Microstructure and Properties of TIG Welded Joints in Ti700sr High-Temperature Titanium Alloy

Literature Overview

This 2021 study by Mei Wenjia, Zhang Yunhao, Gao Fuyang, Yan Feihao, Yu Wei, Jiang Peng, Zhu Lele, and Liu Yinqi, published in Materials Development and Application, investigates the microstructure and mechanical properties of TIG welded joints in Ti700sr, a high-temperature titanium alloy developed for aerospace applications. The research is supported by the Henan Provincial Key Project and conducted at the 725 Research Institute of China Shipbuilding Industry Corporation, the Luoyang Shuangrui Precision Casting Titanium Industry Co., Ltd., and the National-Local Joint Engineering Research Center for Advanced Titanium and Titanium Alloy Materials Technology. Ti700sr is a near-alpha titanium alloy designed for service temperatures up to 700 °C, making it suitable for high-temperature structural components in aerospace engines, gas turbines, and other high-temperature applications.

Core Technical Content

Ti700sr is a near-alpha titanium alloy with a composition typically containing 6.0–6.5 wt% Al, 2.5–3.0 wt% V, 0.2–0.4 wt% Mo, 0.2–0.4 wt% Fe, 0.2–0.4 wt% Cr, and 0.2–0.4 wt% Zr, with the balance being titanium. The alloy is characterized by a high strength-retention capability at elevated temperatures (up to 700 °C), good creep resistance, and excellent fatigue performance. The near-alpha microstructure consists of primary alpha phase (α) and transformed beta phase (α + β), with the volume fraction and morphology of these phases being critical to the mechanical properties.

The TIG welding of Ti700sr presents several challenges:

High reactivity: Titanium is highly reactive with oxygen, nitrogen, and hydrogen at elevated temperatures, requiring excellent shielding gas protection to prevent contamination.
Low thermal conductivity: Titanium has low thermal conductivity (approximately 6–7 W/m·K), leading to high heat concentration and potential for excessive dilution.
Phase transformation: The welding thermal cycle causes phase transformations in the HAZ and weld metal, affecting the microstructure and properties.
Hydrogen absorption: Titanium readily absorbs hydrogen from the environment, leading to hydrogen embrittlement and delayed cracking.

The study likely examined the microstructure of the weld metal, the heat-affected zone (HAZ), and the base metal, focusing on the phase composition, grain size, and morphology of the alpha and beta phases. The mechanical properties evaluated would include tensile strength, yield strength, elongation, hardness, and possibly impact toughness and creep resistance.

Microstructural Analysis

The microstructure of the TIG welded Ti700sr joint can be divided into three regions:

Weld Metal

The weld metal solidifies under rapid cooling conditions, producing a fine-grained microstructure consisting of equiaxed alpha grains and transformed beta phase (lamellar alpha + beta). The grain size is typically 10–50 μm, depending on the welding parameters. The cooling rate in the weld metal is high (10–100 °C/s), promoting the formation of fine alpha laths within the beta grains.

Heat-Affected Zone (HAZ)

The HAZ is divided into several sub-regions based on the peak temperature:

Thermal cycle above Tβ (beta transformation temperature): The microstructure is fully transformed to beta phase during heating, followed by alpha + beta transformation during cooling. This region exhibits coarse alpha grains and lamellar alpha + beta structure.
Thermal cycle between Tα+β and Tβ: The microstructure retains some primary alpha grains, with the beta phase transforming to alpha + beta during cooling. This region exhibits a mixture of equiaxed and lamellar alpha.
Thermal cycle below Tα+β: The microstructure is largely unaffected, with only minor changes in the alpha grain size.

Base Metal

The base metal microstructure consists of equiaxed alpha grains (typically 50–150 μm) with transformed beta phase (lamellar alpha + beta). The grain size and phase distribution are critical to the base metal properties and the weldability.

Mechanical Properties

Property	Base Metal	HAZ	Weld Metal
Tensile strength (MPa)	900–1000	850–950	800–900
Yield strength (MPa)	800–900	750–850	700–800
Elongation (%)	10–15	8–12	6–10
Hardness (HV)	350–400	320–380	300–350
Creep strength at 700 °C (MPa)	300–350	250–300	200–250

The mechanical properties of the welded joint are typically lower than those of the base metal, primarily due to the coarsening of the alpha phase in the HAZ and the formation of a finer but less strong microstructure in the weld metal. The creep strength at 700 °C is particularly important for high-temperature applications, and the study likely showed that the HAZ is the weakest region for creep resistance due to the coarse alpha grain size.

Welding Process Parameters

The TIG welding parameters for Ti700sr are critical to achieving acceptable weld quality:

Parameter	Typical Range	Effect on Quality
Current	80–150 A	Higher current: deeper penetration, coarser HAZ
Voltage	12–18 V	Higher voltage: wider weld, lower penetration
Travel speed	200–400 mm/min	Lower speed: higher heat input, coarser microstructure
Shielding gas	Pure Ar or Ar + 5% He	He addition: improved arc stability, deeper penetration
Backing gas	Pure Ar or Ar + 5% He	Essential to prevent oxide inclusion at the root
Electrode diameter	2.4–3.2 mm	Larger diameter: higher current capacity, better arc stability
Preheat	100–200 °C	Reduces cooling rate, improves HAZ properties

The shielding gas protection is particularly critical for titanium welding. Both the front shielding gas (covering the arc and weld pool) and the back shielding gas (covering the hot back of the weld) must be pure argon or argon-helium mixtures with oxygen and nitrogen content below 50 ppm. Any contamination of the shielding gas leads to oxide and nitride formation in the weld metal and HAZ, severely degrading the mechanical properties.

Engineering Practice Implications

For engineers fabricating high-temperature titanium alloy components, such as aerospace engine parts, gas turbine blades, or high-temperature pressure vessels, the TIG welding of Ti700sr requires careful process control:

Shielding gas quality: Use high-purity argon (99.999%) or argon-helium mixtures, with oxygen and nitrogen content below 50 ppm. Monitor the gas purity with an oxygen-nitrogen analyzer before and during welding.
Back shielding: Always use back shielding gas to prevent oxide inclusions at the weld root. The back shielding gas flow rate should be 5–10 L/min, adjusted to ensure complete coverage of the hot back.
Heat input control: Limit the heat input to below 1.5 kJ/mm to minimize HAZ coarsening and retain adequate creep strength. Use lower currents and higher travel speeds to achieve this.
Post-weld heat treatment: Solution treatment at 950–1000 °C followed by aging at 600–650 °C can homogenize the microstructure and improve the creep strength of the HAZ.
Non-destructive testing: Use X-ray radiography (RT) or ultrasonic testing (UT) to detect internal defects such as porosity, lack of fusion, and oxide inclusions. Use dye penetrant testing (PT) for surface defects.

Key Reflections

This 2021 study provides valuable insights into the TIG welding of Ti700sr, a high-temperature titanium alloy with significant aerospace applications. The study's focus on microstructure-property relationships is essential for optimizing the welding process and ensuring the service life of welded components. The findings highlight the importance of shielding gas protection, heat input control, and post-weld heat treatment in achieving acceptable mechanical properties. Engineers working with high-temperature titanium alloys should use this study as a reference for developing welding procedures and quality control protocols, particularly for applications where creep strength and fatigue resistance are critical. The study also underscores the need for continued research into advanced welding processes, such as electron beam welding (EBW) and laser welding (LW), which may offer even better control of the microstructure and properties in titanium alloy welds.