Phased Array Ultrasonic Testing of Ultrasound-Assisted Plasma Arc Surfaced Ni60AA Coating

Literature Overview

The paper by Zhang Ning and colleagues from Xuzhou University of Technology, published in the Journal of Xuzhou University of Technology (Natural Science Edition) in 2022, investigates the application of phased array ultrasonic (PAUT) testing to Ni60AA coatings produced via ultrasound-assisted plasma arc surfacing. The study is supported by the National Natural Science Foundation of China (Grant 52105403) and the Jiangsu Provincial Natural Science Foundation (Grant BK20200174), indicating a well-funded and rigorous research program. The work addresses a practical gap in non-destructive testing (NDT) for thin overlay welds, where conventional TOFD (Time of Flight Diffraction) and straight-beam ultrasonic testing often struggle with adequate defect resolution in single-pass welds.

Core Technical Content

The study compares three PAUT scanning approaches for detecting defects in single-pass Ni60AA overlay welds: TOFD probe scanning, PA fan-beam scanning, and straight-probe linear electronic scanning. The research demonstrates that PA fan-beam scanning and straight-probe linear electronic scanning are superior to TOFD for single-pass weld inspection. This finding is significant because in overlay welding applications, the weld geometry is inherently asymmetric—the weld cap sits on one side of the base metal with no symmetric counterpart—making TOFD's reliance on diffraction from both weld boundaries problematic.

The ultrasound-assisted plasma arc surfacing process introduces ultrasonic vibration into the arc zone during welding. The key experimental variable is ultrasonic power, tested at increasing levels. The results reveal a clear trend: as ultrasonic power increases, the number and severity of welding defects decrease progressively. At lower power levels, large lack-of-fusion defects dominate, but as power increases, these transition to smaller porosity and inclusion defects. When ultrasonic power reaches 800 W, no significant defects are observed, and the weld quality is optimal.

Microstructure and Performance Analysis

The microstructural analysis reveals that ultrasound assistance refines the grain structure of the Ni60AA overlay. The ultrasonic vibration promotes nucleation and suppresses grain growth during solidification, resulting in a finer and more uniformly distributed microstructure. This microstructural refinement correlates directly with improved mechanical properties: both microhardness and wear resistance show substantial increases with higher ultrasonic power. The mechanism is well understood—ultrasound induces cavitation in the molten pool, which breaks up dendrites, promotes homogeneous temperature distribution, and eliminates porosity by collapsing gas bubbles.

The PAUT inspection demonstrated a high defect detection rate for the overlay layer, offering more intuitive and visual results compared to conventional ultrasonic testing. This is particularly valuable in engineering practice where rapid assessment of overlay quality is needed for critical components such as pump impellers, valve seats, and drill collars.

Engineering Practice Implications

For engineers working with Ni-based overlay welds, this study provides actionable guidance on process optimization. The ultrasonic-assisted approach requires specialized equipment—an ultrasonic transducer coupled to the arc region—which may not be readily available in all fabrication shops. However, the benefit of defect-free overlay welds at 800 W ultrasonic power justifies the capital investment for high-value components. The PAUT testing methodology described offers a practical alternative to destructive coupon testing for quality assurance, enabling in-situ inspection of as-welded overlays on production components.

One practical consideration is the calibration of PAUT equipment for thin overlay welds. The stand-off distance between the probe and the weld surface must be carefully controlled to avoid signal loss in the thin weld cap. The fan-beam approach allows electronic steering of the beam to cover the full weld cross-section without physical probe movement, which is advantageous for automated inspection systems.

Study Insights and Reflections

This research bridges materials science and NDT engineering in a way that is directly applicable to industrial practice. The finding that ultrasonic power of 800 W eliminates detectable defects provides a clear process window for operators. However, engineers should be cautious about extrapolating this result to thicker multi-pass overlays, where the ultrasonic energy may not penetrate the full weld depth uniformly. The study's focus on single-pass welds is both a strength and a limitation—it demonstrates the principle clearly but does not address the more complex multi-pass scenarios common in production. The combination of PAUT with ultrasound-assisted welding represents a promising pathway toward zero-defect overlay manufacturing, and further work on multi-pass applications and different Ni-based alloy compositions would extend the practical value of these findings.