Steel-Lead Bimetallic Structure via Droplet Deposition Hybrid TIG Arc Additive Manufacturing

Literature Overview and Research Context

The paper by Zhang Yongheng and colleagues from the State Key Laboratory of Mechanical Manufacturing Systems Engineering at Xi'an Jiaotong University, published in Rare Metal Materials and Engineering (2023, Vol. 52, No. 3), presents research on the additive manufacturing of steel-lead bimetallic structures using a droplet deposition hybrid TIG arc process. Supported by multiple funding sources including the Civil Aerospace Technology Preliminary Research Program and the Jiangsu Provincial Key Laboratory Open Research Project, this study addresses the challenge of creating metallurgically bonded interfaces between dissimilar metals with vastly different physical and chemical properties. Steel and lead have extremely different melting points (approximately 1500°C for steel versus 327°C for lead), thermal conductivities, and densities, making their direct joining a significant metallurgical challenge.

Core Technical Approach and Methodology

The researchers employed a droplet deposition hybrid TIG arc additive manufacturing process to create 45 steel/tin-lead alloy bimetallic structures. The process involves depositing molten lead alloy droplets onto a 45 steel substrate using a TIG arc as the heat source, with the droplet deposition providing the filler metal in a controlled manner. The interface microstructure was characterized using optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) to examine the microstructure, elemental distribution, and intermetallic compound (IMC) phases at the steel-lead interface.

Key Findings and Technical Interpretation

The study produced several significant findings regarding the steel-lead interface metallurgy:

• The steel-lead bimetallic structures produced by the droplet deposition hybrid TIG process exhibit no obvious macroscopic metallurgical defects such as cracks or porosity at the interface. This is a notable achievement given the extreme property mismatch between the two metals.
• Two distinct wetting mechanisms coexist at the steel-lead interface during the droplet impact and spreading process: "reactive wetting" and "inert wetting."
• In the reactive wetting regions, the interface exhibits minor undulations, and intermetallic compounds (IMCs) are formed at the interface. The identified IMCs are FeSb2 and FeSn2, which form through chemical reactions between the lead alloy and the steel substrate.
• In the inert wetting regions, no reaction products are formed, and micro-porosity defects are present. This indicates that in these regions, the lead alloy wets the steel surface through physical mechanisms alone without chemical interaction.
• The IMC layer thickness is greatest at the center of the molten pool, reaching approximately 6 μm. As the measurement position moves away from the molten pool center, the IMC layer thickness decreases in a nonlinear manner.

The coexistence of reactive and inert wetting at the same interface is a particularly interesting finding. It suggests that the local conditions during droplet impact and spreading (such as local temperature, cooling rate, and droplet velocity) vary across the interface, leading to different wetting behaviors in different regions.

Engineering Practice Implications

For applications requiring steel-lead bimetallic structures, such as radiation shielding components, vibration damping devices, and nuclear fuel storage containers, this research provides a viable manufacturing route. The absence of macroscopic cracks and porosity at the interface is encouraging for structural applications, although the presence of micro-porosity in the inert wetting regions may need to be addressed through process optimization or post-processing.

The identification of FeSb2 and FeSn2 as the interfacial IMCs is important for understanding the long-term stability of the interface. These intermetallic compounds are brittle phases, and their thickness and distribution directly affect the mechanical properties and failure behavior of the bimetallic interface. The nonlinear decrease in IMC thickness away from the molten pool center suggests that the thermal history of the interface varies significantly across the deposit, which should be considered when designing the geometry and loading conditions of the final component.

Study Insights and Reflections

This research represents a significant advance in the field of dissimilar metal joining and additive manufacturing. The successful creation of a metallurgically bonded steel-lead interface without macroscopic defects demonstrates the potential of the droplet deposition hybrid TIG process for challenging bimetallic applications. The finding of coexisting reactive and inert wetting mechanisms at the interface provides new insight into the fundamental wetting and reaction processes that govern dissimilar metal joining. Engineers working with bimetallic structures should pay close attention to the IMC layer thickness and distribution, as these factors directly influence the interface strength and long-term durability. The nonlinear variation of IMC thickness with distance from the molten pool center highlights the importance of controlling the thermal history during the additive manufacturing process to achieve uniform interface properties throughout the component.