Evaluation of the influence of post-processing on the optical inspection accuracy of additively manufactured parts

[EN] Optical measurement systems are important techniques for rapid inspecting additively manufactured parts by techniques such as selective laser melting (SLM). Depending on their application, SLM parts require post-processes such as sandblasting or heat treatment, commonly applied in order to improve their surface finish or mechanical properties, respectively. These post-processes modify the parts surface characteristics, and therefore the suitability for optical inspection. This work evaluates the influence of these SLM post-processes on optical inspection. For this, a test part, manufactured in 17-4PH stainless steel using a 3DSystems ProX100 machine, was optically measured using a structured light scanner and compared to the values obtained from contact measurements (reference values). Both optical and contact measurements were performed under three conditions: as-built, post sandblasting, and post sandblasting and subsequent heat treatment. The analysis results show that applying the sandblasting postprocessing provides a surface finish to the SLM parts suitable for optical inspection. This postprocess allows precise inspection of this type of parts, reaching values close to those obtained by contact. Likewise, it is concluded that the used structured blue-light scanner is suitable for inspecting SLM parts.SIMinisterio de Ciencia, Innovación y UniversidadesInstituto Universitario de Tecnología Industrial (SV-18-1-GIJON-1-06 and SV-19-GIJON-1-14

Influence of 17-4 PH stainless steel powder recycling on properties of SLM additive manufactured parts

[EN] Metal Additive Manufacturing (AM) processes are developing quickly. These processes have several attractive qualities, however, the quality of manufactured parts still remains a major issue that needs to be addressed if it is to become a prevalent technology in the industry. In some powder bed fusion techniques, such as Selective Laser Melting (SLM), there is a portion of initial powder that does not melt and it can be recycled to ensure the economic and environmental viability of the process. In previous research, we demonstrated the morphological, chemical and microstructural change suffered by 17-4 PH stainless steel powder after reusing it in a SLM manufacturing process. In this work, the properties of 17-4 PH stainless steel parts, printed from powder in different recycling states (virgin powder (P0) and 20 times reused powder (P20)), were evaluated, in order to establish good recycling procedures and optimise the SLM process performance. Analyses of the properties revealed a slight decrease in roughness and pore size with powder recycling. The external porosity of the samples is similar in both powder states; however, internal porosity decreases by increasing the number of reuse cycles. Regarding the microstructural analysis, a slight increase in the γ-phase is observed with the powder recycling, which leads to a slight increase in ductility and decrease in hardness of the samples. Therefore, it is concluded that the 17-4 PH powder recycling process in SLM manufacturing is adequate and recommended to ensure the economic and environmental viability of the process without adversely affecting the properties of the parts.S

New procedure for qualification of structured light 3D scanners using an optical feature-based gauge

[EN] This work evaluates the performance and operative limits to the dimensional accuracy of 3D optical scanning based on blue-light fringe projection technology. This technology, also known as structured light 3D scanning, is widely used in many reverse engineering applications. It allows the user to quickly capture and create point-clouds, by using images taken at different orientations of white-or blue-light fringe projected patterns on the part. For the survey, a large and feature-based gauge has been used with specific optical properties. The gauge is endowed with canonical geometrical features made of matt white ceramic material. The gauge was calibrated using a coordinate measuring machine (CMM) by contact. Therefore, it is possible to compare the measurements obtained by the structured blue-light sensor with those obtained by the CMM, which are used as reference. In the experimentation, the influence of the scanner software in the measurement results was also analysed. Besides, different tests were carried out for the different fields of view (FOV) of the sensor. The survey offers some practical values and limits to the accuracy obtained in each configuration.SIMinisterio de Economía y Competitividad de EspañaJunta de Castilla y Leó

Influence of the scanning strategy parameters upon the quality of the SLM parts

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Proposal of design rules for improving the accuracy of selective laser melting (SLM) manufacturing using benchmarks parts

