一体化压铸技术迫切需求高强韧性免热处理压铸铝合金材料。本论文研究了重力铸造条件下和高压铸造条件下亚共晶铝硅合金的细化行为，并根据上述结果对本课题组开发的THAS-1压铸合金进行优化，开发设计了新型压铸亚共晶铝硅合金THAS-4。首先，以Al-9Si-0.4Fe-0.3Mn为基础进行合金开发设计，研究了Ti、V、Zr、Nb和Ce单元素添加、Ti、V和Zr复合元素添加以及Al-5Ti-1B、Al-2Ti-0.5C和Al-3B晶粒细化剂添加对该基础合金的细化影响。研究结果表明，单元素对α-Al晶粒的细化能力排序为Ti＞Zr＞Nb＞Ce＞V，异质形核相较于溶质生长限制对α-Al晶粒的细化效果更好。兼具异质形核能力与生长限制能力的Ti元素对于α-Al晶粒的细化作用最强。复合添加Ti和Zr元素可与Al和Si元素形成AlSi(Ti,Zr)异质核心，对α-Al晶粒具有较强的细化能力。此外，Al-3B细化剂相比于Al-5Ti-1B和Al-2Ti-0.5C细化剂对亚共晶铝硅合金的细化效果更明显。在此基础上，提出了THAS-1合金的优化策略，一方面提高Fe元素含量以满足压铸的脱模需求，同时还可以减少熔体的除铁成本；另一方面继续引入其他细化元素，促进晶粒细化，目标为实现合金的强韧化。其次，对THAS-1压铸合金进行组织表征，并以该合金作为基础合金，研究了Al-5Ti-1B细化剂、Nb和Ce元素添加对组织的细化影响。结果表明，在合金元素添加后组织中的ESCs得到了一定程度的细化和球化。其细化机理为通过促进异质形核和提高对晶粒的生长限制能力两方面协同作用。在添加细化元素后，熔体中将原位自生或者引入大量异质核心，作为ESCs的形核位点。同时，溶质元素会富集在晶粒的前沿，限制晶粒生长，促进ESCs球化。综合上述研究结果，开发设计了THAS-4压铸合金。THAS-4合金中的细化元素协同为晶粒形核提供了大量的异质核心和较大的成分过冷，促进了晶粒细化。AlSi(Ti,Zr)相是有效的异质核心，该相的大量析出促进了铝晶粒的细化，减小了ESCs的尺寸。AlSi(Ti,Zr)相的主要三维形貌有短棒状和“V”字状两种。同时，研究发现工艺参数中低速速度对压铸铝硅合金的细化效果也具有一定影响。低速速度越高，金属液在压室内的停留时间越短，ESCs的数量越少，晶粒尺寸越小，圆整度越高，且在组织内分布更为均匀。

There is an urgent demand for high strength, high toughness and non-heat treatable aluminum alloy materials for large single-piece die casting technology. This paper studied the refinement behavior of hypoeutectic Al-Si alloys under gravity casting and high-pressure die-cast conditions, and based on the results obtained, the THAS-1 die-cast alloy, which was developed by our research team, was optimized and a new die-cast hypoeutectic Al-Si alloy, THAS-4, was designed.Initially, the development of the alloy was based on Al-9Si-0.4Fe-0.3Mn, and the effects of single element addition of grain refining elements Ti, V, Zr, Nb and Ce, composite element addition of Ti, V and Zr, and grain refiners of Al-5Ti-1B, Al-2Ti-0.5C and Al-3B on the refinement of the base alloy were studied. The results showed that the ability of single elements to refine α-Al grains was in the order of Ti>Zr>Nb>Ce>V, and heterogeneous nucleation was more effective in refining α-Al grains than growth restriction effect. Ti element was found to have the strongest refining effect on α-Al grains, which had both heterogeneous nucleation capability and growth restriction effect. Composite addition of Ti and Zr could form AlSi(Ti,Zr) heterogeneous nucleation cores with Al and Si elements, which had strong refining ability on α-Al grains. In addition, the Al-3B grain refiner was found to have a more significant refining effect on hypoeutectic Al-Si alloys compared to Al-5Ti-1B and Al-2Ti-0.5C grain refiners. On this basis, an optimization strategy for the THAS-1 alloy was proposed. On one hand, the Fe content was increased to meet the demolding requirements of die casting, and reduce the cost of iron removal from the melt. On the other hand, other refining elements were introduced to promote the effect on grain refinement and achieving the goal of strengthening and toughening.Subsequently, the THAS-1 die-cast alloy was characterized, and the effects of Al-5Ti-1B grain refiner, Nb, and Ce element additions on the refinement of the microstructure were studied. The results showed that after the addition of refining elements, the externally solidified crystals (ESCs) in the microstructure were refined and spheroidized. The refinement mechanism was found to be a synergistic effect of promoting heterogeneous nucleation and increasing the growth restriction capacity. After the addition of refining elements, in situ or introduced heterogeneous nucleation cores were formed in the melt as nucleation sites for ESCs. Meanwhile, solute elements were enriched at the grain boundaries, which restricted the growth of ESCs and promoted the spheroidization.Based on the above research findings, the THAS-4 die-cast alloy was developed. The refining elements in THAS-4 provided numerous heterogeneous nucleation cores and a large component undercooling for ESCs, which promoted grain refinement. AlSi(Ti,Zr) phase was an effective heterogeneous nucleation core, and its abundant precipitation promoted the refinement of aluminum grains and reduced the size of ESCs. The AlSi(Ti,Zr) phase was mainly in the form of short rods and "V" shapes. At the same time, it was found that the slow shot speed of process parameters also had a certain influence on the refinement of die-cast Al-Si alloys. The higher the slow shot speed, the shorter the residence time of metal liquid in the shot sleeve, which resulted in fewer ESCs number, smaller ESCs size and higher the roundness. Furthermore, higher low shot speed resulted in a more uniform distribution of ESCs in the microstructure.