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Significance Statement
In recent years, advances in fabrication technology have led to a significant growth of the additive manufacturing (AM) industry. Compared to other production processes such as casting, forging, etc. AM offers advantages to produce relatively complex parts for low volume batches, making it an ideal process for prototyping, on-demand parts and tooling production. Powder feedstocks used in AM are generally gas atomized powders as they are less oxidized and naturally more spherical compared to water atomized powders. These characteristics are desired as they provide powders with an improved flowability, a significant factor for AM. However, gas atomization is quite expensive which limits the growth of this industry for certain markets.
Herein, a novel technique for the production of ferrous water atomized powders for additive manufacturing applications is presented. The technique utilized a magnesium treatment of the melt before atomization which generates conditions that favor the direct formation of more spherical powders.
Researchers Mathieu Boisvert and colleagues from the Materials Engineering Program at École Polytechnique de Montréal in Canada, proposed a study on the effects of a magnesium treatment on ferrous melts before water atomization on powder properties such as their flow, shape, apparent and tap densities. Their study was based on studying the effects of the treatment on three different types of ferrous alloys which included: a 304-stainless steel, a high carbon steel alloyed with silicon and a hypereutectic cast iron. The researchers chiefly aimed at improving the sphericity of water atomized powders. Their work is now published in the peer reviewed journal, Materials and Design.
For empirical purposes, the three types of alloys were produced with and without a magnesium treatment before water atomization. The raw materials for each alloy were melted in an induction furnace and then water atomized so as to generate powder in a laboratory-scale water atomizer. Varying temperatures and water pressures were utilized for each type of alloy. Later, size distributions were obtained using a particle size analyzer. The flow rates and apparent densities and tap densities were then measured. After mounting and polishing of the powders, scanning electron micrographs were obtained on which image analysis was carried out to measure the sphericity improvement that a magnesium treatment provides.
The researchers performed thermodynamic computations to support their proposed mechanisms explaining the effect of magnesium on the sphericity of ferrous water-atomized powders. From the work presented, the research team was able to observe that all the powders that were treated with magnesium were more spherical and contained minimal and smaller internal pores. They also portrayed better flow and larger apparent and tap densities. The better sphericity of the magnesium treated particles is the result of a larger solidification time and a smaller spheroidization time of the droplets. The increased solidification time was considered to be the result of the generation of a continuously renewed insulating magnesium gas layer during solidification of the droplets. The reduced spheroidization time was observed to be the result of an increased surface tension of the melt from the reaction of magnesium with dissolved sulfur. Finally, the smaller amount of smaller internal porosities was also attributed to the larger surface tension of the melt.
The novel technology presented here has advantages that are not limited to this specific process but can also be incorporated in other atomization processes, to improve the sphericity and lower the frequency of internal porosities. This technique is highly economical and can contribute to the growth of the additive manufacturing market by significantly reducing the cost of powders.
Reference
Mathieu Boisvert, Denis Christopherson, Philippe Beaulieu, Gilles L’Espérance. Treatment of ferrous melts for the improvement of the sphericity of water atomized powders. Materials and Design. Volume 116 (2017) pages 644–655.
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