Porosity in castings is a major defect that reduces product strength, sealing, and lifespan.
The following six common nondestructive testing (NDT) methods are available, and selection can be based on defect location, cost, and efficiency:
- Radiographic Testing (RT): Suitable for detecting internal pores, providing intuitive two-dimensional images with high accuracy, but also relatively high cost.
- Ultrasonic Testing (UT): Suitable for internal and near-surface defects, effective for thick-walled parts, but requires specialized operators.
- Magnetic Particle Testing (MT): Suitable only for detecting surface and near-surface defects in ferromagnetic materials, with high sensitivity and low cost.
- Penetrant Testing (PT): Specialized for detecting open surface defects, low cost and simple operation, but unable to detect closed internal pores.
- Eddy Current Testing (ET): Used for rapid, automated screening of near-surface defects, particularly for non-ferrous metals.
- Industrial Computed Tomography Scanning (CT): Provides the most accurate three-dimensional size and distribution analysis of internal pores. The equipment is expensive and is used for high-value parts.
Selecting a method requires a comprehensive consideration of the testing requirements, casting material, dimensions, and budget.
The following article will provide a detailed analysis of the principles, steps, advantages, and disadvantages of each casting porosity testing method, along with selection recommendations.
Casting Porosity Detection Methods
Radiographic Testing (RT)
Principle
X-rays or gamma rays are used to penetrate castings. Due to the different densities of pores and the metal matrix, their absorption capacity for the radiation varies.
The radiation attenuates less after passing through the pores, resulting in a stronger signal received by the film or digital detector, creating a dark shadow on the image.
Radiographic Testing (RT) Steps
1. Preparation of Work Area: Establish and secure a radiation control area with clear warning signs. Ensure all personnel are evacuated from the controlled area before exposure.
2. Equipment and Parameter Selection: Select an appropriate X-ray generator or gamma ray source (e.g., Iridium-192). Set exposure parameters (kV, mA, time) based on casting material, thickness, and expected defect size.
3. Setup of Exposure Geometry: Position the radiation source on one side of the casting and the film in a cassette or a digital detector (DR/CR plate) on the opposite side. Minimize the source-to-object and object-to-detector distances to optimize image clarity.
4. Placement of Image Quality Indicators (IQIs): Place wire-type or hole-type IQIs (penetrameters) on the source side of the casting to verify imaging sensitivity and quality.
5. Exposure: Conduct the exposure after ensuring the area is clear. Radiation penetrates the casting; areas with porosity absorb less radiation.
6. Film Processing / Image Reading:
- Film (RT): Develop, fix, wash, and dry the exposed film in a darkroom.
- Digital Detector (DR/CR): Read the digital detector plate or scan the Imaging Plate (CR) to obtain a digital image.
7. Image Interpretation: A certified technician interprets the radiograph on a light viewer or computer monitor. Porosity appears as dark, irregularly shaped spots or areas on the image.
8. Documentation and Reporting: Record the results, generate a report, and archive the radiographs or digital files for traceability.
Advantages
- Strong Structural Adaptability: Particularly effective in inspecting castings with complex internal cavities and irregular flow channels.
- Intuitive Results: 2D images clearly demonstrate the size, shape, and distribution of pores.
- Recordable Results: Film or digital images serve as a permanent record, facilitating archiving, traceability, and customer review.
- High Accuracy: High detection rate and measurement accuracy for volumetric defects such as pores and shrinkage cavities.
Disadvantages
- Strict Safety Requirements: Requires specialized radiation protection measures and a segregated work area, increasing the complexity of inspection implementation.
- High Overall Cost: Significant equipment investment, high operating and maintenance costs, and expensive inspection consumables.
- Limited Operating Space: The radiation source and detector typically need to be placed on opposite sides of the workpiece, making on-site operation sometimes inconvenient.
The following diagram illustrates radiographic testing for casting porosity:
Radiographic Testing for Casting Porosity
Ultrasonic Testing (UT)
Principle
High-frequency sound waves (ultrasonic waves) are transmitted into the casting. When the sound waves encounter the interface of a defect (such as a pore), they are reflected.
By receiving and analyzing these echo signals, the location, size, and general shape of the defect can be determined.
