The porosity acceptable standard for castings represents a dynamic techno-economic decision. Two fundamental dimensions govern these standards: the intrinsic properties of the casting material itself and the inherent capabilities of the casting process employed. A deep understanding of these dimensions forms the essential foundation for establishing a scientific, rational, and cost-effective porosity acceptance standard.
The following sections will detail how the two dimensions of casting material and casting process influence the development of casting porosity acceptance standards. It will also systematically introduce a three-step framework to facilitate the establishment of clear, fair, and efficient quality agreements between you and your suppliers.
Porosity Types and Key Inspection Standards
Types of Porosity
Gas Porosity: Caused by gas entrapment in the melt or poor mold venting, gas is trapped during solidification. Porosity is typically spherical or elliptical with smooth inner walls.
Shrinkage Porosity: Caused by volumetric shrinkage during metal solidification and insufficient shrinkage feeding, it often appears in the last solidification area, such as hot spots.
Shrinkage porosity has an extremely irregular shape, a rough inner wall, and is often accompanied by dendritic structures.
The following image illustrates these two main types of porosity in castings:
Gas Porosity & Shrinkage Porosity
Want to learn more about the types and causes of casting porosity? Check our other article:
What Is Die Casting Porosity? Types, Distinct Causes, and Impact on Die Castings
Inspection Standards for Gas and Shrinkage Porosity
Both porosity and shrinkage porosity are evaluated using a defect grading system, but with different assessment focuses.
The acceptance level for internal defects is primarily based on reference radiographs in ASTM E155 (aluminum/magnesium castings) or ASTM E446 (steel castings).
For gas porosity, the focus is on size, number, and distribution density, while for shrinkage porosity, the focus is on morphology, concentration, and depth of depressions.
For surface defects, the acceptance level is based on ASTM A802/A802M (steel castings) or ISO 11971 (steel/iron castings), which define acceptable defect limits for gas and shrinkage porosity at different levels.
Porosity Defect Limit Levels
In casting standards, the permissible defect limits are not fixed values, but are defined dynamically through a classification system. The specific limits depend on the selected acceptance level.
The following table shows the steel casting porosity inspection standards and grades:
|
Quality Level |
Internal Defects Standard (ASTM E446) | Surface Defects Standard (ASTM A802) |
|
Level 1 (Highest) |
Single Defect ≤ 0.8 mm Density ≤ 3 pieces/in² Area ≤ 0.5% |
Depth ≤ 0.8 mm Diameter ≤ 1.0 mm Density ≤ 2 pieces/in² |
| Level 2 (High) | Single Defect ≤ 1.5 mm Density ≤ 5 pieces/in² Area ≤ 1% |
Depth ≤ 1.2 mm |
|
Level 3 (Commercial) |
Single Defect ≤ 2.5 mm Density ≤ 8 pieces/in² Area ≤ 2% |
Depth ≤ 2.0 mm Diameter ≤ 3.0 mm Density ≤ 6 pieces/in² |
| Level 4 (General) | Single Defect ≤ 4.0 mm Density ≤ 12 pieces/in² Area ≤ 3% |
Depth ≤ 3.0 mm |
| Level 5 (Low) | Single Defect ≤ 6.0 mm Density ≤ 20 pieces/in² Area ≤ 5% |
– |
The following table shows the Aluminum/Magnesium casting porosity inspection standards and grades:
|
Quality Level |
Internal Defects (ASTM E155) | Surface Defects (Typical Company Standard) |
|
Level 1 (Highest) |
Single Defect ≤ 0.5 mm Density ≤ 2 pieces/in² Area ≤ 0.3% |
Depth ≤ 0.5 mm |
|
Level 2 (High) |
Single Defect ≤ 1.0 mm Density ≤ 4 pieces/in² Area ≤ 0.8% |
Depth ≤ 0.8 mm Diameter ≤ 1.5 mm Density ≤ 4 pieces/in² |
| Level 3 (Commercial) | Single Defect ≤ 1.8 mm Density ≤ 6 pieces/in² Area ≤ 1.5% |
Depth ≤ 1.2 mm |
|
Level 4 (General) |
Single Defect ≤ 2.5 mm Density ≤ 10 pieces/in² Area ≤ 2.5% |
Depth ≤ 1.5 mm Diameter ≤ 3.5 mm Density ≤ 8 pieces/in² |
| Level 5 (Low) | Single Defect ≤ 3.5 mm Density ≤ 15 pieces/in² Area ≤ 4% |
– |
The following table shows the iron casting porosity inspection standards and grades:
|
Quality Level |
