Effect of Welding Current on Microstructure and Properties of Aluminum Bronze Powder Plasma Cladding Deposits

Literature Overview

This 2015 study by Wang Bo from the Department of Materials at Bohai Shipbuilding Vocational College investigates the influence of welding current on the microstructure, hardness, and corrosion resistance of aluminum bronze powder plasma cladding deposits. The work addresses a practical challenge in the marine and offshore engineering industry, where aluminum bronze alloys (such as CuAl10Fe5Ni5) are widely used for their excellent resistance to seawater corrosion and cavitation erosion. The study provides valuable insight into how process parameters affect the performance of plasma-clad aluminum bronze coatings.

Aluminum Bronze in Marine Applications

Aluminum bronze alloys are valued in marine engineering for their combination of good mechanical properties, excellent seawater corrosion resistance, and resistance to cavitation erosion. These alloys are commonly used for propeller blades, pump impellers, valve seats, and other components exposed to aggressive marine environments. However, the high cost of solid aluminum bronze components often motivates the use of cladding or overlay to apply a thin, corrosion-resistant surface layer onto a more economical base material such as carbon steel or low-alloy steel.

Plasma arc cladding is an attractive option for aluminum bronze overlay because it offers low dilution, high deposition rates, and the ability to produce dense, metallurgically bonded coatings with minimal porosity.

Experimental Configuration and Process Parameters

The study employed a plasma arc cladding system with a consumable aluminum bronze powder (composition: Cu balance, Al 10%, Fe 5%, Ni 5%) fed into the plasma arc. The following table summarizes the process parameters investigated:

The current was varied as the primary independent variable, with other parameters adjusted proportionally to maintain a consistent heat input where possible. Cross-sectional specimens were prepared for metallographic examination, hardness testing, and corrosion testing.

Microstructural Response to Current Variation

The study identified three distinct microstructural regimes corresponding to low, medium, and high current values:

At low currents, the plasma arc energy was insufficient to fully melt the aluminum bronze powder, resulting in unmelted or partially melted particles embedded in the deposit. These particles acted as stress concentrators and reduced both hardness and corrosion resistance. At high currents, excessive heat input caused increased dilution from the steel substrate, altering the effective alloy composition and destabilizing the alpha + delta phase structure characteristic of aluminum bronze.

In the optimal current range (180-220 A), the deposit exhibited a fine dendritic microstructure with a well-bonded interface to the substrate. The microstructure consisted of an alpha (Cu-Al solid solution) matrix with delta (Fe-Al intermetallic) phase particles distributed along the dendrite boundaries. This microstructure is characteristic of well-controlled aluminum bronze and provides the best combination of hardness, ductility, and corrosion resistance.

Corrosion Performance

Electrochemical polarization tests in simulated seawater (3.5% NaCl solution at 35 °C) demonstrated that the optimal current range produced deposits with a corrosion potential of approximately -0.2 to -0.3 V vs. SCE and a corrosion current density of 1-5 μA/cm². These values are significantly better than those of the uncoated Q235 substrate (corrosion current density of 50-100 μA/cm²) and approach the performance of cast aluminum bronze.

The deposits produced at low and high currents exhibited higher corrosion current densities (10-30 μA/cm²) due to the presence of unmelted particles, excessive dilution, and microstructural inhomogeneity. Localized corrosion was observed at the boundaries between unmelted powder particles and the matrix in the low-current deposits, and at the dilution zone near the substrate interface in the high-current deposits.

Engineering Practice Considerations

For marine applications, the optimal plasma cladding parameters identified in this study provide a practical process window for applying aluminum bronze coatings to steel substrates. The key parameters to control are plasma current (180-220 A), powder feed rate (80-100 g/min), and travel speed (200-250 mm/min). Preheating the substrate to 150 °C is recommended to reduce residual stress and minimize cracking at the interface.

Engineers should also consider the long-term performance of the coating in actual seawater service, including exposure to biofouling, temperature cycling, and mechanical abrasion. Post-cladding heat treatment (solution treatment at 850-900 °C followed by water quenching) may be beneficial for thick coatings to homogenize the microstructure and dissolve any incipient delta phase precipitation, though this must be balanced against the risk of substrate distortion.

Key Reflections

This study provides a clear demonstration of how welding current governs the quality of plasma-clad aluminum bronze deposits through its influence on dilution rate, powder melting efficiency, and solidification microstructure. The identification of an optimal current window (180-220 A) is practically valuable for process development and quality control in marine repair and manufacturing operations. The work underscores the importance of parameter optimization in thermal spray and cladding processes, where small variations in input parameters can lead to significant differences in deposit quality and performance.