Microstructure and Properties of Plasma Cladding Nickel-Based Alloy Powder

Literature Overview

This 2018 study by Xu Guojian and colleagues from Shenyang University of Technology and Nanjing Zhongke Yuchen Laser Technology Co., Ltd., published in the Journal of Shenyang University of Technology, investigates the microstructure and mechanical properties of a nickel-based alloy overlay deposited using plasma transferred arc (PTA) cladding technology. The research was supported by the Liaoning Provincial Science and Technology Innovation Major Special Project (2014371) and addresses the growing demand for high-performance surface coatings in power generation, petrochemical, and aerospace industries. PTA cladding has become a preferred technology for depositing nickel-based alloys due to its precise thermal control, high deposition efficiency, and excellent metallurgical bonding.

Core Technical Findings

The study examined a nickel-based alloy powder with a composition tailored for high-temperature oxidation resistance and hot corrosion resistance, typical of alloys used in gas turbine components and chemical processing equipment. The PTA process parameters were optimized through a series of experiments, and the resulting overlay was characterized through comprehensive metallographic, mechanical, and corrosion testing.

Microstructure Characterization

The microstructure of the PTA-cladded nickel-based alloy overlay was examined at multiple depths and positions across the deposit. Key observations included:

• The overlay exhibited a columnar dendritic microstructure near the fusion line, transitioning to an equiaxed structure toward the surface, with a columnar-to-equiaxed transition (CET) occurring at approximately 60% of the overlay thickness.
• The grain size in the overlay was in the range of 50–150 μm, significantly finer than the substrate grain size, indicating rapid solidification conditions during the PTA process.
• Precipitation of γ' (Ni₃(Al,Ti)) and γ'' (Ni₃Nb) phases was observed within the dendritic matrix, contributing to the high-temperature strength of the overlay.
• The dilution rate was measured to be approximately 5–10%, significantly lower than that achieved with conventional arc welding methods, indicating excellent process control and minimal substrate contamination.

Mechanical Properties

The mechanical properties of the PTA-cladded overlay were evaluated through microhardness testing, tensile testing, and high-temperature creep testing:

The post-weld heat treatment (PWHT) significantly improved the creep life by promoting the formation of coherent γ' precipitates and relieving residual stresses. The tensile strength increased slightly after PWHT, while the elongation improved substantially, indicating enhanced ductility and toughness.

Corrosion Resistance

The corrosion resistance of the overlay was evaluated through potentiodynamic polarization testing and salt spray testing:

• The corrosion potential of the as-deposited overlay was approximately -0.25 V vs. SCE, while after PWHT it shifted to -0.18 V vs. SCE, indicating improved nobility.
• The corrosion current density decreased from 2.5 × 10⁻⁶ A/cm² (as-deposited) to 1.2 × 10⁻⁶ A/cm² (after PWHT), demonstrating a significant improvement in corrosion resistance.
• The salt spray test (ASTM B117) showed no significant corrosion after 500 hours for the PWHT-treated overlay, compared to noticeable pitting on the as-deposited overlay after 300 hours.

Process Analysis and Optimization

PTA Process Parameters

The PTA process parameters were optimized through a systematic experimental approach, considering the following factors:

Quality Control Considerations

The study emphasized several quality control measures essential for producing high-quality PTA cladding:

1. Pre-weld cleaning: The substrate surface must be thoroughly cleaned to remove oxidation, scale, and contaminants that could lead to interfacial defects or reduced bond strength.
2. Powder quality control: The nickel-based alloy powder must meet strict specifications for particle size distribution, flowability, and chemical composition. Spherical powders with a size distribution of 45–150 μm are preferred for consistent melting and deposition.
3. Process monitoring: Real-time monitoring of arc current, arc voltage, and travel speed is essential to maintain consistent process conditions throughout the cladding operation.
4. Post-weld inspection: Non-destructive testing (NDT) using ultrasonic testing (UT) or magnetic particle inspection (MT) should be performed to detect any internal defects or interfacial cracks.
5. PWHT control: The PWHT cycle must be carefully controlled to achieve the desired precipitate distribution without causing grain growth or excessive carbon segregation.

Engineering Practice Implications

The findings of this study have direct implications for the design and fabrication of components requiring high-temperature corrosion resistance and oxidation resistance, such as:

• Gas turbine hot section components (combustor liners, turbine blades, and ducts)
• Petrochemical reactor linings and heat exchanger tubes
• Aerospace engine components and exhaust systems
• Nuclear reactor internal components

For engineers involved in the application of PTA cladding technology, the following practical considerations are important:

• The low dilution rate of PTA cladding (5–10%) is a significant advantage over conventional welding methods, as it preserves the integrity of the substrate and minimizes the risk of interfacial cracking.
• The PWHT treatment is not optional but essential for achieving the full performance potential of the nickel-based alloy overlay, particularly in terms of creep resistance and corrosion resistance.
• The columnar-to-equiaxed transition observed in the overlay microstructure is beneficial, as it reduces the risk of intergranular cracking and improves the transverse mechanical properties.
• The overlay thickness should be carefully controlled to balance performance requirements with economic considerations; excessive thickness increases cost without providing proportional performance benefits.

Key Questions and Reflections

The study raises several important questions for future research and engineering practice. First, the long-term performance of the PTA-cladded overlay under cyclic thermal and mechanical loading conditions, such as those experienced in gas turbine engines, requires further investigation through accelerated life testing. Second, the effect of multi-layer PTA cladding on the microstructure and properties of the overlay, particularly at the interlayer interfaces, should be systematically studied, as most practical applications require multiple layers to achieve the desired thickness. Third, the development of advanced monitoring and control systems for PTA cladding processes, capable of real-time adjustment of process parameters based on feedback from the deposition zone, could significantly improve the consistency and quality of the overlay.

Study Insights and Implications

The work by Xu et al. provides a comprehensive characterization of PTA-cladded nickel-based alloy overlays and establishes clear guidelines for process optimization and quality control. The systematic investigation of microstructure, mechanical properties, and corrosion resistance provides engineers with a solid foundation for the rational design of PTA cladding processes. The emphasis on PWHT as a critical step in achieving optimal performance is particularly noteworthy, as it underscores the importance of post-processing in surface engineering. For industries relying on nickel-based alloy coatings for high-temperature and corrosive environments, this study reinforces the value of PTA cladding as a reliable and controllable technology for achieving superior surface properties. The findings also highlight the need for application-specific process development and rigorous quality assurance to ensure the reliability and durability of PTA-cladded components in demanding service conditions.