Internal Defects (VDG P 201/P 202) | Surface Defects (ISO 11971) |
|
Level 1 (Highest) |
– | Very smooth surface, only very small, sparsely distributed defects are allowed |
| Level 2 (High) | Single Defect ≤ 3 mm Defect Area ≤ 1% |
Smooth surface, allowing a small amount of fine pores |
|
Level 3 (Commercial) |
Single Defect ≤ 5 mm Defect Area ≤ 3% |
Good surface quality, and a certain number and size of defects are allowed, but they do not affect general use |
| Level 4 (General) | Single Defect ≤ 8 mm Defect Area ≤ 5% |
Average surface quality, obvious defects such as large pores or slag inclusions are allowed |
| Level 5 (Low) | Single Defect ≤ 12 mm Defect Area ≤ 8% |
Rough surface, large or severe defects are allowed |
Two Key Dimensions Determining Porosity Acceptance Standard
Dimension 1: Casting Material
Different materials have different matrix strengths and microstructures. This difference in intrinsic physical properties leads to differences in the numerical values of the porosity acceptance standard.
High-Strength Materials
The mechanical properties of high-strength aluminum alloys, cast steel, and ductile iron are highly dependent on a complete and dense matrix.
Porosity, as a geometric discontinuity, can become a significant source of stress concentration, severely weakening the material’s strength and fatigue properties.
Therefore, their porosity acceptance standards are extremely stringent.
Value Example (Internal Pore Diameter):
High-Strength Cast Steel: Per standards like ASTM E446 Level 1, the single pore diameter is typically limited to ≤ 0.8 mm.
High-Strength Aluminum: Per ASTM E155 Level 1, it also sets the limit of single pore diameter within a narrow range of ≤ 0.5 mm.
Materials with Inherent Matrix Defects
Taking gray cast iron as an example, there are already a large number of graphite flakes in the microstructure of this type of material.
The flake graphite itself is like natural microcracks or voids, which significantly reduces the material’s sensitivity to porosity defects.
Value Example (Internal Pore Diameter):
Gray Iron: General quality standards often allow single pores up to 3-5 mm or more, contrasting sharply with the sub-millimeter limits for high-strength materials.
Dimension 2: Casting Process
The choice of casting process directly determines the metal filling, solidification, and related degassing capabilities.
Therefore, the porosity acceptance standard must be matched to the specific casting process.
Die Casting
The die-casting process uses high speed and high pressure to inject molten aluminum into a die-casting mold, which can easily trap air.
Therefore, porosity is an inherent characteristic of the die casting process. The core of the acceptance standard is not the pursuit of “zero defects” but rather “zoning control.”
Acceptance Standard:
Sealing/Functional Surfaces: No penetrating pores are allowed; surface pore depth ≤ 0.3 mm, diameter ≤ 0.5 mm, density ≤ 1 pore/cm².
High-Stress Areas: Single pore diameter ≤ 1.0 mm; linear or chain-like pores are not allowed.
General Areas: A small number of isolated pores are permitted, with the maximum diameter of each pore ≤ 2.5 mm, but the number of pores with a diameter greater than 1 mm per square inch is ≤ 5.
CEX Casting’s Solution: Squeeze Casting
CEX Casting‘s patented squeeze casting technology combines casting and forging advantages. High pressure applied during solidification drastically suppresses gas and shrinkage porosity.
Request the Squeeze Casting Case Study
The following video shows the squeeze casting process at CEX Casting:
Squeeze Casting Workshop at CEX Casting
Porosity Testing Results of Squeeze Castings
Porosity testing results for CEX Casting squeeze castings (see figure below) show that the pore area is only 0.003 mm² (approximately φ0.06 mm), and the overall porosity is as low as 0.3%, far exceeding the highest grade requirement of ASTM E446.
CEX Casting Porosity Testing Results
Investment Casting
Investment casting uses the principles of hot shell pouring and sequential solidification.
The original intention of the process is to produce near-net shape and high internal quality castings.
