CLADDING TECHNOLOGY SHANXI CO., LTDTIG Cladding of QA19-2 Aluminum Bronze on 38CrA Rudder Shaft Substrate
Literature Overview
This 1993 publication by Shi Ning from the context of Chinese ship repair addresses the application of gas tungsten arc welding (GTAW/TIG) cladding of QA19-2 aluminum bronze on 38CrA steel rudder shaft substrates. Rudder shafts in marine applications are subjected to severe combined loading from bending, torsion, and corrosion in seawater environments. The adoption of aluminum bronze overlays on steel shafts provides an economical solution that combines the structural strength of steel with the corrosion resistance and wear resistance of aluminum bronze.
Technical Background
Material Selection Rationale
| Component | Material | Key Properties |
|---|---|---|
| Substrate | 38CrA steel | Tensile strength 600–750 MPa, good toughness, weldable |
| Overlay | QA19-2 aluminum bronze | Cu-19Al-2Fe, excellent seawater corrosion resistance, high wear resistance |
| Service environment | Seawater, mechanical loading | Corrosion, erosion, fatigue |
The QA19-2 aluminum bronze (equivalent to CuAl10Fe5Ni5 or similar grades per GB/T 1176) offers superior cavitation resistance and seawater corrosion resistance compared to standard copper alloys. The 38CrA steel substrate provides the necessary structural strength for rudder shaft applications while maintaining acceptable weldability.
Dissimilar Metal Welding Challenges
The combination of aluminum bronze and carbon steel presents significant metallurgical challenges:
- Large difference in thermal conductivity (copper alloys: ~90 W/m·K; steel: ~45 W/m·K)
- Different coefficients of thermal expansion leading to residual stress
- Formation of brittle intermetallic compounds at the interface (FeAl, Fe₂Al₅)
- High dilution rates when using consumable electrodes in TIG welding
- Sensitivity to hydrogen-induced cracking in the steel substrate
Process Development
GTAW Cladding Parameters
The TIG process was selected for this application due to its precise heat input control, which is essential for managing dilution and preventing cracking in dissimilar metal joints. The recommended process parameters are as follows:
| Parameter | Value | Rationale |
|---|---|---|
| Welding current | 150–220 A | DCEN for tungsten electrode life and arc stability |
| Arc voltage | 12–16 V | Depends on electrode diameter and gap |
| Travel speed | 60–120 mm/min | Controls bead width and dilution |
| Tungsten electrode | CeLa₂O₂ or ThO₂, 2.4–3.2 mm | Stable arc, good wetting |
| Shielding gas | 100% Ar or 98% Ar + 2% H₂ | Reduces oxide formation |
| Gas flow rate | 12–18 L/min | Adequate protection of weld pool |
| Filler wire | QA19-2 aluminum bronze, 2.4–3.2 mm | Matches overlay composition |
| Preheat temperature | 200–300°C | Reduces cracking tendency in steel |
Multi-Pass Deposition Strategy
For achieving adequate overlay thickness (typically 3–5 mm for rudder shaft applications), a multi-pass approach is necessary. The first pass is critical for establishing a sound metallurgical bond and controlling dilution. Subsequent passes build thickness with progressively lower dilution as the aluminum bronze layer acts as a buffer between the steel substrate and the surface.
The dilution profile across a multi-pass overlay typically follows this pattern:
- First pass: 25–40% dilution (highest, directly on steel)
- Second pass: 15–25% dilution
- Third pass: 5–15% dilution
- Final surface pass: <5% dilution
Metallurgical Analysis
Interface Microstructure
The bond line between aluminum bronze and 38CrA steel exhibits a characteristic microstructure consisting of:
- A narrow diffusion zone (10–50 μm) with intermetallic compounds
- A transition zone with mixed copper-iron solid solution
- The aluminum bronze overlay with dendritic α phase and eutectic Al₂Cu particles
The presence of intermetallic compounds at the interface is inevitable but can be controlled through process optimization. Excessive heat input leads to wider intermetallic zones and reduced bond strength. The TIG process, with its concentrated heat input and relatively low total energy, provides better control than arc processes such as MIG or submerged arc welding.
Mechanical Properties
The overlay layer properties are significantly influenced by dilution:
| Dilution (%) | Hardness (HV) | Tensile Strength (MPa) | Corrosion Rate (mm/y) |
|---|---|---|---|
| <10 | 250–300 | 450–550 | <0.05 |
| 10–25 | 200–250 | 350–450 | 0.05–0.15 |
| 25–40 | 150–200 | 250–350 | 0.15–0.30 |
The target overlay properties for rudder shaft applications require hardness above 200 HV for adequate wear resistance and corrosion rate below 0.1 mm/y in seawater.
Engineering Practice Considerations
Surface Preparation
The substrate surface must be prepared to a minimum Ra of 6.3 μm, with all oxide, paint, and contamination removed. For repair applications on existing rudder shafts, grinding to bare metal is essential. The presence of residual hydrogen from surface contamination can lead to cold cracking in the 38CrA steel, particularly in the heat-affected zone.
Post-Weld Heat Treatment
A post-weld stress relief treatment at 550–600°C for 2–4 hours is recommended to:
- Reduce residual stresses from differential thermal expansion
- Stabilize the microstructure
- Prevent delayed cracking
However, the heat treatment must be performed carefully to avoid softening the 38CrA steel substrate or causing excessive grain growth in the aluminum bronze overlay.
Key Reflections
This literature addresses a specific and practically important application in marine engineering. The use of TIG cladding for aluminum bronze on steel substrates remains a valid approach today, though modern practice has evolved to include hot-wire TIG and pulsed TIG processes that offer higher deposition rates while maintaining the precise heat input control of conventional TIG. The fundamental metallurgical challenges identified in this 1993 work—dilution control, intermetallic formation, and residual stress management—remain relevant and require careful attention in current practice.
The work also highlights the importance of material compatibility assessment in dissimilar metal welding applications. The selection of QA19-2 aluminum bronze for seawater service demonstrates sound engineering judgment, as this alloy offers superior cavitation resistance compared to simpler copper alloys. Engineers involved in marine repair and maintenance should refer to this work when evaluating cladding options for rudder shafts and other submerged steel components exposed to seawater.