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5.1.1 This guide is intended to support PBF-LB process and parameter development, part acceptance criteria, and process control tests.

5.1.2 Flaws and Defects—Fabricating fully dense parts continues to be a challenge in AM as the process intrinsically introduces volumetric flaws into a part reducing the part relative density (that is, increasing porosity or the presence of small voids in a part making it less than fully dense) and mechanical performance.

5.1.3 Uncertainty and Error—Users should consider that each measurement technique considered in this guide has differing sensitivities to various sized features. The measurement methods will also have different potential systematic errors or measurement uncertainties due to sampling sizes, detection resolution, effect of surface condition, experimental set-up, or reliance on a theoretical material density. It is important that these effects are taken into consideration as well as the natural statistical variability in the measurements. Multiple measurements of nominally identical test specimens should be made to enable the quantification of statistical uncertainty. Systematic uncertainty contributions will not be reduced by greater numbers of repeated measurements. When measuring specimens with relative densities close to 100 % quantification of systematic uncertainty for the selected measurement technique(s) becomes more critical to separate measurement and systematic variation from variation driven by the AM process. Differing levels of rigor can be applied when determining the role of uncertainty and variation depending on whether the measurement is in support of process development (for example, identifying appropriate fabrication parameters) or part acceptance (for example, part qualification).

5.1.4 Repeatability and Reproducibility—As uncertainty and error can be introduced into the measurement process through operator variation. Performing gage repeatability and reproducibility (Gage R&R), a process that determines a test method’s repeatability and reproducibility, is recommended for methods that rely on significant manual specimen preparation or operation such as Archimedes, pycnometry, ultrasonic, and metallography. Refer to Guide E2782 for guidance on performing this process evaluation.

5.2.1 When evaluating methods, it may be beneficial to understand how the various attributes compare from method to method. In Fig. 1, a summary matrix comparing these various methods and their qualities is given.

FIG. 1 Comparison Matrix of the Test Methods Evaluated in This Guide

5.2.2 Using Multiple Methods—It can be desirable to use multiple methods to determine relative density. For example, using low-resolution XCT to measure larger part flaws and metallography to identify the quantity of smaller process flaws could prove to be a highly useful way of producing accurate flaw data. Another approach to strengthen measurement accuracy is by implementing multiple methods that operate on similar principles, such as pycnometry and Archimedes.

5.2.3 Non-destructive Methods—Archimedes, ultrasonic, pycnometry, and XCT are nondestructive methods, while metallographic methods require part destruction to get relative density measurements. All the nondestructive methods can be used to characterize part relative density; however, as part size increases, these methods can become cumbersome to use. Archimedes requires a much larger and dedicated setup for relative density calculation that can be expensive for the appropriate accuracy but remains the least cost-intensive option, XCT and ultrasonic results are highly geometry and size dependent, and many pycnometry devices cannot handle larger part volumes (many pycnometers are equipped to handle specimen volumes of 1 cm³ to 3.5 cm³, however there are some that can handle up to 10 cm³). While several of these methods may not be suitable for characterizing larger part volumes, all can provide relative density. Low-cost and quick measurement methods, such as Archimedes, can be used as a means of process development or data for statistical process control during production.

5.2.4 Pore Morphology Data—Metallographic and XCT methods can provide relative density measurements and specific geometric details (that is, size, aspect ratio, and shape) of individual flaws in addition to the overall part relative density. However, metallographic and XCT measurements are highly dependent on the resolution of the data, whether that is the sections examined, quantity of images, or microscope resolution, or a combination thereof, for metallographic methods or voxel size used for XCT. Archimedes, ultrasonic, and pycnometry methods do not provide these types of data when measuring relative density.

5.2.5 Relative Density Measurements Relying on Theoretical Material Density—Archimedes, ultrasonic, and gas pycnometry methods rely on theoretical material density values in the calculation of relative density. The theoretical material density value selected is a possible source of systematic error. Material density is composition dependent. Each material will have a compositional specification and an allowable variation of that composition. This combined with material vaporization during fabrication could lead to a different material density value than the reported value by a material vendor or online source. The user should use caution on the reliance of a reported value and ensure the theoretical density is representative of the material (that is, from the specific material lot, measured from final material, or from a reliable database such).

5.3.1 Archimedes Method—The Archimedes method is highly cost effective, nondestructive, and relatively non-geometry dependent; however, a significant amount of variation can be introduced into the process from the operator, part size, surface finish of the part, fluid entrapment, evaporation of fluid, temperature, water purity, absorbed gases, surface pores or cracks, and bubbles. Uncertainties of approximately 0.1 % for relative density measurements can be achieved for fully dense materials using this method. The sources of variation combined with part size will increase this uncertainty. However, training and consistent practices can minimize the effects of variation between measurements. Additionally, there are two main ASTM International standards for Archimedes measurements, Test Methods B962 and B311. Test Method B311 is specifically designed for measuring material density of parts with less than 2 % porosity volume and is, therefore, recommended as the measurement method for PBF-LB parts. The major difference between the two methods is that Test Methods B962 require fluid impregnation to deal with surface-connected porosity and Test Method B311 does not. If a specimen increases in mass while submerged in water, use Test Methods B962, and if the specimen does not gain mass, then Test Method B311 is applicable. Agitating the PBF-LB specimens while submerged is recommended to reduce any air pockets that may exist on the part’s surface. Additionally, a benefit of AM is the ability to achieve high complexity—a potential source of error using this method would be internal channels or the ability for the liquid to cover the entire volume. It is recommended to take multiple measurements when using this method and compute a standard deviation.

