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Which Flowmeter for Powder-Bed-Based Additive Manufacturing?
In SLS (selective laser sintering), SLM (selective laser melting) and EBM (electron beam melting) techniques, successive thin layers of powder are created with a ruler or with a rotating cylinder...
Powder Additive Manufacturing
Selective Laser Sintering
In SLS (selective laser sintering), SLM (selective laser melting) and EBM (electron beam melting) techniques, successive thin layers of powder are created with a ruler or with a rotating cylinder.
Each layer is partially sintered or melted with an energy beam (laser or electron beam). The layer thickness defines the vertical resolution. Therefore, a thin layer leads to a better resolution.
In order to obtain a thin layer, the powder is as fine as possible. Unfortunately, when the grain size decreases, the cohesiveness increases and the flowability decreases. Moreover, the powder becomes more and more sensitive to moisture.
Therefore, a compromise between grain size and flowability has to be found. The quality of the parts build with AM is directly related to the powder flowing properties. The flowability must be good enough to obtain homogenous successive powder layers.
Processing Method
For all the processing method dealing with powder, the measurement method used to characterize the powder should be as close as possible to the process. In particular, the stress state and the flow field of the powder should be comparable in the measurement cell and in the process.
Different recent publications are evidencing that the classical flowmeters are unable to give pertinent information about powder flow behaviour in powder-bed-based powder additive manufacturing. The existence of a compressive load is incompatible with the free surface flow used in AM devices in shear cell testers. The measurement method based on the rotating drum is a good candidate as the powder flow is analysed precisely at the powder/air interface without any compressive load.
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Moreover, the rotating drum geometry allows studying the natural aeration of the powder during the flow.
In addition to the flowability, the powder packing fraction is an important parameter. Indeed, a high packing fraction reduces the porosity of the produced part. Therefore, a precise measurement of the range of packing fractions accessible by the powder is also necessary.