Based on this process capability, the market has high expectations for the internal density of investment castings.
Acceptance Standard:
Internal Quality: Referring to ASTM E192, commercial-grade (Level 3) castings typically require individual pores to be ≤ 1.5 mm in diameter, with dense clusters of pores not permitted in thin-walled areas.
Surface Quality: Surface pores ≤ 1.0 mm in diameter and ≤ 0.5 mm in depth. For the visible A-side, the number of pores permitted per 100 cm² is strictly limited (e.g., ≤ 3).
Sand Casting
Sand casting has the greatest process variability, resulting in a wide range of pore shapes, sizes, and locations.
Acceptance Standard:
Critical Areas: Required to meet ASTM E446 Level 2 or higher; that is, individual pores ≤ 1.5 mm in diameter, no defects larger than 1/5 of the wall thickness, and a total pore cluster area ≤ 1%.
General Areas: ASTM E446 Level 3 is acceptable, allowing individual pores ≤ 2.5 mm in diameter, but defects must not significantly reduce the designed wall thickness.
Positioning Requirements: On the machined surface, the residual depth of pores must not be less than 50% of the machining allowance.
The following table compares the porosity acceptance standard for four casting processes:
|
Process |
Critical Area Standard | General Area Standard |
|
Squeeze Casting |
Single diam. ≤ 0.5mm
Porosity ≤ 0.5% Density ≤ 3 pieces/25cm²
|
Single diam. ≤ 2.0mm
Porosity ≤ 1.5% |
| Die Casting | Sealing/stress-bearing area:
Single diam. ≤ 0.5-1.0mm Chain-shaped pores are prohibited |
Single diam. ≤ 2.5mm Density ≤ 5 pieces/in² (＞1mm) |
|
Investment Casting |
Ref. ASTM E192 Level 3
Single diam. ≤ 1.5mm (Internal) Surface diam. ≤ 1.0mm |
Small, isolated pores are allowed, but high uniformity is required |
| Sand Casting | Ref. ASTM E446 L2
Single diam. ≤ 2.0mm Area ≤ 2% |
Ref. ASTM E446 L3 Single diam. ≤ 4.0mm |
Three-Step Approach to Developing Customized Casting Porosity Acceptance Standards
Step 1: Functional Risk Assessment
Function: Determine the core function of the casting, such as whether it is intended to withstand fatigue loads, provide static sealing, or serve a decorative purpose.
Failure Consequences: The potential for failure, such as fatigue cracking, pressure leakage, or dimensional overshoot; and the impact of failure, such as safety risks or financial losses.
Step 2: Benchmarking Against Standards
After assessing the risk level, seek reference to existing industry or technical standards.
Industry Standards: Investigate whether applicable industry standards exist, such as the VDA 6.x series for the automotive industry, AMS or MIL standards for the aerospace industry, and API standards for the pump and valve industry.
Technical Standards: Examples include ASTM E446 for internal porosity inspection in steel castings and ASTM E155 for internal porosity inspection in aluminum/magnesium castings.
Internal Standards: Reference the customer’s or supplier’s existing porosity acceptance standards for similar castings.
Step 3: Developing Customized Standards
Referenced Standards: Clearly indicate the referenced standard number, version, and specific level. For example, internal porosity acceptance is based on ASTM E446-2023, Level 2.
Differentiated Developing: Different levels of porosity acceptance standards are established based on the stress conditions and functional requirements of different areas of the casting.
For example, load-bearing surfaces may be subject to ASTM E446 Level 2, while non-critical areas may be subject to Level 4.
This approach is key to achieving cost-effective optimization and ensuring precise quality and budget delivery.
The following figure illustrates the three-step framework for developing casting porosity acceptance standards:
Three-Step Framework for Casting Porosity Acceptance Standards
Conclusion
Scientific porosity acceptance standards rely on a comprehensive consideration of two key dimensions: material properties and casting process.
The core principle is to make economically sound decisions based on an understanding of the part’s function and application.
CEX Casting‘s squeeze casting technology precisely controls the solidification process, eliminating porosity defects at the source.
This provides customers with significant advantages in standard setting and quality control.
Feel free to contact us if you require a porosity standard assessment or process optimization for your project.
Our technical team will provide professional and feasible aluminum die casting & squeeze casting solutions tailored to your needs.