Ultrasonic Testing (UT) Procedure
1. Surface Preparation: Clean the inspection area to remove scale, paint, dirt, and other contaminants to ensure smooth coupling.
2. Equipment and Probe Selection: Select a suitable ultrasonic flaw detector and transducer (probe) based on material properties and thickness. Higher frequencies offer better resolution for near-surface defects.
3. Calibration: Calibrate the instrument’s time base (range) and sensitivity (gain) using a reference standard block with known artificial defects.
4. Couplant Application: Apply a couplant (e.g., oil, gel, glycerin) to the surface to facilitate the transmission of ultrasonic energy into the casting.
5. Scanning: Move the probe over the surface in a consistent pattern with constant speed and pressure. Maintain beam direction perpendicular to the expected defect orientation.
6. Defect Signal Identification: Monitor the A-scan display. The reflection from a pore (defect echo) will appear between the initial pulse and the back wall echo.
7. Defect Evaluation: Evaluate the location (depth), amplitude (equivalent size), and characteristics of the echo signals using Distance Amplitude Correction (DAC) or similar techniques.
8. Documentation and Reporting: Document the location, depth, size, and type of indications. Generate an inspection report.
Advantages
- Strong Penetration: Effectively inspects thick and large-section steel and iron castings, offering a unique advantage in inspecting large workpieces.
- High Sensitivity: Detects common, small defects such as millimeter-level pores and shrinkage cavities in castings.
- Safety: Eliminates radiation risk, eliminating the need for special radiation protection measures during inspection.
- Portability: The device is lightweight and portable, allowing flexible inspection in various working environments, including foundries and processing sites.
Disadvantages
- Not Intuitive: Results are based on waveform analysis, requiring experienced operators. Qualitative and quantitative characterization of defects is not as intuitive as with RT.
- Couplant Requirements: A couplant (such as oil or gel) is required between the probe and the workpiece to transmit the sound waves.
- Shape Limitations: Detecting castings with complex shapes and rough surfaces is difficult and prone to generating interference signals.
The following figure shows a schematic diagram of ultrasonic testing for casting porosity:
Ultrasonic Testing for Porosity in Castings
Magnetic Particle Testing (MT)
Principle
Magnetic particle testing utilizes the property that, after magnetization of ferromagnetic materials, a leakage magnetic field is generated at defects.
When magnetic particles are applied, the leakage magnetic field attracts the particles, forming visible magnetic traces, which visually reveal surface and near-surface defects.
Magnetic Particle Testing (MT) Steps
1. Surface Preparation: Thoroughly clean the surface to remove all oil, grease, rust, sand, and paint that could trap particles or interfere with the test.
2. Magnetization: Magnetize the casting using a suitable method (e.g., prod contacts, coil, yoke). Perform magnetizations in at least two directions approximately 90° apart to detect defects in various orientations.
3. Application of Magnetic Particles: Apply dry magnetic powder or wet magnetic particle suspension (visible or fluorescent) to the surface during magnetization.
4. Inspection: Examine the surface under adequate lighting conditions (white light for visible particles, UV-A/black light for fluorescent particles). Porosity will collect magnetic particles, forming visible indications.
5. Interpretation and Evaluation: Interpret the shape, sharpness, and size of the particle accumulation to identify and assess defects.
6. Post-Test Cleaning and Demagnetization:
- Demagnetization: Demagnetize the part if required for subsequent processing or service.
- Cleaning: Remove all residual magnetic particles from the surface.
7. Documentation and Reporting: Record the location, nature, and extent of indications. Prepare an inspection report.
Advantages
- Extremely High Sensitivity: Detects micron-level surface and near-surface defects and is particularly sensitive to dense porosity.
- High Detection Efficiency: Suitable for rapid on-site screening of large steel and iron castings, with a single magnetization pass capable of inspecting a large area.
- Intuitive Results: Porosity formed by accumulated magnetic traces is clearly visible, and the defect morphology is clearly visible.
- Cost-Effective: Low equipment investment and maintenance costs make it particularly suitable for inspecting large quantities of castings.
Disadvantages
- Material Limitations: Applicable only to ferromagnetic castings (cast steel and cast iron), not non-ferrous metals such as aluminum alloys and magnesium alloys.
