Electron Microscopy Analysis of 17-4 PH Powder for Additive Manufacturing
Metal additive manufacturing (MAM) has evolved into a production-ready manufacturing technology over a period of just a few years. The strong interest in MAM mainly derives from the ability to manufacture complex parts without the tooling and material waste typically associated with machining components from bulk alloys. Most of the attention for MAM has focused on Ti-6Al-4V and IN718 alloys, but several other alloy classes, such as stainless steels have gained traction as well. The stainless steels that have been considered for MAM include: 304, 316, and 17-4PH alloys, but to date none of these has been studied in great detail. 17-4PH is a heat-treatable martensitic stainless steel. Since MAM starts with the spreading or feeding of powders, a careful characterization of the starting powder is necessary to understand the way in which the microstructure and defects develop in the additively manufactured components, and how these lead to variations in component properties.
Characterization of 17-4PH stainless steel powders produced by supersonic gas atomization
Abstract17-4PH stainless steel powders were prepared using a supersonic nozzle in a close-coupled gas atomization system. The characteristics of powder particles were carried out by means of a laser particle size analyzer, scanning electron microscopy (SEM), and the X-ray diffraction (XRD) technique. The results show that the mass median particle diameter is about 19.15 μm. Three main types of surface microstructures are observed in the powders: well-developed dendrite, cellular, and cellular dendrite structure. The XRD measurements show that, as the particle size decreases, the amount of fcc phase gradually decreases and that of bcc phase increases. The cooling rate is inversely related to the particle size, i.e., it decreases with an increase in particle size.
Preparation of Fe-, Co-, and Ni-Based amorphous alloy powders by high-pressure gas atomization and their structural relaxation behavior
By using newly constructed high-pressure gas atomization equipment, the effect of atomizing gas (Ar, N2, or He) on the formation tendency of amorphous alloy powders and their Curie temperature(Tc), structural relaxation, glass transition(Tg), and crystallization(Tx) was examined for Fe-P-C, Fe-Si-B, Fe-Cr-Mo-P-C, Co-Si-B, Ni-Si-B, Ni-Pd-P, and Cu-Zr alloys with easy glass-forming capacity. Amorphous powders are formed in all the alloy systems and the critical diameter for the formation of amorphous powders is 25 to 44 µm for Ar and 44 to 100 µm for He. The average particle size is considerably smaller for He than for Ar and N2, probably because of large kinematic viscosity of He. No appreciable differences inTc,Tg, andTx of Fe77P13C10 amorphous powders with the kind of atomizing gas are seen and these values are the same as those of the amorphous ribbon with 20 µm thickness. However, He-atomized Fe77P13C10 powders exhibit a lower onset temperature of relaxation and larger heats of relaxation and crystallization as compared with the Ar- and N2-atomized powders, and the values of the He-atomized powders are comparable to those of the ribbon sample, indicating that He-gas atomization has a higher cooling capacity and results in the formation of a more disordered amorphous structure. This difference agrees with the tendency estimated under the assumption that heat removal from atomized droplets occurs by Newtonian cooling caused only by the convective heat transfer. Accordingly, the greater heat removal capacity of He gas is concluded to be a dominant factor for the formation of amorphous alloy powders with a more unrelaxed atomic configurations.