It can be seen that, when the A205 foundry alloy is chosen as the reference, depending on the experimental conditions, the calculation leads to relative densities higher than 100%, which is inconsistent. Results on the change in the relative density of the samples as a function of the volume energy density ψ applied when they were produced are given for the two reference densities in Table 5. As with the first alloy studied, we calculated this relative density in two ways: (i) by selecting the density of the A205 foundry alloy as the reference density, and (ii) by taking the density of the AM205 powders as the reference. The relative density of the SLM samples was determined using the Archimedes method, according to the same protocol as for the AlSi7Mg0.6 alloy. The true density of the AM205 powder that we measured by pycnometry was 2.88, which represents a considerable deviation from the density of the foundry alloy that corresponds to the grade studied, i.e., that of the A205: 2.86 g.cm −3. The measured value is therefore not an intrinsic value of the material that makes up the powder and cannot therefore be selected as a reference. However, the value obtained (2.66) is characteristic of the powder, while the particles of this powder have very clear porosities, as demonstrated by micrographic observation ( Figure 8). In this last case, density ρ theoretical is the density of the powders, which is determined by helium pycnometry. We might then consider taking the powder material as the reference, as it has the same composition. In the case of the AlSi7Mg0.6 alloy, we can assume that the reference is the foundry alloy and hence that ρ theoretical = 2.68 however, we note that the composition of the foundry alloy and that of the powders are not perfectly identical ( Table 1). Yet this is an important question because the value of the theoretical density has a strong influence on relative density. In the literature, even in very recent studies, this choice is never discussed. This then raises the question of the choice of reference. The ratio of this extracted area to the total area of the image gives the porosity rate of the sample, from which the relative density of the sample can be determined. By adjusting the threshold level, it is possible to extract the area specific to the porosities, within the resolution limit of the device. The limit of separation is defined by the operator. Depending on the gray level defined above, each pixel is converted into either black or white. The next stage, thresholding, consists in transforming the digital image into a binary image. The information captured in each pixel is coded into a specific level of gray, with the user able to adjust the scaling. This digital image is made up of pixels containing specific information such as light intensity or color, which will then be converted into a grayscale. For each polished section observed, the “real” image is transformed into a digital image via a camera. This method takes a polished surface and determines the ratio of the surface area that corresponds to porosities to the total surface area observed. Analyzing micrographs is a destructive process used to characterize porosities both morphologically and quantitatively. One method used to determine the relative density of a sample was image analysis. This article shows that it is essential that a result of relative density obtained from Archimedes measurements be supplemented by an indication of the reference density used. It can be observed, for example, that, depending on the experimental conditions, the calculation can lead to relative densities higher than 100%, which is inconsistent. In addition, the results show that the Archimedes method has limitations, particularly related to the choice of reference materials for calculating relative density. The study concludes that an analysis of the metallographic images to calculate the relative density of the part depends on the areas chosen for the analysis. To investigate this, two different grades of aluminum alloy, AlSi7Mg0.6 and AM205, were used in this study. To achieve this aim, two experimental methods are used: the image analysis method, which provides local information on the distribution of porosity, and the Archimedes method, which provides access to global information. This article deals with the limitation of the relative density results to conclude on the quality of a part manufactured by additive manufacturing and focuses on the interpretation of the relative density result. Micrographic image analysis, tomography and the Archimedes method are commonly used to analyze the porosity of Selective Laser Melting (SLM)-produced parts and then to estimate the relative density.
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