Materials

8-2001

Design Application

Light alloys go from strength to strength

Tom Shelley looks at the latest leading edge trends and developments in metal alloys based on magnesium, aluminium and other light metals

Driven on by the needs of aerospace and the car industry, light alloys continue to advance into new territory

Not only are existing alloys continuing to be developed, along with improved fabrication methods, but scientists are now researching metal matrix composites based on magnesium, and totally new glass-like alloys with amazing properties.

Aluminium based metal matrix composites are already in use in components which include connecting rods and pistons for diesels. Now, the German research institute, GKSS Forschungszentrum in Geesthacht, just outside Hamburg, is studying fabrications based on magnesium, which is even lighter. The base method is the same as that used for aluminium MMCs, which is to squeeze cast, forcing molten metal under pressure into ceramic fibre pre-forms. GKSS says that it is also investigating the manufacture of magnesium alloy components using die casting, extrusion and thixocasting.

Aluminium alloy fabrications tend to be about two thirds the weight of equivalent constructions made in steel, but magnesium fabrications come out at about half. Reinforcing with ceramic or carbon fibre lowers the coefficient of thermal expansion, and increases Young's modulus and creep resistance. Complications are caused by chemical reactions between magnesium and carbon and ceramics, producing magnesium carbides at metal/fibre interfaces in the case of carbon, and magnesium silicides in the case of silica reinforcement. Hajo Dieringa of GKSS told Eureka that "We think it will take some time for these products to become commercial, but one of our targets is to make a completely fibre reinforced metal matrix composite magnesium piston as opposed to one which is only partially reinforced." More information about GKSS may be found on its Web site at www.gkss.de , which also mentions research into alloys based on titanium and aluminium.

Further into the future, but already attracting interest from automotive companies is bulk production of glassy light alloys.

It has been known for some time that if certain metal alloys are cooled very rapidly, such as by pouring them onto a rapidly spinning copper disk, they solidify in an amorphous, glass-like form instead of crystals. Thin ribbons of suitable glassy alloys can be made with very useful magnetic properties, for example, focussing weak magnetic fields into magnetic sensors.

According to Dr Lindsay Greer, Deputy Head of the Department of Materials Science and Metallurgy at Cambridge, it has now become possible to cast bulk ingots of glassy metal alloys up to 100mm across. Currently, he says, there are two main areas of interest: true bulk glasses based on zirconium, titanium and magnesium, and glasses based on aluminium which have to be rapidly quenched into thin ribbon form.

Bulk Metallic Glasses, or BMGs, behave very differently from their normal crystalline equivalents. Dropping a hard, steel ball onto a normal metal surface will cause it to bounce a few times. Dropping it onto a glassy alloy surface leads to its bouncing many, many times, because the metal surface absorbs very little energy during each impact.

There is thus much interest in the possibility of using such materials as the basis of springs, and they could, if anyone wanted them, be used to make chimes and bells which would go on ringing and ringing. So far, the main commercial glassy alloy products we are aware of are inserts for golf clubs, which makes them able to hit golf balls 10% further, and so have been banned by the PGA. More information can be found about them on their maker's Web site, www.liquidmetalgolf.com. The alloy used is one based on zirconium, titanium, nickel, copper and beryllium. Other alloys of interest include one containing magnesium, copper and yttrium, and another based on zirconium, nickel, aluminium and copper.

All the alloys are typically made up of three to five different metals, with very different atom sizes, and compositions close to deep eutectics, where the solidification temperature is way below that of any of the parent metals. When molten, they all form dense liquids with very little free space between the atoms and viscosities much greater than those of their parent metals or most other normal alloys. When they solidify, it may be presumed that the individual atoms have trouble getting past each other to form crystalline arrangements, and thus get stuck wherever they happen to be in the liquid.

Liquidmetal is a subsidiary of Amorphous Technologies International, a company which has spun out of research originally conducted at the California Institute of Technology. Liquidmetal says its alloy is two to three times stronger than stainless steel or titanium, and two or three times more resistant to plastic deformation. The Web site says that the US Government is working with ATI on an armour piercing rocket to be used as an anti-tank weapon in M1-19 missiles and 120mm tank shells. The amorphous material self sharpens as it pierces armour. NASA is also said to be working with ATI on a Liquidmetal coated drill bit to take core samples during the next Mars landing exhibition.

Even glassy metal alloys that are not so long term stable are of considerable interest in that they can be heat treated to decompose in such a way as to form nanometre sized precipitates which in the case of aluminium alloys, can triple their strengths.

Dr Greer's researches into BMG alloys based on magnesium and zirconium are supported by the UK-Korea Science and Technology fund in collaboration with unnamed UK and Korean companies. While we have no detailed information on the details of his investigations, Dr Greer did mention, in reply to a question about their use as springs, that one of the areas where the alloys were a little lacking was fatigue life, but added that this was "improving".

Aluminium takes the strain

The fronts of new Siemens trains for use in South London will be made by Superform Aluminium in Worcester.

Seimens_train_part.jpg (16463 bytes)

The train type will be called, Desiro UK, with carriage dimensions, voltage systems and entry heights for boarding and exiting specially adapted for the UK railway market. The first order has come from leasing company, Angel trains, which is the world's largest leasing company for passenger trains. They will be suitable for operation throughout the entire Greater London system, and will be able to work on both voltage systems.

DesiroUK_station.jpg (22738 bytes)

The front of Each Desiro UK is made of two superformed panels, 2.5m high, 1m wide and 700mm deep. The Desiro train front is the largest and deepest component Superform has ever made.

Siemens Project Manager, Hans Stiedl said, "The group of parts, 'front complete' was tendered to several suppliers, who underwent a rigorous selection process. We finally chose Superform as we had worked with them on a previous project and their price was extremely competitive. We are pleased with the parts they have supplied so far and will be considering them for future projects". More information at www.superform-aluminium.com .

Back to Materials