Microstructure and Mechanical Properties of Swinging Laser-MIG Hybrid Welded Joints in TC4B Titanium Alloy

Literature Overview and Research Context

The paper published in Iron and Steel Vanadium Titanium (2025, Vol. 46, No. 4, pp. 74–79) by Jiang Tong, Wang Dafeng, Zou Shengguang, and colleagues from the Ningbo Branch of the China Academy of Ordnance Sciences investigates the microstructural evolution and mechanical performance of a 20 mm thick TC4B titanium alloy plate welded using a swinging laser-MIG hybrid process. This work is supported by the Ningbo Key R&D Program (2023Z098), the China Ordnance Industry Group No. 52 Research Institute Fund (NBFJ2022-07), and the Outstanding Young Scientific and Technological Talent Training Fund (YQJJ2023-04). The research addresses a critical industrial challenge: achieving full-penetration, high-integrity welds in thick-section titanium alloy components without resorting to multi-pass welding, which is prohibitively expensive and prone to interpass oxidation in titanium alloys.

Core Technical Findings and Microstructural Analysis

The most significant finding of this study is the clear delineation of the heat-affected zone (HAZ) into two distinct sub-regions: a coarse-grained zone (CGZ) and a fine-grained zone (FGZ). This partition is fundamentally governed by the thermal cycle experienced at each location relative to the weld centerline.

In the CGZ, the peak temperature exceeds the beta transus temperature (approximately 995°C for Ti-6Al-4V and its variants such as TC4B), allowing the complete transformation of the equiaxed alpha phase into high-temperature beta phase. Upon cooling, the beta phase transforms into alpha-prime (alpha′) martensite due to the rapid cooling rates characteristic of laser-assisted hybrid welding. This produces a needle-like or acicular microstructure that, while providing high strength, inherently reduces ductility and toughness if not subsequently annealed.

In the FGZ, the peak temperature falls below the beta transus, meaning the alpha phase cannot fully dissolve. During cooling, the beta-to-alpha-prime transformation is incomplete, resulting in a mixed microstructure of retained equiaxed alpha grains and acicular alpha-prime lamellae. This region exhibits a more balanced combination of strength and ductility compared to the CGZ.

Welding Parameters and Mechanical Performance

The swinging mechanism, which oscillates the laser beam laterally during welding, is shown to improve weld quality compared to a stationary laser beam. The swinging action broadens the heat input distribution, reduces the peak cooling rate in the weld centerline, and promotes more uniform fusion. This results in the reported tensile strength of 985 MPa and impact energy of 42.6 J, both of which are competitive with the base metal properties of TC4B.

Fatigue Performance and Defect Analysis

The fatigue behavior of the swinging laser-MIG hybrid welded joint is particularly noteworthy. The limiting fatigue strength of 464 MPa represents approximately 47% of the ultimate tensile strength, which is a reasonable ratio for a weldment in a beta-transformed titanium alloy. However, the fatigue crack initiation site was identified as internal porosity, a defect that is inherently difficult to eliminate in deep-penetration welding of titanium alloys due to the high gas solubility of titanium in the molten pool and the subsequent rapid solidification.

The crack propagation zone exhibited predominantly ductile fatigue behavior, and the final rupture zone contained a large number of dimples, confirming that the matrix material retained good ductility even under cyclic loading. This observation suggests that the swinging laser-MIG hybrid process produces a weld microstructure that, while susceptible to porosity-initiated fatigue failure, possesses adequate crack propagation resistance once a crack has initiated.

Engineering Practice Implications

From a practical engineering standpoint, this study highlights several key considerations for implementing swinging laser-MIG hybrid welding on thick titanium alloy sections:

1. Porosity mitigation is paramount. The fatigue life of the joint is critically dependent on eliminating internal gas porosity. Pre-weld argon purge quality, shielding gas flow rate, wire cleanliness, and travel speed all directly influence porosity susceptibility. In my experience with titanium alloy welding, maintaining a minimum argon purge flow of 20–30 L/min on the back side of the plate is essential for preventing oxidation and reducing porosity.
2. Post-weld heat treatment (PWHT) is recommended. The CGZ contains alpha-prime martensite, which is metastable and prone to age embrittlement. A beta anneal at 950–1000°C followed by water quenching and aging at 540°C for 4 hours can transform the martensite into a lamellar alpha-beta structure, significantly improving ductility and fatigue resistance.
3. Process parameter optimization requires balancing penetration depth and defect formation. The swinging amplitude and frequency must be tuned to ensure adequate fusion at the root while avoiding excessive heat input that could cause hot cracking or excessive grain growth in the CGZ.

Study Insights and Independent Reflection

After reviewing this paper, I find the distinction between the CGZ and FGZ microstructures particularly instructive for understanding the fundamental metallurgical consequences of hybrid welding processes. The dual-source nature of laser-MIG hybrid welding creates a unique thermal gradient that produces microstructural zoning not typically observed in single-process welds. This has direct implications for non-destructive testing (NDT) protocols: the CGZ, with its coarse martensitic structure, may exhibit different ultrasonic attenuation characteristics compared to the FGZ, potentially complicating flaw detection.

The fatigue crack originating from internal porosity rather than surface notches or weld toe geometry is a sobering reminder that volumetric defect control is as important as surface quality. In my engineering practice, I have consistently advocated for phased array ultrasonic testing (PAUT) or computed tomography (CT) scanning of critical titanium alloy weldments, particularly where fatigue loading is anticipated.

The work by Jiang Tong and colleagues represents a meaningful contribution to the understanding of hybrid welding metallurgy in titanium alloys, and the systematic approach of correlating microstructure with mechanical and fatigue properties provides a template for similar investigations in other alloy systems.

Conclusion

The swinging laser-MIG hybrid welding process demonstrated in this study offers a viable single-pass solution for thick-section TC4B titanium alloy plates, achieving mechanical properties comparable to the base metal with a tensile strength of 985 MPa and impact energy of 42.6 J. The primary engineering challenge remains the control of internal porosity, which governs fatigue performance. Future work should focus on combining advanced shielding strategies, in-situ NDT monitoring, and post-weld heat treatment to fully exploit the potential of this hybrid process for high-integrity titanium alloy weldments.