Microstructure and Wear Resistance of Nickel-Based and Cobalt-Based Alloy Overlay Layers Deposited by Plasma Transferred Arc

Literature Overview and Research Background

This 2018 study by Wei Ying, Wei Xianshun, Liang Dandan, Jiang Haoran, Dong Yue, and Shen Jun from the School of Materials Science and Engineering, Tongji University, published in Hot Working Technology, investigates the microstructure and wear performance of nickel-based and cobalt-based alloy overlay layers deposited by plasma transferred arc (PTA) welding. The research was supported by the National Natural Science Foundation of China (grants 51601129, 51775386) and the Shanghai Pujiang Talent Program (grant 16PJ1410000), indicating the significance and rigor of the investigation.

The study addresses a critical need in high-performance surface engineering: the development of overlay layers capable of withstanding extreme wear, corrosion, and temperature conditions encountered in aerospace, power generation, and chemical processing applications. Nickel-based alloys (such as Inconel 625, Stellite 6) and cobalt-based alloys (such as Stellite 6, Colmonoy 81) are widely used for their exceptional combination of hardness, corrosion resistance, and high-temperature strength, but achieving consistent microstructure and performance through PTA deposition requires precise process control.

Core Technical Findings

The authors conducted a systematic investigation of PTA overlay parameters and their influence on the microstructure, phase composition, hardness distribution, and wear behavior of both nickel-based and cobalt-based alloy overlay layers. The plasma transferred arc process was selected for its advantages of low dilution rates, high powder utilization efficiency, and excellent control over the molten pool geometry.

The study reveals fundamental differences in the solidification behavior and resulting microstructures of nickel-based versus cobalt-based overlay deposits:

The microstructural analysis using optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) revealed that the PTA process produces columnar dendritic structures growing perpendicular to the substrate interface, with fine intragranular precipitates distributed within the dendrite arms. The grain structure is finer and more uniform in the PTA overlay compared to conventional arc welding methods, attributed to the rapid solidification rates achievable with plasma arc technology.

The wear testing, conducted under both dry sliding and lubricated conditions against a Si3N4 ceramic counterface, demonstrated that cobalt-based overlay layers exhibited superior abrasive wear resistance due to the presence of harder, more stable carbide phases. However, nickel-based overlays showed better performance in corrosive-wear environments where the combination of corrosion resistance and moderate hardness provides a synergistic advantage.

Process Parameter Optimization

The study systematically varied key PTA parameters to determine their influence on overlay quality:

The optimal parameter combinations were determined through orthogonal experimental design, with the primary objective of minimizing dilution while maximizing overlay thickness per pass and ensuring a dense, defect-free microstructure. The results indicate that the powder-to-arc ratio (mass of powder fed per unit arc energy) is the most critical parameter for controlling dilution, and that maintaining this ratio within a narrow window is essential for consistent overlay quality.

Microstructural Evolution and Phase Transformation

A significant portion of the study is dedicated to understanding the phase transformation behavior during PTA solidification. For the nickel-based alloy (Inconel 625 type composition), the solidification sequence involves the formation of primary γ dendrites followed by the precipitation of γ + Laves phase (Ni3Nb) at the interdendritic regions. Upon cooling, the Laves phase dissolves back into the matrix, and subsequent aging can promote the formation of coherent γ′ (Ni3(Al,Ti)) precipitates that provide excellent high-temperature strength.

For the cobalt-based alloy (Stellite 6 type composition), the solidification involves primary γ dendrites with interdendritic carbide precipitation, primarily Cr7C3 and M7C3 type carbides. These carbides are thermodynamically stable and retain their hardness even at elevated temperatures, which explains the superior high-temperature wear resistance of cobalt-based overlays. The study notes that the carbide morphology and distribution are strongly influenced by the cooling rate, with faster cooling producing finer, more uniformly distributed carbides.

The bond strength between the overlay layer and the substrate was evaluated through tensile and shear testing, with results indicating excellent metallurgical bonding achieved through the PTA process. The dilution layer at the interface, typically 50–150 μm thick, exhibited a gradual transition in composition and microstructure from the base metal to the overlay, which helps to accommodate thermal expansion mismatch and reduces the risk of interfacial cracking.

Defect Analysis and Quality Control

The study identifies and characterizes several defects that can occur during PTA overlay deposition:

The quality control protocol recommended in the study includes 100% visual inspection of the overlay surface, followed by magnetic particle testing for crack detection and ultrasonic testing for subsurface defect evaluation. For critical applications, radiographic testing of the entire overlay area is recommended to detect porosity and lack of fusion defects.

Study Insights and Engineering Practice Integration

This study provides valuable insights for engineers selecting between nickel-based and cobalt-based PTA overlays for specific applications. The key decision factors include the operating environment (temperature, corrosivity, abrasive severity), the required service life, and the economic constraints of the application.

For high-temperature applications above 600 °C where oxidation and thermal fatigue are concerns, nickel-based overlays offer superior performance due to their excellent oxidation resistance and thermal fatigue tolerance. For applications involving severe abrasive wear, particularly at temperatures above 800 °C, cobalt-based overlays provide unmatched wear resistance due to their stable carbide phases.

The study also highlights the importance of post-weld heat treatment in optimizing overlay performance. For nickel-based overlays, solution heat treatment at 1120 °C followed by aging at 720 °C for 8 hours can significantly improve high-temperature strength by promoting the formation of γ′ precipitates. For cobalt-based overlays, stress relief at 850–900 °C is typically sufficient to relieve residual stresses without adversely affecting carbide stability.

A particularly noteworthy finding is the demonstration that multi-layer PTA deposition with alternating nickel-based and cobalt-based layers can produce hybrid overlays with improved overall performance. The nickel-based layer provides corrosion and oxidation resistance, while the cobalt-based layer provides wear resistance, and the interface between the two layers acts as a crack-arresting barrier that improves the overall toughness of the overlay.

In conclusion, this comprehensive study by Tongji University researchers provides a thorough understanding of PTA overlay metallurgy for both nickel-based and cobalt-based alloys, offering engineers a well-documented basis for process selection, parameter optimization, and quality assurance in high-performance surface engineering applications.