[EN] Purpose: Among the different methodologies used for performance control in precision manufacturing, the measurement of metrological test artefacts becomes very important for the characterization, optimization and performance evaluation of additive manufacturing (AM) systems. The purpose of this study is to design and manufacture several benchmark artefacts to evaluate the accuracy of the selective laser melting (SLM) manufacturing process. Design/methodology/approach: Artefacts consist of different primitive features (planes, cylinders and hemispheres) on sloped planes (0°, 15°, 30°, 45°) and stair-shaped and sloped planes (from 0° to 90°, at 5° intervals), manufactured in 17-4PH stainless steel. The artefacts were measured optically by a structured light scanner to verify the geometric dimensioning and tolerancing of SLM manufacturing. Findings: The results provide design recommendations for precision SLM manufacturing of 17-4PH parts. Regarding geometrical accuracy, it is recommended to avoid surfaces with 45° negative slopes or higher. On the other hand, the material shrinkage effect can be compensated by resizing features according to X and Y direction. Originality/value: No previous work has been found that evaluates accuracy when printing inwards (pockets) and outwards (pads) geometries at different manufacturing angles using SLM. The proposed artefacts can be used to determine the manufacturing accuracy of different AM systems by resizing to fit the build envelope of the system to evaluate. Analysis of manufactured benchmark artefacts allows to determine rules for the most suitable design of the desired parts.SIThe authors gratefully acknowledge the financial support provided by the Junta de Castilla y León and FEDER (project ref. LE027P17)

Impact of the laser scanning strategy on the quality of 17-4PH stainless steel parts manufactured by selective laser melting

[EN] A significant correlation between scanning strategies and the quality of parts manufactured additively with Selective Laser Melting (SLM) technology is shown. Therefore, a in deep study of the influence of scanning strategy is of great value for the manufacturing process in order to promote SLM technology in applications with high service requirements. In particular, this research is carried out on 17-4PH stainless steel parts, which is an alloy widely used in sectors such as aerospace or automotive for its excellent mechanical properties. This research proposes to evaluate the properties of 17-4PH parts manufactured using three usual scanning strategies (normal, concentric and hexagonal) in order to optimize the SLM process depending on the final part application. According to the obtained results, the following general conclusions have been drawn. Hexagonal strategy can be considered the most appropriate in terms of porosity. Both hexagonal and normal strategy have good mechanical properties, as well as geometrical and dimensional quality. Regarding surface finish of top face (where the used scanning pattern is visible), normal strategy is the most appropriate. In general, concentric strategy presents different results from the others: larger and irregular pores, ductile tensile behaviour, low roughness on lateral faces, and high geometrical errors in samples with large scanning vectors. As a future work, it is proposed to manufacture SLM parts combining these strategies in order to improve their properties. In addition, it is proposed to evaluate the influence of different post-process operations on the quality of parts printed by SLM using different strategies.S

Analysis of Modern Optical Inspection Systems for Parts Manufactured by Selective Laser Melting

[EN] Metal additive manufacturing (AM) allows obtaining functional parts with the possibility of optimizing them topologically without affecting system performance. This is of great interest for sectors such as aerospace, automotive, and medical–surgical. However, from a metrological point of view, the high requirements applied in these sectors constitute a challenge for inspecting these types of parts. Non-contact inspection has gained great relevance due to the rapid verification of AM parts. Optical measurement systems (OMSs) are being increasingly adopted for geometric dimensioning and tolerancing (GD&T) verification within the context of Industry 4.0. In this paper, the suitability (advantages and limitations) of five different OMSs (based on laser triangulation, conoscopic holography, and structured light techniques) for GD&T verification of parts manufactured by selective laser melting (SLM) is analyzed. For this purpose, a specific testing part was designed and SLM-manufactured in 17-4PH stainless steel. Once the part was measured by contact (obtaining the reference GD&T values), it was optically measured. The scanning results allow comparing the OMSs in terms of their inspection speed as well as dimensional and geometrical accuracy. As a result, two portable systems (handheld laser triangulation and structured blue-light scanners) were identified as the most accurate optical techniques for scanning SLM parts.S

Laser Defocusing Effect on the Microstructure and Defects of 17-4PH Parts Additively Manufactured by SLM at a Low Energy Input

