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

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.

Treatment of ferrous melts for improvement of the sphericity of water atomized powders-Advances in Engineering

About The Author

Denis Christopherson, Jr. is currently the Director of Research and Development, Federal-Mogul Powertrain, Valve Seats and Guides Group. Christopherson is responsible for leading technology and innovative material and process development in support of new business and advanced customer applications for the Valve Seat and Guides Group of Federal-Mogul Powertrain.
Prior to joining Federal-Mogul, Christopherson held various management and leadership positions at GKN Sinter Metals and the U.S. Department of Energy Superconducting SuperCollider project.
Christopherson earned a bachelor’s degree in Metallurgical Engineering from the University of
Wisconsin-Madison in 1988, and a master’s degree in Engineering Management from the University of Wisconsin-Madison in May 2010. He has also earned the Power Metal Technologist (PMT) certification from APMI International, and a certificate in Mid-Management Development from University of Wisconsin-Madison.
Christopherson was elected to the APMI International Board of Directors in 2007 and served as
chairman of the APMI International Publications Committee for four years. He served as co-chair of the 2013 International Conference on Powder Metallurgy and Particulate Materials, and is currently on the Board of Directors for the Powder Metal Parts Association (PMPA) of MPIF.

He has authored several articles in the areas of powder metallurgy and superconducting materials, including the PM machinability sections of the 2015 edition of the ASM Metals Handbook, Volume 7, and co-authored the testing and measurement section of the MPIF Design Manual. Christopherson is active in several powder metal industry societies including APMI International, Metal Powder Industries Federation (MPIF), and the American Society for Testing and Materials (ASTM), and is a frequent speaker at seminars and conferences. He has earned thirteen patents in the area of powder metal technology, and has several more pending.

About The Author

Professor L’Espérance obtained his bachelor degree in 1978 in industrial metallurgy and Materials Science at the University of Birmingham in England. He obtained his doctorate in 1981 at the same University.

He joined Ecole Polytechnique de Montréal in 1982 where he became a professor in 1983.  He co-founded in 1986 the Center for Characterization and Microscopy of Materials, (CM)² which is equipped with state of the art electron microscopes.  From 1998 to 2004 he spent 50% of his time in industry working with a Metal Powder producer and its customers.

His main research activities include: 1) the development and application of electron microscopy techniques to establish the relationships between the processing of materials, their nano/microstructure and their properties; 2) powder metallurgy particularly the development of metal powders for high performance press and sinter applications and additive manufacturing.

He was elected Fellow of the Canadian Academy of Engineers (FCAE) in 2004, Fellow of the American Metal Powder Institute (AMPI) in 2006 and he received in 2013 the distinguished service award to Powder Metallurgy from the Metal Powder Industries Federation (MPIF).  Over his 33 years of career, he supervised 51 post graduate students all of whom work in engineering and 5 have become university professors. He currently supervises 3 Ph.D. students, 2 M.Sc.A. students and 4 research associates.

About The Author

Dr. Mathieu Boisvert earned his bachelor degree in materials engineering in 2008 and his doctoral degree in powder metallurgy from École Polytechnique de Montréal in 2016. His doctoral research focused on:

  • Detailed electron microscopy of powder metals and sintered parts.
  • Investigation of the growth mechanism of nodular graphite in cast iron powder.
  • Characterization of oxide bifilms in water-atomized powders.
  • The effect of a magnesium treatment of the melt on the morphology of water atomized particles.
  • Development and implementation of sintering processes of new cast iron powders.
  • Development of powder metal automotive parts with improved wear resistance.

Since September 2016, Dr. Mathieu Boisvert is working as a R&D metallurgist at Federal-Mogul Powertrain on the development of new powder metal alloys for valve seat inserts and valve guides.

About The Author

Dr. Philippe Beaulieu joined Federal-Mogul Powertrain in 2012 as a research and development metallurgist in the valve seats and guides group on material development & tribology. Since 2017, he is the research & development manager for Europe for the valve seats and guides group of Federal-Mogul Powertrain. He is supporting material development and tribology within the valve seat & guides group and support the European market for new materials and new customer applications.

He earned his bachelor degree in materials engineering in 2000 and his doctoral degree in metallurgical engineering from École Polytechnique of Montréal in 2012. His doctoral research focused on:

  • Development of water atomized highly alloyed ferrous powders for high wear resistance applications.
  • Development of PM sintered materials for high temperature and high wear resistance valve seat insert applications.
  • Detailed characterization of nanostructures in water atomized high alloy tool steel 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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