Treatment of ferrous melts for the improvement of the sphericity of water atomized powders

Significance Statement

Advances in fabrication technology have led to significant utilization of additive manufacturing compared to the conventional processes in recent years. Additive manufacturing offers improved reliability, reduction in equipment price and allows for alternative manufacturing processes to be used. These additive manufacturing processes are either powder-fed or powder-bed based systems. Powder feedstocks used in the additive manufacturing processes are either gas atomized or water atomized powders. Gas atomized powder feeding technique yields less oxidized and more spherical products but is quite expensive for large scale commercialization as compared to the water atomized technique.

Herein, a novel technique for the production of ferrous water atomized powders for additive manufacturing applications is presented. The technique utilized the 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, École Polytechnique de Montréal, Édouard-Montpetit blvd., Montréal, Canada, proposed a study on the effects of a magnesium treatment on ferrous melts before water atomization on powder properties such as their flows, shapes, 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 nominal chemical composition for the three elements was obtained by combustion. 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 powder, optical micrographs were obtained. Eventually, the researchers performed thermodynamic computations and concluded their experiment when they performed equilibrium calculations of the liquids of each alloy so as to estimate the ΔTa−l that was used for the atomization process.

From the work presented work 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 instigated by larger solidification duration and a smaller spheroidization period of the droplets. The increased solidification time was noted to be as a result of the generation of a continuously renewed insulating magnesium gas layer during solidification of the droplets. The reduced spheroidization duration was observed to be as a result of an increased surface tension of the melt from the reaction of magnesium with dissolved Sulphur. The larger surface tension of the melt was noted to be the cause of the decline in the amount and size of internal porosities.

The novel technology presented here has advantages that are not limited to this specific process only but can also be incorporated in other atomization processes, so as to improve on the sphericity and lower the frequency of internal porosities. This technique is highly economical and can contribute to the advancements of the additive manufacturing market.

 

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.

Go To Materials and Design