REQUIREMENTS FOR METAL AND ALLOY POWDERS FOR 3D PRINTING (Review)

Abstract

There are five 3D printing methods in which metal or alloy powders can be used. The most promising methods are melting in the powder layer, directional energy deposition, and spraying of the binder material. General requirements for powders and their most important characteristics (size, particle shape, flowability of powders), 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 spreaders (as a knife or a platen) are analyzed. It shown that the d₉₀ size does not meet the actual requirements, and d_max needs to be taken into account. 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. Insufficient attention is paid to the shape of powder particles. In additive processes, powders with nonspherical particles (produced by grinding and other methods) have been already used but, in most cases, the shape indicators are not determined or calculated and their dispersion is not determined. The main criteria for the particle shape that correlate with the powder flowability should be identified. The standard flowability value (determined by flow test) does not sufficiently characterize the dynamic behavior of powders, nor does it allow comparison of powders with significantly different bulk densities and particle materials, and requires adjustment. The most important characteristic for the processes considered is the ability of powders to form a thin flat layer under certain conditions. A new characteristic of the powder dynamic behavior has been proposed: prevalence. It includes two criteria: the coating fraction of the building board and the dynamic flow angle of the powder, each having its drawbacks. But to date, there is no accepted technique for testing prevalence, nor is there an agreed indicator that would characterize it. There is only an understanding that the research method should best reproduce the powder behavior in a 3D printer during its operation. Techniques such as rotating the drum with powder (GranuDrum device) or the long-established classification of pharmaceuticals by fluidity, which is being tried to be apply to metal powders, are involved. According to classification, excellent fluidity is inherent in powders having an angle of natural slope varying from 25 to 30 degrees, Hausner coefficient 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 main methods of producing powders of various metals and alloys used in 3D printers are considered: gas atomization of the melt with melting in a crucible without vacuum and with vacuum and with induction melting, plasma atomization using a prefabricated rod, gas or plasma atomization of a rapidly rotating rod, etc. Gas atomization as an industrial method remains the most popular. Powders of greater quality, made from highly active elements, allow the production of new high-quality items but also leads to additional costs.