Zhuojin Pipe Fitting Co., LtdTIG Surfacing of QA19-2 Aluminum Bronze on 38CrA Rudder Shaft Base
Literature Overview
This technical paper by Shi Ning, published in China Ship Repair (1993, Vol. 6, No. 4, pp. 27-30), describes the application of tungsten inert gas (TIG/GTAW) welding for surfacing QA19-2 aluminum bronze onto a 38CrA steel rudder shaft base in a marine repair context. The paper addresses a critical dissimilar metal welding challenge encountered in ship repair: restoring worn or damaged rudder shaft surfaces using a high-performance copper alloy overlay to improve corrosion resistance, wear resistance, and cavitation resistance in seawater environments.
Technical Background and Material Selection
Rudder shafts in marine propulsion systems are subjected to severe mechanical and environmental loading, including cyclic bending, torsion, cavitation erosion, and seawater corrosion. The 38CrA steel base material provides adequate structural strength but is vulnerable to pitting and crevice corrosion in marine conditions. QA19-2 aluminum bronze (approximately 9% Al, 1% Fe, 1% Ni, balance Cu) offers excellent resistance to cavitation, seawater corrosion, and wear, making it an ideal surfacing material for rudder shaft applications.
The selection of TIG welding for this application is justified by several factors:
- Precise heat input control, essential for minimizing dilution between the dissimilar base and overlay metals
- High-quality weld appearance with minimal spatter
- Ability to weld thin sections without burn-through
- Flexibility in handling curved and restricted geometries typical of rudder shafts
Welding Process Parameters and Technique
Based on standard GTAW practice for copper alloys and dissimilar steel-copper combinations, the following process parameters and techniques are relevant to this application:
| Parameter | Typical Value | Rationale |
|---|---|---|
| Current type | DCEN | Provides deep penetration and stable arc |
| Current range | 120-200 A | Sufficient for 1-2 mm wire diameter |
| Shielding gas | Pure argon (99.99%) | Prevents oxidation of copper and aluminum |
| Wire diameter | 1.6-2.4 mm | Balances deposition rate and control |
| Stick-out length | 2-3 mm | Minimizes arc wandering |
| Travel speed | 50-80 mm/min | Controls heat input and dilution |
| Preheat temperature | 200-300°C | Reduces thermal stress and cracking |
Dilution Control
The primary technical challenge in surfacing aluminum bronze onto steel is controlling the dilution rate. Excessive dilution leads to:
- Formation of brittle intermetallic phases (Fe-Cu, Fe-Al) at the weld interface
- Reduced corrosion resistance of the overlay
- Cracking in the heat-affected zone due to high thermal stress
To minimize dilution, the following measures are recommended:
- Use of a low heat input per pass to limit the volume of base metal melted
- Application of multiple thin passes rather than a single thick deposit
- Pre-deposition of a transition layer (e.g., Ni-based or bronze with higher Ni content) to reduce Fe-Cu intermetallic formation
- Backing with copper or bronze to ensure full penetration without excessive base metal melting
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Hot cracking | High thermal stress, impurity segregation | Reduce heat input, use low-sulfur filler, preheat |
| Cold cracking | Hydrogen absorption, martensitic transformation in HAZ | Post-weld heat treatment, low-hydrogen procedure |
| Porosity | Gas entrapment, insufficient shielding | Improve gas flow, clean surface, use pure argon |
| Excessive dilution | High heat input, single thick pass | Multi-pass thin deposits, reduce current |
| Undercut | Travel speed too fast, arc too close to edge | Reduce travel speed, adjust torch angle |
Engineering Practice Considerations
In ship repair operations, the rudder shaft is typically removed from the vessel and repaired in a workshop or dockyard facility. The surfacing operation must be performed with careful attention to the following:
- Surface preparation: The shaft surface must be ground smooth, free of oil, rust, and scale. Any existing cracks or severe pitting must be repaired before surfacing.
- Geometric accuracy: The rudder shaft must maintain precise concentricity and dimensional tolerances after surfacing. Post-weld machining is usually required to restore the original dimensions.
- Post-weld heat treatment: A stress-relief anneal at 500-550°C for 1-2 hours is recommended to reduce residual stresses and improve the ductility of the weld interface.
- Quality verification: The surfacing quality should be verified by magnetic particle testing (MT) for surface cracks, ultrasonic testing (UT) for subsurface defects, and hardness testing to confirm the overlay properties.
Study Insights and Implications
This 1993 paper represents an important early contribution to the practice of dissimilar metal surfacing in marine engineering. The approach described—using TIG welding to apply aluminum bronze onto steel rudder shafts—is still widely practiced today, although modern techniques have incorporated additional refinements such as laser cladding, plasma arc surfacing, and advanced filler metal compositions.
The key insight from this work is that the success of dissimilar metal surfacing depends not only on the selection of appropriate materials but also on meticulous process control to manage dilution, residual stress, and interface metallurgy. For modern engineers, this paper serves as a reminder that fundamental welding principles—heat input management, dilution control, and interface engineering—remain central to successful surfacing operations, regardless of the specific technology employed.