Unbonded/Delamination Assessment of Bimetallic Clad Interface by Ultrasonic Testing
1. Definition and Fundamental Principles
Unbonded or delamination defects at the clad interface represent the most critical failure mode in bimetallic composite materials. An unbonded zone is defined as a region where metallurgical bonding between the overlay (cladding) layer and the backing (base) material has not been achieved, leaving a void or discontinuity at the interface. These defects may manifest as localized unbonded areas, planar delamination along the full interface, or partial bonding with scattered voids.
Ultrasonic Testing (UT) is the primary non-destructive evaluation method for detecting interface defects in clad plates and pipes. The fundamental principle relies on the acoustic impedance mismatch at the clad-base interface. When an ultrasonic pulse is directed perpendicularly or at a controlled angle toward the interface, a properly bonded region produces a coherent reflected signal from the interface and back-wall. In contrast, an unbonded zone introduces a high-impedance air gap that causes a distinct, high-amplitude reflection from the unbonded surface, often accompanied by a loss of the back-wall echo or a shift in the time-of-flight pattern.
Modern phased-array UT (PAUT) and conventional contact UT methods employ either pulse-echo or through-transmission configurations. In pulse-echo mode, the time delay and amplitude of the interface reflection versus the back-wall reflection are compared. In through-transmission mode, the attenuation of the transmitted signal across the clad interface serves as the bonding indicator.
2. Category and Business Positioning
Within the corporate capability taxonomy, this entry falls under the category of Welding Defect Assessment (焊接缺陷判定), specifically targeting Interface Defects (界面缺陷). It is classified as a core acceptance item (复合材料核心验收项) for all composite material products, meaning no clad plate or pipe can be certified for delivery without passing this assessment.
Business positioning is critical: interface bonding quality is the single most discriminating quality parameter that separates a commercially viable clad product from a rejected lot. For Cladding Technology Shanxi Co., Ltd., mastery of unbonded/delamination assessment enables:
- Confident product qualification for demanding end-users in oil & gas, power generation, and chemical processing
- Reduced warranty exposure by providing quantified, standards-based bonding certifications
- Accelerated process optimization through rapid feedback loops between manufacturing and inspection
- Credibility in WPS/PQR documentation when submitting qualifications to third-party inspectors and regulatory bodies
3. Technical Purpose and Value
The stated technical purpose is composite bonding quality assurance (复合结合质量). The value proposition is threefold:
3.1 Quantitative Bonding Certification
UT assessment transforms the qualitative question "Is it bonded?" into a quantitative, auditable metric. By measuring individual unbonded area dimensions, spacing between adjacent unbonded zones, and the total unbonded fraction per unit area, the assessment provides a numerical bonding rate that can be compared directly against contractual and standard acceptance thresholds.
3.2 Grade-Based Classification
The two-tier acceptance system—Grade I (≥99% bonding rate) for critical applications and Grade II (≥95% bonding rate) for general industrial service—allows customers to specify performance expectations and enables the manufacturer to route products to appropriate service conditions. This grading also supports insurance underwriting and regulatory compliance in safety-critical environments.
3.3 Traceability and Continuous Improvement
Each UT assessment generates a permanent record (A-scan data, coverage maps, and defect logs) that supports root-cause analysis when defects exceed limits. Pattern recognition across production lots enables proactive process adjustments before defects accumulate, reducing scrap rates and improving first-pass yield.
4. Key Process and Implementation Points
4.1 UT Scanning Methodology per GB/T 8165 and ASTM A263
Both GB/T 8165 (Ultrasonic Testing Method for Bonding Quality of Clad Plates) and ASTM A263 (Standard Practice for Ultrasonic Examination of Clad Steel Plates) prescribe systematic scanning procedures. The implementation workflow is as follows:
- Surface Preparation: The clad surface must be ground or polished to remove scale, oxide, and surface roughness that would attenuate or scatter the ultrasonic signal. Surface flatness should be within ±0.1 mm over the transducer beam width.
