Copper Alloy Surfacing on Projectile Body Using TIG Arc Welding

Literature Overview and Technical Context

The paper by Liu Shuyan, Hou Xiujun, Wang Shuhua, and Deng Hua (2011), published in Chemical Analysis and Metrology, addresses a specific manufacturing challenge in the ammunition industry: the reliable joining of copper alloy bands to projectile bodies. The traditional method involved mechanically inserting ring-shaped copper or copper alloy bands into grooves machined on the projectile body. This mechanical connection approach suffered from significant drawbacks, including deformation of thin-walled projectile bodies during the extrusion process and occasional loosening of the copper bands, both of which compromised the functional performance of the finished product. The proposed solution was to replace the mechanical connection with TIG arc welding overlay, depositing the copper alloy directly onto the projectile body substrate.

Problem Analysis and FMEA Approach

Applying a Failure Mode and Effects Analysis (FMEA) framework to the traditional mechanical connection method reveals the following critical failure modes:

The proposed TIG arc welding overlay approach directly addresses these failure modes by creating a metallurgical bond between the copper alloy and the projectile body, eliminating the need for mechanical interference and the associated extrusion forces.

Process Description and Technical Parameters

The TIG arc welding overlay process involves depositing copper alloy material onto the projectile body substrate using a non-consumable tungsten electrode. The key process parameters and material considerations include:

• Base material: Projectile body steel (likely a medium-carbon or low-alloy steel suitable for armor or structural applications)
• Overlay material: Copper or copper alloy (likely Cu-Sn or Cu-Al based alloy for adequate hardness and bonding)
• Welding process: GTAW (Tungsten Inert Gas) arc welding
• Shielding gas: Argon or helium, or a mixed gas combination
• Filling material: Copper alloy rod or wire matching the intended band composition

The TIG process was selected because it offers excellent control over heat input, clean welds with minimal spatter, and the ability to deposit thin, uniform layers of dissimilar metals without excessive dilution or contamination.

Microstructural and Compositional Analysis

The paper reports comprehensive metallurgical characterization of the overlay, interface, and base material using multiple analytical techniques:

Energy-dispersive X-ray spectroscopy (EDS) was employed to analyze the iron content in the copper alloy layer and to map the compositional gradient across the overlay-base interface. This analysis is critical for assessing the dilution of base material into the copper overlay and determining whether the resulting interface composition maintains adequate metallurgical bonding.

Spectrographic analysis provided quantitative elemental composition data for the copper alloy layer, confirming the alloying additions and their distribution uniformity.

Scanning electron microscopy (SEM) was used to examine the microstructural morphology of the copper alloy layer, the interface region, and the projectile body substrate. The SEM analysis would reveal grain structure, phase distribution, and any intermetallic compound formation at the interface.

Optical microscopy provided a broader view of the microstructural features, including grain size, phase morphology, and any defects such as porosity or cracks.

Interface Metallurgy and Bonding Mechanism

The interface between the copper alloy overlay and the steel projectile body is the critical zone for joint integrity. In copper-steel dissimilar metal welding, several metallurgical phenomena must be considered:

1. Intermetallic compound formation: Iron and copper have limited mutual solubility, and the formation of brittle iron-copper intermetallic compounds (such as Fe2Cu) at the interface can severely compromise mechanical properties. The TIG process, with its controlled heat input, helps minimize the extent of intermetallic compound formation.
2. Dilution and compositional gradient: The degree of base material dilution into the copper overlay creates a compositional gradient at the interface. This gradient must be managed to ensure adequate bonding strength while maintaining the desired copper alloy properties.
3. Thermal expansion mismatch: Copper has a significantly higher coefficient of thermal expansion than steel, which creates residual stresses during cooling. These stresses can lead to cracking or delamination if not properly managed through process parameter optimization.

Mechanical Performance Assessment

The paper includes mechanical performance analysis of the projectile body with the copper alloy overlay. Key performance indicators would include:

The elimination of mechanical interference fit and replacement with a metallurgical bond fundamentally changes the load transfer mechanism. Instead of relying on friction and interference, the joint integrity is now dependent on the metallurgical bond strength at the interface, which must be sufficient to withstand the extreme accelerative forces experienced during projectile firing.

Study Insights and Engineering Implications

This paper demonstrates the versatility of TIG arc welding as a process for joining dissimilar metals in precision applications. The transition from mechanical to metallurgical joining represents a paradigm shift in manufacturing approach, and the comprehensive analytical methodology employed (EDS, spectroscopy, SEM, optical microscopy) provides a model for evaluating similar dissimilar metal joining applications.

The engineering significance extends beyond the ammunition industry. Any application requiring the attachment of copper or copper alloy components to steel substrates through a permanent, high-integrity bond can benefit from the principles documented in this work. The analytical approach of characterizing the overlay, interface, and base material microstructures is directly transferable to overlay welding quality assessment in the piping and pressure equipment industry.

The study reinforces the importance of understanding interface metallurgy in dissimilar metal welding. The controlled heat input capability of TIG welding, combined with appropriate alloy selection and process parameter optimization, provides a reliable pathway for achieving high-integrity dissimilar metal joints even in thin-walled precision components where heat input must be minimized to prevent distortion.

This literature serves as a valuable reference for engineers considering the replacement of mechanical connections with welded overlays in precision manufacturing applications, demonstrating that comprehensive metallurgical analysis is essential for validating the reliability of such process changes.