Numerical Simulation of Temperature and Residual Stress Fields in FSW of AA2195-AZ31B Dissimilar Joints

Background and Significance of FSW in Dissimilar Aluminum Joining

Friction stir welding (FSW) has emerged as a preferred solid-state joining process for dissimilar aluminum alloy combinations, particularly where conventional fusion welding introduces excessive intermetallic compound (IMC) formation, hot cracking, and poor mechanical integrity. The specific combination of AA2195 (a 2xxx-series Al-Cu-Li structural alloy) and AZ31B (a magnesium alloy) represents a challenging lightweight joint system used in aerospace and advanced transportation applications. The large difference in melting points, thermal conductivity, and solid solubility between these two alloys creates a complex thermomechanical environment during FSW that demands rigorous numerical modeling to predict joint quality and performance.

Core Findings on Temperature Field Distribution

The numerical simulation study reveals that the temperature field in FSW of AA2195-AZ31B is highly asymmetric due to the significant difference in thermal properties between the two alloys. The peak temperature at the tool shoulder reaches approximately 400–450 °C, well below the melting point of either alloy, confirming the solid-state nature of the process. However, the temperature gradient is markedly steeper on the AZ31B side due to its lower thermal conductivity (approximately 70 W/m·K) compared to AA2195 (approximately 130 W/m·K). This asymmetry directly influences the flow pattern of the material at the tool shoulder and pin interface, leading to uneven plastic deformation zones and potential defects such as tunnel voids or incomplete bonding on the AZ31B side.

The following table summarizes the key thermal parameters observed in the simulation:

Residual Stress Field Analysis

The residual stress distribution following FSW exhibits a characteristic pattern of longitudinal compressive stress at the weld centerline transitioning to tensile stress at the weld edges. The peak longitudinal compressive stress reaches approximately -180 MPa at the nugget center, while transverse residual stresses remain predominantly compressive throughout the weld zone with magnitudes up to -80 MPa. The asymmetry in residual stress between the two alloy sides is pronounced: the AZ31B side exhibits higher tensile residual stresses near the weld boundary due to its lower modulus of elasticity and greater thermal expansion coefficient. This stress asymmetry has direct implications for fatigue performance and stress corrosion cracking resistance of the joint.

Engineering Implications and Process Optimization

The simulation results provide actionable guidance for FSW parameter selection in dissimilar aluminum-magnesium joints. The tool shoulder diameter should be optimized to ensure sufficient plasticization on the AZ31B side without excessive material flow on the AA2195 side. A slight tilt angle (2–3°) toward the AA2195 side can compensate for the asymmetric flow pattern and improve bonding quality. Furthermore, the residual stress analysis suggests that a low-temperature stress relief treatment (below 200 °C to avoid IMC degradation) may be beneficial for fatigue-critical applications. These findings underscore the value of coupled thermo-mechanical finite element analysis in FSW process development, particularly for dissimilar material combinations where experimental characterization alone would be insufficient to capture the full complexity of the thermomechanical interactions.

Study Insights and Practical Recommendations

The numerical simulation of FSW temperature and residual stress fields in AA2195-AZ31B joints offers a powerful predictive tool for process optimization and quality assurance. The key takeaway is that the inherent asymmetry in material properties between aluminum and magnesium alloys cannot be ignored in FSW process design. Engineers should leverage simulation results to pre-identify critical parameters — particularly tool geometry, tilt angle, and welding speed — that govern bonding quality and residual stress distribution. For production applications, the simulation-derived parameter windows should be validated through systematic experimental trials, including macrosectioning, microhardness mapping, and tensile testing of the joints. This integrated approach of simulation-guided experimentation represents the most efficient pathway to developing reliable FSW processes for dissimilar lightweight alloy joints in aerospace and transportation sectors.