- Direction Dependence: Multiple magnetizations are required based on the potential direction of pore formation. Before testing, the force direction and pore generation patterns of the casting must be clearly understood.
- High Surface Requirements: The casting surface must be thoroughly cleaned of impurities such as oil, paint, and scale; otherwise, the visualization of pore defects will be severely affected.
- Demagnetization Requirements: After testing, the workpiece is often demagnetized to prevent residual magnetic fields from affecting subsequent processing and performance.
The following figure shows a schematic diagram of magnetic particle testing for casting porosity:
Magnetic Particle Testing for Porosity in Castings
Liquid Penetrant Testing (PT)
Principle
Penetrant testing involves applying a specific penetrant liquid to the casting surface, allowing it to penetrate open surface defects.
After removing excess liquid, a developer is applied to draw out the penetrant liquid within the defect, creating a clear color contrast on the surface, visually highlighting the defect.
Liquid Penetrant Testing (PT) Steps
1. Pre-Cleaning: Clean and dry the surface thoroughly to remove any contaminants that might block the openings of surface defects.
2. Penetrant Application: Apply the penetrant (visible dye or fluorescent) by spraying, brushing, or dipping. Ensure full coverage and allow sufficient dwell time for the penetrant to seep into defects.
3. Excess Penetrant Removal:
- Water-Washable: Carefully rinse with water.
- Solvent-Removable: Wipe with a clean cloth lightly moistened with remover.
- Post-Emulsifiable: Apply emulsifier first, then rinse with water. Avoid over-cleaning.
4. Developer Application: Apply a thin, even layer of developer (dry, wet, suspendable, or soluble) to draw the penetrant out of defects and create a visible indication.
5. Development: Allow the developer to work for the specified development time.
6. Inspection:
- Visible Dye: Examine under adequate white light.
- Fluorescent: Examine in a darkened area under ultraviolet (UV-A/black) light. Porosity will be shown by brightly colored or fluorescent indications.
7. Post-Cleaning: Clean the part after inspection to remove all residual penetrant and developer.
8. Documentation and Reporting: Document the size, location, and type of indications. Generate a report.
Advantages
- Simple Operation: Low equipment cost and a simple process flow make it ideal for rapid batch testing on the production site.
- Intuitive Results: The specific contours, size, and distribution of defects such as pores can be directly and clearly revealed.
- Wide Material Compatibility: Suitable for magnetic materials such as cast steel and cast iron, it is also the preferred method for surface porosity detection in non-magnetic materials such as aluminum alloys and magnesium alloys.
Disadvantages
- Limited Defect Types: Only open defects connected to the surface can be detected; internal pores and shrinkage cavities cannot be identified.
- High Surface Cleanliness Requirements: Oil, scale, and coatings on the workpiece surface must be thoroughly removed before testing; otherwise, the test results will be severely affected.
- Environmental Compliance: The chemical reagents used require professional handling, and certain environmental and operator safety requirements are in place.
The following figure shows a schematic diagram of liquid penetrant testing for casting porosity:
Liquid Penetrant Testing for Casting Porosity
Eddy Current Testing (ET)
Principle
Utilizes the principle of electromagnetic induction. When a test coil carrying an alternating current is brought close to a conductive casting, eddy currents are induced.
The distribution and magnitude of the eddy currents are affected by the conductivity, magnetic permeability, and defects of the casting.
Defects such as pores interfere with the eddy currents, causing changes in the coil’s impedance, which can be detected by the instrument.
Eddy Current Testing (ET) Steps
1. Equipment and Probe Selection: Select an appropriate eddy current instrument and probe (absolute or differential) based on the application and material conductivity.
2. Calibration and Setup: Calibrate the instrument on a reference standard with known artificial defects. Set test parameters (frequency, gain, phase rotation, filters).
3. Scanning: Scan the surface by moving the probe consistently while maintaining a stable lift-off (distance between probe and part). No couplant is needed.
4. Signal Monitoring: Monitor the impedance plane display (signals on an X-Y screen). Defects like porosity cause characteristic phase and amplitude shifts in the signal.
5. Analysis and Evaluation: Analyze the signal response to differentiate between defect signals and false indications from geometry changes or material variations.
