Plasma Arc Remelting of Nickel-Based Thermal Spray Coatings

Literature Overview and Background

This study focuses on the plasma arc remelting (PAR) process applied to nickel-based thermal spray coatings, a technique widely used to improve coating integrity, reduce porosity, and enhance metallurgical bonding between the coating and the substrate. Nickel-based thermal spray coatings, typically applied by high-velocity oxygen-fuel (HVOF) or plasma spraying, often exhibit inherent porosity, oxide inclusions, and weak inter-splat bonding due to the rapid solidification during the spraying process. The literature under review systematically investigates the effect of plasma arc remelting parameters—such as plasma current, arc voltage, travel speed, and torch standoff distance—on the microstructure, mechanical properties, and corrosion resistance of the remelted coating.

Core Technical Findings

The study demonstrates that plasma arc remelting significantly reduces the porosity content from the typical 3–8% found in as-sprayed coatings to below 1%. This reduction is achieved through the complete melting and re-solidification of the coating surface, which eliminates the layered structure and oxide-rich inter-splat boundaries characteristic of thermal spray deposits. The remelting depth is typically controlled in the range of 0.5 to 2.0 mm, depending on the plasma current (80–200 A) and travel speed (100–400 mm/min). A deeper remelt zone improves homogeneity but increases the risk of substrate dilution and cracking, particularly when the base material has a different thermal expansion coefficient.

The microstructural evolution during remelting involves the dissolution of fine oxide particles and the formation of a columnar-to-equiaxed grain transition. At higher plasma currents, the remelted zone exhibits a more equiaxed dendritic structure with refined grain sizes, which contributes to improved toughness. The hardness profile shows a gradient from the remelted surface (typically 250–320 HV) to the as-sprayed substrate interface, with a slight softening zone near the heat-affected region due to grain growth.

Process Parameter Optimization

The study emphasizes that the optimal remelting parameters are highly dependent on the specific nickel-based alloy composition and the substrate material. For Inconel 625-based coatings on carbon steel substrates, a plasma current of 120–150 A with a travel speed of 200–300 mm/min provides the best balance between porosity elimination and substrate protection.

Engineering Practice Implications

From a practical standpoint, the plasma arc remelting process is particularly valuable for pressure vessel internals, heat exchanger tubes, and pump components where the as-sprayed coating porosity would compromise long-term corrosion resistance. However, the process introduces residual stresses that must be managed through post-remelting stress relief treatment, typically performed at 600–800°C for nickel-based alloys. The study also notes that multiple remelting passes may be required for thicker coatings (>1.5 mm), and each subsequent pass must be carefully controlled to avoid excessive substrate dilution.

Key Reflections and Study Insights

One of the most significant insights from this literature is the recognition that plasma arc remelting is not merely a surface treatment but a process that fundamentally alters the metallurgical nature of the coating. The transition from a thermally sprayed, oxide-laden microstructure to a fully remelted, homogeneous deposit represents a paradigm shift in how we think about the integrity of overlay coatings. In engineering practice, this means that the design specifications for remelted coatings should be distinguished from those of as-sprayed coatings, with different acceptance criteria for porosity, hardness, and bond strength.

The study also highlights an important practical consideration: the geometric complexity of the component can significantly affect remelting quality. Sharp corners, thin walls, and complex contours may require specialized torch designs or multi-axis positioning systems to ensure uniform remelting coverage. This has direct implications for the fabrication of pressure vessel components with complex geometries, where manual or robotic remelting operations must be carefully planned.

Summary

The plasma arc remelting of nickel-based thermal spray coatings represents a mature and effective technology for enhancing coating performance in demanding service environments. The key to successful application lies in the careful optimization of process parameters, thorough understanding of microstructural evolution, and appropriate post-processing to manage residual stresses. Engineers working in the field of pressure vessel fabrication and overlay welding should consider this technique as a viable alternative to traditional weld overlay methods, particularly when dealing with thin coatings or components where weld dilution is a concern.