Polymer powder bed fusion surface texture measurement

Polymer laser powder bed fusion (LPBF) surfaces can be challenging to measure. These surfaces comprise complex features including undercuts, deep recesses, step-like transitions, a large range of measurement scales and unfavourable optically materials properties. While recent research has begun to examine the nature of these surfaces, there has not yet been significant effort in understanding how different measurement instruments interact with them. In this paper, we compare the results of LPBF surface topography measurements using a series of different instrument technologies, including contact stylus, focus variation microscopy, coherence scanning interferometry, laser scanning confocal microscopy and X-ray computed tomography. Measurements are made on both side and top surfaces of a cubic polyamide-12 LPBF sample. Different instrument behaviours are highlighted through qualitative visual inspection of surface reconstructions. Further comparisons are then performed through evaluation of profile and areal surface texture parameters and statistical modelling of surface topographies. These analyses allow for the identification both of discrepancies between texture parameters and discrepancies between local topographies reconstructed from measurements. Instrument repeatability metrics are also presented for each measurement of the test surfaces. Results show that discrepancies in measurements made on the acquired datasets are often similar in magnitude to the size of the features present on the surfaces. Conclusions are drawn regarding the suitability of various surface measurement instruments for polymer LPBF surfaces

Shape defect analysis from volumetric data - Application to lattice struts in additive manufacturing

International audienceAdditive manufacturing (AM) revolutionises the way parts are produced as it offers a variety of design freedom. Lattice structures are an illustration of this AM freedom, allowing for the production of complex geometries that are being investigated in many applications. However, lattice structures present different typology of defects such as surface quality, porosity or dimensional inaccuracies. The most adapted measurement technology to reveal AM internal defects falls into X-ray computed tomography (XCT). Although there have been significantefforts in modelling lattice structure defects from XCT, a direct and accurate link between volumetric data of the part being measured and the CAD model is still required. This direct link would have the noteworthy advantage of not involving XCT surface determination tool, which choice may be discussed. In this paper, shape defects from metal laser powder bed fusion (PBF) strut-based lattice structures are studied. Different struts are printed as representative of BCCz lattice cells. Struts are successively measured by XCT and focus variation (FV). A virtualvolume correlation (V2C) method is presented where shape defect contained in XCT volumetric data is successively approached by modal decomposition relying on a generated defect basis. The modal decomposition approach is firstly validated by comparing its efficiency towards least square cylinder approximation. Then, correlation intrinsic parameters are found, by conducting 2-dimensional sensitivity studies to identify optimal V2C parameters. V2C is further applied to the entire XCT measurements for each considered strut. Comparisons between correlated envelopes and registered FV and XCT measurements are performed to numerically estimate RMS errors. Results show that RMS errors between correlated envelopes and registered measurements are in the same order as the XCT resolution. Conclusions can then be drawn regarding the ability of V2C to estimate lattice strut shape defect relying on an user-defined shape defect basis