Thermal Behavior of Powder Particles Under Plasma Arc Powder Cladding Conditions

Literature Overview

Published in the Chinese Journal of Welding in 2002 by researchers from the Center for Surface Engineering at Tianjin University and Hebei Provincial Installation Company, this study represents a foundational contribution to the understanding of plasma transferred arc (PTA) powder cladding thermodynamics. Supported by the Tianjin Natural Science Foundation (Grant F200008), the work focuses on the heat transfer mechanisms between individual powder particles and the plasma arc during the cladding process — a critical aspect that governs powder melting efficiency, dilution control, and microstructure evolution.

PTA cladding is one of the most widely used thermal spray-like processes for applying corrosion-resistant and wear-resistant overlays in the power generation, chemical processing, and aerospace industries. Despite its industrial maturity, the fundamental thermal interactions at the powder-particle level have remained challenging to characterize due to the extremely short interaction times (millisecond scale) and high temperature gradients involved.

Core Technical Content

Heat Transfer Mechanisms

The study identifies three primary heat transfer mechanisms between the plasma arc and powder particles:

1. Convective heat transfer: Dominant mechanism for particle heating, characterized by forced convection from the high-velocity plasma jet (typically 100–150 m/s at the nozzle exit). The convective heat transfer coefficient ranges from 1000–3000 W/(m²·K) depending on plasma current and nozzle geometry.
2. Radiative heat transfer: Significant for larger particles (diameter > 50 μm) and at distances beyond 5 mm from the arc centerline. Radiative contribution typically accounts for 15–30% of total heat input to the particle.
3. Conductive heat transfer: Negligible during flight but becomes significant upon particle impact with the molten pool, where thermal contact resistance governs the initial heating rate.

Particle Thermal Response Analysis

The study models the temperature evolution of powder particles during flight using a lumped capacitance approach modified for non-uniform heating:

The analysis reveals that particle diameter is the most critical parameter affecting thermal response. Particles below 30 μm achieve near-complete melting before impact, while particles above 60 μm often arrive at the substrate surface in a partially molten state, leading to incomplete fusion and potential porosity.

Plasma Arc Parameters and Their Influence

Dilution and Bonding Quality

The thermal behavior of individual particles directly determines the dilution ratio in the cladding layer. The study establishes the following relationship:

• For complete particle melting (surface temperature > melting point + 200°C): dilution typically 10–20%, excellent metallurgical bond.
• For partial melting (surface temperature within 100–200°C of melting point): dilution 25–40%, risk of unmelted particles and poor bonding.
• For minimal melting (surface temperature < melting point - 50°C): dilution > 40%, mechanical bonding only, unacceptable for most applications.

Engineering Practice Implications

Process Optimization Recommendations

Based on the thermal analysis, the following process optimization strategies are recommended for PTA cladding operations:

1. Powder preparation: Use spherical or near-spherical powder with controlled size distribution (80% within 20–50 μm range) to ensure uniform thermal response.
2. Preheating: Substrate preheating to 150–200°C reduces thermal gradient at the interface and improves bonding quality.
3. Multi-pass strategy: For thick overlays (> 3 mm), employ multiple passes with interpass temperature control at 150–250°C to manage residual stress.
4. Powder feeding system: Use gravity-fed or pneumatic powder feeders with consistent powder flow rate (coefficient of variation < 5%).

Quality Control Considerations

For PTA cladding applications in pressure vessel fabrication (per NB/T 47002 and ASME VIII Div.1), the following quality assurance measures should be implemented:

Study Insights and Reflections

This research provides fundamental thermal data that is essential for process modeling and optimization of PTA cladding. The lumped capacitance approach, while simplified, captures the essential physics of particle heating and provides practical guidance for powder selection and process parameter setting.

A particularly important insight is the recognition that particle size distribution uniformity is more critical than average particle size. In industrial practice, powder lot-to-lot variability can lead to inconsistent cladding quality even with identical process parameters. This has implications for powder supplier qualification and incoming inspection requirements.

The study also highlights the challenge of achieving low dilution with large-diameter powders, which is often necessary for high-deposition-rate applications. Engineers must balance productivity requirements against quality constraints, and the thermal data presented provides a quantitative basis for this trade-off analysis.

For pressure vessel overlay applications, the thermal behavior data should be incorporated into weld procedure qualification testing per NB/T 47014. The dilution ratio, being a function of particle thermal response, must be verified through metallographic examination of qualification coupons.