Info:- Alloy Wheel Construction Techniques
Nowadays, there are basically three ways in which alloy automotive wheels are constructed. The three types of alloy wheels can be referred to simply by their common names: cast, billet and forged. Let's take a look at these manufacturing processes, and how they make the end product better or worse.
Casting is a relatively inexpensive way to produce a high-quality, fairly strong alloy wheel; many aftermarket alloy wheels designed for street use are made this way.
In common gravity casting, the wheel maker begins with a prototype "plug" that is used as the positive to produce the mold. This plug is usually made by machining a piece of material (often plastic or other phenolic material) on CNC machining equipment to produce a highly precise model.
The "negative" is then made from the positive by pressing casting sand around it. The sand is actually a composite slurry that, when compressed under high pressure, becomes quite hard. Think of what happens when you walk down the beach: Your foot compacts the sand and makes a very accurate, very stable negative impression. It's the same principle, just with high-tech sand.
Next, molten aluminum alloy is poured into the sand mold and allowed to cool. When the sand is broken away, you're left with a wheel that only needs minor finishing (like drilling and possibly trimming of some excess metal) to be considering complete.
Negative pressure casting is a similar process, but instead of pouring the molten material into the mold, the molten alloy is drawn up into the mold using a high-pressure vacuum. This eliminates much of the trapped gas found in the gravity casting process, producing a stronger wheel that is much less porous than a gravity-cast one.
Billet wheels are machined from a solid chunk, or "billet," of material. First, a telephone pole-sized piece of aluminum alloy is produced (or bought from a vendor). Since this piece of stock is generally extruded, the grain runs through the stock, much like the fibers within a single strand of wire. The stock aluminum is then sliced up into sections which are machined down into either complete wheels or just wheel centers.
Since they retain the grain structure of the extruded stock material, billet wheels are extremely strong. This grain structure, which is not present in a cast wheel, gives the final product a backbone--makes the wheel even stronger without adding weight.
Of course, billet wheels are also extremely expensive to produce because much of the original material is wasted. A lot of time is also spent machining the original stock down to a finished wheel, which only adds to the cost of the final product.
Actually, most "billet" wheels are actually billet centers bolted into stamped or spun rim halves. Entire wheels forged from a single billet are so rare as to be almost nonexistent, and are usually seen only on show cars. Billet centers on multi-piece wheels, however, are common.
Unlike casting or machining (billet), forging uses intense heat and pressure to transform a slug of alloy material into the final shape of a wheel. Under this heat and pressure, the original grain structure of the stock material is forced from the center of the wheel towards the outer edge. This grain structure is even stronger than the one found in a billet wheel because it runs along the spokes and serves to further strengthen the forged wheel's spokes, while the grain in a billet wheel simply runs through the spokes. Thanks to this process, a forged wheel can be up to 300 percent stronger than a cast wheel. Additionally, since forged aluminum is stronger than cast aluminum, less material is needed to produce the wheel, resulting in a lighter product.
When shopping for a forged wheel, you may want to ask how close to net the forging is--the closer the forging is to the final product, generally the stronger the wheel.
Because of the basic limitations inherent in forging, most forged wheels are two- or three-piece units. In two-piece construction, a center is forged and then welded or bolted into a spun or stamped outer rim. In a three-piece wheel, the center is bolted to an inner and an outer rim half. Three-piece wheels have the advantage of being easily customizable for a variety of widths and offsets. Crash damage in the form of bent rim outers can also be repaired.
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