CLADDING TECHNOLOGY SHANXI CO., LTDMechanism of New Plasma Transferred Arc Powder Cladding Process
Literature Overview
This pioneering research by Wang Hongying from Shenzhen Polytechnic and Cheng Zhiguo, Zhao Kun, and Dong Zuyu from the Harbin Welding Research Institute, published in 2002 in the journal Acta Metallurgica Sinica (Welding Journal) under the Ministry of Machinery Industry Technology Development Fund (96JA0404), presents a novel plasma transferred arc (PTA) powder cladding process and elucidates its underlying mechanisms. This work represents a significant advancement in the understanding of PTA cladding, a process that has become one of the most important thermal spray alternatives for producing high-quality, metallurgically bonded overlay layers.
Core Technical Content
The study introduces a modified PTA process that improves upon conventional PTA by optimizing the interaction between the plasma arc, the powder feed, and the molten pool. The key innovation lies in the controlled powder delivery system and the plasma arc parameters that produce a stable, well-defined molten pool with minimal dilution and excellent powder capture efficiency.
Process Mechanism Analysis
The PTA cladding mechanism involves several coupled phenomena:
- Plasma arc stabilization: The plasma arc is generated by constricting an electric arc through a water-cooled copper nozzle, producing a high-temperature, high-velocity plasma jet with temperatures exceeding 10,000°C. The arc is transferred from a tungsten electrode to the workpiece, creating a deep, narrow molten pool.
- Powder injection and melting: Powder is fed into the plasma arc through a gas-assisted nozzle, where it is heated to its melting point by the plasma jet. The molten powder particles are then directed onto the workpiece surface, where they melt into the existing molten pool.
- Molten pool dynamics: The interaction between the plasma arc and the incoming molten powder creates a complex fluid flow pattern within the molten pool, governed by electromagnetic forces, surface tension, and buoyancy-driven convection.
- Solidification and microstructure formation: As the plasma arc traverses the workpiece, the molten pool solidifies behind the arc, forming a dense, metallurgically bonded overlay layer with controlled microstructure.
Key Process Parameters and Their Effects
| Parameter | Typical Range | Effect on Microstructure | Effect on Quality |
|---|---|---|---|
| Plasma current | 100–400 A | Higher current → deeper penetration, coarser grains | Dilution increases with current |
| Travel speed | 100–500 mm/min | Higher speed → thinner layer, finer grains | Too high → incomplete melting |
| Powder feed rate | 200–800 g/min | Higher rate → thicker layer, more unmelted particles | Too high → poor bonding |
| Shielding gas flow | 10–20 L/min | Affects arc stability and oxide formation | Insufficient → oxidation |
| Powder particle size | 45–150 μm | Smaller particles → better melting, finer grains | Too small → poor flowability |
Dilution Control Mechanism
One of the most critical aspects of PTA cladding is the control of dilution, which determines the composition of the final overlay layer. The study identifies several mechanisms for dilution control:
- Molten pool geometry: The depth-to-width ratio of the molten pool determines the volume of base metal melted per unit length of travel. A shallower, wider pool results in lower dilution.
- Powder preheating: Preheating the powder before injection reduces the thermal energy required for melting, allowing more energy to be directed into the base metal and reducing dilution.
- Multi-layer deposition: Depositing multiple thin layers, each with controlled dilution, allows the composition of the final overlay to be tailored by adjusting the powder composition of each layer.
- Arc oscillation: Oscillating the plasma arc laterally creates a wider, shallower molten pool, reducing dilution while maintaining adequate bonding strength.
Microstructure Characteristics
The PTA process produces overlay layers with distinctive microstructural features:
- Columnar grain structure: The solidification pattern is typically columnar, with grains growing perpendicular to the substrate surface. This is a direct consequence of the directional heat flow from the workpiece into the overlay layer.
- Fine grain size: The high cooling rates achieved in PTA (100–1000°C/s) produce fine grain sizes, typically 10–50 μm, which contribute to high hardness and good mechanical properties.
- Low porosity: The plasma arc provides excellent shielding and the molten pool is protected from atmospheric contamination, resulting in porosity levels below 1% when proper shielding gas flow is maintained.
- Metallurgical bonding: The overlay layer is metallurgically bonded to the substrate, with no oxide interlayer or mechanical interlocking, resulting in superior bonding strength compared to thermal spray processes.
Comparison with Other Cladding Processes
| Process | Dilution (%) | Bonding Strength (MPa) | Hardness (HV) | Surface Roughness (Ra) | Cost |
|---|---|---|---|---|---|
| PTA | 5–15 | 200–350 | 400–1200 | 1.6–6.3 | Medium |
| Laser cladding | 5–20 | 250–400 | 500–1500 | 0.8–3.2 | High |
| SAW overlay | 15–30 | 150–250 | 300–900 | 3.2–12.5 | Low |
| Thermal spray (HVOF) | 0 | 50–150 | 600–1200 | 0.2–1.6 | Medium-High |
PTA offers a favorable balance of low dilution, high bonding strength, good surface quality, and moderate cost, making it suitable for a wide range of industrial applications including aerospace, energy, and mining.
Key Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Excessive dilution | High current, low travel speed | Reduce current, increase travel speed, use multi-layer deposition |
| Porosity | Insufficient shielding gas, wet powder | Increase shielding gas flow, dry powder thoroughly |
| Cracking | High residual stress, incompatible composition | Reduce heat input, use compatible powder composition |
| Poor surface quality | Unstable arc, uneven powder feed | Stabilize arc parameters, use controlled powder delivery |
| Unmelted particles | Low current, high feed rate | Increase current, reduce feed rate, preheat powder |
Study Insights and Reflections
The research by Wang Hongying et al. provides fundamental insights into the PTA cladding mechanism that are directly applicable to process optimization and quality control. The key contribution is the systematic understanding of how process parameters influence the molten pool dynamics, dilution, and final microstructure. For engineers implementing PTA cladding in production, the study emphasizes the importance of precise parameter control and the use of multi-layer deposition strategies to achieve the desired overlay composition and properties. The PTA process remains one of the most versatile and reliable methods for producing high-quality cladding layers, and the mechanistic understanding presented in this work continues to guide process development and application expansion.