PM processing of elemental and prealloyed 6061 aluminium alloy with and without common lubricants and sintering aids M. Youseffi* and N. Showaiter A comparison has been made between compaction, sintering, microstructural and mechanical properties of the 6061 aluminium alloy prepared via premixed elemental (EL) and prealloyed (PA) powders (as received and degassed) with and without additions of sintering aids and various solid and/or liquid lubricants. Both EL and PA powders were cold pressed at different pressures, ranging from 250 to 770 MPa, and sintered under vacuum in the range 580–640uC for 30–120 min. and then under pure nitrogen atmosphere for comparison. Vacuum degassing of the PA powder provided better compressibility and thus higher green densities than those for the as received PA or the premixed EL powder compacts pressed at compaction pressures >340 MPa. Near full sintered densities of ,98%TD were obtained for both EL and PA 6061 Al alloys. Degassed PA Al with 0 . 6 wt-% paraffin wax (PW) or with only 0 . 12 wt-%Pb addition as sintering aid and no lubricant, and premixed EL with only 0 . 12 wt-%Pb addition and no lubricant gave the best optimum properties. It became apparent that additions of some solid lubricants such as lithium stearate (LS) and acrawax to both the premixed EL and PA powders provided reasonable green densities, but had deleterious effect on sintered densities and microstructures, particularly under vacuum sintering. Heating data curves during the sintering cycle, revealed formation of both transient and persistent liquid phases for the EL and mainly supersolidus liquid phase sintering (SLPS) mechanism for the PA. Tensile properties of the degassed, vacuum or nitrogen sintered PA Al alloy in T6 condition were higher than those of the equivalent alloy prepared by EL mixing with the former giving a tensile strength of 330 MPa and 6–8% elongation to failure, which are similar to those of the commercial (wrought) 6061 Al alloys. Keywords: 6061 Al alloy powder, Elemental and prealloyed, Sintering aids, Solid and liquid lubricants, Compaction, Vacuum and nitrogen sintering, Microstructure, Mechanical properties Introduction There is a significant rise in the application of net shape powder processing of aluminium alloys using press and sinter technology, especially in the automotive industry where, nowadays, low cost and light weight are two important design criteria. These applications include camshaft belt pulleys, cam bearing caps, shock absorber pistons, timing pulleys, couplings, oil pump gears, sprockets, connecting rods and camshafts as engine parts employed by companies such as GM, Nissan and Honda Motors. 1–3 Also, in non-automotive applica- tions, Al parts produced by powder metallurgy (PM) may replace small Al die castings where better material utilisation and closer tolerances are advantageous. Any further growth in PM Al alloy, therefore, requires development of material systems with higher and more consistent performance than those available presently. Recently, a comparison of pulleys manufactured using uniaxial pressing made from PM Fe–Cu–C, Al–Cu–Mg– Si and Al–Si alloys showed that a structural part made from Al–Cu–Mg–Si alloy with a sintered density of about 2 . 35–2 . 45 g cm 23 can fully compete with a part made from PM steel with a carbon content of ,0 . 3% and sintered density of about 6 . 6–6 . 8 g cm 23 , with at least 50% of weight advantage of Al maintained (www.sinterstahl.com). The goal is, therefore, production of densified PM Al parts using prealloyed (PA) or elementally premixed powders. However, consolidation by conventional com- paction and sintering of aluminium based powders to full density can be extremely difficult owing to the inherent oxide layer present on aluminium powder particles usually of large surface area owing to fine particle size. Other problems include swelling owing to the presence of entrapped gases, particularly in the elementally admixed alloys, and high solubility of the alloying elements leading to rapid dimensional changes, i.e. volume expansion instead of shrinkage. There is also poor compressibility problem owing to rather high School of Engineering, Design and Technology, Engineering Materials Unit, University of Bradford, Bradford BD7 1DP, UK *Corresponding author, email m.youseffi@bradford.ac.uk 240 ß 2006 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 19 January 2006; accepted 22 February 2006 DOI 10.1179/174329006X152460 Powder Metallurgy 2006 VOL 49 NO 3