Experimental and Numerical Studies of Titanium Foil–Steel Explosively Welded Clad Plate

1. Definition and Fundamental Principles

1.1 Explosive Welding of Titanium–Steel Systems

Explosive welding is a solid-state joining process in which two dissimilar metal surfaces are accelerated by a detonating charge to a high collision velocity, producing a metallurgical bond through plastic deformation, jetting, and hydrodynamic flow at the interface. In the titanium foil–steel explosively welded clad plate system, a thin titanium (Ti) foil or sheet is applied over a carbon steel or low-alloy steel backing plate. The collision velocity typically ranges between 200 m/s and 600 m/s, depending on the stand-off distance, charge geometry, and material pair characteristics. At these velocities, the titanium and steel surfaces undergo severe plastic deformation, oxide layer disruption, and interfacial mixing, forming a continuous metallurgical bond without melting or dilution.

1.2 Interfacial Microstructure and Bond Quality

The resulting interface in titanium–steel explosive welds is characterized by a wavy or sinusoidal morphology, driven by Kelvin–Helmholtz instabilities at the collision point. The amplitude and wavelength of these waves are governed by the collision velocity, angle, and material properties. A well-formed interface exhibits a high wave amplitude-to-wavelength ratio, indicating strong mechanical interlocking. However, excessive collision velocities can produce intermetallic compounds (IMCs) such as TiFe, Ti₂Fe, and Ti₃Fe along the interface, which may compromise ductility and fatigue performance. Numerical studies—typically employing finite element methods (FEM) with Johnson–Cook constitutive models—allow prediction of interfacial temperature, strain, and IMC formation zones, enabling optimization of process parameters before physical trials.

1.3 Role of Titanium Foil as a Cladding Layer

Titanium foil cladding provides corrosion resistance, biocompatibility, and low-density advantages to steel substrates. In industrial applications, titanium's passive oxide film (TiO₂) offers exceptional resistance to chlorides, sulfuric acid, and marine environments. When applied as a thin foil (typically 0.5 mm to 3.0 mm) via explosive welding, the titanium layer retains its full microstructural integrity while being firmly bonded to the structural steel substrate, combining the economic and mechanical advantages of steel with the surface performance of titanium.

2. Category and Business Positioning

2.1 Classification Within Cladding Technology Shanxi's Technology Portfolio

This entry falls squarely within the company's explosion welding technology route. Unlike TIG/MIG weld overlay (which builds up cladding layers through arc melting) or hydraulic explosive bonding (which uses controlled water-jet-assisted explosive energy), traditional explosion welding relies on shaped explosive charges to achieve the required collision velocity. The titanium foil–steel system represents a high-value, technically demanding application that demonstrates the company's capability in handling reactive metals, thin-film cladding, and precision interface control.

2.2 Strategic Value in Qualification Building

Conducting and publishing experimental and numerical studies on titanium–steel explosive welds serves multiple strategic purposes:

2.3 Market Positioning

Titanium–steel clad plates are in demand in the chemical processing, marine, pharmaceutical, and nuclear industries where corrosion resistance is critical but full-titanium construction is cost-prohibitive. By mastering this specific material system, Cladding Technology Shanxi can serve niche, high-margin markets that require certified, high-integrity titanium cladding solutions.

3. Technical Purpose and Value

3.1 Objective of the Experimental and Numerical Study

The primary objectives of such a study are:

  1. To characterize the interfacial microstructure and bonding quality of titanium foil–steel explosive welds under various process parameters (stand-off distance, charge thickness, explosive type, collision velocity).
  2. To predict interfacial temperatures and strain states using coupled Eulerian–Lagrangian (CEL) or smooth particle hydrodynamics (SPH) numerical methods.
  3. To identify the process window boundaries that avoid interfacial fracture (low velocity) and excessive intermetallic compound formation (high velocity).
  4. To correlate numerical predictions with experimental outcomes (metallography, hardness profiling, shear testing, bend testing).

3.2 Value to Product Delivery

By establishing a validated process window through combined experimental and numerical approaches, the company can:

4. Key Process and Implementation Points

4.1 Process Parameter Optimization

Parameter Typical Range for Ti–Steel Influence on Bond Quality Optimization Strategy
Stand-off distance (SOD) 15–50 mm Controls collision velocity; too small = excessive velocity, too large = insufficient velocity Iterative FEM simulation + velocity measurement (Hawk-Eye or laser Doppler)
Charge thickness 50–150 mm Higher thickness increases energy input and collision velocity Balance with safety regulations and economic constraints
Explosive type TNT, PETN, or industrial equivalent Detonation velocity and pressure determine energy transfer efficiency Select based on available safety permits and required energy level
Titanium foil thickness 0.5–3.0 mm Thinner foil = lower mass, higher acceleration; affects wave morphology Match to application requirements (corrosion service life vs. mechanical load)
Steel backing plate Q235, Q345, 16Mn, or equivalent Density and strength affect momentum transfer and collision dynamics Select based on structural requirements; ensure surface cleanliness
Collision velocity 250–450 m/s (typical for Ti–steel) Below ~200 m/s: no bond; above ~500 m/s: excessive IMC and possible fracture Validate via Hawk-Eye measurement; correlate with FEM predictions
Collision angle 15°–25° (effective) Too low = poor deformation; too high = excessive shear Derived from SOD and charge geometry; confirmed by FEM

