Comparative Study of TIG Surfacing Processes for Soft Iron Cartridge Belts

Literature Overview

This paper by Lv Shixiong and colleagues from Harbin Institute of Technology's State Key Laboratory of Advanced Welding and Joining, along with collaborators from Zhejiang Qianjiang Motorcycle Co., Ltd. and Heilongjiang Hua'an Machinery Co., Ltd., presents a comparative study of three TIG surfacing variants for applying soft iron deposits on projectile steel substrates. Published in Acta Armamentarii in 2013, the work addresses a critical military manufacturing challenge: replacing traditional copper cartridge belts with soft iron to improve performance and reduce costs.

Technical Background

Cartridge belts (弹带) are helical or circumferential metal strips applied to the exterior of artillery projectiles. They serve to seal the gap between the projectile and the gun barrel bore, ensuring gas containment during firing. Traditionally, copper has been the material of choice due to its excellent formability, low hardness, and ability to conform to the bore surface during firing.

However, copper belts present several disadvantages:

• High material cost due to copper price
• Potential for copper contamination in the barrel bore affecting subsequent rounds
• Environmental and recycling concerns
• Difficulty in TIG surfacing due to interfacial penetration cracking

The interfacial penetration cracking problem in copper TIG surfacing is particularly challenging. Copper and steel have significantly different thermal expansion coefficients and form brittle intermetallic compounds at the interface, leading to cracks that propagate into the substrate during the thermal cycling of the welding process.

Experimental Design

The authors selected soft iron as an alternative to copper for the cartridge belt material. Three TIG surfacing variants were compared:

The hot wire TIG process (热丝TIG焊) was newly designed by the authors for this application. In this variant, the filler wire is pre-heated before entering the arc, effectively increasing the heat input per unit of electrical energy and allowing higher wire feed speeds. The vibration wire TIG process (振动焊丝TIG焊) introduces mechanical vibration to the wire during welding, which has a fragmentation effect on the solidifying weld metal.

Results and Analysis

Microstructural Effects

The three processes produce distinctly different microstructures:

• Cold wire TIG: Baseline microstructure with moderate grain size at both the interface and in the surfacing layer.
• Hot wire TIG: Both the interface and surfacing layer exhibit coarser grains compared to cold wire. The higher heat input per unit length promotes grain growth.
• Vibration wire TIG: The mechanical vibration of the wire introduces a fragmentation effect that refines both the interface and surfacing layer grains compared to cold wire TIG.

Mechanical Properties

• Hardness: Vibration wire TIG produced the lowest average hardness of the soft iron surfacing layer at HV160, approximately HV15 lower than the other two methods. This is beneficial because cartridge belts require relatively low hardness to deform and seal during firing.
• Interface strength: All three methods achieved interface strength values exceeding the strength of the projectile base material itself, indicating that the bond is not the weak link in the system.

Functional Performance

Live firing tests confirmed that soft iron belts produced by all three methods performed satisfactorily, demonstrating that soft iron is a viable replacement for copper as a cartridge belt material.

Technical Interpretation

The vibration wire TIG process emerges as the most promising variant for this application. The mechanical vibration of the wire during welding achieves several beneficial effects:

1. Grain refinement: The vibration disrupts the dendritic solidification pattern, producing finer grains that improve toughness and formability.
2. Hardness reduction: The finer microstructure and potentially different phase distribution result in lower hardness, which is desirable for a sealing element that must deform under pressure.
3. Current reduction: The vibration process required only a 10 A reduction in current compared to cold wire, while achieving a higher wire feed speed of 1.6 m/min. This suggests improved thermal efficiency.

The hot wire TIG process, while achieving the highest wire feed speed (1.8 m/min), produced coarser microstructures. This is a classic trade-off: higher deposition rates often come at the cost of microstructural quality. For cartridge belt applications where the material must deform plastically during firing, coarse grains could be detrimental to formability and fatigue resistance.

Engineering Considerations

Several practical considerations emerge from this study:

• Substrate compatibility: The 20# steel substrate (equivalent to 20CrMo or similar low-alloy steel) is representative of projectile materials. The soft iron deposit must bond reliably to this substrate without introducing brittle intermetallic phases.
• Formability requirements: The cartridge belt must deform radially inward during firing to seal the bore gap. The HV160 hardness achieved by vibration wire TIG is well-suited for this purpose.
• Barrel compatibility: Soft iron, being ferrous, may leave iron residues in the bore rather than copper residues. This is generally considered acceptable or even preferable in many barrel designs.
• Manufacturing scale: The vibration wire TIG equipment requires additional mechanical components for wire vibration, but the process parameters are relatively simple to implement and control.

Key Questions and Reflections

1. How does the vibration frequency and amplitude affect the microstructure and properties of the surfacing layer? The paper does not provide detailed vibration parameter optimization.
2. What is the long-term durability of soft iron belts compared to copper after multiple firings?
3. Can the vibration wire TIG concept be extended to other ferrous surfacing applications where grain refinement is desired?
4. What are the residual stress levels at the interface for each process, and how do these affect the reliability of the belt under firing conditions?

Study Insights and Implications

This paper demonstrates that process innovation in TIG welding—specifically the introduction of wire vibration—can produce microstructural improvements that are difficult to achieve through parameter optimization alone. The vibration wire TIG process represents a practical engineering solution that balances deposition rate, microstructural quality, and functional performance. For defense manufacturing and related industrial applications, this approach offers a pathway to replacing expensive or problematic materials with more economical alternatives while maintaining or improving functional performance. The work also highlights the importance of matching surfacing process selection to the specific functional requirements of the application, rather than simply maximizing deposition rate or hardness.