6. Documentation and Reporting: Record the location and characteristics of any signals. Generate a report. Defect verification with another NDT method is often required.
Advantages
- High-Speed, Automated Inspection: Enables high-speed, high-volume porosity screening of non-ferrous metal castings, such as aluminum alloys, on the production line.
- Non-Contact Inspection: Requires no coupling agent and can be completed without contacting the workpiece surface, thus preventing damage or contamination.
Disadvantages
- Limited Inspection Depth: Affected by the skin effect, this method is primarily suitable for near-surface porosity detection within a range of 2-3 mm below the surface.
- High Material Sensitivity: Sensitive to changes in material composition, heat treatment status, and other factors, requiring parameter adjustment for different materials.
- Difficulty in Defect Characterization: Distinguishing porosity from other types of defects can be difficult, requiring verification using other inspection methods.
The following figure illustrates eddy current testing for casting porosity:
Eddy Current Testing for Casting Porosity
Industrial Computed Tomography Scanning (CT)
Principle
A X-ray source is used to project and scan the casting from multiple angles.
A detector collects a large amount of projection data, and a computer reconstruction algorithm is used to synthesize a three-dimensional image of the casting’s internal structure, enabling spatial localization and morphological analysis of defects such as porosity.
Industrial Computed Tomography (CT) Scanning Steps
1. Sample Mounting: Securely mount the casting on the rotary stage in the scanning chamber, ensuring it is stable and centered.
2. Parameter Setup: Set the X-ray source parameters (voltage, current, filtration) and detector settings based on material density, part size, and desired resolution/voxel size.
3. Scan Planning: Define the region of interest (ROI), number of projections (angles), and exposure time per projection.
4. Data Acquisition: The system rotates the casting 360 degrees while the X-ray source and detector remain stationary, capturing a series of 2D radiographic images from all angles.
5. Image Reconstruction: A computer uses specialized software and algorithms (e.g., Filtered Back Projection, Iterative Reconstruction) to reconstruct the 2D projections into a 3D volumetric dataset (voxel model).
6. Data Analysis:
- Use analysis software to view cross-sectional slices in any orientation.
- Apply density thresholding and segmentation tools to isolate, identify, and label porosity defects.
- Perform quantitative analysis (pore volume, position, sphericity, distribution).
7. Reporting: Generate a comprehensive report containing 2D slices, 3D renderings, and quantitative data tables.
Advantages
- Precise 3D Imaging: Provides comprehensive 3D visualization of porosity defects, enabling virtual sectioning and accurately depicting the spatial distribution and morphological characteristics of internal pores.
- Quantitative Analysis: Accurately measures 3D parameters such as pore volume, location, and distribution density, enabling precise quantification and statistical analysis of defects.
- Nondestructive Testing: Obtains detailed defect information comparable to destructive testing without damaging the workpiece.
Disadvantages
- Large Equipment Investment: Industrial CT systems are expensive to purchase, and the cost of a single inspection project is significantly higher than conventional inspection methods.
- Limited Inspection Efficiency: 3D data acquisition and reconstruction are time-consuming processes, making it challenging to meet the rapid inspection requirements of production lines.
- Limited Workpiece Size: The inspection capability for large castings is limited by the scanning chamber size and X-ray source energy.
The following figure illustrates industrial CT scanning for casting porosity:
Industrial CT Scanning for Casting Porosity
The following table compares six NDT methods for detecting casting porosity:
|
Inspection Method |
Primary Detection Defect Location | Advantages | Disadvantages | Applicability |
|
Radiographic Testing (RT) |
Internal, Near-Surface | Intuitive, recordable, reliable results | Radiation, high cost, limited operating space | Almost all materials, complex internal structures |
| Ultrasonic Testing (UT) | Internal, Near-Surface | Strong penetration, good for thick sections, portable | Unintuitive, requires couplant, experience-dependent |
Large thick-walled castings (cast steel, iron) |
|
Magnetic Particle Testing (MT) |
Surface, Near-Surface | High sensitivity, low cost, intuitive results | Ferromagnetic materials only, requires demagnetization | Ferromagnetic materials (cast steel, iron) |
| Penetrant Testing (PT) | Surface Opening Only | Simple, low-cost, intuitive | Surface defects only, environmental pollution |
Magnetic and non-magnetic materials’ surface defects |
|
Eddy Current Testing (ET) |
Near-Surface | Fast, automated | Shallow depth, high material sensitivity |
Non-ferrous metals rapid screening |
| Industrial CT | Internal (3D) | Most intuitive, precise measurement | Extremely expensive, slow inspection |
High-value precision castings |
How to Select the Appropriate Casting Porosity Inspection Method?
