Ultrasonic-Assisted Plasma Arc Overlay Welding of Ni60AA Coatings: PAUT Inspection and Performance Study

Literature Overview

Published in the Journal of Xuzhou University of Engineering (Natural Science Edition) in 2022, this paper by researchers from Xuzhou University of Engineering investigates the application of ultrasonic vibration assistance during plasma arc overlay welding of Ni60AA coatings. The study combines advanced inspection technology (phased array ultrasonic testing) with metallurgical analysis and tribological characterization to evaluate the effects of ultrasonic power on weld quality and coating performance. 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), this work represents a multidisciplinary approach to improving overlay welding quality.

Technical Background and Motivation

Ni60AA is a widely used nickel-based overlay alloy for applications requiring high wear resistance, corrosion resistance, and thermal stability. Plasma arc overlay welding (PAOW) is the preferred process for depositing Ni60AA coatings due to its high deposition rate, good dilution control, and ability to produce dense, well-bonded coatings. However, conventional PAOW often produces defects such as lack of fusion, porosity, and inclusions, which compromise coating performance. The introduction of ultrasonic vibration during welding is a well-established technique in solid-state joining, but its application to plasma arc overlay welding remains relatively limited.

Experimental Methodology

Ultrasonic-Assisted Plasma Arc Overlay Welding

The experimental setup involved coupling an ultrasonic transducer to the welding workpiece, generating controlled vibrations during the plasma arc welding process. The ultrasonic power was varied systematically to study its influence on weld quality.

Phased Array Ultrasonic Testing

Three PAUT scanning modes were evaluated:

Microstructural and Tribological Characterization

The study included comprehensive microstructural analysis (optical microscopy, SEM), microhardness mapping, and wear testing to correlate weld quality with coating performance.

Key Findings

Defect Evolution with Ultrasonic Power

The most significant finding is the systematic improvement in weld quality with increasing ultrasonic power:

At 800 W ultrasonic power, no significant defects were detected, representing the optimal welding condition. The transition from large lack-of-fusion defects to small pore and inclusion defects with increasing ultrasonic power is attributed to the enhanced fluid flow in the molten pool, which promotes better wetting and fusion, and the acoustic cavitation effect, which helps to break up and remove large gas bubbles and inclusions.

Microstructural Characteristics

The ultrasonic-assisted overlay layers exhibited:

• Finer grain structure compared to conventional PAOW
• More uniform distribution of hard phases (carbides)
• Reduced segregation of alloying elements
• Better metallurgical bonding with the substrate

Hardness and Wear Performance

The microhardness and wear resistance improved significantly with ultrasonic assistance:

• Hardness increased due to grain refinement and more uniform carbide distribution
• Wear resistance improved substantially, correlating with defect reduction
• The 800 W condition showed the best combination of hardness and wear resistance

PAUT Inspection Methodology Insights

The comparative evaluation of PAUT scanning modes provides valuable guidance for overlay weld inspection:

1. Straight probe linear electronic scan was found to be most suitable for single-pass overlay welds, providing the best sensitivity and defect characterization capability.
2. PA sector scan offered good coverage but required more careful setup and interpretation.
3. TOFD was less effective for thin overlay layers due to the limited thickness range and the complexity of interpreting diffraction signals from thin layers.

The study demonstrates that PAUT provides significantly more intuitive and informative inspection results compared to conventional ultrasonic testing, with better defect sizing and characterization capabilities.

Engineering Practice Integration

For engineers implementing ultrasonic-assisted overlay welding in production, several practical considerations emerge:

• Process window optimization: The optimal ultrasonic power of 800 W identified in this study should be verified for each specific application, as the optimal power depends on substrate thickness, overlay layer thickness, and welding parameters.
• Equipment integration: The ultrasonic system must be properly integrated with the plasma arc welding equipment, with careful attention to electrical isolation and mechanical coupling.
• Inspection qualification: The PAUT procedure should be qualified according to relevant standards (such as ISO 13588 or ASME V) before use in production inspection.
• Parameter monitoring: Real-time monitoring of ultrasonic power and welding parameters is essential to maintain consistent quality.

Study Insights and Implications

This research demonstrates the transformative potential of ultrasonic assistance in plasma arc overlay welding. The ability to virtually eliminate weld defects through ultrasonic vibration assistance opens new possibilities for producing high-quality Ni60AA coatings for demanding applications in power generation, oil and gas, and mining industries. The integration of PAUT inspection provides a reliable quality assurance tool for verifying the integrity of ultrasonic-assisted overlay welds. However, further research is needed to understand the long-term performance of ultrasonic-assisted coatings under actual service conditions, particularly regarding thermal cycling resistance and corrosion fatigue behavior. The findings also suggest that ultrasonic assistance could be beneficial for other overlay welding processes and materials, warranting broader investigation.