CLADDING TECHNOLOGY SHANXI CO., LTDInterfacial Characterization and Mechanical Property of Ti-Cu Clad Sheet Produced by Explosive Welding and Annealing Study Note
Literature Overview
This study investigates the interfacial bonding characteristics and mechanical properties of titanium-copper (Ti-Cu) clad sheets produced by explosive welding followed by annealing treatment. Explosive welding is a high-strain-rate solid-state bonding process that produces metallurgical bonds between dissimilar metals without melting, making it uniquely suitable for joining materials that form brittle intermetallic compounds when melted together. Ti-Cu clad materials find applications in marine engineering, electrical connectors, heat exchangers, and aerospace components where the combination of titanium's corrosion resistance and copper's electrical conductivity is required.
Core Technical Findings
Explosive Welding Process Parameters
The explosive welding parameters used in the study are summarized below:
| Parameter | Value |
|---|---|
| Explosive charge (RDX) | 50–80 g/cm² |
| Standoff distance (SD) | 8–12 mm |
| Flyer plate velocity (Vf) | 2.5–3.5 m/s |
| Target plate velocity (Vt) | 0.3–0.5 m/s |
| Impact angle (θ) | 15–25° |
| Relative velocity (Vr) | 2.0–3.0 m/s |
The relative velocity (Vr = Vf × sin θ - Vt × cos θ) is the critical parameter determining the quality of the explosive weld. For Ti-Cu systems, a relative velocity of 2.0–3.0 m/s produces the optimal conditions for jet formation and interface cleaning without excessive material loss or damage.
Interfacial Microstructure
The as-welded interface exhibits a characteristic wavy (or "wave") morphology with a wavelength of 0.5–2.0 mm and amplitude of 0.1–0.5 mm. The wave amplitude and wavelength are directly influenced by the impact angle and relative velocity:
- Low relative velocity (<2.0 m/s): Shallow waves, incomplete bonding, oxide inclusions
- Optimal relative velocity (2.0–3.0 m/s): Deep waves, clean metallurgical bond, minimal oxide inclusion
- High relative velocity (>3.0 m/s): Excessive jetting, material loss, damaged interface
The interface region contains a thin intermetallic layer (TiCu, Ti₂Cu, TiCu₃) formed during the high-strain-rate impact. In the as-welded condition, this intermetallic layer is discontinuous and thin (<5 μm), distributed along the wave crests where the local temperature and pressure are highest.
Effect of Annealing on Interface
Annealing at elevated temperatures promotes the growth and coarsening of intermetallic phases at the interface:
| Annealing Temperature (°C) | Time (h) | Intermetallic Layer Thickness (μm) | Phase Composition | Bond Strength (MPa) |
|---|---|---|---|---|
| 400 | 2 | 2–5 | TiCu, Ti₂Cu (discontinuous) | 280–320 |
| 500 | 2 | 5–15 | TiCu, Ti₂Cu, TiCu₃ (semi-continuous) | 220–260 |
| 600 | 2 | 15–30 | TiCu₃, Ti₂Cu (continuous) | 150–180 |
| 650 | 4 | 30–50 | Ti₂Cu, TiCu₃ (continuous, thick) | 80–120 |
The annealing treatment serves a dual purpose: it relieves residual stresses from the explosive welding process and allows for controlled diffusion at the interface. However, excessive annealing temperatures (>600°C) lead to the formation of thick, continuous brittle intermetallic layers that severely degrade bond strength and fracture toughness.
Process Parameter Analysis
Critical Impact Parameters
The success of explosive welding depends on achieving the correct combination of impact velocity, angle, and standoff distance. The study identifies the following critical relationships:
- Impact angle: The optimal angle of 15–25° ensures that the relative velocity is sufficient for interface cleaning while avoiding excessive jetting. Angles below 15° produce insufficient shear deformation, while angles above 25° cause excessive material loss.
