CLADDING TECHNOLOGY SHANXI CO., LTDPhased Array Testing Application in Explosively Welded Metal Clad Plates
Literature Overview and Significance
Explosively welded clad plates are widely used in the chemical processing, hydrogen energy, and nuclear industries where a corrosion-resistant facing layer is bonded to a structural backing plate through the high-strain-rate impact of an explosive detonation. The bonding quality of the explosive weld interface is critical to the service performance of the clad plate, yet the interface is inherently difficult to inspect due to the metallurgical complexity of the bond zone. This study explores the application of phased array ultrasonic testing (PAUT) to detect bonding defects—such as unbonded areas, voids, and interfacial cracks—in explosively welded metal clad plates.
Technical Challenges of Explosive Clad Plate Inspection
The explosive welding process creates a distinctive wave-patterned bond interface, typically characterized by copper-tin type bonding with localized areas of mechanical interlocking. The interface contains:
- Solid-state bonds formed through plastic instability waves
- Microvoids and gas pockets trapped during the high-velocity collision
- Residual stress concentrations along the wave peaks and troughs
- Potential for interfacial cracks if the collision velocity falls outside the optimal window
These features make conventional contact ultrasonic testing unreliable because the acoustic impedance mismatch at the wave-patterned interface causes severe signal scattering. The phased array approach addresses this challenge through focused beam control and advanced signal processing.
Process Development and Key Parameters
| Parameter | Value | Notes |
|---|---|---|
| Probe type | 2.25 MHz, 32 elements, linear array | Optimized for 1–10 mm clad thickness |
| Wedge material | Plexiglass (PMMA) | Acoustic impedance matched to steel |
| Beam steering range | -15° to +15° from normal | Covers full clad thickness from both sides |
| Focus depth | 1.5–8 mm | Variable focusing tracks the interface |
| Scan resolution | 0.1 mm axial, 0.2 mm lateral | Sufficient for detecting sub-millimeter voids |
| Threshold level | 30% full scale for screening | 50% FSC for defect evaluation |
| Couplant | Glycerin-based gel | Ensures consistent acoustic coupling |
The study demonstrates that dual-probe PAUT configurations—scanning from both the clad side and the backing side—provide complementary information. Defects at the interface that appear as echo signals from one side may appear as signal loss from the other side, enabling cross-verification of indications.
Defect Classification and Acceptance
Based on the study's findings, bonding defects in explosively welded clad plates are classified as follows:
- Type A (Critical): Complete unbonded areas exceeding 50 mm² in any single location or total unbonded area exceeding 1% of the plate surface.
- Type B (Major): Partial bonding with voids larger than 0.5 mm diameter, or interfacial cracks longer than 10 mm.
- Type C (Minor): Small voids less than 0.5 mm diameter, isolated unbonded areas less than 25 mm², acceptable under most standards.
The acceptance criteria align with ASTM E2770 and GB/T 31902, which specify that the bond area must exceed 95% of the total area for most applications and 99% for nuclear-grade components.
Engineering Practice Integration
In my work with bimetal pressure vessels, the explosive welding process is often selected when the clad material has a significant melting point difference from the backing plate—for example, titanium on carbon steel or zirconium on stainless steel. The PAUT inspection results directly inform the subsequent forming and welding operations. If unbonded areas exist near a planned weld line, the cladding layer may delaminate during thermal cycling in the welding process.
The study's finding that PAUT can detect interfacial defects with a minimum detectable size of approximately 0.3 mm diameter represents a significant advancement over conventional pulse-echo methods, which typically require defects larger than 1.0 mm for reliable detection.
Key Reflections
The most valuable contribution of this study is the demonstration that PAUT can serve as a viable replacement for the traditional radiographic testing (RT) method used for explosive clad plate inspection. RT is expensive, involves radiation hazards, and cannot be performed in the field. PAUT provides equivalent or superior detection capability with real-time imaging and no radiation concerns. However, the study also candidly acknowledges that the wave-patterned interface creates acoustic complexity that requires sophisticated signal processing and experienced interpretation—this is not a technology that can be deployed without substantial training investment.