Sintering and mechanical properties of prealloyed 6061 Al powder with and without common lubricants and sintering aids

Abstract

Physical and mechanical properties for prealloyed 6061 Al powder processed with and without additions of solid and/or liquid lubricants and sintering aids (Pb, Sn, Ag) are presented. For comparison, both vacuum and nitrogen sintering were carried out on as received (gas atomised) and degassed powder compacts pressed at 340 and 510 MPa. Vacuum degassing of the prealloyed powder provided better compressibility and thus higher green densities than those for the as received powder. Highest sintered densities of ∼98–99% of theoretical were obtained for the prealloyed (and degassed) Al compacts by sintering under pure nitrogen with an addition of 0·6 wt-% paraffin wax as solid lubricant or 1·33 vol.-% liquid paraffin, or with a 0·12 wt-%Pb addition as sintering aid and no lubricant. It was found that additions of solid lubricants such as lithium stearate and acrawax to both the premixed (elemental) and prealloyed powders provided reasonable green densities of ∼94·5–95·5% TD, but had deleterious effect on sintered densities and microstructures, particularly under vacuum sintering. Other lubricants such as zinc stearate, stearic acid and liquid paraffin provided similar green densities, but higher sintered densities and less porous microstructures, particularly by sintering under pure nitrogen. The prealloyed compacts sintered under pure nitrogen consistently provided much higher sintered densities than elementally premixed compacts sintered under pure nitrogen or vacuum. It is therefore concluded that both lubricant type and sintering atmosphere will have a major effect on the sintered properties of the 6061 Al powder. Sintering under pure nitrogen resulted in higher sintered densities as compared with vacuum sintering for this grade of Al alloy. Tensile properties of the degassed and vacuum sintered (and T6 tempered) prealloyed powder compacts were higher than those of the equivalent alloy prepared by elemental mixing and comparable with those of the commercial (wrought) 6061 Al alloys.