BACKGROUND ART Powder metallurgy is the technology of transforming metal powders into semi-finished or finished products by mechanical and thermal operations. Advantages of using powder metallurgy techniques include the ability to fabricate specialty alloys with unique compositions, microstructures and properties; to make parts of complex shape to close tolerances without secondary processing; and to produce alloys, such as the refractory and reactive metals, which can only be fabricated in the solid state as powders. Standard powder metallurgy techniques involve the pressing of metal powders in a die, the removal of the green part from the die, and the sintering of the part in a furnace under a controlled atmosphere. The starting powder can be a blend of pure elemental powders, a blend of master alloy powders, fully alloyed powders or any combination thereof. Non-metallic particulate materials can be added to make composites. The sintering process causes atomic bonds to form between the powder particles.

This provides most of the strength. Bonding and/or densification may be aided by the development of liquid phases during sintering. These may or may not persist to the completion of sintering. These liquid phases may form by melting of elements or compounds, by incipient melting, or by the melting of eutectics which form by diffusional processes during sintering. The alloy can be used in the as sintered state or may be further processed. Secondary processes include coining, sizing, re-pressing, machining, extrusion and forging. They can also be surface treated and/or impregnated with lubricating liquids. Many metals are fabricated this way, including iron and steel, copper and its alloys, nickel, tungsten, titanium and aluminium.

Aluminium alloys for powder metallurgy applications are prepared using pre-alloyed powders or by mixing aluminium powder with alloy powders in elemental form or as master alloy blends. The mixture typically consists of particles which differ considerably in size, shape and density. This particle size distribution is required to optimise the properties of the resulting alloy and it is therefore desirable to maintain these particle sizes. However, the differences in size, shape and density mean that the powder mixture is susceptible to segregation during transport and handling. This segregation leads to compositional variations in green compacts manufactured from the powder, and thus to varying dimensional changes during the sintering operation and to varying mechanical properties in the as-sintered

product. Moreover, and most importantly, some of the aluminium and most of the alloying element powders are present in the form of very fine particles. The mixtures therefore have a tendency to dust, which can lead to difficult environmental and safety problems. Indeed, the dust given off by aluminium alloy powders is explosive which has impeded the use of aluminium alloys in the powder metallurgy industry. It is therefore highly desirable that an effective treatment is developed that can bond the fine particles present in an aluminium powder mixture to each other or to larger particles. The process should be simple, inexpensive and have little or no effect on sintering and the resultant mechanical properties.

A similar problem has been investigated by the ferrous powder industry. US Patent Numbers 4 483 905,4 834 800,5 069 714,5 256 185 and 5 432 223 describe methods for producing improved iron based powders and powder mixes using organic binder agents. The aforementioned patents teach that the inclusion of a binder reduces segregation and dusting of alloy mixture components. However, the dusting phenomenon addressed in these patents is the dusting and segregation of alloying ingredients and not the dusting of the metal component per se which is also a problem with aluminium alloy powders.

SUMMARY OF THE INVENTION An object of the invention is to provide an aluminium-based powder mixture in which dusting and segregation are eliminated or at least reduced to inconsequential levels.

According to a first embodiment of the invention, there is provided a powder composition comprising an aluminium-based powder and a binding agent.

According to a second embodiment of the invention, there is provided a powder composition comprising an aluminium-based powder, at least one alloying powder and a binding agent.

The invention further includes within its scope sintered aluminium alloy products formed from the composition according to the foregoing embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a chart showing the effect of various bonding treatments on weight loss from an aluminium powder mixture in a dusting test procedure detailed in Example 1.

Figure 2 is a graph showing the effect of binder content on weight loss from an aluminium powder mixture in the dusting test procedure detailed in Example 1.

Figure 3 is a graph showing the effect of binder content on the sintered density of an alloy formed from the aluminium powder mixture of Example 1.

Figure 4 is a graph showing the effect of binder content on the ultimate tensile strength of an alloy in the T4 temper formed from the aluminium powder mixture of Example 1.

Figure 5 is a graph showing the effect of paraffin wax on weight loss from an

aluminium powder mixture in a dusting test procedure detailed in Example 2.

Figure 6 is a graph showing the effect of binder content on the sintered density of an alloy formed from the aluminium powder mixture of Example 2.

Figure 7 is a graph showing the effect of binder content on the ultimate tensile strength of an alloy formed from the powder mixture of Example 2.

