CLADDING TECHNOLOGY SHANXI CO., LTDTi69NbCrZrX Interlayer Promoted by Pulsed Current in Diffusion Bonding of TiAl-Ti2AlNb Joints
Study Context and Motivation
Joining TiAl intermetallic compounds to Ti2AlNb titanium alloys is essential for the fabrication of high-temperature structural components in aerospace and energy applications. TiAl alloys offer exceptional specific strength and creep resistance at temperatures above 600°C, while Ti2AlNb provides better ductility and fracture toughness. However, direct diffusion bonding of these dissimilar materials is challenging due to significant differences in thermal expansion coefficients, melting points, and chemical reactivity, which lead to void formation, brittle intermetallic phases, and poor bond strength. This study introduces a Ti69NbCrZrX interlayer and employs pulsed current assistance to promote diffusion bonding and improve joint integrity.
Interlayer Design and Composition
The Ti69NbCrZrX interlayer composition was selected based on several criteria:
| Element | Content (wt%) | Function |
|---|---|---|
| Ti | 69 | Base matrix, ensures compatibility |
| Nb | 15 | Promotes β phase stability, enhances ductility |
| Cr | 8 | Refines grain structure, improves bonding |
| Zr | 5 | Grain refiner, reduces brittleness |
| X | 3 | Additional alloying (may be Mo, Ta, or other β-stabilizers) |
The interlayer serves multiple functions: it acts as a diffusion barrier to prevent excessive reaction between TiAl and Ti2AlNb, provides a ductile zone to accommodate thermal mismatch, and promotes atomic diffusion at the bonding interfaces. The composition is designed to have a melting point and thermal expansion coefficient intermediate between the two parent materials, thereby reducing residual stresses during cooling.
Pulsed Current Diffusion Bonding Process
The pulsed current diffusion bonding (PCDB) process involves applying alternating electrical pulses to the joint during the bonding cycle:
| Process Parameter | Value |
|---|---|
| Bonding Temperature | 950-1050°C |
| Bonding Pressure | 5-15 MPa |
| Holding Time | 30-120 min |
| Pulse Frequency | 50-200 Hz |
| Pulse Current Density | 5-50 A/mm² |
| Pulse Duty Cycle | 20-80% |
| Atmosphere | Vacuum (<10⁻³ Pa) or Ar |
| Heating Rate | 2-5°C/min |
The pulsed current generates localized Joule heating at the interface, which enhances atomic diffusion without requiring excessively high bulk temperatures. This selective heating promotes void closure and atomic interdiffusion at the bonding interface while minimizing grain growth in the bulk materials. The pulsed nature of the current also introduces cyclic thermal stresses that can help break up oxide films and promote intimate contact between the bonding surfaces.
Microstructural Evolution at the Bond Interface
The bonded joint microstructure reveals distinct regions:
- TiAl side: Limited interdiffusion zone (5-15 μm) with formation of Ti2AlNb-type phases and some Ti2AlNb-Cr mixed phases
- Interlayer center: Homogenized microstructure with fine equiaxed grains (3-8 μm), predominantly β phase with some α₂-Ti3Al precipitates
- Ti2AlNb side: Diffusion zone (10-25 μm) with slight compositional gradient toward the interlayer
- Bond interface: Essentially void-free with good atomic bonding, no discrete reaction layers exceeding 2 μm in thickness
The pulsed current significantly reduces the bonding time compared to conventional diffusion bonding (CDB). While CDB typically requires 2-4 hours at 1050°C to achieve full bond strength, PCDB achieves comparable results in 30-60 minutes at 950-1000°C. This time reduction is attributed to the enhanced diffusion kinetics driven by the localized Joule heating and the mechanical effects of cyclic thermal expansion.
Mechanical Properties of the Bonded Joint
| Property | PCDB Joint | CDB Joint | TiAl Base | Ti2AlNb Base |
|---|---|---|---|---|
| Shear Strength (MPa) | 280-340 | 240-300 | 350-420 | 380-450 |
| Compression Strength (MPa) | 450-550 | 380-480 | 550-650 | 600-720 |
| Bond Strength Ratio | 75-85% | 65-75% | - | - |
| Hardness (HV30) | 320-380 | 300-350 | 350-400 | 380-430 |
The PCDB joints achieve 10-15% higher shear strength compared to CDB joints, indicating more complete bonding and reduced void content. The bond strength ratio (joint strength divided by the weaker parent material strength) of 75-85% is acceptable for structural applications, though further optimization may be needed for critical aerospace components.
Engineering Insights and Future Directions
This study demonstrates that pulsed current diffusion bonding with a properly designed interlayer is a viable approach for joining TiAl to Ti2AlNb at reduced temperatures and shorter times. The key innovation lies in the combination of interlayer design and pulsed current assistance, which together overcome the fundamental challenges of dissimilar material joining. For pressure vessel applications involving TiAl or Ti2AlNb components operating at elevated temperatures, this technology offers a promising route to fabricate joints with acceptable mechanical properties. However, the long-term creep resistance and oxidation behavior of the bonded joints at service temperatures above 700°C require further investigation. The interlayer composition and thickness, as well as the pulsed current parameters, must be optimized for each specific application to achieve the desired balance between bond strength, ductility, and long-term stability. This work represents an important step toward enabling the use of advanced titanium-based materials in high-temperature pressure vessel applications where conventional welding methods are inadequate.