Mechanical Properties of TIG Welded Joints in Marine Ti75 Titanium Alloy

Literature Overview

The research conducted by Xi Quan, Li Wei, and Zhang Yidian from Jiujiang Vocational and Technical College investigates the mechanical properties of TIG welded joints fabricated from Ti75 marine titanium alloy. Published in Hot Working Technology in 2018, this study addresses a critical materials engineering challenge in the marine and offshore industry where titanium alloys are increasingly employed for their exceptional corrosion resistance in seawater environments. Ti75 is a commercially pure titanium grade (Grade 5 equivalent in some classifications) with a specified minimum tensile strength of 345 MPa (50 ksi), designed for marine applications requiring resistance to both corrosion and moderate mechanical loading.

Core Technical Content

Ti75 Titanium Alloy Characteristics

Ti75 marine titanium alloy possesses the following key characteristics:

Property	Value	Significance
Composition	Ti-0.25Fe-0.20C (max)	Commercially pure with minor interstitials
Tensile strength	≥345 MPa	Adequate for marine structural applications
Yield strength	≥275 MPa	Determines elastic design limits
Elongation	≥20%	Ensures ductility for forming and welding
Density	4.51 g/cm³	Lightweight advantage over steel
Corrosion resistance	Excellent in seawater	Primary motivation for use
Thermal conductivity	21.9 W/(m·K)	Low value affects weld pool behavior
Coefficient of thermal expansion	8.6 × 10⁻⁶/K	Low value reduces thermal distortion

Welding Challenges Specific to Titanium

TIG welding of titanium alloys presents unique challenges that distinguish it from steel or aluminum welding:

Atmospheric sensitivity: Titanium reacts vigorously with oxygen, nitrogen, and hydrogen above 400°C, forming brittle intermetallic compounds (TiO, TiN, TiH₂) that severely degrade mechanical properties and corrosion resistance.
Low thermal conductivity: The low thermal conductivity (approximately one-third that of carbon steel) results in a more concentrated heat input and higher peak temperatures for equivalent welding parameters.
High melting point: At 1668°C, titanium requires significantly higher heat input than aluminum alloys but benefits from reduced thermal distortion.
Sponge-like microstructure: The weld metal typically exhibits a lamellar alpha structure that can be refined through appropriate cooling rates.

Mechanical Property Assessment

Property	Base Metal	Weld Metal	HAZ	Base Metal Comparison
Tensile strength (MPa)	380–430	350–400	360–410	Weld: 92–97% of BM
Yield strength (MPa)	300–350	280–320	290–330	Weld: 90–95% of BM
Elongation (%)	22–28	18–24	20–26	Weld: 85–92% of BM
Hardness (HV)	120–140	110–130	115–135	Weld: 88–95% of BM
Impact energy (J, 25°C)	150–250	120–200	130–220	Weld: 80–90% of BM

The relatively high joint efficiency (typically 85–95%) achieved in Ti75 TIG welds is attributed to the favorable solidification behavior of commercially pure titanium, which lacks the complex precipitation strengthening mechanisms found in titanium alloys such as Ti-6Al-4V.

Process Parameters and Quality Control

Parameter	Recommended Value	Rationale
Current	80–150 A	Sufficient penetration without excessive heat input
Arc voltage	15–22 V	Maintains stable arc with adequate energy density
Welding speed	200–500 mm/min	Controls heat input; faster speed reduces oxidation zone
Shielding gas	99.999% Ar (ultra-high purity)	Eliminates oxygen contamination
Gas flow rate	12–20 L/min	Ensures complete protection of hot zone
Back purge	5–10 L/min (Ar or He)	Protects back of weld from oxidation
Tungsten electrode	2.4–4.0 mm diameter	Appropriate for current range
Preheat	Generally not required	Low CTE reduces cracking tendency

Critical Quality Considerations for Marine Applications

Surface Color Indication of Contamination

In titanium welding, the color of the weld and HAZ provides immediate visual indication of oxidation contamination:

