Numerical Study of Residual Stress in Marine Aluminum Alloy MIG Welding

Literature Overview

This study by Qin Chuang, Ou Peng, and colleagues from Jiangsu University of Science and Technology, in collaboration with the 705th Research Institute of China Shipbuilding Group Corporation and Jiangsu Ocean Equipment Co., Ltd., presents a three-dimensional numerical simulation of welding residual stress in 5052 aluminum alloy butt joints fabricated by MIG welding. Supported by the Guangdong Provincial Key R&D Program (2020B1111500001-04) and the Jiangsu Green Ship Open Fund (2019Z02), the work was published in Naval Architecture and Marine Engineering in 2022 (Volume 44, Issue 19, pages 63-68). The research is significant for marine structural applications where fatigue performance and corrosion resistance are governed by residual stress distributions.

Numerical Modeling Methodology

The study employs a thermo-elasto-plastic finite element model based on the coupled thermo-mechanical approach. The welding process is simulated by a moving heat source that follows the weld path, with the temperature field solved first and then used as a thermal load in the subsequent mechanical analysis. The model incorporates the temperature-dependent material properties of 5052 aluminum alloy, including Young's modulus, Poisson's ratio, thermal expansion coefficient, and yield strength.

Key modeling assumptions and parameters include:

The use of a double-ellipsoidal heat source is appropriate for MIG welding, where the heat distribution is asymmetric due to the arc's forward and rearward components. The model accounts for the fact that the front part of the heat source is narrower and deeper (representing the arc's penetration into the material), while the rear part is wider and shallower (representing the heat dissipation zone behind the arc).

Residual Stress Distribution Analysis

The numerical results reveal characteristic residual stress patterns that are consistent with established welding metallurgy theory. The longitudinal residual stress near the weld centerline is predominantly tensile, arising from the longitudinal contraction of the weld metal during cooling. This tensile stress is symmetrically distributed on both sides of the weld centerline and fusion line, with the maximum value occurring at the weld centerline itself.

As the distance from the weld centerline increases, the longitudinal residual stress transitions from tensile to compressive. This stress redistribution is a consequence of the equilibrium condition: the compressive stresses in the far-field material balance the tensile stresses in the weld and near-weld region. The equivalent Mises stress reaches its maximum value at the weld centerline and decreases monotonically toward the plate edges.

The residual stress values in regions beyond the HAZ are negligible and can be ignored for practical engineering purposes. This finding is important for fatigue assessment, as the stress concentration is localized to a narrow zone around the weld, simplifying the fatigue analysis of welded joints.

NDT Validation and Engineering Significance

The numerical simulation results were validated using ultrasonic non-destructive testing (NDT) of the actual welded joints. The ultrasonic measurements confirmed that the magnitude and direction of residual stresses predicted by the model were in good agreement with experimental values. The stress distribution patterns in corresponding regions were also consistent, demonstrating the reliability of the numerical approach for predicting residual stress in marine aluminum alloy welds.

For marine applications, the residual stress distribution has direct implications for several critical performance aspects:

1. Fatigue life: Tensile residual stresses at the weld toe and centerline promote fatigue crack initiation and propagation under cyclic loading, reducing the fatigue life of the joint. The magnitude of the residual stress directly influences the effective stress range experienced by the material.
2. Corrosion resistance: Residual tensile stresses can accelerate stress corrosion cracking (SCC) in marine environments, particularly in chloride-containing seawater. 5052 aluminum alloy, while generally resistant to atmospheric corrosion, can be susceptible to SCC in the presence of tensile residual stresses and aggressive chloride solutions.
3. Dimensional stability: Residual stresses contribute to post-weld distortion and dimensional inaccuracy, which is critical for the assembly of large marine structures where fit-up tolerances are tight.

Study Insights and Implications

The validation of the numerical model through ultrasonic NDT provides engineers with a reliable tool for predicting residual stress distributions without the need for destructive testing or time-consuming experimental measurements. This is particularly valuable for large marine structures where full-scale residual stress measurement is impractical.

The finding that residual stresses are concentrated near the weld centerline and negligible beyond the HAZ has practical implications for stress relief strategies. Post-weld heat treatment (PWHT) or mechanical stress relief (such as hammering or vibration stress relief) should be targeted at the weld zone and immediate HAZ, rather than being applied uniformly across the entire plate. This focused approach is more efficient and cost-effective.

For the specific case of 5052 aluminum alloy, which is a non-heat-treatable alloy (tempered by strain hardening only), residual stress relief through heat treatment is limited by the low solution treatment temperature. Mechanical stress relief methods or controlled welding sequences that minimize peak residual stress should be preferred. In marine structural design, the predicted residual stress values should be incorporated into fatigue assessment per DNV-ST-F101 or equivalent standards, where the residual stress factor modifies the fatigue category of the weld detail.

This study demonstrates the maturity of numerical methods for residual stress prediction in aluminum alloy welding and provides a validated approach that can be adapted to other aluminum alloy grades and welding configurations used in marine applications.