ADVANCED MATERIALS & PROCESSES | MARCH 2025 18 of metal feedstock onto the previously melted layer. Upon build completion, the test coupons, commonly referred to as density cubes, are removed from the printer still on the build plate (Fig. 2), subsequently de-powdered and cut off the plate using a band saw or electrical-discharge machine (EDM). Once removed from the build plate, they are sent to the laboratory for inspection. Some best practices for metal- lographic inspection and observations from a LPBF-focused AM lab follow. Clean at several key points during preparation. After test coupons, or density cubes, are delivered to the laboratory, images are taken upon receipt. Laboratory-grade detergent, like that used to clean fracture surfaces, is put in a container with the cubes in an ultrasonic bath prior to metallurgical sectioning. The detergent removes the EDM fluid, and the ultrasonic cleaning removes partially melted powder from the density cube’s outer surface along with powder that may be contained in surface-connected pores. Surface-connected pores are only seen during early stages of material and parameter development, unless the structure is intentionally porous. However, if left uncleaned, this powder transfers and sullies the sectioning saw and its fluid. Cleaning after sectioning laboratories. To enable these demands labs perform tasks including, but not limited to, powder characterization, mechanical testing, and metallographic inspection. POWDER SAMPLING CONSIDERATIONS Obtaining a representative sample of metal powder feedstock is vital for an accurate snapshot of the powder used during a manufacturing process and for annotating its quality over the course of its use in processes such as LPBF. Different AM processes have different definitions of powder quality, but generally the shape of the powder, its size (and distribution of sizes), and how the powder behaves as a bulk are the universal types of quality indicators. The following best practices focus on the universal but little discussed areas of powder sampling and the considerations for sampling metal powder in accordance with ASTM B 215. Take images of packaging. The first step to sampling a metal powder is unpacking the powder. Taking images upon receipt and of each step during unpacking will assist with resealing the powder after sampling and is also a process record, noting the condition the powder arrived in prior to testing. Bring a variety of tools to open and close powder containers. Three types of containers are commonly used to pack powder: HDPE containers with lobed lids, HDPE jug containers, and metal drums. A variety of tools are necessary to access the powder. The successful tool belt includes a rubber mallet, knife with a partially serrated blade, needle-nose pliers or narrow slip joint pliers, and either aviation snips or heavy-duty wire snips. Additionally, a scale, clean, pre-weighed containers for the powder and a sampling device such as a hollow tube sampler (sampler thief) are necessary. Be aware of unannounced desiccant packets within the powder container. Many metal powders will readily adsorb moisture from the environment and desiccant packets, clay, or silica gel are packaged with the powder to mitigate this. As shown in Fig. 1, these packets could be directly on the powder, or simply in the container with the powder. However, it is not common practice to announce on the packaging or documentation that desiccant packets are present. A punctured desiccant packet can result if the person sampling is unaware of a desiccant packet and uses a pointed sampling tool at different depths as recommended in ASTM B 215. While the powder is recover- able by using a sieve to remove the desiccant particles, time spent retrieving the damaged packet and spilled contents slows the process considerably. If the desiccant particles are similar in size to the powder removal becomes much more complicated. The variation in metal powder feedstock packaging containers can require adaptation, patience, and atten- tion to detail by the person sampling to effectively sample the powder without spilling, damaging the packaging or damaging desiccant packets. To stay consistent with ASTM B 215, sampling heights will vary depending on the container as will the amount of powder obtained in each increment. On containers where the opening is the same size as the sampling device, the adaptation must be made that the location of sampling will always be the same, because sampling at the prescribed distance from the center of the container opening is not possible. METALLOGRAPHIC INSPECTION CONSIDERATIONS Metallographic inspection is an essential tool for material and process development for LPBF technology. Improving optimal machine parameters with the requisite material benefits from metallography as the main tool for characterizing the location, shape and extent of porosity with a LPBF part, especially during developmental stages. The path of metal powder feedstock to final part or a test coupon works like this: the powder is fused to the build plate and built up layer-by-layer using laser(s) to melt newly deposited layers Fig. 2 — Test coupons, commonly referred to as density cubes, are removed from the printer still on the build plate.
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