Requirements for Metal and Alloy Powders for 3D Printing (Review)

There are five 3D printing methods that use metal or alloy powders. The most promising methods are powder bed fusion, directed energy deposition, and binder jetting. General requirements for the powders and their most important characteristics (particle size and shape, powder flowability), as well as the chemical composition of nickel alloy powders from two manufacturers, are addressed. Features peculiar to the behavior of powders in use of two types of recoater (as a blade or a roller) are analyzed. It is shown that the d 90 size does not meet the actual requirements and d max needs to be taken into account instead. Powders with nonspherical particles (mixtures of spherical and nonspherical particles) are known to be reused, but there are still no clear recommendations for their use. Inadequate attention is paid to the shape of powder particles. In additive manufacturing processes, powders with nonspherical particles (produced by grinding and other methods) have been already used but, in most cases, the shape indicators or their dispersion are not determined. Basic criteria for the particle shape that correlate with the powder flowability should be identified. The standard flowability value (determined by flow test) does not adequately characterize the dynamic behavior of powders, nor does it allow the powders with significantly different bulk densities and particle material to be compared, and thus requires adjustment. The most important characteristic for the processes considered is the ability of powders to form a thin flat layer in certain conditions. A new characteristic of the powder dynamic behavior has been proposed: spreadability. It includes two criteria: build plate coverage ratio and powder dynamic flow angle, each having its drawbacks. To date, there is no accepted technique for testing spreadability, nor is there an agreed indicator that would characterize it. There is only an understanding that a research method should best reproduce the powder behavior in a 3D printer in operation. Methods such as powder drum rotation (GranuDrum instrument) or long-established classification of pharmaceuticals by flowability, which was tried to be applied to metal powders, are involved. According to the classification, excellent flowability is inherent in powders having an angle of repose varying from 25 to 30 deg, Hausner ratio lower than 1.11, and Carr index lower than 5–15. The validity of this application requires thorough verification. The advantages and disadvantages of the following basic methods for producing powders of various metals and alloys used in 3D printers are addressed: gas atomization of melts in crucibles without vacuum and with vacuum melting or induction melting, plasma atomization using feedstock rods, rotation electrode gas or plasma atomization, etc. Gas atomization as a commercial method remains the most popular. Powders of greater quality made from reactive elements allow the production of new high-quality parts but also involve additional costs.