Aluminium metallurgy made simple

Materials World magazine
,
3 Jun 2011
Optical micrograph showing the dual phase microstructure

A simplified powder metallurgy process for mechanically alloyed aluminium (MA AI) that uses magnesium may lead to faster and efficient production of small engineering component parts.

The new method would mean a single cycle of cold pressing and sintering could be employed to produce engineering parts to near net shapes. This is achieved by enhancing the sinterability of the MA Al powder with the addition of elemental magnesium combined with a high compacting pressure, or by changing the sintering characteristics of the MA Al powder by milling in an ammonia atmosphere.

Lead Professor Jose Rodriguez at the Universidad de Sevilla, in Spain, explains, ‘Aluminium base powder particles are difficult to sinter due to the presence of a tenacious surface oxide film. This oxide layer is a barrier to sintering, since it inhibits material transport and, therefore, the formation of necks between aluminium particles. Aluminium oxide films are also stable at normal sintering temperatures and atmospheres. The magnesium works to reduce these oxide skins.’

He adds, ‘The MA Al powders are consolidated by the press and sinter technique. The as milled Al powder, eventually in the annealed condition, is blended with a very small amount of magnesium powder (0.15%Mg)’. In terms of production efficiency he says, ‘The new process [involves] only a 20 minutes mixing stage for sintering and a heat treatment that takes two hours.’

According to Rodriguez, magnesium can form with aluminium during sintering at temperatures higher than the eutectic temperatures of 450 and 437ºC, both these factors facilitate sintering.

On the other hand, metal–metal contacts are increased at the compaction stage when the MA Al powder is used in its annealed condition. This enables a lower compacting pressure to be used, thus facilitating the consolidation processing of the powder blend.

To test the technique, the team applied pressures ranging between 600 and 1,120 MPa. In some experiments, the MA Al powder was vacuum annealed at (five pascal pressure units) at 600ºC for two hours, before blending with magnesium. Both cylindrical (12mm diameter), and mass (less than four grammes) and flat ‘dog-bone’ tensile specimens (net dimensions – 25x4x4mm) are directly consolidated, without machining, resulting in a yield of ultrafine grained bulk powder metallurgy aluminium with a grain size of 700nm.

The blending of magnesium was found to improve the mechancial properties of the nanocomposite aluminium powder and increase both the ductility and tensile strength of the parts, argues Rodriguez.

‘The extent of the increase depends on the compaction strength of the consolidated pressure employed. For a cold pressure of 1,120MPa, the improvement in elongation to failure is 175%, with a simultaneous increase in ultimate tensile strength of 120%. The favourable effect of magnesium is especially remarkable in improving tensile properties. Bulk samples have so far had fairly satisfactory ductility (3%) and strength (223MPa). The samples have also performed remarkably under high temperature.’

However, he adds that tensile strength could be improved further by a better control of the annealing atmosphere.

Another important structural feature of powder metallurgy products is porosity – amount, pore size and shape, he notes. Ductility and resistance to fatigue and fracture are increased when the volume of porosity is small, and the pores are fine and round. Tests of the material MA AlMg/700 reveal a good average pore size of 5.5μm and an aspect ratio of 0.43, says Rodriguez.

The next step will involve completing more studies on mechanical properties such as fracture toughness and fatigue. Further developments will consist of adding new elemental powders as sintering aids.

Bob Blake, Technology Expert for the Materials Knowledge Transfer Network, comments, ‘The transport sector’s needs for lightweighting and low cost could be strong drivers for structural parts made using the press and sinter route with mechanically alloyed aluminium alloys. As well as the attainment of the necessary properties, the relatively high cost of powder preparation of these powders compared with iron powder, needs to be considered.’