RE变质对ZA35-4Si-3Sn合金凝固组织的影响

Effect of RE Modification on Microstructure of ZA35-4Si-3Sn Alloy

利用普通铸造法制备了不同RE含量的ZA35-4Si-3Sn合金,研究了RE含量对该合金凝固特性及其组织的影响。ZA35-4Si-3Sn合金凝固结晶过程主要包括液相中析出Si相和α-Al相,包析反应和共析反应。RE变质会显著影响液相析出α-Al相的结晶过程,RE加入降低其初晶形核温度和生长时间。当RE含量为0.2%时效果较佳,此时过冷度最大,初晶生长时间最短,再辉温差最大。同时,α-Al树枝晶和二次枝晶臂间距均显著细化。变质机制主要为稀土元素富集于固液界面处,增大了合金成分过冷,提高晶核形核率。

ZA35-4Si-3 Sn alloy with different RE contents were fabricated by ordinary gravity casting. Effect of RE content on solidification process and microstructure was investigated through thermal analysis. The solidification process of ZA35-4Si-3Sn alloy mainly contains the precipitation of Si phase and α-Al phase, peritectoid reaction and eutectoid reaction. The precipitation of α-Al phase was remarkably influenced by the RE modification. The nucleation of α-Al primary crystal and the growth time decreased firstly and then increased with increasing in RE content. It was found that the optimal RE content was 0.2%, resulting in the highest undercooling degree, the shortest crystal growing time and the largest recalescence temperature difference. Meanwhile, the dendrite and second dendrite arm spacing were obviously refined. The microstructure was refined mainly owing to the compositional undercooling caused by the oversaturated RE element gathered at the solid/liquid interface, increasing the nucleation rate of the crystal nucleus.