Numerical Analysis of Stress Evolution at the Interface of Aluminum-Steel Dissimilar MIG Arc Melting-Brazing

Literature Overview

The paper by Qin Guoliang, Geng Peihao, Chen Yong, and Ren Wenjian, published in the Journal of Mechanical Engineering in 2021 (Vol. 57, No. 2, pp. 87–96), presents a comprehensive numerical analysis of stress and strain field evolution during MIG arc melting-brazing of dissimilar aluminum-steel lap joints. The study was supported by the National Natural Science Foundation of China (Grant No. 52075297) and conducted at the Key Laboratory of Liquid-Solid Structure Evolution and Processing, Ministry of Education, Shandong University, in collaboration with Shandong Aotai Electric Co., Ltd. The research focuses on a specific configuration: 1 mm thick 5052 aluminum alloy lap jointed with 2 mm thick galvanized steel, using a combination of asymmetric four-ellipse surface heat source and uniform volume heat source to simulate the welding thermal field.

Thermal-Structural Finite Element Modeling Approach

The numerical modeling approach employed in this study is sophisticated and reflects a deep understanding of the physics of arc melting-brazing. The key innovation is the development of a combined heat source model that accounts for two distinct thermal effects:

1. Asymmetric four-ellipse surface heat source: This models the arc heat input on the aluminum side, where melting occurs. The four-ellipse distribution captures the non-uniform energy deposition characteristic of a MIG arc, with the front and rear heat fluxes adjusted to account for the travel direction effect. The asymmetry reflects the fact that the arc heat is concentrated ahead of the travel direction due to the interaction between the arc plasma and the molten pool flow.
2. Uniform volume heat source: This models the thermal effect of overheated droplets impinging on the brazing zone. In melting-brazing, the MIG wire melts in the arc and the molten droplets transfer to the joint. On the steel side, the molten aluminum wets and flows along the steel surface without melting the steel. The thermal energy carried by these droplets is distributed over a volume rather than concentrated on a surface, hence the volume heat source formulation.

This dual heat source approach is more physically realistic than a single heat source model and is particularly important for melting-brazing processes, where the thermal coupling between the two materials is asymmetric. The thermal-elastic-plastic finite element analysis was then used to compute the stress-strain field evolution throughout the welding and cooling process.

Residual Stress Distribution and Interface Analysis

The numerical results reveal several important findings regarding the residual stress distribution in the aluminum-steel lap joint:

Stress Distribution on the Steel Side

As the distance from the weld center increases, the internal stress in the galvanized steel near the brazing weld zone transitions from compressive stress to tensile stress. This is consistent with the general pattern of residual stress in welded joints, where the weld zone and immediate HAZ experience compressive residual stresses due to constrained contraction during cooling, while the far field experiences tensile stresses to maintain equilibrium.

Stress Distribution on the Aluminum Side

The residual stress in the aluminum weld zone is characterized by a tensile stress equal to the yield strength of aluminum at room temperature. This is a critical finding because it indicates that the aluminum weld zone is at the limit of its elastic capacity. The residual stress decreases to zero or transitions to compressive stress at the weld root, which is consistent with the constraint imposed by the steel backing.

Interface Stress Discontinuity

Perhaps the most significant finding is the large residual stress difference across the brazing interface. The stress difference distribution on both sides of the brazing interface exhibits an "n-shaped" profile under different welding heat inputs. This n-shaped distribution is a direct consequence of the asymmetric thermal field: the aluminum side experiences much higher temperatures and greater thermal strain than the steel side, leading to a stress mismatch at the interface.

The inverse correlation between interface residual stress difference and joint shear strength is a critical engineering insight. As the heat input increases, the thermal gradient becomes more severe, the stress mismatch at the interface increases, and the shear strength of the joint decreases. This relationship provides a clear basis for heat input optimization in aluminum-steel brazing applications.

Engineering Practice Implications

The findings of this study have direct implications for the design and fabrication of aluminum-steel dissimilar joints, which are increasingly common in automotive, aerospace, and energy applications:

• Heat input control is paramount: The inverse relationship between heat input and shear strength indicates that the lowest practicable heat input should be used to maximize joint strength. This aligns with general welding practice but is particularly critical for dissimilar joints where thermal mismatch is inherent.
• Interface stress monitoring: The large stress discontinuity at the brazing interface represents a potential failure initiation site. In design, this should be considered in fatigue life assessment and damage tolerance analysis.
• Process parameter optimization: The numerical model can be used as a design tool to predict residual stress distributions for different parameter combinations, reducing the need for extensive experimental trial-and-error.
• Material selection: The use of 5052 aluminum alloy and galvanized steel represents a common industrial combination. The numerical results provide a baseline for evaluating alternative material combinations.

Key Questions and Critical Reflections

Several aspects of the modeling approach warrant further discussion. First, the use of a uniform volume heat source to represent the droplet thermal effect is a simplification that may not fully capture the complex fluid dynamics of droplet impingement and spreading. The actual thermal distribution of droplets on the brazing surface is likely non-uniform and dependent on droplet velocity, temperature, and the wetting behavior of molten aluminum on galvanized steel. Second, the model assumes a sequential thermal-structural analysis, which is appropriate for the present purposes but may not capture the full coupling between thermal and mechanical fields during the welding process. Third, the study focuses on a specific geometry (lap joint) and material combination; the generalizability of the findings to other joint configurations and material combinations should be verified through additional studies.

Study Insights and Conclusions

The research by Qin Guoliang and colleagues provides a valuable numerical framework for understanding the stress evolution in aluminum-steel MIG arc melting-brazing joints. The development of a physically motivated combined heat source model represents a significant methodological contribution, and the finding of an inverse correlation between interface stress difference and shear strength provides a clear design guideline. The n-shaped stress distribution profile at the brazing interface is a distinctive feature that can serve as a diagnostic indicator for process optimization. For engineers involved in dissimilar metal joining, this study offers both theoretical insight and practical guidance, emphasizing the critical role of heat input control in achieving reliable aluminum-steel joints. The numerical approach demonstrated here can be extended to other welding processes and material combinations, making it a broadly applicable tool for welding process development and optimization.