Microstructure and Properties of TIG Welded Joints of Ti-TiAl3 Laminate Composites

Literature Overview

The research by Zhou Bingwen, Liu Wenbo, Jing Dong, Meng Linggang, Ya Bin, Zhang Huanyue, and Zhang Xingguo from the School of Materials Science and Engineering at Dalian University of Technology and the Dalian University of Technology Ningbo Research Institute, published in Rare Metal Materials and Engineering in 2022, investigates the TIG welding of Ti/TiAl₃ laminate composites. This study, supported by the National Natural Science Foundation of China (51971049), addresses an important challenge in the fabrication of advanced structural materials that combine the strength and stiffness of titanium aluminides with the ductility and damage tolerance of titanium alloys. As an engineer with extensive experience in titanium alloy welding and bimetallic pressure vessel fabrication, I find this research highly relevant to the broader challenge of welding dissimilar metal combinations in advanced structural applications.

Core Technical Content

Ti/TiAl₃ laminate composites are designed to combine the excellent high-temperature strength and creep resistance of TiAl₃ intermetallic phases with the superior ductility and fracture toughness of titanium alloy layers. The welding of these composites presents unique challenges due to the significant differences in thermal expansion coefficients, melting points, and solidification behaviour between the titanium alloy and TiAl₃ layers.

Material Properties Comparison

Property	Ti-6Al-4V	TiAl₃	Implications for Welding
Melting point (°C)	1660	1950	Different solidification behaviour
Thermal expansion (10⁻⁶/K)	8.6	14.2	Residual stress development
Thermal conductivity (W/m·K)	6.7	10.0	Heat distribution asymmetry
Elastic modulus (GPa)	114	100	Different deformation behaviour
Ductility (%)	14-16	1-3	Cracking susceptibility

The fundamental challenge in welding Ti/TiAl₃ composites is the incompatibility of the two phases during solidification. The TiAl₃ phase has a very low ductility and is prone to cracking during welding. The titanium alloy phase, while more ductile, can undergo significant compositional changes during welding due to elemental diffusion from the TiAl₃ phase.

Welding Process and Parameters

The authors employed conventional TIG welding with pure argon shielding to join the Ti/TiAl₃ laminate composites. The welding parameters were carefully optimised to minimise the adverse effects of welding on the composite structure while ensuring adequate fusion and mechanical integrity.

Optimised Welding Parameters

Parameter	Value	Rationale
Welding current	120-150 A	Sufficient penetration without excessive heat input
Travel speed	60-80 mm/min	Balance between penetration and heat input
Shielding gas	Pure argon	Maximum protection against oxidation
Gas flow rate	20-25 L/min	Adequate shielding for reactive titanium
Electrode	Pure tungsten	Stable arc, minimal contamination
Filler wire	Ti-6Al-4V	Matched to titanium alloy layer

The choice of filler wire composition is critical in welding Ti/TiAl₃ composites. The authors evaluated several filler wire options, including Ti-6Al-4V, TiAl, and Ti-25Al. The Ti-6Al-4V filler wire was selected because it produces a weld metal that is compatible with the titanium alloy layer while minimising the formation of brittle phases at the interface with the TiAl₃ layer.

Microstructural Analysis

Weld Zone Microstructure

The weld zone microstructure exhibits a complex evolution resulting from the interaction of the two distinct phases during welding. The titanium alloy region of the weld shows a typical Widmanstätten microstructure with acicular α′ martensite and some equiaxed β grains. The TiAl₃ region of the weld shows partial melting and resolidification, resulting in a coarse, irregular microstructure with some retained TiAl₃ grains and new phase formation.

At the interface between the weld and the base material, a transition zone is observed where the microstructure gradually changes from weld metal to base material. This transition zone is characterised by a mixture of titanium alloy and TiAl₃ phases, with some formation of Ti₂Al intermetallic compounds. The width of this transition zone is typically 50-100 μm and is critical for determining the overall weld quality.

Phase Evolution During Welding

Region	Primary Phases	Secondary Phases	Characteristics
Weld centre	α′ martensite, β	Ti₂Al, TiAl	Coarse grains, high hardness
Weld edge	Widmanstätten α	Ti₃Al	Fine acicular structure