- Calibration: Reference blocks with known unbonded areas (typically manufactured with controlled air gaps of varying sizes) are used to calibrate instrument gain, time-gain compensation (TGC), and defect detection sensitivity. The calibration covers the full thickness range and both material combinations (clad thickness and base thickness).
- Scan Configuration: The transducer (typically 2.25 MHz to 5 MHz contact probe) is traversed in a grid pattern with a beam overlap of at least 50% in both longitudinal and transverse directions. For large-format plates, automated scanning rigs ensure consistent lift-off and coupling.
- Signal Acquisition: For each scan position, the instrument records the interface reflection amplitude and time-of-flight, the back-wall echo amplitude, and any intermediate reflections indicative of internal voids.
- Defect Mapping: All indications exceeding the detection threshold are marked on a coverage map with coordinates, dimensions (projected on the surface), and amplitude values.
4.2 Measurement Parameters and Their Significance
| Parameter | Definition | Measurement Method | Typical Grade I Limit | Typical Grade II Limit |
|---|---|---|---|---|
| Maximum Individual Unbonded Area | Largest contiguous unbonded zone (projected area on clad surface) | 6 dB drop method or amplitude threshold on A-scan | ≤ 0.5% of plate area or ≤ specified dimension per standard | ≤ 1.0% of plate area or ≤ specified dimension per standard |
| Spacing Between Adjacent Unbonded Zones | Minimum distance between two separate unbonded indications | Coordinate difference on coverage map | Must exceed specified minimum (e.g., 50 mm) to prevent coalescence | Must exceed specified minimum (e.g., 30 mm) |
| Unit Area Unbonded Rate (Bonding Rate) | Total unbonded area divided by total scanned area, expressed as a percentage | Summation of all mapped defect areas / total plate area | Bonding rate ≥ 99% (unbonded ≤ 1%) | Bonding rate ≥ 95% (unbonded ≤ 5%) |
| Edge Unbonded Zone Width | Width of unbonded region along plate edges (common in explosive cladding) | Direct measurement from scan profile | ≤ 5 mm from edge | ≤ 10 mm from edge |
4.3 Grade Classification Decision Logic
The final grade assignment follows a hierarchical decision process:
- If the total bonding rate is below 95%, the plate is rejected regardless of individual defect sizes.
- If the bonding rate is ≥95% but <99%, the plate qualifies as Grade II, provided no individual unbonded zone exceeds the Grade II size limit and spacing requirements are met.
- If the bonding rate is ≥99%, the plate qualifies as Grade I, provided no individual unbonded zone exceeds the Grade I size limit and spacing requirements are met.
- If any single unbonded zone exceeds the maximum allowable dimension for the target grade, the plate is downgraded or rejected per the applicable specification.
5. Applicable Standards and Acceptance Criteria
5.1 Primary Standards
| Standard | Title / Scope | Key Provisions for Unbonded Assessment |
|---|---|---|
| GB/T 8165 | Ultrasonic Testing Method for Bonding Quality of Clad Plates | Defines UT scanning procedure, calibration requirements, defect sizing methodology, and acceptance classification for welded and explosion-clad plates |
| ASTM A263 | Standard Practice for Ultrasonic Examination of Clad Steel Plates | Specifies transducer frequencies, scan patterns, signal interpretation, and acceptance limits for bonded and unbonded areas; widely referenced in North American and international projects |
| ASTM A490 | Standard Specification for Multilayer Steel Plate | Material-level specification that references bonding quality requirements and may specify minimum bonding percentage |
| ASME BPV Section II Part D | Acceptance Standards for Fusion Welding | Provides general NDT acceptance philosophy; UT acceptance for clad interfaces in pressure vessel service may reference this section for signal amplitude criteria |
| NACE SP0775 | Standard Practice for Surface Preparation and Coating of Carbon Steel for Corrosion Protection | Indirectly relevant: specifies surface quality requirements that affect UT coupling and signal quality during pre-inspection preparation |
5.2 Acceptance Criteria Summary
For explosion-welded clad plates, the industry-standard acceptance thresholds are:
- Grade I: Bonding rate ≥ 99%, with no individual unbonded area exceeding the dimensional limit specified in the applicable product specification (commonly ≤ 0.5% of plate area or a maximum projected dimension of 25–50 mm depending on plate thickness and application criticality).
