Distortion in a 7xxx Aluminum Alloy during Liquid Phase Sintering

Abstract

The distortion in a sintered 7xxx aluminum alloy, Al-7Zn-2.5Mg-1Cu (wt. pct), has been investigated by sintering three rectangular bars in each batch at 893 K (620°C) for 0 to 40 minutes in nitrogen, followed by air or furnace cooling. They were placed parallel to each other, equally spaced apart at 2 mm, with their long axes being perpendicular to the incoming nitrogen flow. Pore evolution in each sample during isothermal sintering was examined metallographically. The compositional changes across sample mid-cross section and surface layers were analyzed using energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy depth profiling, respectively. The two outer samples bent toward 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 around the large pores as well as lower magnesium and zinc contents than the interior and inner surface of the same sample, while the inner surface was distinguished by the presence of AlN. The differential shrinkage was caused by different oxygen contents in local sintering atmosphere and unbalanced loss of magnesium and zinc between the outer and inner surfaces.

CFD modelling of nitrogen gas flow pattern during sintering of Al–7Zn–2.5Mg–1Cu alloy and its effect on distortion

“…5 Assuming that the oxygen concentration is homogeneous in the incoming nitrogen gas, the convective oxygen flux (J) depends simply on the oxygen concentration (C) and flow velocity (v): J5CNv. Thus, the oxygen flux is proportional to the flow velocity.…”

“…17 Earlier work has shown that the distortion in the as sintered Al-7Zn-2.5Mg-1Cu samples predominantly occurs during isothermal sintering at 620uC. 5 Hence, the 3D CFD modelling focused on the isothermal stage. It should be noted that powder compacts have lower thermal conductivity than fully dense material.…”

“…[1][2][3][4] A previous experimental study has shown that three rectangular Al-7Zn-2.5Mg-1Cu (in wt-%) compacts within the same batch exhibit different degrees of distortion after sintering in flowing nitrogen. 5 It was suggested that the distortion was a result of the differential shrinkage between the outer and inner surfaces of each sample, due to the different oxygen contents in local sintering atmosphere and unbalanced evaporation rates of the volatile elements Mg and Zn. 5 Since the green compacts were fabricated from the same powder blend under the same procedure and sintered simultaneously, it is likely that the different sintering responses are a consequence of nitrogen gas flow pattern over each sintering surface.…”

The Effect of Nitrogen Linear Flow on Lubricant Removal and Sintering Densification of Alumix 431D Grade Powder

“…Thus, besides chemical and physical interactions with aluminium [12], the removal of the lubricant's vapour and volatile products of its decomposition seems to be the main role of the flowing nitrogen sintering atmosphere in the manufacturing of sintered parts made of aluminium alloys. Yuan et al demonstrated [13] the influence of the nitrogen gas flow pattern on the sintering of a 7xxx aluminium alloy produced from elemental powders with compacts' distortion. In particular, they analysed the effect of the sample position in the furnace on its distortion.…”

Effects of Post-Sinter Processing on an Al–Zn–Mg–Cu Powder Metallurgy Alloy

“…This step (termed "sizing") is typically completed at room temperature utilizing rigid tooling and press technology comparable to that used during powder compaction. While studies have looked into the mechanisms of sintering-induced distortion [11][12][13], there exists a clear void in the open literature on the effects of the actual sizing operation itself. Furthermore, the incorporation of post-sinter sizing and heat-treatment processes introduces the unique challenge of whether to apply sizing before, during, or after the heat-treatment cycle.…”