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

Literature Overview

Plasma transferred arc (PTA) cladding using aluminum bronze (Al-bronze) powder is a widely used technology for producing wear-resistant and corrosion-resistant overlay layers on industrial components. The literature under review investigates the systematic effect of welding current on the microstructure, hardness, wear resistance, and corrosion resistance of Al-bronze PTA cladding layers. This study note examines the key findings, process parameters, metallurgical mechanisms, and practical implications.

Technical Background

Aluminum bronze (Al-bronze) alloys are known for their excellent combination of:

• High hardness and strength (up to 400 HB in solution-treated and aged condition)
• Outstanding wear resistance in sliding and abrasive conditions
• Excellent corrosion resistance in seawater and acidic environments
• Good thermal conductivity
• Non-magnetic properties

The Al-bronze PTA cladding layer is widely applied in:

• Marine propellers and pump impellers
• Mining and construction equipment components
• Chemical processing equipment
• Hydraulic components
• Valve seats and trim

PTA cladding is particularly well-suited for Al-bronze applications because:

1. The inert gas shielding provides excellent protection against oxidation
2. The dilution rate is low (typically 5–15%), preserving the cladding layer composition
3. The process is easily automated for consistent quality
4. The heat input is controllable, allowing optimization of microstructure

Process Parameters and Experimental Design

The welding current is the primary parameter investigated because it directly controls:

• Heat input (Q = I × V × t / v)
• Penetration depth into the substrate
• Dilution rate of the cladding layer
• Cooling rate of the molten pool
• Solidification microstructure

Effect of Current on Microstructure

Low Current (150–180 A)

At low welding current:

• Heat input is low, resulting in high cooling rates (10–50 °C/s)
• Penetration depth is shallow (0.5–1.0 mm)
• Dilution rate is low (5–10%)
• Microstructure consists of fine dendritic grains with inter-dendritic precipitates
• The matrix is primarily α-Cu (solid solution of Al, Fe, Ni in copper)
• ε-phase (Cu₅Al₈) precipitates are fine and uniformly distributed
• Hardness is 250–300 HV due to fine precipitate dispersion

Medium Current (200–250 A)

At medium welding current (optimal range):

• Heat input is moderate, resulting in moderate cooling rates (5–15 °C/s)
• Penetration depth is moderate (1.0–2.0 mm)
• Dilution rate is moderate (10–15%)
• Microstructure consists of medium-sized dendritic grains
• ε-phase precipitates are coarser and more distinct
• Some η-phase (CuAl₂) may appear at higher Al content
• Hardness is 300–380 HV, representing the optimal balance

High Current (280–350 A)

At high welding current:

• Heat input is high, resulting in low cooling rates (2–8 °C/s)
• Penetration depth is deep (2.0–3.5 mm)
• Dilution rate is high (15–25%)
• Microstructure consists of coarse dendritic grains with significant interdendritic segregation
• ε-phase precipitates are coarse and irregularly shaped
• η-phase (CuAl₂) is abundant, forming a network at grain boundaries
• Hardness may decrease to 250–300 HV due to coarse precipitates and increased iron dilution
• Cracking susceptibility increases due to high residual stress and coarse microstructure

Effect of Current on Properties

The optimal current range of 200–250 A provides the best combination of:

• High hardness (320–350 HV)
• Excellent wear resistance (1.8–2.2 times the substrate)
• Acceptable dilution rate (10–14%)
• Low crack susceptibility
• Good bond strength (> 400 MPa)

Metallurgical Mechanisms

The relationship between welding current and cladding layer properties is governed by several metallurgical mechanisms:

1. Solidification microstructure: The cooling rate, controlled by current and travel speed, determines dendrite spacing and precipitate morphology. Optimal cooling rates (5–15 °C/s) produce medium-sized dendrites with fine ε-phase precipitates, providing high hardness.
2. Phase transformation: The Al-bronze system exhibits several intermetallic phases (ε, η, κ) whose formation depends on composition and cooling rate. The ε-phase (Cu₅Al₈) is the primary hardening phase, while the η-phase (CuAl₂) is harder but more brittle. Current affects the relative proportion of these phases.
3. Dilution effect: Higher current increases dilution, introducing more iron from the steel substrate. Iron is beneficial in moderate amounts (enhancing hardness through solid solution strengthening) but detrimental in excess (promoting brittle phases and reducing ductility).
4. Residual stress: Higher current produces greater thermal gradients and residual stresses, increasing crack susceptibility. The combination of high dilution (reducing ductility) and high residual stress (increasing crack driving force) at high current levels explains the increased cracking tendency.

Engineering Practice Implications

Based on the study findings, several practical recommendations emerge:

1. Process window definition: The optimal process window for Al-bronze PTA cladding on carbon steel substrates is welding current 200–250 A, travel speed 150–200 mm/min, and powder feed rate 150–200 g/min. This window should be established through qualification testing for each specific application.
2. Multi-layer strategy: For thick cladding layers (3–5 mm), a multi-layer approach is recommended. The first layer (bond layer) should use slightly lower current (180–200 A) to ensure good bond strength, while subsequent layers can use optimal current (220–250 A) for maximum hardness and wear resistance.
3. Heat treatment: The as-cladded Al-bronze layer can be further strengthened by solution treatment (950–1000 °C, 1–2 hours) followed by aging (400–500 °C, 2–4 hours). This can increase hardness from 350 HV to 400–450 HV. However, heat treatment must be carefully controlled to avoid distortion of the substrate.
4. Inspection requirements: The cladding layer should be inspected by:

• MT or PT for surface cracks
• UT or bond strength testing for interface integrity
• Hardness mapping to verify uniformity
• Metallographic examination for microstructure and defects

1. Substrate preparation: The substrate surface should be ground to remove scale and contamination. A slight bevel (1–2 mm) may be beneficial to ensure proper bond. Preheating to 150–200 °C is recommended to reduce cracking risk.

Study Insights and Reflections

The systematic investigation of welding current effects on Al-bronze PTA cladding provides valuable insights into the process-property relationships. The key finding is that there exists an optimal current range (200–250 A) that balances competing factors: low enough to minimize dilution and cracking, but high enough to ensure adequate bond strength and hardening precipitate formation.

The study also highlights the importance of understanding the metallurgical mechanisms behind property variations. Without this understanding, process optimization becomes purely empirical and may not be transferable to different substrates, powder compositions, or component geometries.

From an engineering practice perspective, the study reinforces the importance of:

• Establishing and documenting process windows through systematic qualification testing
• Understanding the interaction between process parameters and material properties
• Implementing rigorous quality control procedures to ensure consistent cladding layer quality
• Considering post-weld heat treatment as a means to further optimize properties

In conclusion, the effect of welding current on Al-bronze PTA cladding is a complex but well-understood phenomenon. The optimal current range of 200–250 A provides excellent hardness, wear resistance, and crack resistance, making it the recommended process parameter for most Al-bronze PTA cladding applications.