Residual Stress Distribution in FSW and TIG Welded Aluminum Alloy Joints

Literature Overview and Research Significance

The study by Wang Xun-hong, Wang Kuai-she, Shen Yang, and Xu Ke-wei, published in Materials in Mechanical Engineering in 2007, provides a comparative analysis of residual stress distributions in friction stir welding (FSW) and TIG welded joints of aluminum alloys. Residual stresses are an inherent consequence of welding processes and play a critical role in determining the service performance of welded structures, particularly with respect to fatigue life, distortion, stress corrosion cracking, and dimensional stability. This research was supported by the Shaanxi Provincial Science and Technology Plan (Grant 2003K07-G8), and the authors leveraged X-ray diffraction (XRD) techniques to non-destructively measure surface residual stress distributions.

The comparison between FSW and TIG is particularly meaningful because these two processes represent fundamentally different approaches to joining aluminum alloys. TIG welding is a fusion process that involves melting and solidification, while FSW is a solid-state process that joins materials without melting. This fundamental difference leads to markedly different residual stress patterns, microstructures, and mechanical properties.

Residual Stress Measurement Methodology

X-ray diffraction is a well-established technique for measuring surface residual stresses in metallic materials. The method is based on the principle that the interplanar spacing of crystal planes changes under stress according to Bragg's law, and this change can be measured by varying the specimen tilt angle. The sin²ψ method is commonly used, where the diffraction peak position is measured at multiple tilt angles and the stress is calculated from the slope of the peak position versus sin²ψ plot.

The use of XRD for residual stress measurement is advantageous because it is non-destructive, provides localized measurements, and can be applied to production components. However, it is limited to near-surface measurements (typically within 50–200 μm depth) and requires access to the measurement surface.

Residual Stress Distribution Patterns

The key finding of this study is that both FSW and TIG welded joints exhibit a characteristic "W"-shaped residual stress distribution in the direction parallel to the weld axis. This distribution pattern can be described as follows:

The "W" shape arises from the thermal contraction sequence during welding. As the weld pool solidifies, it contracts and is restrained by the cooler surrounding metal, generating tensile stresses in the weld metal. As the heat-affected zone cools, it contracts and generates additional tensile stresses. The regions immediately adjacent to the weld experience compressive stresses due to plastic deformation and the elastic constraint from the surrounding material. This creates the characteristic alternating tensile-compressive pattern.

The most significant finding is that FSW joints exhibit residual stresses that are 15–25% lower than TIG joints in the heat-affected zone. This reduction is attributed to the fundamentally different thermal cycles of the two processes. FSW operates entirely in the solid state, avoiding the phase transformations and extreme thermal gradients associated with fusion welding. The plastic deformation and dynamic recrystallization that occur during FSW also contribute to stress relaxation, as the material is continuously worked and recrystallized under the搅拌 tool.

Implications for Structural Integrity

Residual stresses have profound implications for the structural performance of welded joints. The following table summarizes the key effects:

The lower residual stresses in FSW joints translate directly into improved fatigue resistance and reduced susceptibility to stress corrosion cracking. For aerospace and automotive applications where fatigue life and corrosion resistance are critical, this advantage of FSW over TIG welding can be decisive. However, FSW also has limitations, including restricted weldability of certain material combinations, limited joint geometry flexibility, and higher equipment costs.

Engineering Practice and Process Selection

The choice between FSW and TIG welding should be guided by the specific application requirements. For applications where fatigue life and corrosion resistance are paramount, such as aircraft fuselage panels, battery enclosures, and cryogenic pressure vessels, FSW is generally preferred. For applications requiring complex joint geometries, dissimilar material joining, or thick section welding, TIG welding remains a viable option, provided that residual stress management strategies such as post-weld stress relief, shot peening, or laser shock peening are employed.

Post-weld stress relief techniques can partially mitigate the high residual stresses in TIG joints. Mechanical methods such as shot peening and laser shock peening introduce beneficial compressive surface stresses that counteract the tensile residual stresses. Thermal stress relief, while effective, may compromise the strength of precipitation-hardened aluminum alloys due to over-aging. The selection of the appropriate stress relief method depends on the alloy system, joint configuration, and service requirements.

This comparative study provides valuable quantitative data for engineers making process selection decisions. The 15–25% reduction in residual stress in FSW joints is a substantial advantage that should be considered in life prediction and damage tolerance analyses. As FSW technology continues to mature and equipment costs decrease, its application in industrial welding is expected to expand, particularly in the aerospace and transportation sectors where lightweight, high-integrity joints are essential.