In a recent article published in the journal Additive manufacturing Lettersresearchers have discussed the usefulness of chemical etching of stainless steel spatter to extend powder life in additive manufacturing.
Study: Extending the life of powders in additive manufacturing: chemical etching of stainless steel splashes. Image Credit: MarinaGrigorivna/Shutterstock.com
Background
Metal laser powder bed fusion (LPBF) produces spatter particles from molten droplets expelled from the weld pool or powder particles heated near or above the melting point as they pass through the beam laser.
Despite the use of an inert medium, the strong reactivity of metals near their melting temperature promotes oxidation. Although the ejected spatter particles are melted at least on the surface for a brief duration during the LPBF, diffusion of volatile elements to the surface is likely to occur, and these high oxygen affinity elements create thick oxide layers.
Since the partial pressures of oxygen in LPBF are often higher than in gas atomization, the possibility of binding with oxygen is increased.
Spatter of stainless steel and nickel-based alloys are well known to oxidize rapidly, forming islands up to a few meters thick. Additionally, stainless steels and nickel-based alloys, such as those that produce island-type oxide spatter, are much more commonly processed materials in LPBF, and it is critical that this approach be applied to more typical LPBF metal splashes to demonstrate chemical turnover. of powder generically.
(a) SEM image of a stainless steel spatter particle, (b) experimental approach to heated chemical etching, (vs) LPBF treatment of deoxidized spatter particles. Image credit: Murray, J.W et al., Additive Manufacturing Letters
About the study
In this study, the authors used a novel chemical etching technique to remove oxides from the surface of oxidized stainless steel blasting powder. Dissolution of the metal surrounding and under the oxide islands on the powder was used as the primary oxide removal mechanism, allowing for more aggressive oxide removal. Splashed, etched and blank powders were sieved into the same powder size range for LPBF processing.
The team demonstrated how to remove oxides from stainless steel spatter particles, specifically those that separate to create Si and Mn-rich oxide islands on the surface of the powder using a chemical technique. The 316L spatter was collected from the powder bed of the LPBF prints and etched chemically by immersion. After sieving all the particles in the same size range, LPBF processed them in a single lane, with the optimally etched splashes and pristine stainless steel.
The researchers studied temperature as well as two different etchants for stainless steel. After sieving to an equal sized range, single LPBF tracks were created using analog blank powder, blasting powder, and efficiently etched blasting powder.
Unique LPBF tracks generated from splatter, etched splatter and blank powder. The high magnification images reveal that the oxide layer that prevails on the projection tracks is somewhat removed on the etched projection tracks. The virgin powder still shows some presence of oxide. Image credit: Murray, J.W et al., Additive Manufacturing Letters
Comments
Ralph’s reagent, heated at 65°C in a water bath for 1 hour, resulted in a 10-fold reduction in surface coverage of oxides on 316L stainless steel blasting powder, from 7% to 0, 7%. Using large-area mapping, EDX data revealed a reduction in oxygen presence from 13.5% to 4.5%.
Compared to splashes, etched splashes had a lower oxide slag coating on the track surfaces. In addition, the chemical attack of the powder increased the assimilation of the powder on the tracks. Chemical etching had the potential to improve the reusability and durability of heavily used spatter or powder made from widely used, corrosion-resistant stainless steel powder.
The remaining agglomeration particles among the etched and non-etched spatter powders throughout the sieved size range of 45–63 µm explained why the track volumes of the etched spatter powder and the spatter were similar, whereas the virgin powder was about 50% larger in volume. It was observed that powders that were agglomerated or orbited affected the packing density and therefore followed the volume.
Compared to splashes, etched splashes had a lower oxide slag coating on the track surfaces. When the oxides were chemically removed, the semi-embedded and stripped powder demonstrated better incorporation of the reduced oxides, which was attributed to better wetting.
Diagram showing the benefits of LPBF treatment when oxides are chemically removed from powder spray for the stainless steel system. Superior wetting is made possible by the removal of oxides. Image credit: Murray, J.W et al., Additive Manufacturing Letters
conclusion
In conclusion, this study used a chemical etching procedure to chemically rejuvenate highly oxidized stainless steel spatter powder with immersion in Ralph’s Reagent, which is a solution of iron and copper chlorides in acid. hydrochloric. Immersion in a heated Ralph’s etchant solution for one hour has been observed to result in a 10-fold reduction in oxide zone coverage on the sprayed powder.
The authors believe that chemical etching has the potential to be improved and used on a larger scale to refresh spatter particles or LPBF powder that has been reused many times, thereby increasing the value of expensive powder-like materials.
Source
Murray, JW, Speidel, A., Spierings, A., et al. Extending the life of powders in additive manufacturing: chemical etching of stainless steel splashes. Additive Manufacturing Letters 100057 (2022). https://www.sciencedirect.com/science/article/pii/S2772369022000317
