| As a new type of metal forming process, semi-solid processing technology is widely used in the field of non-ferrous metal processing, the key point of it is semi-solid slurry preparation. The problem is how to prepare qualified semi-solid slurry under the condition of energy saving and high efficient, which has very important practical significance for the development and application of semi-solid processing technology. In this context, the A356aluminum alloy was selected as the object of study, the effects of insulation process parameters, electromagnetic stirring and the mixed rare earth refiner were researched in the process of preparation hypoeutectic Al-Si alloy semi-solid slurry. The laws of alloy’s microstructure, especially the primary a phase’s morphology evolution and distribution pattern under the above conditions were discussed in detail, so that to obtain the new semi-solid preparation process of hypoeutectic Al-Si alloy and its corresponding theoretical basis.In order to obtain the small, round and uniformly distributed primary a phase of A356alloy, A356melt poured by low superheat by using different thermal conditions, determine the best insulation process parameters under the comparison of different experimental results. The effects of different holding temperature and time on the phase morphology and distribution of primary a-Al alloy were analyzed. There were the good primary a-Al phase in semisolid A356alloy poured at620℃and isothermally held at615℃for10min. Reasonable insulation process can fuse the arborescent crystal to improve the nucleation rate and solute distribution, interfacial tension tends to be same in all directions, so that the primary α-Al can get rounded shape.A356melt poured by low superheat was slightly electromagnetically stirred for short time with an electromagnetic stirrer which was controlled by frequency conversion. The effects of electromagnetic stirring time and frequency on microstructure in semisolid A356alloy were researched to obtain the best electromagnetic stirring process parameters. The results indicated that the best electromagnetic stirring process parameter is electromagnetic stirring for15seconds under the condition of30Hz electrical frequency. The primary a-Al phase is high-speed rotated and broken by electromagnetic stirring, under the "erosion" process, the grains’size are polished rounded and decreased. The broken arborescent crystals are uniformly distributed in the melt under electromagnetic stirring, they can be the nucleation of solidification, so the rate of nucleation has been increased. Too high or too low frequency is not good for the penetration of magnetic field into alloy melt.Under the condition of above results, the effects of mixed rare-earth addition on microstructure under electromagnetic stirring in semisolid A356alloy were researched. The results showed that there were the best primary α-Al in the alloy added with0.7%La and0.4% Ce, poured at620℃, stirred for15seconds at30Hz in electric current, and isothermally held at610℃for10min. RE elements are easy to be uniformly distributed around the primary a-Al phase under electromagnetic stirring, the proeutectic a phase before the reaction of Al with RE can be the nucleation of the primary α-Al phase in Al-Si eutectic reaction, Al11Ce3can be directly used as nucleation of α-Al phase during Al-Si eutectic reaction. The refinement effect of mixed RE has a micro-inhomogeneity.The features of this paper are:by using electromagnetic stirring process, the improvement effect of the morphology at different addition of mixed RE was researched. The morphology evolution laws and mechanisms of A356aluminum alloy’s solidification structure especially the primary a-Al phase under the dual process conditions were discussed in detail. Finally, the paper compared the mixed rare earth with conventional Al-Ti-B refiner in terms of efficiency and economy, the theoretical analysis was combined with production practices. |
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