Zhuojin Pipe Fitting Co., LtdEffect of Surfacing Current on Microstructure and Properties of Aluminum Bronze Powder Plasma Surfacing Layer
Literature Overview
Wang Bo (2015, Hot Working Technology, Vol. 44, No. 23) investigated the influence of surfacing current on the microstructure, hardness, and wear resistance of aluminum bronze powder plasma surfacing layers deposited on 20G boiler steel substrate. The study employed reverse-polarity plasma arc surfacing (PAW) and systematically varied the current to identify optimal process parameters. This research is particularly relevant for the repair and refurbishment of boiler components subject to abrasive and erosive wear.
Technical Background
Aluminum bronze (typically Cu-Al-Ni-Fe system) is an excellent wear-resistant material for applications involving sliding wear, abrasive wear, and cavitation erosion. The plasma arc surfacing process offers several advantages for depositing aluminum bronze coatings:
| Advantage | Description |
|---|---|
| High energy density | Plasma arc provides concentrated heat input, enabling rapid melting of powder feedstock |
| Low dilution | Compared to arc surfacing methods, PAW achieves dilution ratios of 10–25% |
| Fine microstructure | Rapid solidification produces refined grain structures |
| Good adhesion | Strong metallurgical bonding with the substrate |
| Flexible geometry | Suitable for complex component geometries |
The 20G boiler steel substrate is a low-carbon, low-alloy steel commonly used in high-temperature pressure vessels. Its relatively low hardness and strength make it susceptible to wear in erosive service, motivating the application of hard surfacing layers.
Experimental Results and Analysis
The study examined multiple current levels and observed clear trends in microstructure evolution:
| Current (A) | Microstructure | Hardness (HV) | Relative Wear Resistance |
|---|---|---|---|
| 90 | Laminar structure | Lower | Lower |
| 100 | Transition structure | Medium | Medium |
| 110 | Finest polygonal structure | High | Highest (tied) |
| 120 | Coarse polygonal structure | Highest | Highest (tied) |
| 130+ | Coarse, irregular structure | Decreasing | Decreasing |
Microstructure Evolution
The transition from laminar to polygonal structure with increasing current is explained by the changing thermal conditions during solidification:
- At lower currents (90–100 A), the heat input is insufficient to fully melt the powder, resulting in a partially melted, laminar structure with retained powder particle boundaries.
- At intermediate currents (110–120 A), complete melting occurs with optimal solidification rates, producing a fine, uniform polygonal grain structure.
- At higher currents (>130 A), excessive heat input causes grain coarsening and potential substrate dilution, degrading the coating properties.
Hardness and Wear Resistance
The hardness maximum at 120 A and the wear resistance optimum at 110–120 A are explained by:
- Solid solution strengthening: Aluminum and nickel atoms dissolved in the copper matrix provide solid solution strengthening.
- Precipitation hardening: Intermetallic compounds (CuAl₂, Cu₅Al₈) form during solidification and contribute to hardness.
- Grain refinement: The Hall-Petch relationship indicates that finer grains produce higher hardness.
- Wear mechanism: At optimal parameters, the coating exhibits a balanced combination of hardness and toughness, resisting both abrasive and adhesive wear.
Process Parameter Optimization
Based on the study results, the recommended process parameters for aluminum bronze plasma surfacing are:
| Parameter | Recommended Value |
|---|---|
| Current | 110–120 A |
| Voltage | 20–25 V |
| Travel speed | 100–200 mm/min |
| Powder feed rate | 150–250 g/min |
| Powder diameter | 63–125 μm |
| Shielding gas | Argon |
| Gas flow rate | 15–25 L/min |
| Torch standoff distance | 5–8 mm |
Quality Control Considerations
- Porosity: Inadequate shielding gas flow or excessive travel speed can lead to gas porosity. Maintain consistent gas flow and torch alignment.
- Cracking: Aluminum bronze coatings are generally crack-resistant due to the ductile copper matrix, but excessive dilution with the steel substrate can increase cracking susceptibility.
- Surface quality: Travel speed and powder feed rate must be matched to produce a uniform, smooth surface. Excessive feed rate produces a rough, uneven surface.
Reflections and Study Value
This paper provides valuable insight into the current-dependent microstructure evolution in plasma arc surfacing of aluminum bronze. The clear relationship between current, microstructure, and properties is consistent with fundamental solidification principles and provides a practical guideline for process optimization.
The finding that the optimal current range (110–120 A) produces the finest microstructure and highest wear resistance is particularly useful for engineers working on boiler component repair. The study confirms that plasma arc surfacing is a viable technology for depositing hard, wear-resistant coatings on low-carbon steel substrates with good adhesion and low dilution.
One area for further investigation would be the long-term performance of the coating under thermal cycling conditions representative of boiler service. Aluminum bronze has a relatively high thermal expansion coefficient (~17×10⁻⁶/°C) compared to 20G steel (~12×10⁻⁶/°C), which could lead to thermal fatigue cracking at the interface during repeated heating and cooling cycles. Engineers should consider this when selecting coatings for high-temperature applications.
The study also highlights the importance of powder characteristics in plasma surfacing. The powder diameter, flowability, and chemical composition all influence the melting behavior and final coating properties. For production applications, consistent powder quality is essential for reproducible results.