Laboratory X-ray tomography for metal additive manufacturing: Round robin test

Abstract

This paper reports on the results of a round robin test conducted by ten X-ray micro computed tomography (micro-CT) laboratories with the same three selected titanium alloy (Ti6Al4V) laser powder bed fusion (L-PBF) test parts. These parts were a 10-mm cube, a 60-mm long and 40-mm high complex-shaped bracket, and a 15-mm diameter rod. Previously developed protocols for micro-CT analysis of these parts were provided to all participants, including suggested scanning parameters and image analysis steps. No further information on the samples were provided, and they were selected from a variety of parts from a previous different type of round robin study where various L-PBF laboratories provided identical parts for micro-CT analysis at one laboratory. In this new micro-CT round robin test which involves various micro-CT laboratories, parts from the previous work were selected such that each part had a different characteristic flaw type, and all laboratories involved in the study analyzed the same set of parts. The 10-mm cube contained subsurface pores just under its top surface (relative to build direction), and all participants could positively identify this. The complex bracket had contour pores around its outer vertical sides, and was warped with two arms deflected towards one another. Both of these features were positively identified by all participants. The 15-mm diameter rod had a layered stop/start flaw, which was also positively identified by all participants. Differences were found among participants for quantitative evaluations, ranging from no quantitative measurement made, to under and overestimation of the values in all analyses attempted. This round robin provides the opportunity to highlight typical causes of errors in micro-CT scanning and image analysis as applied to additively manufactured parts. Some workflow variations, sources of error and ways to increase the reproducibility of such analysis workflows are discussed. The ultimate aim of this work is to advance the efficient use of micro-CT facilities for process optimization and quality inspections for additively manufactured products. The results provide confidence in the use of laboratory micro-CT but also indicate the need for further development of standards, protocols and image analysis workflows for quantitative assessment, especially for direct and quantitative comparisons between different laboratories.