- Standoff distance: A standoff distance of 8–12 mm provides the correct flyer plate velocity. Too small a distance results in insufficient impact energy, while too large a distance causes the flyer plate to decelerate excessively before impact.
- Explosive charge density: The charge density (50–80 g/cm²) must be sufficient to accelerate the flyer plate to the required velocity. Insufficient charge density leads to incomplete bonding, while excessive charge density causes damage to the plates.
Residual Stress Distribution
The explosive welding process introduces significant residual stresses in the clad plate:
| Location | Residual Stress (MPa) | Stress Type |
|---|---|---|
| Ti layer (near interface) | 300–450 (tensile) | Tensile |
| Cu layer (near interface) | 250–400 (compressive) | Compressive |
| Ti layer (far from interface) | 50–100 (tensile) | Tensile |
| Cu layer (far from interface) | 20–50 (compressive) | Compressive |
These residual stresses can cause distortion, cracking, or delamination during subsequent machining or service. Annealing at 400–500°C for 2 hours reduces the residual stresses by 40–60% without significantly affecting the interfacial integrity.
Engineering Practice Integration
Application in Pressure Vessel and Heat Exchanger Fabrication
Ti-Cu clad materials are used in:
- Marine heat exchangers: Titanium tubes on copper-clad carbon steel shells, combining corrosion resistance with thermal conductivity
- Electrical connectors: Ti-Cu transition joints for high-current applications where low contact resistance and corrosion resistance are required
- Nuclear components: Ti-Cu thermal shields and heat exchangers in reactor cooling systems
- Aerospace fuel systems: Ti-Cu tubing for cryogenic fuel transfer
Quality Control and Inspection
| Inspection Method | Acceptance Criteria | Frequency |
|---|---|---|
| Visual examination | No visible cracks, delamination, or excessive jetting | 100% |
| Bend test (180°) | No cracking or delamination at interface | Per batch |
| Peel test | Bond strength ≥ 200 MPa | Per batch |
| Metallographic examination | Continuous metallurgical bond, intermetallic < 10 μm | Per batch |
| UT scanning | No delamination or voids | 100% |
| Hardness mapping | No excessive hardening at interface | Per batch |
Standards Compliance
- ASTM A263: Specification for clad plate, strip, and sheet
- ASTM A264: Specification for clad plate and strip
- ASTM A265: Specification for clad plate and strip, weld-overlay
- EN 10028-7: Clad plate for pressure equipment
- NB/T 47002: Fabrication of pressure vessels
- GB/T 150: Pressure vessel design and fabrication
- ASME VIII Div.1: Construction of pressure vessels
Key Questions and Reflections
The study highlights a fundamental challenge in Ti-Cu bonding: the formation of brittle intermetallic phases is inevitable due to the thermodynamic driving force for Ti-Cu compound formation. The key question for engineers is not whether intermetallics will form, but how to control their thickness, distribution, and morphology to maintain acceptable mechanical properties. The annealing treatment offers a means to control intermetallic growth, but the window between stress relief and excessive intermetallic formation is narrow.
A practical consideration is the effect of subsequent machining on the clad plate. Machining operations can introduce local heating and residual stresses that may affect the interfacial integrity. Engineers should specify appropriate cutting parameters (low cutting speed, high feed rate, sharp tools) and consider stress-relief annealing after machining if the component is to be used in critical applications.
Study Insights and Implications
The explosive welding of Ti-Cu clad sheets offers a reliable method for producing materials that combine the best properties of both metals. The interfacial characterization revealed in this study provides engineers with the knowledge needed to predict and control the bonding quality. The annealing treatment, while beneficial for stress relief, must be carefully controlled to avoid excessive intermetallic formation. For pressure vessel and heat exchanger applications, engineers should specify the maximum allowable intermetallic layer thickness in the fabrication drawing and require metallographic verification at the interface. The wavy interface morphology produced by explosive welding is not a defect but a feature that provides mechanical interlocking and enhances bond strength; engineers should recognize this and avoid treating it as an anomaly during quality inspection.