Research on Explosives Used in Explosive Welding

Classification and Selection of Explosives for Cladding

The selection of explosive material is perhaps the most critical decision in explosive welding process design, as it directly governs the collision velocity, shock wave characteristics, and ultimately the quality of the metallurgical bond. The literature on explosives for explosive welding provides a comprehensive framework for matching explosive properties to specific material combinations and cladding geometries.

Explosives used in explosive welding can be broadly classified into high explosives and propellants, each offering distinct advantages and limitations:

Detonation Characteristics and Their Impact on Bonding

The detonation characteristics of an explosive determine the shock wave profile transmitted to the flyer plate, which in turn controls the collision dynamics. Key detonation parameters include detonation velocity, detonation pressure, charge density, and the shape of the pressure-time profile at the flyer interface.

The detonation velocity must be sufficiently high to accelerate the flyer plate to the critical bonding velocity while maintaining a stable wave front. For most steel-based cladding applications, detonation velocities between 6000 and 8000 m/s provide adequate flyer velocities in the 1000-2500 m/s range. However, the relationship between detonation velocity and flyer velocity is not linear; it depends on the acoustic impedance matching between the explosive and the flyer material.

The detonation pressure profile is equally important. A sharp, well-defined pressure pulse produces a more uniform flyer acceleration, resulting in consistent collision velocities across the plate area. In contrast, explosives with irregular detonation profiles can produce velocity gradients that compromise bonding uniformity. This is particularly critical for large-diameter cladding operations where maintaining uniform collision conditions across the entire plate area becomes increasingly challenging.

Safety Considerations and Handling Requirements

The safety profile of explosives used in welding operations differs significantly from military or mining applications due to the proximity of personnel and equipment to the detonation event. The literature emphasizes several safety-critical aspects:

1. Stability: Explosives must remain stable under normal storage and handling conditions without sensitivity to friction, impact, or electrostatic discharge.
2. Detonation reliability: The explosive must detonate reliably under all expected environmental conditions, including temperature variations and humidity exposure.
3. Blast containment: The explosive configuration must direct energy efficiently toward the flyer plate while minimizing collateral damage to surrounding equipment and structures.
4. Residual effects: The explosive must not produce toxic gases or residual materials that could contaminate the weld interface or create occupational health hazards.

These safety requirements have led to the development of composite explosive formulations specifically designed for welding applications, incorporating binders and additives that improve stability while maintaining adequate detonation performance.

Process Design Parameters Governed by Explosive Selection

The explosive choice directly influences several key process design parameters:

• Charge thickness: Determined by the required detonation energy and the need to maintain stable detonation propagation. Typical charge thicknesses range from 20 to 100 mm depending on the explosive type and flyer plate dimensions.
• Standoff distance: The gap between the flyer and base plates must be precisely controlled to achieve the desired collision velocity. This distance is calculated based on the explosive's detonation velocity and the material's shock impedance.
• Detonation sequence: For large plates, multi-point initiation systems ensure simultaneous or sequential detonation that produces uniform flyer acceleration. The timing between initiation points is calculated from the explosive's detonation velocity.
• Support plate thickness: The backing plate behind the flyer must be thick enough to provide uniform confinement pressure while thin enough to not absorb excessive detonation energy.

Engineering Practice and Material-Specific Recommendations

Based on extensive engineering experience, the following recommendations emerge for explosive selection in specific cladding applications:

For stainless steel 304/316 on carbon steel cladding, TNT-based formulations with detonation velocities around 6900 m/s provide reliable bonding at collision velocities of 1200-1800 m/s with good process margins. For titanium-clad steel applications, higher-performance explosives such as PETN or RDX formulations are necessary to achieve the higher collision velocities (2000-3000 m/s) required for bonding titanium to steel without producing brittle intermetallic phases.

In large-scale production environments, ANFO-based formulations offer the best combination of cost-effectiveness, handling safety, and adequate performance for standard steel-to-stainless steel cladding. However, for critical applications requiring tight quality tolerances, such as nuclear pressure vessel cladding, high-purity RDX or HMX formulations are preferred despite their higher cost.

Summary

The research on explosives for explosive welding establishes that the explosive material is not merely an energy source but a process engineering parameter that must be carefully matched to the target material combination, plate geometry, and quality requirements. Proper explosive selection, combined with precise control of charge geometry and detonation sequencing, is fundamental to achieving consistent metallurgical bonding quality in industrial cladding operations. Engineers involved in bimetallic product manufacturing must develop deep understanding of explosive properties and their interaction with collision dynamics to optimize both process reliability and product quality across the full range of cladding applications from standard industrial plates to critical pressure vessel components.