IMPROVING THE PROPERTIES OF SINTERED Al-Zn-Mg-Cu FROM MIXED POWDER THROUGH SYNCHRONIC EFFECTS OF SINTERING AND HEAT TREATMENT

摘要

Among the lightweight metals most prevalent in the powder metallurgy industry, aluminium parts cover the widest range of applications. Aluminium P/Mparts are used in automotive, aerospace, business machine and appliance applications. Mostly, excellent mechanical properties are of primary importance, and here, Al-Zn-Mg-Cu alloys are particularly attractive, mainly because of their high strength as-heat treated. This article describes a study on densification and microstructural changes occurring during liquid phase sintering of Al-Zn-Mg-Cu mixed powder compacts. The objective was to find the optimum sintering temperature for obtaining higher densities and improved mechanical properties, the latter being further enhanced by a subsequent T6 heat treatment. The effect of the sintering temperature was evaluated by measuring sintered density, dynamic Young's modulus, transverse rupture strength and macrohardness. It could be observed that the sintered density increases with higher sintering temperature up to a certain limit, particle rearrangement as well as pore elimination due to liquid phase sintering being helpful. On the other hand, too high sintering temperatures are unwelcome either, apparently as a consequence of microstructural changes such as grain growth. It was found that the maximum sintered density for this material is attained by sintering at about 610°C at which temperature the amount of liquid phase is satisfactory. Furthermore, the behaviour of dynamic Young's modulus with respect to sintering temperature is very similar. The results of transverse rupture strength and hardness measurements indicate that the sintering temperature has a crucial effect on the mentioned mechanical properties of this alloy, and proper heat treatment improves both transverse rupture strength and apparent hardness compared to the as sintered state. It can also be concluded that during heat treatment, the alloying constituents are first dissolved in the matrix, then on cooling/aging precipitate from solid solution as a fine dispersion of intermetallic phases in an attempt to reach equilibrium. For this material under the defined aging the equilibrium T phase, Mg_(32)(Al,Zn)_(49), was formed.