TIG Surfacing of QA19-2 Aluminum Bronze on 38CrA Rudder Shaft Substrate

Literature Overview

This 1993 paper by Shi Ning, published in China Ship Repair, documents a practical application of tungsten inert gas (TIG) arc welding for surfacing QA19-2 aluminum bronze onto a 38CrA steel rudder shaft substrate in a ship repair context. Although the paper is concise, it addresses a classic dissimilar metal welding challenge that remains relevant in marine engineering to this day. The rudder shaft is a critical component in ship steering systems, and its conical section interfaces with the rudder stock, requiring both mechanical strength and corrosion resistance in a marine environment.

Dissimilar Metal Welding Challenges

The combination of 38CrA steel (a medium-carbon chromium-molybdenum steel with approximately 0.35% C and 0.8–1.1% Cr) and QA19-2 aluminum bronze (approximately 9–11% Al, 5–7% Fe, with the remainder copper) presents several metallurgical challenges:

Challenge	Description	Mitigation Approach
Diffusion intermetallic formation	Fe-Cu and Fe-Al intermetallics form at the interface during cooling	Limit heat input; control cooling rate
Thermal conductivity mismatch	Aluminum bronze has higher thermal conductivity than steel	Preheating of steel substrate
Dilution control	Excessive steel dilution degrades bronze corrosion resistance	Use appropriate filler wire composition; limit penetration depth
Hot cracking susceptibility	Copper-based alloys are prone to solidification cracking	Add small amounts of nickel or zinc to filler

Process Parameters and Technique

The paper describes the use of TIG welding (GTAW) with argon shielding for the surfacing operation. While specific numerical parameters are not exhaustively tabulated, the following technique points can be extracted from the context of the era and the material system:

Welding current: Typically in the range of 120–200 A for a rudder shaft of this size, using AC or DCEN depending on the filler wire configuration.
Shielding gas: Pure argon, with a flow rate of approximately 8–12 L/min to ensure adequate coverage of the weld pool and the heat-affected zone.
Tungsten electrode: Thoriated tungsten (WT20) was standard practice at the time, though modern practice would favor ceriated tungsten (WLCe-2) for lower thorium content.
Filler wire: QA19-2 aluminum bronze wire, matched to the composition of the target surfacing layer.
Preheating: The steel substrate should be preheated to 150–250°C to reduce thermal gradients and minimize the risk of cracking in the steel HAZ.

Quality Control Considerations

For a rudder shaft application, the following quality criteria are essential:

Bond strength: The interface between the steel substrate and the aluminum bronze overlay must achieve full metallurgical bonding. Partial bonding or cold shuts would compromise the structural integrity of the rudder shaft.
Crack resistance: Both the overlay and the interface are susceptible to cracking. Post-weld inspection should include magnetic particle testing (MT) on the steel side and liquid penetrant testing (PT) on the bronze overlay, as MT is not applicable to non-ferrous materials.
Dimensional accuracy: The surfacing must restore the conical geometry of the rudder shaft to within specified tolerances, typically ±0.05 mm per meter of length for the taper.
Corrosion resistance: The aluminum bronze overlay must provide adequate resistance to seawater corrosion. Salt spray testing in accordance with ISO 9227 would be an appropriate verification method.

Engineering Practice Reflection

This paper represents an important historical document in ship repair welding practice. The technique described—TIG surfacing of aluminum bronze onto steel—remains in use today, though modern practice would incorporate several improvements:

Use of ceriated tungsten electrodes to eliminate thorium exposure risks.
Application of pulse TIG welding to better control heat input and reduce dilution.
Post-weld heat treatment (PWHT) of the steel HAZ to relieve residual stresses and prevent delayed cracking.
Metallographic examination of the interface to verify the absence of intermetallic compounds and incomplete bonding.

The enduring relevance of this work lies in its demonstration that dissimilar metal surfacing, when properly executed, can successfully restore critical marine components without the need for complete replacement. For today's engineers, this paper serves as a reminder that the fundamental metallurgical principles governing dissimilar welding have not changed, even as equipment and techniques have evolved.