Aluminium metal injection moulding (MIM) is considered to have great potential to successfully compete with aluminium die casting because of the ability of the injection moulding process to produce parts more economically and with much thinner walled sections and more intricate shapes. However, many MIM aluminium parts, such as heat sinks, already in production do not have adequate mechanical properties for load bearing applications because they are made from pure rather than alloyed powder. Aluminium alloy MIM parts would offer mechanical properties approaching those of conventionally processed Al alloys and open up prospective applications in consumer electronic products, office equipment, hand tools and even automotive parts.
The key issue facing MIM producers wanting to develop aluminium alloy MIM parts is sintering and the need to overcome the oxide film on aluminium powder particles. The oxide film is a sintering barrier and needs to be removed or disrupted in order to achieve densification and high density sintered MIM parts. Researchers at the ARC Centre of Excellence for Design in Light Metals at the University of Queensland, Australia, have succeeded in developing and patenting (Wipo Patent WO/2008/017111) a process for sintering of Al alloy MIM parts using AA6061 alloy powders (Al-Fe-Si-Cu-Mg-Cr) with near full density and with good mechanical properties in the sintered, T4 and T6 heat treated condition.
Graham Schaffer and Zhenyun Liu report in a paper published the June 2008 issue of Powder Metallurgy (pp78-83) that under appropriate conditions, including compositional and environment control, the oxide barrier can be overcome without the need for mechanical shear to the aluminium alloy (AA6061) powder. They state that spherical AA6061 grade powders having D50 of 13.4µm were used and 2 wt% tin powder having <43µm particle size was added to the aluminium alloy powder as a liquid phase sintering aid. The feedstock produced by premixing the powders and binders, compounding and extruding, is said to have a powder loading of 82.9 wt%.
The binder system is said to consist of 3% stearic acid, 52% palm oil wax, and 45% high density polyethylene. The feedstock was used to produce injection moulded test bars and demonstration parts. Solvent debinding was conducted in hexane to extract more than 90% of the wax and stearic acid. The thermal debinding of the remaining binder and sintering was done in one furnace cycle with sacrificial magnesium blocks placed on the sintering tray surrounding the parts. The magnesium blocks serve as an oxygen getter and without these magnesium pieces the outer surface of the aluminium alloy MIM parts would not sinter.
Schaffer and Liu report that the addition of 2 wt% tin powder was an effective aid to sintering by significantly increasing the sintered density and expanding the sintering window for the aluminium alloy used in MIM. The combination of adding tin powder and sintering in nitrogen atmosphere resulted in ~95% or higher sintered density in the sintering temperature window of 600-6300C for 2 hours. The nitrogen atmosphere used also allowed the formation of AlN in the microstructure which the researchers state provides structural rigidity, enhanced dimensional accuracy and prevention of grain growth. Shrinkage of the injection moulded parts was uniform and parts were distortion free.
The tensile strength achieved for the AA6061 MIM material was around 160, 210 and 300 MPa and elongation to failure is about 9.5%, 10.4% and 1.5% in as-sintered, T4 and T6 condition respectively. However, the researchers stress that the process is not restricted to AA6061 alloy and that other grades of aluminium alloys may also be used with this technology.
The project to develop the MIM aluminium alloy process was funded by Cooltemp Pty Ltd, the Aluminium Powder Company, and the Australian Research Council.
Professor Graham Schaffer may be contacted at [email protected]