I know the difference between cast and forged(cranks etc..). Where does billet come in? What is stronger billet aluminum or forged?

Can someone explain the difference between cast, forged, billet, spun (centerline wheels?), hammer forged in plain terms? and the advantage one has over the other.

Thanks in advance, Ray

Lets get rid of the odd man out here, which is spun.

A spun wheel starts life as aluminum alloy in a powdered form. It is loaded into a die extruding machine (Imagine a heavy duty tube of tooth paste full of powdered aluminum) and the pressure is applied. At the end of the die the aluminum appears as a plastic (near molten liquid but not quite) which is attached to a spinneret (bobbin or aluminum hub) and as the it rotates the line of extruded material adheres to what has been extruded before it forming a disc shape. By moving the disc through a plane any round shape desired can be formed. This is the exact opposite of a billet process. In spinning a wheel (somebody probably got the idea from watching a spider) we create a shape in space with no (well very little, it still has to be machined to final form) wasted material.

Billet is just that a billet, or ingot, or slug, or whatever word you would use to describe a big gob of material. The automotive parts we use inside our engine are machined out of a solid billet which has usually been forged prior to being machined. An ingot is a word used to describe what comes out of a hole in the ground that someone just poured molten metal into (after it has cooled to a solid). If we take a pure ingot of a metal like steel and reheat it to red-hot or above (nearly plastic) and then put that in a big forge and drop the hammer (forges are larger scale versions of an anvil and a hammer that used to be water wheel powered) we will compress all the grains of metal in the steel to flow in one direction. By rotating the piece being worked on, and repeatedly hammering it until it cools to the lower limit of it's working temperature we align all the steel grains in one direction. That big gob of material is now called a forged billet.

Any force trying to work across those lined up grains has a harder time bending or deforming it than a similar cross section of unworked raw steel of a similar composition (metallurgy). We have mechanically altered the normal grain structure of the metal by work hardening it. If we now take our billet and whittle away all of the excess material to form a shaped piece that you or I would recognize as a connecting rod, you would have a billet connecting rod. We would also have a lot of random pieces on the floor that cost a lot of money to get there. That is why billet pieces cost more than forged pieces (someone has to pay for time and material even if its now just scrap).

Another more cost effective way of getting a forged connecting rod is for someone to take that ingot of uniform steel material we poured earlier and carve it up into a bunch of connecting rod shaped blanks. That requires a lot of work and we now have a lot of connecting rod shaped blanks, which no one really wants. But if we invest some more money we can make a connecting rod shaped die (think of it as a very precisely machined and shaped Jell-O™ mold) and put that in our forge. Now if we heat all those of connecting rod shaped blanks in an oven till they are all red hot, and throw them in the die one at a time, we can hit them with our forge forcing the piece to conform to our die. That lines up a lot of the metal grains into the shape of our of connecting rod (but not all of them). Now we have a whole bunch of forged connecting rod shapes which if we machine each one we can have a whole bunch of connecting rods (economy of scale). They are not as strong individually as the billet, but they cost less to make on a per unit basis.

Now if we really want cheap, we avoid all this silly forging stuff, and go right for the mold; pouring the molten metal into that. Out pops a bunch of connecting rod shaped blanks ready for machining. Of course they are made of cast iron usually, but we could have poured any metal or metal alloy (that would be using metallurgy to increase strength).

Casting has inclusions (stuff that we don't want, but it gets there anyway) which are inherent to the casting process. Castings are generally not as strong, and are more brittle (the cooling process could be controlled through annealing and tempering to obtain less stress in the native metal, but that cost more money and casting implies economy class) than machining from bar stock. Forging, especially when properly heat treated yields a much stronger, and potentially lighter piece (since it's stronger we use less of it). Billet is the ultimate in strength and we can machine the desired part out of any alloy billet we choose for the ultimate in strength (but by it's very nature is not cost effective being every piece is one of a kind).

