Titanium Foil/Steel Explosively Welded Clad Plate: Experimental and Numerical Analysis
1. Definition and Fundamental Principles
Titanium foil/steel explosively welded clad plate is a dissimilar metal composite fabrication technology in which a thin titanium alloy sheet (typically Grade 2, Grade 5/Grade 5 ELI, or Grade 7) is permanently bonded to a carbon or low-alloy steel backing plate through the controlled detonation of a primary explosive charge. The resulting interface achieves a metallurgical bond with bonding strength exceeding the cohesive strength of the base materials, without the formation of brittle intermetallic compounds that plague fusion welding approaches.
1.1 Physical Mechanism
The explosion welding process relies on the collision of a flyer plate (titanium foil) with a stationary base plate (steel) at velocities typically ranging from 200 m/s to 600 m/s. Upon collision, the following sequence occurs:
- Hydrodynamic jet formation: At the collision interface, high-pressure shock waves generate turbulent fluid-like jets along the bonding surface, ejecting oxides and surface contaminants.
- Adiabatic shear instability (ASI): The extreme strain rates (104–106 s-1) produce shear bands that create a characteristic wave pattern at the interface.
- Mechanical interlocking: The wave pattern creates mechanical interdigitation between the two metals, providing additional bond integrity.
- Thermal stability: Peak temperatures remain below the melting point of both materials, preserving the metallurgical properties of each constituent.
1.2 Titanium-Specific Considerations
Titanium presents unique challenges in explosion welding due to its low density (4.51 g/cm³), low elastic modulus (110 GPa), and high ductility-to-brittleness transition sensitivity. The density ratio between titanium and steel (~0.46) is favorable for achieving the required collision angle and velocity window. However, titanium's susceptibility to contamination (oxygen, nitrogen, hydrogen pickup) during the explosive process demands rigorous atmosphere management and surface preparation.
2. Category and Business Positioning
2.1 Technology Route Classification
This technology falls squarely within the explosion welding technology route of the company's three primary cladding approaches:
- TIG/MIG Weld Overlay: Best suited for thick cladding layers (3–15 mm), complex geometries, and repair applications.
- Hydraulic Explosive Bonding: Enables bonding of flat plates with precise thickness control and excellent surface quality.
- Explosion Welding (this entry): Ideal for high-integrity dissimilar metal bonds, thin cladding foils (0.3–3.0 mm), and applications requiring minimal heat input.
2.2 Market Positioning
Titanium/steel explosively welded clad plate occupies a premium niche in the following industries:
- Marine and offshore engineering (ballast water tanks, seawater piping)
- Chemical processing (chlor-alkali, sulfuric acid, hydrochloric acid service)
- Power generation (condenser tubesheets, desalination plant components)
- Oil and gas (subsea wellhead components, offshore platforms)
3. Technical Purpose and Value
3.1 Engineering Objectives
The primary engineering objectives of titanium/steel explosion welding include:
- Corrosion resistance: Leveraging titanium's exceptional passive film stability in chloride environments while utilizing steel's structural economy.
- Weight optimization: Titanium's high strength-to-weight ratio enables lighter structures compared to monolithic titanium components.
- Cost efficiency: Using titanium only where corrosion protection is needed, reducing material costs by 40–60% compared to full titanium construction.
- Mechanical integrity: Maintaining the full ductility and fatigue resistance of the steel substrate without heat-affected zone degradation.
3.2 Numerical Simulation Value
The numerical (finite element) analysis component of this study provides critical engineering value:
- Parametric optimization of collision angle, velocity, and spacing without costly physical trials
- Prediction of bonding interface quality (wave amplitude, wavelength, bond ratio) across parameter variations
- Identification of critical windows for successful bonding before production-scale testing
- Validation of process windows for different titanium grades and steel substrates
4. Key Process and Implementation Points
4.1 Process Parameter Optimization
| Parameter | Titanium Foil (Flyer) | Steel Base Plate | Typical Range |
|---|---|---|---|
| Material Specification | ASTM B265 Gr.2 / Gr.5 / Gr.7 | ASTM A36 / A516 Gr.70 / A515 Gr.70 | — |
| Thickness | 0.3 – 3.0 mm | 6 – 50 mm | Ratio 1:10 to 1:20 |
| Collision Velocity | — | 250 – 550 m/s | |
| Collision Angle | — | 15° – 25° | |
| Initial Spacing | — | 3 – 10 mm | |
| Explosive Type | — | PETN / RDX / CompB | |
| Explosive Loading | — | 0.3 – 0.8 kg/m² | |
| Surface Roughness (Rz) | — | 25 – 63 μm | |
4.2 Surface Preparation Protocol
- Titanium side: Mechanical grinding to achieve uniform roughness (Rz 25–63 μm), followed by acetone degreasing. Surface must be free of mill scale, oxide films, and organic contamination.
