What is the minimum defect size that X-ray inspection can detect?

The minimum defect size detectable by X-ray inspection, specifically computed tomography (CT), is not a single, universal value, but rather a variable dependent on a sophisticated interplay between the component being scanned and the capabilities of the inspection system. For high-quality industrial CT systems used in additive manufacturing quality control, the detectable flaw size typically ranges from 5 to 50 micrometers (µm) under optimal conditions.

Key Factors Influencing Detection Sensitivity

The resolution of an X-ray CT scan is a function of several critical parameters that we meticulously control based on the part's requirements.

1. Geometric Magnification and System Resolution

This is the most fundamental principle. The detector's pixel size sets a theoretical limit, but effective resolution is achieved by magnifying the part's features onto the detector.

• Voxel Size: The fundamental 3D pixel in a CT scan. A smaller voxel size allows for the detection of smaller defects. We achieve this by placing the part close to the X-ray source and far from the detector, magnifying its projection.

• Part Size vs. Detector Capability: Larger parts require a larger field of view, which typically increases the minimum achievable voxel size. For a small, critical feature in a Powder Bed Fusion process, we can achieve a voxel size of 5-10 µm. For a large assembly, it may be 100 µm or more.

2. Material Density and Thickness

The material's ability to absorb X-rays directly impacts contrast and detectability.

• High-Density Materials: Inspecting dense materials, such as tungsten or Copper Alloys, requires higher-energy X-rays, which can sometimes reduce contrast for very small, low-density defects, like voids.

• Low-Density Materials and Thickness: For Aluminum Alloys or Plastics, or thinner wall sections in any material, lower-energy X-rays can be used, providing high contrast that reveals defects at the lower end of the detection range (e.g., 5-15 µm).

3. Defect Type and Contrast

The nature of the defect itself is a major factor.

• High-Contrast Defects: Inclusions of a denser material (e.g., tungsten in an aluminum matrix) are significantly easier to detect and can be found even when they are smaller than the voxel size due to the strong contrast.

• Low-Contrast Defects: Lack-of-fusion pores, micro-cracks, or delaminations have very similar density to the surrounding material. Detecting these, especially cracks with widths below 1 µm, is extremely challenging and often pushes the limits of the technology, requiring ultra-high resolution and sophisticated analysis.

Practical Implications for Quality Assurance

Understanding these variables allows us to tailor the inspection process to validate part integrity for specific applications.

• Aerospace and Medical Validation: For critical Aerospace and Aviation turbine blades or medical and healthcare implants, we configure the CT scan to achieve a voxel size small enough to detect porosity that could initiate fatigue failure, often targeting resolutions better than 30 µm.

• Correlation with Other Data: CT findings are often correlated with mechanical test data from [Witness Coupons](### The Purpose and Importance of Testing Final Parts) to establish a quantitative relationship between defect size/distribution and mechanical performance.

• Process Improvement: By identifying sub-50 µm lack-of-fusion pores, we can provide feedback to optimize Powder Bed Fusion parameters, and if necessary, validate the effectiveness of Hot Isostatic Pressing (HIP) in closing these defects.

In summary, while we can push detection limits to single-digit micrometers for small, critical features, a practical and reliable detection threshold for most engineering-grade metal 3D printed parts is in the 10-30 µm range, contingent on a detailed technical discussion about the specific part geometry and material.