- Grade II: Bonding rate ≥ 95%, with individual unbonded areas within the Grade II dimensional limit and adequate spacing between adjacent defects.
For weld-overlay clad products (TIG/MIG), acceptance criteria are typically more stringent due to the thinner overlay layer and greater sensitivity to porosity-induced unbonding. Many specifications require ≥99.5% bonding rate with zero tolerance for through-thickness unbonded areas.
5.3 Customer-Specific and Project-Specific Criteria
In addition to the baseline standards, end-users frequently impose project-specific acceptance criteria that may exceed standard requirements. Examples include:
- Zero-tolerance for unbonded areas in cryogenic service (e.g., LNG tanks per API 620)
- 100% UT coverage (vs. standard 50% or spot-check) for nuclear-grade clad components per ASME Section III
- Phased-array UT with full digital data retention for traceability in aerospace or defense applications
6. Common Risks and Control Measures
6.1 False Positive Indications (Over-Rejection)
Risk: Surface roughness, residual scale, or uneven coupling can produce signal amplitude changes that mimic unbonded areas, leading to unnecessary rejection of acceptable product.
Controls:
- Mandate surface preparation to Ra ≤ 1.6 μm (or as specified) before UT scanning
- Use reference blocks machined from the same material combination and heat treatment condition
- Implement dual-operator verification for marginal indications (amplitude within ±2 dB of threshold)
- Employ phased-array UT with focused beam steering to distinguish surface noise from true interface defects
6.2 False Negative Indications (Under-Rejection)
Risk: Small or closely-spaced unbonded zones may produce signals below the detection threshold, or the scan pattern may miss defects between scan lines.
Controls:
- Ensure scan overlap ≥ 50% in both directions to eliminate coverage gaps
- Calibrate detection sensitivity to a known small defect (e.g., 6 mm diameter flat bottom hole equivalent or equivalent unbonded area)
- Supplement UT with magnetic particle testing (MT) or penetrant testing (PT) at high-risk zones (edges, weld start/stop points, transition zones)
- For critical applications, require 100% UT coverage with automated scanning and full A-scan data archiving
6.3 Edge Effects and Corner Defects
Risk: In explosion welding and hydraulic explosive bonding, edge regions and corners often exhibit higher unbonded fractions due to incomplete jetting and debris removal. These zones are frequently excluded from UT scanning due to geometric constraints, creating blind areas.
Controls:
- Define a minimum bonded-width requirement from each edge (e.g., ≥ 20 mm for Grade I) and verify by UT or by sectioning
- Use angle probes or immersion UT for edge regions where normal perpendicular scanning is impractical
- Apply consistent edge trimming after bonding to remove the known unbonded margin
6.4 Material-Dependent Signal Interpretation Challenges
Risk: Dissimilar material combinations (e.g., stainless steel clad on carbon steel, nickel alloy on copper, aluminum on steel) have different acoustic impedances, attenuation coefficients, and grain structures, complicating signal interpretation.
Controls:
- Develop material-specific calibration blocks and acceptance curves for each clad-base combination
- Train UT technicians on the expected signal patterns for each material pair (normal bonded echo, partial bonding echo, unbonded echo)
- Maintain a library of reference A-scan and B-scan images for each material combination and thickness range
7. Application Across the Three Technology Routes
7.1 TIG/MIG Weld Overlay Cladding
In weld overlay cladding, the clad layer is deposited as a series of TIG or MIG weld beads on the prepared base surface. Interface bonding quality is governed by weld penetration into the base material, interpass temperature, and the absence of porosity or lack-of-fusion at the root of the first weld pass.
