| Adding ultrafine ceramic particles to liquid aluminum can provide a large number of dispersed particle in order to promote heterogeneous nucleation of grain, thus can refine the as-cast microstructure of aluminum alloy. The refining effect of ceramic particles has a good resistance to recession because of its excellent thermal stability. It is controllable and can overcome the uncertainty of the size and quantity of generated particles. The method has a wide application prospect in refining aluminum ally. However, poor wettability between ceramic particle and liquid aluminum makes it difficult to disperse particles into the melt uniformly. Therefore, to improve the wettability of ceramic particles and liquid aluminum is one of the key technology in application of ultrafine ceramic grain refiner.A new-type of Ti cladding nano-size TiC(TiC/Ti)composite grain refining agent was prepared by high energy ball milling. The effect of this refining agent on industrial aluminum was investigated, the influence of solidification process on pure Al and its refinement mechanism was also researched. Then, discussed the impact on microstructure and mechanical properties of as-cast 7050 alloy with different amount of refiner added. At last, influence on refining effect of TiC/Ti composite refiner in different cooling rate was discussed. The following results were obtained.TiC/Ti composite refiner has a remarkable grain refiner effect on pure aluminum. microstructure of ? 40 mm pure aluminum ingot casting transform into isometric crystal with an average grain size of 135?m when added 0.5wt% TiC/Ti into melt, while microstructures of ingot without refiner were thick columnar grain, which is better than Al-5Ti-1B and Ti powder. Furthermore, composite refiner can reduce the sensitivity of pure aluminum to cooling rate.Nano-size TiC composite refiner can reduce the degree of supercooling of pure Al while Ti increase it, and the degree of supercooling change slightly when adding TiC/Ti compares to adding nothing. But the grain size was obviously refined and crystal platform on cooling line was obviously shorten. The melting of Ti powder can improved the wettability between nano-size TiC and liquid aluminum, and form a Ti rich layer around TiC particle, the Ti rich layer can stimulation to the nucleation of ?-Al.The grain size of ?-Al in 7050 alloy decreases with the increase of TiC/Ti composite refiner. With 0.5wt% addition of TiC/Ti composite refiner, grain size of ?-Al in ? 40 mm ingot decreases to 120?m while it is 525?m without refinement treatment, and the grain size starts to largen when addition quantity increase to 0.7wt%. Refiner can improve the distribution of second phase without changing its composition.With the increase of TiC/Ti refiner which adding into melt alloy, tensile strength and hardness of the as-cast specimens of 7050 alloy increased first and then decrease. With 0.5wt% TiC/Ti added, the tensile strength and hardness was 249.5MPa and 137.3HV, which means an advance of 32.9% and 16.4% for respectively compare to those without refined. The tensile fracture surface of as-cast specimen show an obvious characteristic of brittle fracture. TiC/Ti composite refiner can also significantly reduce the sensitivity of as-cast microstructure and hardness of 7050 alloy to cooling rate. |
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