CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure Analysis of Plasma Cladded Ni-SiCp Reinforced Cobalt-Based Overlay
Literature Overview
This 2016 study by Pan Chenggang, Xiao Qin, Yang Huqun, Ma Wenchao, Chang Qingming, and Wang Huachang from the Ministry of Education Key Laboratory of Ferrous Metallurgy and Resources Utilization at Wuhan University of Science and Technology and the School of Materials Science and Engineering at Wuhan University of Technology represents an advanced investigation into particle-reinforced composite overlay claddings produced by plasma transferred arc (PTA) welding. Funded by the National Natural Science Foundation of China (Project 51375353) and the Key Laboratory of Refractory Materials and Metallurgy (Project 2014QN03), this research addressed the challenge of developing overlay claddings with enhanced wear resistance through the incorporation of silicon carbide (SiCp) particulate reinforcement into a nickel-cobalt-based matrix. Particle-reinforced metal matrix composites (MMCs) are recognized as promising materials for severe wear applications, but their production by cladding techniques presents unique challenges related to particle distribution, particle-matrix bonding, and microstructural integrity.
Composite Overlay Design and Process Parameters
The study investigated the effects of SiCp particle size, particle volume fraction, and PTA process parameters on the microstructure, hardness, and wear resistance of the composite overlay. The Ni-Co base alloy served as the matrix material, providing good bonding with the substrate and adequate ductility to accommodate the SiCp particles. The SiCp particles were added to the powder feed in volume fractions ranging from 0% to 30%, with particle sizes of 5–15 micrometers and 20–45 micrometers.
The PTA process parameters were carefully controlled to ensure complete particle melting or dispersion while minimizing particle agglomeration and void formation. The key challenge in PTA cladding of particle-reinforced composites is achieving uniform particle distribution throughout the overlay thickness while maintaining sound metallurgical bonding between the particles and the matrix. The PTA process offers advantages over other cladding processes for this application because the plasma arc provides intense, localized heat that can effectively melt and disperse the particles, and the process parameters can be precisely controlled to manage the thermal cycle.
| Parameter | Range Investigated | Optimal Value | Effect on Overlay Quality |
|---|---|---|---|
| SiCp volume fraction | 0–30 vol% | 10–20 vol% | Higher fractions increase hardness but reduce toughness |
| SiCp particle size | 5–15 μm, 20–45 μm | 5–15 μm | Finer particles provide better dispersion and bonding |
| PTA current | 250–450 A | 350–400 A | Higher current improves particle melting but increases dilution |
| Travel speed | 100–300 mm/min | 150–200 mm/min | Higher speed reduces dilution and improves particle dispersion |
| Powder feed rate | 100–300 g/min | 150–200 g/min | Higher feed rate increases dilution and may cause incomplete melting |
Microstructural Characteristics
The microstructure of the Ni-SiCp reinforced Co-based overlay was characterized by a cellular dendritic matrix with dispersed SiCp particles. The cellular structure was a result of the rapid solidification conditions inherent to the PTA process, with typical cell spacings of 2–8 micrometers depending on the process parameters. The SiCp particles were distributed throughout the matrix, with some particles located at cell boundaries and others within the cellular structure.
The particle-matrix interface was a critical feature of the composite overlay. The study revealed that the Ni-Co matrix formed a good metallurgical bond with the SiCp particles, with a thin interfacial reaction layer of 0.1–0.5 micrometers in thickness. This reaction layer consisted of Ni-Si and Co-Si compounds that formed during the melting and solidification process. The presence of this reaction layer was beneficial for particle-matrix bonding but excessive reaction could lead to the formation of brittle intermetallic phases that weakened the composite.
The distribution uniformity of the SiCp particles was found to be strongly dependent on the powder feed rate and travel speed. At high powder feed rates and low travel speeds, particle agglomeration was observed, particularly at the top surface of the overlay where the particles were more concentrated. At lower powder feed rates and higher travel speeds, the particle distribution was more uniform, but the overlay thickness was reduced. The optimal combination of powder feed rate and travel speed was found to be 150–200 g/min and 150–200 mm/min, respectively, which produced uniform particle distribution with minimal agglomeration.
Hardness and Wear Resistance
The hardness of the Ni-SiCp reinforced Co-based overlay increased significantly with increasing SiCp volume fraction. At 0 vol% SiCp, the base Ni-Co alloy had a hardness of approximately 280–320 HV. At 10 vol% SiCp, the hardness increased to 380–420 HV, and at 20 vol% SiCp, the hardness reached 450–500 HV. The hardness increase was attributed to the load-sharing effect of the hard SiCp particles and the Hall-Petch strengthening effect of the refined cellular structure.
The wear resistance of the composite overlay was evaluated using a pin-on-disk wear test under dry sliding conditions. The wear volume loss decreased by 60–70% at 20 vol% SiCp compared to the unreinforced Ni-Co alloy. The wear mechanism of the composite overlay was a combination of abrasive wear by the SiCp particles, adhesive wear at the particle-matrix interface, and matrix plastic deformation. The SiCp particles acted as load-bearing elements that reduced the contact area between the wear pin and the overlay surface, thereby reducing the wear rate.
However, the study also identified a critical limitation: at high SiCp volume fractions above 25 vol%, the overlay toughness decreased significantly, and the particles became prone to pull-out during wear, creating voids that accelerated wear. The optimal SiCp volume fraction for the best balance of hardness and wear resistance was found to be 15–20 vol%, which provided significant hardness and wear resistance improvements without excessive toughness reduction.
Process Challenges and Quality Control
The production of particle-reinforced composite overlays by PTA cladding presents several quality challenges that must be addressed for reliable engineering applications. The primary challenges include particle agglomeration, incomplete particle melting, void formation at the particle-matrix interface, and non-uniform particle distribution.
Particle agglomeration occurs when the powder feed rate is too high or the plasma arc energy density is insufficient to fully disperse the particles. This results in localized regions of high particle concentration that act as stress concentrators and reduce the effective wear resistance of the overlay. The study recommended the use of a pre-mixed powder with thoroughly homogenized particle distribution, achieved through ball milling or planetary mixing, to minimize agglomeration.
Incomplete particle melting is another challenge, particularly for larger particles at lower plasma arc energy densities. Unmelted or partially melted SiCp particles can create weak interfaces and voids that reduce the mechanical integrity of the overlay. The study found that plasma arc currents above 350 A were required to ensure complete melting of SiCp particles up to 45 micrometers in size.
Key Questions and Reflections
The study raises important questions about the long-term durability of particle-reinforced composite overlays under severe wear conditions. While the laboratory wear tests demonstrated significant improvements in wear resistance, the actual performance under field conditions may be influenced by factors such as temperature, lubrication, and loading conditions that are not fully replicated in laboratory tests. Additionally, the question of how the particle-matrix interface evolves during prolonged wear service remains an open area for further investigation.
Study Insights and Implications
This study demonstrates the effectiveness of SiCp particle reinforcement in enhancing the hardness and wear resistance of Ni-Co based PTA cladding overlays. The key finding that 15–20 vol% SiCp with fine particle sizes (5–15 micrometers) provides the optimal balance of hardness, wear resistance, and toughness has direct implications for the design of composite overlays for severe wear applications such as mining equipment, cement mill liners, and industrial pumps. The study also highlights the importance of careful process parameter control to achieve uniform particle distribution and sound particle-matrix bonding, which are critical for realizing the full potential of particle-reinforced composite overlays.