The continuous increase in complexity and size of 3D printed parts requires a careful analysis of the methodologies used to verify their integrity. Among these, special importance is covered by X-ray inspections. Traditionally, without destructive testing, full metrology has only been performed on the exterior dimensions of components, such as with a coordinate-measuring machine (CMM) or with a 3D vision system to map exterior surfaces.
In many cases, x-ray inspection, one of the oldest and most widely used non-destructive testing method, is required to check the quality of 3D printed parts, generating a 2D or 3D internal and external representation of the scanned object.
Industrial x-ray digital scanning is used for flaw detection, failure analysis, metrology, assembly analysis and reverse engineering applications. Displaying information in 2D or 3D without destroying the part, it’s used to detect defects such as porosities, inclusions or cracks.
2D and 3D x-ray scannings have very different complexity, times and costs, as well as offering different results. So let’s try to give some details on how and when it is convenient to use 2D or 3D.
2D X-ray inspection
2D digital X-ray imaging is used in many applications and industries. The technology is slowly replacing conventional film radiography usage. 2D digital radiography produces real-time 2D grayscale images that allow us to identify objects:
- quality level of internal features
X-ray radiation is generated by an X-ray source and after passing through the object, is captured by a digital detector that translates it into an electronic signal and grayscale values. The variation in grayscale values inside the 2D image illustrates the variations in density (or thickness) of the part.
One of the main advantages of 2D radiography is the presence of several standard practices (ASTM E 1742, ASTM E 2033, UNI EN ISO 5579, UNI EN ISO 16371-2) which clearly define the quality requirements of radiography and allow the test to be performed under “controlled” conditions.
The limitation of 2D digital X-ray imaging is its inability to provide precise information such as the depth and location of the inside features of an object. It is impossible to locate any material defect along the Z-axis of the inspected volume (i.e. determine its depth) unless subsequent views from different angles are taken.
Another limitation could arise due to the complex geometry of the components. In case of several features with different thicknesses or section changes, dedicated radiographic views are required to cover the 100% volume of the part.
The solution to this issue is to use 3D digital X-ray imaging, even known as Computed Tomography (CT).
3D X-ray inspection (CT – Computed Tomography)
3D CT is based on the ability of scanners to acquire 2D X-ray images of an object at different rotation steps. Basically, the object rotates while being irradiated by radiation, then the computer merges all the images to recreate the entire volume of the inspected part and synthesises a stack of virtual cross-section slices through the object using a reconstruction algorithm. Scrolling through cross-sections and visualising internal structures allows to inspect it from different planes, precisely locating discontinuity along all three axes.
Example of 3D Computed Tomography on a water pump in AlSi10Mg
3D computed tomography is really useful to inspect complex geometries, especially for geometric dimensioning and tolerancing analysis. Dimensional data of the sample can be obtained by performing CAD comparison or wall thickness analysis, thus evaluating quotes with respect to the drawing and checking for unexpected variations of material thicknesses along the whole volume.
Another application for 3D CT is for assembly or visual analysis. CT scanning provides views inside components in their functioning position, without disassembly. Some software programs for industrial CT scanning allow for measurements to be taken from the CT dataset volume rendering. These measurements are useful for determining the clearances between assembled parts or the dimension of an individual feature.
The main drawbacks of this technique are that, even if it’s gaining more and more attention and its usage in industrial application is growing, standard rules specifying the quality requirements of a CT inspection are not available yet. Also, with respect to 2D radiography, CT is more time consuming, due to the complexity of the analysis being performed and the data that can be achieved at the end of the inspection.
Which is the best inspection?
It is not possible to define whether a technique is better than the other or vice-versa. It is widely believed that 2D radiography and 3D CT exclude each other. By contrast, the two techniques are often used as complementary techniques, depending on the final goal of the inspection.
The technique should be chosen according to the information that is needed to achieve from the component. In a practical way, for First Article Inspection (FAI) of components, a 3D Computed Tomography is preferable due to the wide range of information that can be achieved:
- Location of discontinuity along X, Y & Z axis
- Possibility to perform CAD Comparison and dimensional measurements
- Capability to inspect 100% of the volume
On the other hand, if on a sample, even with complex geometry, critical areas are clearly defined, and only that critical areas are required to be inspected, a 2D radiography is still one of the most reliable techniques of analysis.