Metal AM Spring 2017

Page 103

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World PM2016: Water atomised powders

Additive Manufacturing at World PM2016: Opportunities for the use of water atomised metal powders At the World PM2016 Congress, held in Hamburg, Germany, 9-13 October, 2016, two papers discussed the potential for the replacement of gas-atomised powders with water-atomised powders as the raw material for Selective Laser Melting of different alloy types. Dr David Whittaker reports on the work undertaken to assess the viability of the water atomisation process for these materials which could, in turn, offer significant cost savings.

Microstructures and mechanical properties of Selective Laser-Melted metals with different processed powder The first of these papers, from Naoyuki Nomura, Keiko Kikuchi and Akira Kawasaki (Tohoku University, Japan), considered the processing of Co-Cr-Mo alloys. Co-Cr-Mo alloys have been widely used as metallic biomaterials in applications such as bone fixation devices, artificial knee joints and dental appliances, because of their excellent strength and good corrosion resistance. Such alloys have been used in the as-cast state with slow cooling and, therefore, show limited ductility because of the precipitation of carbides in the inter-dendritic regions. Also, when carbon is not added to the alloy, a martensitic transformation from the γ phase (f.c.c) to the ε phase (h.c.p) occurs during cooling after casting. The resultant ε phase restricts further deformation. To improve ductility, the

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Tohoku University group has developed a nitrogen-containing Co-Cr-Mo alloy, where nitrogen acts as a γ phase stabiliser. In alloys with added nitrogen, elongation was dramatically improved when nitrogen and chromium contents were increased in tandem. The group has also reported that the mechanical properties of a Co-29Cr-6Mo alloy were improved by applying a Powder Bed Fusion (PBF) process using a laser compared to a conventional dental casting. The γ phase was predominant in the SLM-built microstructure.

(a)

In the present reported study, the focus was mainly on the difference in responses of gas and water atomised powder variants. Co-33Cr-5Mo-0.4N (mass%) alloy powders were prepared by gas and water atomisation. A spherical particle shape with a smooth surface and dendritic traces was observed in the gas atomised powder, whereas an ellipsoidal particle shape with a dark contrast area was seen in the water atomised powder (Fig. 1). The median diameters (d50) of the gas and water atomised powders were 27.5 µm and 24.5 µm, respectively.

(b)

Fig. 1 Scanning electron micrographs of (a) gas and (b) water atomised Co-33Cr-5Mo-0.4N powders [1]

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