Phased Array Ultrasonic Detection and Performance of Ni60AA Coating by Ultrasonic-Assisted Plasma Arc Cladding

Literature Overview

Published in 2022 in the Journal of Xuzhou University of Technology (Natural Science Edition), this study by Zhang Ning, Liu Xiaowen, Chu Jie, Shi Duanhu, and Song Wei from the School of Mechanical and Electrical Engineering at Xuzhou University of Technology investigates the application of phased array ultrasonic testing (PAUT) for defect detection in Ni60AA coatings produced by ultrasonic-assisted plasma arc cladding. Funded by the National Natural Science Foundation of China (52105403), the Jiangsu Provincial Natural Science Foundation (BK20200174), and the Jiangsu Provincial College Student Innovation and Entrepreneurship Training Program (xcx2022126), the research addresses the critical challenge of quality assurance in advanced cladding processes.

Core Technical Content

Ni60AA is a cobalt-chromium-tungsten-based hardfacing alloy renowned for its exceptional wear resistance, high-temperature strength, and corrosion resistance. The ultrasonic-assisted plasma arc cladding (UAPAC) process introduces high-frequency ultrasonic vibrations into the welding arc, which refines the microstructure, reduces dilution, and improves coating properties.

Parameter	Conventional PTA	Ultrasonic-Assisted PTA (UAPAC)	Improvement
Grain size	50–100 μm	20–50 μm	40–60% refinement
Dilution rate	15–25%	8–15%	30–40% reduction
Hardness (HV)	400–500	450–600	10–20% increase
Microcracking	Moderate	Minimal	Significant improvement
Coating thickness uniformity	±0.2 mm	±0.1 mm	Better control

The ultrasonic vibration mechanism operates through several pathways:

Acoustic cavitation: Microbubbles in the molten pool collapse, generating local high temperatures and pressures that refine the microstructure.
Stirring effect: Ultrasonic vibration stirs the molten pool, promoting homogeneous mixing and reducing microsegregation.
Grain refinement: Ultrasonic-induced nucleation sites increase the number of crystal nuclei, producing finer grains.
Stress relief: Vibration reduces solidification stresses and suppresses microcracking.

Technical Interpretation

The integration of PAUT with UAPAC represents a holistic approach to advanced cladding quality assurance. Conventional ultrasonic testing (UT) using single-element probes has limited capability to detect and characterize defects in thin cladding layers, particularly:

Delaminations at the substrate-coating interface
Microcracks within the coating layer
Porosity clusters
Inclusions from powder contamination

PAUT overcomes these limitations through:

Multi-element phased array: An array of 32–128 ultrasonic elements allows electronic beam steering and focusing, enabling detailed imaging of thin layers.
High-frequency transducers: 10–25 MHz transducers provide the resolution necessary to detect small defects in coatings 1–3 mm thick.
Advanced imaging modes: B-scan, C-scan, and S-scan modes provide comprehensive defect mapping.
Signal processing: Time-gain compensation and harmonic filtering enhance signal-to-noise ratio for thin-layer inspection.

Process and Standards Analysis

The relevant standards for ultrasonic testing of cladding layers include:

NB/T 47013-3: Non-destructive testing of pressure vessel components—Ultrasonic testing methods.
JB/T 4730.3: Non-destructive testing of pressure vessels—Ultrasonic testing.
ISO 17640: Non-destructive testing—General principles for the application of phased array techniques.
ASTM E2316: Standard practice for phased array contact ultrasonic examination of steel castings.
EN ISO 13588: Non-destructive testing—Ultrasonic testing—General principles.

Inspection Parameter	Recommended Value for Ni60AA Coating
Transducer frequency	10–20 MHz
Array configuration	64 elements, 1 mm pitch
Scan speed	10–30 mm/s
Lift-off	0.5–1.0 mm
Couplant	Water or glycerin
Acceptance criteria	No defects > 0.5 mm equivalent diameter

Integration with Engineering Practice

For practical implementation of UAPAC with PAUT inspection:

Process parameter optimization: The ultrasonic power (typically 500–2000 W), frequency (20–40 kHz), and coupling method (contact, immersion, or through-transmission) must be optimized for the specific substrate geometry and coating thickness.
Reference standard preparation: Calibration blocks with known defect sizes (flat-bottom holes, side-drilled holes) embedded in Ni60AA coatings are essential for establishing acceptance criteria.
Scanning strategy: Multi-angle scanning from different orientations provides comprehensive coverage. For curved surfaces, conformable phased array probes or water-immersion setups are recommended.
Data interpretation: Trained inspectors must distinguish between true defects and artifacts such as grain noise, geometric reflections, and surface roughness effects.
Documentation: All inspection results should be documented according to the applicable code requirements, including scan maps, defect locations, sizes, and disposition decisions.

Key Questions and Reflections

A significant challenge in PAUT inspection of thin cladding layers is the separation of back-wall echoes from defect indications. When the coating thickness is less than 2 mm, the pulse-echo method may produce overlapping echoes that obscure small defects. Engineers must consider:

Using pulse-echo through-transmission (PET) techniques for very thin coatings.
Employing time-of-flight diffraction (TOFD) for crack detection at the coating-substrate interface.
Combining PAUT with other NDT methods (MT, PT, or radiographic testing) for comprehensive inspection.

Another reflection: the ultrasonic-assisted process itself may introduce artifacts that complicate inspection. The vibration-induced microstructure refinement creates a more homogeneous material, which generally improves ultrasonic signal quality. However, the increased density of fine grain boundaries may increase acoustic scattering, requiring careful signal processing to maintain detection sensitivity.

Study Insights and Implications

This research demonstrates the synergy between advanced cladding processes and advanced inspection techniques. The ultrasonic-assisted plasma arc cladding process produces superior coatings, and phased array ultrasonic testing provides the means to verify and document their quality. Engineers should recognize that investing in both advanced process technology and advanced inspection capability is essential for reliable production of high-performance cladding layers. The integration of real-time PAUT monitoring during the cladding process could enable in-process quality control, allowing immediate detection and correction of defects before they propagate through subsequent layers.