Zhuojin Pipe Fitting Co., LtdMicrostructure and Mechanical Properties of Atmospheric Plasma Sprayed Fe-Based Coatings and Their TIG Remelting Layers
Literature Overview
This study by Dong Tianshun et al. (2019), published in Materials Reports (Vol. 33, No. 4, pp. 679–683), investigates the effect of TIG remelting on atmospheric plasma-sprayed (APS) Fe-based coatings. The authors address a well-recognized limitation of plasma-sprayed coatings: their inherently porous, layered microstructure with unmelted particles and inclusions. By applying a post-spraying TIG remelting treatment, the research demonstrates that the coating microstructure and mechanical properties can be substantially improved, transforming a mechanically bonded, porous deposit into a metallurgically bonded, dense layer.
Core Technical Findings
The key quantitative results reported in this paper are summarized below.
| Parameter | APS Coating (Before Remelting) | TIG Remelting Layer (After Remelting) | Improvement |
|---|---|---|---|
| Porosity | 4% | 0.4% | ~90% reduction |
| Microstructure | Lamellar, with pores, unmelted particles, inclusions | Dense, single-crystal regions and (Fe,Cr)₂₃C₆ phase | Fundamental transformation |
| Crystallinity | Poor, disordered atomic arrangement | Good, regular atomic arrangement | Significant improvement |
| Coating-substrate bonding | Mechanical bonding with visible gaps | Metallurgical bonding with "white bright band" | Fundamental improvement |
| Average microhardness | Baseline | +33.4% | Notable increase |
| Elastic modulus | Baseline | +53.2% | Significant increase |
| Surface roughness | Baseline | -43.2% | Roughly halved |
The APS coating itself consists of three distinct zones: a microcrystalline region, a nanocrystalline region, and a transition region. The remelting layer, by contrast, is composed of single-crystal areas and (Fe,Cr)₂₃C₆ intermetallic phase precipitates. Importantly, no microcracks were observed at the interface between the precipitate phase and the matrix, indicating good cohesion within the remelted microstructure.
Interpretation of Microstructural Mechanisms
The microstructural evolution observed in this study is consistent with the fundamental thermodynamic and kinetic principles governing solidification. During atmospheric plasma spraying, the rapid solidification of molten droplets results in a lamellar structure with trapped gas pores and partially melted particles. The crystallinity is inherently poor because the cooling rate is too high for orderly nucleation and growth.
When TIG remelting is applied, the heat input provides sufficient thermal energy to fully melt the deposited coating material. The subsequent controlled solidification allows for a more orderly crystal growth process. The formation of (Fe,Cr)₂₃C₆ carbide precipitates is thermodynamically favorable given the Fe-Cr composition of the coating, and the absence of interfacial microcracks suggests that the thermal expansion mismatch between the precipitate and matrix is within acceptable limits.
The transition from mechanical to metallurgical bonding at the coating-substrate interface is particularly significant. The "white bright band" observed at the interface is indicative of diffusion bonding, where atomic interdiffusion between the coating and substrate has occurred during the remelting process. This represents a fundamental improvement in joint integrity compared to the simple mechanical interlocking observed in the un-remelted coating.
Engineering Practice Implications
For engineers working in the surface engineering and corrosion protection fields, this study has several practical implications:
- Hybrid coating processes: The APS-plus-TIG-remelting combination offers a viable route to produce dense, well-bonded coatings that combine the deposition flexibility of plasma spraying with the metallurgical quality of conventional welding.
- Wear and corrosion resistance: The 33.4% increase in microhardness and 53.2% increase in elastic modulus suggest improved wear resistance. The elimination of pores and unmelted particles reduces potential corrosion initiation sites, which is critical for coatings applied in aggressive environments such as oil and gas pipelines or marine structures.
- Process parameters: The study implies that careful control of TIG remelting parameters (current, voltage, travel speed) is essential to avoid excessive melting of the substrate while achieving complete remelting of the coating. Overheating could lead to substrate dilution and potential cracking.
- Application to pipe and fitting repair: In the context of pipeline and fitting maintenance, this hybrid technique could be applied to restore worn or corroded surfaces on critical components such as valve bodies, flange faces, and pipe fittings without the need for complete component replacement.
Key Questions and Reflections
A critical question arises regarding the scale-up of this technique. While the laboratory results are compelling, the practical application of TIG remelting over large coating areas presents challenges related to thermal distortion, heat input management, and process repeatability. The study does not address the effects of multiple remelting passes on the microstructure, nor does it discuss the long-term durability of the remelted coating under cyclic loading or thermal fatigue conditions.
Furthermore, the formation of (Fe,Cr)₂₃C₆ carbide, while contributing to hardness, may introduce concerns regarding intergranular corrosion susceptibility in certain environments, particularly in chloride-containing solutions. Future work should investigate the corrosion behavior of the remelted coating in relevant service environments.
Study Insights and Implications
This study exemplifies the power of hybrid surface engineering approaches, where combining two established processes yields results superior to either process alone. The concept of using a low-energy post-treatment to transform a rapidly solidified deposit into a metallurgically sound layer has broad applicability beyond Fe-based coatings. Similar approaches could be explored for oxide ceramic coatings, Ni-based superalloy coatings, and other functionally graded surface systems used in high-performance engineering applications. The fundamental insight is that the quality of a surface coating is not solely determined by the deposition process but can be significantly enhanced by post-deposition thermal treatments that promote full melting and controlled solidification.