Three synchronized methods are combined for studying in situ the foaming process of AlSi11 powder precursors containing either as-received TiH2 or pre-oxidized 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-oxidized TiH2−x are revealed as well. The oxide shell of pre-oxidized 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.