Plasma Cladding of FeCoNiAlTi High Entropy Alloy Coating Microstructure Evolution and Mechanical Properties

Literature Overview

This study published in Materials Protection (2025, Vol. 58, Issue 4, pp. 56-69) by Wang Yonghong, Zhang Chunlin, Zhang Shihan, Yu Jianping, Liu Yingfu, and Xie Zhiwen from Liaoning University of Science and Technology investigates the effect of plasma cladding current on the microstructure and mechanical properties of FeCoNiAlTi high-entropy alloy (HEA) coatings. Supported by the National Key R&D Program (2021YFB3702003), the research systematically evaluates five different cladding current levels (140, 160, 180, 200, and 220 A) and their influence on porosity, elemental segregation, phase composition, hardness, wear resistance, and tensile properties of the coatings.

Core Technical Findings

Effect of Cladding Current on Microstructure

The cladding current is identified as the primary process parameter governing the quality of the plasma-cladded HEA coating. As the cladding current increases from 140 A to 220 A, both the porosity and elemental segregation in the coating are significantly improved. This improvement is attributed to the increased energy input at higher currents, which promotes more complete melting of the powder feedstock and better fluidity of the molten pool, thereby reducing gas entrapment and promoting more homogeneous mixing of the alloying elements.

Notably, the phase composition remains consistent across all current levels, consisting of the typical FCC solid solution phase and Co3Ti precipitate phase. This indicates that the current level primarily affects the microstructural quality (porosity and segregation) rather than the fundamental phase constitution of the coating.

Mechanical Performance at Optimal Current

The T3 coating deposited at 180 A demonstrates the best overall mechanical performance:

• Surface microhardness: 322.77 HV0.2, approximately twice the base material hardness.
• Wear resistance: Maximum wear depth of only 2.144 μm, average friction coefficient of 0.362, and minimum wear rate of 3.83×10^-6 mm³/(N·m).
• Tensile properties: Tensile strength of 948 MPa and elongation of 26.61%, demonstrating excellent strength-ductility combination.

The combination of high hardness, excellent wear resistance, and good tensile ductility at 180 A makes this condition particularly attractive for engineering applications requiring both wear protection and structural integrity.

Hardness and Wear Performance Relationship

The hardness maximum at 180 A coincides with the best wear resistance, which is consistent with the general correlation between hardness and abrasive wear resistance. However, the wear performance is also influenced by the microstructure quality, including porosity and phase distribution. The improved porosity and reduced segregation at 180 A contribute to a more uniform microstructure that resists crack initiation and propagation during sliding wear.

High Entropy Alloy Coating Technology Analysis

Plasma Arc Cladding Process Considerations

Plasma arc cladding is a well-established technique for depositing alloy coatings on steel substrates. The process involves the use of a plasma torch to create a high-temperature plasma arc that melts both the substrate surface and the powder feedstock, creating a metallurgically bonded coating. Key process parameters include:

• Plasma arc current: Controls the energy input and melt pool size.
• Powder feed rate: Determines the deposition rate and dilution.
• Travel speed: Affects the thermal cycle and cooling rate.
• Shielding gas flow: Protects the molten pool from atmospheric contamination.

The study's finding that 180 A provides the optimal balance of microstructural quality and mechanical performance establishes a clear process window for FeCoNiAlTi HEA coating deposition.

Microstructural Interpretation

The FCC solid solution phase is the dominant phase in the FeCoNiAlTi HEA coating, which is consistent with the high-entropy alloy design philosophy of stabilizing a single solid solution phase through the high configurational entropy effect. The Co3Ti precipitate phase forms as a secondary phase due to the strong Co-Ti bonding affinity, which provides additional strengthening through precipitation hardening.

The progressive improvement in porosity with increasing current is explained by the increased melt pool energy, which provides sufficient thermal energy to expel entrapped gases and promote complete powder melting. The reduction in elemental segregation at higher currents is attributed to the enhanced convective mixing in the larger, more energetic melt pool.

Engineering Practice Implications

Application Potential

FeCoNiAlTi HEA coatings offer several advantages for engineering applications:

• The combination of high strength (948 MPa) and good ductility (26.61%) makes these coatings suitable for structural components subjected to both wear and mechanical loading.
• The excellent wear resistance (minimum wear rate of 3.83×10^-6 mm³/(N·m)) is particularly valuable for tribological applications such as valve seats, pump components, and pipeline internal surfaces.
• The high hardness (322.77 HV0.2, approximately 2× base material) provides effective protection against abrasive and adhesive wear.

Process Development Recommendations

• For production implementation, the plasma cladding current should be set at 180 A with appropriate powder feed rate and travel speed parameters to maintain the optimal process window.
• Quality control should include porosity assessment via metallographic examination or ultrasonic testing, hardness profiling, and periodic wear testing.
• The coating thickness should be optimized based on the expected service life and wear rate calculations, with allowance for periodic recoating.

Key Questions and Reflections

The study demonstrates that 180 A provides optimal performance, but the sensitivity of the process to small variations in current (e.g., ±10 A) is not fully characterized. In production environments, maintaining precise current control is essential to ensure consistent coating quality. Additionally, the study focuses on single-layer coatings, but multi-layer deposition may introduce different thermal cycling effects that could alter the microstructure and properties.

The tensile properties reported (948 MPa and 26.61% elongation) are measured on the coating-base metal composite, which is an important metric for evaluating the structural integrity of the coated component. However, the interfacial bond strength between the coating and substrate should also be evaluated through adhesion testing, as coating delamination is a common failure mode in plasma-cladded components.

Study Insights and Implications

This study provides a systematic investigation of the plasma cladding current effect on FeCoNiAlTi HEA coating performance, establishing 180 A as the optimal parameter for achieving the best combination of microstructural quality, hardness, wear resistance, and tensile properties. The consistent FCC + Co3Ti phase composition across all current levels indicates that the HEA design is robust and not sensitive to moderate variations in process parameters. The excellent mechanical performance, particularly the combination of high strength and good ductility, positions FeCoNiAlTi HEA coatings as a promising material system for demanding engineering applications in the oil and gas, aerospace, and heavy equipment industries. The work contributes to the growing body of knowledge on HEA coatings and provides practical guidance for process development and quality control in plasma arc cladding operations.