ADVANCED MATERIALS & PROCESSES | MARCH 2025 19 the cubes in half is a good idea, especially if the sample is expected to be noticeably porous. If the samples belong to material or parameter development projects, or applications with intentional internal porosity, there could be unmelted powder in pores in the new plane revealed by sectioning. Figure 3 shows the sectioned density cubes in a container with the detergent and the powder removed during ultrasonic cleaning. Polishing cloths and time are conserved if steps are taken early and often to minimize the number of errant particles of unknown size. The particles of unknown size released from edges or internal pores will complicate both grinding and polishing stages of preparation, and even with the best cleaning regimen, some scratches are inevitable in density cubes with sizeable porosity. The cleaning steps also help keep density cubes of materials further along in the technology readiness level scale from having scratches that complicate imaging and density analysis process. Figure 3 shows the process of cleaning density cubes, sectioned density cubes, and the polished cross- section showing porosity encountered during a development project. Understand the nature of defects. After polishing, the density cubes are inspected in a plane perpendicular to the build direction for porosity in one or more planes of polish, in the as- polished condition. This inspection reveals the frequency and distribution of specimen defects (pores). The shape, size, orientation, and location of these phases, or grain structure expected in each state. An adoption of terminology reflecting the purpose and/or state of the defect inspection such as “as-polished density analysis,” or “optical density analysis,” implies the purpose of the analysis is the porosity and its corresponding density. Etched metallographic inspection occurs on additive samples, but many quality assurance tests focus on the quantitative as-polished density analysis. Efforts are being made to incorporate the analysis of additively manufactured samples into industry standards such as ASTM E 3: Standard Guide for Preparation of Metallographic Specimens and this may be an area on which it provides guidance. Consider innovative fixturing and measurement. Many different sized and shaped samples are submitted to an additive-focused laboratory. Other analyses besides the standard is essential to understand the effect of machine parameters, which in turn guide the development process. If trying to calibrate a new layer thickness for an existing material, differences may exist in the types of porosity seen as the laser power and scan speed, or hatch distance changes. This holds for nearly any manipulation of the combination of material, machine parameters, or deposition rate. Watch nomenclature: as-polished structure versus etched microstructure. One area where terminology contradicts with previously established metallographic terminology is in the use of the word “microstructure” (in additive manufacturing) to refer to the as-polished structure where pores within a given sample are visible. In metallographic inspection of conventional manufactured specimens, the term “microstructure” will be qualified with “as-polished” or “etched” to distinguish between the types of inclusions, Fig. 3 — Left to right: Sectioned density cubes during ultrasonic cleaning; powder released from internal porosity; final polished density cubes showing a range of densities from a developmental design of experiments. Fig. 4 — Left: As-printed LPBF part, a typically difficult-to-print geometry made successful by build control software. Right: Metallographic cross-section of the difficult geometry, used for optical density analysis and qualitative assessment of the build software’s efficacy.
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