PSI - Issue 10

I.G. Papantoniou et al. / Procedia Structural Integrity 10 (2018) 243–248 I.G. Papantoniou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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2.3. Foaming stage

In all the experiments the foaming process was being monitored by a camera, connected to a computer for recording images (at a rate of 60 frames per second). The foaming time was 10 minutes in order to observe all the foaming stages (nucleation, growth, peak, coarsening and decay) (Fig.2).

Fig. 2. Aluminium foaming stages (specimen: fine aluminium powder, 700 MPa compaction pressure and 800 o C foaming temperature) Using the open-source image processing software ImageJ and the data from the camera we were able to create the porosity-time (P f -t) diagrams for each specimen (Fig.3). The porosity (P f ) and the foaming efficiency (η=P fmax ) was quantitatively expressed by the measure of macroscopic volumetric expansion. Hence, the porosity and the foaming efficiency were evaluated based on the relative volume of the obtained foam (V r ), calculated by dividing the initial volume of the aluminium precursor (V i ) to the final volume of the foamed specimen (V f ). Thereof, the porosity and the foaming efficien cy were expressed as Eq. (1):

(1)

Fig. 3. P f and η calculations using open-source image processing software ImageJ

2.4. Compressive properties

Six additional specimens with the optimum parameters (parameters that led to maximum porosity) were created and foamed at a holding time which corresponded to the maximum porosity. Electrical Discharge Machining (EDM) was used to extract the samples out of the foamed batch and to create specimens of specific geometry (20 mm diameter and 20 mm height). The tests were conducted using an Instron 4482 test machine of 100 kN capacity at a constant crosshead speed of 5 mm/ min. The samples were compressed to 70% strain. The machine’s axis was parallel to the direction of the compression axis and the samples were placed on the steady press base. A preload was applied to the samples until the gaps between the sample and jaws disappeared. The compressive stress was defined from Eq. (2):

(2)