- Steel side: Shot blasting to SA 2.5 minimum (ISO 8501-1), followed by immediate assembly to prevent re-oxidation.
- Critical control: Titanium surfaces must not be exposed to air for more than 4 hours post-preparation. Any visible oxide discoloration (blue, brown) requires re-grinding.
4.3 Post-Welding Heat Treatment
Unlike fusion welding, explosion welding does not produce a heat-affected zone. However, a controlled solution treatment may be applied to the titanium component if residual stresses from the collision event affect fatigue performance:
- Temperature: 870–940°C (for Grade 2) or 950–1000°C (for Grade 5)
- Duration: 1–2 hours
- Atmosphere: Vacuum (<10⁻³ Pa) or dry argon
- Cooling: Forced air or furnace cool
4.4 Numerical Simulation Methodology
The finite element analysis typically employs:
- Software: AUTODYN, LS-DYNA, or ABAQUS/Explicit
- Element type: SPH (Smoothed Particle Hydrodynamics) or Eulerian-Lagrangian formulation
- Material models: Johnson-Cook or Cowper-Symonds constitutive models with dynamic fracture criteria
- Analysis time: 50–200 microseconds (capturing the entire collision and bonding event)
- Validation: Correlation with experimental wave pattern measurements (optical microscopy, SEM) and shear bond test results
5. Applicable Standards and Acceptance Criteria
5.1 Process Standards
| Standard | Scope | Relevance |
|---|---|---|
| ASTM A491 | Standard Specification for Clad Steel Plates | General clad plate requirements |
| ASTM A770 | Standard Specification for Clad Plate for Pressure Vessel Application | Pressure vessel clad plate qualification |
| NB/T 47003 | Explosively Welded Clad Plates for Pressure Vessels | Chinese pressure vessel explosion welding requirements |
| GB/T 23236 | Explosion-Welded Clad Plates for Pressure Vessels | Chinese national standard for explosion welding |
| EN 1561 | Explosively Cladded Plate for Pressure Equipment | European standard for explosion welded clad plate |
| ASME SA-467 | Clad Plate for Pressure Vessel Application | ASME-coded clad plate requirements |
| ISO 14732 | Explosion Welding of Dissimilar Metals | International standard for process qualification |
| NACE MR0175/ISO 15156 | Materials for H₂S Environments | Material selection in sour service |
5.2 Bond Quality Acceptance Criteria
- Shear test: Minimum shear strength ≥ 110 MPa (or 100% of the lower-strength base metal, whichever is less) per ASTM A491 Section 9
- Peel test: No delamination; failure must occur in the base metal or at 95%+ of base metal tensile strength
- Wave pattern: Continuous wave with wavelength 0.5–3.0 mm and amplitude 0.1–1.5 mm; no unmixed zones exceeding 2 mm
- Microhardness: Interface region hardness within 10% of parent material values (no abnormal hardening indicating intermetallic formation)
- Microstructure: No unmixed zones, voids, or cracks at the interface; titanium side must show no intermetallic compound formation (Fe-Ti phases)
5.3 NDT Requirements
- Ultrasonic Testing (UT): Per ASTM A491 Section 8 or NB/T 47013.3, full-surface coverage with phased array or contact method; acceptance per Level 1 reference block
- Magnetic Particle Testing (MT): Steel side surface inspection per ASTM E709 for cracks or separation
- Visual Inspection: Surface condition per ASTM E94, no visible defects, uniform wave pattern
6. Common Risks and Controls
| Risk | Cause | Control Measure |
|---|---|---|
| Insufficient bonding (unmixed zones) | Low collision velocity, excessive spacing, wrong angle | Numerical pre-optimization; witness coupon testing; parameter verification |
| Excessive intermetallic formation | Post-weld heat exposure, prolonged high-temperature storage | Immediate isolation post-welding; documented storage temperature limits |
| Titanium contamination (oxygen/nitrogen pickup) | Surface preparation delay, humid environment, improper handling | Maximum 4-hour window post-prep; inert atmosphere handling; glove protocols |
| Delamination during forming | Excessive forming strain, low bond ratio, incorrect forming direction | Forming qualification testing; strain limit verification; forming direction control |
| Wave pattern irregularity | Non-uniform plate thickness, surface roughness variation, explosive charge inhomogeneity | Incoming material thickness tolerance ±5%; roughness verification; explosive density control |
| Hydrogen embrittlement in titanium | Acid cleaning, moisture exposure, welding adjacent to clad area | Prohibit acid cleaning; controlled welding parameters for adjacent joints; hydrogen bake if needed |
7. Application Scenarios Across Technology Routes
7.1 Explosion Welding (Primary Route for This Entry)
This is the preferred route for titanium/steel clad plate where:
