Microstructure and Mechanical Properties of TIG-FSW Cross Joints in 2A14 Aluminum Alloy

Literature Overview

This study by Zhao Yingjie, Meng Zhanxing, Sun Guangda, Liang Tao, and Zhou Li, published in Aerospace Materials and Technology (Volume 50, Issue 4, 2020, pp. 98-102), investigates the microstructure and mechanical properties of TIG-FSW (Tungsten Inert Gas welding followed by Friction Stir Welding) cross joints in 6 mm thick 2A14 aluminum alloy. The research was conducted at Tianjin Aerospace Long March Rocket Manufacturing Co., Ltd. and Harbin Institute of Technology (Weihai), supported by the National Science and Technology Major Project. The work addresses the challenge of creating reliable cross joints in aluminum alloy structures where different welding processes are combined in a single joint configuration.

Core Technical Findings

Microstructural Analysis

The cross joint exhibits distinct microstructural zones corresponding to the two welding processes:

Zone	Location	Microstructure Characteristics
TIG weld zone	Left side	Fusion weld characteristics with visible porosity defects at upper portion
FSW weld zone	Right side	"Bowl-shaped" distribution with good weld profile
HAZ	Both zones	Grain coarsening and precipitation changes
Base metal	Away from weld	Original heat-treated condition

The TIG weld zone shows obvious porosity defects in the upper portion, which is a known challenge in aluminum alloy fusion welding due to hydrogen porosity formation. The FSW weld zone exhibits a "bowl-shaped" distribution with good weld profile, reflecting the solid-state nature of friction stir welding and its inherent resistance to porosity formation.

Mechanical Properties

Zone	Hardness Characteristics	Notes
TIG weld zone	Significantly lower than FSW zone	Typical of fusion weld softening in Al alloys
FSW weld zone	Higher hardness, "U-shaped" distribution at weld nugget right side	Characteristic FSW hardness profile
Base metal (M temper)	Highest hardness	Heat-treated condition

Hardness testing reveals that the TIG weld zone has significantly lower microhardness values compared to the FSW weld zone. The FSW weld nugget right side exhibits a "U-shaped" hardness distribution, which is characteristic of the complex thermomechanical processing in friction stir welding.

Tensile Behavior

Tensile testing performed in the fusion weld direction shows fracture occurring at the M-temper base metal location, with the fracture surface exhibiting typical ductile fracture characteristics. This indicates that the joint strength is governed by the base metal rather than the weld zones, which is a favorable outcome for joint design.

Process Analysis and Technical Insights

The combination of TIG and FSW in a single cross joint configuration represents a hybrid approach that leverages the strengths of both processes. TIG welding provides the initial joint formation with good accessibility and flexibility, while FSW provides consolidation and strengthening through solid-state deformation. The sequential application of TIG followed by FSW allows the FSW process to potentially repair or improve the TIG weld zone through severe plastic deformation and dynamic recrystallization.

The porosity observed in the TIG weld zone is a critical concern for joint integrity. In aluminum alloy fusion welding, hydrogen porosity is a common defect caused by hydrogen absorption from moisture in the atmosphere, surface oxides, or flux residues. The porosity in the upper portion of the TIG weld may be related to gas entrapment during solidification, particularly in the thicker sections of the cross joint geometry.

The "U-shaped" hardness distribution in the FSW zone reflects the complex thermomechanical processing history. The weld nugget experiences the highest temperatures and most severe plastic deformation, leading to recrystallization and softening. The thermomechanically affected zone (TMAZ) experiences lower temperatures and different deformation conditions, resulting in varying degrees of precipitation modification and dislocation density.

Engineering Practice Implications

For aerospace and structural applications involving aluminum alloy cross joints, this hybrid TIG-FSW approach offers several potential advantages:

The FSW consolidation step can potentially repair porosity defects from the TIG weld through severe plastic deformation.
The combination allows for greater geometric flexibility in cross joint configurations.
The joint strength is governed by the base metal, providing predictable and reliable mechanical behavior.
The FSW zone provides higher hardness and potentially better fatigue resistance than the TIG zone.

However, several challenges must be addressed:

Porosity control in the TIG weld zone remains critical for joint quality.
The interaction between the TIG weld and subsequent FSW processing must be carefully controlled to avoid introducing new defects.
Process parameters for both welding steps must be optimized for the specific cross joint geometry.
Quality assurance procedures must account for the heterogeneous microstructure of the hybrid joint.

Key Reflections

This research highlights the complexity of hybrid welding joints where different welding processes interact in a single configuration. The porosity observed in the TIG weld zone is a reminder that fusion welding of aluminum alloys remains challenging, and process optimization is essential for defect-free welds. The fact that the joint fractures in the base metal rather than the weld zones is a positive indicator of joint design adequacy, but the porosity in the TIG zone could still be a concern for fatigue life and pressure tightness in service.

The "U-shaped" hardness distribution in the FSW zone is characteristic of the process but also indicates a region of potential weakness. The softening in the weld nugget due to recrystallization and overaging of precipitates is a well-known issue in friction stir welding of precipitation-hardened aluminum alloys. The TIG-FSW combination may partially mitigate this through the additional thermomechanical processing, but careful parameter optimization is required.

Reference Value and Outlook

This research provides valuable insights into the microstructure and mechanical behavior of hybrid TIG-FSW cross joints in 2A14 aluminum alloy. The findings are directly relevant to aerospace structural fabrication where cross joints are common and where the combination of welding processes may be necessary for geometric or process reasons. Future work should focus on optimizing the TIG welding parameters to minimize porosity, investigating the effects of FSW processing on TIG weld porosity, and evaluating the fatigue performance of the hybrid joint under cyclic loading. The study also underscores the importance of understanding the interaction between sequential welding processes in hybrid joint configurations.