Plasma Arc Surfacing Application in Gun Band Manufacturing

Literature Overview

This paper by Wang Long from a military research facility under the General Staff Department, published in Welding journal (1997, Vol. 7, pp. 25–26), describes an innovative application of plasma arc surfacing technology to the manufacturing of gun bands (also known as driving bands) for military projectiles. Gun bands serve two critical functions: providing gas seal between the projectile and the gun barrel bore, and imparting rotational spin to the projectile through engagement with the rifling grooves. The conventional manufacturing method for gun bands did not meet national military standard requirements for projectile strength testing, specifically regarding the dimensional tolerance of the projectile outer diameter at the band location during proof testing.

Technical Background and Problem Statement

Gun bands are typically manufactured as integral components of the projectile body, formed by pre-stressing or swaging operations that create a raised band with precise dimensions. The conventional method of pre-stressing the projectile body to form the gun band resulted in unacceptable dimensional changes during the projectile strength test (proof test). The proof test subjects the projectile to pressures exceeding those encountered in actual firing, and the pre-stressed band location experienced dimensional changes that exceeded the national military standard limits.

Plasma Arc Surfacing Process for Gun Bands

The plasma arc surfacing approach to gun band manufacturing involves the following key process steps:

1. Surface preparation: The projectile body surface at the band location is cleaned and prepared to ensure proper metallurgical bond between the base metal and the overlay.
2. Plasma arc surfacing: A suitable alloy powder is fed into the plasma arc, which melts the powder and a small amount of the base metal surface, creating a metallurgical bond. The plasma arc provides a highly concentrated heat source with excellent process control.
3. Layer control: The overlay is deposited in controlled layers to build up the gun band to the required height and geometry. The plasma arc's precise heat input allows for thin, uniform deposits.
4. Post-deposition machining: The deposited band is machined to the precise dimensional requirements specified by the military standard.

The key advantage of this approach is that the gun band is built up as an additive deposit rather than formed by plastic deformation of the projectile body. This eliminates the residual stresses and dimensional instabilities associated with the pre-stressing method.

Metallurgical Considerations

The plasma arc surfacing of gun bands raises several important metallurgical considerations:

• Dilution control: The dilution of the overlay with the projectile body material must be carefully controlled. Excessive dilution reduces the hardness and wear resistance of the overlay, while insufficient dilution may result in poor metallurgical bond.
• Overlay material selection: The overlay material must provide adequate gas seal properties, wear resistance against the rifling grooves, and compatibility with the projectile body material. Typical materials include copper alloys, brass, or specialized proprietary compositions.
• Thermal effects: Although plasma arc surfacing has lower heat input than conventional arc welding, the thermal effects on the projectile body must be evaluated. The projectile body is typically made of steel, and excessive heating could affect the mechanical properties of the body material.
• Residual stresses: The residual stress field in the deposited band must be evaluated to ensure it does not compromise the structural integrity of the projectile during firing.

Engineering Practice Implications

This paper represents a creative application of surfacing technology to solve a manufacturing problem that conventional methods could not adequately address. The plasma arc surfacing approach offers several advantages for gun band manufacturing:

• Dimensional precision: The additive nature of surfacing allows for precise control of the band geometry, which is critical for meeting tight military specification tolerances.
• Material flexibility: The overlay material can be selected independently of the projectile body material, allowing optimization of the band properties for gas seal and wear resistance.
• Reduced residual stresses: Unlike the pre-stressing method, surfacing does not introduce the complex residual stress fields associated with plastic deformation of the projectile body.
• Scalability: The plasma arc surfacing process can be adapted for different projectile sizes and band geometries with relatively minor process modifications.

Study Insights and Reflections

This paper demonstrates the versatility of plasma arc surfacing technology beyond its traditional applications in wear-resistant overlay and repair welding. The application to gun band manufacturing represents a shift from thinking of surfacing as a repair or enhancement technology to recognizing it as a viable manufacturing process for precision components. In my view, this is a forward-looking approach that anticipates the modern concept of additive manufacturing. The plasma arc surfacing process, with its ability to deposit metal layer by layer with precise control, is essentially a form of directed energy deposition, which is one of the primary technologies used in modern additive manufacturing. This paper, written in 1997, represents an early example of this concept being applied to a practical manufacturing problem. The success of this approach in meeting military specification requirements demonstrates the maturity and reliability of plasma arc surfacing technology for precision applications.