1. Introduction
Titanium alloys, especially ZTC4 titanium alloy, have found extensive applications in the aerospace, medical, and marine industries due to their outstanding properties such as low density, high strength – to – weight ratio, and excellent corrosion resistance. In the aerospace field, ZTC4 titanium alloy is commonly used in the manufacturing of complex – structured components like gearbox housing castings. However, during the production process of these castings, issues such as low casting defect detection rates and poor consistency in multiple inspections during fluorescence inspection have emerged, posing challenges to ensuring product quality.
Fluorescence inspection is a crucial non – destructive testing method for detecting surface – opening defects in products. But for ZTC4 titanium alloy gearbox housing castings with complex structures, the presence of surface contaminants like oil stains and oxide scales, as well as the complex geometry, makes it difficult to accurately detect defects. This has led to the situation where some defective products pass through multiple inspection processes, resulting in potential quality risks. Therefore, it is of great significance to conduct in – depth research on improving the fluorescent defect detection technology for ZTC4 titanium alloy gearbox housing castings.
2. Characteristics of ZTC4 Titanium Alloy Gearbox Housing Castings
2.1 Material Properties
ZTC4 titanium alloy combines several advantageous properties. Its low density contributes to weight reduction in aerospace components, which is crucial for improving fuel efficiency and overall performance. The high strength – to – weight ratio ensures that the gearbox housing can withstand the mechanical loads during operation without sacrificing structural integrity. Additionally, its strong corrosion resistance makes it suitable for use in harsh environments, whether it is in the high – altitude aerospace environment or the marine environment with high salt content.
| Property | Value/Description |
|---|---|
| Density | Relatively low, contributing to weight – saving in aerospace applications |
| Strength – to – weight ratio | High, enabling it to withstand mechanical loads effectively |
| Corrosion resistance | Strong, suitable for harsh operating environments |
2.2 Structure Complexity
The gearbox housing castings made of ZTC4 titanium alloy often have complex structures. They contain numerous intricate details such as multiple cavities, thin – walled sections, and complex curved surfaces. These complex structures pose challenges during the casting process, leading to issues like uneven casting, especially at the junctions of complex profiles. As shown in Figure 1 (Gearbox 3D Model), the irregular shape and various connecting parts make it difficult to ensure uniform material distribution and solidification during casting.
[Insert Figure 1: Gearbox 3D Model here]
3. Current Situation of Fluorescent Inspection
3.1 Inspection Process
In the current production process, multiple rounds of fluorescent inspections are typically carried out on ZTC4 titanium alloy gearbox housing castings. The first inspection is usually conducted on the raw materials. After that, inspections are carried out after each major processing step, such as after welding and mechanical processing. However, despite these multiple inspections, defective products still manage to pass through the inspection process.
| Inspection Stage | Time of Inspection | Purpose |
|---|---|---|
| First inspection | After receiving raw materials | Detect initial casting defects in raw materials |
| Second inspection | After the first defect – removal by grinding and completion of welding oil – circuit process | Identify defects that may have emerged during welding and heat – treatment |
| Third inspection | After all mechanical processing and before final inspection | Ensure all surfaces of the product are free from defects |
3.2 Existing Problems
- Low Defect Detection Rate: Direct fluorescent inspection can only identify some obvious surface cracks, sand holes, and inclusions. For small or hidden defects, especially those masked by surface contaminants, they are often overlooked.
- Poor Consistency: The inspection results are greatly affected by the surface state of the product. Contaminants such as oil stains and oxide scales can cause false positives or false negatives, resulting in inconsistent inspection results during multiple inspections.
4. Process Analysis and Experiments
4.1 Process Analysis
The low defect detection rate and poor consistency are mainly due to the following reasons:
- Residual Contaminants: Surface residues like oil stains and oxide scales are difficult to identify during fluorescent inspection. These contaminants can block the defect openings, preventing the fluorescent agent from entering and highlighting the defects.
- Complex Structure: The complex geometry of the gearbox housing castings makes it challenging to ensure comprehensive and accurate inspection. Some areas may be difficult to access, leading to undetected defects.
- Incomplete Defect Removal: During the defect – removal process by grinding, new defects may be generated, and the metal debris from grinding can cover existing defects, making them undetectable during subsequent inspections.
4.2 Process Improvement Verification
4.2.1 Adding Heat Treatment Process
Heat treatment can decompose surface contaminants such as grease on ZTC4 titanium alloy gearbox housing castings. By subjecting the raw materials that have passed the initial fluorescent inspection to a heat treatment process at the same temperature as the stress – relief temperature after welding (450 °C ± 3 °C), subsequent fluorescent inspections can more effectively reveal opening defects.
After heat treatment, when the gearbox housing castings were inspected again, multiple defects that did not meet the acceptance criteria were found. These included linear inclusions and lack of fusion in some repaired welding areas. Some defects had a maximum length exceeding 20 mm. The following table summarizes the defects found after heat treatment:
| Defect Type | Number of Defects | Maximum Length (mm) | Location |
|---|---|---|---|
| Linear inclusion | [X] | Over 20 | Product surface |
| Lack of fusion in repaired welding area | [X] | – | Product surface |
[Insert Figure 2: Defects Appearing on the Surface of Raw Materials after Vacuum Heat Treatment here]
4.2.2 Adding Acid Pickling Corrosion
Acid pickling corrosion is usually carried out before the welding oil – circuit process to ensure a good welding surface environment. To verify whether acid pickling can fully expose casting defects, an experiment was conducted. A batch of ZTC4 gearbox housing casting raw materials with more than 6 fluorescent – showing defects were selected. After manually grinding the defect areas and ensuring that the exposed defects were removed, acid pickling corrosion was carried out, followed by another fluorescent inspection.
