| A356 alloy is a hypoeutectic alloy in the Al-Si-Mg system,which has a wide range of applications in the automotive industries mainly due to its good casting properties,comprehensive mechanical properties and high corrosion resistance.However,the mechanical properties of the A356 alloy are limited by coarse primary ?-Al dendritic structure and acicular-shaped eutectic Si.It is a good way to improve the properties of aluminum alloy by adding the grain refiner during the smelting process.The commonly used grain refiner is Al-5Ti-B master alloy.However,for A356 alloy,the grain refinement effect of Al-5Ti-B on the alloy is significantly weakened due to its Si content over 3wt.%.Therefore,it is urgent to develop a grain refiner suitable for A356 alloy.In this paper,a novel refiner Al-3wt.%VN master alloy was prepared by the combination of mechanical stirring and high-energy ultrasonic treatment using pure Al and VN powders as raw materials.The preparation process of Al-3wt.%VN master alloy was optimized by studying its microstructure.The refinement effect and refinement mechanism of Al-3wt.%VN on A356 alloy were analyzed.The influence of different cooling rates on the microstructure of A356 alloy before and after refinement was explored.The refining effect of the self-made refiner Al-3wt.%VN and the traditional refiner Al-5Ti-B was investigated by the comparative test.It was confirmed that the new refiner had a good refining effect.The pre-heated VN particles were added into the aluminum melt smoothly by mechanical stirring.When the stirring time was 4min,the particles dispersed uniformly in the melt,but there were many pores in the solidified structure.Then the mixed slurry was subjected to high-energy ultrasonic treatment for 4min to further disperse the particles in the melt and eliminate the pores,thereby the Al-3wt.%VN master alloy with uniform structure and low porosity was successfully prepared.The master alloy was mainly composed of ?-Al and VN phases.Al-3wt.%VN master alloy could refine A356 alloy effectively.When 1.0wt.% of Al-3wt.%VN master alloy was added into A356 alloy and held for 15 min,the grain size of ?-Al decreased from 1060±230?m to 350±95?m.In addition,the extra addition level did not improve the refining effect.When 1.0wt.% of Al-3wt.%VN was added into A356 alloy and held for 5min,the good grain refinement effect could be achieved.However,the refining effect faded significantly after holding for more than 30 min,and the grain size of ?-Al reached 455±98?m after holding for 60 min.The effective refinement of ?-Al grains by Al-3wt.%VN master alloy was due to the fact that VN particles could be stable in the aluminum matrix,and could be acted as the heterogeneous nuclei of ?-Al grains.Similar to the unrefined A356 alloy,the ?-Al grain size and eutectic Si size both decreased with the increase of cooling rate for the A356 alloy with Al-3wt.%VN master alloy addition.The higher the cooling rate,the higher the volume fraction of eutectic Si phase.In addition,with the change of cooling rate,compared with the unrefined A356 alloy,the variation range of ?-Al grain size was smaller for A356 alloy refined by Al-3wt.%VN master alloy.The good modification effect could be obtained for A356 alloy with 0.1wt.% of Al-10 wt.%Sr addition.In A356 alloy,Sr and VN particles did not react with each other.When 1.0wt.% of Al-3wt.%VN and 0.1wt.% of Al-10 wt.%Sr were added into the A356 alloy together,?-Al grains with grain size of 300±85?m and fine granular or fibrous eutectic Si uniformly distributed at ?-Al grain boundary could be obtained at the same time.No matter the addition level was 0.5wt.% or 1.0wt.%,the grain refinement effect of Al-3wt.%VN on A356 alloy was better than that of Al-5Ti-B.Specially,when the addition level was 1.0wt.%,the ?-Al grain size of A356 alloy refined by Al-3wt.%VN was 300±85?m while that of A356 alloy treated by traditional Al-5Ti-B was 400±89?m.After T6 heat treatment,the eutectic Si phases in A356 alloy refined by different master alloys were passivated or spheroidized,and the morphology of the eutectic Si phase of the unrefined A356 alloy was also the same.Due to the refinement strengthening,the strength and ductility of the A356 alloy were improved.Regardless of the as-cast or T6 state,the mechanical properties of A356 alloy refined by Al-3wt.%VN were higher than those of A356 alloy refined by Al-5Ti-B.Moreover,after T6 heat treatment,the overall mechanical properties of A356 alloy were improved due to Orowan strengthening.The yield strength,ultimate tensile strength and elongation of A356 alloy refined by 1.0wt.% of Al-3wt.% VN in T6 state were 125±3MPa,250±4MPa and 9.0±0.2% respectively,compared with the mechanical properties in as-cast state,those were improved by 17.9%,14.2% and 25% respectively. |
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