DETAILED DESCRIPTION OF THE INVENTION The following abbreviations are used herein: CA cellulose acetate PVA polyvinyl alcohol PVAc polyvinyl acetate In this specification, unless the context requires otherwise, the word"comprise"or variations thereof such as"comprises"or"comprising", will be understood to mean the inclusion of a stated integer or integers but not the exclusion of any other integer or integers.

As indicated above, the present inventors have found that the addition of a polymeric binding agent to a powder composition comprising an aluminium-based powder, or an aluminium-based powder and at least one alloying powder, prevents dusting and segregation of the components of the composition. Surprisingly, this prevention of dusting and segregation enhances the"green"properties of the powder mixture without detrimentally affecting the ultimate mechanical properties of the alloy formed from the composition.

With reference to the first and second embodiments of the invention, the aluminium- based powder can be pure aluminium or aluminium alloyed with other elements such as, but not limited to zinc, magnesium, copper, silicon, lithium, cobalt, nickel, titanium, zirconium, silver, tin or chromium, or any combination thereof. For compositions of the second embodiment, the aluminium-based powder can be an aluminium master alloy containing zinc, magnesium, copper, silicon, lithium, cobalt, nickel, chromium, titanium, zirconium, silver or tin, or any combination thereof.

The alloying powder or powders of a composition according to the second embodiment can be any of the alloying powders known to those of skill in the art. For example, the alloying powder can be, but is not limited to, zinc, magnesium, copper, silicon, tin, chromium or titanium, or any combination thereof.

Binding agents suitable for inclusion in compositions according to the invention include paraffin wax, cellulose acetate, polyvinyl alcohol and polyvinyl acetate, polyethylene glycol, polypropylene glycol, cellulose acetate butyrate, polyurethane, alkyd resin or polyvinyl pyrrolidone. The binding agent is typically present in the composition at a concentration of 0.01 to 2.0 wt%. A more preferred concentration range is 0.1 to 1.0 wt%.

Compositions can also optionally include lubricants such as stearic acid, zinc

stearate, lithium stearate and ethylene bis-stearamide, and other additives known to those skilled in the art. In other cases, the binding agent, such as paraffin wax, can also act as the lubricant.

When using a composition according to the second embodiment of the invention, the aluminium-based powder and at least one alloying powder can be combined in the dry state.

Preferably, the binder is added to compositions as a liquid or as a solution as this facilitates mixing and homogenous distribution of the binder through the powder mixture.

Typical solvents for non-liquid binding agents are water, ethanol, methanol, glycerol, acetone, heptane and ethyl acetate. However, any suitable solvent can be used as will be appreciated by one of skill in the art.

Following mixing, the solvent can be removed by evaporation to provide a powdered, homogeneous composition.

Compositions. according to the invention can be used for the preparation of all sintered aluminium alloys, aluminium metal matrix composites and the parts made from them. Alloys include the 7xxx AI-Zn-Mg and AI-Zn-Mg-Cu alloys exemplified below as well as the 2xxx Al-Cu, Al-Cu-Mg and Al-Cu-Mg-Si alloys, the 6xxx Al-Mg-Si alloys and novel alloys such as Al-Sn-Mg. Composites are not confined to a specific reinforcement but can include all possible ceramics such as Api203, SiC, AIN, TiB2, silicon, graphite, glass, flyash and ZrSiO4, as will be appreciated by one of skill in the art.

Having broadly described the invention, non-limiting examples will now be provided.

EXAMPLE 1 A mixture of an Al-8wt% Zn-2.5wt% Mg-1wt% Cu-0.07wt% Pb alloy, as described in International Patent Application No. PCT/AU96/00256 (publication No. WO 96/34991), was prepared by mixing powders in the correct proportions with stearic acid as a die lubricant in standard laboratory bottle-mixing equipment for 20 minutes. This led to a relatively uniform powder mixture. Binders were then added to and blended with the powder mixture. Manual pre-mixing was applied when the binder volume was < 3%. This pre-mixing assisted in spreading the binder uniformly through the mixture. Blending continued for a further 20 minutes, until the mixture had a uniform appearance. The mixture was then spread out on a sheet and allowed to dry in air. After drying, the mixture was milled using 10 x 8 mm steel balls in the same bottle mixing equipment for 10 minutes, and then sieved through a 300 pm screen.

