Zhuojin Pipe Fitting Co., LtdTIG Oscillating Remelting Treatment of FeCrBSiWNb Thermal Spray Coating
Literature Overview
Published in Surface Technology (Vol. 46, No. 7, 2017, pp. 195–200) by Zong Xuemei and colleagues from Jiangsu Xuzhou Construction Machinery Research Institute and the State Key Laboratory of Intelligent Manufacturing for High-End Construction Machinery, this paper presents a novel TIG oscillating remelting process for FeCrBSiWNb high-velocity arc spray (HVAF) coatings. The work addresses critical quality issues in traditional remelting processes, including narrow and uneven single-pass width, porosity and cracking at multi-pass overlap regions, and excessive residual stress leading to coating failure.
Technical Background
FeCrBSiWNb is a high-performance wear-resistant alloy coating widely used in construction machinery and mining equipment. The coating contains multiple hard phases including borides and Laves phases, which provide excellent wear resistance but also introduce challenges during subsequent heat treatment and remelting operations.
Traditional TIG remelting of HVAF coatings suffers from several limitations:
- Narrow single-pass width: Limited by the arc energy distribution and thermal diffusivity of the coating.
- Uneven remelting depth: Inconsistent penetration leads to variable coating properties.
- Overlap defects: Multi-pass welding creates weak interfaces with porosity and cracking.
- High residual stress: Rapid cooling from the remelting temperature causes tensile residual stresses that can crack the coating.
The oscillating remelting approach proposed in this paper addresses these issues through controlled lateral movement of the tungsten electrode.
Process Design and Parameters
Geometric Analysis and Process Design
The process design is based on a geometric analysis of the arc energy distribution and the desired remelting pattern. The oscillation pattern is designed to ensure complete overlap between adjacent passes while maintaining uniform remelting depth.
Optimized Process Parameters
| Parameter | Value | Description |
|---|---|---|
| Welding current | 53–55 A | Moderate current for controlled remelting |
| X-direction speed | 65–68 mm/min | Lateral oscillation speed |
| Y-direction speed | 485–500 mm/min | Forward travel speed |
| Oscillation amplitude | Not specified | Determined by geometric analysis |
| Shielding gas | Argon (implied) | Standard TIG shielding |
The relatively low welding current (53–55 A) is notable, as it suggests that the process is designed for remelting of a thin coating layer rather than deep penetration into the substrate. The high forward travel speed relative to the lateral oscillation speed indicates a fine, elongated remelting pattern.
Microstructural Analysis
Phase Composition
The remelted coating exhibits the following phase structure:
| Phase | Type | Role |
|---|---|---|
| Martensite | Matrix | Base structural phase |
| Residual austenite | Matrix | Retained high-temperature phase |
| Fe-Cr solid solution | Matrix | Solid solution strengthening |
| Borides | Hard phase | Wear resistance |
| Laves phases (Fe₂W, FeCrW) | Hard phase | Wear resistance and strengthening |
Strengthening Mechanisms
The remelted coating benefits from multiple strengthening mechanisms:
- Solid solution strengthening: From Fe-Cr solid solution and alloying elements in solution.
- Precipitation strengthening: From fine borides and Laves phases dispersed in the matrix.
- Grain refinement: From the rapid solidification during remelting.
- Dislocation strengthening: From the high dislocation density in the martensitic matrix.
Microhardness Profile
| Region | Microhardness (HV0.3) | Relative to Substrate |
|---|---|---|
| Remelted coating | 820 | 3.4× |
| Heat-affected zone | 563 | 2.3× |
| Base material | 242 | 1.0× |
The microhardness values indicate significant hardening in the remelted coating, consistent with the presence of hard phases and multiple strengthening mechanisms. The HAZ also shows substantial hardening, likely due to the thermal cycling effects on the substrate microstructure.
Defect Analysis and Countermeasures
Traditional Process Defects
| Defect | Cause | Traditional Process | Oscillating Process |
|---|---|---|---|
| Cracking | High residual stress | Common | Eliminated |
| Porosity | Incomplete fusion, gas entrapment | Common at overlaps | Eliminated |
| Uneven width | Arc energy variation | Common | Uniform |
| Tungsten pickup | Poor arc control | Possible | Minimized |
Crack Prevention Mechanism
The elimination of cracking in the oscillating remelting process is attributed to:
- Reduced residual stress: The oscillation pattern distributes heat input more uniformly, reducing thermal gradients and associated residual stresses.
- Improved metallurgical bonding: The controlled remelting ensures complete fusion between adjacent passes, eliminating weak interfaces.
- Reduced cooling rate variation: The oscillation pattern creates a more uniform thermal history, reducing the likelihood of stress concentration.
Engineering Practice Implications
For industrial application of FeCrBSiWNb coatings in construction machinery, the oscillating remelting process offers several practical advantages:
- Increased productivity: Wider single-pass width reduces the number of passes required, decreasing processing time.
- Improved quality: Elimination of cracks and porosity reduces rework and improves coating reliability.
- Enhanced wear resistance: The optimized microstructure with multiple hard phases provides superior wear performance.
- Process consistency: The geometrically designed oscillation pattern ensures repeatable results across different production runs.
Process Optimization Considerations
For further optimization, the following parameters should be investigated:
- Oscillation amplitude and frequency for different coating thicknesses
- Electrode diameter and stick-out length effects
- Shielding gas flow rate and nozzle design
- Substrate preheating temperature for thick sections
- Post-remelting heat treatment for stress relief
Key Questions and Reflections
A fundamental question arises: what is the optimal oscillation pattern for different coating geometries and substrate configurations? The present study focuses on flat surfaces, but practical applications often involve curved surfaces, corners, and edges where the oscillation pattern must be adapted. The development of adaptive oscillation patterns based on real-time monitoring of remelting pool geometry would be a valuable extension of this work.
Another consideration is the scalability of the process. The relatively low welding current (53–55 A) and specific travel speeds suggest that the process is optimized for thin coatings on flat surfaces. For thicker coatings or larger areas, the process parameters may need significant adjustment, and the defect elimination benefits may not be maintained.
Study Insights and Outlook
This work demonstrates that process innovation through geometric analysis and controlled electrode movement can significantly improve the quality of thermal spray coating remelting. The elimination of cracks and porosity in the oscillating remelting process is a major achievement that addresses longstanding quality challenges in coating production. For construction machinery and mining equipment manufacturers, this process offers a path to more reliable and higher-performing wear-resistant coatings. Future research should focus on adaptive process control, multi-axis oscillation patterns for complex geometries, and integration with automated coating production systems to maximize the industrial impact of this technology.