Ultrasonic-Assisted Plasma Arc Cladding Ni60AA Coating with PAUT Inspection and Performance Evaluation

Literature Overview

This study by Zhang Ning, Liu Xiaowen, Chu Jie, Shi Duanhu, and Song Wei (2022), published in the Journal of Xuzhou University of Technology (Natural Science Edition), examines the effects of ultrasonic vibration assistance on plasma arc cladding of a Ni60AA (Ni-Cr-Mo alloy) coating, with particular emphasis on phased array ultrasonic testing (PAUT) for defect detection and characterization of coating performance. The research was supported 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 work was conducted at the School of Mechanical and Electrical Engineering, Xuzhou University of Technology.

Core Technical Analysis

Ultrasonic vibration assistance in cladding introduces mechanical vibrations into the molten pool, which can refine grain structures, reduce porosity, and improve wetting between the cladding and substrate. For Ni60AA coatings, which are widely used in high-wear and high-temperature applications such as pump impellers, valve seats, and chemical processing equipment, achieving a dense, crack-free, and uniformly hard coating is critical. The ultrasonic frequency typically employed ranges from 15–25 kHz, with amplitude controlled to avoid disrupting the molten pool stability.

The ultrasonic vibration promotes equiaxed grain nucleation by breaking up columnar dendrites, resulting in a more isotropic mechanical response. The refined microstructure also improves the signal-to-noise ratio during PAUT inspection, as the reduced grain scattering allows for more accurate detection of internal defects such as lack of fusion, porosity clusters, and microcracks.

PAUT Inspection Methodology

Phased array ultrasonic testing offers significant advantages over conventional contact UT for cladding inspection, particularly in terms of imaging resolution and defect characterization. For Ni60AA cladding layers, which typically range from 0.5 to 3.0 mm in thickness, the selection of transducer frequency (typically 5–10 MHz), element count (16–64 elements), and scan geometry (linear or phased array) are critical. The high acoustic impedance mismatch between the Ni-based cladding and carbon steel substrate can create strong interface reflections that mask internal defects, making PAUT with focused beam steering essential.

Performance Characterization

The wear resistance of ultrasonic-assisted Ni60AA coatings is evaluated through dry sliding wear tests against alumina (Al2O3) or steel counterfaces. The improved hardness and reduced porosity contribute to a 20–40% reduction in wear volume compared to conventionally deposited coatings. Microstructural analysis reveals that the ultrasonic-assisted coatings exhibit a higher volume fraction of primary carbides (Cr7C3, Mo2C) distributed in a fine martensitic matrix, which provides excellent resistance to abrasive and adhesive wear.

Corrosion resistance testing in 3.5% NaCl solution demonstrates that the denser microstructure of ultrasonic-assisted coatings reduces the rate of pitting initiation by 30–50%, attributed to fewer porosity-related initiation sites and a more uniform carbide distribution that minimizes galvanic couples between carbides and matrix.

Engineering Practice Implications

The integration of ultrasonic assistance into plasma cladding processes represents a practical route to enhancing coating quality without requiring exotic consumables or complex equipment modifications. The ultrasonic transducer can be mounted directly on the torch body or the workpiece fixture, with minimal disruption to the existing production setup. For large-scale manufacturing of wear-resistant components such as pump casings, valve bodies, and mixing paddles, the improved defect detectability through PAUT enables more reliable quality assurance, reducing the need for destructive testing and rework.

However, practical implementation requires careful attention to the coupling between the ultrasonic transducer and the workpiece. Thermal cycling during multi-pass cladding can degrade the coupling medium, necessitating periodic reapplication or the use of a water-cooled transducer design. Additionally, the ultrasonic energy must be calibrated to avoid excessive vibration that could disrupt the plasma arc stability or cause workpiece displacement.

Summary and Study Insights

This study demonstrates that ultrasonic vibration assistance is a highly effective means of improving both the metallurgical quality and the inspectability of Ni60AA plasma cladding coatings. The combination of refined microstructure, reduced porosity, enhanced hardness, and improved PAUT signal quality makes this approach particularly attractive for critical applications where coating integrity is paramount. The research highlights the importance of process-structure-property relationships in cladding technology and provides a clear pathway for engineers seeking to optimize cladding quality through the introduction of mechanical energy assistance. Future development should focus on scaling this approach to industrial production environments and establishing standardized inspection protocols that account for the unique acoustic characteristics of ultrasonic-assisted cladding deposits.