This Harley-Davidson billet aluminum air cleaner cover was forged, machined, polished and chrome-plated. The forging is on
the left, the machined piece on the right, and the finished piece in the center.
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Few Accessories are more commonly found on customized motorcycles, ATVs and UTVs than those made out of billet aluminum. That
said, few people working in parts or service know what billet really is or what advantages it has over the cast-aluminum parts
they often replace.
First, the difference between cast aluminum and wrought billet aluminum. Cast-aluminum parts are made by heating the aluminum
alloy to a molten state so it can be poured into a mold. The type of mold and the processes used determine the type of casting
and the characteristics of the finished product:
- Sand-cast parts, made using sand molds, have the most surface porosities.
- Permanent-mold parts, made using metal or ceramic molds, can have very good surface characteristics.
- Die-cast parts are similar to permanent mold parts, but their makers use force to push the molten aluminum into often intricate
molds, such as one for a cylinder head with thin fins. The die-cast process produces surfaces that are typically smoother
and more detailed than those found on parts made with a permanent mold.
The advantage to cast aluminum is speed of production. The disadvantage to cast aluminum is the surface of a cast part is
different than its center in quality and characteristics. That's because during production the molten aluminum that touches
the mold (the exterior) cools to a solid state quicker than the aluminum in the center.
Then comes billet aluminum. It too starts as a molten mix of pure aluminum and other metals and minerals to create the desired
aluminum alloy. But here, the molten mix is cooled to the consistency of putty and then forced through a die to produce shapes
such as plate, square and round billet stock. Thousands of different shapes, sizes and alloys can be purchased from metal
yards around the country. The billet stock is then machined into the desired part. The big advantage billet has over cast is the characteristics of the surface are the same as the center's. That means when
you're machining or polishing the billet, you won't run into porosities or areas of extreme weakness as you machine into the
center.
6061-T6 BILLET ALUMINUM
The aluminum alloy 6061-T6 is used most often by manufacturers of custom powersports parts because it
- has excellent strength,
- weighs about half to one-third the weight of steel,
- resists corrosion,
- is economical compared to other alloys, and
- is very machineable, meaning it has a smooth surface after being machined, without excessive dulling of the cutting tools.
The designation 6061-T6 refers to the ingredients of the alloy (around 10 different minerals and metals added to the aluminum)
and the heat treatment (T6 refers to an artificial aging process that's used to produce desired characteristics of strength).
For most customers who ask, it's easier just to say, "6061 is aircraft-quality aluminum."
ALL BILLET IS NOT THE SAME
Billet aluminum can be categorized into three groups: good, better and best.
- Good: As mentioned, parts machined from billet stock (plate, bar, square, etc.) are good because the qualities of their surface
and center are consistent.
- Better: Extruded billet aluminum is stronger than billet stock. An extruded part is created by heating the billet stock until it's
like putty and then forcing it through a die to create a final shape. This improves the grain of the metal, which now follows
the final shape, making the part less likely to fracture under load.
- Best: Forged billet aluminum is the strongest because the grain is improved to follow the finished shape, and the metal is made
denser from the forging process. Forging is the act of slamming the hot, putty-like billet stock into a die to create the
new shape, like a wheel, sprocket or air cleaner cover.
