Microstructure and Properties of Ni60A/WC Composite Coating Prepared by Plasma Surfacing with Dual Powder Feeding

Literature Overview

Published in Metal Heat Treatment (2025, Vol. 50, No. 10, pp. 271-275), this study by Ma Zhaoyang, Wang Xuanguo, Jiang Xinyu, Xie Bing, and Wang Huajun from the School of Materials Science and Engineering at Wuhan University of Technology investigates a novel dual powder feeding approach for plasma surfacing of Ni60A/WC composite coatings. Funded by the National Natural Science Foundation of China (Grant No. 51475346), the research addresses a well-recognized limitation in conventional single-feed plasma surfacing: the inability to independently control the distribution and volume fraction of WC particles in the composite coating. The study is highly relevant to engineers in the tool and die industry, where H13 hot work steel components require hard, wear-resistant, and thermally stable surface layers.

Core Technical Findings

Dual Feed Configuration

The authors developed a system that combines an external powder feeding channel with the built-in powder delivery of a standard plasma surfacing torch. This dual-feed arrangement allows the operator to independently adjust the Ni60A alloy powder flow rate (delivered internally) and the WC particle flow rate (delivered externally). This separation provides unprecedented control over the WC volume fraction in the final coating, which is the primary determinant of coating hardness and wear resistance.

WC Distribution Quality

A critical finding is that when the external WC feed rate exceeds 6 g/min, the WC particles achieve uniform distribution throughout the coating cross-section. Below this threshold, agglomeration and segregation of WC particles are observed, particularly near the fusion line. This threshold value provides a clear process parameter guideline for production settings.

Phase Composition

XRD analysis reveals that both dual-feed and conventional single-feed coatings contain the same phases: γ-(Fe,Ni) solid solution matrix, Cr₇C₃, Cr₂₃C₆, Cr₃C₂, and Cr₄Ni₁₅W. The absence of unreacted WC in the XRD patterns suggests complete dissolution of WC particles during the plasma surfacing process. However, the dual-feed method produces a higher volume fraction of these carbide phases due to the greater WC input, which directly correlates with the observed hardness increase.

Hardness and Thermal Fatigue Performance

The dual-feed coatings exhibit higher hardness than single-feed coatings, with hardness increasing monotonically with external WC feed rate. However, thermal fatigue testing reveals a trade-off: as WC feed rate increases, the number and density of thermal fatigue cracks also increase. Cracks concentrate at the fusion line and around WC particle sites, indicating that the thermal mismatch between the hard WC-rich regions and the softer matrix creates stress concentrations that promote crack initiation.

Optimal Parameter Selection

The authors identify an external WC feed rate of 8 g/min as the optimum, providing the best overall balance between hardness, wear resistance, and thermal fatigue resistance. At this rate, the coating achieves high hardness while maintaining acceptable crack density.

Process Parameter Comparison

Engineering Practice Integration

The dual-feed plasma surfacing approach has significant implications for the production of hot work tool coatings. The following practical considerations should be incorporated into engineering workflows:

1. Equipment modification: Existing plasma surfacing systems can be retrofitted with an external powder feeder, which is a relatively low-cost modification. The external feeder should be positioned to introduce WC particles into the molten pool at the trailing edge of the torch, ensuring adequate mixing.
2. Process monitoring: In-line monitoring of powder feed rates is essential to maintain the WC feed rate at or above the 6 g/min threshold. Flow meters with closed-loop control should be employed.
3. Post-weld inspection: Thermal fatigue testing should be performed on production samples to verify crack density. Non-destructive testing methods such as ultrasonic testing or magnetic particle inspection can detect surface and near-surface cracks.
4. Application-specific optimization: For components subjected to severe thermal cycling, the 8 g/min WC feed rate should be used as a starting point, with further optimization based on the specific thermal loading conditions. For components with primarily mechanical wear requirements, higher WC feed rates may be acceptable if thermal fatigue is not a concern.

Key Questions and Reflections

The study does not address the long-term wear performance of the dual-feed coatings under actual hot work conditions, such as in die casting or forging applications. Tribological testing under elevated temperatures would provide more definitive evidence of the coating's practical advantages. Additionally, the study does not examine the effect of multiple coating passes on the thermal fatigue crack density, which is important for thick coatings required in heavy-duty applications.

Another reflection is that the crack formation mechanism around WC particles is likely related to the coefficient of thermal expansion mismatch between WC (approximately 5.6 × 10⁻⁶ /°C) and the Ni60A matrix (approximately 13 × 10⁻⁶ /°C). This mismatch generates residual stresses during cooling, which act as crack driving forces during thermal cycling. Future research could explore the use of graded WC distribution or the addition of ductile phases to mitigate this issue.

Study Insights and Implications

This research demonstrates that the dual-feed plasma surfacing technique offers a practical and effective solution for producing high-performance Ni60A/WC composite coatings. The ability to independently control WC feed rate provides a powerful tool for tailoring coating properties to specific application requirements. The identification of the 8 g/min optimal WC feed rate provides a clear process parameter for production implementation. For engineers in the tool and die industry, this approach represents a significant advancement over conventional single-feed methods, offering improved hardness and wear resistance at the cost of slightly increased thermal fatigue crack density, which is a manageable trade-off for most hot work applications.