In this work, the current state of understanding of grain structure evolution during friction-stir welding is briefly reviewed. The broad aspects of this process and experimental techniques for its examination are critically addressed. The specific character of the microstructural evolutions in body-centered cubic, face-centered cubic and hexagonal close-packed metals are considered in details. In all cases, the grain structure evolution is shown to be a relatively complex process, which usually involves geometric effect of strain, continuous recrystallization and discontinuous recrystallization. Moreover, mechanical twinning, annealing twinning and grain convergence may also occur in particular cases. It is also demonstrated that activation of a specific microstructural mechanism is primarily governed by crystal structure and stacking fault energy but may also be influenced by welding temperature. Specifically, microstructure evolution in cubic metals with high stacking-fault energy is primarily governed by the continuous recrystallization whereas grain structure development in materials with low stacking-fault energy is mainly driven by the discontinuous recrystallization. In the case of transient stacking-fault energy, the materials may experience a transition from the continuous to the discontinuous mechanism. In hexagonal metals, microstructural changes are shown to be directly linked with crystallographic texture. Specifically, a formation of very sharp texture may promote the grain convergence.

中文翻译：

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摩擦搅拌焊接过程中的晶粒结构演变

在这项工作中，简要回顾了对搅拌摩擦焊接过程中晶粒结构演变的理解现状。这个过程的广泛方面和用于其检查的实验技术得到了批判性的解决。详细考虑了体心立方、面心立方和六方密排金属微观结构演化的具体特征。在所有情况下，晶粒结构演化都被证明是一个相对复杂的过程，通常涉及应变、连续再结晶和不连续再结晶的几何效应。此外，在特殊情况下也可能发生机械孪晶、退火孪晶和晶粒收敛。还证明了特定微观结构机制的激活主要受晶体结构和层错能控制，但也可能受焊接温度的影响。具体而言，具有高堆垛层错能的立方金属的微观结构演变主要受连续再结晶控制，而低堆垛层错能材料的晶粒结构发展主要受不连续再结晶驱动。在瞬态堆垛层错能的情况下，材料可能会经历从连续机制到不连续机制的转变。在六方金属中，微观结构的变化与晶体结构直接相关。具体而言，非常尖锐的纹理的形成可以促进晶粒收敛。