Process control in powder bed fusion of reactive metal powders: oxides, hydroxides, hydrated oxides and water films
The limited understanding of the complex relationships between alloy composition, temperature, relative humidity, oxide coverage, and the effects of these parameters on processing and powder flow motivates a key question: how much variation can be expected during powder bed fusion. In other words, how are additively manufactured parts and their properties affected at the current level of atmosphere control during storage and machining of powders?
In terms of manufacturing and process control, one important aspect of powder bed fusion technologies are the exposure of powder to external environment. The atmosphere in the storage containers and within the building chambers, especially during machining, affects the surface condition of the powder particles. In turn, this might impact not only the effective densities and the powder flow during recoating, but also the chemical composition of the melt pool and the final properties of the finished product .
For reactive powder materials such as titanium-based or aluminum-based powders, oxide layers form under all but the most severe vacuum conditions. Argon flows typically found in SLM machines and vacuum levels of about 10^-5 mbar found in electron beam equipments are inadequate to prevent oxides formation.
Aside from the formation of oxide layers, the adsorption of water on the powder particle surfaces must be considered [2, 3]. Adsorbed water films impact the powder flow behavior  and can modify the melt pool chemistry. From publications on electron beam welding [5, 6], it is known that when an electron beam hits a surface water film, water molecules can desorb and dissociate. Hydrogen can then enter the melt pool and, depending on the solubility of hydrogen gas in the liquid alloy as a function of temperature, can cause the formation of bubbles that might freeze in or dissolve in the base alloy upon freezing.
Adsorbed water films can also have a negative impact on powder flow and effective density of the powder beds [4, 7, 8].
Hydroxides and hydrated oxides
While oxide layers tend to be hard and brittle, hydroxide layers can assume a gel-like consistency with lubricating effects; these changes in the surface phases profoundly affect the wear behaviour of metals . It should be expected that the surface reactions and formation of hydroxides, as opposed to oxides, would also affect the flow of articles and possibly their agglomeration.
If the water vapor pressure decreases or the temperature increases, the hydroxides can dry out and crystallize to form oxides and, in the process, change the powder flow characteristics. The water coming off the hydroxides or hydrated oxides at increasing temperatures could moreover contaminate the atmosphere in the additive manufacturing build chambers or interact with the laser or electron beam.
Impact on process repeatability
The oxide, hydroxide, hydrated oxide, and water vapour formation represent complex processes that are likely to be interrelated. At present, only a few individual (but detailed) studies investigate the cause–effect relations between powder flow behaviour and the presence of oxide and water films on particle surfaces.
Humidity and temperature variations might be too small to affect powder flow, melting and solidification mechanisms and final properties of additively manufactured parts. Or they could turn out to be critical.
 Rainer J. Hebert, Viewpoint: metallurgical aspects of powder bed metal additive Manufacturing, J Mater Sci, 5 October 2015 - DOI 10.1007/s10853-015-9479-x
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 Matei G, Claussen N, Hausner HH (1974) Influence of relative humidity on flow behavior of metal and ceramic powders. Mod Dev Powder Met 8:5–11
 Mohandas T, Banerjee D, Kutumba Rao VV (1999) Fusion zone microstructure and porosity in electron beam welds of an alpha plus beta titanium alloy. Metall Mater Trans A 30A:789–798
 Gouret N, Dour G, Miguet B, Ollivier E, Fortunier R (2004) Assessment of the origin of porosity in electron-beam-welded TA6V plates. Metall Mater Trans A 35A:879–889
 Karde V, Panda S, Ghoroi C (2015) Surface modification to improve powder bulk behavior under humid conditions. Powder Tech 278:181–188
 Kwek JW, Ng WK, Tan CL, Chow PS, Tan RBH (2006) The effects of particle surface properties and storage condition on powder flow properties. In: AIChE Spring national meeting—5th world congress on particle technology, April 23, 2006–April 27, 2006, Orlando
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