Microstructure and Properties of TA2 Sheet Through-Crack TIG Welded Joints

Literature Overview

Published in 2018 by Yin Yayun and colleagues from the 725th Research Institute of China Shipbuilding Industry Corporation, this study examines the microstructure and mechanical properties of through-crack (small hole) TIG welded joints in TA2 titanium sheet. The research was supported by the Marine Engineering Innovation Project of the 725th Institute. TA2 (commercially pure Grade 2 titanium) is widely used in marine engineering, nuclear applications, and chemical processing due to its excellent corrosion resistance and good formability.

Core Technical Content

Through-Crack TIG Welding Process

The through-crack TIG welding technique, also known as full-penetration TIG or keyhole TIG welding, achieves complete weld penetration in a single pass by carefully controlling heat input to create a controlled keyhole effect. This method is particularly attractive for titanium sheet applications where:

Single-pass welding reduces heat-affected zone (HAZ) extent
Minimal distortion is critical for thin-sheet marine structures
Back-side protection with inert gas is essential to prevent oxidation

Microstructure Analysis

The study examined the weld metal, HAZ, and base metal microstructures. TA2 titanium exhibits a characteristic microstructure evolution during welding:

Region	Microstructure	Grain Size	Mechanical Characteristics
Weld metal	Acicular alpha + residual beta	Fine, dendritic	Good toughness, lower strength
HAZ (coarse grain zone)	Widmanstätten alpha	Coarse	Reduced ductility, potential crack initiation site
HAZ (partially transformed)	Equiaxed alpha + transformed beta	Medium	Balanced properties
Base metal	Equiaxed alpha + small amount of beta	Fine	Reference properties

Mechanical Property Evaluation

Key mechanical properties evaluated include tensile strength, elongation, hardness distribution across the weld cross-section, and potentially impact properties. The through-crack TIG weld typically shows:

Weld metal tensile strength comparable to or slightly below base metal
Elongation values that may be reduced due to acicular microstructure
Hardness peaks in the HAZ due to martensitic-like alpha' transformation in cooling
Potential for lamellar alpha' formation if cooling rates are excessive

Interpretation of Technical Points

The through-crack TIG technique represents a significant advancement in titanium sheet welding productivity. The critical process parameters include:

Current density: Must be sufficiently high to establish stable keyhole formation, typically in the range of 400–600 A for 3–6 mm sheet
Travel speed: Controls heat input and keyhole stability; too fast causes incomplete penetration, too slow causes excessive HAZ
Shielding gas: Pure argon or argon-helium mixtures; helium addition increases arc energy and penetration
Back-purge: Essential for preventing oxide formation on the weld root; flow rate typically 2–5 L/min
Polarity: DCEN provides deep penetration with stable arc; DCEP offers shallower but wider welds

The microstructure of the HAZ is of particular concern for pressure vessel applications. The formation of coarse Widmanstätten alpha in the coarse grain zone of the HAZ can significantly reduce fracture toughness and increase susceptibility to stress corrosion cracking in chloride-containing environments.

Connection with Engineering Practice

For bimetal pressure vessel fabrication involving titanium cladding or titanium-lined components, the through-crack TIG technique offers several advantages:

Reduced dilution of titanium cladding layers when welding to steel substrates
Precise control of heat input minimises carbon pickup in titanium overlay layers from carbon steel backing
Single-pass capability reduces the risk of intergranular corrosion sensitisation in titanium welds

However, several challenges arise in practical application:

Maintaining adequate back-purge in complex geometries such as vessel heads, nozzles, and channels
Controlling dilution at titanium-steel transition zones requires careful parameter adjustment
Hydrogen contamination from atmospheric moisture must be strictly controlled to prevent delayed cracking

Application Scenario	Key Challenge	Recommended Approach
Ti-clad pressure vessel shell	Dilution control at interface	Use backfill with compatible filler; limit heat input
Marine heat exchanger tubesheet	Back-purge accessibility	Use internal purge fixtures; consider multi-pass approach
Chemical processing vessel	Corrosion resistance of HAZ	Post-weld heat treatment to refine microstructure
Nuclear application	Fracture toughness requirements	Strict PWHT schedule; consider lower HAZ temperature

Key Questions and Reflections

The study raises important questions about the long-term performance of through-crack TIG welds in service. Specifically:

How does the coarse HAZ microstructure affect fatigue performance under cyclic loading typical of pressure vessels?
What is the impact of the acicular weld microstructure on stress corrosion cracking resistance in marine environments?
Can post-weld heat treatment effectively refine the HAZ microstructure without introducing new problems such as sigma phase formation or excessive grain growth?

From a quality assurance perspective, the examination of these joints requires specialised NDE techniques. Titanium welds are challenging for radiographic testing due to high gamma ray absorption, making ultrasonic testing (particularly phased array UT) the preferred method for detecting internal defects.

Study Insights and Implications

This research demonstrates that through-crack TIG welding can produce acceptable joints in TA2 sheet, but the microstructural heterogeneity inherent in the weld zone demands careful consideration for critical applications. For pressure vessel engineers, the key insight is that achieving full penetration in a single pass does not automatically guarantee equivalent properties throughout the weld zone. Post-weld heat treatment, typically at 650–800°C for 1–2 hours followed by controlled cooling, may be necessary to homogenise the microstructure and restore corrosion resistance. The work provides valuable baseline data for process qualification under standards such as ASME IX and NB/T 47014, which require demonstration of acceptable mechanical properties and microstructural integrity for titanium weld procedures.