Relationship between degassing conditions and tensile properties of Al-20Si-X P/M products

Abstract

Results are reported which show the effect of different degassing modes on the properties of the AI-20Si-3Cu-1 Mg powder. The paper complements previous papers [1-31 concerning the conventional and modified degassing of the same powder. This research was mainly directed to study the influence of temperature on the tensile properties, ultimate tensile strength, Cyuv s, and elongation, ~, of extrudates obtained of AI-20Si-3Cu-1 Mg compacts non-degassed, conventionally degassed, and treated by a modified process, namely degassing assisted by flushing with a depurative gas such as argon or nitrogen. The processing of the AI-20Si-3Cu-1 Mg P/M powder must include a degassing step which significantly improves the tensile properties, at room and elevated temperatures, of the products of compacted powder with respect to those of the products whose compacts were non-degassed. It is apparent that degassing assisted by flushing with argon or nitrogen gives products with higher tensile properties than those of the products conventionally degassed under optimal conditions of temperature and time and much higher than those of the non-degassed products. The tensile results are in agreement with the theoretical approach to the gas entrapment and evolution of the aluminium powders presented in previous papers.

Degassing Behavior of Nanostructured Al and Its Composites

“…Reduction of the gas content in Al alloy powders prior to compaction is, therefore, critically important. [3][4][5][6] Thermodynamic calculations reveal that it is practically impossible to produce oxide-free powder because in the Al-O system, the partial pressure of O 2 in equilibrium with Al 2 O 3 is far below the range attainable under protective gases or in vacuum systems (Al oxide requires an oxygen partial pressure of 10 À144 atm at 100°C and 10 À64 atm at 500°C [7] ). Consequently, oxidation always takes place on the fresh surface of Al powders.…”

“…The chemical reactions involved in the outgassing process are [3] Figure 11 shows the Gibbs free energy for Eqs. [3] through [6] as a function of temperature and pressure. The standard state free energy data are from Reference 7.…”

“…Reactions [3] and [4] are likely to occur at elevated temperatures or under vacuum. When the hydroxide is embedded in the matrix, Reactions [5] and [6] are kinetically favored. Therefore, one sees a significant decrease in chemisorbed water and a concurrent increase in hydrogen.…”