Microstructure and Mechanical Properties of TC4 Titanium Alloy Laser-MIG Hybrid Welded Joints

Literature Overview

This study published in the Journal of Jiangsu University of Science and Technology (Natural Science Edition), Volume 39, Issue 1, 2025, investigates the microstructure and mechanical properties of laser-MIG hybrid welded joints in 5 mm thick TC4 (Ti-6Al-4V) titanium alloy. Authored by researchers from Jiangsu University of Science and Technology in collaboration with aerospace and shipbuilding industry partners, the work provides detailed metallurgical characterization of the hybrid welded joint under optimized process parameters.

The research is funded by the Jiangsu Provincial Industry-University-Research Cooperation Project (BY2020449), reflecting the strong industry-academia collaboration that characterizes advanced welding research in China. TC4 titanium alloy is one of the most widely used titanium alloys in aerospace, medical, and marine applications due to its excellent strength-to-weight ratio, corrosion resistance, and fatigue performance.

Welding Process Parameters and Configuration

Optimized Parameter Set

The study identifies the following process parameters as optimal for welding 5 mm thick TC4 titanium alloy:

The laser-wire distance of 2 mm is particularly critical as it determines the degree of interaction between the laser-induced keyhole and the MIG arc plasma. At this distance, the arc provides effective stabilization of the keyhole while contributing appropriate additional heat input for complete groove filling.

Joint Quality Assessment

The welded joint exhibits excellent quality characteristics:

• Surface appearance: Silver-white color indicating proper gas protection without oxidation
• Bead geometry: Uniform and well-formed with no undercut or excessive reinforcement
• Internal quality: No porosity detected, indicating effective gas shielding and stable keyhole operation
• Penetration: Complete fusion through the full 5 mm thickness

Microstructural Analysis

Weld Metal Zone

The weld metal zone is characterized by a predominantly α′ + β phase composition. The primary microstructural features include:

The columnar β grain structure in the weld metal is characteristic of the rapid solidification conditions produced by laser-MIG hybrid welding. Within these β columns, the α′ martensite phase forms as acicular needles oriented perpendicular to the columnar growth direction. This morphology results from the high cooling rates (typically 100-500 K/s) experienced during solidification and subsequent cooling below the β-transus temperature.

The perpendicular orientation of α′ needles within β columns is a distinctive feature of laser-MIG hybrid welded titanium joints. This results from:

• The directional heat flow during solidification establishing columnar β grain orientation
• The rapid cooling below T_β-transus (~995°C for TC4) triggering martensitic transformation
• The constrained transformation within existing β grain boundaries

Coarse Grain Heat Affected Zone (CGHAZ)

The CGHAZ exhibits distinct characteristics compared to the weld metal:

• β grains: Equiaxed morphology (contrasted with columnar structure in weld metal)
• α′ phase: Smaller size compared to weld metal α′
• Phase composition: Predominantly α′ + β

The equiaxed β grain morphology in the CGHAZ reflects the thermal cycling experienced by this zone. The grains were originally equiaxed in the base material, and the thermal cycle during welding (reaching temperatures above T_β-transus) caused grain growth while maintaining the equiaxed character. The smaller α′ phase size compared to the weld metal results from the different cooling history: the CGHAZ cools more slowly than the weld metal but faster than the base material, producing intermediate α′ morphology.

Fine Grain Heat Affected Zone (FGHAZ)

The FGHAZ shows a distinct microstructural character:

• Base matrix: α + β phases (original base material morphology)
• α′ phase: Small quantity of acicular martensite α′ distributed on the α + β matrix
• Grain size: Retains original base material grain size

The limited α′ formation in the FGHAZ indicates that peak temperatures in this zone remained below the β-transus but were sufficient to cause localized martensitic transformation during cooling. This partial transformation creates a microstructure with characteristics intermediate between the fully transformed CGHAZ and the untransformed base material.

Mechanical Properties

Hardness Distribution

The CGHAZ exhibits the highest hardness at 359 HV, representing the peak hardness location in the joint. This is consistent with the full martensitic transformation occurring in this zone, where the β-transus temperature was exceeded but cooling rates were sufficient to suppress α-phase precipitation during cooling.

Tensile Properties

The tensile test results reveal a critical finding: all specimens fractured in the base material rather than at the weld or HAZ. This indicates that:

• The weld metal strength is adequate relative to base material properties
• The HAZ does not represent a weakness in the joint
• The joint achieves near-base-material tensile performance
• The hybrid welding process does not introduce significant softening in the HAZ

This result is particularly significant for titanium alloy welding, where HAZ softening is a common concern with conventional welding processes due to excessive grain growth at high temperatures.

Process-Microstructure-Property Relationships

Cooling Rate Effects

The laser-MIG hybrid process produces cooling rates that are intermediate between pure laser welding (very high, 500-2000 K/s) and conventional arc welding (low, 10-50 K/s). This intermediate cooling rate produces:

• Fine enough α′ morphology to maintain high hardness
• Grain sizes that do not excessively coarsen
• Sufficient time for complete solidification without excessive segregation
• Weld metal properties that approach base material levels

Comparison with Conventional Welding

Engineering Practice Implications

Applicability Assessment

The successful welding of 5 mm TC4 titanium alloy using laser-MIG hybrid welding demonstrates the technology's capability for aerospace-grade titanium applications. Key implications include:

• Single-pass capability: 5 mm thickness welded in a single pass, eliminating multi-pass complications
• Low distortion: Narrow HAZ and reduced heat input minimize geometric distortion
• High productivity: Travel speed of 0.8 m/min significantly exceeds conventional TIG welding rates
• Quality consistency: Stable process parameters produce repeatable results

Quality Control Considerations

For production implementation of this process, the following quality control measures are recommended:

1. Pre-weld: Ensure surface cleanliness (no oxide scale, oil, or contamination) and proper joint fit-up (root gap 1-2 mm)
2. During welding: Monitor laser power stability, gas flow rate (≥15 L/min argon), and travel speed consistency
3. Post-weld: Perform visual inspection, dye penetrant testing, and ultrasonic testing for internal defects
4. Documentation: Record all process parameters for traceability and qualification purposes

Limitations and Considerations

Despite the excellent results achieved, several limitations should be noted:

• The study focuses on 5 mm thickness; thicker sections may require different parameters or multiple passes
• Long-term fatigue properties were not evaluated, which is critical for aerospace applications
• Creep and stress corrosion resistance under service conditions remain to be characterized
• The narrow process window requires precise equipment control for consistent results

Study Insights and Practical Relevance

This study provides valuable experimental data on laser-MIG hybrid welding of TC4 titanium alloy that directly supports technology qualification for aerospace and marine applications. The comprehensive microstructural analysis across all joint zones establishes a clear understanding of the process-microstructure-property relationships.

The finding that the CGHAZ represents the hardest zone (359 HV) rather than the weakest zone is particularly encouraging for structural applications. In many welding processes, the HAZ represents the critical location for crack initiation; here, the hybrid process produces a HAZ that is at least as strong as the base material.

For piping engineers working with titanium alloy components, this technology offers a pathway to higher productivity welding of thin-to-medium wall thickness sections while maintaining the metallurgical quality required for demanding service environments. The single-pass capability for 5 mm thickness is particularly relevant for titanium heat exchanger tubes, cryogenic piping, and aerospace fuel system components.

The collaboration between academia and industry partners (aerospace engineering, shipbuilding, and intelligent manufacturing institutes) exemplifies the effective technology transfer model that is essential for advancing welding capabilities in critical industries.