Issue 50

I. Papantoniou et alii, Frattura ed Integrità Strutturale, 50 (2019) 497-504; DOI: 10.3221/IGF-ESIS.50.41 499 Figure 1 : Powder metallurgy route Al foam manufacturing process. Foaming Process An especially designed setup was used to characterize the free expansion behaviour of the precursors during the foaming stage. The setup consisted a ceramic-glass window at the front side of the furnace and a high definition camera mounted at a close distance behind the glass. The camera was connected to a computer for recording images (at a rate of 60 frames per second). Using this setup, we were able to monitor the foaming process in al the experiments. The foaming time used was 10 minutes in order to observe all the foaming stages. The basic stages of the foaming process as seen in Fig.2 are the nucleation and growth, the peak, the pore coarsening and finally the porous structure decay. Figure 2 : Main stages during the foaming process (nucleation, growth, peak, coarsening and decay). By analyzing the data from the camera using the open-source image processing software ImageJ, we were able to create the porosity-time (P f-t ) diagrams for each specimen by using an image analysis procedure (Fig.3). The procedure included application of median edge preserving filters and special area detecting tools. Volume expansion was estimated by calcu- lating the change of the solid precursor to quasi-spherical shape (liquid foam state). Hence, the porosity (P f ) and the foaming efficiency (η=P fmax ) was quantitatively expressed by the measure of macroscopic volumetric expansion. 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 efficiency were expressed as Eq.(1): Figure 3 : P f – Time diagram and foaming efficiency (η) calculation using open-source image processing software ImageJ and Al-foam cross-sections at the peak and decay stages (corresponding to 180 and 500 seconds respectively).