Metal foaming studied in situ by energy dispersive X ray diffraction of synchrotron radiation, X ray radioscopy and optical expandometry

Three synchronised methods are combined for studying in situ the foaming process of AlSi11 powder precursors containing either as received TiH2 or pre oxidised TiH2 x. The phase transformations are followed by energy dispersive X ray diffraction ED XRD of synchrotron radiation. The internal structure of the foam is monitored by X ray radioscopy and a video camera records the overall foam expansion. Complementary mass spectrometry follows the H2 gas release. Phase transformations of TiH2 and TiH2 x particles inside Al Si foams are reported in this work for the first time and they are different than the ones observed for loose powders under Ar flow because the solid metal matrix initially retards H2 outgassing, but also because after melting the liquid reacts with TiH2 forming the ternary compounds Ti AlxSi1 x 2 in the semi solid state and Ti Al1 xSix 3 in the liquid state of the alloy. Further reasons for the larger expansion obtained when using pre oxidised TiH2 are revealed as well. The oxide shell of pre oxidised TiH2 x not only shifts the onset of H2 release towards higher temperatures, but it also hinders the reaction between blowing agent and melt, stabilizing the structure of TiH2 x thus preserving its role as a good blowing agent for foaming Al based alloys