Friction stir welding (FSW) is an important technology for manufacturing large-scale aluminum alloy propellant tank. Due to stress corrosion and cyclic loads, the key FSW joints require online monitoring to ensure the structural integrity and service safety of long-term storage propellant tanks. FSW joints in the propellant tank are regarded as a type of circumferential or longitudinal long and narrow region. In order to detect the flaws with high efficiency and fewer sensors, the propagation characteristics of ultrasonic guided waves in the FSW joint of same material is investigated in this paper. The weld of a FSW joint is characterized by concave cross-sectional shape and different microstructure-mechanical parameters. The micro-structure, micro-hardness, and Young's modulus of a real FSW joint are analyzed, and a two-dimensional semi-analytical finite-element (SAFE) method is employed to study the effects of different parameters on the modal characteristics of weld-guided waves in the FSW joint. In the studied fundamental modes (symmetric (S0), anti-symmetric (A0), and shear-horizontal (SH0)), an almost non-leaky A0-like weld-guided wave at a certain frequency range from 100 kHz to 210 kHz is discovered in the welded zone of a specific FSW model and shows a potential for long-distance detection. Parametric simulation results show that A0-like, SH0-like and S0-like modes at 120 kHz always exist when the weld width is changed while the moduli of the welded zone and base metal zone are maintained the same. Additionally, the simulations demonstrate that some weld-guided waves only exist if the modulus value of the welded zone is lower than that of the base metal zone when the cross section is geometrically continuous (i.e. the shoulder plunge depth is zero). Comparing with weld-guided waves affected by weld width, the weld-guided waves affected by the modulus change shows less obvious energy leakage during propagation. The experiments are conducted to validate the existence of A0-like weld-guided mode with a primary energy trapping effect.

中文翻译：

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超声波焊接导波在同种材料搅拌摩擦焊接头中的传播特性

搅拌摩擦焊（FSW）是制造大型铝合金推进剂罐的重要技术。由于应力腐蚀和循环载荷，关键FSW接头需要在线监测，以确保长期储存推进剂罐的结构完整性和使用安全。推进剂罐中的FSW接头被认为是一种周向或纵向长而窄的区域。为了高效、少传感器地检测缺陷，本文研究了超声导波在相同材料的FSW接头中的传播特性。FSW 接头焊缝的特点是横截面呈凹形，显微组织-力学参数不同。分析了真实 FSW 接头的微观结构、显微硬度和杨氏模量，采用二维半解析有限元（SAFE）方法研究了不同参数对FSW接头焊缝导波模态特性的影响。在所研究的基本模式（对称 (S0)、反对称 (A0) 和剪切水平 (SH0)）中，在 100 kHz 至 210 kHz 的特定频率范围内几乎不泄漏的类 A0 焊接导波在特定 FSW 模型的焊接区域中发现，并显示出远距离检测的潜力。参数化仿真结果表明，在焊区和母材区的模量保持不变的情况下，改变焊缝宽度时，在120 kHz频率下始终存在A0-like、SH0-like和S0-like模式。此外，模拟结果表明，当横截面几何连续（即肩部插入深度为零）时，只有当焊接区的模量值低于母材区的模量值时，才会存在一些焊接导波。与受焊缝宽度影响的焊缝导波相比，受模量变化影响的焊缝导波在传播过程中的能量泄漏不明显。进行实验以验证具有初级能量俘获效应的类似 A0 的焊接引导模式的存在。