In the manufacturing of deep-sea pressure-resistant structures, aluminum alloy shell castings play a critical role due to their excellent strength-to-weight ratio and corrosion resistance. However, these shell castings often suffer from unavoidable casting defects such as slag inclusions, porosity, pinholes, and shrinkage, which significantly reduce the yield rate. To minimize scrap, save raw materials and energy, shorten production cycles, and lower costs, it is essential to develop effective repair welding techniques for defective shell castings. This study focuses on the repair welding of defects in aluminum alloy shell castings used for deep-sea pressure hulls, employing tungsten inert gas (TIG) welding with suitable filler materials. We investigate the crack resistance, mechanical properties, and corrosion resistance of the welded zones, ensuring they meet or exceed the base alloy performance. Furthermore, full-scale repair welding trials on actual shell castings segments have been successfully conducted, validating the practicality of our approach.
Repair welding of aluminum alloy shell castings presents significant technical challenges. Cast aluminum alloys, especially multi-component ones, contain low-melting eutectic structures that make them prone to solidification cracks and liquation cracks in the weld and heat-affected zone (HAZ) during fusion welding. Additionally, shell castings often have high gas content, primarily hydrogen, which remains dissolved in the solid solution or trapped in interdendritic regions. During welding, this hydrogen can increase gas content in the molten pool, leading to weld porosity, and cause hydrogen accumulation in the HAZ, resulting in intergranular porosity. The complex shapes and structural stresses of shell castings, combined with irregular repair positions, non-linear weld paths, and difficulties in cleaning contaminants and oxide films, further exacerbate the risk of defects like cracks, pores, and slag inclusions. These factors have limited the widespread application of repair welding for shell castings in industry. Our research addresses these challenges by optimizing welding processes and filler materials for two specific cast aluminum alloys used in deep-sea pressure-resistant shell castings.
The base materials for this study are two cast aluminum alloys, designated here as Alloy A and Alloy B, which are commonly used in deep-sea pressure-resistant shell castings. Their chemical compositions and mechanical properties, measured after heat treatment according to the specifications for the shell castings segments, are summarized in Table 1. The heat treatment regimes are as follows: for Alloy A, solution treatment at 535°C for 8 hours followed by water quenching, then artificial aging at 175°C for 6 hours with air cooling; for Alloy B, solution treatment at 525°C for 8 hours followed by water quenching, then artificial aging at 175°C for 8 hours with air cooling. These treatments are crucial for achieving the desired microstructure and properties in the shell castings.
| Alloy Designation | Chemical Composition (wt.%) | Mechanical Properties | Heat Treatment Regime |
|---|---|---|---|
| Alloy A | Si: 6.5-7.5, Mg: 0.25-0.45, Mn: 0.3-0.5, Ti: 0.1-0.2, Al: balance | Tensile Strength (σ_b): ≥220 MPa, Yield Strength (σ_{0.2}): ≥180 MPa, Elongation (δ): ≥2% | 535°C/8h WQ + 175°C/6h AC |
| Alloy B | Si: 4.5-5.5, Mg: 0.4-0.6, Mn: 0.3-0.5, Ti: 0.1-0.2, Al: balance | Tensile Strength (σ_b): ≥240 MPa, Yield Strength (σ_{0.2}): ≥200 MPa, Elongation (δ): ≥3% | 525°C/8h WQ + 175°C/8h AC |
Filler materials for repair welding were provided in two forms: cast rods and wires. For Alloy A, which served as a backup material for the shell castings segments, only cast rods with composition identical to the base alloy were used. For Alloy B, the primary material for the actual shell castings, both cast rods and a specially designed wire, designated as Filler W, were employed. The design of Filler W aimed to ensure good crack resistance when welded to the base alloy, while matching the main chemical composition to maintain consistent anodized color after surface treatment of the shell castings. The wire was produced through melting, casting, extrusion, and drawing processes, resulting in a diameter of 3.0 mm. Its designed and analyzed chemical compositions are listed in Table 2.
