Testing and Analysis of Copper Strip Surfacing on Projectile Bodies

Literature Overview

This paper by Liu Shuyan, Hou Xiujun, Wang Shuhua, and Deng Hua, published in Chemical Analysis and Metering (2011, Vol. 20, No. S1, pp. 66-69), addresses a specialized application of surfacing welding in the defense industry: the attachment of copper alloy strips to projectile bodies. The traditional method involves mechanically inserting ring-shaped copper or copper alloy strips into grooves machined into the projectile body, which suffers from deformation during the extrusion process and potential loosening of the copper strips. The authors propose a TIG arc welding (GTAW) surfacing approach to directly weld copper alloy onto the projectile base material, then characterize the resulting joint through energy dispersive spectroscopy (EDS), spectroscopic analysis, scanning electron microscopy (SEM), optical microscopy, and mechanical property testing.

Technical Challenges and Solutions

The joining of copper alloy to steel projectile bodies presents several metallurgical challenges:

Challenge	Description	Solution
High thermal conductivity of copper	Rapid heat dissipation reduces weld pool temperature	High current density, concentrated arc
Thermal expansion mismatch	Cu (17 ppm/K) vs. steel (12 ppm/K)	Controlled cooling rate, low heat input
Intermetallic compound formation	Brittle Fe-Cu intermetallics at interface	Thin interlayer, controlled diffusion
Dilution	Iron contamination of copper layer	Low dilution process parameters
Cracking	Hot cracking in Cu-rich weld metal	Alloy design, preheat control

The GTAW process was selected because it provides excellent arc stability, minimal spatter, and precise heat input control, all of which are essential for joining dissimilar metals with significant thermal property differences.

Microstructural Analysis

The characterization results reveal several important features:

Interface region: SEM examination shows a thin interfacial zone (approximately 50-100 μm) where iron from the base metal diffuses into the copper layer. EDS line scanning confirms a gradient in iron content across this zone, with iron concentration decreasing from the base metal side toward the pure copper surface. The presence of iron in the copper layer is quantified through spectroscopic analysis, with typical values of 0.5-3% Fe depending on welding parameters.

Copper alloy layer: The microstructure of the copper layer depends on the specific alloy composition used. For pure copper (C11000), the structure consists of equiaxed grains with some elongation in the direction of solidification. For copper alloys (such as CuSn or CuAl), secondary phases (metallic compounds of Sn or Al with Cu) are visible as fine precipitates that enhance hardness without significantly reducing ductility.

Projectile base metal: The heat-affected zone (HAZ) in the projectile base material shows grain coarsening near the weld interface, with grain size increasing from approximately 20 μm in the base metal to 40-60 μm at the fusion line. This grain coarsening can reduce local mechanical properties but is generally acceptable for the application.

Mechanical Property Evaluation

The mechanical testing provides critical data for process validation:

Hardness profile: Hardness increases from the base metal (approximately 200 HV for typical projectile steel) through the HAZ (approximately 180-200 HV) to the copper layer (approximately 80-120 HV for pure copper, higher for copper alloys). The hardness gradient is smooth, indicating good metallurgical compatibility.
Tensile strength: The copper layer exhibits tensile strength of 250-350 MPa (depending on alloy), which is lower than the projectile base material but adequate for the application requirements.
Bond strength: Shear testing of the interface reveals bond strengths of 150-200 MPa, significantly exceeding the minimum requirements for mechanical integrity during projectile operation.
Impact resistance: The interface region shows no evidence of brittle fracture, confirming that the GTAW process produces a ductile, crack-resistant joint.

Engineering Practice Implications

For engineers working on similar dissimilar metal joining applications, this study offers several practical insights:

Process selection: GTAW is the preferred process for copper-on-steel surfacing due to its low dilution and precise control. FCAW or SAW would produce excessive iron contamination.
Heat input management: Low heat input (0.5-1.5 kJ/mm) is essential to minimize intermetallic formation and thermal distortion. This requires careful control of current (150-250 A), voltage (12-18 V), and travel speed (100-200 mm/min).
Shielding gas: Pure argon or argon-helium mixtures (75/25 or 50/50) provide adequate shielding. The higher thermal conductivity of helium can help maintain pool temperature when welding copper.
Preheat: Mild preheating (100-150°C) of the projectile body reduces thermal shock and cracking risk without promoting excessive intermetallic growth.
Filler metal selection: The filler metal should match or slightly exceed the composition of the intended copper strip to ensure adequate alloying and mechanical properties.

Key Reflections

This work demonstrates that surfacing welding can successfully replace traditional mechanical fastening methods in precision applications. The elimination of the mechanical groove and insertion process not only improves production efficiency but also eliminates the risk of strip loosening during high-stress operation. The comprehensive characterization approach (EDS, SEM, OM, mechanical testing) provides a model for validating dissimilar metal surfacing joints in other applications, such as copper-clad steel wire, electrical contact materials, and thermal barrier coatings.