The results showed that 4 defects were detected. Only 1 defect was located in the original grinding position, while the other 3 were in the un – ground areas that were previously judged as qualified during fluorescent inspection. This indicates that acid pickling corrosion can effectively expose the casting defects on the surface of ZTC4 gearbox housing castings. The details of the defects found after acid pickling are presented in the table below:
| Defect Location | Number of Defects | Remarks |
|---|---|---|
| Original grinding position | 1 | – |
| Un – ground areas (previously judged as qualified) | 3 | – |
[Insert Figure 3: Defects Appearing on the Surface of Raw Materials after Acid Pickling here]
4.2.3 Adding Ultrasonic Cleaning
Since the final processed gearbox housing castings are not suitable for corrosion treatment, ultrasonic cleaning was added to remove the grinding residues in the defects. A batch of 8 welded gearbox components was selected. After ultrasonic cleaning, fluorescent defect inspections were carried out.
The results showed that casting defects such as cracks and point – like defects existed on the non – processed surfaces of all 8 gearbox housing castings. This indicates that ultrasonic cleaning can fully expose some casting defects that were not fully revealed during the fluorescent inspection in the raw material state. The table below shows the defects found after ultrasonic cleaning:
| Defect Type | Number of Defects | Location |
|---|---|---|
| Cracks | [X] | Non – processed surfaces |
| Point – like defects | [X] | Non – processed surfaces |
[Insert Figure 4: Defects Appearing on the Surface of Raw Materials after Ultrasonic Cleaning here]
5. Process Improvement and Verification
5.1 Process Improvement
Based on the experimental results, the following process improvements were made:
- Pre – arrival Heat Treatment: Before the ZTC4 raw materials arrive at the factory, a heat treatment process is added, followed by a fluorescent inspection.
- Acid Pickling and Multiple Inspections: For the defects shown in the fluorescent inspection, a fitter is arranged to carry out grinding and repair. Then, acid pickling corrosion is performed on the entire product surface, and another fluorescent inspection is carried out. If the welding area has undergone acid pickling for contamination removal before welding, an additional fluorescent inspection is required. After welding, a full – surface fluorescent inspection of the gearbox housing is carried out.
- Ultrasonic Cleaning for Multiple Acid – pickled Castings: For gearbox housing castings that have undergone acid pickling more than 2 times, ultrasonic cleaning is added before the fluorescent inspection to fully expose the fluorescent defects.
The following flow chart illustrates the improved process:
[Insert a flow chart showing the improved process here]
5.2 Verification of Improvement Effect
A subsequent batch of more than 20 ZTC4 gearbox housing casting components was produced using the improved process. The results showed a significant improvement in the inspection pass rate. The one – time detection pass rate of ZTC4 raw materials increased by more than 70%, and the one – time inspection pass rate of the final products increased by more than 20%. The comparison of pass rates before and after the improvement is presented in the table below:
| Inspection Stage | Pass Rate before Improvement | Pass Rate after Improvement | Increase Percentage |
|---|---|---|---|
| ZTC4 raw materials inspection | [X]% | [X + 70]% | More than 70% |
| Final product inspection | [X]% | [X + 20]% | More than 20% |
6. Significance of Surface Pretreatment
Fluorescent inspection requires the inspected surface to be clean and dry. Contaminants such as dust, oil stains, and chemical residues on the surface can cause false displays during fluorescent inspection and may block the defects, resulting in missed detections. Therefore, adding surface pretreatment operations before fluorescent inspection is of great significance for improving the accuracy of fluorescent defect detection technology.
For products made of cast titanium alloy, surface pretreatment measures such as acid etching or decomposition of contaminants (such as grease) in a high – temperature environment can fully expose the casting defects on the product surface. The addition of acid pickling corrosion, ultrasonic cleaning, and heat treatment processes in the process improvement for ZTC4 titanium alloy gearbox housing castings can open the defect openings, laying a solid foundation for subsequent surface fluorescent inspections. This pretreatment process effectively solves the technical problems of low defect detection rates and poor stability in the fluorescent inspection of complex – structured ZTC4 titanium alloy castings.
7. Conclusion
This article has systematically studied the problem of improving the fluorescent inspection efficiency of ZTC4 – based complex – structured gearbox housing castings. Through process improvements, the addition of pretreatment processes such as heat treatment, acid pickling for contamination removal, and ultrasonic cleaning before fluorescent inspection can significantly increase the defect detection rate of product fluorescent inspections. This has basically solved the problems of low defect detection rates and low inspection efficiency in the fluorescent inspection of ZTC4 cast titanium alloy components, providing a new approach for enhancing the surface defect inspection technology of cast titanium alloys. Future research can focus on further optimizing these pretreatment processes, exploring more advanced inspection techniques, and improving the overall quality control system for ZTC4 titanium alloy products.
