铝合金作为现代工业的关键结构材料，其密度小、比强度高、耐腐蚀性强、铸造性能好，在航空航天、新能源汽车等领域应用广泛。随着我国工业技术的发展，铝合金铸件向大型化、复杂化、精密化、薄壁化方向演进，这些趋势引起了铸件在凝固过程中薄壁区域的液相补缩不足及变截面处的液相流动加剧，导致铸件内部微观孔洞和偏析缺陷增多，目前研究人员针对两种缺陷已分别建立对应模型对其形成过程和影响因素进行研究，但针对多缺陷耦合机制的跨尺度研究尚未形成系统理论。本文以Al-4.5 wt.% Cu合金作为研究对象，利用垂直向上定向凝固实验和数值模拟，研究不同熔体初始氢含量条件下微观孔洞与枝晶组织协同生长及微观孔洞对溶质偏析的影响。主要研究内容如下：

         通过将连续性方程与动量方程、相场方程相结合，建立气液两相流中氢气泡上浮动力学的二维多场耦合模型，利用数值模拟研究凝固过程中微观孔洞的产生并揭示气-液界面的相互作用机制。揭示了气泡在熔体中上浮过程的形貌演变机制，探究了Al-Cu合金中微观孔洞大小对毗邻区温度场和流场的影响，且通过实验研究了微观孔洞对枝晶生长方向及局部微观组织的影响。仿真及实验结果表明：在氢气泡的上浮过程中，氢气泡形貌由球状向椭球状与非规则状转变；在熔体中存在的微观孔洞会影响马兰戈尼对流和热传导路径，并间接改变局部流场，从而导致枝晶生长方向发生偏移和溶质元素的不均匀分布；且微观孔洞越大，对周围熔体温度场和流场的影响越大，对枝晶生长方向的影响越剧烈，对补缩液流及溶质微观偏析的影响越显著。

针对Al-Cu合金垂直向上定向凝固实验，通过调控合金熔炼环境湿度来控制熔体氢含量(0.28、0.36、0.43、0.54 mL/100 g)。利用COMSOL Multiphysics对凝固过程进行仿真，发现模拟结果与实验所测冷却曲线结果基本吻合。利用光学显微镜和扫描电镜对铸锭纵剖面微观组织进行观测，发现随着距离底部激冷面的高度增加，凝固组织发生了柱状晶向等轴晶转变（CET）；在柱状晶区，随着距离底部激冷面的高度增加，一次枝晶臂间距和二次枝晶臂间距增大，但微观孔洞体积分数逐渐减少，且形貌圆整度逐渐增加，微观孔洞的特征尺寸受一次枝晶臂间距影响较大；在等轴晶区内，随着远离激冷面，二次枝晶臂间距增大，微观孔洞体积分数逐渐增大，圆整度逐渐增加，微观孔洞的特征尺寸受二次枝晶臂间距影响较大。随着熔体氢含量的增加，定向凝固试样中微观孔洞体积分数整体升高，其特征尺寸会增大，圆整度会降低。

         基于考虑微孔析出的“局部溶质再分配方程”，将模拟计算的Al-Cu合金垂直向上定向凝固过程糊状区演化与柱状晶糊状区补缩液流模型进行联立求解，计算不同熔体氢含量时柱状晶区的补缩液流及最终的平均溶质含量。针对以柱状晶定向凝固的Al-4.5 wt.% Cu合金，使用扫描电子显微镜(SEM)和能谱仪(EDS)分析柱状晶区域微孔存在对溶质偏析的影响，将数值计算结果与实验结果进行对照分析，研究发现：Al-Cu合金中的宏观偏析与到激冷面的距离有关，铸锭底部线状偏析程度加剧；随着熔体氢含量的增加，微观孔洞的增加可以抑制凝固过程中糊状区的反向补缩液流，降低溶质元素在枝晶间的富集程度，导致最终凝固组织中平均合金成分降低，降低铸件近表面的逆偏析程度。