Dimension 1: Selecting the Inspection Method Based on the Inspection Objective
To select the appropriate inspection method, first clarify the type of information you hope to obtain. Depending on the objective, the following levels of methods can be selected:
Qualitative Screening: Rapid “Presence” or “Absence” determination
Applications: Rapid initial screening of large batches of castings on the production line, incoming inspection of low-stress, non-critical castings.
Information Required: Confirmation of surface or near-surface defects is sufficient; precise size and shape are not necessary.
Preferred Method: Penetrant Testing (PT) or Eddy Current Testing (ET).
- Penetrant Testing(PT): Rapidly detects surface-open pores and cracks.
- Eddy Current Testing(ET): High-speed detection of near-surface defects, easily automated.
Quantitative Assessment: Accurately Measures “Where” and “How Big”
Applications: Quality assessment of key components, conformity testing to determine if defects exceed standards, and process stability verification.
Information Requirements: Quantitative data, such as the precise location, size, and distribution of defects, is required for quality assessment.
Preferred Methods: Radiographic Testing (RT) or Ultrasonic Testing (UT).
- Radiographic Testing(RT): Provides intuitive 2D images and accurately measures the size and distribution of pores on the projected surface.
- Ultrasonic Testing(UT): Detects internal defects and measures their depth and pore size.
Precise Analysis: 3D Insight into “True Morphology and Precise Data.”
Applications: Failure analysis, root cause investigations, new product process development, and identification of high-reliability requirements (such as aerospace).
Information Requirements: Requires complete 3D morphology, precise volume, spatial location, and relative relationship of defects to other structures.
Preferred Methods: Industrial CT scanning.
- Industrial Computed Tomography Scanning (CT): Provides non-destructive three-dimensional images, enabling virtual sectioning and the most accurate quantitative analysis of porosity.
Dimension 2: Selecting the Inspection Method Based on Casting Characteristics
The appropriate porosity inspection method can also be selected based on the characteristics of the casting itself.
The following table recommends porosity inspection methods based on casting characteristics:
|
Casting Characteristics |
Recommended Method | Reasoning |
|
[Material] |
||
| Non-Ferromagnetic Materials (e.g., Aluminum Alloys, Magnesium Alloys) |
PT (surface) ET (near-surface) RT UT |
Magnetic particle testing (MT) cannot be used. PT and ET are highly effective for non-magnetic materials. |
|
Ferromagnetic Materials |
MT (surface/near-surface) RT UT |
MT is the preferred method for surface inspection of ferromagnetic materials, with faster inspection speeds than PT. |
|
[Size and Thickness] |
||
| Large and Heavy Parts | UT |
RT has difficulty penetrating thick and heavy parts, resulting in high inspection costs; UT offers high penetration and portability. |
|
Small/Medium Parts with |
RT (X-ray) | Provides clear images unaffected by complex structures and accurately reveals internal porosity distribution. |
| Micro-Precision Parts | Microfocus RT or Industrial CT |
Requires extremely high spatial resolution and 3D reconstruction capability. |
|
[Surface Condition] |
||
| Rough (As-Cast Surface) | RT, UT |
PT/MT requires a smooth surface; rough surfaces can produce interference signals. |
| Machine-Finished Surfaces | PT, MT, RT, UT |
The surface condition meets the implementation requirements for all testing methods. |
Note: PT (Penetrant Testing); ET (Eddy Current Testing); RT (Radiographic Testing); UT (Ultrasonic Testing); MT (Magnetic Particle Testing); CT (Industrial Computed Tomography Scanning).