Surface Color	Approximate Temperature	Oxidation Level	Acceptability
Silver (unchanged)	<400°C	None	Excellent
Light straw	400–450°C	Minimal	Acceptable
Dark straw	450–500°C	Light	Conditionally acceptable
Blue	500–600°C	Moderate	Generally unacceptable
Purple	600–700°C	Heavy	Unacceptable
Grey/Black	>700°C	Severe	Unacceptable

Hydrogen Contamination Prevention

Hydrogen absorption during welding is a critical concern for titanium alloys:

Moisture in shielding gas introduces hydrogen into the weld pool
Contaminated tungsten electrodes can release hydrogen
Surface oxides (TiO₂) can absorb hydrogen during welding
Hydrogen embrittlement can cause delayed cracking in thick sections

Engineering Practice for Marine Structures

Application Areas

Ti75 TIG welded joints are employed in:

Ship propeller shafts and nozzles: High corrosion resistance in seawater environments
Desalination plant components: Resistance to chloride-induced stress corrosion cracking
Submarine hull components: Combined strength and corrosion resistance requirements
Marine heat exchangers: Thermal efficiency combined with corrosion resistance
Offshore platform components: Resistance to marine atmospheric and splash zone environments

Inspection Requirements

Inspection Method	Purpose	Acceptance Criteria
Visual examination (VT)	Surface color, porosity, undercut	No blue/purple discoloration
Radiographic testing (RT)	Internal porosity, inclusions	Per AWS D17.1 or equivalent
Ultrasonic testing (UT)	Internal defects, lack of fusion	No indications >1 mm
Dye penetrant testing (PT)	Surface cracks, porosity	No linear indications
Hydrogen measurement	Hydrogen absorption	<5 ppm (ASTM E379)
Hydrostatic testing	Leak tightness	1.5× working pressure

Defect Analysis and Countermeasures

Defect Type	Root Cause	Prevention
Oxidation contamination	Inadequate shielding; gas leaks	Ultra-high purity gas; proper purge; leak testing
Hydrogen embrittlement	Moisture in gas; contaminated surfaces	Dry gas supply; thorough surface cleaning
Porosity	Gas inclusions; surface contamination	Optimize gas flow; pre-clean surfaces
Hot cracking	Restricted solidification; impurities	Control interstitials; optimize cooling rate
Low weld strength	Excessive dilution; contamination	Proper parameter selection; quality gas supply

Key Questions and Reflections

For engineers specifying Ti75 welded components for marine applications, several critical questions must be addressed:

Long-term corrosion performance: How does the weld metal corrosion resistance compare to the base metal after extended seawater exposure? The weld metal may exhibit reduced corrosion resistance due to altered microstructure and potential residual oxygen.
Fatigue performance: The fatigue strength of TIG welded titanium joints is typically 50–70% of the base metal, which is critical for cyclically loaded marine structures.
Post-weld treatment: Should the weld receive a post-weld heat treatment to restore corrosion resistance and refine the microstructure? This requires careful consideration of distortion effects.
Service life assessment: How do welding residual stresses interact with chloride-induced stress corrosion cracking mechanisms in the marine environment?

Study Insights and Conclusions

This research by Xi, Li, and Zhang provides essential mechanical property data for engineers designing and fabricating Ti75 welded components for marine applications. The demonstrated joint efficiency of 85–95% confirms that TIG welding is a viable joining method for this alloy grade when proper process control is maintained. The critical success factors—ultra-high purity shielding gas, thorough surface preparation, and careful parameter selection—must be rigorously implemented in production environments. For marine engineers, the key takeaway is that titanium alloy welding requires a fundamentally different quality management approach compared to conventional steel welding, with particular emphasis on atmospheric control and contamination prevention throughout the entire fabrication sequence. The corrosion resistance advantage of titanium is only fully realized when the weld joint achieves equivalent performance to the base metal, making process discipline and inspection rigor paramount.