Green strength of powder compacts provided for production of highly porous titanium parts

The production route of highly porous near net shape titanium parts includes the green machining of compacts made of metallic and pore former (often ammonium bicarbonate) powder mixtures. Success in green machining greatly depends on compact strength. The aim of the present work was to determine and quantitatively evaluate factors influencing the green strength of titanium-ammonium bicarbonate compacts. The substantial dependence of green strength on the amount of pore former and compaction pressure was demonstrated. Some influence of the particle size of ammonium bicarbonate on compact strength was also observed. A theoretical explanation of these experimental results is given. A sharp decrease of green strength owing to decomposition of ammonium bicarbonate in compacts during holding in air was experimentally found. The factors influencing decomposition, i.e. time, compaction pressure and particle size of pore former, were investigated in detail. The catalytic action of titanium on decomposition of ammonium bicarbonate is discussed.