Dimension Three: Selecting an Inspection Method Based on Practical Conditions
|
Key Factor |
Method Characteristics | Recommended Method |
| Equipment and Personnel Costs | RT and CT equipment are expensive and require certified personnel to operate. PT/MT/UT equipment investment is relatively low. |
Limited budget: UT, PT |
|
Testing Efficiency and Batch Volume |
ET and automated RT/UT are extremely fast and suitable for batch online testing. Conventional RT and UT are slower and suitable for spot checks or laboratory analysis. |
Batch testing: ET, automated UT/RT Laboratory analysis: Conventional RT, UT |
| Safety and Environmental Protection | RT carries radiation risks and requires licensing and protective facilities. PT uses chemical reagents and requires environmentally friendly disposal. |
No radiation license: UT, ET |
|
Standards and Customer Requirements |
Customer drawings or industry standards (such as ASTM, ASME, and GB) often mandate the use of specific methods and acceptance levels. |
Mandatory: Strictly follow the specified methods in the standard. |
Note: PT (Penetrant Testing); ET (Eddy Current Testing); RT (Radiographic Testing); UT (Ultrasonic Testing); MT (Magnetic Particle Testing); CT (Industrial Computed Tomography Scanning).
How Does CEX Casting Achieve Near-Zero Porosity Aluminum Die Castings?
To achieve nearly-zero porosity in aluminum die casting parts, the first step is to understand what aluminum die casting porosity is. Click here to learn more about this topic.
Patented Squeeze Casting Process
CEX Casting‘s self-designed squeeze casting technology applies continuous, ultra-high pressure immediately after the molten aluminum is filled into the mold.
Request the Squeeze Casting Case Study
This powerful pressure effectively “squeezes out” any potential porosity and shrinkage defects, ensuring the molten metal remains dense throughout the solidification process.
The video below demonstrates CEX’s full squeeze casting service process:
CEX Casting Squeeze Casting Workshop
Centralized Raw Material Melting System
We utilize a centralized raw material melting system, employing refining processes such as vacuum degassing and rotary degassing during the melting process.
Combined with precise temperature control, this system minimizes hydrogen and oxide inclusions from the molten aluminum, minimizing the root causes of porosity.
In-House Mold Design and Manufacturing
Leveraging our in-house mold design and manufacturing team, CEX can deeply optimize the mold’s venting, gating, and cooling systems.
Simulations and verification using advanced mold flow analysis software ensure that the molten metal fills the cavity smoothly and orderly, and that gases are expelled smoothly, significantly reducing air entrapment caused by improper mold design.
Regular Mold Maintenance
CEX Casting has established a strict mold maintenance system. Regular cleaning of venting grooves, repair of surface wear, and inspection of cooling channels ensure that each mold is in optimal working condition.
This ensures smooth venting during production and minimizes the risk of porosity.
CEX Casting Porosity Rate Testing Results
The figure below shows the porosity test results for our A356.2-T6 material sample, issued by a third-party authoritative testing agency. This fully demonstrates our exceptional quality of near-zero porosity.
Test Summary:
- Test Sample: A356.2 Specimen
- Material Grade: A356.2-T6
- Test Standard: GB/T 13298-2015
- Test Result: Porosity 0.3%
CEX Porosity Rate Testing Results
This result demonstrates that CEX Casting, through its patented squeeze casting process, meticulous melting process control, and comprehensive quality management, has successfully controlled the porosity of its castings to an extremely low level (0.3%), far exceeding common industry standards and providing customers with a high-density, high-reliability product guarantee.
Conclusion
Detecting porosity in castings relies on a comprehensive balance between the inspection objectives, casting characteristics, and actual conditions.
A variety of nondestructive testing methods, including radiographic, ultrasonic, magnetic particle, penetrant, eddy current, and industrial CT, constitute a comprehensive technical framework.
Each method is an effective tool within its applicable scope. Decision-makers must simultaneously consider factors such as defect location requirements, part properties, and resource constraints to select the most appropriate testing solution.
As a leading manufacturer of custom aluminum alloy castings in China, CEX Casting leverages its patented squeeze casting process and comprehensive quality control system to achieve an industry-leading porosity of 0.3%, providing you with high-density, highly reliable casting solutions.
Contact us today for more technical information or a specific quote, and our engineering team will be happy to assist you.