Thermal Behavior of Powder Particles in Plasma Arc Powder Overlay Welding

Literature Overview

This paper by Wang Xibao, Wang Xiaofeng, and Jia Fengsuo from Tianjin University and Hebei Installation Company, published in the Transactions of the Welding Institute of China in 2002, presents a theoretical analysis of the thermal behavior of powder particles during plasma arc powder overlay welding. The study focuses on two types of powder particles: iron-based (Fe-based) particles and boron carbide (B4C) particles, which represent two fundamentally different categories of overlay materials. The researchers applied heat transfer theory to calculate and analyze the heating behavior of these particles as they travel through the plasma arc column, examining the effects of plasma current, particle size, and particle material properties on the degree of particle heating and melting.

Theoretical Framework and Heat Transfer Analysis

The thermal behavior of powder particles in plasma arc overlay welding is governed by the heat transfer mechanisms operating within the plasma arc column. As powder particles are injected into the arc, they are subjected to convective heat transfer from the hot plasma gas, radiative heat transfer from the arc column and electrode, and potentially conductive heat transfer if the particles come into contact with other particles or the arc electrodes. The rate of heating depends on the particle size, material properties, plasma current, and the position of the particle within the arc column.

The researchers developed a heat transfer model that accounts for the convective and radiative heat fluxes acting on the particle surface. The convective heat transfer coefficient depends on the plasma gas velocity, viscosity, and thermal conductivity, while the radiative heat flux depends on the temperature of the surrounding plasma and the emissivity of the particle surface. The particle temperature rises as it absorbs heat, and the rate of temperature increase depends on the particle's thermal mass, which is a function of its mass, specific heat capacity, and temperature.

Key Parameters Affecting Particle Thermal Behavior

Results for Iron-Based Powder Particles

The analysis reveals that the plasma current has a much greater effect on the degree of particle heating than on the particle transport velocity. This finding is significant because it means that increasing the current primarily improves particle melting rather than particle delivery speed, which is important for process optimization.

For Fe-based powder particles at a plasma current of 200 A, the thermal behavior is strongly size-dependent:

• Particles ≤ 50 μm: Completely vaporized within the arc column. These very small particles have extremely high surface-to-volume ratios and are rapidly heated to their boiling point, transitioning directly to the vapor phase.
• Particles 50-150 μm: Completely melted within the arc column. These particles reach their melting point and become fully liquid, which is the desired condition for good bonding with the base metal and uniform overlay composition.
• Particles > 200 μm: Not fully melted within the arc column. These larger particles retain solid cores even after traversing the arc, which can result in poor bonding, unmelted inclusions, and heterogeneous overlay microstructure.

At a lower plasma current of 100 A, the heating capacity of the arc is significantly reduced:

• Particles < 50 μm: Can be completely melted within the arc column, but only a small number near the arc center reach the melting temperature.
• Particles > 50 μm: Cannot be completely melted; only partial melting occurs near the arc center.

These results clearly demonstrate that the plasma current must be sufficiently high to ensure complete melting of the powder particles, and that particle size selection is critical for achieving uniform overlay properties.

Results for Boron Carbide (B4C) Powder Particles

The thermal behavior of B4C particles is fundamentally different from that of Fe-based particles due to the much higher melting point of B4C (approximately 2450°C compared to 1538°C for iron). Even at a plasma current of 200 A, only particles with diameter ≤ 50 μm can be completely melted within the arc column center region. Particles larger than this size are only preheated and do not reach their melting temperature.

This finding has important implications for the use of B4C as a reinforcing phase in overlay welding. The inability to melt B4C particles means that they must be delivered to the weld pool in a preheated but still solid state, where they are then melted by the molten metal in the weld pool. This requires careful control of the powder injection parameters to ensure that the B4C particles are delivered to the weld pool in a manner that promotes good bonding and uniform distribution.

Comparison of Thermal Behavior: Fe-Based vs. B4C Particles