During the direct chill (DC) casting process, primary cooling from the mold and bottom block, and secondary cooling from the waterjets produce a concave solid shell. The depth of this liquid pocket and mushy zone not only depends on the solidification range of the alloy but also the boundary conditions such as cooling rates. Al-Li alloys solidify in a long solidification range increasing the susceptibility of porosity nucleation in the semi-solid region. In this study, the effects of cooling rate on the porosity formation were quantified for the large ingot casting using X-ray computed tomography (XCT). By characterizing pore size distributions at four different cooling conditions, the correlation between the mechanical properties at both room and high temperatures and the microstructure features was identified. The constitutive equations were constructed. It is found that increasing the cooling rate reduces the grain size, increases the number density of micropores, and minimizes the number of large pores, thereby improving the mechanical performance. Therefore, long mushy zones and deep liquid pockets in Al-Li alloys can be effectively controlled by controlling the boundary conditions of the DC casting solidification process, thereby obtaining castings with excellent mechanical properties.

铝合金材料广泛应用于航空航天等领域,但其高温强度相对于钢铁材料还有一定的差距.通过添加过渡族金属和稀土金属元素可有效控制铝合金高温条件下的析出强化相(Al3X),从而提升其高温力学性能.概述了微合金元素对铝合金高温强化影响的国内外研究进展,并结合第一性原理计算阐述了Al3X强化相热力学参数及其性能方面研究的最新进展.

To improve the prediction of hot cracking, fluid flow, and dendritic growth must be investigated quantitatively and thus an integrated 2D model of coupling phase-field model together with CFD calculations was proposed in this work. First, a phase-field model for thermodynamics of binary Al-3.3wt.%Li was developed based on the CALPHAD database. The columnar dendrites with different orientation angles (0°-28°), different grain sizes of equiaxed grains, and columnar to equiaxed transition (CET) were simulated. Then, CFD calculations were performed based on geometries from simulated microstructure features to predict the inter-dendritic pressure distribution. The RDG model and Kou model were used to evaluate the hot cracking susceptibility (HCS) of these simulated microstructure features. Comparing to the RDG model, our simulations have shown that the feeding pressure drop varies as a function of grain orientation, while it is a constant in the RDG model. It is found that equiaxed crystals with large grain sizes have high HCS due to their large pressure drop as well as the narrow liquid channel. The Kou model is considered unable to calculate the HCS of CET because the value of dT/dfs1/2 near fs=1 of CET and pure equiaxed crystals are the same. The predicted pressure drop of CET using our integrated model shows that the equiaxed crystals have a great impact on a the liquid feeding of columnar crystals when fs=0.51.

铝合金广泛应用于航空航天、轨道交通、汽车轻量化等领域.半连续直接冷却铸造技术是制备大型铝合金铸锭的重要方法.在此制备工艺中热裂是较为严重的铸造缺陷,该缺陷的控制是制备优质大型铸锭的一项关键技术.针对半连续铸造的发展概况及热裂形成机理和预测模型进行了概述,分析了基于几种热裂判据的热裂敏感性计算结果并与铸造实践对比.结果表明没有任何热裂判据能够定量预测热裂的产生,在定性预测方面,RDG判据具有最大的应用潜力.提出了微观 – 宏观理论计算与先进观测手段集成的未来展望,在发展热裂判据时,应将相变动力学、断裂力学、流体传热以及合金热力学相结合,并考虑三维糊状区热裂形核 – 扩展 – 愈合的复杂机制.

To predict hot tearing susceptibility (HTS) during solidification and improve the quality of Al alloy castings, constitutive equations for AA6111 alloys were developed using a direct finite element (FE) method. A hot tearing model was established for direct chill (DC) casting of industrial AA6111 alloys via coupling FE model and hot tearing criterion. By applying this model to real manufacture processes, the effects of casting speed, bottom cooling, secondary cooling, and geometric variations on the HTS were revealed. The results show that the HTS of the billet increases as the speed and billet radius increase, while it reduces as the interfacial heat transfer coefficient at the bottom or secondary water-cooling rate increases. This model shows the capabilities of incorporating maximum pore fraction in simulating hot tearing initiation, which will have a significant impact on optimizing casting conditions and chemistry for minimizing HTS and thus controlling the casting quality.

Macro-segregation is a type of unacceptable defects, which normally occurs for aluminum alloys and other nonferrous alloys. In this work, the macro-segregation susceptibility (MAS) of commercial 5xxx and 6xxx alloys and Al-Mg and Al-Mg-Si alloys have been investigated via a criterion based on the solidification curves from thermodynamic calculations. Equilibrium solute distribution coefficients, k, with different initial alloying concentrations and their relationships with the MAS are discussed. Of all commercial 5xxx and 6xxx aluminum alloys, the AA5754, AA5182, AA6181A and AA6016 alloys have shown less susceptible to macro-segregation than others. Of all Al-Mg and Al-Mg-Si alloys in this study, Al-Mg alloys with higher Mg, and Al-Mg-Si alloys with both higher Mg and Si have shown less susceptible to macro-segregation than other compositions. Finally, the role of mass transfer behavior in improving macro-segregation is discussed from the perspective of thermodynamic and kinetic calculations.