UT Assessment Specifics:
- Unbonded areas in weld overlay typically manifest as lack-of-fusion at the root interface or gas porosity between the first pass and the base metal
- UT scanning is performed on the clad surface after machining to final thickness; the signal from the clad-base interface is identified by time-of-flight
- Acceptance is commonly ≥99.5% bonding rate for high-integrity applications (e.g., heat exchanger tubesheets, nuclear components)
- Multi-pass weld overlays require assessment of interpass bonding as well as the final clad-base interface
Typical Defect Patterns:
| Defect Type | UT Signature | Likely Cause | Corrective Action |
|---|---|---|---|
| Root lack-of-fusion | High-amplitude interface reflection, loss of back-wall echo in affected zone | Inadequate heat input, improper torch angle, insufficient base surface preparation | Adjust WPS parameters; increase heat input; improve base preparation |
| Gas porosity at interface | Cluster of small-amplitude indications at interface time-of-flight | Contaminated shielding gas, wet flux, surface oxide not removed | Improve gas flow; dry consumables; grind and re-weld affected areas |
| Crack at clad-base interface | Sharp, high-amplitude linear indication | Thermal stress, HAZ cracking, incompatible metallurgy | Review WPS for thermal control; consider preheat; evaluate metallurgical compatibility |
7.2 Hydraulic Explosive Bonding
Hydraulic explosive bonding uses a shaped charge or propellant-driven explosive charge contained within a hydraulic housing to generate the collision velocity needed for metallurgical bonding between clad and base plates. The process is an alternative to free-air explosion welding, offering improved safety and control in urban or constrained environments.
UT Assessment Specifics:
- Bonding quality is determined by the formation of a jetted, wavy interface (jetting pattern) visible in cross-section; UT detects the presence or absence of this bonded interface
- Unbonded areas in hydraulic explosive bonding often appear as localized zones where collision velocity was below the critical threshold, resulting in no jetting and no metallurgical bond
- Edge and corner regions are particularly susceptible due to non-uniform pressure distribution in the hydraulic chamber
- Acceptance follows the same Grade I (≥99%) and Grade II (≥95%) framework, with additional scrutiny on edge bonded width
Key Process Variables Affecting Bonding:
| Variable | Effect on Bonding | Optimal Range (Typical) | UT Consequence if Out of Range |
|---|---|---|---|
| Collision angle | Controls jetting mechanism and interface wave formation | 5°–25° (material-dependent) | Too low: no bonding; Too high: spalling and fragmentation |
| Collision velocity | Determines whether critical jetting velocity is exceeded | 300–600 m/s (material-dependent) | Below critical: unbonded zones; UT detects as unbonded area |
| Surface cleanliness | Contaminants at collision interface prevent metallurgical bonding | Free of oil, grease, oxide; Ra ≤ 3.2 μm | Contaminated surfaces produce localized unbonded areas |
| Plate flatness | Non-parallel plates create variable collision angles across the surface | ≤ 0.5 mm/m (typical) | Wavy unbonded patterns following plate geometry deviations |
7.3 Explosion Welding (Free-Air)
Conventional explosion welding involves the detonation of a free-standing explosive charge positioned between the clad strip and base plate. The explosive gases accelerate the clad strip to high velocity, causing it to collide with the base plate and form a metallurgical bond through adiabatic shear jetting.
