Production of Aluminium Based MMCs by Short-Time Ammonia Gas Flow Milling

In order to produce metal matrix composites (MMCs), aluminium powder was milled for a total time of 5 hours. Aluminium nitride was the ceramic reinforcement chosen to improve the mechanical behaviour of the aluminium matrix. In order to form it in situ, an ammonia gas flow was incorporated during a certain period of the milling process. Two different conditions of NH3 flow during milling were studied: short time (5 min) and long time (3 h). In both cases, milling started with a 2 h period of mechanical alloy in vacuum (5 Pa). Then, NH3 was incorporated during the stipulated time (5 min or 3 h), after which the milling process continued under vacuum to complete 5 hours. The powders were cold pressed and vacuum sintered to produce compacts. The results showed that compacts with better mechanical properties are obtained when short duration ammonia gas flow is used. The use of short flows provides good control of the amount of ceramic second phases formed. This allows the produced compacts to reach ultimate tensile strength higher than 400 MPa.