Silicon Alloy Wheels explained

By: Steve Skinner

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Silicon is an essential ingredient in alloy truck wheels, and Steve Skinner visited Australia’s only silicon plant, which in itself involves a massive trucking operation.

Alloy wheels seem to be nearly everywhere these days, and not just because they look good when they’re shiny.

What are the advantages of alloy wheels?

They’re stronger but lighter than steel, and when used around entire truck and trailer combinations they can either save a lot of fuel or allow extra payload, or both.

Another advantage pushed by the makers of alloy wheels is that they allow quicker heat dissipation, which is good for brakes and tyres.

Of course a major catch is they’re also more expensive.

Maybe you’ve occasionally wondered what the term ‘alloy’ wheel actually means. They’re often also called ‘aluminium’ wheels.

Maybe the most correct name is ‘aluminium alloy’, because these wheels are mostly aluminium, but with a couple of other metals thrown in – hence they are a metal ‘alloy’.

One of these other metals is silicon, a hard but brittle metal that has all sorts of beneficial properties when used in truck wheels.

In other forms silicon is a major part of modern life, for example in computers and electronics as well as in optical fibre communications lines. This versatile element is also used in solar panels, and silicon based polymers (silicones) are used in heaps of everyday items.


Australia’s only silicon plant

Getting back to alloy wheels, we recently visited Australia’s only producer of silicon metal — Simcoa, near Bunbury, south of Perth in Western Australia.

Simcoa stands for Silicon Metal Company of Australia, and it’s a Japanese-owned smelter that employs about 180 people directly.

The basic ingredient for producing silicon is quartzite, which is a rock with its key compound scientifically known as silicon dioxide.

High purity quartzite is trucked in from a big Simcoa quarry at Moora, 180km north of Perth.

Low-ash charcoal — which is basically pure carbon — is produced on-site in a giant ‘retort’ from forest residue logs and sawmill off-cuts.IMG_8850

The quartzite, charcoal, coal and pine woodchips are mixed together in three electrically-powered arc furnaces. In a complex set of chemical reactions, with temperatures reaching up to 3,000 degrees Celsius, the carbon in the charcoal, coal and chips ‘extracts’ the oxygen in the quartzite, ultimately producing silicon metal and carbon monoxide and silica fume by-product.

The carbon monoxide is combined with oxygen in the air using large fans to produce the non-toxic carbon dioxide. The very fine silica fume powder is a good admixture to produce high strength concrete.

The pure silicon is poured into moulds and crushed into small pieces after cooling.

It’s then loaded into big bags or loaded bulk, trucked off to the port of Fremantle and shipped overseas – at the rate of 50,000 tonnes a year.


How silicon wheels are made

A lot of that silicon might be back in Australia spinning around in alloy wheels.

Silicon usually makes up about 7 per cent of an alloy wheel, and it has numerous important properties that make it essential.

"Silicon is one of the most important single alloying element used in aluminium alloys," Simcoa’s Senior Metallurgical Engineer, Chris Naudé says.

Silicon, aluminium, magnesium and maybe copper are basically heated together at massive temperatures until they form a liquid mixture.

Because silicon increases the fluidity of the molten alloy, this mixture pours into casts a lot easier and more uniformly than otherwise — more like water than syrup. Then when it solidifies, it doesn’t shrink as much because silicon actually expands when it cools down to solid form.

The lower density of silicon compared with aluminium means the total mixture is also less dense than aluminium — in other words reducing the total weight, at the same time as helping to make it stronger.

Silicon is also very hard, meaning it toughens up the alloy, making it more resistant to abrasion — scratching — than aluminium alone. (That means less polishing on the weekends.)

Silicon also improves the corrosion resistance of the wheel, with aluminium itself in turn being less prone to corrosion than regular steel, which of course can rust.

Last here but not least, Naudé says that by adding silicon, the alloy doesn’t expand as much as aluminium when heated.

The main benefit of that actually comes in when the wheel cools back down, by reducing shrinkage and therefore the likelihood of small hairline fractures which can ultimately lead to cracks and breaks.

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