Effect of Laser Power on TC4 Titanium Alloy Laser-TIG Hybrid Welding

Literature Overview

This research by Fan Hao, Cui Shan, Chen Wenjing, Ni Yu, and Hui Yuanyuan, published in Applied Laser (2023, Vol. 43, No. 8, pp. 48-54), investigates the influence of laser power on the weld geometry, surface morphology, and microstructural characteristics of TC4 titanium alloy welded using the laser-TIG hybrid welding process. Funded by Shaanxi Provincial Higher Education Youth Innovation Team projects, this study addresses a contemporary welding technology that combines the deep penetration capability of laser welding with the stable arc characteristics of TIG welding to achieve superior weld quality in titanium alloys.

Core Technical Findings

The laser-TIG hybrid welding process produces weld joints with excellent surface formation, free from obvious defects such as undercut and weld bead overflow. However, a distinctive feature of this hybrid process is the formation of a depression or crater at the center of the weld surface, caused by the intense molten pool convection generated by the laser beam. This surface depression is a characteristic signature of the laser-TIG interaction and must be considered in post-weld finishing requirements.

The most striking finding relates to the evolution of weld cross-sectional geometry with increasing laser power. At lower laser powers, the weld cross-section exhibits a "mushroom" shape, characteristic of TIG-dominated welding. As laser power increases, the cross-section transitions to a "hourglass" shape, indicating the dominance of laser keyhole welding with a narrow penetration channel. This geometric transition has direct implications for weld strength, fatigue performance, and residual stress distribution.

The porosity analysis reveals that average porosity area decreases with increasing laser power, and porosity is predominantly concentrated at the bottom of the weld. This distribution pattern suggests that porosity formation is related to the solidification behavior at the trailing edge of the molten pool, where gas entrapment is more likely to occur.

Microstructural Analysis

The weld metal microstructure in the laser-TIG hybrid welds consists of bundles of α′ martensite composed of multiple parallel α′ laths, interspersed with narrow residual β phase bands. The width of these β phase bands varies significantly, ranging from approximately 0.45 μm in narrow regions to approximately 1.95 μm in wider regions. This wide variation in β band width indicates non-uniform cooling rates across the weld cross-section, which is expected given the complex thermal field generated by the combined laser and arc heat sources.

The finding that grain size decreases with increasing laser power is particularly important. The more intense molten pool convection generated by higher laser power promotes dendrite fragmentation and increases nucleation sites, resulting in finer grains. This is consistent with the well-established relationship between fluid flow intensity in the molten pool and grain refinement during solidification.

Process Mechanism Interpretation

The laser-TIG hybrid welding process operates through the synergistic interaction of two distinct heat sources. The laser beam provides a high energy density that creates a deep, narrow penetration profile, while the TIG arc provides a broader, more stable heat input that ensures complete fusion at the top of the weld and maintains a stable molten pool. The interaction between these two heat sources creates complex fluid flow patterns in the molten pool, which directly influence grain growth during solidification.

The formation of the surface depression is attributed to the intense electromagnetic and Marangoni convection forces generated by the laser beam on the molten pool surface. The laser-induced plasma plume and the associated recoil pressure create a strong downward flow at the center of the weld surface, which deepens the weld pool at that location. Upon solidification, this deepened region manifests as a surface depression. In engineering practice, this depression can be addressed through proper joint design, backfilling, or post-weld machining.

The transition from mushroom to hourglass weld geometry with increasing laser power reflects the shift in the dominant penetration mechanism. At low laser powers, the TIG arc dominates the heat input distribution, producing a wider, shallower weld. As laser power increases, the keyhole effect becomes more pronounced, creating a narrow, deep penetration channel that narrows the weld cross-section at the root. This hourglass geometry can be beneficial for stress concentration reduction at the weld root but may introduce challenges for full fusion at the bottom of the joint.

Engineering Practice Considerations

For engineers considering laser-TIG hybrid welding for TC4 titanium alloy fabrication, several practical considerations emerge from this study. First, the process offers the advantage of producing welds with finer microstructures compared to conventional TIG welding alone, which can improve both strength and toughness. Second, the porosity reduction achieved at higher laser powers is beneficial for weld integrity, particularly in applications where porosity can act as crack initiation sites under cyclic loading.

However, the surface depression must be accounted for in design and manufacturing. In applications where surface quality is critical, such as aerospace structural components or medical implants, the surface depression may require post-weld machining or filling. The concentration of porosity at the weld bottom is also a concern for applications subject to high tensile stresses perpendicular to the weld axis, as these porosity clusters can serve as fatigue crack initiation sites.

The residual β phase bands, while narrow, represent regions of lower hardness and potentially lower strength within the weld metal. The significant variation in β band width (0.45-1.95 μm) suggests that local mechanical properties within the weld cross-section may vary considerably. This microstructural heterogeneity should be considered when evaluating the fatigue performance of laser-TIG hybrid welded joints.

Key Questions and Reflections

A significant question arising from this study is the optimal laser power setting that balances the competing objectives of grain refinement, porosity reduction, and weld geometry control. While higher laser power produces finer grains and less porosity, it also creates a more pronounced hourglass geometry and a deeper surface depression. The engineering decision on laser power must therefore consider the specific requirements of the application, including the importance of surface quality, the expected loading conditions, and the post-weld processing capabilities.

Another important reflection is the role of residual β phase in the mechanical properties of the weld. The β phase bands, although narrow, represent a softer phase that can accommodate deformation more readily than the surrounding α′ martensite. In the context of fatigue loading, these β phase regions may influence crack initiation and propagation behavior. Further investigation into the fatigue performance of laser-TIG hybrid welded TC4 joints, with specific attention to the role of β phase distribution, would be valuable.

Study Insights and Implications

This study demonstrates that laser-TIG hybrid welding is a promising technology for TC4 titanium alloy fabrication, offering improved weld quality through finer microstructures and reduced porosity compared to conventional TIG welding. The controllable transition from mushroom to hourglass weld geometry provides engineers with a tool to tailor weld cross-sectional profiles to specific design requirements. The key challenge remains the optimization of laser power to achieve the best balance between microstructural refinement, geometric quality, and porosity control. For engineering practice, this technology is particularly attractive for thin-to-moderate thickness TC4 components where the combination of high penetration efficiency and fine microstructure offers significant advantages over conventional welding processes.