Dimensional verification of metal additively manufactured lattice structures by X-ray computed tomography: Use of a newly developed calibrated artefact to achieve metrological traceability

Abstract

Metal additive manufacturing (AM) technologies can produce complex components with unique and controlled lattice architectures and advantageous strength-to-weight properties, for which there is increasing interest by important industrial sectors, including aerospace, automotive and biomedical. One critical open issue of metal AM products is related to dimensional and geometrical verification, not only because the dimensional accuracy of products is often poor, but also because the typical geometrical complexity of such products can be difficult to evaluate via traditional measuring instruments (e.g. tactile and optical coordinate measuring systems), especially in the case of lattice structures. X-ray computed tomography (CT) can be used as a dimensional metrology tool enabling advanced control methods that are fundamental for improving the quality of complex metal AM products and processes. However, the quantification of uncertainty of CT dimensional measurements – fundamental to effectively improve the AM process based on CT metrology – is a very complex problem due to several error sources having complex nature, the lack of well accepted models of the full measurement process and consequently the current dearth of internationally standardized procedures. This work investigates a methodology for achieving metrological traceability in CT measurements of AM lattice structures. In particular, Ti6Al4V lattice structures produced by laser powder bed fusion were measured using a metrological X-ray CT system. A new reference object was designed, produced and calibrated to be similar to the investigated AM structures and to determine the task-specific uncertainty of CT-based dimensional measurements of lattice struts. In particular, the newly developed reference object enables the implementation of the substitution method, which – although being established for determining the uncertainty of CT dimensional measurements – has never been applied before to AM lattice structures. Two different approaches were investigated for the method implementation. Results and advantages were documented through dimensional verification of lattice structure features with respect to specified tolerances. Finally, limitations of the method and future perspectives are also discussed.