The phenomenon of joint softening often occurs in the fusion welding process of Al–Cu–Mg alloy, which predominately derives from the grain coarsening within the joint. But in this paper, it is pointed out that the decrease in the number and proportion of strengthening phases caused by element burning is also one of the causes. The mechanism of performance degradation of the joint was explained from transformation of strengthening phase. It was clarified that the ultrasonic strengthening was due to the fragmentation of surface grains and eutectic phases in the shape of continuous net along grain boundaries. The formation of subcrystals, resulting from multiplication, slip, intersection of dislocations and the interaction with the strengthening phases, further improved the surface properties. Highlights The mechanism of performance reduction of electron beam welding Al–Cu–Mg alloy is explained. The burning loss of magnesium element and the effect on the number and type of strengthening phases are revealed. The strengthening mechanism of the surface modification by ultrasonic impact treatment is expounded. The improvement of properties and the transformation of tensile fracture mode are revealed.

中文翻译：

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Al-Cu-Mg合金电子束焊接性能劣化的根本原因及超声改性机理

Al-Cu-Mg合金熔焊过程中常出现接头软化现象，主要是由于接头内部晶粒粗化所致。但在本文中指出，元素燃烧引起的强化相数量和比例的减少也是原因之一。从强化相的转变解释了接头性能退化的机理。澄清了超声强化是由于表面晶粒和共晶相沿晶界呈连续网状破碎。由增殖、滑移、位错交叉以及与强化相的相互作用导致的亚晶的形成进一步改善了表面性质。亮点 解释了电子束焊接 Al-Cu-Mg 合金性能降低的机制。揭示了镁元素的燃烧损失以及对强化相的数量和类型的影响。阐述了超声冲击处理表面改性的强化机理。揭示了性能的改进和拉伸断裂模式的转变。