mixes. The sintering response of Al-Zn-Mg-Cu (7xxx series) alloys and the development of tensile properties are The sintering characteristics and the tensile properties discussed. The effect of trace additions of selected elements of the 7000 series Al-Zn-Mg-Cu alloys, fabricated are examined first, and then bulk composition effects and using elemental powders in a conventional press and finally the influence of process parameters. Other aspects sinter powder metallurgy process, are examined.in the development of these alloys will be described in Microalloying with 100 ppm of lead or tin enhances two future companion papers. Elsewhere, the influence the sintering response of these alloys significantly, that magnesium has on reducing the oxide film present with a corresponding increase in the tensile strength.on all aluminium powder exposed to the atmosphere,4 The system has aspects of both transient and superthe development of 2xxx series alloys,5,6 and of nonsolidus liquid phase sintering. Zinc melts and eutectic conventional alloys7 are all considered. liquids form during heating to the sintering temperature but these liquid phases are absorbed by the EXPERIMENTAL PROCEDURE aluminium on further heating. Sintering above the solidus induces the formation of additional liquid.Alloys of Al-Zn-Mg-Cu-(Pb, Sn, Bi, Sb, Se) were mixed Because it has aspects of both transient and superas elemental blends in a tumbler mixer for up to 60 min. solidus liquid phase sintering, the system is extremelyThe powder details are presented in Table 1. The zinc sensitive to process variables, including particle size, content was in the range 4-8%, magnesium 0•5-2•5%, sintering temperature, and heating rate, but insensicopper 0•5-3%, and lead, tin, bismuth, antimony, and tive to green density. When the alloy composition and selenium 0-0•16% (all compositions are in wt-%). Two the process variables are optimised, tensile strengths aluminium powder size distributions were used with a d 50 in excess of 400 MPa are possible.PM/0849 of 60 and 100-120 mm. The particle size distributions are presented in Table 1. Three different sources of zinc were The authors are in the Department of Mining, used: vacuum distilled powder, air atomised powder, and Minerals, and Materials Engineering, The Univeran air atomised master alloy powder which had been sity of Queensland, Queensland 4072, Australia. prealloyed with 0•9%Pb. Admixed stearic acid (1%, −120 Manuscript received 15 March 1999; accepted 12 mesh) was used as a lubricant. Zn-Pb master alloy powder May 1999. sizes of <45, <75, <125, and <250 mm were used. Cylindrical specimens of 8 mm diameter or tensile bars of © 1999 IoM Communications Ltd.approximately 3 g and 40×9×3•5 mm were cold pressed in a floating uniaxial die at pressures from 50-500 MPa. These were sintered under nitrogen which had been passed through a dessicant of silica gel and magnesium perchloride
No 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.
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