Controlled-Penetration Welding of Thick-Section Titanium Alloys: Mechanisms and High-Efficiency Implementation via K-TIG/Narrow-Gap TIG Hybrid Process

This study develops a novel hybrid welding process integrating keyhole TIG (K-TIG) with narrow-gap TIG (NG-TIG) deposition through an asymmetric U-V groove design for 48-mm-thick TC4 ELI titanium alloy. The optimized groove configuration enabled simultaneous penetration control and deposition efficiency, achieving > 40% productivity enhancement over conventional methods while maintaining joint integrity. Microstructural analysis identified a Widmanstätten structure (α/β lath bundles) in the weld metal (WM) and acicular α′ martensite, primary α-phase, and some β phase in the heat-affected zone (HAZ). Mechanical characterization revealed enhanced weld center hardness (340 HV) surpassing the base metal (310 HV), with a characteristic V-profile hardness minimum (290 HV) at the HAZ/BM interface. The weld exhibited superior tensile strength (915 MPa vs. BM: 860 MPa) and − 20 °C impact toughness (45 J vs. HAZ: 30 J). Fractography demonstrated mixed-mode failure featuring dimples (5–15 μm) and local cleavage planes, confirming the hybrid process’s viability for thick-section titanium alloys in aerospace/marine applications requiring high structural integrity.