Effects of Welding Process and Microalloying on Aluminum Bronze Overlay Microstructure and Bond Strength

Literature Overview

Published in Copper Industry Engineering (2025, No. 5, pp. 83–90) by Ning Shaochen et al. from the Ningbo Intelligent Machine Tool Research Institute (China Machinery Engineering Corporation) and Jiangxi Copper Technology Research Institute, this study investigates the influence of welding process mode and microalloying on the microstructure and bonding strength of aluminum bronze overlay welds on steel substrates. Funded by the National Key R&D Program (2021YFB3401100), this work addresses the critical challenge of dissimilar metal joining between aluminum bronzes and carbon/low-alloy steels—a common requirement in shipbuilding, marine engineering, mining equipment, and chemical processing industries.

Technical Background

Aluminum bronze overlays on steel substrates serve several important engineering functions:

• Corrosion protection: Aluminum bronzes offer excellent resistance to seawater and chemical media
• Wear resistance: High hardness and fatigue resistance in abrasive environments
• Cavitation resistance: Essential for marine propellers, pump impellers, and valve components
• Galvanic compatibility: In certain configurations, aluminum bronzes provide controlled galvanic protection

The challenge lies in the significant differences between aluminum bronze and steel:

• Different thermal expansion coefficients
• Different melting points and solidification ranges
• Potential for intermetallic compound formation at the interface
• Risk of hot cracking in the weld metal
• Dilution effects that can compromise either material's properties

Welding Process Comparison: CMT vs. Pulse

The study compares two advanced welding processes:

S215 Aluminum Bronze Results

For the S215 aluminum bronze alloy, CMT welding produced superior results:

• Lower heat input led to finer α-phase grains
• More dispersed second-phase particles
• Bond strength approximately 38 MPa higher than pulse mode
• Fracture consistently occurred on the copper side (indicating good metallurgical bonding)

S216 Aluminum Bronze Results

For the S216 aluminum bronze alloy (containing Mn and Ni microadditions), the results were reversed:

• Pulse mode produced higher bond strength than CMT
• Fe diffusion from steel substrate into copper alloy was more pronounced in pulse mode
• Heterogeneous nucleation from Fe diffusion and grain boundary pinning by Fe enrichment produced fine columnar and equiaxed α-phase grains
• Dislocation pinning further enhanced strength
• Bond strength approximately 32 MPa higher than CMT mode

Microalloying Effects

The S216 alloy contains Mn and Ni microadditions that fundamentally alter the welding behavior:

1. Melting point depression: Mn and Ni lower the alloy melting point, affecting heat absorption distribution
2. Heat distribution: With lower melting point, more welding heat is absorbed by the steel substrate rather than the copper alloy
3. Diffusion behavior: In pulse mode, Fe from the steel substrate diffuses more readily into the copper alloy, becoming the dominant diffusing species
4. Nucleation effects: Fe diffusion causes heterogeneous nucleation and grain boundary Fe enrichment, pinning grain boundaries
5. Dislocation strengthening: The refined microstructure with Fe enrichment creates additional dislocation pinning

This reversal of behavior between S215 and S216 demonstrates that welding process selection cannot be generalized across all aluminum bronze compositions—microalloying fundamentally changes the thermodynamic and kinetic driving forces for diffusion and phase transformation.

Interface Metallurgy

Both welding modes achieved good metallurgical bonding at the aluminum bronze-steel interface. The fracture consistently occurred on the copper side, which is a positive indicator:

• Metallurgical bonding confirmed: No interfacial separation observed
• Bond strength adequate: Fracture in the bulk copper alloy rather than at the interface
• Intermetallic formation: Limited intermetallic compound formation at the interface, though not explicitly quantified

The interface reaction in aluminum bronze-steel systems typically involves:

• Fe dissolution into the copper melt
• Possible formation of Fe-Cu intermetallics (Cu3Fe, CuFe)
• Grain boundary wetting and penetration
• Dilution zone formation

Engineering Practice Implications

Process Selection Guidelines

Based on this study, the following process selection guidelines can be proposed:

1. For standard aluminum bronzes (S215-type): CMT welding is preferred for its lower heat input and finer microstructure
2. For microalloyed aluminum bronzes (S216-type with Mn, Ni): Pulse welding may produce superior bond strength due to enhanced diffusion and grain refinement
3. Always consider: Substrate preheating, interpass temperature control, and post-weld heat treatment as complementary measures

Quality Control Considerations

• Bond strength testing: Shear or tensile bond strength tests should be included in qualification procedures
• Fracture analysis: Examination of fracture surfaces to confirm metallurgical bonding vs. interfacial failure
• Microstructural characterization: Metallographic examination of the interface to assess intermetallic formation and grain structure
• Hardness profiling: Hardness measurements across the overlay, dilution zone, and interface

Standards and Specifications

Relevant standards for aluminum bronze welding include:

• AWS D10.9 (Welding of Copper and Copper Alloys)
• AWS D17.1 (Welding of Nickel and Nickel Alloys—applicable by analogy)
• ISO 1127 (Welding of copper and copper alloys)
• EN ISO 15614-2 (Qualification test procedures for welding of metallic materials)
• Various ship classification society requirements (DNV, Lloyd's Register, ABS)

Key Questions and Reflections

1. The study does not report specific welding parameters (current, voltage, pulse frequency, travel speed) for each mode. These are critical for reproducibility and for understanding the quantitative relationship between process parameters and microstructural outcomes.
2. The long-term durability of the bonded joints under thermal cycling and cyclic loading is not addressed—intermetallic growth and stress relaxation at the interface could degrade bond strength over time.
3. The role of substrate preparation (surface cleanliness, roughness, preheating) is not discussed, yet these factors significantly influence bonding quality in dissimilar metal welding.
4. The study focuses on two specific aluminum bronze compositions; the generalizability of the findings to other aluminum bronze grades (Al-B-2, Al-B-4, Al-B-9, etc.) requires further investigation.

Study Insights and Implications

This research provides valuable insight into the complex interplay between welding process, alloy composition, and bonding performance in aluminum bronze-steel dissimilar metal joints. The key finding—that microalloying can reverse the relative performance of different welding processes—has significant implications for process development and qualification. Engineers designing aluminum bronze overlay systems must carefully match the welding process to the specific alloy composition, rather than applying generalized process recommendations. The demonstration of good metallurgical bonding with fracture on the copper side in both cases is encouraging, suggesting that reliable aluminum bronze-steel joints are achievable with appropriate process selection. For marine and offshore applications where aluminum bronze overlays are commonly used for corrosion and cavitation protection, this work provides a technical basis for optimizing welding procedures to maximize joint performance.