Mechanical Properties of Ti75 Titanium Alloy TIG Welded Joints in Marine Applications

Literature Overview

The study by Xi Quan, Li Wei, and Zhang Yidian from Jiujiang Vocational and Technical College, published in the journal Hot Working Technology in 2018, investigates the mechanical performance of TIG (GTAW) welded joints fabricated from Ti75 titanium alloy plates intended for marine engineering applications. Ti75, also known commercially as commercially pure Grade 5 titanium, contains approximately 99.5 wt% titanium with residual oxygen, iron, and other interstitial elements. This alloy is widely specified for marine heat exchangers, desalination systems, and structural components exposed to seawater environments due to its excellent corrosion resistance and favorable strength-to-weight ratio. The research addresses a critical gap in weld quality assurance for marine-grade titanium structures, where weld integrity directly governs service life and safety.

Core Technical Findings

The primary objective of the research is to characterize the tensile strength, hardness distribution, and microstructural evolution across the weld zone of Ti75 TIG joints. The authors conducted systematic tensile tests in accordance with standard coupon geometry, Vickers hardness traverses across the weld centerline, and optical metallographic examination of the fusion zone and heat-affected zone (HAZ).

Key findings include:

• The ultimate tensile strength (UTS) of the base metal Ti75 was measured at approximately 320–340 MPa, while the weld metal exhibited a UTS in the range of 300–330 MPa, indicating a slight reduction of 5–10% relative to the parent material.
• The yield strength of the weld metal was found to be marginally lower than that of the base metal, consistent with the well-documented phenomenon of weld metal softening in commercially pure titanium alloys due to the dissolution and subsequent coarsening of interstitial oxygen and nitrogen during the welding thermal cycle.
• Vickers hardness measurements revealed a hardness profile with the base metal at approximately 140–160 HV, the weld center at 120–140 HV, and a transition zone showing gradual hardness recovery toward the base metal value. This softening in the weld metal is attributed to the homogenization of the microstructure during melting and the absence of the cold-worked or precipitate-strengthened condition present in the as-supplied plate.
• Metallographic examination confirmed a fine acicular Widmanstätten-like microstructure in the fusion zone, characteristic of rapid solidification under TIG conditions without post-weld heat treatment.

Welding Process Parameters and Their Influence

The TIG welding process parameters employed in the study are summarized below:

The study highlights that the selection of DC+ polarity is critical for titanium welding because it provides cathodic cleaning action on the tungsten while avoiding excessive tungsten erosion associated with AC welding. The high-purity argon shielding is non-negotiable for titanium, as even trace contamination with oxygen, nitrogen, or hydrogen leads to severe embrittlement of the weld metal and HAZ. The authors emphasize that the back-side gas shielding must be maintained at a flow rate not less than 8 L/min to prevent oxidation of the root pass, which is the most common cause of weld rejection in titanium fabrication.

Defect Analysis and Countermeasures

A systematic defect analysis was conducted, and the following common defects were identified and discussed:

The authors note that porosity is the most frequently encountered defect in Ti75 TIG welds, and its occurrence is directly correlated with the quality of surface preparation and gas shielding integrity. In marine applications, where welds are subjected to cyclic loading and corrosive seawater, even minor porosity can act as a stress concentrator and crack initiation site, making rigorous quality control essential.

Engineering Practice Integration

From a practical standpoint, the findings of this study are directly applicable to the fabrication of marine heat exchanger tubesheets, desalination plant components, and shipbuilding structural elements using Ti75. The following engineering considerations emerge from the study:

• The weld metal strength reduction of 5–10% relative to the base metal must be accounted for in design calculations, particularly for pressure-containing components governed by standards such as ASME VIII Div. 1 or GB/T 150.
• The hardness softening in the weld zone, while not directly harmful in terms of static strength, may affect the fatigue performance of the joint under cyclic marine loading. Engineers should consider the implications for fatigue design curves and allowable stress intensification factors.
• The microstructural observations confirm that no post-weld heat treatment (PWHT) is required for Ti75 welds of moderate thickness (up to approximately 6 mm) when the welding parameters are properly controlled. However, for thicker sections or when residual stress relief is mandated by the applicable code, a solution treatment at 650–700°C for 1 hour followed by air cooling may be specified.
• The importance of consumable quality cannot be overstated. The filler wire must be supplied in a degreased and sealed condition, and any contamination during handling must be removed by solvent cleaning immediately before welding.

Key Questions and Reflections

Several questions arise from the study that warrant further investigation in engineering practice:

1. The study does not report intergranular corrosion (IGC) or stress corrosion cracking (SCC) test results, which are of paramount importance for Ti75 components in seawater service. Engineers should supplement the mechanical data with corrosion testing, particularly ASTM G15 IGC tests and potentiodynamic polarization measurements in simulated seawater.
2. The effect of welding sequence and thermal history on residual stress distribution in multi-pass welds was not addressed. For thick-section marine components requiring multiple weld passes, residual stress management through weld sequence optimization and interpass temperature control becomes critical.
3. The study focuses on single-pass or limited multi-pass welds. For heavier sections (above 6 mm), the thermal cycling from multiple passes may alter the weld metal microstructure and properties, and the findings should not be extrapolated without additional validation.

Study Insights and Implications

The study provides a solid baseline for understanding the weldability and mechanical performance of Ti75 in marine TIG applications. The most significant insight is that the weld metal, while slightly weaker than the base metal, retains adequate strength for most marine structural and pressure-containing applications when proper welding practice is followed. The emphasis on shielding gas quality and surface cleanliness as primary quality drivers is consistent with the well-established understanding of titanium welding metallurgy. For engineers involved in the design and fabrication of marine titanium equipment, this study reinforces the necessity of rigorous process qualification in accordance with NB/T 47014 or ASME IX, thorough welder performance qualification, and strict adherence to consumable handling and storage protocols. The data presented can serve as a reference for establishing acceptance criteria in weld quality specifications and for conducting weld procedure qualification tests.