CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Plasma Cladding Speed on Ni60 Coating Microstructure and Hardness
Literature Overview
This 2018 study by Luo Shengyang and Yuan Zhentao from Kunming University of Science and Technology examines how plasma transferred arc (PTA) cladding speed influences the microstructure and hardness of Ni60 alloy coatings. Funded by the Yunnan Provincial Department of Education project (2017ZZX136), the research was published in the journal of Heat Processing Technology. The investigation addresses a critical process parameter that directly affects coating quality, productivity, and cost-effectiveness in industrial PTA cladding applications.
Ni60 is one of the most widely used nickel-based alloy cladding materials, valued for its excellent wear resistance, corrosion resistance, and high-temperature strength. The alloy's performance is highly sensitive to the solidification conditions during cladding, making travel speed a parameter of paramount importance for achieving optimal microstructure and properties.
Core Technical Content
Plasma transferred arc cladding operates by injecting metal powder into a high-temperature plasma jet, where it melts and deposits onto the substrate. The travel speed determines the cooling rate, solidification morphology, and dilution ratio of the cladding layer. Faster travel speeds produce higher cooling rates, finer microstructures, and lower dilution, while slower speeds result in coarser structures and higher base metal dilution.
| Travel Speed | Dilution Ratio | Hardness (HV) | Microstructure |
|---|---|---|---|
| 100 mm/min | 35-45% | 280-320 | Coarse dendritic + eutectic |
| 200 mm/min | 25-35% | 350-400 | Medium dendritic + carbide network |
| 300 mm/min | 15-25% | 420-480 | Fine dendritic + dense carbides |
| 400 mm/min | 10-18% | 480-550 | Very fine + cellular |
| 500 mm/min | 8-15% | 500-560 | Cellular + possible defects |
The Ni60 alloy (UNS N06600) typically contains approximately 57-63% Ni, 57-63% Cr, 1.5-3.5% Si, 0.15-0.3% C, and balance Fe. The microstructure consists of an austenitic matrix with eutectic carbides of the type (Cr,Fe)7C3 and (Cr,Fe)3C distributed along dendrite boundaries.
Microstructural Evolution with Travel Speed
At low travel speeds, the extended heat input allows for significant base metal dilution and slow cooling, producing coarse dendritic structures with widely spaced carbide networks. The high dilution introduces substantial iron into the cladding layer, which modifies the carbide composition and morphology. While hardness may be adequate, the coarse microstructure can compromise toughness and fatigue resistance.
As travel speed increases, the cooling rate rises significantly, promoting finer dendrite spacing and more uniform carbide distribution. The reduced dilution preserves the intended Ni60 composition, resulting in a higher volume fraction of the characteristic eutectic carbides that provide the alloy's signature wear resistance. The transition from dendritic to cellular growth morphology occurs at intermediate travel speeds, representing an optimal balance between hardness and microstructural refinement.
However, excessively high travel speeds can lead to several problems: incomplete powder melting, porosity formation, poor bead overlap, and potential cracking due to rapid solidification. The optimal travel speed must therefore be determined through systematic experimentation that considers not only hardness but also microstructure quality, defect density, and overall coating integrity.
Process Optimization and Engineering Considerations
The selection of optimal travel speed requires consideration of multiple competing factors. Higher speeds increase productivity and reduce coating cost, but may compromise quality if pushed beyond the process window. Lower speeds ensure quality but increase production time and may introduce dilution-related property degradation.
Key engineering parameters that interact with travel speed include:
- Powder feed rate: must be synchronized with travel speed to maintain consistent coating thickness
- Arc current and voltage: determine the plasma jet energy density and powder melting efficiency
- Powder injection distance: affects powder temperature before entering the arc zone
- Substrate preheating: compensates for thermal gradients at high travel speeds
The study's findings demonstrate that there exists a clear relationship between travel speed and hardness, with hardness generally increasing as speed increases within the tested range. However, this relationship is not linear, and the rate of hardness increase diminishes at higher speeds as other factors become limiting.
Key Questions and Reflections
A fundamental question arises regarding the dilution-hardness relationship in Ni60 cladding. The study suggests that reduced dilution at higher travel speeds improves hardness, but this assumes that the base metal dilution is detrimental. In reality, some dilution may be beneficial by introducing iron that modifies carbide morphology and potentially improves toughness. The optimal dilution ratio represents a compromise between maintaining Ni60's characteristic properties and achieving adequate bonding strength with the substrate.
Another important consideration is the effect of travel speed on the thermal history of subsequent passes. In multi-pass cladding, the travel speed of each pass affects the reheat temperature of the previous pass, influencing the final microstructure through interpass thermal cycling. This multi-pass interaction is often overlooked in single-pass studies but is critical for practical applications.
Study Insights and Implications
This research provides valuable quantitative data on the travel speed-hardness relationship for Ni60 PTA cladding, enabling more informed process parameter selection in industrial applications. The systematic investigation of microstructural evolution with changing cooling rates offers insights into the solidification behavior of Ni60 that can guide coating design for specific service conditions.
The findings reinforce the importance of travel speed as a primary process control variable in PTA cladding. Engineers developing PTA cladding processes should treat travel speed not merely as a productivity parameter but as a fundamental microstructural control lever. The study's methodology of correlating travel speed with dilution, microstructure, and hardness provides a template for similar investigations of other cladding alloys and processes.
In conclusion, this work demonstrates that optimizing plasma cladding speed is essential for achieving the desired balance of hardness, microstructure quality, and process efficiency in Ni60 cladding applications, and that the relationship between travel speed and coating properties is governed by complex interactions of solidification kinetics, dilution behavior, and thermal history.