Abstract Refinement and uniform dispersion of the free machining elements within the matrix is always desirable for aluminium billets produced by direct chill (DC) casting. The conventional grain refiner addition cannot ensure uniformity of microstructure and homogeneous distribution of insoluble or intermetallic phases. Melt conditioned direct-chill (MC-DC) casting is a key technology for producing uniform microstructure and refinement of secondary phases. This physical approach combines conventional vertical direct chill (DC) casting with a high shear device directly immersed in the sump for in situ microstructural control. Increased heat extraction rate due to forced convection, a larger temperature gradient at the solidification front and a shallower sump contribute to fine equiaxed dendritic grains with fine dendritic arm spacing that favor the fine and uniform distribution of second phase particles in the as-cast billet. This trend of the microstructural features was preserved even after thermomechanical downstream processing, giving rise to much improved machinability for the extruded solid bars.

中文翻译：

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通过控制直接冷硬铸造过程中的凝固来提高铝合金的断屑性

摘要 对于通过直接冷硬 (DC) 铸造生产的铝坯而言，基体中的自由加工元素的细化和均匀分布总是需要的。常规的晶粒细化剂添加不能确保显微结构的均匀性和不溶性或金属间相的均匀分布。熔融条件直冷 (MC-DC) 铸造是生产均匀微观结构和细化二次相的关键技术。这种物理方法将传统的垂直直冷 (DC) 铸造与直接浸入集水槽中的高剪切装置相结合，以进行现场微观结构控制。由于强制对流增加了热提取率，凝固前沿较大的温度梯度和较浅的贮槽有助于形成细小的等轴枝晶，枝晶臂间距细，有利于第二相颗粒在铸态钢坯中的精细和均匀分布。即使在热机械下游加工之后，这种微观结构特征的趋势也得以保留，从而大大提高了挤压实心棒材的可加工性。