Microstructure and Tensile Properties of TIG Welds in Titanium Matrix Composites

Literature Overview

This study by Pan Yuwei, Mao Jianwei, and Zhang Lixin, published in Materials in Mechanical Engineering (2020, Vol. 44, No. 5, pp. 1-5), investigates the TIG welding of discontinuously reinforced titanium matrix composites (TMCs) and characterizes the resulting weld joint microstructure and tensile properties. The research was funded by the National Natural Science Foundation of China (Grants U1602274, 51875349, 51741108) and the Chinese Aviation Science Foundation (Grant 20173625005), underscoring the strategic importance of titanium composites for aerospace structural applications. The collaboration between Tangshan Steel Group and the State Key Laboratory of Metal Matrix Composites at Shanghai Jiao Tong University reflects the cross-sector interest in composite welding technology.

Core Technical Findings

The study demonstrates that TIG welding can successfully join discontinuously reinforced titanium matrix composites with acceptable weld quality and mechanical performance. Key findings include:

The weld joint was free of microcracks, porosity, and other visible welding defects, indicating that the TIG process was well-controlled and the shielding gas coverage was adequate. The weld bead exhibited good formation with a uniform and clean surface appearance.

Microstructural Characteristics of the Weld Joint

The weld joint consists of three distinct regions: weld metal zone (WMZ), heat-affected zone (HAZ), and base material zone (BMZ). The most notable microstructural features are:

1. Reinforcement phase distribution: In the WMZ and the HAZ adjacent to the weld, TiB reinforcement particles exhibit a high aspect ratio (length-to-diameter ratio) and refined morphology. This refinement is attributed to the melting and resolidification of the reinforcement particles during welding, followed by rapid solidification that promotes elongated particle formation along the solidification direction.
2. Martensitic alpha-prime phase: The WMZ and adjacent HAZ contain a large quantity of acicular martensitic alpha-prime (α') phase. This is formed by the rapid cooling of the weld pool, which transforms the high-temperature beta phase into the martensitic alpha-prime structure without diffusion. The alpha-prime phase is characterized by its needle-like morphology and provides significant strength through solid solution strengthening and transformation hardening.
3. Phase transformation sequence: During welding, the base material (which typically contains alpha + beta phases with TiB reinforcements) is heated above the beta transus temperature in the WMZ, dissolving the alpha phase and potentially melting the TiB particles. Upon cooling, the beta phase transforms to alpha-prime through a diffusionless martensitic transformation, and the TiB particles resolidify with modified morphology.

Technical Analysis of Welding Metallurgy

Effect of TiB Reinforcement on Welding Behavior

The presence of TiB reinforcement particles significantly influences the welding process and resulting microstructure:

• Melting behavior: TiB has a melting point of approximately 3223°C, which is well above the melting point of titanium alloys (approximately 1600-1670°C). During TIG welding, the TiB particles do not fully melt but may partially melt at their interfaces, leading to redistribution and resolidification of titanium and boron.
• Particle refinement: The rapid solidification in the weld pool causes the TiB particles to resolidify with higher aspect ratios and finer dispersion. This refinement can be beneficial for mechanical properties, as finer and more uniformly distributed particles provide better strengthening and crack resistance.
• Compositional segregation: The differential melting and solidification behavior of TiB and the titanium matrix can lead to local compositional variations, particularly in the boron and titanium concentrations near the particles. This can create localized regions of enhanced strength or potential crack initiation sites.

Alpha-Prime Martensite and Mechanical Properties

The formation of alpha-prime martensite in the WMZ and HAZ is a direct consequence of the high cooling rates achieved with TIG welding of titanium alloys. The alpha-prime phase:

• Provides high strength through solid solution strengthening (by dissolved interstitial oxygen, nitrogen, and carbon) and transformation hardening.
• Is inherently brittle and susceptible to cracking under tensile or bending loads.
• Can be tempered to form more ductile alpha + beta phases through post-weld heat treatment (PWHT) at temperatures between 500-700°C.

The observed tensile strength of 1137 MPa (92% of base material) indicates that the weld joint retains most of the base material strength, which is a significant achievement for a composite material weld. However, the elongation of 2.20% is relatively low, reflecting the brittleness of the alpha-prime martensite and the potential for stress concentration around the refined TiB particles.

Fracture Analysis

The fracture occurring in the base material region rather than in the weld or HAZ is a positive indicator, suggesting that the weld joint is at least as strong as the base material. The mixed fracture mode (predominantly ductile with some intergranular regions) indicates that while the base material retains its ductile fracture characteristics, localized intergranular cracking occurs, possibly due to:

• Prior beta grain boundary embrittlement from welding thermal exposure.
• Localized depletion of beta-stabilizing elements at grain boundaries.
• Hydrogen or oxygen pickup along grain boundaries during welding.

Process Optimization and Quality Control

For successful TIG welding of titanium matrix composites, the following process parameters and quality control measures are recommended:

Quality Control Considerations

1. Visual inspection: Verify weld bead uniformity, absence of porosity, and proper fusion with base material.
2. Non-destructive testing: Perform dye penetrant testing (PT) for surface cracks and radiographic testing (RT) or ultrasonic testing (UT) for subsurface defects.
3. Metallographic examination: Examine cross-sections for microstructure characterization, reinforcement particle distribution, and phase identification.
4. Mechanical testing: Conduct tensile testing on witness coupons to verify strength retention and ductility.
5. Hardness mapping: Perform microhardness measurements across the weld joint to identify hardness variations and potential brittle zones.

Study Insights and Implications

The successful TIG welding of titanium matrix composites with 92% strength retention represents a significant advancement in composite joining technology. The key insight is that the welding process, while introducing microstructural changes (alpha-prime martensite formation, TiB particle refinement), does not fundamentally compromise the joint integrity. The fracture occurring in the base material confirms that the weld is not the weak link in the joint.

For aerospace engineering applications, where titanium composites are used in high-performance structural components such as engine mounts, airframe fittings, and landing gear components, this study provides confidence that TIG welding is a viable joining method. However, the relatively low elongation (2.20%) and mixed fracture mode indicate that post-weld heat treatment should be considered for applications requiring high ductility or fatigue resistance.

The refinement of TiB particles in the weld zone is a particularly interesting finding. While the refined particles contribute to strength, they may also create stress concentration sites if the particle-matrix bonding is imperfect. Future research should investigate the particle-matrix interface quality in the weld zone and its influence on fatigue crack initiation and propagation. Additionally, the long-term performance of the weld joint under cyclic loading and elevated temperature conditions should be evaluated to qualify the technology for demanding aerospace applications.