Microstructure and Mechanical Properties of TC4 Titanium Alloy K-TIG Welded Joints

Literature Overview

This research, published in the Transactions of Nonferrous Metals Society of China (2021) by Cui Shu-wan, Shi Yong-hua, and Zhang Cheng-shi from Guangxi University of Science and Technology, South China University of Technology, and SAIC Liuzhou Automobile Transmission Co., investigates the microstructure and mechanical properties of TC4 (Ti-6Al-4V) titanium alloy welded joints produced using K-TIG (Keyhole TIG or Kinetic TIG) welding technology. The study was supported by multiple funding sources including the Key R&D Program of Guangdong Province, the Natural Science Foundation of Guangdong Province, and the Guangxi Science and Technology Base and Talent Special Project.

Core Technical Content

TC4 (Ti-6Al-4V) is the most widely used titanium alloy in aerospace, biomedical, and automotive applications due to its excellent specific strength, corrosion resistance, and biocompatibility. Conventional TIG welding of titanium alloys is limited by low deposition rates and susceptibility to porosity and cracking. K-TIG welding represents an advanced variant that utilizes high current densities to create a keyhole effect in the weld pool, enabling deeper penetration and higher deposition rates while maintaining the clean weld quality characteristic of TIG processes.

K-TIG Process Characteristics

K-TIG welding differs from conventional TIG welding in several key aspects:

The keyhole effect in K-TIG welding is achieved through the high current density that vaporizes the base metal, creating a deep cavity in the weld pool. The high gas flow rate provides enhanced shielding and helps stabilize the keyhole, while the high arc pressure drives the molten metal to the rear of the weld pool, creating a deep, narrow weld bead.

Microstructural Evolution

The K-TIG welded TC4 joints exhibit distinct microstructural characteristics compared to conventionally welded joints:

• Fusion Zone: Characterized by acicular martensite (α') needles formed during rapid cooling from the β phase field. The needle width is typically 0.1-0.5 μm, significantly finer than in conventionally welded joints.
• HAZ: Exhibits a mixture of Widmanstätten α and β phases, with grain coarsening in the coarse grain HAZ region.
• Base Metal: Retains the equiaxed α + β microstructure of the as-received condition.

The rapid cooling rates achieved in K-TIG welding (estimated at 10^2-10^3 K/s in the fusion zone) promote the formation of fine acicular martensite, which provides excellent strength but may compromise ductility if not properly controlled.

Mechanical Property Assessment

The mechanical properties of K-TIG welded TC4 joints demonstrate:

• Tensile Strength: 900-1100 MPa in the fusion zone, comparable to or slightly exceeding the base metal (950-1050 MPa)
• Yield Strength: 850-1000 MPa, indicating high strength retention
• Elongation: 8-15%, representing a reduction of 20-40% compared to the base metal (14-20%)
• Hardness: 350-420 HV in the fusion zone, 320-380 HV in the HAZ, and 340-380 HV in the base metal

The reduced ductility in the welded joint is attributed to the fine martensitic microstructure in the fusion zone and the grain coarsening in the HAZ. The high strength is maintained due to the fine martensite needles and the high dislocation density retained from the rapid solidification.

Process Optimization and Defect Control

K-TIG welding of TC4 requires careful parameter control to avoid common defects:

Defect Analysis and Countermeasures

Critical Parameter Windows

For TC4 K-TIG welding, the following parameter ranges have been identified as producing sound welds with acceptable properties:

• Current: 150-250 A (depending on thickness)
• Voltage: 18-25 V
• Travel Speed: 5-15 mm/s
• Gas Flow: 20-30 L/min (argon or argon-helium mixture)
• Interpass Temperature: Below 150°C for multi-pass welds
• Shielding Gas: 99.99% Ar or 75% Ar / 25% He

Engineering Practice Integration

For bimetal pressure vessel fabrication involving titanium alloy cladding on steel substrates, the K-TIG process offers advantages in terms of deposition rate and penetration depth. However, the high current densities and rapid cooling rates require careful management of the clad-base metal interface to prevent the formation of brittle intermetallic compounds.

The microstructural control achieved through K-TIG parameters provides opportunities for optimizing the mechanical properties of titanium alloy overlays. For example, adjusting the current density and travel speed can control the martensite needle width, which directly influences the ductility-strength balance of the overlay.

Study Insights and Implications

This research demonstrates that K-TIG welding is a viable technology for TC4 titanium alloy fabrication, offering improved deposition rates and sound weld quality compared to conventional TIG welding. The key engineering insight is that the high current densities and rapid cooling rates of K-TIG produce fine martensitic microstructures that provide excellent strength but may compromise ductility. For pressure vessel applications, this trade-off must be carefully evaluated, and post-weld heat treatment may be necessary to restore ductility through martensite decomposition and β phase recovery. The research also highlights the importance of parameter optimization and defect control in achieving reliable K-TIG welds, which is essential for the technology's adoption in critical pressure vessel fabrication.