The apparent density, the Hall flow rate and the dusting resistance of the powder mixtures were measured. In the absence of a standard dusting test, the procedure described in US Patent No. 4 834 800 was applied. In this test, the mixture was elutriated with a controlled flow of nitrogen. The test apparatus consists of a cylindrical glass tube vertically

mounted on a two-litre Erlenmeyer flask equipped with a side port to receive a flow of nitrogen. The glass tube (17.5 cm in length; 2.5 cm inside diameter) was equipped with a 400-mesh screen plate positioned about 2.5cm above the mouth of the Erlenmeyer flask. A 20-25 gram sample of the powder mixture to be tested was placed on the screen plate, and nitrogen was passed through the tube at a rate of 2 litres per minute for 15 minutes. At the start and conclusion of the test, the powder mixture was weighed to 0.001g. The dusting resistance is expressed as a percentage weight loss.

The effects of the bonding treatments on the sintered density, densification and tensile strength of the Al-8wt% Zn-2.5wt% Mg-1 wt% Cu-0.07wt% Pb powder mixture were examined. For these tests, powder was pressed in a floating steel die and sintered in dry nitrogen (dew point <-40°C). The density of the green compacts were determined from weight and dimensional measurements, which were accurate to within + 0.001g and + 0.001 mm, respectively. The sintered density was measured using the Archimedes method.

Tensile samples were solution treated at 490°C for 1 hr, quenched into cold water and aged at room temperature for 6 days. Tensile specimens were machined from the sintered bars and tested to failure in an Instron tensile testing machine using a cross head speed of 0.5 mm/min. The nominal dimensions of the gauge section were 13.5 mm x 4.5 mm x 2.6 mm.

Polyvinyl acetate (PVAc), cellulose acetate (CA) and polyvinyl alcohol (PVA) were trialled as binders. The PVAc was diluted in acetone at a concentration of 5 wt%, CA in ethyl acetate at a concentration of 5 wt% and PVA in water at concentrations ranging from 1.25 to 10 wt%. The dilutions were carried out by heating the binders in the solvents at 60-100°C while stirring.

The untreated powder mixture had a substantial dusting tendency. A smoke-like dust was observed when the untreated powder mixture was tested: the average weight loss for the untreated powder was 4.3%. Stearic acid had no influence on the weight loss. All of the bonding treatments substantially reduced the weight loss, as shown in Figure 1. The treatment using the PVA was the most effective. No visible dust was observed in these tests.

Figure 2 shows the weight loss as a function of the binder content. At a binder content of 0.10wt%, the weight loss was reduced to less than 0.5wt%. Greater binder concentrations had little effect.

Figure 3 shows the effect of the bonding treatments on the sintered density of the powder mixture. The PVA had little effect on the sintered density at concentrations <0.5 wt%. The PVA is detrimental to the sintered density at higher concentrations. The decrease in the sintered density was associated with a decrease in the tensile strength, as shown in Figure 4. The mechanical properties generally decrease significantly with increasing PVA content. At low concentrations however,-0.2 wt%, there was a small increase in the

sintered strength.

EXAMPLE 2 A mixture of an Al-4.4wt% Cu-0.8wt% Si-0.5wt% Mg alloy was prepared by mixing elemental powders in the correct proportions in standard laboratory bottle mixing equipment for 10 minutes. Up to 1wt% paraffin wax, as a solution in heptane, was added to the powder mixture and mixed for a further 10 minutes. The powder was then dried in air for three days and sieved through a 300 gm screen. For comparative purposes, 1wt% stearic acid was substituted for the paraffin wax in some instances. Here the stearic acid was added as a dry powder with the alloying elements and the powders were mixed uninterrupted for 20 minutes.

The apparent density, Hall flow rate and dusting resistance were measured, as per Example 1. For bulk density measurement and tensile testing, samples were prepared and tested as per Example 1. Sintering was performed at 600°C for 20 minutes.

As shown in Figure 5, paraffin wax is an effective de-dusting agent. The weight loss was reduced from 3.6% where stearic acid was used to 0.04% at 1wt% paraffin wax.

Paraffin wax had a positive influence on the sintered density, as shown in Figure 6. The sintered density increased from a mean of with stearic acid to 3 with 1wt% paraffin wax. This increase in sintered density produced a 20% improvement in the tensile strength from a mean of 20011MPa with stearic acid to 2438MPa with 1wt% paraffin wax and a doubling of the tensile ductility from a mean of with stearic acid to with 1wt% paraffin wax. This is shown in Figure 7.

It will be appreciated that many changes can be made to the compositions as exemplified above without departing from the broad ambit and scope of the invention.