July_August_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 1 9 2 2 A mong the various additive manu- facturing (AM) processes either in use or under development, e.g., metal powder bed fusion (PBF) and ce- ramic binder jetting, several employ powder feedstocks. As a result of manu- facturing artifacts and inter-particle forc- es, powder materials exhibit properties that depart from those of bulk materi- als in other forms. Manufacturing condi- tions can affect all aspects of individual particle properties, which in turn affect attractive particle interactions such as adhesion and non-bonded van der Waals forces [1,2] . Differences in these properties manifest themselves in the dynamics of powder flow and subsequent particle fu- sion, which must be optimal to achieve fully densified parts. In some cases, conventional pow- der characterization techniques pro- vide useful information when adapting powders to AM processes. Yet there are many cases where the techniques fail to adequately distinguish enough in- formation to enable process optimiza- tion [2,3] . The most common example is the use of static flow characterization methods (such as Hall or Carney fun- nel flowrate tests) to quantify powder flowability or spreadability. Additive manufacturers have found that these methods often provide little or no use- ful information to explain observed differences in flow behavior between virgin and recycled powders or powders with varying levels of moisture content, for example. A recent study from the Massachu- setts Institute of Technology [2] present- ed a dramatic difference in the surface energy of three Ti-6Al-4V PBF powders (0.1 mJ/m 2 ) compared to bulk metals (40-50 mJ/m 2 ). Results were attributed to parameters such as surface rough- ness, surface chemistry, and surface oxidation, and the study explained why conventional metrics such as angle of repose fail to properly represent bulk behavior in metal AM powders. There- fore, a suite of advanced characteri- zation techniques is now being de- ployed across the full range of the AM powder supply chain. In most cases, these are well-established methods that have found common use in fields such as catalysis, mining, pharmaceuticals, and glass. A joint effort between ASTM and ISO is working toward standardization of powder characterization techniques for AM, borrowing from existing stan- dards in other fields as well as devel- oping new standards unique to AM. The Additive Manufacturing Consortium operated by EWI is concurrently pro- viding data to support standardization through a varied portfolio of R&D proj- ects. Nonetheless, several notable gaps in AM-specific powder characterization still exist: sample conditioning, particle morphology, moisture analysis, and powder flow characterization. In this article, a few of the myriad characteri- zation options available to particle sci- entists will be discussed. It is useful to describe individu- al powder properties with respect to the scale at which they are manifest- ed—the core material level, the formed particle level, or the bulk powder level. As an example, a few of the most com- mon powder characterization tech- niques are categorized using a matrix of material scale and four property cate- gories (Table 1). SAMPLING AND CONDITIONING Note the inclusion of sampling and conditioning methods at the top of the matrix in Table 1. Although not characterization techniques per se, it is critical that these are not overlooked or discounted when dealing with pow- der samples. Proper powder sampling techniques must be used in order to produce representative samples for subsequent laboratory analysis. The unique nature of powders can also lead to self-segregation during conveyance, storage, packing, and handling [4] . In one example, a 2014 NIST study revealed significant variability in chemical com- position among four sub-samples taken from a single lot of a cobalt-chromium ADVANCED POWDER CHARACTERIZATION FOR ADDITIVEMANUFACTURING As powder-based additive manufacturing (AM) processes continue to mature, reliable powder characterization techniques will be key to optimizing both AM processes and final part performance. Dave van der Wiel NSL Analytical Services Inc., Cleveland