Microstructure and Mechanical Properties of Narrow-Gap Laser-MIG Hybrid Welds in 60 mm TC4ELI Titanium Alloy Plates

Literature Overview

This paper, published in Hot Working Technology (2026, Vol. 55, No. 11, pp. 82–87), investigates the narrow-gap laser-MIG hybrid welding of 60 mm thick TC4ELI titanium alloy plates. The research is funded by the National Defense Basic Research Program (JCKY2023206A002) and conducted by the Luoyang Ship Material Research Institute in collaboration with the National Key Laboratory of Marine Corrosion and Protection. TC4ELI (Ti-6Al-4V ELI) is a widely used aerospace and marine grade titanium alloy known for its excellent strength-to-weight ratio, but welding thick sections of this alloy has long been a challenge due to its low thermal conductivity, high reactivity, and susceptibility to hydrogen embrittlement and cracking.

Core Technical Points

The narrow-gap laser-MIG hybrid welding process combines the deep penetration capability of laser welding with the high deposition rate of MIG (Metal Inert Gas) welding. This hybrid approach is particularly advantageous for thick-plate applications where conventional autogenous laser welding would require excessive power or multiple passes. The key findings of the study are:

• The weld metal and fully transformed heat-affected zone (HAZ) microstructure consists primarily of acicular martensitic α′ phase and α phase, with small amounts of β phase present between grains.
• Due to the multi-pass welding thermal cycle, lamellar or blocky α phase is also observed in the central weld region and HAZ.
• The tensile strength of the weld center deposited metal exceeds that of the base metal across all thickness layers.
• Microhardness and Charpy impact energy generally decrease from the weld center toward the base metal.
• Bend test results are acceptable, indicating adequate ductility.

Welding Process Parameters and Metallurgical Analysis

The narrow-gap configuration reduces the amount of filler metal required while maintaining structural integrity. For 60 mm thick TC4ELI plates, the process parameters would typically fall within the following ranges:

The acicular martensitic α′ structure in the weld metal and fully transformed HAZ is a direct consequence of the rapid cooling rates achieved by the laser component. This microstructure provides high strength but can be susceptible to cracking under certain conditions. The presence of lamellar α phase in the multi-pass regions indicates that subsequent welding passes partially reheat and soften the previously deposited layers, creating a gradient in microstructure and properties through the weld thickness.

Quality Control Considerations

Welding TC4ELI titanium alloy imposes strict quality control requirements:

1. Atmospheric protection: Titanium is highly reactive above 400°C. Any oxygen or nitrogen ingress can cause embrittlement. The shielding gas coverage must be comprehensive, including back-purging for thick plates.
2. Hydrogen control: Hydrogen pickup from moisture or contaminants can lead to delayed cracking. The filler wire and base metal must be thoroughly cleaned, and the shielding gas must have low moisture content.
3. Crack sensitivity: The acicular α′ structure can be prone to hot cracking, particularly in the centerline of the weld. Controlling the sulfur and oxygen content of the base metal and filler is essential.
4. Residual stress: The high thermal gradients in laser-MIG hybrid welding can generate significant residual stresses, which must be managed through proper welding sequence, back-stress welding, or post-weld stress relief.

Engineering Practice Implications

The study demonstrates that narrow-gap laser-MIG hybrid welding is a viable approach for joining thick TC4ELI plates, offering a good combination of strength, toughness, and production efficiency. However, several practical challenges remain:

• The process requires sophisticated equipment with precise laser-MIG synchronization, which increases capital costs.
• The narrow gap configuration demands high-quality edge preparation and fit-up, as any misalignment can lead to incomplete fusion or excessive dilution.
• The multi-pass nature of thick-plate welding means that thermal management (interpass temperature control) is critical to maintaining consistent microstructure and properties throughout the weld.

For marine and aerospace applications, the welds must also satisfy stringent non-destructive testing requirements, typically including ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) for internal defects and visual inspection or dye penetrant testing (PT) for surface defects.

Study Insights and Outlook

This research represents a meaningful step toward enabling the fabrication of large, thick-section titanium alloy components that were previously impractical to produce by welding. The key insight is that the hybrid laser-MIG approach effectively bridges the gap between the deep penetration of laser welding and the high deposition rate of arc welding, making it possible to achieve full-penetration welds in 60 mm thick plates with acceptable mechanical properties. Future work should focus on optimizing the process parameters to further improve toughness, particularly in the HAZ, and on developing standardized welding procedures that can be reliably applied in production environments. The findings are particularly relevant to engineers working on submarine hulls, aircraft structures, and other high-value titanium alloy applications where weight reduction and structural integrity are paramount.