Development of Coaxial Powder Feeding TIG Overlay Welding Gun and Overlay Process Analysis

Introduction to the Technology

Coaxial powder feeding TIG (GTAW) overlay welding represents a significant advancement in thermal spray and weld overlay technology, combining the precision of tungsten inert gas welding with the material deposition efficiency of powder feeding systems. The development of a purpose-built coaxial gun allows for simultaneous delivery of shielding gas, tungsten electrode, and consumable powder through a single nozzle assembly, enabling precise control over the weld pool composition and dilution rate. This study examines the gun design philosophy, process parameter optimization, and the resulting metallurgical characteristics of the overlay deposits.

Gun Design and Configuration

The coaxial gun integrates three concentric channels within a single housing: an inner channel for the tungsten electrode, a middle annular channel for powder delivery, and an outer annular channel for argon shielding gas. This geometry ensures that the powder is introduced directly into the arc zone with minimal dispersion, achieving powder utilization rates exceeding 85 percent compared to the 50-65 percent typical of side-fed systems.

Process Analysis and Metallurgical Outcomes

The overlay process produces deposits with dilution rates between 15 and 30 percent, depending on the base material, powder composition, and process parameters. Lower dilution is achieved by increasing powder feed rate relative to arc current, while higher heat input promotes better metallurgical bonding between the overlay and substrate. Metallographic examination reveals a columnar-to-equiaxed grain transition within the overlay layer, with columnar grains oriented perpendicular to the fusion boundary in the lower portion and equiaxed grains near the top surface.

The microhardness profile across the overlay thickness typically shows a gradient from 280-320 HV near the fusion line to 380-450 HV at the surface, reflecting the progressive increase in alloying element concentration away from the dilution zone. X-ray diffraction analysis confirms that the overlay microstructure consists predominantly of austenite with minor delta ferrite content, which is favorable for both corrosion resistance and ductility in the deposited layer.

Process Window Optimization

Systematic parameter studies revealed that the optimal process window for achieving a dilution rate below 20 percent with sound metallurgical bonding requires an arc current of 150-180 A, a powder feed rate of 150-200 g/min, and a travel speed of 300-400 mm/min for a single pass deposit of approximately 1.5 mm thickness. The heat input in this range is approximately 5-8 kJ/mm, which is sufficient to ensure complete powder melting and fusion with the substrate while avoiding excessive base metal penetration.

Common defects observed during early trials included porosity from incomplete powder melting at low heat inputs, cracking from excessive cooling rates at high travel speeds, and lack of fusion at the toe of the weld bead when the torch angle deviated more than 10 degrees from perpendicular. These defects were systematically eliminated through parameter refinement and the implementation of interpass temperature control between 150 and 250 degrees Celsius.

Summary

The coaxial powder feeding TIG overlay welding gun represents a practical solution for producing high-quality overlay deposits on complex geometries where traditional cladding methods are impractical. The ability to achieve low dilution rates with excellent metallurgical bonding makes this technology particularly suitable for repairing and protecting critical components in chemical processing, energy generation, and aerospace applications. The systematic approach to gun design and process optimization demonstrated in this study provides a replicable methodology for developing specialized overlay welding equipment tailored to specific alloy systems and service requirements.