Hope that helped


Larger Dave

Dave, I'm pretty sure spun where a flat piece of metal is spun and formed very much like a jug on a clay-wheel.. Many pots, pans, bowls etc are made this way and no doubt the technique has been tried on countless other items including wheels. In fact, I think the rims of steel wheels are spun. They're definitely not cast, forged, billet or sintered.

Your explanation of spun sounds like sintered.. Like a brake pad or many OE connecting rods now.
:)

The process you are describing I was taught was called peening (as in Ball Peen Hammer) were a piece of metal is stretched to shape by working it with either rollers or hammers (like in a scale removing pneumatic hammer). That is possible and matched the tool marks I see looking at the wheels. The process I described is used in aerospace to make many small spherical or round parts; could be very mistaken about the wheel. Sorry for the confussion, I will have to research this and get back with you.


Larger Dave

Great Reading this stuff from Larger Dave well written and worth while

I spent many years running large steam and air hammers. I made thousands of crankshafts and other parts for auto and defense industries. Hard, dirty and dangerous work but forgings are probably the strongest pieces out there....

Thanks Larger Dave!
Awesome reading. Just printed it out and will be taking to work on monday.

The reason I asked the question is because we kind of had a discussion/disagreement at work about it. I think it started with which wheel is stronger then progressed from there LOL.

Went something like this... "If billet wheels are stronger then why are all the bling bling type street wheels billet while all sfi drag wheels/racing wheels forged?!....."

Hehehee then it turned into a cross between Tim "the tool man" Taylor and Cliff from Cheers explaining metallury!if you know what I mean. LOL!!

Thanks again, Ray

Because it takes a lot of money to 1) make the forging die, and 2) to get the SFI certification. It's less expensive to whip out a set of rims on a HAAS than build a forging die, especially on short production runs. How many OEM billet machined, vs. billet forged, wheels are out there?

How much would you trust a rim that is whittled out by someone who thinks they have a sense of style but no sense of engineering? Take a look at some of the really blingy wheels, and imagine what they would do if pushed hard. Kinda scary.

Actually there really is a spinning process that has a flat piece of aluminum turning and it is formed over a wooden form.:thumbsup:

I agree:

With wooden bucks, a dolly hammer, English Wheel and a solder torch you could make a lot of sheet metal parts in any shape you want. Perfect example; the AC Bristol better known to Amerikuns, as the AC Cobra (Invented out of thin air single handedly by Carroll Shelby).


Larger Dave

I agree with GaryL on the spinning process of wheels. It has nothing to do with english wheels, dollys and hammers. It is a true production process. I understand that parts like the Cobra was a designed with hammers, dollys, and english wheels by Shelby, which is the same process NASCAR teams use to form fenders and quarters on Cup cars. But, there is a spinning process that for mass producing products like wheels. Kevin

Here's another indicator of Chevrolet's attention to detail: Cast spun wheels replace last year's (2001) forged aluminum (Corvette) rims for 1.3 pounds of weight reduction and GM Cadillac is going to spun wheels for 2007.

"In the spinning process, the wheel's center disc, or spider, is revolved rapidly in a machine that is equipped with a mandril and pressure tools. The mandril is shaped like the inboard surface of the spider itself, and the pressure tools, which are round, are pressed against the outside surface of the wheel during the production cycle, forcing the component to take the shape of the mandril." Here is were to find the full article.

http://www.findarticles.com/p/articles/mi_m3MKT/is_v92/ai_3281952

Another article I read in a Circle Track Mag states:

"The spun process is done a little differently. The original tube of metal is larger than the die and placed outside of it. It is then spun into the die and tightly against it. Since the die is machined perfectly and the shell is forced to conform to that shape, the result is a much better, almost perfect run-out with control of critical thicknesses in areas where we would see the most flexing and stress.

Spun wheels are more expensive due to the extra time in the process, the expense of the CNC machinery, and the quality control that is incorporated."

So it's more like I described using powdered technology than a clump of aluminum on a pottery wheel I think.


Larger Dave

That's exactly what I was refering to Dave. Interesting article especially since the article was written in 1984. The spinning process for mass production has been around for some time now. :D Kevin