Effect of Welding Current on Microstructure and Properties of Aluminum Bronze Powder Plasma Surfacing Layer

Literature Overview

This paper by Wang Bo from Bohai Shipbuilding Vocational College, published in 2015 in Hot Working Technology, investigates the influence of plasma arc surfacing current on the microstructure, hardness, and wear resistance of aluminum bronze (Al-bronze) overlays deposited on 20G boiler steel. The study employs reverse-polarity plasma arc surfacing with aluminum bronze powder and systematically varies the welding current to identify optimal parameters.

Technical Background

Aluminum bronze alloys are widely used for wear-resistant and corrosion-resistant applications in marine, chemical, and mining industries. Their excellent properties stem from the formation of hard CuAl₂ (θ-phase) precipitates within a copper-rich matrix. However, aluminum bronze is difficult to cast and machine, making plasma surfacing an attractive alternative for applying these properties to existing components.

The 20G base material (a Chinese standard boiler steel equivalent to ASTM A192 Gr. C) contains approximately 0.2% C, 0.3-0.6% Mn, and 0.17-0.37% Si. This low-carbon steel provides a good substrate for plasma surfacing due to its low hardenability and minimal cracking susceptibility.

Experimental Design and Results

The study varies the plasma surfacing current while keeping other parameters constant. The key findings are summarized below:

The paper identifies 110 A as producing the finest microstructure and 120 A as producing the highest hardness. The optimal wear resistance range is 110-120 A.

Microstructural Evolution Mechanism

The transition from lamellar to polygonal structure with increasing current is explained by the following mechanisms:

1. Low current (90-100 A): Lower heat input results in slower cooling rates and less thermal driving force for nucleation. The θ-phase (CuAl₂) precipitates in a lamellar arrangement, typical of eutectic solidification.
2. Intermediate current (110-120 A): Increased heat input provides higher thermal energy, promoting more uniform powder melting and better mixing. The cooling rate increases due to the higher energy density, resulting in finer grain structures.
3. High current (130+ A): Excessive heat input leads to excessive dilution with base metal, reducing the effective aluminum content in the overlay. The cooling rate may decrease due to heat accumulation, resulting in coarser polygonal structures.

The dilution rate is a critical parameter that varies with current. At higher currents, more base metal melts and mixes with the powder, reducing the aluminum content in the overlay. This dilution effect explains why hardness decreases at currents above 120 A—the reduced aluminum content limits θ-phase formation.

Plasma Surfacing Process Characteristics

Reverse-polarity plasma arc surfacing offers specific advantages for this application:

The reverse polarity (negative electrode) configuration produces a concentrated, high-energy arc suitable for surfacing thin layers with low dilution. The argon shielding gas prevents oxidation of the aluminum in the powder, which would otherwise form Al₂O₃ inclusions that degrade mechanical properties.

Wear Mechanism Analysis

The wear resistance of aluminum bronze overlays is primarily governed by:

• θ-phase (CuAl₂) precipitates: These hard, coherent precipitates (HRC 40-50) provide the primary wear resistance mechanism through micro-ploughing resistance.
• Matrix hardness: The Cu-rich solid solution matrix (HRC 30-35) provides the base hardness.
• Microstructure refinement: Finer precipitate spacing increases the number of obstacles to dislocation motion, improving both hardness and wear resistance.

At 110-120 A, the combination of fine microstructure, optimal θ-phase volume fraction, and controlled dilution produces the best wear performance. The intermetallic wear test (金属间磨损试验) mentioned in the paper likely involves sliding against a hardened steel counterface, simulating abrasive wear conditions.

Engineering Practice Considerations

For practical implementation of aluminum bronze plasma surfacing:

1. Surface preparation: The 20G substrate should be machined to Ra 3.2 μm and cleaned with acetone to remove oils and contaminants.
2. Powder quality: Aluminum bronze powder must be free of oxidation. Powders with >0.5% O content should be rejected, as oxide inclusions significantly reduce hardness and wear resistance.
3. Layer thickness control: Each pass deposits approximately 0.3-0.5 mm. Multiple passes should be applied with interpass temperature below 150°C.
4. Post-weld heat treatment: Aging at 500-550°C for 2-4 hours can precipitate fine θ-phase, improving hardness by 10-15%.
5. Application limitations: Plasma surfacing is best suited for axisymmetric or flat surfaces. Complex geometries may require alternative processes.

Study Insights and Implications

This study provides valuable quantitative data on the current-microstructure-property relationship for aluminum bronze plasma surfacing. The identification of 110-120 A as the optimal range for 20G substrate with aluminum bronze powder is directly applicable to engineers selecting process parameters for similar applications.

The finding that hardness peaks at a slightly higher current (120 A) than the finest microstructure (110 A) is interesting from a materials science perspective. It suggests that the hardness maximum is not solely governed by microstructural refinement but also by the optimal balance between θ-phase volume fraction and matrix composition. At 110 A, the microstructure is finest but the θ-phase fraction may not be maximized. At 120 A, slightly more dilution occurs, but the θ-phase volume fraction reaches its peak, resulting in higher overall hardness.

This study is particularly relevant for marine applications where aluminum bronze overlays are used on propeller shafts, pump impellers, and valve components. The ability to precisely control the overlay properties through current selection provides a powerful tool for tailoring performance to specific service conditions.