Aluminum alloy, as a key structural material in modern industry, features low density, high specific strength, strong corrosion resistance, and good casting performance, and is widely used in aerospace, new energy vehicles and other fields. With the development of China's industrial technology, aluminum alloy castings are evolving towards larger, more complex, more precise and thinner-walled designs. These trends have led to insufficient liquid phase feeding in the thin-walled areas and intensified liquid phase flow at the variable cross-sections during the solidification process of the castings, resulting in an increase in internal porosities and segregation defects in the castings. At present, researchers have established corresponding models for the two kinds of defects respectively to study their formation process and influencing factors, but the cross-scale research on the coupling mechanism of multi-defects has not formed a systematic theory. In this paper, Al-4.5 wt.% Cu alloy was studied by up-vertical undirectional solidification experiment and numerical simulation to study the synergic growth of porosity and dendrite structures and the effects of porosity on solute segregation under different initial hydrogen content of melt. The main research contents are as follows:

By combining the continuity equation with the momentum equation and the phase field equation, a two-dimensional multi-field coupling model of the floating dynamics of hydrogen bubbles in gas-liquid two-phase flow was established. The formation of porosities during solidification was studied by numerical simulation and the interaction mechanism of gas-liquid interface was revealed. The mechanism of the morphology evolution during the floating of bubbles in the melt was revealed, and the effect of the size of the porosities on the temperature field and the flow field in the adjacent region was investigated. Moreover, the effect of the porosities on the dendrite growth direction and local microstructure was investigated experimentally. The simulation and experimental results show that the morphology of hydrogen bubbles changes from spherical to elliptic and irregular during the floating process. The presence of porosities in the melt can affect the Marangoni convection and heat conduction paths, and indirectly change the local flow field, which leads to the deviation of dendrite growth direction and the uneven distribution of solute elements. The larger the porosity, the greater the influence on the temperature field and flow field of the surrounding melt, the more severe the influence on the direction of dendrite growth, and the more significant the influence on the feed flow and solute microsegregation.

For the up-vertical undirectional solidification experiment of Al-Cu alloy, the melt hydrogen content (0.28, 0.36, 0.43, 0.54 mL/100 g) was controlled by adjusting the alloy melting environment humidity. COMSOL Multiphysics was used to simulate the solidification process, and it was found that the simulation results were basically consistent with the experimental cooling curve. The microstructure of the longitudinal section of the ingot was observed by optical microscope and scanning electron microscope. It was found that the solidification structure changed from columnar to equiaaxial crystal (CET) with the increase of the distance from the bottom cold surface. In the columnar region, the distance between primary dendrite arms and secondary dendrite arms increases with the increase of the height from the bottom cold surface, but the volume fraction of porosities decreases gradually, and the shape circularity increases gradually. The characteristic dimensions of porosities are greatly affected by the distance between primary dendrite arms. In the isometric zone, the distance between secondary dendrite arms increases, the volume fraction and the circularity of porosities increase gradually, and the characteristic dimensions of porosities are greatly affected by the distance between secondary dendrite arms. With the increase of hydrogen content in the melt, the volume fraction of porosities in the directionally solidified sample increases, the characteristic size increases, and the roundness decreases.

Based on the "local solute redistribution equation" considering porosity precipitation, the simulation model of mushy zone evolution during the up-vertical undirectional solidification of Al-Cu alloy and the feed flow model in cylindrical crystal mushy zone were solved simultaneously, and the feed flow in cylindrical crystal region and the final average solute content in different melts with different hydrogen content were calculated. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to analyze the influence of porosities in the columnar region on solute segregation for Al-4.5 wt.% Cu alloy, which was directionally solidified with columnar crystals. The numerical results were compared with the experimental results, and it was found that: The macroscopic segregation in Al-Cu alloy is related to the distance to the cold surface, and the linear segregation at the bottom of the ingot is intensified. With the increase of hydrogen content in the melt, the increase of porosities can inhibit the reverse feeding liquid flow in the mushy zone during solidification, reduce the enrichment degree of solute elements in the dendrites, lead to the decrease of the average alloy composition in the final solidification structure, and reduce the degree of inverse segregation near the surface of the casting.

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