Aluminum Alloy Laser Multi-Strand Wire MIG Hybrid Welding Joint Microstructure and Performance Analysis

Literature Overview

This study by Xu Kai, Wu Pengbo, Li Linlin, Huang Ruisheng, Liang Xiaomei, and Liang Yu, published in the Journal of Welding (2022, Vol. 43, No. 11, pp. 43-49), investigates the microstructure and mechanical properties of aluminum alloy weld joints produced by laser-multi-strand wire MIG hybrid welding. The study used 5A06 aluminum alloy plates with ER5356 1x3 multi-strand twisted wire as the filler metal, producing 20 mm thick butt weld joints. The research was supported by the Heilongjiang Provincial Head Goose Action Plan - Advanced Welding Technology Innovation Team for Energy Equipment (Grant 201916120). The work was conducted at the Harbin Welding Research Institute, China Ordnance Industry Group Aviation Ammunition Research Institute, and Jiangsu Lianjie Welding Technology Co., Ltd.

Hybrid Welding Process Characteristics

Laser-MIG hybrid welding combines the deep, narrow penetration of laser welding with the high deposition rate and process flexibility of MIG welding. The synergy between the two energy sources creates a weld pool with characteristics that neither process can achieve alone. The laser provides deep penetration and narrow heat-affected zone, while the MIG arc provides a wider weld bead with good surface profile and the ability to fill gaps.

The use of multi-strand twisted wire (1x3 configuration, meaning three strands twisted together) is an innovative approach that increases the effective wire diameter while maintaining flexibility. The 1x3 ER5356 wire provides a larger cross-sectional area for metal deposition compared to a single solid wire of equivalent outer diameter, resulting in higher deposition rates. The twisted configuration also provides additional turbulence in the wire melting process, potentially improving arc stability and droplet transfer characteristics.

Microstructural Characterization

The study employed comprehensive characterization techniques including optical metallography, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), tensile testing, and microhardness mapping:

The weld metal consists primarily of the α(Al) matrix with dispersed AlMg second phase particles. The equiaxed grain structure with an average size of 34.83 μm indicates effective grain refinement during solidification. The AlMg particles serve as nucleation sites for equiaxed grain formation and also provide precipitation strengthening in the as-welded condition.

The HAZ shows fine grains with an average size of 10.21 μm, significantly smaller than the weld metal grains. This refinement results from the recovery and recrystallization of the base metal microstructure under the welding thermal cycle. The recrystallization produces a new grain structure with lower stored energy and smaller grain size, which provides good strength-ductility balance in the HAZ.

Mechanical Properties

The tensile strength of 292 MPa, representing 84% of the base metal strength, is excellent for an aluminum alloy weld joint without post-weld heat treatment. The ductile fracture characteristics, with fracture occurring near the fusion zone rather than in the weld center, indicate that the weld metal has adequate ductility and that the fusion zone, despite being the weakest zone in terms of hardness, is not the weakest in terms of fracture resistance.

The hardness profile shows a minimum at the fusion zone (84.6% of base metal hardness), with the weld metal center having slightly higher hardness. This is somewhat unusual, as the weld metal center is typically the softest zone. The higher hardness in the weld center may be attributed to the dispersed AlMg second phase particles, which provide solid solution and dispersion strengthening even in the as-welded condition.

Process-Structure-Property Relationships

The hybrid welding process produces a unique combination of process advantages that translate into favorable microstructural and mechanical outcomes:

1. The laser provides deep, narrow penetration, creating a high aspect ratio weld pool that promotes columnar-to-equiaxed transition (CET) through rapid cooling at the weld root.
2. The MIG arc provides a wider weld pool with better surface profile and the ability to fill the gap created by the laser.
3. The multi-strand wire increases the deposition rate while maintaining good arc stability.
4. The combination results in a weld joint with fine equiaxed grains in the weld metal, fine recrystallized grains in the HAZ, and high strength-ductility balance.

The use of EBSD for grain orientation analysis provides additional insight into the recrystallization behavior in the HAZ. The grain orientation distribution reveals the preferred orientations developed during recrystallization, which can influence anisotropic mechanical properties. The fine recrystallized grains in the HAZ suggest that the welding thermal cycle provided sufficient energy for complete recrystallization without excessive grain growth.

Engineering Practice Implications

For thick-section aluminum alloy welding (20 mm in this study), the laser-MIG hybrid process offers significant advantages over conventional MIG welding alone. The higher penetration rate reduces the number of passes required, improving productivity and reducing the risk of interpass defects. The narrower HAZ minimizes the volume of material affected by the welding thermal cycle, preserving more of the base metal properties.

The 1x3 multi-strand wire configuration is particularly attractive for industrial applications because it can be used with existing wire feed systems with minimal modification. The twisted wire provides the same outer diameter as a single solid wire but with a larger cross-sectional area, enabling higher deposition rates without requiring changes to the torch design or shielding gas flow rates.

Study Insights and Reflections

This study demonstrates the maturity of laser-MIG hybrid welding for thick-section aluminum alloy applications. The combination of laser and arc energy sources, together with the multi-strand wire innovation, produces weld joints with properties approaching those of the base metal without post-weld heat treatment. This is a significant achievement, as post-weld heat treatment of large aluminum alloy structures is often impractical due to distortion concerns and cost.

The use of EBSD for microstructural characterization represents a methodological advancement in welding research. EBSD provides quantitative information about grain orientation, texture, and recrystallization behavior that is not accessible through conventional optical metallography. This information is valuable for understanding anisotropic mechanical properties and predicting fatigue behavior in different directions.

The ductile fracture characteristics observed in this study are particularly encouraging for structural applications. The fracture occurring near the fusion zone, rather than in the weld center, indicates that the weld metal has adequate toughness to accommodate plastic deformation. The dimple morphology on the fracture surface confirms that the failure mechanism is ductile, involving microvoid nucleation, growth, and coalescence. This is the desired failure mode for structural components, as it provides warning (through plastic deformation) before catastrophic failure.

The study also highlights the importance of filler metal selection in hybrid welding. The ER5356 wire, with its Mg-rich composition, provides hot crack resistance through the formation of low-melting-point Al-Mg eutectic, while also contributing to the AlMg second phase particles that provide strengthening. The 1x3 configuration maintains the beneficial composition while increasing the deposition rate, demonstrating that wire geometry and wire composition can be independently optimized for different process objectives.