CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Temperature Field in Dissimilar Metal Friction Stir Welding of 6061 Aluminum Alloy and AZ31 Magnesium Alloy
Literature Overview and Research Context
The friction stir welding (FSW) of dissimilar aluminum and magnesium alloys represents one of the most challenging yet practically significant joining problems in lightweight structural engineering. The specific pairing of 6061-T6 aluminum alloy with AZ31 magnesium alloy is of particular interest because both materials are widely used in aerospace, automotive, and marine applications where weight reduction is critical. However, the large difference in melting points (660°C for Al-6061 versus 650°C for AZ31, but with vastly different thermal conductivities, thermal expansion coefficients, and solidus temperatures) creates a highly asymmetric temperature field during FSW. The literature under study presents a finite element numerical simulation of the temperature distribution during this dissimilar FSW process, providing critical insight into process window optimization and defect prediction.
Core Technical Points and Thermal Analysis
The numerical model employs a coupled thermo-mechanical finite element approach to capture the transient temperature evolution during FSW. The key thermal parameters considered include the tool rotational speed, welding speed, shoulder diameter, pin geometry, and the asymmetry of material properties on either side of the weld.
| Parameter | 6061 Al Alloy | AZ31 Mg Alloy |
|---|---|---|
| Thermal conductivity (W/m·K) | 205 | 72 |
| Specific heat (J/kg·K) | 896 | 1023 |
| Density (kg/m³) | 2700 | 1738 |
| Melting point (°C) | 660 | 650 |
| Thermal expansion (10⁻⁶/°C) | 23.6 | 26.2 |
The simulation reveals that the temperature gradient across the weld line is extremely steep due to the nearly threefold difference in thermal conductivity between the two materials. The aluminum side dissipates heat rapidly, while the magnesium side retains heat locally, leading to a peak temperature zone shifted toward the magnesium side. This asymmetry has direct implications for material flow, microstructural evolution, and potential defect formation.
Temperature Field Characteristics and Process Implications
The numerical results demonstrate that the peak temperature in the stir zone typically reaches 480–550°C depending on the process parameters, well below the melting point of both materials, confirming the solid-state nature of the process. However, the temperature on the magnesium side consistently exceeds that on the aluminum side by 40–80°C under identical input conditions. This temperature asymmetry drives unequal material flow rates and can lead to asymmetric stir zone morphology.
The literature highlights three critical temperature-dependent phenomena:
- Intermetallic compound formation risk: The interface region between Al and Mg is susceptible to the formation of brittle intermetallic phases such as Al₃Mg₂ and Al₁₂Mg₁₇ when local temperatures exceed certain thresholds. The simulation helps identify process windows that minimize interfacial temperatures below the critical formation threshold.
- Material flow imbalance: The lower thermal conductivity of AZ31 results in more localized heat accumulation, causing the magnesium material to flow more readily than the aluminum material. This can lead to incomplete mixing at the Al/Mg interface and potential bonding defects.
- Residual stress asymmetry: The differential cooling rates between the two sides generate asymmetric residual stress patterns, which may influence post-weld distortion and fatigue behavior.
Defect Prediction and Process Optimization
The temperature field simulation enables the prediction of several characteristic FSW defects in dissimilar Al-Mg joints:
| Defect Type | Mechanism | Mitigation Strategy |
|---|---|---|
| Tunnel defect | Insufficient material flow on Al side | Increase tool rotation speed or reduce welding speed |
| Flash defect | Excessive material flow on Mg side | Reduce shoulder pressure or increase welding speed |
| Void at interface | Incomplete mixing at Al/Mg boundary | Optimize pin geometry and tool tilt angle |
| Intermetallic layer | Excessive interfacial temperature | Reduce heat input, use variable speed welding |
A particularly important finding is that the tool tilt angle plays a decisive role in managing the temperature asymmetry. Tilting the tool toward the aluminum side (typically 1–3 degrees) helps compensate for the thermal conductivity difference by directing more heat into the aluminum side, thereby reducing the temperature gradient across the interface.
Engineering Practice Implications and Reflections
From a practical engineering perspective, this numerical study provides several actionable insights for cladding and bimetal component fabrication involving aluminum-magnesium dissimilar joints:
- The simulation results validate the necessity of process parameter optimization tailored specifically to dissimilar material combinations rather than applying parameters developed for homogeneous joints.
- The predicted temperature fields can be used to design thermocouple placement for experimental validation, ensuring that critical temperature zones are monitored during actual welding.
- The identification of intermetallic compound formation risk informs post-weld heat treatment strategies; a controlled solution treatment may be necessary to dissolve undesirable intermetallic phases at the interface.
- The residual stress predictions should be incorporated into design calculations for pressure vessels or structural components where fatigue life is a critical concern.
The study also raises an important question regarding the scalability of FSW from laboratory specimens to production-scale components. The numerical model assumes uniform tool rotation and straight-line welding, but in practice, curved weld paths, varying plate thicknesses, and complex geometries introduce additional thermal asymmetries that may not be captured by simplified models. Engineers should therefore use simulation results as a starting point for parameter development rather than as definitive process specifications.
Study Insights and Conclusions
The numerical simulation of the temperature field in 6061-Al/AZ31-Mg dissimilar FSW provides a fundamental understanding of the thermal asymmetry inherent in this material combination. The key takeaway is that the large difference in thermal conductivity between aluminum and magnesium alloys creates a non-equilibrium thermal state that must be actively managed through process parameter selection. The simulation serves as an essential design tool for predicting defect formation, optimizing heat input, and ensuring adequate bonding at the dissimilar interface. For engineers involved in lightweight bimetal component fabrication, this type of numerical analysis should be an integral part of the process qualification workflow, complementing but not replacing physical experimental validation. The ultimate goal is to achieve a sound metallurgical bond with minimal intermetallic formation, adequate mechanical properties, and acceptable fatigue performance in the final component.