Balasubramani Nagasivamuni, Venezuela Jeffrey, Yang Nan, Wang Gui, StJohn David, Dargusch Matthew
Centre for Advanced Materials Processing and Manufacturing (AMPAM), School of Mechanical and Mining Engineering, University of Queensland, St Lucia, QLD 4072, Australia.
Centre for Advanced Materials Processing and Manufacturing (AMPAM), School of Mechanical and Mining Engineering, University of Queensland, St Lucia, QLD 4072, Australia.
Ultrason Sonochem. 2022 Sep;89:106151. doi: 10.1016/j.ultsonch.2022.106151. Epub 2022 Aug 30.
A refined, equiaxed grain structure and the formation of finer primary intermetallic phases are some of the notable benefits of ultrasonic processing of liquid/solidifying melts. Ultrasonic treatment (UST) has been widely explored in Al and Mg-based alloys due to its operational versatility and scalability. During UST, the refinement of grain and primary intermetallic phases occurs via cavitation-induced fragmentation mechanisms. In addition, UST improves the efficiency (activation of particles) of the conventional grain refinement process when potent particles are added through master alloys. Though the UST's ability to produce refined as-cast structures is well recognized, the understanding of the refinement mechanisms is still debated and unresolved. Significant efforts have been devoted to understanding these mechanisms through the use of sophisticated techniques such as in-situ/ real-time observation, lab-scale and commercial-scale casting processes. All these studies aim to demonstrate the significance of cavitation, fragmentation modes, and alloy chemistry in microstructure refinement. Although the physical effects of cavitation and acoustic streaming (fluid flow) are primary factors influencing the refinement, the dominant grain refinement mechanisms are affected by several solidification variables and casting conditions. Some of these include melt volume, solute, cooling rate, potent particles, grain growth (equiaxed, columnar or dendritic), and the cold zones of the casting where the onset of nucleation occurs. This review aims to provide a better insight into solidification variables emphasizing the importance of cold zones in generating fine structures for small- and large-volume (direct chill) castings. Another important highlight of this review is to present the relatively less explored mechanism of (acoustic) vibration-induced crystallization and discuss the role of cavitation in achieving a refined ingot structure.
细化的等轴晶组织以及更细小的初生金属间相的形成是对液态/凝固熔体进行超声处理的一些显著优点。由于其操作的多功能性和可扩展性，超声处理（UST）已在铝基和镁基合金中得到广泛研究。在超声处理过程中，晶粒和初生金属间相的细化是通过空化诱导破碎机制实现的。此外，当通过中间合金添加强效晶粒细化剂时，超声处理提高了传统晶粒细化过程的效率（颗粒活化）。尽管超声处理产生细化铸态组织的能力已得到充分认可，但对细化机制的理解仍存在争议且尚未解决。人们已投入大量精力，通过使用复杂技术，如原位/实时观察、实验室规模和商业规模的铸造工艺来理解这些机制。所有这些研究旨在证明空化、破碎模式和合金化学成分在微观结构细化中的重要性。尽管空化和声流（流体流动）的物理效应是影响细化的主要因素，但主导的晶粒细化机制受几个凝固变量和铸造条件的影响。其中一些变量包括熔体体积、溶质、冷却速率、强效晶粒细化剂、晶粒生长（等轴晶、柱状晶或枝晶）以及铸造过程中形核开始的冷区。本综述旨在更好地洞察凝固变量，强调冷区对小体积和大体积（直接激冷）铸件生成精细组织的重要性。本综述的另一个重要亮点是介绍相对较少探索的（声学）振动诱导结晶机制，并讨论空化在获得细化铸锭组织中的作用。
