Steel-Lead Bimetallic Structure by Droplet Deposition Hybrid TIG Arc Additive Manufacturing and Interface Microstructure Study

Literature Overview

This paper, authored by Zhang Yongheng et al. from Xi'an Jiaotong University's State Key Laboratory of Mechanical System and Vibration, published in Rare Metal Materials and Engineering (2023, Vol. 52, No. 3, pp. 1079-1086), presents an innovative approach to fabricating steel/lead bimetallic structures using a droplet deposition hybrid TIG arc additive manufacturing process. The work is particularly significant for applications in radiation shielding, nuclear engineering, and vibration damping, where the combination of structural steel strength with lead's high density and neutron absorption capability is highly desirable.

Process Description and Mechanism

Droplet Deposition Hybrid TIG Process

The process combines conventional TIG arc welding with controlled droplet feeding of molten lead alloy. The TIG arc melts the steel substrate (45 steel) while a separate mechanism delivers molten lead alloy droplets onto the molten pool. This hybrid approach enables direct metallurgical bonding between the steel and lead phases without requiring intermediate bonding layers or complex pre-treatment.

The process parameters include:

Interface Wetting Mechanisms

The paper identifies two coexisting wetting mechanisms at the steel/lead interface:

1. Reactive wetting — Occurs where lead alloy droplets chemically interact with the steel surface, forming intermetallic compounds (IMCs) such as FeSb₂ and FeSn₂. The reactive wetting interface shows minor undulations and good metallurgical bonding.
2. Inert wetting — Occurs where lead alloy droplets spread on the steel surface without chemical reaction. No reaction products form at the interface, but micro-porosity defects are observed due to insufficient wetting and bonding.

The coexistence of both mechanisms explains the non-uniform distribution of IMCs observed at the interface. The molten pool center exhibits the thickest IMC layer (approximately 6 μm), with thickness decreasing nonlinearly as the measurement position moves away from the center.

Microstructure Analysis

Metallographic Examination (OM)

Optical microscopy reveals:

• A clearly defined interface between the steel substrate and lead alloy deposit.
• No macroscopic cracks or voids at the interface, indicating successful metallurgical bonding.
• Gradient transition zones where the microstructure gradually changes from steel grain structure to lead alloy grain structure.

Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)

SEM analysis identifies three distinct regions at the interface:

1. Steel side — Ferrite-pearlite structure of 45 steel, with slight grain refinement near the interface due to thermal effects.
2. IMC transition zone — 2–8 μm thick layer of intermetallic compounds (FeSb₂, FeSn₂), with varying thickness depending on position relative to the molten pool center.
3. Lead alloy side — Sn-Pb eutectic microstructure with lead-rich dendrites in a tin-rich matrix.

X-Ray Diffraction (XRD)

XRD analysis confirms the presence of:

• FeSb₂ (orthorhombic structure)
• FeSn₂ (tetragonal structure)
• No evidence of FePb compounds, suggesting that lead does not directly participate in IMC formation but rather acts as a diluent.

Interface Quality Assessment

Defect Analysis

The absence of macroscopic cracks is a significant achievement, as steel-lead systems are inherently challenging due to their large coefficient of thermal expansion mismatch (steel: ~12×10⁻⁶/K; lead: ~29×10⁻⁶/K). The hybrid droplet deposition process appears to mitigate this mismatch by allowing controlled, gradual thermal cycling.

Engineering Applications and Implications

Radiation Shielding Applications

The steel/lead bimetallic structure is directly applicable to:

• Nuclear reactor shielding components (neutron and gamma radiation).
• Medical X-ray and gamma-ray shielding panels.
• Particle accelerator components requiring high-density shielding.

Vibration Damping Applications

The lead phase provides inherent damping capacity, making the bimetallic structure suitable for:

• Vibration isolation mounts in precision equipment.
• Seismic isolation bearings.
• Noise reduction components in aerospace structures.

Process Optimization Recommendations

Based on the findings, the following optimization strategies are recommended:

1. Increase the proportion of reactive wetting by optimizing droplet temperature and impact velocity.
2. Apply a thin pre-deposited layer of antimony or tin on the steel surface to promote uniform IMC formation.
3. Implement controlled cooling between deposition passes to reduce thermal stress.
4. Optimize the droplet feed rate to ensure consistent molten pool geometry.

Study Insights

This research demonstrates a novel and promising approach to fabricating bimetallic components that combine structural integrity with functional properties. The identification of dual wetting mechanisms (reactive and inert) provides fundamental insight into the metallurgical bonding process, which is essential for process optimization. The nonlinear decrease of IMC thickness away from the molten pool center suggests that the thermal field distribution is a critical process variable. Future work should focus on achieving uniform reactive wetting across the entire interface, possibly through optimized droplet delivery strategies or surface pre-treatment of the steel substrate. The ability to directly bond steel and lead without intermediate layers represents a significant advancement in bimetallic manufacturing technology.