In the production of aerospace casting parts, aluminum alloys are widely employed due to their low density, high strength, and excellent corrosion resistance. These properties make them ideal for critical components in the aerospace industry, where reliability and performance are paramount. However, during the manufacturing process, surface defects such as oxide scales, sand inclusions, and other contaminants can compromise the integrity of these castings aerospace. To address this, surface cleaning techniques like sandblasting and manual polishing are commonly used to ensure the desired surface quality. Despite these efforts, an issue arises when fluorescence penetration inspection (FPI) is conducted post-heat treatment, where abnormal fluorescence imaging occurs, obscuring the detection of actual defects. This study investigates the root causes of this phenomenon and proposes effective solutions through experimental analysis and process optimization.
Fluorescence penetration inspection is a non-destructive testing method that relies on capillary action to detect surface-breaking defects in materials. When applied to aluminum alloy castings, the process involves applying a fluorescent penetrant that seeps into any surface discontinuities, such as cracks, cold shuts, or pores. After removing excess penetrant and applying a developer, the trapped penetrant is drawn back to the surface, creating an amplified indication that fluoresces under ultraviolet light. However, the surface condition of the casting significantly influences the accuracy of FPI. In the case of aerospace casting parts, surface treatments like sandblasting and polishing can alter the surface morphology, leading to false indications or background noise that masks genuine defects. This problem is particularly prevalent in castings aerospace after heat treatment, where thermal cycles exacerbate surface imperfections.
To systematically address this issue, we designed a series of experiments using ZL114A aluminum alloy castings, which are representative of typical aerospace casting parts. The objective was to evaluate the impact of various surface treatment parameters on fluorescence imaging. The experimental approach included comparing different sandblasting conditions, such as abrasive grain size and blasting pressure, as well as assessing the effects of polishing tools applied before and after heat treatment. After each treatment, the castings underwent FPI, and any abnormal imaging locations were sectioned for detailed analysis using optical microscopy and scanning electron microscopy (SEM). This allowed us to examine the cross-sectional and surface characteristics of the affected areas, identifying the mechanisms behind the fluorescence anomalies.
The initial observations revealed that abnormal fluorescence imaging manifested as dense green spots on the casting surface, often with a powdery texture. In severe cases, these spots merged into a continuous background, making it impossible to distinguish actual defects. Through microscopic analysis, we identified embedded foreign particles and surface deformations as the primary contributors. For instance, SEM images showed irregularly shaped particles, ranging from 10 to 100 micrometers in size, pressed into the surface. Energy-dispersive X-ray spectroscopy (EDS) confirmed that these particles were composed of aluminum and oxygen, consistent with Al2O3 abrasives used in sandblasting. The presence of gaps between these particles and the base material allowed fluorescent penetrant to accumulate, resulting in the persistent green fluorescence. This highlights the critical role of surface integrity in ensuring reliable FPI results for castings aerospace.
To quantify the effects of sandblasting parameters, we conducted tests with varying abrasive grain sizes and blasting pressures. The results are summarized in Table 1, which illustrates the relationship between these parameters and the occurrence of abnormal fluorescence imaging. As the data shows, higher pressures and larger grain sizes increased the likelihood of surface embedding and subsequent fluorescence issues. This can be modeled using a simple equation for the kinetic energy of abrasive particles during blasting: $$ E_k = \frac{1}{2} m v^2 $$ where \( E_k \) is the kinetic energy, \( m \) is the mass of the abrasive particle, and \( v \) is the velocity proportional to blasting pressure. Higher \( E_k \) values correlate with greater surface damage, explaining why aggressive blasting conditions lead to more severe fluorescence anomalies in aerospace casting parts.
