Effect of Welding Process and Micro-Alloying on Aluminum Bronze Overlay Microstructure and Bond Strength

Literature Overview

This study published in Copper Industry Engineering (2025, Issue 5) by Ning Shaochen and colleagues from the Ningbo Intelligent Machine Tool Research Institute and Jiangxi Copper Technology Research Institute addresses the challenging problem of aluminum bronze overlay welding onto steel substrates. Funded by a National Key R&D Program project (2021YFB3401100), this research tackles the complex interfacial reactions and stress concentration issues inherent in dissimilar material joining, which is a critical challenge in the manufacturing of high-performance machine tool components and marine equipment.

Research Configuration

Two aluminum bronze alloys (S215 and S216) were overlaid onto steel substrates using two welding processes:

Cold Metal Transfer (CMT): A pulsed short-circuiting process with very low heat input, characterized by controlled wire feeding and periodic wire retraction
Pulsed GMAW: Conventional pulsed metal transfer with higher heat input per unit length

The key differentiating factor between S215 and S216 is the micro-alloying with Mn and Ni elements, which fundamentally alters the thermal behavior and interfacial reaction dynamics.

Microstructural Evolution at the Interface

Parameter	S215 (CMT)	S215 (Pulsed)	S216 (CMT)	S216 (Pulsed)
α-phase grain size	Finer	Coarser	Coarser	Finer
Second phase distribution	More dispersed	Less dispersed	Less dispersed	More dispersed
Bond strength relative to reference	+38 MPa	Baseline	Baseline	+32 MPa
Fracture location	Copper side	Copper side	Copper side	Copper side

S215 Behavior Analysis

For S215 alloy, CMT mode produces superior results because:

Lower heat input results in finer α-phase grains with more dispersed second-phase particles
The reduced thermal cycle minimizes interfacial diffusion and avoids excessive intermetallic compound formation
The controlled solidification rate promotes uniform nucleation and growth of the α-phase
Bond strength improvement of approximately 38 MPa over pulsed mode is achieved

S216 Behavior Analysis

For S216 alloy, the opposite trend is observed because:

Mn and Ni additions lower the alloy melting point
At identical welding parameters, more heat is absorbed by the steel substrate rather than the bronze filler
In pulsed mode, the higher heat input promotes Fe diffusion from steel into the copper filler material
Fe diffusion causes heterogeneous nucleation and Fe enrichment at grain boundaries creates pinning effects
The resulting α-phase exhibits fine columnar and equiaxed grains with dislocation pinning
Bond strength improvement of approximately 32 MPa over CMT mode is achieved

Interfacial Reaction Mechanism

The fundamental mechanism driving the different behaviors is the Fe-Cu interdiffusion at the dissimilar interface:

Low heat input (CMT, S215): Limited Fe diffusion → fewer intermetallic compounds → cleaner interface → fracture in copper side with higher strength
High heat input with alloying (Pulsed, S216): Enhanced Fe diffusion → Fe enrichment at grain boundaries → heterogeneous nucleation → refined microstructure → dislocation pinning → higher strength

The fracture consistently occurs on the copper side of the interface, confirming that the metallurgical bond at the steel-bronze interface is stronger than the bulk bronze material. This is a desirable failure mode indicating good interfacial integrity.

Engineering Practice Implications

Design Consideration	Recommendation
Process selection for S215	CMT for maximum bond strength
Process selection for S216	Pulsed GMAW for maximum bond strength
Heat input control	Critical parameter—must be optimized for each alloy system
Interface quality	Verified by fracture location analysis
Micro-alloying strategy	Mn and Ni additions change optimal process selection

Study Insights

This research demonstrates a counterintuitive but important principle: the optimal welding process for dissimilar material overlay is not universal but depends on the specific alloy chemistry. The same process that produces superior results for one alloy system (CMT for S215) produces inferior results for another (S216). This underscores the necessity of process-alloy matching studies rather than adopting a one-size-fits-all approach. The Fe diffusion mechanism identified in S216 is particularly elegant—it shows how micro-alloying can be leveraged to promote beneficial interfacial reactions rather than merely suppressing them. For engineers developing aluminum bronze overlay processes for machine tool and marine applications, this study provides a framework for rational process selection based on alloy-specific thermal and diffusion behavior.