TIG Arc Compound Droplet Deposition Additive Manufacturing of 45 Steel Lead Alloy Bimetallic Structures

Literature Overview

This study, authored by Du Jun, Jiang Minbo, Zhang Yongheng, Xu Siyuan, and Wei Zhengying from the School of Mechanical Engineering at Xi'an Jiaotong University, was published in Materials Reports in 2022 and supported by the National Natural Science Foundation of China (Grant No. 51775420) and the Civil Aerospace Pre-research Project (D020208). The work investigates a novel TIG arc compound droplet deposition additive manufacturing process for fabricating 45 steel/lead alloy bimetallic structures. This research sits at the intersection of additive manufacturing and bimetallic composite fabrication, addressing the challenge of joining dissimilar metals with vastly different melting points and thermal properties through a single-arc multi-feedstock approach.

Core Technical Concept

The compound droplet deposition technique differs fundamentally from conventional single-wire TIG welding by simultaneously introducing two distinct molten metal streams into a single arc zone. In this configuration, the 45 steel base material and the lead alloy filler are independently melted and transferred to the weld pool, where they coalesce to form a bimetallic composite structure. The key innovation lies in the controlled interaction between the two molten streams at the arc-weld pool interface, which determines the interfacial bonding quality, microsegregation patterns, and the overall metallurgical compatibility of the resulting composite.

Lead alloys, with melting points in the range of 280–330 °C, present extreme challenges when combined with ferrous alloys (45 steel melting point approximately 1495 °C) because of the enormous thermal gradient, potential for liquid lead embrittlement, and the risk of intermetallic compound formation at the interface. The compound droplet approach allows independent thermal management of each material stream, potentially mitigating these issues through precise control of arc parameters and wire feed rates.

Process Parameters and Metallurgical Analysis

The following table summarizes the critical process parameters and their expected effects on the bimetallic interface:

The microstructure at the steel-lead interface is expected to exhibit a gradient zone characterized by progressive solidification from the steel side (ferrite-pearlite microstructure) through a transition region containing possible intermetallic phases to the lead-rich zone. The cooling rate at the interface, which can reach 10^2–10^3 K/s in additive manufacturing conditions, plays a decisive role in determining whether a continuous or discontinuous interfacial layer forms.

Defect Analysis and Countermeasures

Using an FMEA (Failure Mode and Effects Analysis) framework, the following defect modes are identified:

• Porosity: Lead vaporization at the high temperatures near the arc can generate gas bubbles that become trapped in the solidifying composite. Countermeasures include reducing arc current to minimize lead-side thermal exposure and optimizing wire feed synchronization.
• Cracking: Thermal stresses arising from the mismatch in thermal expansion coefficients between steel and lead can induce cracking at the interface. The coefficient of thermal expansion for lead (approximately 29 × 10^-6 /K) is nearly three times that of 45 steel (approximately 12 × 10^-6 /K).
• Insufficient bonding: Incomplete coalescence between the two molten streams results in weak interfacial bonding. This can be mitigated by ensuring adequate overlap of the two droplet streams within the weld pool.
• Spatter: Excessive arc energy can cause violent metal transfer and spatter, particularly on the lead alloy side. Controlled pulsed current waveforms help stabilize droplet transfer.

Engineering Practice Implications

The practical significance of this research extends to applications where lead-containing bimetallic structures are required, such as radiation shielding components in aerospace applications, bearing bushings, and vibration-damping elements. The additive manufacturing approach offers the advantage of near-net-shape fabrication, reducing the material waste inherent in traditional explosive cladding or electroplating methods for lead-containing composites. However, the current deposition rates achievable with TIG-based compound droplet processes are likely in the range of 0.5–2.0 kg/h, which limits scalability for large structural components.

Study Insights and Reflections

This research represents a meaningful exploration of multi-material additive manufacturing using conventional welding equipment. The compound droplet concept effectively bridges the gap between single-material AM and true bimetallic fabrication, though the fundamental challenge of joining materials with such disparate properties remains formidable. The aerospace funding context suggests that the intended applications involve radiation shielding or specialized bearing applications where lead's density and lubricity are advantageous. A critical question that emerges from this study is whether the interfacial bond strength, which is inherently limited by the thermodynamic incompatibility of iron and lead, can be made sufficient for load-bearing applications or whether these composites are limited to non-structural uses. Further investigation into post-weld thermal treatment and surface modification of the interface would be valuable for enhancing long-term durability.