The effect of the atmosphere and the role of pore filling on the sintering of aluminium

Alloys of Al–3.8Cu–1Mg–0.7Si, Al–4Cu–0.6Si–0.1Mg, Al–4Cu–1.2Mg and Al–1.9Mg–1.9Si were made using air atomised powder and conventional press-and-sinter powder metallurgy techniques. These were sintered under nitrogen with a controlled water content which varied from 3 to 630 ppm (a dew point of −69 to −25 °C), nitrogen–5%hydrogen, argon and argon–5%hydrogen, all at atmospheric pressure, or a vacuum of <10−2 torr. Dry nitrogen is the most efficacious atmosphere. Vacuum is more effective than argon while hydrogen, and thus water, is extremely prejudicial to sintered density. The minimum dew point for optimum sintering is −60 °C (10.5 ppm H2O). The key feature in the beneficial effect of nitrogen is the formation of aluminium nitride. This reduces the pressure in the pore spaces relative to the external atmosphere, which induces pore filling at grain sizes that are smaller than those required for sintering in inert atmospheres. It is suggested that pore filling is an important densification mechanism during the sintering of aluminium.