Comparative Study of Soft Iron Strip TIG Surfacings on Projectile Steel

Literature Overview

This study, published in the Acta Armamentarii in 2013 by researchers from the Harbin Institute of Technology State Key Laboratory of Advanced Welding and Joining, addresses a critical engineering challenge in ammunition manufacturing: the replacement of traditional copper strips on projectile bodies with soft iron. The motivation is both economic and strategic, as copper is expensive and supply-constrained, while soft iron offers comparable functional performance at significantly lower cost. The authors compared three TIG welding variants—cold wire TIG, hot wire TIG (HWT), and vibration wire TIG (VWT)—to determine which process yields the most reliable soft iron overlay on 20# carbon steel projectile bodies.

Core Technical Findings

The study systematically varied wire feed rate, current, and wire vibration parameters to evaluate their influence on interface integrity, grain morphology, hardness, and mechanical strength. The key results are summarized below.

The most significant finding is that vibration wire TIG produces the finest microstructure at both the fusion interface and within the overlay. The mechanical vibration of the wire introduces turbulence into the molten pool, which acts as a grain-refining mechanism. This results in a hardness reduction of approximately HV15 compared to the other two methods, bringing the overlay hardness down to HV160. The lower hardness is actually desirable for soft iron applications, as it ensures the strip deforms appropriately during projectile flight without cracking.

Process Mechanism Analysis

The hot wire TIG process delivers additional heat input through a heated wire electrode, which increases the effective energy density at the weld pool. While this dramatically improves wire feed rate to 1.8 m/min, the consequence is excessive thermal input that promotes grain coarsening at the interface. The coarsened interface grains are more susceptible to interfacial penetration cracks—the very defect that plagued the original copper strip TIG process. This is a classic example of how process optimization can inadvertently introduce new failure modes if metallurgical consequences are not carefully monitored.

In contrast, the vibration wire TIG process introduces a controlled mechanical oscillation to the welding wire. The vibration amplitude and frequency create periodic disturbances in the molten pool, which serve two purposes: first, they break up the solidification front and nucleate new grains, thereby refining the microstructure; second, they reduce the effective current requirement by 10 A while maintaining adequate penetration. The combination of grain refinement and slightly reduced heat input produces a more ductile, crack-resistant interface.

Engineering Practice Implications

For engineers working on similar overlay applications, several lessons emerge from this study. First, process selection should not be based solely on productivity metrics such as wire feed rate; metallurgical quality must be the primary criterion. Second, mechanical wire vibration is a simple yet effective means of microstructure control that does not require complex equipment modifications. Third, the fact that interface strength exceeds the base metal strength in all three processes confirms that the soft iron/20# steel system is metallurgically compatible when proper process parameters are used.

The successful live-fire test results demonstrate that soft iron strips can functionally replace copper strips in projectile bodies. This finding has direct implications for ammunition manufacturing cost reduction and supply chain resilience. Engineers should note that the vibration wire TIG process, while producing the best microstructure, requires careful control of vibration amplitude and frequency to avoid wire instability or arc interruption. A systematic parameter optimization study, possibly using response surface methodology, would be a logical next step to define the robust processing window for production applications.

This study exemplifies how welding process innovation can solve practical engineering problems by combining fundamental metallurgical understanding with practical manufacturing constraints. The soft iron TIG surfacing approach opens new possibilities for cost-effective, high-performance overlay applications beyond ammunition manufacturing, including wear-resistant surfacing on steel components where ductility at the interface is critical.