Plasma Cladding Dual Powder Feeding Preparation of Ni60A/WC Composite Coating Microstructure and Properties

Literature Overview

This research, conducted by researchers from Wuhan University of Technology and supported by the National Natural Science Foundation of China (Grant No. 51475346), investigates the microstructure and mechanical properties of Ni60A/WC composite coatings fabricated using plasma transferred arc (PTA) cladding with a dual powder feeding system. The study addresses the challenge of combining the excellent corrosion resistance and high-temperature oxidation resistance of Ni60A (a Ni-Cr-Mo alloy) with the exceptional hardness and wear resistance of tungsten carbide (WC) particles in a single overlay layer. The dual powder feeding approach allows independent control of the two powder streams, enabling precise adjustment of the WC content in the final coating composition.

Process Configuration and Parameters

The PTA cladding process employs a dual powder feeder system where Ni60A powder and WC powder are fed simultaneously but independently into the plasma arc. The plasma arc serves as the heat source, melting the substrate surface and the incoming powders to form a dilution-controlled overlay layer. The dual feeding configuration provides significant advantages over single-feed systems with pre-mixed powders, as it allows real-time adjustment of the WC content without changing the powder supply.

Process Parameters and Their Effects

The study systematically varies the WC content from 10 wt% to 40 wt% in the total powder blend, maintaining constant plasma parameters. This variation allows the establishment of a clear relationship between WC content and coating properties, which is essential for process optimization in different application scenarios.

Microstructural Characterization

The microstructure of the Ni60A/WC composite coating is characterized by three distinct phases: the Ni-Cr-Mo binder matrix, the WC carbide particles, and a reaction layer formed at the interface between WC and the binder matrix during the plasma melting process. The binder matrix consists primarily of austenite with some delta ferrite, as confirmed by X-ray diffraction analysis. The austenite structure is retained due to the high nickel content in Ni60A, which stabilizes the face-centered cubic phase even at room temperature.

The WC particles in the as-cladded coating exhibit varying degrees of thermal decomposition. At lower plasma current settings (180 to 200 A), a significant fraction of the WC particles remain intact, with minimal reaction with the binder matrix. At higher current settings (240 to 260 A), the WC particles undergo partial decomposition, forming W2C and Fe7W6 intermetallic phases at the particle-matrix interface. This decomposition is accompanied by the formation of a reaction layer with a thickness of 0.5 to 2 micrometers around each WC particle.

Hardness and Wear Performance vs. WC Content

The data reveals a clear trend where coating hardness increases with WC content up to 30 wt%, after which the hardness begins to decline. This decline at 40 wt% WC is attributed to insufficient binder wetting of the high volume of WC particles, leading to poor particle-matrix bonding and the formation of micro-voids at particle boundaries. The optimal WC content for maximum hardness and wear resistance is therefore identified as 30 wt%.

The dilution rate, which represents the fraction of substrate material melted and incorporated into the overlay, increases slightly with WC content. This is because the higher thermal conductivity of WC particles promotes heat transfer into the substrate, increasing the melt pool depth. However, the dilution rate remains within acceptable limits (below 20%) for all tested WC contents, indicating that the PTA process provides adequate dilution control for this composite system.

Phase Evolution and Reaction Mechanisms

The study provides detailed insight into the chemical reactions occurring between WC particles and the Ni-Cr-Mo melt during plasma cladding. The primary reaction is the decomposition of WC into tungsten and carbon, followed by the dissolution of tungsten into the Ni matrix and the formation of various tungsten carbide and intermetallic phases. The reaction can be summarized as:

WC + Ni → Ni3W + C (in solution)

2WC + 3Ni → Ni3W2 + 2C (in solution)

WC + 7Fe → Fe7W6 + C (in solution)

The extent of these reactions is governed by the plasma arc energy density, the residence time of the WC particles in the melt pool, and the cooling rate of the solidifying overlay. Higher energy density and longer residence time promote more complete WC decomposition, while rapid cooling preserves more of the original WC particle morphology.

The carbon released from WC decomposition plays a dual role in the coating microstructure. Some of the carbon dissolves into the Ni-Cr-Mo matrix, providing solid solution strengthening. Excess carbon, however, forms secondary carbides such as M7C3 and M23C6 (where M = Cr, Mo, W), which contribute to the overall hardness of the coating. The balance between dissolved carbon and secondary carbide formation is critical for achieving the desired combination of hardness and toughness.

Defect Analysis and Process Optimization

The study identifies several defects that can compromise coating quality. Porosity is the most common defect, occurring due to gas evolution from the decomposition of organic binders in the powder feedstock or from moisture absorption by the WC powder. The recommended countermeasure is to pre-dry the WC powder at 150 degrees C for 2 hours before use and to ensure the Ni60A powder is stored in a desiccant environment.

Cracking in the coating layer is primarily associated with high WC content and rapid cooling rates. The thermal expansion mismatch between the WC particles and the Ni matrix creates localized tensile stresses during cooling, which can initiate micro-cracks at the particle-matrix interface. The study recommends limiting the WC content to below 35 wt% and using a travel speed that provides adequate cooling rate without being excessive.

Delamination at the coating-substrate interface is another potential defect, particularly problematic when the substrate material has significantly different thermal properties from the coating. The study recommends preheating the substrate to 200 to 300 degrees C to reduce thermal gradients and improve metallurgical bonding. Post-weld stress relief at 400 to 500 degrees C for 1 hour is also recommended to reduce residual stresses that can compromise coating adhesion.

Engineering Practice Applications

The Ni60A/WC composite coating developed in this study is particularly suitable for applications requiring simultaneous corrosion resistance and wear resistance. Typical applications include pump impellers in chemical processing, valve seats in oil and gas production, and wear plates in mining equipment exposed to corrosive slurries. The PTA process provides excellent control over coating thickness (typically 1 to 3 mm per pass) and dilution, making it suitable for precision cladding of complex geometries.

For industrial implementation, the study recommends a multi-pass cladding strategy where the first pass uses pure Ni60A powder to establish a clean, dilution-controlled bonding layer, followed by subsequent passes with the Ni60A/WC blend to build up the wear-resistant composite layer. This approach ensures reliable bonding while achieving the desired composite microstructure in the functional layer.

Study Insights and Implications

This study provides valuable process-microstructure-property relationships for the PTA cladding of Ni60A/WC composite coatings, establishing clear guidelines for powder blend composition and process parameter selection. The identification of 30 wt% WC as the optimal reinforcement content, combined with specific plasma parameters, provides a reliable starting point for production implementation. The study also highlights the importance of understanding the chemical reactions between WC and the Ni matrix, as these reactions fundamentally determine the coating's microstructure and properties. For engineers designing composite overlay solutions for demanding service environments, the practical takeaway is that the dual powder feeding PTA process offers a flexible and effective approach to combining corrosion and wear resistance in a single overlay layer, with the process parameters providing the necessary control to tailor the coating to specific application requirements.