5.3.2 Gas Pycnometry Method—Gas pycnometry requires that specimens be free of contaminants that may outgas during the test, shall not react with the displacing gas, and shall have sufficient strength to avoid deformation in the pressurized gas environment. Additionally, this method should only be used to measure parts with high relative densities since this method uses a gas to determine volume. Specimens with surface porosity or interconnected pore structures (whether through process defect or by design) will measure the skeletal volume, resulting in an inaccurate relative density measurement. This method functions on similar principles to that of Archimedes; however, it does not possess as many potential sources of error related to using a liquid for volume displacement. Uncertainty in this method is a function of part size and equipment capacity. There are several equations to calculate uncertainty from the equipment manufacturer; however, it will be equipment and part specific. Note that pycnometry determines a volume that can be compared directly to theoretical part volume based upon CAD dimensions of the part being produced. Differences can point directly to the volume of closed porosity in the produced part. Test Method B923 is used for measurement of skeletal or material density. While this test method is primarily for determination of skeletal density of metal powders, it has also been found to be useful for determination of skeletal volume and density of parts produced by traditional powder metallurgy methods. Note that it is best to try to choose a pycnometer capacity and specimen container configuration that result in the specimen under test occupying as much of the specimen container volume as possible. It can occur that a produced part is too large for any commercial gas pycnometers, and if so, another listed method shall be selected. Multiple measurements should be taken when using this method and compute a standard deviation.

5.3.3 Ultrasonic Method—Ultrasonic relative density characterization is limited in application by part geometry and suffers errors induced by small part sizes and surface roughness inherent in AM parts. Larger specimen parts with simple geometries (for example, cubes) and polished surfaces to measure from should be used for data capture. Note that this method is typically used as an NDT method and not for relative density measurement. Changes in measured velocity can be indicative of cracking or flaws within parts. These data received through ultrasonic testing can be evaluated into several equations provided in Practice E494 to calculate material density. This can then be compared to theoretical material density to compute a relative density measurement. However, several material constants such as Poisson’s ratio or Young’s modulus are required. To determine accurate values, additional testing is required, which can be cumbersome. Otherwise, vendor or online sources may need to be used to estimate these values, which may not be representative of the true values or include uncertainty considerations. Because of these factors combined with measurement variability inherent to measuring as-built AM parts, this is not a preferred method for measuring relative density of PBF-LB parts.

5.3.4 XCT Method—XCT provides highly descriptive data, such as pore size, shape, and distribution. However, it does require costly equipment and is time intensive. High-resolution (voxel size of ~1 µm to 5 µm) analysis is obtainable using XCT; however, there is a limitation on specimen size, requiring smaller parts, longer scanning times, and often more cost. Low resolution XCT can evaluate larger parts but is unable to detect fine details or smaller flaws. Successful parameters and software processing steps should be recorded to ensure repeatability. Additionally, the software tools used to filter noise and the grayscale thresholding can lead to uncertainty and variation as some pores can be missed in the analysis. Significant iterating may be required to find the proper scanning and processing parameters and operators should be trained. Resolution is governed by voxel size in this method; however, uncertainty can arise from this as well. Smaller gas pores can be unaccounted for in relative density computation if a larger voxel size is selected. Additionally, the thresholding criteria selected for identifying pores are another source of error.

5.3.5 Metallography and Serial Sectioning Method—Metallurgical examination of porosity requires mechanical sectioning of the part or test specimen and an apparatus such as described in Practice E1245, a light microscope equipped with brightfield objectives and digital imaging. Image analysis software produces a binary image and counts black pixels against a white background or vice versa. This is a relatively low-cost relative density measurement technique. However, as it is a destructive method, it is, therefore, more applicable to test specimens than actual parts in a production setting. If sectioning a production part, examine critical design locations and areas prone to porosity. The microscopic images from which relative density data are derived are 2D and require mathematical relationships of stereology for planar surfaces to determine uncertainty. Multiple images of the full specimen area in several orientations and at multiple polishing depths should be taken when using this method. A significant amount of uncertainty exists within this method from systematic and sampling sources. Critical pores can be missed as the whole specimen is not measured. Another method, such as pycnometry, should be used to generate reliable relative density measurements of a specimen. Automated serial sectioning and imaging allows for expansive data collection that captures a significant number of micrographs from a specimen using metallurgical methods. However, the capital equipment involved and time to generate a large quantity of images makes this method costly. Serial sectioning images can be stitched in the polishing direction to render high resolution 3D data of the specimen. Serial sectioning can be used to reduce the uncertainty of the metallographic method by taking a large quantity of image data. Special care should be taken during the specimen preparation procedure to ensure minimal residual scratching.