Stress and Deformation Analysis of Medium-Thick Magnesium-Aluminum Dissimilar Alloy Laser-MIG Hybrid Welding

Research Context and Technical Challenges

The study by Wang Liang, He Xiaohan, Zhang Mengyao, Hu Yiwen and Zhou Peishan from Southwest Petroleum University and Sichuan Chuanguo Boiler Co., Ltd. addresses the welding of dissimilar magnesium and aluminum alloys using a laser-MIG hybrid process. This is a technically demanding application because magnesium and aluminum have fundamentally different physical and chemical properties, including melting points (650 °C for Mg versus 660 °C for Al), thermal conductivities (156 W/m·K for Mg versus 237 W/m·K for Al), coefficients of thermal expansion (26×10⁻⁶/K for Mg versus 23×10⁻⁶/K for Al) and densities (1.74 g/cm³ for Mg versus 2.70 g/cm³ for Al). The research was supported by the Nanchong City University-Industry Collaboration Project (SWJBGS004) and published in 2023, reflecting current industrial interest in lightweight structural welding.

Hybrid Welding Process Configuration

The laser-MIG hybrid welding process combines the deep penetration capability of a high-power laser with the high deposition rate and filler metal flexibility of MIG welding. In this configuration, the laser serves as the primary heat source for keyhole formation, while the MIG arc provides additional heat input and delivers filler metal into the keyhole. The synergistic interaction between the laser and the arc creates a molten pool that is deeper and more stable than either process alone.

The key to successful hybrid welding is the precise alignment of the laser beam and the MIG arc relative to the joint. The laser beam is typically positioned slightly ahead of the MIG arc in the travel direction, creating a leading keyhole that is then filled by the MIG arc and filler metal. The offset between the laser and the arc is typically 0.5–2.0 mm, and the angle between the MIG wire and the vertical axis is 5–15° in the direction of travel.

Residual Stress Distribution

The residual stress distribution in the hybrid welded joint was analyzed using the X-ray sin²ψ method and finite element simulation. The longitudinal residual stresses in the weld metal reached peak values of approximately 200–280 MPa, while the transverse residual stresses were approximately 150–220 MPa. The compressive residual stresses in the HAZ were approximately 80–150 MPa, which is beneficial for fatigue resistance but may contribute to distortion if the compressive zone is asymmetric.

The residual stress distribution in dissimilar Mg-Al welds is more complex than in homogeneous welds because of the asymmetric thermal expansion and the different elastic moduli of the two materials. The magnesium side, having a lower modulus (45 GPa) compared to aluminum (70 GPa), experiences higher plastic strains during cooling, which results in a shift of the peak tensile stress toward the magnesium side. This asymmetry can lead to uneven distortion and potential cracking at the Mg/Al interface if the thermal mismatch is not properly managed.

Deformation Behavior

The angular and longitudinal distortions of the welded plates were measured using laser scanning and coordinate measurement machines. For 8 mm thick plates, the angular distortion was approximately 1.5–2.5° and the longitudinal shrinkage was approximately 0.3–0.5% of the weld length. The distortion was asymmetric, with the magnesium side deflecting more than the aluminum side due to the lower stiffness of magnesium.

The root cause of the asymmetric distortion is the difference in the elastic modulus and the coefficient of thermal expansion between the two materials. During cooling, the aluminum side contracts more in absolute terms due to its higher thermal conductivity and higher modulus, while the magnesium side, having a lower modulus, deforms more easily and absorbs more of the plastic strain. This results in a curl toward the aluminum side.

The hybrid welding process offers advantages over pure laser welding or pure MIG welding in terms of distortion control. The lower heat input per unit length compared to pure MIG welding reduces the overall thermal distortion, while the higher deposition rate compared to pure laser welding allows for thicker joints without excessive multi-pass distortion. The laser provides the initial keyhole with minimal thermal spread, and the MIG arc provides the additional heat needed for full penetration and fusion with the filler metal.

Interfacial Reaction and Metallurgical Bonding

A critical concern in Mg-Al dissimilar welding is the formation of intermetallic compounds (IMCs) at the Mg/Al interface. The primary IMCs formed are Mg₂Al₃ (β phase) and Mg₁₇Al₁₂ (η phase), which are brittle and can significantly reduce the fracture toughness of the joint. The thickness of the IMC layer is strongly dependent on the peak temperature and the holding time at elevated temperature during the welding thermal cycle.

The hybrid welding process can be optimized to minimize IMC formation by controlling the heat input and the cooling rate. A lower laser power with higher travel speed reduces the peak temperature and the time spent at high temperature, thereby limiting the diffusion of Mg and Al atoms across the interface. However, this also reduces the penetration depth, so a balance must be struck between IMC control and joint integrity.

Study Insights and Practical Recommendations

This research demonstrates that laser-MIG hybrid welding is a viable process for joining magnesium and aluminum alloys, but the success depends on careful process optimization to manage the metallurgical and mechanical challenges of the dissimilar interface. The residual stress distribution is asymmetric and must be accounted for in the design of welded structures to avoid excessive distortion or cracking.

For engineering practice, the following recommendations emerge from this study: preheating the magnesium side to 150–200 °C can reduce the thermal gradient and minimize IMC formation; using a laser power of 2–3 kW with a travel speed of 500–600 mm/min provides an optimal balance between penetration and heat input; and post-weld stress relief at 200 °C for 1 hour can reduce the residual stresses by 30–40% without significantly affecting the joint strength. The research also highlights the importance of numerical simulation in predicting and managing the complex stress and deformation behavior of dissimilar metal welds, and engineers should invest in validated simulation models before committing to production welding.