Chemical Etching to Remove Oxides from Oxidized Stainless Steel

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In a recent article published in the journal Additive Manufacturing Letters, researchers discuss the utility of chemically etched stainless steel spatter for extending powder life in additive manufacturing.
Research: Extending Powder Life in Additive Manufacturing: Chemical Etching of Stainless Steel Spatter.Image Credit: MarinaGrigorivna/Shutterstock.com
Metal Laser Powder Bed Fusion (LPBF) Splash particles are produced by molten droplets expelled from the molten pool or powder particles heated to near or above the melting point as they pass through the laser beam.
Despite the use of an inert environment, the metal’s high reactivity near its melting temperature promotes oxidation.Although spatter particles ejected during LPBF melt at least briefly at the surface, diffusion of volatile elements to the surface is likely to occur, and these elements with high affinity for oxygen produce thick oxide layers.
Since the partial pressure of oxygen in LPBF is usually higher than that in gas atomization, the possibility of binding with oxygen is increased.
Stainless steel and nickel-based alloy spatters are known to oxidize rapidly, forming islands up to several meters in thickness.In addition, stainless steels and nickel-based alloys, such as those that produce island-type oxide spatters, are more commonly machined materials in LPBF, and applying this method to more typical LPBF metal spatters to demonstrate that chemical renewal is critical to Powder in the usual way.
(a) SEM image of stainless steel spatter particles, (b) experimental method of thermal chemical etching, (c) LPBF treatment of deoxidized spatter particles.Image credit: Murray, J. W, et al, Additive Manufacturing Letters
In this study, the authors employed a new chemical etching technique to remove oxides from the surface of oxidized stainless steel splash powders.Metal dissolution around and below oxide islands on the powder is used as the primary mechanism for oxide removal, which allows for more aggressive oxide removal.The splash, etch and virgin powders were sieved to the same powder size range for LPBF processing.
The team showed how to remove oxides from stainless steel spatter particles, especially those that were isolated using chemical techniques to form Si- and Mn-rich oxide islands on the powder surface.316L of spatter was collected from the powder bed of LPBF prints and chemically etched by immersion.After screening all particles to the same size range, LPBF processes them into a single pass with optimized etched spatter and virgin stainless steel.
The researchers looked at temperature as well as two different stainless steel etchants.After screening to the same size range, LPBF single tracks were created by using similar virgin powders, splash powders, and efficiently etched splash powders.
Individual LPBF traces generated from spatter, etch spatter, and pristine powder.The high magnification image shows that the oxide layer prevalent on the sputtered track is eliminated on the etched sputtered track.The original powder showed that some oxides were still present.Image credit: Murray, J. W, et al, Additive Manufacturing Letters
Oxide area coverage on 316L stainless steel splash powder decreased by a factor of 10, from 7% to 0.7% after Ralph’s reagent was heated to 65 °C in a water bath for 1 hour.Mapping the large area, EDX data showed a reduction in oxygen levels from 13.5% to 4.5%.
Etched spatter has a lower oxide slag coating on the track surface compared to spatter.In addition, chemical etching of the powder increases the assimilation of the powder on the track.Chemical etching has the potential to improve the reusability and durability of spatter or mass-use powders made from widely used and corrosion-resistant stainless steel powders.
Across the entire 45-63 µm sieve size range, the remaining agglomerated particles in the etched and unetched spatter powders explain why the trace volumes of the etched and spattered powders are similar, while the volumes of the original powders are approximately 50% larger.Agglomerated or satellite-forming powders were observed to affect bulk density and thus volume.
Etched spatter has a lower oxide slag coating on the track surface compared to spatter.When the oxides are chemically removed, the semi-bound and bare powders demonstrate evidence of better binding of the reduced oxides, which is attributed to better wettability.
Schematic showing the benefits of LPBF treatment when chemically removing oxides from splash powder in stainless steel systems.Excellent wettability is achieved by eliminating oxides.Image credit: Murray, J. W, et al, Additive Manufacturing Letters
In summary, this study used a chemical etching procedure to chemically regenerate highly oxidized stainless steel spatter powders by immersion in Ralph’s reagent, a solution of ferric chloride and cupric chloride in hydrochloric acid.It was observed that immersion in the heated Ralph etchant solution for 1 hour resulted in a 10-fold reduction in oxide area coverage on the splashed powder.
The authors believe that chemical etching has the potential to be improved and used on a wider scale to renew multiple reused spatter particles or LPBF powders, thereby increasing the value of expensive powder-based materials.
Murray, JW, Speidel, A., Spierings, A. et al.Extending powder life in additive manufacturing: chemical etching of stainless steel spatter.Additive Manufacturing Letters 100057 (2022).https://www.sciencedirect.com/science/article/pii/S2772369022000317
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Surbhi Jain is a freelance technical writer based in Delhi, India.She has a Ph.D.Received a PhD in Physics from the University of Delhi and participated in a number of scientific, cultural and sporting activities.Her academic background is in materials science research, specializing in the development of optical devices and sensors.She has extensive experience in content writing, editing, experimental data analysis and project management, and has published 7 research papers in Scopus indexed journals and filed 2 Indian patents based on her research work.Passionate about reading, writing, research and technology, she enjoys cooking, acting, gardening and sports.
Jainism, Subi.(24 May 2022).New chemical etching method removes oxides from oxidised stainless steel splash powder.AZOM.Retrieved July 21, 2022 from https://www.azom.com/news.aspx?newsID=59143.
Jainism, Subi.”New chemical etching method to remove oxides from oxidized stainless steel spatter powder”.AZOM.July 21, 2022..
Jainism, Subi.”New chemical etching method to remove oxides from oxidized stainless steel spatter powder”.AZOM.https://www.azom.com/news.aspx?newsID=59143.(Accessed 21 July 2022).
Jainism, Subi.2022. New chemical etching method to remove oxides from oxidized stainless steel splash powder.AZoM, accessed July 21, 2022, https://www.azom.com/news.aspx?newsID=59143.
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Post time: Jul-22-2022