UT Assessment Specifics:
- Explosion welding typically produces the highest bonding rates among the three routes, with Grade I (≥99%) being achievable for most material combinations when process parameters are optimized
- Unbonded areas are more likely to occur at the leading edge (where the clad strip first contacts the base) and trailing edge (where collision geometry changes)
- The characteristic wavy jetted interface produces a distinctive UT signature that differs from weld overlay flat interfaces
- Large-format plates (up to 2000 mm width × 6000 mm length) require systematic grid scanning, often using automated UT rigs with robotic transducer positioning
Comparison of Bonding Performance Across Routes:
| Parameter | TIG/MIG Weld Overlay | Hydraulic Explosive Bonding | Explosion Welding |
|---|---|---|---|
| Typical Bonding Rate (Grade I) | ≥ 99.0% – 99.5% | ≥ 99.0% | ≥ 99.5% – 99.9% |
| Common Unbonded Zone Location | Weld root, between passes | Edges, corners, centerline | Leading/trailing edges, impact point vicinity |
| UT Scan Complexity | Moderate (flat interface, uniform thickness) | High (wavy interface, edge effects) | High (wavy interface, large format, edge effects) |
| Typical Clad Thickness Range | 1 – 20 mm | 2 – 50 mm | 2 – 100 mm |
| Typical Plate Size | Up to 3000 × 6000 mm | Up to 2000 × 6000 mm | Up to 2000 × 6000 mm (limited by charge size) |
| Grade I Achievability | Achievable with optimized WPS | Achievable with careful process control | Most consistently achievable |
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
Systematic unbonded/delamination assessment is integral to building a robust WPS/PQR (Welding Procedure Specification / Procedure Qualification Record) qualification portfolio. Each UT assessment provides:
- Evidence of process capability: Consistent Grade I results across multiple production lots demonstrate that the manufacturing process is stable and repeatable
- Defect database for root-cause analysis: Accumulated UT data enables statistical process control (SPC) of bonding quality, identifying trends and enabling predictive maintenance of equipment
- Third-party audit readiness: Maintained UT records, coverage maps, and defect logs provide the documentation required by ASME, API, or customer-specific quality audit programs
8.2 Product Delivery
For each production lot, the UT assessment serves as the gatekeeping inspection that determines whether product can be released for delivery:
- Release certification: A signed UT report with bonding rate, defect map, and grade classification is attached to the material test report (MTR) and accompanies the product to the customer
- Non-conformance management: When defects exceed acceptance limits, the UT assessment provides the precise location and extent of non-conformance, enabling targeted repair (grinding and re-welding, re-bonding, or trimming) rather than full-lot rejection
- Traceability: Unique identification of each plate with its UT data ensures that any field failure can be traced back to the manufacturing and inspection record
8.3 Customer Value
The unbonded/delamination assessment directly translates to customer value in several ways:
- Service life assurance: Unbonded areas are stress concentration sites that can initiate corrosion, fatigue cracking, or debonding under thermal cycling. By certifying ≥99% bonding, the manufacturer guarantees extended service life and reduced maintenance intervals
- Reduced risk of catastrophic failure: In pressure-containing equipment, interface delamination can lead to sudden loss of containment. Grade I bonding certification provides the confidence needed for safety-critical applications
- Regulatory compliance: Many industries (nuclear, aerospace, oil & gas) require documented UT assessment of clad interfaces as a condition of regulatory approval. Providing this certification removes a barrier to market access
- Cost optimization: By providing quantified bonding data, the manufacturer enables customers to make informed decisions about whether Grade I or Grade II product meets their application requirements, avoiding over-specification and unnecessary cost
9. Conclusion
Unbonded/delamination assessment by ultrasonic testing is not merely an inspection step—it is the definitive quality gate that validates the entire cladding manufacturing process. Whether the product is produced by TIG/MIG weld overlay, hydraulic explosive bonding, or explosion welding, the UT assessment provides the quantitative evidence that the clad-base interface has achieved the required metallurgical bond. Mastery of this capability, aligned with GB/T 8165 and ASTM A263 standards, positions Cladding Technology Shanxi Co., Ltd. to deliver certified, high-integrity composite materials that meet the most demanding customer specifications and regulatory requirements. The systematic application of this assessment across all three manufacturing routes creates a unified quality framework that supports qualification building, ensures reliable product delivery, and delivers measurable value to end-users in critical industrial applications.