A novel semi-solid processing technique, called new vacuum suction casting (NVSC), is used to manufacture high-quality components of AZ91D Mg alloy directly from a liquid metal. The resulting apparent morphologies and microstructures of castings are characterized in detail and linked to the corresponding mold-filling behavior and subsequent solidification behavior. It is revealed that the semi-solid metal (SSM) with higher viscosity can be caused to fill the mold with “solid-front fill”, as compared with the liquid metal “spraying” in the conventional vacuum suction casting (CVSC) process. The smooth filling achieved in the NVSC process diminishes some disadvantages inherent for the CVSC sheets, and generates castings with better surface finish and structures with high integrity. The microstructure of the CVSC sheet consists of the fine and homogeneous supersaturated α-Mg solid solution due to the extremely high cooling rate. In the NVSC microstructure, the “preexisting” primary solid particles, with the morphology of near-globules or rosettes, disperse in the homogeneous matrix consisting of fine near-equiaxed secondary α-Mg grains and fine precipitates of β-Mg17Al12 intermetallics. In addition, owing to rapid solidification, the volume fraction of the β phase in the sheets obtained by both the processes is much lower than that in the as-cast ingot.
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机译：一种新颖的半固态加工技术，称为新型真空吸铸（NVSC），用于直接从液态金属制造AZ91D镁合金的高质量组件。对铸件产生的表观形貌和微观结构进行了详细表征，并与相应的铸型行为和随后的凝固行为相关。结果表明，与常规真空吸铸（CVSC）工艺中的液态金属“喷涂”相比，具有较高粘度的半固态金属（SSM）可以用“固态前填充”填充模具。 NVSC工艺中实现的平滑填充消除了CVSC片材固有的一些缺点，并生成了具有更好的表面光洁度和高完整性的结构的铸件。由于极高的冷却速度，CVSC板的微观结构由细小且均匀的过饱和α-Mg固溶体组成。在NVSC微观结构中，“已存在的”初级固体颗粒具有近球状或玫瑰状的形态，分散在由细小的近等次生α-Mg晶粒和β-Mg 17的细小析出物组成的均匀基质中 Al 12 金属间化合物。另外，由于快速凝固，通过两种方法获得的片材中β相的体积分数远低于铸锭中的β相的体积分数。
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