Microstructural Evolution and Mechanical Properties of FeCoNiAlTi High-Entropy Alloy Plasma Arc Cladding Coatings

Literature Overview and Research Significance

This paper, published in Materials Protection (2025, Vol. 58, No. 4), investigates the effect of plasma arc cladding current on the microstructure and mechanical properties of FeCoNiAlTi high-entropy alloy (HEA) coatings. Funded by a national key R&D program project, the study was conducted by researchers at Liaoning University of Science and Technology. High-entropy alloys represent a frontier in materials science, characterized by their unique combination of high entropy stabilization, sluggish diffusion, severe lattice distortion, and cocktail effects. The application of HEA coatings to engineering components through plasma arc cladding is a rapidly developing area with significant potential for improving surface performance in demanding environments.

Core Technical Content

Effect of Welding Current on Microstructure

The study systematically investigates five welding current levels (140, 160, 180, 200, and 220 A) and their effects on coating properties:

The key findings are:

1. Increasing welding current significantly improves porosity and elemental segregation.
2. The phase composition remains relatively unchanged across all current levels, consisting primarily of FCC solid solution phase and Co3Ti precipitate phase.
3. The FCC solid solution phase is the dominant matrix phase, characteristic of high-entropy alloys with equiatomic or near-equiatomic composition.

Hardness and Wear Resistance

The hardness and wear resistance data reveal an optimal current level:

The coating at 180 A (designated T3) achieves:

• Surface hardness of 322.77 HV0.2, approximately twice the substrate hardness.
• Maximum wear depth of only 2.144 μm.
• Average friction coefficient of 0.362.
• Minimum wear rate of 3.83×10⁻⁶ mm³/(N·m).

Tensile Properties

The tensile properties of the T3 coating (180 A) are particularly noteworthy:

The combination of high tensile strength (948 MPa) and good ductility (26.61% elongation) is exceptional for a plasma arc cladding coating. This indicates that the coating maintains good metallurgical bonding with the substrate and possesses adequate strain capacity to accommodate deformation without cracking.

Microstructural Mechanisms

The optimal performance at 180 A can be attributed to several microstructural mechanisms:

1. Optimal grain refinement: The welding current of 180 A provides sufficient thermal input to promote complete melting and good fluidity, while not being excessive enough to cause grain coarsening.
2. Reduced porosity: Lower porosity at 180 A compared to lower currents reduces stress concentration sites and improves coating integrity.
3. Homogeneous elemental distribution: Reduced elemental segregation at 180 A leads to more uniform mechanical properties and reduced risk of local corrosion or cracking.
4. Adequate Co3Ti precipitation: The Co3Ti precipitate phase provides precipitation strengthening without excessive embrittlement.

Engineering Practice Implications

Process Parameter Selection for HEA Cladding

Based on the study findings, the recommended process parameters for plasma arc cladding of FeCoNiAlTi HEA coatings are:

Application Potential

FeCoNiAlTi HEA coatings are suitable for applications requiring:

1. High wear resistance in abrasive environments.
2. Good mechanical integrity under cyclic loading.
3. High-temperature stability (HEAs generally exhibit good high-temperature strength).
4. Corrosion resistance (the presence of Ni and Co provides good corrosion resistance).

Potential applications include:

• Turbine blade coatings for aerospace engines.
• Wear-resistant coatings for mining equipment.
• Corrosion-resistant coatings for chemical processing equipment.
• High-temperature components in power generation.

Quality Control Considerations

For plasma arc cladding of HEA coatings, the following quality control measures are recommended:

1. Porosity inspection: Visual and radiographic examination to ensure porosity is within acceptable limits.
2. Hardness mapping: Hardness testing across the coating thickness to ensure uniform properties.
3. Tensile testing: Cross-sectional tensile testing to verify coating-substrate bonding strength.
4. Wear testing: Pin-on-disc or block-on-ring wear testing to verify wear resistance.
5. Microstructural examination: Metallographic examination to confirm FCC matrix with Co3Ti precipitates.

Key Questions and Reflections

1. The study focuses on single-pass cladding. The effect of multi-pass cladding on microstructure evolution, residual stress, and mechanical properties requires further investigation.
2. The long-term stability of the coating under thermal cycling and fatigue loading is not addressed. Accelerated thermal cycling and fatigue tests would provide valuable data for engineering applications.
3. The effect of substrate preparation (surface roughness, cleaning, preheating) on coating quality and properties should be systematically investigated.
4. The cost-effectiveness of HEA coatings compared to conventional coatings (e.g., Stellite, Ni-based alloys) should be evaluated for specific applications.

Study Insights and Summary

This study provides a comprehensive investigation of the effect of welding current on the microstructure and mechanical properties of FeCoNiAlTi high-entropy alloy plasma arc cladding coatings. The identification of 180 A as the optimal welding current is supported by comprehensive data on porosity, elemental segregation, phase composition, hardness, wear resistance, and tensile properties. The exceptional combination of high hardness (322.77 HV0.2), low wear rate (3.83×10⁻⁶ mm³/(N·m)), high tensile strength (948 MPa), and good ductility (26.61% elongation) at 180 A demonstrates the potential of HEA coatings for demanding engineering applications. The consistent phase composition (FCC + Co3Ti) across all current levels indicates that the HEA coating properties are primarily governed by microstructural features (grain size, porosity, elemental distribution) rather than phase composition changes. For engineers, the key takeaway is that plasma arc cladding of FeCoNiAlTi HEA coatings at 180 A provides excellent surface performance, but careful process control and quality assurance are essential to achieve the optimal results demonstrated in this study.