Desorption of hydrogen from blowing agents used for foaming metals

Hydrogen desorption from TiH2, ZrH2, and MgH2 was studied by thermal desorption spectroscopy (TDS), differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Loose hydride powders as well as powder compacts of zinc and various hydrides were studied. It was found that during the powder compaction process free surfaces on the hydride powder particles were created. As a consequence, the desorption temperature of the hydride in the precursor was lowered in comparison to loose powder exposed to air. Foam expansion of zinc was highest for TiH2 which also exhibits the highest desorption rate at the melting point of zinc, followed by ZrH2 and MgH2 which decompose at lower temperatures and are therefore less effective for foaming. The desorption kinetics of Al and AlSi7 compacts containing TiH2 were also studied for matters of comparison. The much lower foam expansions compared to Zn foams could be explained by higher hydrogen losses at temperatures below the melting point of Al and AlSi7.