Effect of TIG Repair Welding on Microstructure and Properties of 2219 Aluminium Alloy FSW Joints

Literature Overview

The paper by Liu Yuning, Zhang Jian, Liu Debo, Wang Feifan, and Zhou Li, published in Aerospace Materials and Technology in 2022, investigates the effects of TIG repair welding on the microstructure and mechanical properties of friction stir welded (FSW) joints of 2219 aluminium alloy. This alloy is widely used in aerospace applications due to its excellent combination of strength, fracture toughness, and fatigue resistance. The study is supported by the Beijing Aerospace System Engineering Research Institute and addresses a critical practical issue: the repair of defects in FSW joints using conventional TIG welding.

Core Technical Content

The study examines the microstructural evolution in the FSW joint after TIG repair welding of defects such as lack of fusion, voids, or porosity. The 2219 alloy is a precipitation-hardened alloy strengthened by the formation of eta-phase particles during aging. The TIG repair process introduces a new thermal cycle that can dissolve or coarsen these strengthening particles, leading to a loss of strength and toughness in the repair zone.

The authors conducted metallographic analysis, X-ray diffraction, and mechanical testing including tensile, hardness, and fracture toughness measurements on the repaired joints. The results show that the TIG repair weld zone exhibits a coarse-grained microstructure with partially dissolved eta-phase particles, resulting in hardness values significantly lower than the base metal and the original FSW joint. The heat-affected zone of the repair weld shows a gradient of microstructural changes, with the most severe softening occurring within 1 to 2 millimeters of the weld centerline.

Technical Parameter Analysis

The hardness profile across the repair zone reveals a distinct softening band, with the minimum hardness occurring at the weld centerline and gradually recovering toward the base metal. This softening is attributed to the dissolution of eta-phase precipitates during the TIG welding thermal cycle and the subsequent lack of adequate re-aging to restore the precipitation hardening.

Engineering Practice Implications

In aerospace manufacturing, FSW joints are increasingly used for structural components due to their superior fatigue performance and the absence of filler metal. However, defects in FSW joints inevitably occur, and repair welding is often required. The findings of this study have direct implications for repair procedure qualification and acceptance criteria. The significant loss of strength and toughness in the TIG repair zone suggests that repair welding of 2219 alloy FSW joints must be followed by a post-weld aging treatment to restore the mechanical properties.

The study also highlights the importance of repair weld geometry and parameter selection. A narrower weld bead with reduced heat input can minimize the extent of the softened zone. The use of cold wire TIG or hot-wire TIG techniques, which allow independent control of heat input and filler metal deposition, may offer improved repair quality compared to conventional TIG welding.

Key Reflections and Study Insights

This paper provides a critical assessment of the practical viability of TIG repair welding for 2219 alloy FSW joints. The results clearly demonstrate that the repair weld introduces a significant metallurgical disturbance that degrades the mechanical properties well below the original joint quality. This finding has important implications for the design of repair procedures and for the qualification of repair welding operators in aerospace applications.

From a broader perspective, the study underscores the challenge of repairing advanced joints with conventional welding techniques. The thermal cycle of TIG welding is inherently different from the severe plastic deformation and dynamic recrystallization of friction stir welding, and the two processes produce fundamentally different microstructures. Any repair strategy must account for this metallurgical mismatch, and the post-weld aging treatment is a necessary step to restore the precipitation hardening that was dissolved during the repair welding thermal cycle. This work serves as a valuable reference for engineers developing repair procedures for aerospace aluminium alloy structures.