The purpose of this study was to optimise the tensile properties and design of 2xxx series aluminium alloys fabricated by the press and sinter process. Alloy constituents,
compaction pressure, atmosphere type, lubrication method and thermal cycles are critical parameters that influence both the sintering and the tensile properties. Therefore the interaction of these parameters throughout sintering were investigated using multivariate and univariate statistical experimental designs, microstructural analysis, tensile testing, densitometry and a range of analytical techniques.
A model was developed to predict the relationships between the porosity and TS or elongation. This suggested that pore shape is a critical parameter, particularly for the porosity/elongation relationship. The model was applied to pressed and sintered aluminium. The relationships between porosity and the tensile properties were similar to those in
other ductile alloy systems.
Sintering aluminium compacts in a nitrogen atmosphere caused the formation of AIN if the pore network remained open. This occurred without the presence of a getter such as magnesium. An unsintered layer without nitrides surrounded the specimens and the aluminium in this layer was suggested to scavenge the oxygen, reducing the partial pressure deep in the specimen to levels where nitridation became possible.
The evolution of the porosity and liquid during sintering was investigated. Liquid formed at interfaces between additive elements and the aluminium, creating pools. When using a die wall lubrication method, the pools were round in shape. When an internal lubricant
was used, a few select bridges enabled the incorporation of adjacent particles into the liquid and resulted in irregular shaped liquid pools. Secondary pores formed and grew inside the liquid pools. Their early development was suggested to be a result of the liquid moving into the primary pore network and additive elements diffusing from the liquid into the solid matrix. Their later elimination was consistent with pore filling theory. Pore filling theory and pore formation sequences were incorporated into a combined model and adapted to include effects of nitridation and early pore network closure. This model predicted the effects of the atmosphere type and alloy constituents on the sintered porosity. Using an internal lubricant resulted in irregular pore shapes, which was attributed to the irregular shapes of liquid pools from which they formed.
Multiple regression models were developed that predicted the influence of alloy constituents and sintering temperature on the tensile properties of 2xxx alloys. Both porosity and brittle phases were found in the microstructure, combining to reduce the tensile properties. Increasing magnesium concentration decreased the elongation, which was attributed to the spatial distribution of the pores that formed at magnesium particle sites. Increasing silicon concentration to 0.4wt% increases the elongation, which was attributed to the greater densification due to an increase in the liquid. Further increases in silicon concentration decreased the elongation, possibly due to the increase in brittle phases found on the grain boundaries. Modifying the thermal cycle to decrease the liquid present when cooling begins increased both strength