转棒诱导形核对Al-25%Si合金凝固组织的影响

Influence of rotating-rod induced nucleation on solidification microstructure of Al-25%Si alloy

采用转棒诱导形核对过共晶Al-25%Si合金熔体进行处理获得半固态坯料,研究转棒转速对坯料组织的影响,并结合熔体在转棒表面的运动状态,分析转棒诱导形核细化凝固组织的机理。结果表明,转棒转速为500~600 r/min条件下,熔体能够最大程度地铺展于棒的表面,在棒表面很快获得铺展面积最大、厚度最小的熔体薄膜。在棒的激冷作用下,整体薄膜迅速获得过冷并促使其内部初生硅同时快速并大量形核,单位面积初生硅颗粒个数较铸态和其它转速条件下显著增加,初生硅平均尺寸从常规铸态的250~300μm细化至27μm左右。初生硅大量形核后,剩余液相后续凝固过程发生离异共晶反应形成针片状硅,没有出现典型的片层状共晶组织,同时片状硅得到一定程度细化。转棒诱导形核能够获得细小初生硅和针片状硅均匀弥散分布于α-Al基体的半固态组织。

Molten metal of hypereutectic Al-25% Si alloy was treated to fabricate semi-solid slab by rotating-rod induced nucleation( RRIN). Influence of rotating speed on its microstructure was studied,and the mechanism of microstructure refining was discussed according to the motion of melt on the surface of rotating-rod. The results show that,at a rotating speed of 500 r /min or 600 r /min,the melt spread around the surface of the rod as much as possible. A melt film on the rod surface with the largest spreading area and the thinnest thickness can be obtained shortly. A supercooling degree of the whole film can be obtained rapidly by the chilling of the rod,which can promote a large number of primary Si nucleating rapidly at the same time in the whole melt film. The number per unit area of primary silicon particles can increase significantly compared to that in the microstructure both as cast and under other rotating speed. The average size of primary Si is reduced to about 27 μm from 250-300 μm in the as-cast microstructure. Due to the primary Si nucleating dramatically,following divorced eutectic silicon needles are formed,and the typically lamellar eutectic microstructure is not formed.Meanwhile the needle Si is also refined partly. The microstructure with dispersed fine primary Si particles and divorced eutectic Si needles distributing in α-Al matrix can be obtained by RRIN.