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 5 (loss-on-drying) techniques are usual- ly too imprecise. While the Karl Fischer titration method is water-specific, it is comparatively complex and laborious. The very small sample sizes used in TGA methods often lead to poor precision. Halogen IR methods (already standard- ized for determining moisture in plas- tics) are perhaps the most promising technique, especially because they can accommodate fairly large sample sizes (up to 200 g) and are simple to operate. Nonetheless, the low moisture levels in metal powders require the most ad- vanced (and expensive) versions of these instruments, with some users still reporting insufficient sensitivity. See Table 4. Bulk powders are often character- ized according to one or more types of density. Each type may be useful in an- alyzing powder impacts on the various stages of material densification that oc- cur in powder AM processes, from pow- der spreading to final sintering. Along with apparent (or bulk) density (both loose and tapped), skeletal density continues to be an important powder property for optimizing final AM part density. This parameter is measured through the well-established technique of gas displacement pycnometry, most often using a helium pycnometer with microcell chambers to improve preci- sion in smaller sample volumes. An at- tractive aspect of this technique is that the same instrument can be used to measure the true (or absolute) density of a sample from a final part. Perhaps the most active area of development in powder characteriza- tion for AM is the field of powder flow analysis. Table 5 summarizes the var- ious static, quasi-static, and dynam- ic techniques commonly in use, along with the author’s opinions on their ap- plication to AM powders. Dynamic flow analysis techniques are of particular relevance to pow- der AM applications, where recoater spreading is an example of a dynamic environment under a low stress state. In many cases, when conventional stat- ic funnel testing cannot distinguish be- tween different powders (or when the powders will not flow through the fun- nels), avalanche/drum type rheometers will consistently differentiate between samples. Further, the instruments allow for parametric evaluation to identify “sweet spots” in powder flow behavior, correlating with recoater roller trans- versal speed or rotation speed/direc- tion, for example. The ASTM powder metallurgy sub- committee recently sponsored a round robin test to evaluate the ability of three commercial instruments (one quasi- static and two dynamic) to distinguish between six different AM powder sam- ples. Dynamic avalanche-type instru- ments were best able to differentiate samples based on material type and history (virgin, recycled, or blended). Data presented in the ASTM study, as well as the author’s own experience using an avalanche rheometer, shows that most parameters can achieve good reproducibility. SUMMARY As powder-based AM processes continue to mature, various combina- tions of the powder characterization techniques discussed here will be key to optimizing AM processes and final part performance. At the same time, some of these methods can provide rapid feedback for manufacturing control, powder qualification, and yield optimi- zation. It is important for those involved in the powder supply chain to appre- ciate the unique nature of powders, which results in their distinct behav- iors—making proper powder sampling crucial. Well-established advanced characterization techniques such as krypton BET adsorption, helium pyc- nometry, and powder XRF can provide TABLE 5 — POWDER FLOW ANALYSIS TECHNIQUES AND CHARACTERISTICS Powder Flow Analysis Techniques Static Quasi-static Dynamic Methods Hall, Carney, etc. Shear cell, impeller Avalanche/drum Qualitative Yes/No Flow regimes Semi-quantitative Flowability Flow rate Angle of repose Stress curves Flowability energy Flow function Wall friction angle Surface fractal Avalanche energy Cohesive index Avalanche angle Rest angle Quantitative Carr index Hausner ratio Volume changes Volume change Volume memory Apparent density Conditioned density Consolidation index Dynamic density Vibrated density Stress state Low Moderate to high Low Ease of operation Simple Complex Simple Data interpretation Simple Complex Values Parametric analysis No Yes Yes Standardization ASTM, ISO ASTM (plastics) No