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.
USING THIS INFORMATION
First, when confronted with a choice, consider that billet aluminum is typically superior to cast aluminum in strength and surface quality. That can be important when shopping for components to use in high-stress locations or when your customer desires the very best surface finish. When considering strength, remember that not all billet aluminum is the same. Again, extruded billet is stronger than billet stock, and forged billet is stronger than extruded billet.
Consider the cost. Permanent mold cast-aluminum wheels, for example, are usually half the cost of forged billet wheels. Not every customer wants or needs the strongest wheel made. A cast-aluminum wheel may do just fine.
Realize that while alloys like 6061-T6 have good corrosion resistance, they are not as easy to maintain as when they have been chrome-plated, powder-painted or anodized. If I were doing business near the ocean, I wouldn't sell bare billet aluminum parts; it's too much work to protect them from corrosion.
Another thing to consider is stress risers. This is a term that relates to the way a part is machined and how that can create a weak area that's prone to fracture. When shopping for accessories or inspecting billet and cast-aluminum parts, take notice of any areas that have been machined to a sharp corner. If that sharp corner is located where it experiences high loads or intense vibration, you have a recipe for failure. I've seen fractures in engine cases, wheels and brake pedals start at sharp edges in the machining. Look for machined edges with a radius that will reduce stress risers.
One way to avoid weak products is to use aluminum products that have been thoroughly tested before going into production. Ask company representatives about the testing they perform. Ask whether they perform strength and durability tests prior to going into production, and whether all new designs are tested. I've talked to some wheel manufacturers that use forged wheel blanks that are tested, but then they machine those blanks, and that changes the characteristics. When you're shopping products for use in high-risk areas, like wheels, brakes, suspension and drivetrain, it pays to stick with the best and avoid the rest. Don't turn your customer into a test dummy.