Effect of Surfacing Current on Microstructure and Properties of Aluminum Bronze Powder Plasma Arc Overlay

Literature Overview

The paper by Wang Bo, published in Hot Working Technology (2015, Vol. 44, No. 23, pp. 237–239), investigates the effect of surfacing current on the microstructure and mechanical properties of aluminum bronze powder plasma arc surfacing layers deposited on 20G boiler steel substrates. The study employs reverse polarity plasma arc surfacing and examines current levels that produce different microstructural transitions, identifying an optimal current range of 110–120 A for maximum wear resistance. This work is significant for its systematic investigation of the process parameter-microstructure-property relationship in plasma arc surfacing, a process that is widely used for depositing wear-resistant and corrosion-resistant overlays on industrial components.

Plasma Arc Surfacing Process and Parameters

Plasma arc surfacing (PAS) is a specialized welding process that uses a transferred or non-transferred plasma arc to melt a wire or powder feedstock onto the substrate surface. The key advantages of plasma arc surfacing include:

• High energy density, which produces deep penetration and good metallurgical bonding
• Precise control of the molten pool through adjustment of plasma current, gas flow, and travel speed
• Low dilution rates compared to conventional arc processes, preserving the alloy composition of the overlay
• Suitability for depositing a wide range of alloy compositions, including high-alloy and refractory materials

In this study, the reverse polarity configuration was used, meaning the plasma electrode was connected to the positive terminal and the workpiece to the negative terminal. This configuration produces a more constricted plasma jet with higher energy density at the arc spot, which is advantageous for achieving deep penetration and good fusion with the substrate.

The substrate material, 20G boiler steel (equivalent to ASTM A210 Gr. A or similar), is a low-carbon, low-alloy steel with approximately 0.17–0.24% C and 0.35–0.65% Mn. This material is commonly used in boiler tubes and pressure vessels and represents a typical substrate for wear-resistant surfacing applications in the power generation industry.

Microstructure Evolution with Current

The most important finding of this study is the systematic microstructural evolution observed with increasing surfacing current:

At lower currents, the cooling rate is relatively moderate, and the aluminum bronze solidifies in a lamellar structure consisting of alternating layers of alpha (Cu-Al solid solution) and beta (Cu-Al intermetallic) phases. As the current increases, the energy input increases, but the interaction with the powder feedstock and the plasma jet dynamics lead to changes in the solidification conditions. At 110 A, the structure transitions to a polygonal morphology, which the authors identify as the finest structure. This fine polygonal structure is associated with the highest hardness and wear resistance.

Above 120 A, the increased energy input leads to excessive melting and coarsening of the microstructure. The molten pool becomes larger and the cooling rate decreases, allowing for greater grain growth and phase coarsening. This results in a reduction in hardness and wear resistance despite the higher energy input.

Wear Mechanism Analysis

The wear resistance of aluminum bronze overlays is primarily governed by the hardness and distribution of the beta phase (Cu-Al intermetallic compound, typically CuAl2), which is significantly harder than the alpha phase (Cu-Al solid solution). The fine dispersion of hard beta phase particles within the softer alpha matrix provides an effective combination of wear resistance and toughness.

At the optimal current of 110–120 A, the fine polygonal structure ensures that the beta phase is finely dispersed throughout the matrix, maximizing the hardness-wear resistance response. The wear testing methodology, which involved metal-on-metal abrasion, simulates a common wear condition in industrial applications such as valve seats, pump impellers, and hydraulic cylinder liners.

The relationship between current and wear resistance is not simply monotonic but rather exhibits an optimum, which is a characteristic feature of many surfacing processes. This non-monotonic behavior is important for process development, as it demonstrates that there is a specific process window that must be identified and maintained for optimal performance.

Study Insights and Reflections

This paper provides valuable insight into the process parameter optimization of plasma arc surfacing for aluminum bronze overlays. The identification of the 110–120 A optimal range is practically useful for engineers setting up plasma arc surfacing operations for aluminum bronze deposits.

One aspect that deserves further consideration is the role of other process parameters such as travel speed, powder feed rate, and plasma gas flow rate. While the current is clearly the dominant parameter for microstructural control, the interaction between current and these other parameters can significantly affect the final overlay quality. A more comprehensive parameter optimization study would be beneficial for production applications.

The study also highlights the importance of microstructural characterization in understanding and predicting overlay performance. The transition from lamellar to polygonal structure with increasing current is a direct consequence of the changing solidification conditions, and understanding this relationship is essential for rational process development rather than purely empirical optimization.

From an engineering practice perspective, this work demonstrates that careful control of plasma arc surfacing parameters can significantly improve the performance of aluminum bronze overlays. The optimal current range identified provides a starting point for process development in specific applications, and the microstructural characterization methodology can be applied to other surfacing systems to establish similar process-parameter-property relationships.