Fast cooling yields extra high strength

Tom Shelley reports on an advanced light alloy technology commercially available after many years of R&D



Rapidly solidified aluminium alloys, where molten metal is quenched by pouring it onto a rapidly rotating copper wheel, are at last available commercially in 8 inch (200mm) billets with strengths 1.4 to 1.5 times stronger than billets produced by conventional methods.

In a process that represents the fruits of decades of research and development, the molten metal is solidified at between one and ten million degrees C per second. This produces a material that is not amorphous, as in some research experiments, but microcrystalline, with a grain size of about two microns.

Research and development projects aimed at producing such materials by similar methods may be found in most of the world's university metallurgy department laboratories. The number of companies using the technology to produce commercial products for sale, however, is somewhat smaller.

According to Dutch manufacturers, RSP Technology, their initially produced metal ribbon is chopped and then compacted to produce billets, which may then be extruded. Intermetallic phases and non soluble constituents are refined and homogenously distributed in the matrix, improving ductility, and permitting extrusion of parts with wall thicknesses down to 0.2mm. As well as being applicable to conventional alloy compositions, the ultra fast cooling make the process amenable to exotic compositions such as AlSi40%X and AlFe15%X. Examples of high strength alloys include RSA-70X aluminium zinc with a UTS up to 900MPa, comparable to the strength of titanium. Examples of high stiffness at medium strength include RSA-40X, AlSi7 aluminium silicon. RSA-42X aluminium silicon alloy exhibits high strength and low thermal expansion at temperatures up to 400 deg C. In order to meet particular customer needs, the company says it is able to optimise properties such as wear resistance, strength, expansion, ductility and conductivity.

Automotive applications are mainly in the motor sport end of the business, where they include pistons, cylinders, exhaust valves, connecting rods and space frame parts. The alloys are already to be found in some leading edge military aircraft, but are not yet certified for the civilian aerospace market although it is anticipated that they will be. Other applications are in competition bicycles, skates, golf clubs and mountaineering and archery equipment. In electronics, the technology is used to fabricate thin walled and micro profiled components. Machine builders are able to take advantage of the material's high stiffnesses and low weight to make heat resistant parts and hydraulic components. Fine machining can be used to produce surfaces with roughness down to 5nm. This makes the material applicable for the manufacture of fine mechanical parts, mirrors and tools for high precision injection moulding items such as contact and spectacle lenses. RSP Technology produces alloys as flakes, billets, bars, extrusion profiles or wire. Prices are from around 50 euros/kg.

RSP Technology

Pointers

* Products are available with ultimate tensile strengths up to 850 MPa and yield strengths up to 810 MPa. Future developments are expected to raise these figures to 1000 MPa and 900 MPa

* Thermal expansions can be as low as 15 microns/m deg C

* Elongations are up to 14 per cent, but not for alloys of maximum strength.

Author
Tom Shelley

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