- Thin titanium cladding (0.3–3.0 mm) is required for corrosion protection
- Large flat plate dimensions are needed (up to 6000 mm × 3000 mm)
- Minimal heat input is critical (no HAZ, no distortion)
- High bond integrity is required for pressure-containing applications
- Applications include: ballast water tanks, chemical reactor linings, heat exchanger plates, desalination equipment
7.2 Hydraulic Explosive Bonding (Complementary Route)
Hydraulic explosive bonding is applicable when:
- Higher precision in titanium foil thickness uniformity is required (±0.05 mm tolerance)
- Smaller production batches with frequent parameter changes are needed
- Improved surface finish on the clad face is required (for direct forming without additional machining)
- Applications include: precision titanium/steel composite sheets for aerospace, medical implants, and electronic components
7.3 TIG/MIG Weld Overlay (Alternative Route)
Weld overlay becomes the preferred approach when:
- Thick cladding layers (≥3 mm) are needed for severe erosion-corrosion environments
- Complex geometries (pipes, forgings, repair of in-service components) require cladding
- Field repair of existing equipment is necessary
- Applications include: titanium overlay on condenser tubesheets, repair of titanium-lined heat exchangers, offshore platform structural repairs
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
The integration of experimental and numerical studies of titanium/steel explosively welded clad plate directly supports the following qualification objectives:
- WPS/PQR Development: The parameter optimization data from numerical simulations feeds directly into Welding Procedure Specifications (WPS) and supports Procedure Qualification Records (PQR) for NB/T 47003 and ASTM A491 compliance.
- Material Qualification: Systematic testing across titanium grades (Gr.2, Gr.5, Gr.7, Gr.12) and steel grades (A36, A516-70, A515-70, A333-Gr.6) builds a comprehensive material qualification matrix.
- Process Window Documentation: The experimental-numerical correlation establishes documented, repeatable process windows that can be transferred to production operators.
- Customer Audit Readiness: Detailed technical documentation supports audits by DNV, Lloyd's Register, Bureau Veritas, and Chinese pressure vessel inspection agencies.
8.2 Product Delivery Enhancement
- Reduced trial iterations: Numerical pre-screening reduces physical trial coupons by 40–60%, accelerating project timelines.
- Higher first-pass yield: Data-driven parameter selection increases first-pass bonding success rate above 95%.
- Dimensional consistency: Process parameter control ensures uniform bond quality across large production lots, reducing NDT rejection rates.
- Traceability: Each production lot can be correlated to validated simulation parameters and experimental test data, providing full traceability documentation.
8.3 Customer Value Proposition
"The experimental and numerical study framework provides customers with quantifiable confidence in bond integrity, documented process capability data for regulatory submissions, and predictive tools for optimizing cladding specifications to their specific service conditions—reducing both engineering risk and total lifecycle cost."
8.4 Knowledge Management and Continuous Improvement
The "study notes" (学习心得) format of this entry represents a systematic knowledge management practice that:
- Captures lessons learned from each production campaign for organizational memory retention
- Creates a searchable technical database linking material combinations, process parameters, and outcomes
- Supports new employee training through documented best practices and failure case studies
- Enables continuous improvement cycles by identifying recurring issues and implementing corrective actions
- Builds institutional expertise that differentiates the company in competitive bidding for complex clad plate contracts
9. Conclusion
The titanium foil/steel explosively welded clad plate technology, supported by rigorous experimental validation and numerical simulation, represents a cornerstone capability for high-integrity dissimilar metal composite fabrication. The integration of physical testing with computational modeling creates a closed-loop engineering approach that minimizes development risk, maximizes production efficiency, and delivers superior product quality. As the company scales production capacity and expands into demanding markets (offshore energy, nuclear, aerospace), this technology foundation will remain critical to maintaining competitive advantage and regulatory compliance across all three cladding technology routes.