Aluminium is an important structural material for automotive applications due to its affordability, light weight, good mechanical properties, good corrosion resistance, and ease of machining and recycling. Press-and-sinter aluminium parts have found applications in the automotive industry over the last two decades due to their high performance-to-cost ratio. However, for powder metallurgy (P/M) aluminium parts to compete with aluminium castings in the ever-increasingly competitive automotive market, the near-net shape attribute of P/M must be conserved while satisfying the required mechanical performance. This requires close control of dimensional change during sintering, especially for 7xxx aluminium alloys, which are known to be susceptible to the processing parameters.
The distortion in a sintered 7xxx aluminium alloy, Al-7Zn-2.5Mg-1Cu (wt.%), has been investigated by sintering rectangular bars simultaneously at 620 °C for 0-40 min in nitrogen, followed by air or furnace cooling. These samples were placed parallel to each other, equally spaced 2 mm apart, with the long axes perpendicular to the incoming nitrogen flow. Pore evolution in each sample during isothermal sintering was examined metallographically. The compositional changes after sintering were analysed using energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) depth profiling. The two outer samples always bent towards the middle one while the middle sample was essentially distortion free after sintering. The distortion in the outer samples was a result of differential shrinkage between their outer and inner surfaces during isothermal sintering. The porous outer surface showed an enrichment of oxygen (O) around the large pores as well as lower magnesium (Mg) and zinc (Zn) contents than the interior and inner surface of the same sample, whereas the inner surface was distinguished by the presence of AlN. The differential shrinkage between the outer and inner surfaces of each outer sample was caused by different O contents in the local sintering atmosphere and the unbalanced loss of Mg and Zn from the two surfaces.
To further characterize the effect of the gas flow pattern on the sintering responses, the separation distance between the three samples was varied from 2 mm to 40 mm. Increasing sample separation distance from 2 mm to 20 mm decreased the distortion in the two outer samples after sintering at 620 °C for 45 min. Little distortion was observed in the same samples sintered at a separation distance of 40 mm under the same sintering conditions. The sintered density and pore distribution of each sample were also found to be closely related to the sample position and separation distance. Two-dimensional (2-D) and three-dimensional (3-D) computational fluid dynamics (CFD) models were developed to investigate the gas flow behaviour surrounding each sample during isothermal sintering. The modelling results were presented in terms of streamlines, gas flow rate into each cavity between two adjacent samples and wall shear stress distribution along the surfaces of each sample. It is suggested that the flow patterns surrounding each sample affect the degree of distortion in the sample by changing the O content in the local sintering atmosphere and the evaporation rates of Mg and Zn from the sintering surfaces. This further explains the different sintering responses of the three samples sintered in each batch in terms of the sintered density, pore distribution, and distortion. To demonstrate the fundamental understanding gained from CFD modelling, the 3-D CFD model developed was applied to the design of the sintering of six rectangular samples of Al-7Zn-2.5Mg-1Cu in order to achieve minimum distortion as well as maximum densification. The predictions of CFD simulation were verified experimentally.