Introduction

Stainless steels have many applications (medical equipment, agriculture, aerospace, automotive, and food industries) for their high strength, ductility, and corrosion resistance [1 – 4]. A chromium-rich oxide layer builds up on the surface and protects the steel from corrosion. 03Cr17Ni10M2 (US analog: 316L) is a popular stainless steel grade. To date, it is the most cost-efficient material for corrosive environments. The steel is widely available, easy to manufacture, and has high corrosion resistance [5 – 9].

Advanced additive manufacturing processes can make complex parts without the limitations of conventional subtractive manufacturing [10 – 13]. The raw material is metallic spherical powder. Its quality is paramount for the properties of the final product [14]. Spherical powders for additive manufacturing should have such properties as high fluidity, bulk density, homogeneous chemical composition, and particle size distribution [15 – 18]. The particle size of additive manufacturing powders is less than 160 μm. For example, melting processes use powders with particle sizes less than 60 μm, and surfacing, less than 160 μm [19]. Powders with particle sizes over 160 μm can be used in other processes such as sintering and/or hot pressing.

We refined the plasma spraying process for a higher yield of the particle size fraction suitable for additive manufacturing. We also studied the particle size distribution, morphology, fluidity, bulk, and tapped density of the spherical powder.

Materials and Methods

The powder was made from an industrial-grade wire, 1 mm dia., 03Cr17Nir10M2 (316L) steel grade. We made the spherical powder by wire spraying using a lab plasma spraying system (Russian Federation patent No. 2749403). Fig. 1 shows the system layout.

Air is evacuated from chamber 1 and then the chamber is filled with argon. Next, the plasmatron 2 is activated to generate a plasma jet, and wire feeder 3 feeds wire 4. An electric arc is excited between the free ends of the wires, and the wire melts. The system has feeders 5 and 6 delivering power to the wires to ignite an electric arc. Spherical particles are produced by pulverizing with the plasma jet. The atomized powder particles move along chamber 1 (direction I). Annular nozzle 7 produces a counter flow of cooling gas which slows down and cools the powder particles (direction II). The powder particles are collected in bin 8. Argon was used as the plasma-forming and atomizing gas.

After atomization, the powder was separated into fractions using an ANALYSETTE 3 SPARTAN vibratory sieve shaker (Germany). We used a JEOL JSM-IT500 scanning electron microscope (SEM) to obtain the powder particle images. An Analysette 22 NanoTec laser particle sizer was used for powder particle size analysis. The fluidity and bulk density were measured with an HFlow-1 Hall flowmeter, in accordance with the state standards GOST 20899-98 and GOST 19440-94, Part 1. The tapped density was measured in accordance with the state standard GOST 25279-93 using a BeDensi T1 analyzer.

Results and Discussion

We investigated the effect of electric arc power and gas flow rate on the yield of fraction suitable additive manufacturing (less than 160 μm) using a 1 mm dia. indust­rial-grade wire, 03Cr17H10M2 (316L) steel grade. We selected the process variables to make spherical powder by plasma spraying. The results are shown on Fig. 2.

As the electric power and gas flow rate increase, the yield of the –160 μm fraction increases to over 70 %. When the power exceeds 4 kW, the chamber overheats rapidly, so the process has to be paused for cooling. We found the conditions (power: 4 kW; gas flow rate: 250 l/min) for continuous spherical powder manufacturing.

Then we studied the particle size distribution and morphology of the resulting spherical powder made of the 03Cr17Ni10M2 steel grade wire. It was found that 70 % of the particles are smaller than 167 μm (Fig. 3). The SEM images of the powder particles show that the shape of the particles is almost spherical with no visible defects. The powder is suitable for additive manufacturing (Fig. 4).

We studied the bulk density, fluidity, and tapped density vs. particle size relations. The results are presented in the table.

It can be concluded that the smaller the particle size, the higher the fluidity (16.64 to 22.9 s), while the bulk and tapped density for all particle sizes remain unchanged (average values: 4.15 and 4.38 g/cm3, respectively).

Conclusions

We studied the effect of spraying conditions on the yield of particles smaller than 160 μm suitable for additive manufacturing. As the arc power gas flow rate increase, the –160 μm fraction yield increases to over 70 %.

We refined the spherical powder manufacturing by plasma spraying on a 1 mm dia. industrial-grade wire, 03Cr17Ni10Mo2 (316L) steel grade. The optimum conditions are as follows: 4 kW arc power, 250 l/min gas flow rate. The yield of fractions smaller than 160 μm exceeds 70 %. The resulting powder properties are 17.6 ± 1 s fluidity, 4.15 ± 0.1 g/cm3 bulk density, and 4.36 ± 0.2 g/cm3 tapped density. The powder is suitable for additive manufacturing.

We also studied the effect of the spherical powder particle size on the fluidity, bulk density, and tapped density. The best results (fluidity: 16.64 ± 1 s; bulk density: 4.16 ± 0.1 g/cm3; tapped density: 4.38 ± 0.2 g/cm3) were obtained for a –90 μm fraction.