Numerical Simulation of Temperature Field and Residual Stress in Dual-Pulse MIG Welding of Aluminum Alloy T-Joints

Research Context and Methodology

This study by Cao Shufen and colleagues from Hunan University addresses the thermal and mechanical behavior of dual-pulse MIG welding applied to 6061-T6 aluminum alloy thin plate T-joints. The research, funded by multiple national programs including the National Natural Science Foundation (751229039) and the National 12th Five-Year Science and Technology Support Program (2011BAG03B02), employs elastic-plastic finite element analysis (FEA) to simulate both the temperature field and the stress-strain field during the welding process. The heat source model is simplified as a periodic thermal source that alternates between strong and weak pulse groups at the low-frequency pulse frequency, and the filler wire deposition is simulated using the birth and death element technique.

The dual-pulse MIG welding process represents an advanced variant of conventional MIG welding, where the pulse waveform incorporates two distinct current levels within each cycle. This technique was developed to improve weld quality in aluminum alloys by providing better control over the molten pool dynamics, particularly in terms of penetration depth and weld bead appearance.

Simulation Results and Key Parameters

The simulation results reveal that the maximum residual stress after welding is concentrated at the weld region, reaching a peak value of 273 MPa. This residual stress distribution leads to an angular distortion of 1.61° on the heated side of the T-joint flange. The temperature field and stress field both exhibit periodic variations synchronized with the low-frequency pulse frequency, reflecting the alternating strong and weak pulse groups inherent to the dual-pulse waveform.

The comparison between strong and weak pulse groups reveals distinct differences in molten pool behavior. During the weak pulse group phase, the molten pool temperature is lower and the pool volume is smaller compared to the strong pulse group phase. However, paradoxically, the surrounding metal experiences greater stress during the weak pulse group stage. This counterintuitive result can be explained by the rapid cooling rates associated with the lower energy input during weak pulses, which generate higher thermal gradients and thus larger thermal stresses in the surrounding material.

Periodic Thermal Cycling and Its Metallurgical Benefits

The periodic alternation between strong and weak pulse groups creates a cyclic variation in molten pool dimensions and stress states. This cyclic thermal input is beneficial for weld quality in several ways. First, it promotes the formation of a fish-scale weld bead appearance, which is aesthetically superior and indicates uniform heat distribution. Second, the cyclic solidification conditions favor the formation of fine and uniform weld microstructure, as the repeated heating and cooling cycles provide multiple nucleation opportunities for grain refinement.

From a process control perspective, the dual-pulse approach offers advantages over conventional MIG welding for aluminum alloys. Aluminum's high thermal conductivity and low melting point make it challenging to achieve stable arc conditions and adequate penetration with conventional processes. The dual-pulse waveform effectively manages the energy input by concentrating high energy during strong pulses (for penetration) and reducing energy during weak pulses (for bead control), thereby achieving a balance between penetration and weld bead profile.

The validation of the simulation model against experimental data—showing good agreement between predicted and measured molten pool dimensions, welding thermal cycles, and T-joint welding distortion—confirms the reliability of the numerical approach. This validation is essential for the practical application of FEA in welding process design, as it establishes confidence in using simulation as a predictive tool for process optimization.

Engineering Practice Implications and Reflections

The residual stress of 273 MPa, while significant, is manageable through post-weld stress relief or by incorporating it into the structural design. However, the 1.61° angular distortion is a more practical concern for fabrication, as it may require additional machining or fixture correction. Engineers should consider pre-setting or backing plate strategies to minimize distortion when using dual-pulse MIG welding for T-joints. The simulation approach demonstrated in this study provides a valuable tool for predicting welding distortion and residual stress before physical trials, reducing development time and cost. The key insight is that the periodic nature of dual-pulse welding is not merely an energy management technique but also a microstructure control strategy that leverages cyclic thermal cycling to refine the weld zone grain structure. This work represents a significant advancement in the understanding of advanced welding processes for aluminum alloys and provides a framework for further optimization of dual-pulse parameters for different joint configurations and material thicknesses.