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ASTM F3637-23

Standard Guide for Additive Manufacturing of Metal — Finished Part Properties — Methods for Relative Density Measurement
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1.1 In this standard, guidelines for measuring post-manufacturing relative density of metallic additive manufactured (AM) parts and density assessment test specimens are given.

1.2 In this guide, standard test methods commonly used to measure part relative density and details any procedural changes or recommendations for use with PBF-LB parts are referenced. Extensibility to other types of metallic AM processes may be considered on a case-by-case basis with user discretion.

1.3 This guide is intended to be applied during the selection process of methods to measure the relative density of AM parts to balance cost, accuracy, complexity, part destruction, and part size concerns.

1.4 Pore size, shape, and distribution and their implications relative to the AM process and material are beyond the scope of this guide; however, each method’s ability to obtain these metrics is discussed in the context of the various density measurement methods.

1.5 Units—The values stated in SI units are to be regarded as the standard. No other units of measurement are included in this standard.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

1.1 In this standard, guidelines for measuring post-manufacturing relative density of metallic additive manufactured (AM) parts and density assessment test specimens are given.

1.2 In this guide, standard test methods commonly used to measure part relative density and details any procedural changes or recommendations for use with PBF-LB parts are referenced. Extensibility to other types of metallic AM processes may be considered on a case-by-case basis with user discretion.

1.3 This guide is intended to be applied during the selection process of methods to measure the relative density of AM parts to balance cost, accuracy, complexity, part destruction, and part size concerns.

1.4 Pore size, shape, and distribution and their implications relative to the AM process and material are beyond the scope of this guide; however, each method’s ability to obtain these metrics is discussed in the context of the various density measurement methods.

1.5 Units—The values stated in SI units are to be regarded as the standard. No other units of measurement are included in this standard.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

5.1 General:

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.

5.1.2.1 When a flaw reaches a size, shape, location, or criticality that makes it becomes unacceptable for part acceptance, it will be referred to as a defect.

5.1.2.2 Flaw or defect formation is governed by the manufacturing process, build parameters, feedstock, and geometric factors. Therefore, accurate measurement of fabricated part relative density is an important initial step in determining part and process quality.

5.1.2.3 The quantity, size, and shape of the volumetric flaws influences mechanical performance of a part, particularly under cyclic loading. These data could indicate irregularly shaped (for example, LOF pores or microcracking) or spherical porosity (for example, keyhole or entrapped gas porosity) and determine acceptability by assigning criteria. While these metrics can be quantified, in this guide, the general capabilities of each method to capture this data will be highlighted, but detailed recommendations on these data types will not be made and rather the focus will be on relative density measurements.

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 Method Selection:

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).

5.2.5.1 For methods relying on comparing the measured and theoretical material densities to calculate the relative density of the specimen, the following formula should be used:

Equation F3637-23_1

5.3 Method Specific Recommendations:

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
SDO ASTM: ASTM International
Document Number F3637
Publication Date May 15, 2023
Language en - English
Page Count 6
Revision Level 23
Supercedes
Committee F42.01

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