4.2 Numerical Simulation Methodology

Finite element analysis of explosive welding typically employs:

4.3 Experimental Characterization Protocol

Following explosive welding trials, the following characterization sequence is recommended:

  1. Visual inspection: Check for surface defects, warpage, and dimensional accuracy.
  2. Ultrasonic testing (UT):strong> Full-surface scanning per ASTM E1655 or ASTM E255 to detect lack-of-bond areas.
  3. Macrograph examination: Cross-sectional etching (e.g., Kroll's reagent for titanium, Nital for steel) to assess wave morphology and identify defects.
  4. Micrograph examination: SEM analysis of the interface to identify IMC layers, their thickness, and distribution.
  5. Hardness profiling: Vickers microhardness traverse across the interface to detect IMC formation (hardness peaks indicate intermetallics).
  6. Mechanical testing: Shear testing (ASTM E8), bend testing (ASTM E290), and tensile testing to validate bond strength.
  7. Chemical analysis: EDS or XRD to identify intermetallic phases at the interface.

5. Applicable Standards and Acceptance Criteria

5.1 Welding and Cladding Standards

  • ASTM E255: Standard Practice for Ultrasonic Examination of Explosively Welded Clad Metal.
  • ASTM E1655: Standard Practice for Determining Bond Strength of Explosively Welded Clad Metal by Shear Testing.
  • ASTM E290: Standard Test Method for Bend Test of Clad Metal.
  • ASTM E8: Standard Test Methods for Tension Testing of Metallic Materials.
  • ASME Section VIII, Division 1, UCS-66: Clad Materials — requirements for clad pressure vessels.
  • ASME Section II, Part D: Specifications for cladding materials and welding consumables.
  • GB/T 11268: Chinese standard for explosively welded clad plates (general technical conditions).
  • NB/T 25123: Chinese nuclear industry standard for explosive welding of clad materials.
  • ISO 14224: Guide to the use of explosively welded clad plates in pressure equipment.

5.2 Acceptance Criteria for Titanium–Steel Explosive Welds

Test Method Acceptance Criterion Standard Reference
Ultrasonic Testing (UT) No lack-of-bond indications; minimum bond area ≥ 95% of test area ASTM E255
Shear Test Fracture occurs in the titanium layer or at the interface with ≥ 90% of titanium tensile strength ASTM E1655
Bend Test (Type I) No cracking or delamination at the outer surface of the bend ASTM E290
Bend Test (Type II) No cracking at the interface when bent inward on the clad side ASTM E290
Macrograph Examination Continuous wavy interface; no voids, cracks, or unmixed zones; wave amplitude/wavelength ratio ≥ 0.3 GB/T 11268
Intermetallic Layer Thickness Maximum IMC layer thickness ≤ 20 μm (for ductility-critical applications) Internal specification / customer requirement

6. Common Risks and Controls

6.1 Technical Risks

  • Interfacial fracture due to excessive collision velocity: When the collision velocity exceeds the upper limit of the process window, the titanium–steel interface can experience brittle fracture. Control: Use FEM to predict velocity; validate with Hawk-Eye measurement; limit SOD to maintain velocity below 450 m/s.
  • Excessive intermetallic compound formation: High interfacial temperatures (above ~800°C) promote formation of brittle Ti–Fe intermetallics. Control: Optimize collision velocity to the lower-middle range of the bonding window; use numerical thermal analysis to predict peak interface temperatures.
  • Incomplete bonding (low collision velocity): If the collision velocity is too low, oxide layers are not disrupted, resulting in unbonded or weakly bonded areas. Control: Ensure minimum collision velocity of 250 m/s; verify with UT testing and macrograph examination.
  • Surface contamination: Oxides, oils, or moisture on the titanium or steel surfaces can impair bonding. Control: Implement rigorous surface preparation per ASTM B517 (for titanium) and SSPC-SP 10 (for steel); verify cleanliness with solvent wiping and visual inspection.
  • Plate warpage and distortion: Explosive welding induces residual stresses that can cause plate bowing or twisting. Control: Use post-weld stress relief (per ASTM A703 or equivalent); design charge geometry to minimize asymmetric stress states; implement dimensional control per GB/T 11268.

6.2 Safety and Regulatory Risks

  • Explosive handling and storage: Compliance with local regulations (e.g., GB 12463 for industrial explosives in China) is mandatory. Control: Maintain licensed explosive handling personnel; conduct regular safety audits; maintain compliance with environmental and occupational health regulations.
  • Flyback and debris: Explosive welding generates high-velocity fragments. Control: Establish exclusion zones per OSHA or local regulations; use protective barriers; ensure all personnel are trained and certified.
  • Numerical model inaccuracy: Over-reliance on simulation without experimental validation can lead to process failures. Control: Always validate FEM models against physical test data; maintain a database of validated material models and boundary conditions.

