Research Report Investigates Measuring Internal Surfaces Topography Using XCT

Additive manufacturing (AM) technologies have the ability to produce parts containing complex geometries that were previously impossible to manufacture by conventional means. A number of barriers to increased adoption of AM parts exist, however, relating to the difficulties in applying quality assurance principles, such as dimensional and geometric inspection and verification. In particular, when inspecting AM surfaces, conventional optical and contact surface measurement solutions are often incapable of measuring the inaccessible and internal surfaces. Such conditions are common with typical AM geometries, such as hollow parts and lattice structures.

X-ray computed tomography (XCT) has recently become established as a useful tool in holistic measurement of industrial parts, and is steadily being incorporated into the metrological toolbox. Although much work remains in standardisation of the use of XCT for metrology (ISO 10360-11 is still in the draft stages), XCT has begun to show promise for the verification of internal geometries present in AM parts. Although the spatial resolutions typically achievable by XCT have not historically been at the level required to capture the smaller-scale formations of a surface in addition to the overall shape, advanced systems are approaching these resolutions in their best-case measurement scenarios, and so XCT is becoming an viable option for measurement of surface topography. When
considering the fact that AM parts commonly feature complex, internal geometries, the prospect of using XCT for surface topography measurement appeals further, as a method of overcoming the access requirement problems intrinsic to contact and optical measurements. The use of XCT for surface topography measurement is highlighted in a number of recent studies.

Much work exists in the validation of XCT for internal topography measurement. However, to date and to the  knowledge of report authors’, no research effort had been specifically dedicated to investigating the challenges of measuring topography of internal surfaces. To address this research need, an investigation comparing internal XCT surface measurements and measurements was made using conventional optical surface technologies and presented in the recently published research paper by the UK’s Manufacturing Metrology Team, Faculty of Engineering, at the University of Nottingham.

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To demonstrate the feasibility and potential of XCT for topography measurement, topography datasets were obtained using two XCT systems and compared to those acquired using coherence scanning interferometry and focus variation microscopy. A hollow Ti6Al4V part produced by laser powder bed fusion was used as a measurement artefact. The artefact comprised two component halves that could be separated to expose the internal surfaces. Measured surface datasets were accurately aligned and similarly cropped, and compared by various qualitative and quantitative means, including the computation of ISO 25178-2 areal surface texture parameters, commonly used in part quality assessment. Results showed that XCT can non-destructively provide surface information comparable with more conventional surface measurement technologies, thus representing a viable alternative to more conventional measurement, particularly appealing for hard-to-reach and internal surfaces.

The full report can be downloaded here

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