| Element | Designed Composition (wt.%) | Analyzed Composition (wt.%) |
|---|---|---|
| Si | 5.0-6.0 | 5.5 |
| Mg | 0.4-0.6 | 0.5 |
| Mn | 0.3-0.5 | 0.4 |
| Ti | 0.1-0.2 | 0.15 |
| Al | Balance | Balance |
Test plates were machined from base alloy castings into closed-groove configurations, as illustrated in the methodology, to simulate defect repair in shell castings. Prior to welding, the plates and filler materials were cleaned in a phosphoric acid solution heated to approximately 60°C to remove oils, followed by rinsing with water and drying. Manual TIG welding was performed using an AC TIG welding machine with argon as the shielding gas. Each groove was filled with three weld passes. For each trial, multiple plates were welded simultaneously, and each pass was inspected for surface cracks using magnification. The welding parameters were standardized: welding current of 120-150 A, welding speed of 10-15 cm/min, and argon flow rate of 10-15 L/min. The heat input per unit length, a critical parameter in welding of shell castings, can be expressed as:
$$ Q = \frac{I \cdot V}{v} $$
where \( Q \) is the heat input (J/mm), \( I \) is the welding current (A), \( V \) is the arc voltage (typically around 12-15 V for TIG welding), and \( v \) is the welding speed (mm/s). For our parameters, with \( I = 135 \) A, \( V = 14 \) V, and \( v = 2 \) mm/s (12 cm/min), the heat input is approximately:
$$ Q = \frac{135 \times 14}{2} = 945 \text{ J/mm} $$
This moderate heat input helps control distortion and minimize thermal damage in the shell castings. After welding, all plates were subjected to X-ray inspection to detect internal defects. Additionally, chemical analysis of the weld metal, mechanical testing of weld seams and joints, and stress corrosion cracking tests were conducted.
The crack resistance of the welded shell castings was evaluated first. For both Alloy A and Alloy B, using either cast rods or Filler W, no cracks were observed in the weld or HAZ under magnification, including in multi-pass welds. This indicates that the selected welding process parameters provide sufficient crack resistance for repair welding of these shell castings. X-ray inspection revealed that welds made with cast rods tended to have more porosity, while those with Filler W showed fewer pores, but both met the requirements for Grade 1 weld quality according to relevant standards. This underscores the importance of filler material processing in reducing defects in shell castings repairs.
Chemical composition analysis of the weld metal, as shown in Table 3, confirms that for both alloys, whether using cast rods or Filler W, the weld chemistry falls within the allowable range of the respective base alloys. This compositional consistency is crucial for achieving mechanical and corrosion properties comparable to the base shell castings material.
| Base Alloy | Filler Material and State | Chemical Composition of Weld Metal (wt.%) |
|---|---|---|
| Alloy A | Cast Rod (as-cast) | Si: 7.0, Mg: 0.35, Mn: 0.4, Ti: 0.15, Al: balance |
| Alloy B | Cast Rod (as-cast) | Si: 5.2, Mg: 0.5, Mn: 0.45, Ti: 0.15, Al: balance |
| Alloy B | Filler W (wire) | Si: 5.5, Mg: 0.5, Mn: 0.4, Ti: 0.15, Al: balance |
Mechanical properties of the weld seams and welded joints were extensively tested. The results, summarized in Table 4, demonstrate that for both shell castings alloys, the tensile strength of the weld metal meets or exceeds the base alloy specifications, even when using cast rods with higher gas content. Joint tensile tests consistently fractured in the base material away from the weld zone, indicating that the weld strength is higher than the actual strength of the base alloy in the cast condition. This is attributed to the refined microstructure and reduced porosity in the weld metal due to the welding process. The use of Filler W, which undergoes deformation processing, further enhances the weld properties due to better homogeneity and lower gas content.
