不同冷却条件和B含量对近共晶Al-13.0%Si合金凝固组织的影响

Effects of Cooling Rate and B Content on the Microstructure of Near-Eutectic Al-13.0%Si Alloy

研究了约2℃/s和10℃/s冷却条件下,Al-3B中间合金不同加入量对近共晶Al-13.0%Si合金微观组织的影响。试验结果表明,两种冷却条件下,B的加入都能够细化近共晶铸造Al-13.0%Si合金,并在一定程度上抑制初生相析出,促进共晶体形成;但在冷速增大的情况下,细化效果更加显著,对初生相的抑制也更为明显。未加B条件下,增大冷速有利于枝晶的形成;在较低冷速条件下,B的加入及含量的提高并不能使初生&相由树枝晶转变为等轴晶,但较大的冷速使得这一转变非常明显。B加入后,形核温度TN大大升高,凝固方式由未加B时从型壁向试样中心的近似逐层凝固转变为加B后在整个试样范围内同时大量形核生长。分析认为B的加入为共晶体的形核提供了大量形核基底,导致共晶组织的细化和初生相的减少,增大冷速激发了更多潜在的形核核心,从而提高了B的细化效果和对初生相的抑制作用。

The influence of cooling rate and B content on the microstructure of near eutectic Al- 13.0wt% Si alloys has been investigated, whereas the average cooling rates of melt prior to solidification were about 2 ℃/s and 10 ℃/s. The results showed that under both cooling conditions, addition of boron resulted in refinement of eutectic and suppression of the primary phases. With increased cooling rate, the refinement of eutectic and the suppression of the primary phases were enhanced. When B was not added, high cooling rate condition promoted the （x-Al dendrites formation. Under low cooling rate, the introduction of B didn＇t cause a transition from columnar to equaled, and however, under a high cooling rate, this transition is obvious. After the addition of B, the nucleation temperature TN ascended, the nucleation mode changed apparently from nucleation mode of from the wall towards the centre without B addition to a nucleation mode that the eutectic nucleated evenly throughout the sample with B added. It can be concluded that addition of B affords a great number of nuclei for eutectic solidification, and, as a result, the eutectic become refined. Higher cooling rate will make more nuclei activated, so the refinement of eutectic is advanced.