| Abrasive Grain Size (μm) | Blasting Pressure (MPa) | Fluorescence Imaging Result | Surface Damage Level |
|---|---|---|---|
| 400 | 0.6 | Severe abnormal spots | High |
| 400 | 0.4 | Moderate abnormal spots | Medium |
| 300 | 0.4 | Minor abnormal spots | Low |
| 300 | 0.3 | No abnormal spots | Negligible |
Further analysis involved examining the cross-sectional microstructures of castings subjected to different sandblasting conditions. As depicted in the micrographs, samples treated with 400 μm abrasives at 0.6 MPa showed significant particle embedding and surface cracking, whereas those with 300 μm abrasives at 0.3 MPa exhibited minimal damage. This aligns with the fluorescence results, where lower energy inputs preserved surface integrity. The mechanism can be described by a surface penetration depth formula: $$ d = k \cdot P \cdot \sqrt{t} $$ where \( d \) is the penetration depth, \( P \) is the blasting pressure, \( t \) is the exposure time, and \( k \) is a constant dependent on abrasive properties. For aerospace casting parts, controlling \( d \) through parameter optimization is essential to prevent false FPI indications.
In addition to sandblasting, manual polishing was identified as a major factor contributing to abnormal fluorescence imaging. When polishing was performed before heat treatment, the soft aluminum matrix (with a hardness of approximately HB40) was prone to plastic deformation, resulting in folded or twisted surface layers. After heat treatment, these deformations expanded, creating micro-gaps that trapped fluorescent penetrant. We tested two common polishing tools—silicon carbide and white alumina—and observed that both induced similar surface damage, though the extent varied. The fluorescence images showed large areas of green background noise, directly correlating with the mechanical damage observed in cross-sections. This underscores the importance of sequencing surface treatments for castings aerospace to minimize such issues.
To address the polishing-induced problems, we modified the process sequence by moving the polishing step to after heat treatment. Post-heat treatment, the aluminum alloy achieves a higher hardness (e.g., up to HB90 or more), making it more resistant to deformation during polishing. As a result, the surface remains smoother with fewer micro-gaps, reducing the retention of fluorescent penetrant. The improvement was evident in subsequent FPI tests, where castings exhibited clear backgrounds without abnormal green spots. This adjustment proved crucial for enhancing the reliability of defect detection in aerospace casting parts, ensuring that only genuine defects are highlighted during inspection.
The combined effects of sandblasting and polishing can be modeled using a surface quality index \( Q_s \), which integrates parameters like roughness, embedded particle density, and micro-crack density. We propose the following formula: $$ Q_s = \frac{R_a \cdot N_p}{\sigma_c} $$ where \( R_a \) is the average surface roughness, \( N_p \) is the number of embedded particles per unit area, and \( \sigma_c \) is the critical stress for crack initiation. A lower \( Q_s \) value indicates better surface quality, correlating with reduced abnormal fluorescence in castings aerospace. By optimizing sandblasting and polishing parameters, we can minimize \( Q_s \), as demonstrated in our experiments.
| Treatment Sequence | Surface Roughness \( R_a \) (μm) | Embedded Particle Density \( N_p \) (particles/mm²) | Fluorescence Background | Defect Detectability |
|---|---|---|---|---|
| Polishing before heat treatment | 2.5 | 150 | High green noise | Poor |
| Polishing after heat treatment | 1.2 | 20 | Clean | Excellent |
| Sandblasting only (optimized) | 1.5 | 30 | Minor noise | Good |
In conclusion, our research demonstrates that abnormal fluorescence imaging in aerospace aluminum alloy case castings is primarily caused by surface damage from sandblasting and polishing processes. Through detailed experimental analysis, we established that optimizing sandblasting parameters—specifically, using abrasives no larger than 300 μm and pressures not exceeding 0.4 MPa—significantly reduces surface embedding and associated fluorescence issues. Moreover, relocating the polishing step to after heat treatment prevents mechanical deformation and gap formation, further improving FPI accuracy. These findings provide practical guidelines for enhancing the quality control of castings aerospace, ensuring that fluorescence penetration inspection remains a reliable method for detecting surface defects. Future work could explore advanced surface treatment technologies or real-time monitoring systems to further optimize the manufacturing process for aerospace casting parts.
The implications of this study extend beyond aluminum alloys to other materials used in castings aerospace, as similar surface-related issues may arise. By adopting the proposed modifications, manufacturers can achieve higher yields and better performance in critical applications. Additionally, the methodologies and models developed here, such as the surface quality index, can be adapted for quality assessment in various industrial contexts. Ultimately, this research contributes to the ongoing advancement of non-destructive testing techniques, supporting the production of safer and more reliable aerospace components.