From powder to solid metal pieces - with a bit of technical trickery, processes that are already used successfully for other materials can now also be used for aluminium.
Intricate metal parts are often manufactured using metal powder injection moulding, which involves metal powder being mixed together with plastic, pressed into a mould and baked together at high temperatures to form a solid metal workpiece. This process is known as sintering. Although this technique has worked very well for a long time with steel or titanium, it has always proved to be unsuitable for aluminium. However, TU Wien has now succeeded in developing a powder metallurgy process for aluminium, which can be used to manufacture complex-shaped components in a material-saving manner. This is of particular interest for sectors in which weight-reduction plays an important role - from the automotive industry to space technology.
From powder to solid metal
The team at the Institute of Chemical Technologies and Analytics at TU Wien has been researching sintering technologies for many years and has worked together with some of the world-leading companies in this field with great success. "The raw materials are fine metal particles that react with the oxygen in the air and are therefore usually coated with a thin oxide layer," explains Chemist Christian Gierl-Mayer. In order to make the metal powder fluid and malleable, it is first mixed with a plastic carrier substance and injected into a precast mould. The resulting semi-finished product, known as the "green part", is then heated in a special furnace. In this way, the carrier substance is removed and at high temperatures the oxide layer is reduced. The metal grains come into direct and join to form a solid metal body.
However, a problem arises if aluminium is used: the oxide layer around the aluminium particles can only be removed at extremely high temperatures. At the same time, aluminium has a relatively low melting point which restricts the maximum sintering temperature. It is therefore impossible to remove the oxide layer before the entire metal piece has melted.
The carrier substance, in which the metal powder is bonded, is also removed by thermal processes which only occur at increased temperatures. The overlapping of the temperature ranges for binder removal and sintering means that residues of the binding agent are incorporated into the sintered workpiece if aluminium is processed using the same technique as for other metals.
The ambient atmosphere holds the key
TU Wien has now succeeded in finding a solution to this problem. The key is creating the correct atmosphere in the sintering furnace. A low-oxygen environment is usually used to prevent the complete oxidation of a metal powder. By contrast, with aluminium an oxygen-rich atmosphere is beneficial. "The aluminium oxide layer of the particles is so thick that the particles are protected from complete oxidation. At the same time, the oxygen aids the combustion of the carbon contents of the binding material," explains Gierl-Mayer.
After this first step, the oxygen atmosphere is replaced by nitrogen and the temperature is increased further. With the help of magnesium, the aluminium oxide layer is finally broken. A fluid phase occurs and the aluminium particles are sintered to form a solid metal piece.
"This method allows us to separate the two process steps - the removal of carbon residue and the sintering of the aluminium particles - thereby enabling both steps to run to completion for the first time," explains Christian Gierl-Mayer.
The powder metallurgy process enables complex shapes to be manufactured which cannot be realised in any other way, or only with great effort. The powdered raw material is relatively inexpensive, which means that even relatively large components can be produced at a reasonable cost. In mass production, considerable savings of up to 50% can therefore be made on material and weight compared with conventional production.
There are many industrial uses for this new aluminium sintering method. According to Gierl-Mayer: "Sintering processes with other metals have already asserted themselves in many areas of industry, with Austrian companies taking the lead globally in this field." The low density of aluminium makes it of particular interest for many applications, including in the automotive industries and aerospace engineering, for example, where weight reduction is important. Having said that, this aluminium sintering method could also open up new opportunities in the areas of machine tools and watches.