METALLURGY

Pyrite mineral picture on white surface Roughy square shaped crystal Metal like crystal

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Aluminium: The Metallic Crystal

Aluminium is a metallic crystal and the chemical composition and cooling rate greatly affects its final physical properties.

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We think the metallurgical science behind aluminium is fascinating. On the face of it, it's just melting metal and at best maybe pouring the alloy at the right temperature but it is a complex science and one that you must understand when supplying parts that often have a vital role to play in the functioning of a machine or system.

The elements

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The Elements

All of the elements have both benefits and down sides (even when added at the correct percentages). The common grades of aluminium are balanced to produce a certain set of optimum, physical characteristics and at the same time, be accepting of the trade offs.

Copper, Nickel, Iron, Titanium and Zinc are just a few of the elements that have a real noticeable effect on the performance of aluminium alloys. In just the right amounts they can enhance the performance of aluminium alloys.

Here's a few examples:

Copper - Increases strength AND increases risk of corrosion

Nickel - Improves strength significantly but can make castings hard and brittle.

Iron - Improves strength and wear resistance but can make casting more brittle.

Titanium - Makes microscopic grain structure finer, improves mechanical properties but add too much and casting can have hard spots.

Zinc - Can significantly increase strength but increases cracking risk in corrosive environments.

Tin - Increases softness and machinability but significantly reduces strength.

Silicon

Silicon

Not too dissimilar to ice crystal formation, silicon has a tendency to form large crystalline brittle structures within aluminium so it needs to be controlled. It has essential benefits and some trade offs.

Its key benefits:

Enhances fluidity so high silicon alloys are regarded as being highly castable for complex shapes and thin walled castings.
How does it do this? In LM6 (12%Si) for example, it doesn't enter a partial, early, solidifying phase. It remains liquid all the way down to it's solidification temperature (570C) and then suddenly, boom! Its solid. LM25 (7%Si) for example starts to solidify at 610C and enters a sludge/ solidification phase until it eventually becomes solid at about the same temperature of 570C.
Silicon forms hard particles within the matrix and they give the alloy excellent surface hardness and abrasion resistance in friction moving parts.
Silicon greatly improves heat resistance so for engine components exposed to repeated heat cycles, they will be cast in a high silicon alloy like LM13.

It's key downsides:

Silicon makes aluminium far more brittle so to off-set this, small amounts of sodium or strontium need to be added to alter the silicon structure on a microscopic level.
High silicon alloys are harder to machine so tool wear (and thus) machining costs can be higher.
Unlike lower silicon alloys like LM25, that respond very well to heat treatment, high silicon alloys like LM6 do not but here's where it gets complicated; if copper, magnesium and nickel are present, like in high silicon LM13, alloys with high silicon can be heat treated, strength improves greatly and ductility becomes much lower.

Modification

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When silicon solidifies in an aluminium alloy, it behaves like marathon runners, all wanting to go in a particular direction. Large, straight crystal structures of silicon form, and this is a problem. So, sodium is added (the modifier) and these have the same effect that Tigers would at the London Marathon. Runners (silicon) scarper in all directions and for silicon crystal formation, this is great!

Modification

If a high silicon alloy like LM6 were not treated with a modifier, the silicon would solidify into coarse, needle-like brittle, crystalline structures. Silicon dominates the solidification pattern. The alloy would have very poor flex characteristics and the castings would crack easily.

Modification with either Sodium or Strontium alters the solidification process. Tigers (sodium atoms) send the runners (silicon) scattering in every direction imaginable making them do tight twists and turns as it cools. The end result is not a linear crystal-like path but a tangle of millions of different pathways resulting in a tightly woven network of silicon. This removes the linear pathways that can cause cracking and the new, woven silicon structure can flex and bend when stressed.

So silicon modification is achieved by sodium or strontium bossing/ disrupting the silicon formation pathways while it cools.

Once sodium has done it's job, it largely disappears out the alloy structure and gets pushed out in the form of Sodium Oxide and some very microscopic clusters of sodium form that have little negative effect and remain locked in place.