| Base Alloy | Filler Material | Post-Weld Heat Treatment | Weld Tensile Properties | Joint Tensile Properties | Fracture Location |
|---|---|---|---|---|---|
| Alloy A | Cast Rod | Regime A | σ_b: 235 MPa, δ: 3.5% | σ_b: 225 MPa | Base material |
| Alloy A | Cast Rod | Regime A | σ_b: 230 MPa, δ: 3.2% | σ_b: 220 MPa | Base material |
| Alloy B | Cast Rod | Regime B | σ_b: 250 MPa, δ: 4.0% | σ_b: 245 MPa | Base material |
| Alloy B | Cast Rod | Regime B | σ_b: 255 MPa, δ: 4.2% | σ_b: 240 MPa | Base material |
| Alloy B | Filler W | Regime B | σ_b: 260 MPa, δ: 4.5% | σ_b: 250 MPa | Base material |
| Alloy B | Filler W | Regime B | σ_b: 265 MPa, δ: 4.8% | σ_b: 255 MPa | Base material |
The enhanced mechanical performance can be partially explained by considering the strengthening mechanisms in aluminum alloys. The yield strength \( \sigma_y \) of the weld metal can be modeled using a superposition approach:
$$ \sigma_y = \sigma_0 + \sigma_{ss} + \sigma_{gb} + \sigma_{disp} $$
where \( \sigma_0 \) is the intrinsic strength of pure aluminum, \( \sigma_{ss} \) is solid solution strengthening, \( \sigma_{gb} \) is grain boundary strengthening, and \( \sigma_{disp} \) is dispersion strengthening from precipitates. In repair welding of shell castings, the rapid solidification refines grains and increases solute trapping, boosting \( \sigma_{gb} \) and \( \sigma_{ss} \). Additionally, post-weld heat treatment promotes precipitate formation, enhancing \( \sigma_{disp} \). For instance, in Alloy B, the aging response can be described by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation for precipitate volume fraction \( f \):
$$ f = 1 – \exp(-k t^n) $$
where \( k \) is a rate constant dependent on temperature, \( t \) is aging time, and \( n \) is the Avrami exponent. Optimal aging parameters ensure maximal strengthening without over-aging, which is critical for maintaining the integrity of shell castings under deep-sea pressures.
Corrosion resistance, particularly stress corrosion cracking (SCC) resistance, is vital for deep-sea applications of shell castings. We conducted fork-type SCC tests on welded specimens in a 3.5% NaCl solution under applied stress. The time to failure \( t_f \) for SCC can be related to the stress intensity factor \( K \) and material parameters via an empirical power law:
$$ t_f = A K^{-m} $$
where \( A \) and \( m \) are constants. Results showed that welded zones, especially those using Filler W, exhibited SCC resistance comparable to or better than the base alloy, with no significant reduction in time to failure. This is attributed to the homogeneous microstructure and minimal galvanic differences between the weld and base metal in the shell castings. The corrosion potential \( E_{corr} \) and corrosion current density \( i_{corr} \), measured through polarization tests, also confirmed similar corrosion behavior, ensuring long-term durability of repaired shell castings in seawater environments.
Based on these laboratory findings, full-scale repair welding trials were conducted on actual deep-sea pressure-resistant shell castings segments made of Alloy B. Defects such as porosity and shrinkage were identified by non-destructive testing, and repaired using the optimized TIG process with Filler W. The repaired shell castings segments underwent comprehensive inspection, including ultrasonic testing, X-ray radiography, and pressure cycling tests simulating deep-sea conditions. All repairs met the design and service requirements, with no defects detected in the welded areas. The success of these trials validates the practical applicability of our repair welding methodology for shell castings in critical marine structures.
In conclusion, this study demonstrates that defect repair welding of aluminum alloy shell castings for deep-sea pressure hulls is feasible and effective through careful selection of welding processes and filler materials. The TIG welding technique, combined with tailored filler wires like Filler W, ensures excellent crack resistance, mechanical properties, and corrosion resistance in the weld zone, surpassing the base alloy performance in many cases. The integration of optimized heat input control, post-weld heat treatment, and rigorous quality assurance enables reliable repair of shell castings, reducing waste and enhancing manufacturing efficiency. Future work could explore advanced welding methods such as laser or friction stir welding for shell castings repairs, but the current approach provides a robust solution for industrial applications. The repeated emphasis on shell castings throughout this research underscores their importance in marine engineering and the value of developing reliable repair techniques to extend their service life and ensure structural integrity in challenging deep-sea environments.