Microstructure and Mechanical Properties of Novel Aluminum Alloy MIG Weld Joints with Multi-Layer Multi-Pass Welding

Literature Overview

The paper by Jin Congcong, Huang Libing, Huang Wenbin, Wang Dali, Ma Yueting, Li Peng, and Dong Honggang, published in Welding Journal (Vol. 45, No. 7, 2024, pp. 74-82), investigates the microstructure and mechanical properties of a novel aluminum alloy MIG weld joint produced by single-sided welding of 10 mm thick plate. The study compares two welding configurations: two-layer three-pass (Joint No. 1) and three-layer four-pass (Joint No. 2), examining the effects of layer and pass arrangement on joint performance. This research is motivated by the trend toward aluminum substitution for steel in shipbuilding, driven by the demands for lightweight and high-speed vessel construction.

Core Technical Findings

Microstructural Characteristics

The microstructural analysis reveals several important features:

• Phase composition: Both the upper and lower weld surfaces exhibit good macroscopic morphology with the presence of α-Al, Al₆(Fe,Mn), and Mg₂Si phases.
• HAZ grain structure: Large equiaxed grains form near the fusion line in the HAZ, while columnar grains develop at the weld edge perpendicular to the fusion line, and dendritic grains are found at the weld center.
• Layer-dependent grain size: Each weld layer exhibits different grain sizes, with the first layer having smaller grains than subsequent layers. This is attributed to the thermal effect of subsequent passes remelting and modifying the previously deposited layers.
• Elemental segregation: Fe and Mn elements segregate in the HAZ to form Al₆(Fe,Mn) phase, while Mg elements tend to precipitate as equilibrium phases along weld grain boundaries or within the weld microstructure.

Mechanical Properties

The results demonstrate that the three-layer four-pass configuration (Joint No. 2) significantly outperforms the two-layer three-pass configuration (Joint No. 1) in both strength and ductility, achieving 93.3% of base metal tensile strength and 12.3% elongation compared to 78.4% and 6.56% for Joint No. 1.

Technical Interpretation

Effect of Layer and Pass Arrangement on Microstructure

The difference in microstructure between the two joint configurations can be attributed to several factors:

1. Thermal cycling: The three-layer four-pass configuration subjects the weld joint to more thermal cycles, which promotes grain refinement and phase redistribution. Each subsequent pass acts as a partial annealing treatment for the previously deposited layers.
2. Interpass temperature: With more passes, the interpass temperature management becomes more critical. Proper interpass temperature control helps prevent excessive grain growth and promotes uniform phase distribution.
3. Solidification rate: The first layer, which solidifies against the base metal, experiences a different thermal gradient compared to subsequent layers deposited on already-solidified weld metal. This explains why the first layer has smaller grains and higher hardness.

Hardness Distribution Analysis

The hardness distribution pattern (WM lowest, HAZ intermediate, BM highest) is characteristic of aluminum alloy weld joints and reflects the following metallurgical phenomena:

• Weld metal: The relatively low hardness is due to the solution-treated-like condition of the weld metal, where strengthening phases (Mg₂Si, Al₆(Fe,Mn)) are partially dissolved during the welding heat cycle and have not fully re-precipitated.
• HAZ: The intermediate hardness results from partial dissolution and re-precipitation of strengthening phases, with the degree of softening depending on the peak temperature reached.
• Base metal: The highest hardness reflects the fully strengthened condition of the base metal with intact precipitate distribution.

The Role of First Layer Hardness

The observation that the first layer has the highest hardness among weld layers is significant. This is because the first layer solidifies with a high cooling rate against the relatively cold base metal, promoting fine grain formation and possibly retaining some of the base metal's precipitate structure at the fusion boundary. Subsequent layers solidify against already-warm weld metal, resulting in slower cooling rates and coarser microstructures.

Engineering Practice Integration

Implications for Shipbuilding Applications

For shipbuilding applications where aluminum alloy plate thicknesses of 10 mm and above are common, the following engineering recommendations emerge:

1. Prefer multi-layer multi-pass welding: The superior mechanical properties of the three-layer four-pass configuration justify the additional welding time and cost for critical structural joints.
2. Interpass temperature control: Maintaining interpass temperature below a specified limit (typically 150-200°C for aluminum alloys) is essential to prevent excessive grain growth and strength loss.
3. Welding sequence optimization: The welding sequence should be designed to minimize residual stress and distortion, which is particularly important for thin-walled aluminum structures.
4. Post-weld treatment consideration: Given that the weld metal hardness is significantly lower than the base metal, post-weld aging treatment may be necessary to restore the strength of the weld metal to acceptable levels.

Comparison with Industry Standards

Joint No. 1 marginally fails the typical tensile strength ratio requirement, while Joint No. 2 comfortably exceeds all standard requirements. This has direct implications for welding procedure qualification (WPQ) and welder performance qualification (CPQ).

Critical Reflections and Key Questions

Several important aspects of this study merit further consideration:

1. Fracture mode analysis: The study mentions toughness fracture but does not provide detailed fractography. Understanding the fracture mode (ductile vs. brittle, transgranular vs. intergranular) is essential for assessing the joint's reliability under cyclic loading conditions.
2. Corrosion resistance: The study does not address the corrosion resistance of the weld joint, which is critical for marine applications. The presence of Al₆(Fe,Mn) phase in the HAZ may affect localized corrosion susceptibility.
3. Fatigue performance: Ship structures are subjected to cyclic loading. The fatigue performance of the multi-layer multi-pass weld joint, particularly at the weld toe and fusion line, should be evaluated.
4. Welding parameter sensitivity: The study does not report the specific MIG welding parameters used. Reproducibility of the results requires detailed parameter documentation, including wire feed speed, travel speed, shielding gas composition and flow rate, and voltage settings.
5. Effect of plate thickness: The results are specific to 10 mm plate thickness. The scaling of these results to thicker or thinner plates requires additional investigation.

Study Insights and Implications

This study provides valuable data on the microstructural evolution and mechanical performance of multi-layer multi-pass MIG weld joints in a novel aluminum alloy. The clear superiority of the three-layer four-pass configuration over the two-layer three-pass configuration has direct practical implications for welding procedure development in aluminum alloy fabrication. The finding that the first layer has the finest microstructure and highest hardness among weld layers offers insight into the thermal effects of sequential pass deposition and can guide welding sequence optimization. For shipbuilding and other industries transitioning from steel to aluminum, this type of systematic investigation of welding parameters and their effects on joint performance is essential for developing reliable welding procedures that meet structural and service life requirements. The work underscores the importance of understanding the microstructure-property relationships in welded joints, as these relationships determine the joint's ability to withstand the demanding service conditions of marine and industrial applications.