Mechanical Properties of TIG-Welded Marine Ti75 Titanium Alloy Joints

Literature Overview

This paper by Xi Quan, Li Wei, and Zhang Yidian from Jiujiang Vocational and Technical College investigates the mechanical properties of TIG-welded joints of the marine-grade Ti75 titanium alloy. Published in Hot Working Technology in 2018 (Volume 47, Issue 17, pp. 208-209), the study employed Ti70 filler wire for butt welding of 8 mm thick Ti75 test plates. Both the weld joint and the deposited metal of Ti70 filler wire were characterized for tensile and ductility properties, along with metallographic examination of the weld microstructure.

Material Background

Ti75 is a near-alpha titanium alloy designated for marine applications, characterized by a high strength level derived from a microstructure consisting predominantly of alpha phase with a small volume fraction of transformed beta. The "75" designation indicates a minimum tensile strength of approximately 750 MPa. This alloy is widely used in naval structural components, pressure hulls, and submarine hull sections where a combination of high strength, good corrosion resistance in seawater, and adequate weldability is required.

The selection of Ti70 as the filler metal represents a slightly lower strength matching strategy, which is common practice in titanium welding to accommodate the formation of coarse acicular structures in the heat-affected zone (HAZ) that can reduce local ductility. The Ti70 filler metal, with a minimum tensile strength of approximately 700 MPa, provides a slight undermatching that helps prevent weld cracking during cooling.

Welding Process Parameters

For 8 mm thick titanium alloy plates, the TIG welding process was employed with the following considerations:

The use of DCEN (Direct Current Electrode Negative) provides deep, narrow penetration suitable for the groove geometry of titanium alloy butt joints. The high-purity argon shielding is absolutely critical for titanium welding, as titanium has an extremely high affinity for oxygen, nitrogen, and hydrogen at elevated temperatures. Even trace contamination can severely degrade mechanical properties and corrosion resistance.

Mechanical Properties Results

The tensile strength of both the weld joint and the Ti70 deposited metal was found to be comparable to that of the base metal Ti75, demonstrating that the welding process does not compromise the strength integrity of the joint. The joint exhibited good ductility, indicating that the combination of Ti75 base metal and Ti70 filler wire produces a weldment with balanced strength and toughness.

The metallographic examination revealed a needle-like (acicular) structure in the weld center, which is characteristic of the transformed beta phase in near-alpha titanium alloys. Importantly, no coarse grain growth was observed, which is a significant positive finding. Coarse grain growth in the HAZ of titanium alloys is a well-known degradation mechanism that leads to reduced creep resistance and fatigue strength.

Microstructural Analysis

The acicular structure observed in the weld center is formed by the transformation of the beta phase during cooling. In near-alpha alloys like Ti75, the weld metal undergoes a complete phase transformation to beta during welding, followed by a solid-state transformation upon cooling. The cooling rate in the center of the weld, being the slowest in the weld cross-section, promotes the formation of fine acicular structures of alpha in a beta matrix or vice versa, depending on the specific cooling rate and alloy composition.

The absence of coarse grain growth is attributed to several factors: (1) the relatively low heat input of TIG welding compared to processes such as electron beam welding or friction stir welding; (2) the use of Ti70 filler wire, which may contain microalloying additions that inhibit grain growth; and (3) the appropriate joint preparation and welding sequence that distributes heat input across the weld thickness.

Engineering Practice Considerations

For marine applications governed by standards such as DNV-ST-F101, ASME BPVC Section VIII, or CCS (China Classification Society) rules, the qualification of titanium alloy weld joints requires comprehensive testing including:

• Tensile testing per ASTM E8/E8M at room temperature and potentially at elevated temperatures
• Bend testing per ASTM E139 to evaluate ductility
• Impact testing per ASTM E23 at service temperatures
• Hardness mapping across the weld, HAZ, and base metal per ASTM E18
• Corrosion testing in simulated seawater per ASTM G102 or G103
• Non-destructive examination including ultrasonic testing per ASTM E164 or radiographic testing per ASTM E94

The findings of this study provide confidence that TIG welding with Ti70 filler wire is a suitable process for producing high-quality Ti75 weld joints. However, for thick-section marine components exceeding 20 mm, multi-pass welding with interpass temperature control becomes necessary, and the process parameters would need to be re-optimized for each pass.

Key Reflections

The study demonstrates that the strength matching between Ti75 base metal and Ti70 filler wire is adequate for structural applications, but raises questions about the long-term performance under cyclic loading typical of marine environments. Fatigue testing under seawater exposure would be essential to validate the joint's service life. Additionally, the absence of coarse grain growth is encouraging, but the grain size distribution across the full weld cross-section, including the fusion boundary and HAZ, should be quantified to ensure compliance with grain size requirements specified in ASME or DNV standards.

The use of TIG welding for marine titanium structures is well-established, but the process productivity remains a concern for large-scale fabrication. For production of large marine hull sections, consideration should be given to alternative processes such as plasma arc welding, electron beam welding, or friction stir welding, each of which offers different combinations of productivity, penetration, and microstructural control.