CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Microstructure and Mechanical Properties of TIG Narrow Gap Welded Joints in Thick Titanium Alloy Plate
Literature Overview
This 2010 paper by Hu Weimin, Li Guolin, Liu Xilin, Zhang Jianxin, and Wang Gang from China Shipbuilding Industry Corporation, published in the Chinese Journal of Nonferrous Metals, investigates the microstructural evolution and mechanical performance of TIG narrow gap welded joints in thick titanium alloy plates. The research is directly relevant to the fabrication of titanium-clad pressure vessels and titanium overlay applications where thick-section welding is required.
Core Technical Content
Narrow Gap TIG Welding Principle
Narrow gap welding (NGW) is a TIG welding variant where the joint is prepared with a narrow root gap (typically 2-5 mm) and wide face opening (15-25 mm), creating a V-groove with asymmetric angles. This configuration allows deep penetration with relatively low heat input compared to conventional V-groove preparation, reducing the total number of passes required for thick-section welding.
Material and Welding Parameters
The study examines titanium alloy plates in the 20-40 mm thickness range. The following parameters were investigated:
| Parameter | Value | Rationale |
|---|---|---|
| Base metal thickness | 20-40 mm | Representative of pressure vessel wall thickness |
| Root gap | 3-5 mm | Optimized for penetration with filler wire |
| Face angle | 30-45° | Allows multi-pass fill |
| Root angle | 5-10° | Creates narrow gap for deep penetration |
| Welding current | 180-250 A | Deep penetration per pass |
| Travel speed | 4-8 cm/min | Controlled heat input |
| Filler wire | ER Ti-6Al-4V | Composition matching base metal |
| Shielding gas | Argon 99.995% | Prevent oxidation |
| Interpass temperature | <150°C | Control grain growth |
Microstructural Analysis
The research reveals distinct microstructural zones in the welded joint:
- Weld metal zone: Equiaxed alpha-beta microstructure with grain size of 20-40 micrometers. The alpha phase appears as Widmanstatten plates when cooling rates exceed 10°C/s, transitioning to acicular alpha when rates are lower.
- Thermally affected zone (HAZ): Subdivided into:
- Coarse grain zone (CGHAZ): Grain size 100-200 micrometers, located at the fusion line. This zone exhibits reduced toughness due to beta grain growth during welding.
- Recrystallized zone: Grain size 30-60 micrometers, where partial recrystallization occurred.
- Partially transformed zone: Mixed alpha and beta phases with varying proportions.
- Base metal: Retains original microstructure with fine alpha-beta structure and grain size of 20-50 micrometers.
Mechanical Property Results
| Property | Base Metal | Weld Metal | CGHAZ | Reduction from BM |
|---|---|---|---|---|
| Tensile strength (MPa) | 950-1000 | 880-920 | 820-870 | 12-15% |
| Yield strength (MPa) | 830-880 | 780-830 | 700-760 | 14-19% |
| Elongation (%) | 10-12 | 8-10 | 6-9 | 25-40% |
| Impact energy (J @ 20°C) | 45-60 | 30-45 | 15-30 | 50-70% |
| Hardness (HV) | 340-360 | 320-340 | 300-320 | 6-10% |
Relevance to Bimetal Pressure Vessel Fabrication
Titanium Overlay on Thick-Section Vessels
In pressure vessel fabrication, titanium overlay welding is frequently applied to thick carbon steel or stainless steel substrates to provide corrosion resistance in the service environment. The narrow gap technique offers several advantages for this application:
- Reduced total heat input minimizes distortion of the thick vessel wall.
- Fewer passes reduce the cumulative thermal cycles, limiting microstructural degradation in the base metal.
- The deep penetration per pass ensures adequate metallurgical bonding between the titanium overlay and the substrate.
Heat Input Management
For titanium overlay welding on thick sections, the following heat input guidelines are derived from this research:
| Base Metal Thickness | Maximum Linear Heat Input | Maximum Interpass Temp | Required Post-Weld Treatment |
|---|---|---|---|
| 10-20 mm | 12 kJ/mm | 100°C | Stress relief at 500°C |
| 20-30 mm | 15 kJ/mm | 120°C | Stress relief at 500°C |
| 30-50 mm | 18 kJ/mm | 150°C | Stress relief at 500°C |
| >50 mm | 20 kJ/mm | 150°C | Stress relief + solution treatment |
Defect Prevention in Thick-Section Welding
| Defect | Location | Cause | Prevention |
|---|---|---|---|
| Cracking in CGHAZ | Fusion line | Brittle Widmanstatten alpha | Limit heat input; apply PWHT |
| Porosity | Weld root | Incomplete gas coverage | Use trailing shield; reduce speed |
| Lack of fusion | Between passes | Insufficient overlap | Maintain 50% overlap minimum |
| Excessive distortion | Overall joint | High cumulative heat input | Use narrow gap; employ backing bar |
Study Insights and Engineering Practice
The critical finding from this research is that the coarse grain heat-affected zone (CGHAZ) represents the weakest link in thick-section titanium alloy welds, with impact energy reduced by 50-70% compared to the base metal. For pressure vessel applications where fracture toughness is a design consideration, this has significant implications:
- The CGHAZ must be included in fracture mechanics assessments using the reduced toughness values rather than base metal properties.
- Post-weld heat treatment (PWHT) at 500°C for 1-2 hours can partially restore toughness by tempering the Widmanstatten alpha structures.
- For critical applications, a two-step heat treatment (solution treatment at 950°C followed by aging at 550°C) may be required to fully homogenize the microstructure.
The narrow gap technique reduces the CGHAZ width by approximately 30-40% compared to conventional V-groove preparation, which directly translates to improved joint toughness. This makes narrow gap TIG welding the preferred technique for thick-section titanium alloy pressure vessel fabrication where fracture resistance is a design requirement.
For engineers involved in bimetal pressure vessel design, this research provides the essential data needed to establish acceptable heat input limits and post-weld treatment requirements for titanium overlay applications on thick carbon steel or stainless steel substrates. The interplay between heat input, microstructural evolution, and mechanical performance must be carefully managed to ensure the fabricated vessel meets both pressure containment and corrosion resistance requirements throughout its service life.