Zhuojin Pipe Fitting Co., LtdNumerical Analysis of Stress Evolution at the Interface of Aluminum-Steel Dissimilar Metal MIG Arc Welding-Brazing
Literature Overview
This paper, authored by Qin Guoliang, Geng Peihao, Chen Yong, and Ren Wenjian from Shandong University and Shandong Aotai Electric Co., Ltd., was published in the Journal of Mechanical Engineering in 2021 (Vol. 57, No. 2, pp. 87-96). The study numerically analyzes the stress and strain field evolution during MIG arc welding-brazing of a lap joint between 1 mm thick 5052 aluminum alloy and 2 mm thick galvanized steel. The research is supported by the National Natural Science Foundation of China (Project No. 52075297) and falls under classification TG404, which pertains to welding process simulation.
Dissimilar Metal Welding-Brazing: Technical Background
The welding-brazing process is a specialized technique for joining dissimilar metals where one side melts (the aluminum side) and the other side is heated above its solidus temperature but below its melting point (the steel side). This approach avoids the formation of brittle intermetallic compounds that would result from full melting of both metals. However, the asymmetric nature of the heat source and the thermal properties of the two metals create complex stress states at the interface.
The 5052 aluminum alloy is an Al-Mg alloy with good formability and corrosion resistance, while galvanized steel provides structural strength. The lap joint configuration is commonly used in automotive applications, such as body-in-white structures, where aluminum and steel components must be joined.
Thermal Source Modeling and Numerical Methodology
The researchers developed a combined heat source model that accounts for the asymmetric nature of the welding-brazing process:
| Component | Description | Purpose |
|---|---|---|
| Asymmetric four-ellipsoidal surface heat source | Models the arc heat distribution on the aluminum side | Captures the non-uniform heat input from the arc |
| Uniform volume heat source | Models the heat contribution from overheated droplets | Accounts for the thermal effect of droplets that exceed the melting temperature |
The thermal-elastic-plastic finite element analysis was performed using coupled thermal-mechanical simulation. The key modeling considerations include:
- Temperature-dependent material properties for both aluminum and steel, including Young's modulus, yield strength, thermal conductivity, and thermal expansion coefficient.
- Elastic-plastic constitutive model with kinematic and isotropic hardening.
- Sequentially coupled thermal-mechanical analysis, where the temperature field from the thermal simulation is mapped onto the mechanical model.
- Constraint release and element birth/death techniques to simulate the welding process.
Residual Stress Distribution Characteristics
The numerical results reveal several important findings regarding residual stress distribution:
| Location | Residual Stress Characteristic | Engineering Implication |
|---|---|---|
| Galvanized steel side near brazed zone | Compressive stress near weld center, transitioning to tensile stress with distance | Compressive stress near the weld is beneficial for fatigue resistance |
| Aluminum side weld zone | Tensile stress equal to the yield strength of room-temperature aluminum alloy | Maximum tensile stress at the weld center |
| Aluminum side weld root | Stress decreases to zero or becomes compressive | Stress relief at the root reduces cracking susceptibility |
| Brazing interface (both sides) | Significant residual stress difference across the interface | Critical for interface integrity and joint strength |
A particularly important finding is that the residual stress difference across the brazing interface exhibits an n-shaped distribution pattern at all heat input levels. This n-shaped pattern indicates that the stress difference is highest at the interface center and decreases toward the edges, creating a complex shear stress state at the interface.
Heat Input Effect on Interface Stress and Shear Strength
The relationship between welding heat input and interface performance is critical for process optimization:
| Heat Input Level | Interface Residual Stress Difference | Shear Strength | Trend |
|---|---|---|---|
| Low | Lower | Higher | Optimal balance |
| Medium | Moderate | Moderate | Acceptable range |
| High | Higher | Lower | Deteriorating |
The study demonstrates a negative correlation between the residual stress difference at the brazing interface and the shear strength of the joint. As heat input increases, the residual stress difference across the interface increases, while the joint shear strength decreases. This relationship is attributed to the increased thermal gradient and plastic deformation at higher heat inputs, which create larger differential thermal strains between the aluminum and steel sides. The excessive residual stress at the interface can lead to microcracking or delamination, reducing the effective bonding area and overall joint strength.
Engineering Practice and Optimization Guidance
For practical welding-brazing applications, the following optimization strategies can be derived from this study:
- Heat input control: Select the minimum heat input that achieves adequate bonding, as higher heat input degrades interface strength.
- Joint design: Consider alternative joint configurations that reduce thermal asymmetry, such as using transition pieces or symmetric lap joints with matched thicknesses.
- Post-weld stress relief: Stress relief treatment, such as low-temperature annealing, may be applied to reduce residual stresses, although this must be carefully controlled to avoid softening the aluminum alloy.
- Process monitoring: Real-time monitoring of welding parameters is essential to maintain consistent heat input throughout the weld length.
The 5W2H approach can be applied to process development: What is the target joint strength? Why is welding-brazing selected over other joining methods? Where is the joint located in the assembly? When is the welding performed in the manufacturing sequence? Who performs the welding and what qualifications are required? How are the parameters controlled and verified?
Key Reflections and Study Insights
This study provides valuable insights into the fundamental mechanisms governing the performance of aluminum-steel dissimilar metal joints. The identification of the n-shaped residual stress distribution pattern at the interface is a significant finding that has not been widely reported in the literature. This pattern suggests that the interface is subjected to a complex stress state that includes both normal and shear components, with the shear component being particularly critical for joint integrity.
The negative correlation between residual stress difference and shear strength has direct implications for process parameter selection. In my experience with dissimilar metal joining, the temptation to increase heat input to achieve better wetting and bonding often comes at the cost of increased residual stresses and reduced joint strength. This study quantitatively demonstrates this trade-off, providing engineers with a basis for rational parameter selection.
The use of a combined heat source model that accounts for both the arc and the thermal effect of overheated droplets represents a more realistic modeling approach than conventional symmetric heat source models. Future work should focus on experimental validation of the predicted stress distributions using techniques such as neutron diffraction or X-ray microtomography, and on extending the analysis to include fatigue behavior and long-term aging effects at the interface.