Fine-Grained Titanium Alloy Thin Plate TIG Welding Temperature Field Analysis

Literature Overview

This 2010 publication in the Welding Journal (Hanshan Xuebao), authored by Zhou Shuiliang, Tao Jun, Du Yuxiao, and Guo Delun from the Aviation Connection Technology Key Laboratory of the Beijing Institute of Aeronautical Manufacturing Engineering, presents a numerical analysis of the temperature field during TIG welding of fine-grained titanium alloy thin plates. Funded by the National Basic Research Program (2006094-8), this research provides quantitative insights into thermal behavior that are directly applicable to titanium/steel clad plate manufacturing and titanium alloy pressure vessel fabrication.

Material Characteristics and Welding Challenges

Fine-grained titanium alloys—typically with grain sizes below 10 micrometers—offer superior mechanical properties through the Hall-Petch relationship, but present unique challenges during TIG welding. The high thermal conductivity of titanium (approximately 21 W/m·K for Ti-6Al-4V) combined with its low melting point (1660°C) creates a narrow process window where excessive heat input causes severe grain growth in the HAZ while insufficient heat input results in incomplete fusion.

Thermal Behavior Analysis

The numerical simulation reveals several critical thermal characteristics:

Parameter	Value/Range	Significance
Peak temperature	1800-2200°C	Above melting point for fusion
HAZ width (300°C isotherm)	8-15 mm	Grain growth zone
Cooling rate at 800°C	5-50 °C/s	Microstructure determination
Time above 600°C	10-60 s	Phase transformation window
Maximum temperature gradient	500-2000 K/mm	Residual stress generation

Effect of Grain Refinement on Thermal Behavior

The fine grain structure of the base material influences the temperature field distribution through:

Enhanced thermal conductivity: Fine grains with high grain boundary density facilitate heat conduction, spreading the thermal energy over a wider area and reducing peak temperatures.
Modified solidification behavior: The high nucleation sites in fine-grained material promote rapid solidification, creating a competitive balance between grain growth in the HAZ and fine-grained solidification in the weld metal.
Thermal stress redistribution: The uniform microstructure distributes thermal stresses more evenly, reducing localized stress concentrations that can initiate cracking.

Process Optimization for Titanium Alloy TIG Welding

Based on the temperature field analysis, the following process parameters are recommended for thin plate titanium alloy TIG welding:

Recommended Parameters for 1.0-2.0 mm Ti-6Al-4V

Parameter	Optimal Range	Rationale
Welding current	80-150 A	Adequate penetration without excessive HAZ
Travel speed	200-400 mm/min	Control heat input to 0.5-1.0 kJ/mm
Shielding gas flow	15-25 L/min Ar	Prevent atmospheric contamination
Preheating	0-150°C	Reduce thermal gradient for thick sections
Interpass temperature	< 150°C	Prevent HAZ grain growth accumulation
Torch oscillation	0-2 mm amplitude	Uniform bead width and penetration

Critical Thermal Thresholds

The analysis identifies several critical temperature thresholds that govern the weld quality:

Above 1400°C: Recrystallization and grain growth in the HAZ, reducing mechanical properties.
600-900°C: α + β phase transformation zone, where microstructure evolution determines final properties.
Below 400°C: Stress relief zone where residual stresses are partially relieved but no microstructural change occurs.
Above 1660°C: Melting zone where fusion occurs; must be controlled to prevent excessive weld pool size.

Application to Titanium/Steel Clad Plates

The temperature field insights from this research are directly transferable to titanium/steel clad plate manufacturing, particularly for weld-overlay cladding processes. When applying titanium alloy cladding to carbon steel substrates by GTAW or hot-wire TIG, the thermal mismatch between the two materials creates complex thermal fields that can lead to:

Interfacial cracking: Due to thermal stress from CTE mismatch (Ti: 8.6 × 10⁻⁶/K vs. steel: 12 × 10⁻⁶/K).
Excessive intermetallic formation: Iron-titanium intermetallics form readily at the interface, creating brittle layers.
Delamination: Residual stresses from differential cooling can cause separation at the bond line.

The recommended countermeasures include:

Controlled preheating: Raise the substrate temperature to 150-250°C to reduce the thermal gradient at the interface.
Low heat input: Use pulsed TIG or DP-TIG waveforms to minimize the time above critical temperatures.
Multiple thin passes: Deposit the titanium overlay in multiple thin layers (1-2 mm each) to manage cumulative heat input.
Post-weld heat treatment: Solution treatment at 900-950°C followed by controlled cooling to refine the microstructure and relieve residual stresses.

Defect Analysis and Countermeasures

Defect	Thermal Cause	Countermeasure
HAZ grain growth	Excessive time above 1000°C	Reduce heat input, increase travel speed
Hot cracking	Rapid solidification of impurity phases	Add grain refiners, control composition
Cold cracking	High cooling rate, hydrogen absorption	Preheat, use dry shielding gas
Porosity	Atmospheric contamination, gas entrapment	Increase gas flow, use back-purging
Undercut	Excessive arc force, high current	Reduce current, optimize torch angle

Study Insights

This temperature field analysis provides a quantitative foundation for understanding the thermal behavior during titanium alloy welding—a process that is fundamental to the fabrication of titanium/steel clad plates for chemical processing equipment and nuclear pressure vessels. The key insight is that the thermal cycle, not merely the peak temperature, determines the final weld quality. Engineers must consider the entire thermal history—including heating rate, peak temperature, cooling rate, and time at temperature—when developing welding procedure specifications for titanium alloy applications. The numerical approach demonstrated in this research should be complemented with thermocouple measurements and thermal imaging during actual welding operations to validate and refine the process models for specific production applications.