7. Application Scenarios Across the Three Technology Routes

7.1 Explosion Welding Route (Primary Application)

The titanium foil–steel explosive weld system is the flagship application of the explosion welding route. Typical scenarios include:

  • Chemical processing equipment: Heat exchanger tubesheets, reactor linings, and storage tanks where titanium's corrosion resistance is needed but full titanium construction is uneconomical.
  • Marine and offshore platforms: Clad plates for underwater structural components exposed to seawater, where titanium's biofouling resistance and chloride tolerance are advantageous.
  • Pharmaceutical equipment: Vessels and heat exchangers requiring biocompatible, corrosion-resistant surfaces in aggressive process environments.
  • Nuclear industry: Clad components for secondary containment systems where titanium's resistance to stress corrosion cracking is critical (per NB/T 25123).

7.2 TIG/MIG Weld Overlay Route (Complementary Application)

While explosive welding is preferred for large-area, high-integrity titanium cladding, TIG weld overlay serves as a complementary technology in specific scenarios:

  • Repair and retrofit: When existing steel equipment requires localized titanium protection (e.g., weld repair areas, erosion-damaged zones), TIG overlay with titanium wire (ER Ti-6Al-4V or ER NiTi) provides a practical solution.
  • Small-scale or custom components: For small-diameter pipes, fittings, or intricate geometries where explosive welding is impractical, TIG overlay offers flexibility.
  • Transition layer deposition: In some applications, a nickel or nickel-alloy transition layer is deposited via TIG before titanium cladding to reduce the thermodynamic driving force for IMC formation at the Ti–Fe interface.
  • WPS qualification for hybrid approaches: When explosive welding is followed by localized TIG weld repairs, the combined WPS must be qualified per ASME Section IX, demonstrating that the repair procedure maintains the integrity of the explosive weld bond.

7.3 Hydraulic Explosive Bonding Route (Emerging Application)

Hydraulic explosive bonding (HEB) represents a newer, more controlled approach to explosive welding where water acts as a coupling medium between the explosive charge and the workpiece. For titanium–steel systems, HEB offers potential advantages:

  • Reduced plate distortion: The water layer absorbs some of the shock energy, potentially reducing residual stresses and warpage in thin titanium foils.
  • Improved process control: Water coupling allows more uniform energy distribution, which may be beneficial for thin-film cladding where uniformity is critical.
  • Safety advantages: The water barrier provides additional protection against direct explosive contact with the workpiece, potentially simplifying safety protocols.
  • Experimental and numerical synergy: Numerical studies of titanium–steel explosive welds can be extended to model HEB scenarios by incorporating water as a third material phase in the FEM model, enabling prediction of how water coupling affects collision dynamics and interface quality.

8. Contribution to Qualification Building and Customer Value

8.1 Qualification Building

The experimental and numerical study of titanium foil–steel explosive welds contributes to qualification building in the following ways:

  1. WPS development and qualification: The study provides the fundamental data (collision velocity, interface microstructure, mechanical properties) required to develop and qualify WPS for titanium–steel explosive welds per ASME Section IX or equivalent.
  2. Material specification support: Understanding of IMC formation and interfacial properties enables the company to specify appropriate titanium and steel grades for specific service conditions, ensuring long-term performance.
  3. NDT procedure development: Knowledge of defect formation mechanisms (voids, cracks, unmixed zones) at various process parameters supports the development of tailored UT and radiographic testing procedures with defined acceptance criteria.
  4. Customer audit readiness: Documented research and process understanding demonstrate to customers and third-party inspectors that the company has a scientifically grounded approach to quality, facilitating successful audits and certifications.

8.2 Customer Value

  • Reduced development risk: Customers benefit from a supplier that has already characterized the titanium–steel explosive weld system, reducing the risk of bond failure in critical applications.
  • Faster project timelines: Pre-validated process parameters allow rapid mobilization of production, reducing lead times for clad plate delivery.
  • Design optimization support: Numerical simulation capability enables the company to work with customers on design optimization, predicting how changes in titanium thickness, steel grade, or plate dimensions affect bond quality and mechanical performance.
  • Intellectual property and differentiation: Proprietary knowledge of titanium–steel explosive welding processes creates a competitive moat, positioning the company as a specialist supplier in a niche market with limited competition.
  • Full lifecycle support: From initial process development through production, NDT, and post-delivery quality assurance, the company provides end-to-end support that reduces customer risk and total cost of ownership.

9. Conclusion

The experimental and numerical study of titanium foil–steel explosively welded clad plate represents a critical knowledge asset for Cladding Technology Shanxi Co., Ltd. By combining rigorous experimental characterization with advanced numerical simulation, the company establishes a scientifically validated process window for one of the most challenging dissimilar metal cladding systems. This capability directly supports WPS qualification, product delivery quality, and customer confidence across the chemical, marine, pharmaceutical, and nuclear industries. The integration of this knowledge across all three technology routes—explosion welding, TIG/MIG weld overlay, and hydraulic explosive bonding—ensures that the company can offer comprehensive, flexible solutions tailored to the specific needs of each customer application. Continued investment in research and development in this area will strengthen the company's position as a leading provider of high-integrity clad metal solutions in the global market.