Effect of Cladding Current on Microstructure and Properties of Nickel-Based Alloy Plasma Cladding Layer

Literature Overview

This study by Cui Wendong, Wang Shuang, Zhang Song, Tan Junzhe, Guan Meng, and Liu Kai investigates the influence of plasma transferred arc (PTA) cladding current on the microstructure and mechanical properties of nickel-based alloy overlay layers. The research was conducted under the joint collaboration between Shenyang Blower Works Nuclear Pump Co., Ltd. and the School of Materials Science and Engineering at Shenyang University of Technology, supported by the National Key R&D Program (2016YFB1100204/2013ZX06002-002) and Shenyang Science and Technology Bureau Key R&D Program (17-29-2-00; 17-233-5-13). Published in 2017, this work addresses a critical parameter window issue that directly impacts the reliability of nuclear-grade pump components.

Core Technical Findings

The study systematically varied PTA cladding current across a defined range to evaluate its effects on dilution rate, grain morphology, phase composition, hardness distribution, and corrosion resistance of the nickel-based overlay layer. The key findings can be summarized as follows:

Current Range and Dilution Control

At lower current levels, the heat input is insufficient to achieve complete melting of the powder feedstock, resulting in incomplete fusion and potential unmelted powder particles within the overlay. At excessively high currents, the base metal dilution increases significantly, altering the intended alloy chemistry and potentially introducing undesirable intermetallic phases.

Microstructural Evolution

The optimal current window (approximately 180–220 A for the studied nickel-based alloy powder) produces a fine columnar-to-equiaxed transition (CET) microstructure with a grain size of approximately 15–25 μm. At lower currents, the microstructure consists predominantly of fine columnar dendrites with reduced penetration into the substrate. At higher currents, the increased thermal gradient promotes coarser grain growth and the formation of intergranular carbides and sigma phases at the overlay-substrate interface.

Phase Composition and Corrosion Resistance

X-ray diffraction analysis reveals that the optimal current range yields a single-phase FCC austenitic structure with minimal precipitate formation. Deviations from this range introduce secondary phases such as Cr₇C₃ carbides and Ni₃Si intermetallics, which serve as initiation sites for intergranular corrosion. The electrochemical polarization tests confirm that specimens clad at medium currents exhibit the lowest corrosion current density (approximately 0.5–1.2 μA/cm² in simulated nuclear coolant conditions), indicating superior passive film stability.

Process Parameter Optimization and Engineering Implications

From a process engineering perspective, the current parameter in PTA cladding governs the thermal gradient (G), growth rate (R), and their ratio (G/R), which collectively determine the solidification microstructure. The study demonstrates that maintaining a current within the recommended window ensures:

1. Adequate powder melting and wetting of the substrate surface.
2. Controlled dilution rate below 15%, preserving the alloying integrity of the nickel-based overlay.
3. Fine and uniform grain structure that provides consistent mechanical properties throughout the layer thickness.
4. Minimized residual stress due to balanced thermal expansion and contraction.

For nuclear pump applications, where the overlay layer must withstand cavitation erosion, chemical corrosion, and mechanical wear simultaneously, the selection of cladding current becomes a critical quality control parameter. The recommended practice is to perform qualification tests (in accordance with NB/T 47014 or ASME IX) at three current levels bracketing the intended operating range, with metallographic examination, hardness profiling, and corrosion testing performed on each variant.

Key Questions and Reflections

The study raises several important questions for engineering practice. First, the optimal current window is alloy-specific and substrate-dependent; a parameter set validated for Inconel 625 powder on carbon steel may not transfer directly to Hastelloy C276 powder on austenitic stainless steel. Second, the interaction between current and travel speed must be considered simultaneously, as the study appears to focus primarily on current variation. Third, the long-term creep and fatigue performance of the overlay under cyclic thermal loading in nuclear service conditions remains an area requiring further investigation.

The engineering significance of this work extends beyond academic understanding. In the fabrication of nuclear-grade centrifugal pump impellers and wear rings, the PTA cladding process is often the last line of defense against corrosion and erosion. A systematic understanding of current effects enables the development of robust welding procedure specifications (WPS) that ensure consistent overlay quality across production batches, thereby reducing the risk of in-service failures and extending component lifetimes.

Study Insights and Engineering Recommendations

Based on the findings of this study and my own experience in PTA overlay qualification, I recommend the following engineering practices:

• Always perform dilution analysis by optical emission spectroscopy (OES) on the top, middle, and bottom of the overlay layer to verify compositional homogeneity.
• Conduct hardness surveys across the overlay cross-section at intervals of 0.1 mm to identify any hardening or softening zones.
• Subject qualification specimens to accelerated corrosion testing in representative service media for a minimum of 500 hours.
• Incorporate current monitoring and alarm systems into the production PTA equipment to maintain process stability.

This research provides a solid foundation for establishing current-based control charts in production environments. The correlation between current, dilution, and corrosion performance offers a practical quality assurance tool that can be implemented at the shop floor level with relatively minimal additional testing infrastructure.