CLADDING TECHNOLOGY SHANXI CO., LTDDeep Penetration Welding Mechanism of Medium-Thick Titanium Alloy by Dual-Pulse TIG
Literature Overview
This 2025 publication in Rare Metal Materials and Engineering by researchers from Beihang University and Capital Aerospace Machinery Co., Ltd. investigates the deep penetration welding mechanism of medium-thick titanium alloy plates using a dual-pulse TIG welding technique. Supported by the National Natural Science Foundation of China (Grants U20B2031, 52075024), this work addresses a significant challenge in aerospace and pressure vessel manufacturing where thick-section titanium alloy components require high-quality welds with deep penetration.
Core Technical Content
Medium-thick titanium alloy plates (typically 6–25 mm) present unique welding challenges due to titanium's low thermal conductivity, high reactivity with atmospheric gases at elevated temperatures, and susceptibility to hydrogen embrittlement. The dual-pulse TIG technique combines a high-energy pulse for penetration with a low-energy pulse for filler wire feeding and bead shaping, enabling deep penetration with reduced heat input compared to conventional single-pulse or DC TIG welding.
Dual-Pulse TIG Process Parameters
The dual-pulse configuration operates with two distinct pulse parameters:
| Parameter | High-Energy Pulse | Low-Energy Pulse | Engineering Purpose |
|---|---|---|---|
| Current | 200–400 A | 50–150 A | Penetration / Bead shaping |
| Pulse frequency | 5–20 Hz | 50–200 Hz | Penetration rate / Bead quality |
| Duty cycle | 30–60% | 40–80% | Heat input control |
| Pulse width | 5–20 ms | 5–20 ms | Arc stability |
| Travel speed | 50–200 mm/min | — | Productivity |
| Shielding gas | Ar / He / Ar-He mix | — | Contamination prevention |
| Filler wire | ER Ti-6Al-4V | — | Composition matching |
Welding Mechanism Analysis
The deep penetration mechanism in dual-pulse TIG welding of titanium alloy involves several key physical phenomena:
- Keyhole formation: The high-energy pulse creates a concentrated heat flux sufficient to vaporize titanium and form a keyhole, enabling deep penetration even at moderate travel speeds. The keyhole depth is governed by the balance between surface tension, vapor pressure, and electromagnetic forces.
- Pulse interaction effects: The low-energy pulse stabilizes the keyhole and promotes uniform bead formation. The interaction between consecutive pulses creates a periodic oscillation of the molten pool that enhances fluid flow and inclusion flotation.
- Solidification behavior: The dual-pulse approach creates a distinctive columnar-to-equiaxed transition in the weld microstructure, with the high-energy pulse promoting columnar growth and the low-energy pulse encouraging equiaxed grain formation through thermal oscillation.
Microstructural and Mechanical Properties
| Region | Microstructure | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|---|
| Base metal (Ti-6Al-4V) | Equiaxed α + β | 900–1000 | 10–14 |
| Weld metal | Acicular α + residual β | 850–950 | 8–12 |
| HAZ | Widened α + β grain boundary | 800–900 | 7–10 |
| Fusion line | Fine acicular α | 820–920 | 8–11 |
The dual-pulse approach demonstrates improved weld quality compared to conventional TIG:
- Reduced porosity due to enhanced keyhole stability and bubble flotation during the low-energy pulse phase.
- Lower hydrogen pickup due to reduced total heat input and shorter exposure of titanium to high temperatures.
- Improved weld bead geometry with consistent reinforcement and reduced undercut.
Engineering Practice Implications
For titanium alloy pressure vessel fabrication, this research has direct relevance to:
- Thick-section welding: The dual-pulse technique enables single-pass welding of thicker sections (up to 12–15 mm), reducing the number of weld passes and consequently the total heat input and distortion.
- Weld procedure development: The parameter windows identified in this study provide a starting point for procedure qualification under NB/T 47014 or ASME IX for titanium alloy welds.
- Defect prevention: The reduced porosity and improved bead quality directly address common quality issues in titanium alloy welding, particularly in hydrogenation reactor and cryogenic pressure vessel fabrication.
- Productivity improvement: The ability to achieve deep penetration at higher travel speeds translates to improved welding productivity, which is economically significant for large-scale titanium component manufacturing.
Key Questions and Reflections
Several aspects merit further consideration:
- How does the dual-pulse technique perform when welding dissimilar titanium alloys (e.g., Ti-6Al-4V to Ti-6242S)?
- What is the effect of pulse parameter optimization on weld residual stress distribution and distortion?
- How does the technique scale to thicker sections (20–30 mm) with multiple passes?
- What are the implications for weld NDE, particularly regarding the detection of internal defects in the deep penetration zone?
Study Insights and Implications
This research represents a meaningful advancement in titanium alloy welding technology. The dual-pulse TIG approach offers a practical solution to the deep penetration challenge while maintaining the metallurgical quality essential for titanium alloy applications. For pressure vessel engineers, the technique provides a viable option for welding thick titanium alloy components with reduced heat input, lower distortion, and improved mechanical properties. The findings should be incorporated into welding procedure specifications and qualification programs for titanium alloy pressure vessels, particularly in hydrogenation and cryogenic service where weld quality is paramount.