CLADDING TECHNOLOGY SHANXI CO., LTDAluminum Bronze Powder Plasma Cladding Layer Hardness and Microstructure
Literature Overview
This 2011 study by Liu Zhengjun, Li Jin, and Su Yunhai from the School of Materials Science and Engineering, Shenyang University of Technology, examines the plasma arc transfer (PTA) cladding of aluminum bronze powder alloys. Aluminum bronze (Cu-Fe-Al-Ni system) is a critical engineering material for marine and chemical processing applications due to its excellent combination of strength, corrosion resistance in seawater, and cavitation erosion resistance. The study addresses the fundamental metallurgical behavior of this alloy system during plasma arc deposition, providing insights essential for optimizing overlay performance.
Core Technical Content
Alloy System and Phase Constitution
Aluminum bronze alloys used in this study typically contain 5–11% Al, 5–10% Fe, 5–7% Ni, with balance copper. The phase diagram of the Cu-Fe-Al-Ni system is extremely complex, featuring multiple intermetallic phases:
| Phase | Composition | Hardness (HV) | Role |
|---|---|---|---|
| α (Cu-Al solid solution) | Cu-rich FCC | 100–150 | Ductile matrix |
| η (CuAl₂) | Ordered BCC | 400–500 | Primary strengthening phase |
| θ (Cu₉Al₄) | Ordered | 350–450 | Secondary strengthening |
| κ (Cu₂Al) | Intermetallic | 300–400 | Precipitation strengthening |
| γ' (Cu₃Ni) | Ordered FCC | 250–350 | Ni-stabilized strengthening |
The plasma arc cladding process produces rapid solidification conditions that significantly alter the equilibrium phase constitution. The high cooling rates (50–200°C/s) suppress the formation of equilibrium phases and promote metastable structures with enhanced hardness.
Hardness-Microstructure Correlation
The study reveals that hardness in the cladding layer is governed by three primary mechanisms:
- Solid solution strengthening: Al and Fe atoms in the Cu matrix contribute approximately 80–120 HV
- Precipitation strengthening: Fine η and κ precipitates contribute 150–250 HV
- Grain refinement: Rapid solidification produces grain sizes of 20–50 μm versus 100–200 μm in cast condition, contributing 50–80 HV through Hall-Petch mechanism
The resulting overlay hardness ranges from 250–450 HV depending on composition and process parameters. This represents a 2–3× improvement over the base copper substrate while maintaining adequate ductility for forming operations.
Process Parameter Optimization
| Parameter | Optimal Range | Effect on Hardness |
|---|---|---|
| Arc current | 120–180 A | Moderate increase with current |
| Powder feed rate | 60–120 g/min | Inverse relationship above 90 g/min |
| Travel speed | 300–500 mm/min | Direct correlation with cooling rate |
| Layer thickness | 0.3–0.8 mm | Optimal at 0.5 mm |
| Interpass temperature | <150°C | Prevents over-aging softening |
The dilution rate with the steel substrate is a critical control parameter. At dilution rates above 25%, the copper content in the overlay drops significantly, leading to excessive hardness (>600 HV) with corresponding loss of ductility. The study recommends maintaining dilution below 20% through the use of a backing plate or sacrificial first layer.
Engineering Applications
Aluminum bronze plasma cladding finds primary application in:
- Marine propeller repair: Restoring erosion-damaged surfaces while maintaining cavitation resistance
- Pump impeller refurbishment: Rebuilding worn impellers with enhanced wear resistance
- Valve trim components: Providing hard, corrosion-resistant surfaces for control valves in seawater service
- Shaft sleeve overlays: Protecting rotating shafts in marine propulsion systems
The cladding process offers significant advantages over replacement manufacturing, reducing material costs by 60–70% and eliminating the need for extensive machining of expensive copper alloys.
Key Technical Challenges
The study identifies several challenges specific to aluminum bronze plasma cladding:
- Hot cracking susceptibility: The Cu-Al system is prone to solidification cracking due to its wide freezing range and low ductility at elevated temperatures. Mitigation requires careful control of sulfur content (<0.005%) and the addition of Ti or Zr as grain refiners.
- Oxide inclusion formation: Aluminum readily oxidizes during powder feeding, creating Al₂O₃ inclusions that degrade mechanical properties. Inert gas shielding must be optimized with a minimum flow of 20 L/min Ar.
- Porosity control: Gas porosity from hydrogen absorption requires powder pre-drying at 200°C for 2 hours and vacuum degassing of the shielding atmosphere.
Summary and Reflections
This study provides a comprehensive understanding of the microstructural evolution in aluminum bronze plasma arc cladding layers, establishing clear relationships between processing parameters, phase constitution, and mechanical properties. The work demonstrates that plasma arc cladding can produce overlays with properties comparable to or exceeding wrought aluminum bronze, while offering the flexibility of in-situ repair and component modification. The emphasis on dilution control and microstructural refinement through rapid solidification provides practical guidance for industrial implementation. For marine engineers facing costly component replacement, this technology offers a proven and economical alternative that maintains the essential corrosion and cavitation resistance properties of the original aluminum bronze material.