachness→Ostwald ripening→unmelted solid particles. Basing on the effect of processing parameters on the unmelted solid particles and combining with the results of isothermal holding of AZ91D chips, the transformation process of unmelted solid particles within thixomolded AZ91D is concluded: deformed dendritic→recrystallization, eutectic melting and solute diffusion→melting at the position of solute segregation, liquid immergence and convergence, solid detachness→agglomeration and deagglomeration after collision→spheroidization and Ostwald ripening. Compared to the results of compressed AZ91D during isothermal holding, the extent of AZ91D chips is heavier than the critical point of 2%-3% for recrystallization and the heating temperature in the barrel is higher than the recrystallization temperature provided by literatures. Recrystallization therefore definitely occurs during thixomolding process, driven by the residual energy in the AZ91D resulted from mechanical deformation with the critical size of 2γ/△Es. Due to the consistency of the isothermal holding results for AZ91D chips or compressed AZ91D ingot, the size and sphericity of the unmelted solid particles is quantitative analyzed in the same position of the compressed AZ91D ingot, which represents that irregular solid particles will be rounder and rounder because the melting point of the bulge in the solid particles' surface decreases as a result of the big curvature. And in the end of isothermal holding, the small particles will be smaller and smaller and then disappear because of the concentration grads. The elements in the small particles transfer into big ones and the big ones grow. The total number of the solid particles will reduce and the average size increases. According to the calculation of solution displacement of Al in Mg, spheroidization and Ostwald ripening, driven by the reduction of the interfacial energy, can totally happen in AZ91D chips when heated in the barrel. At the same time, if the semisolid alloy is considered as the suspension of solid particles in the liquid phase, there are four kinds of structures for the semisolid slurry including compact arrangement, percolating network, suspension of clusters and dispersed suspension. The viscosity changes correspondingly to the suspension structures, and then influences the final microstructure and properties.By the results of tensile experiments, it is found that tensile properties are better at higher barrel temperature or higher shot velocity, while they are worse at higher screw rotation speed or higher mold temperature. However, the tensile properties are satisfactory during the wide processing conditions for thixomolded AZ91D and better than the ASTM standard value of high pressure die casting AZ91D, which can be interpreted by the strength improvement ofα-Mg because the dislocation movement is limited in it. Moreover, the low level and uniform distribution of porosity and the refinement of the grain make great contribution to the high strength. The fracture mechanism is complicated for tensile thixomolded AZ91D sample but belongs to quasi-cleavage fracture by the fracture morphology. Irrespective of the influence of porosity, the crack generates and grows in the solidified liquid phase, actually in the interface of primary solid phase and eutectic phase, stops and changes the direction in the interface of unmelted solid phase and liquid phase; and then it can move on along with the interface of unmelted solid phase and liquid phase for the alloy with high solid volume fraction, or move on along with the interface of primary solid phase and go through the eutectic which is mainly for the alloy with low solid volume fraction. The results of heat treated thixomolded AZ91D show that the secondaryβand theβphase in eutectic will dissolve into unmelted solid phase and primary solid phase to form single-phase supersaturated solid solution. The small solid solution will gradually disappear and the average size of the solution solid will increase with the increase of solution temperature. The hardness of the solid solution will get improvement at higher solution temperature because of the solid solution strengthening. The precipitate for the followed aging treatment isα-Mg→β-Mg17Al12. Under the effect of separation driving energy, the secondary phaseβwill separate out more and quickly when the aging temperature is higher, the hardness will correspondingly increase. Increasing the aging time, theβphase will be precipitated discontinuously and be more and more in the grain boundary without going through GP area and middle phase. The morphology will be sheet and cystiform and then be cystiform growing into grain from particle shape. The continuous precipitation will appear in the grain when the discontinuous precipitation is enough. The hardness of the solid particle changes with the solution, the number, the morphology and the precipitation method.In the end of this study, initial viscosity and steady viscosity are modeled with shear rates and solid fraction using the data of semisolid AZ91D slurry in compression tests and Couette viscometer. The results of filling analysis for thixomolded AZ91D show that barrel temperature and shear rate have great effects on viscosity during the preparation of semisolid slurry and then influence the filling process, while the effect of shot velocity is brought by the shear effect of nozzle and gate and the pressure variation during the injection process. Mold temperature has weak effect on the filling results but still can't be neglected. The optimized barrel temperature, screw rotation speed, shot velocity and mold temperature for this study are separately 600℃, 168r/min, 2.09m/s or 2.28m/s, 230℃.The results of this study make great contribution to the basic theory of Thixomolding? technology for magnesium alloys and have important direction to the practical application. |
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