Effect of vanadium carbide on commercial pure aluminum

The effect of vanadium carbide（VC） on the grain size of commercial pure aluminum was experimentally investigated by varying the content of VC, the holding time, and casting temperature. The refining efficiencies of VC and Al5Ti1 B were also compared. The refined samples of commercial pure aluminum were examined using optical microscopy, scanning electron microscopy（SEM） and energy-dispersive spectroscopy（EDS）. The results suggest that VC is a good refiner of commercial pure aluminum. The addition of only 0.3wt% VC can decrease the grain size of aluminum to 102 μm, whereas the casting temperature and holding time have little effect on the grain size. The refining efficiency of VC is better than that of Al5Ti1 B. The VC particles in molten aluminum act as nuclei and the grain refinement of aluminum alloys by VC particles is achieved via heterogeneous nucleation.

Performance comparison of AlTiC and AlTiB master alloys in grain refinement of commercial and high purity aluminum

For further knowledge about the refining performance of AlTiC master alloys, Al5.5Ti0.25C and Al6.5Ti0.5C master alloys containing high Ti and C content were prepared and used in grain refining experiments of 99.8% commercial pure aluminum(CPAl). Their performance was compared with two types of Al5Ti1B refiners whose performance was nowadays considered to be the best. These two types of master alloys show similar refining efficiency at the addition level of 0.2%. However, at the addition level of 0.5%, there still exists great performance difference between AlTiC and Al5TiB alloys in grain refinement of 99.98% and 99.995% high purity aluminum(HPAl). The growth of columnar grains is fully suppressed due to the refinement of AlTiC at the addition level of 0.5%. Also, at the same addition level, the grain refining experiments of Al3Ti0.15C and Al5Ti0.2C master alloys which have found initial commercial applications are conducted in the above-mentioned three types of pure aluminum. According to the experimental results, these two refiners of different compositions are both nonideal. The second phase particles extracted from each refiner were observed through TEM, while the nuclei of grains after grain refinement were observed through SEM. The results were analyzed through computation and comparison of the constitutional-supercooling parameter and the growth-restriction parameter whose values were determined by solute element in aluminum melt with different purity. Apparently, AlTiC master alloys with high content of Ti and C element have great refining potential.

凝固细晶理论与新型细化剂设计研究进展

晶粒细化不仅能增强金属铸件的机械性能,还是控制铸造缺陷的有效机制。因此,了解晶粒细化具有重要的理论和实践意义。本文主要综述了铝合金凝固过程晶粒细化领域的最新理论进展,以及据此开发的新型细化剂和细化技术。根据凝固过程的发生顺序,依次介绍了异质形核、晶粒萌生及限制生长3个阶段的最新认识。同时,从影响这3个环节的主要因素入手,即界面结构、颗粒特征(形态、尺度和空间分布)和溶质元素组成,介绍了近十年来研究者们在新型细化剂设计及细化技术开发上做出的努力和取得的成效。

碳化铪对铸造Mg-3%Al镁合金晶粒度的影响

研究了不同含量碳化铪对Mg-3wt.%Al镁合金晶粒度的影响。结果表明,虽然碳化铪与镁具有非常相似的晶体学关系,但是碳化铪对纯镁没有明显的细化效果,HfC的加入并没有改变纯镁柱状晶的金相特点。然而,碳化铪对Mg-3%Al镁合金却有非常明显的细化效果。当加入0.7%碳化铪到Mg-3%Al镁合金时,平均晶粒度从365μm细化到145μm。异质晶核Al4C3的形成是HfC细化Mg-3%Al的主要机制。

溶质Ti对Al-5Ti-1B晶粒细化剂细晶效果的影响

低合金化铝合金的铸态晶粒往往存在粗晶比例高、内外晶粒尺寸分布不均等问题。在晶粒细化剂添加量不变的前提下,通过改变熔体Ti含量的方式,研究溶质Ti含量对Al-5Ti-1B晶粒细化剂细晶效果的影响。采用试验和计算相结合的方式,利用荧光光谱仪测量熔体在保温炉和铸造流槽的元素变化,对铸锭低倍组织的宏观及微观组织偏光观察,通过热力学计算确定溶质Ti含量、生长限制因子Q、α-Al形核温度及其过冷度,揭示溶质Ti对α-Al的形核率和生长率的作用机制。研究表明:低Ti样品的低倍组织中存在大量粗晶和少量羽毛晶,晶粒尺寸呈现内高外低的趋势,分布区间为750~2500μm;高Ti样品低倍组织细小弥散,内外晶粒分布均匀,呈等轴晶,尺寸区间在100~120μm。此外,计算结果显示,当熔体中溶质Ti含量由1.52×10^(-4)%增长至1.07×10^(-2)%时,TiB_(2)/α-Al界面能逐渐增大,润湿角由约180°降低至近似0°,α-Al的瞬态形核率随之提升数十个数量级。同时,通过降低形核过冷度和提高生长限制因子,更高的溶质Ti含量有助于降低α-Al的生长速率。为充分发挥晶粒细化剂的细晶效果,熔铸工序中溶质Ti含量建议不低于1.07×10^(-2)%。

冷速与稀土协同作用对Al-Sc合金组织及硬度的影响

采用真空感应熔炼和铜模喷铸相结合,研究了不同冷速及稀土含量对Al-Sc合金凝固组织及硬度的影响。结果发现,随Sc含量增加,铸态Al-Sc合金的等轴晶组织发生细化,其中过共晶Al-0.8Sc合金的细化效果最佳,平均晶粒尺寸仅为106μm,相比未添加Sc的纯Al下降了97.6%。由于铜模喷铸的强激冷效果,快冷Al-Sc合金的非平衡凝固组织表现为择优生长的细长柱状晶。加入稀土Sc后发生典型的柱状晶向等轴晶转变,且中心等轴晶比例随Sc含量增加而不断扩大。冷速的提高抑制了初生AlSc相的形成,有利于获得过饱和固溶体,但析出强化效应的减弱导致快冷合金显微硬度相比铸态合金有所下降。