[EN] In this paper, the microstructure, defects, hardness, and tensile strength of the 17-4PH specimens manufactured additively using the selective laser melting (SLM) technique were studied. The analyzed parts (10 mm size cubic specimens and tensile specimens) were manufactured with different defocus parameter values (−1, 0, +1 mm) in order to evaluate this effect with a low power laser (38 W). The study was carried out on three different sections of each cubic specimen, one perpendicular to the laser beam or SLM manufacturing direction (transversal section), and another two parallel to the laser beam direction (longitudinal sections). The specimens microstructures were analyzed with an X-ray diffraction technique, as well as optical, scanning electron, and transmission electron microscopes. Image J software was used to characterize the defects and phase ratio. In addition, hardness and tensile tests were performed according to the corresponding standards. The results show that the amount of austenitic phase and the average grain size varied with defocusing. The percentage of defective area was less than 0.25%. The analyzed defocus distance did not affect the number and average size of the defects. Adjusting the defocusing SLM parameter is important for manufacturing parts with good mechanical properties.S

Comparison of Chemical and Mechanical Surface Treatments on Metallic Precision Spheres for Using as Optical Reference Artifacts

[EN] The improvement of industrial manufacturing processes requires measurement procedures and part inspection tasks to be faster and faster while remaining effective. In this sense, the capabilities of noncontact measuring systems are of great help, not only because of the great amount of data they provide but also for the ease of the integration of these systems as well as their automation, minimising the impact on the industry. This work presents a comparative study on the influence of two surface treatments performed on low-cost, high-precision metallic spheres on the suitability of these spheres to be used as artefacts for the calibration of optical sensors, specifically laser triangulation sensors. The first surface treatment is sandblasting (a mechanical process), whose effect has been studied and presented in previous work. The second treatment focused on in this paper is acid etching (a chemical process). The comparison has been performed by evaluating the same metrological characteristics on two identical groups of spheres of similar type (diameter and accuracy), each of which was subjected to a different treatment. It was necessary to obtain the reference values of the metrological parameters with high accuracy, which involved measuring the spheres with a coordinate measuring machine (CMM) by contact probing. Likewise, spheres were scanned by a laser triangulation sensor mounted on the same CMM. The results derived from both the contact and laser measurements and before and after treating the surfaces were used to compare four parameters: point density, sphere diameter, sphere form deviation, and standard deviation of the best-fit sphere to the corresponding point cloud. This research has revealed that acid etching produces better optical qualities on the surfaces than the mirror-like original ones, thus enhancing the laser sensor capturing ability. However, such chemical etching has affected the metrological characteristics of the spheres to a greater extent than that produced by sandblasting. This difference is due to the variability of the chemical etching, caused by the high aggressiveness of the acid, which makes the process very sensitive to the time of exposure to the acid and the orientations of the spheres in the bath.SIUniversity Institute of Industrial Technology of Asturias, IUTA, through the research project ref. SV-21-GIJON-1-06

Laser line scanner aptitude for the measurement of Selective Laser Melting parts

[EN] When looking for any metrological verification of parts manufactured by metal laser printing with optical equipment, it is necessary to ensure the traceability of the measurements that can be obtained. The difficulty of this process lies in the fact that these measurements are obtained on point clouds captured from surfaces with high form errors and poor surface finishes, even when this type of surface usually undergoes processes to improve the surface finish, such as sandblasting. This research focuses precisely on the analysis of the metrological suitability of a laser line scanner (laser triangulation sensor) on parts manufactured by Selective Laser Melting (SLM). The study starts from the design of a test part specifically oriented to the printing process with SLM metal powder bed. This test part was printed in 17-4PH stainless steel and then sandblasted. The test part was measured in a Coordinate Measuring Machine (CMM), obtaining reference GD&T values. The measurement was carried out under pre-sandblasting ("as built") and post-sandblasting conditions, thus providing interesting information about the erosion rate of this post process. A state-of-the-art laser sensor was employed for the metrological comparison, mounted on the same available CMM that was used for contact measurements. In this research three analyses were carried out: the quality of 3D metal printed parts with respect to CAD model, the effect of the sandblasting post-process, and the accuracy of the measurements obtained with the laser line sensor. In addition, this work conducts an in-depth study about the influence of point cloud treatment and filtering procedures, by comparing the filtering methods applied by different reverse engineering software packages. The study leads to the conclusion that filters based on the standard deviation of the point cloud are the best candidates in order to obtain laser measurements closer to the contact measurements.SIAuthors thank to the financial support provided by the Junta de Castilla y León (project LE027P17-FEDER funds) and also to two student grants awarded by the University Institute of Industrial Technology of Asturias (IUTA, ref. SV-19-GIJON-1-14) and by the Young researcher mobility program of SIF (Manufacturing Engineering Spanish Society)