Epic2112

So, what's the difference? I understand that forged seems to be more desireablt because it's stronger and lighter, but what is the actual difference? What are the processes for forging and casting?

User 41221

Cast is molten metal poured into a mold, whereas forged is a chunk of metal hammered into shape. Forged is MUCH stronger and usually lighter as well.

Regards,

badass951

Apparently forged is also denser and therefore has a higher tolerance to wear. Forged is always more desirable in a high performance engine.

Epic2112

So, what do a set of club sport wheels look like before they are hammered into club sport wheels? Surely not just a cube of aluminum.

Dmitry S.

They look like 17 inch clubsport wheels before they are pounded into being 16 inch clubsport wheels.

Travis - sflraver

Forged starts with a chunk of raw metal that was extruded/rolled into a long bar type shape. The particles are all aligned in the same direction and it is smashed into a shape in a press. When this happens the particle lines shape them selves to the contour of the part making it stronger where it needs it.
Cast parts are made by melting the metal and pouring it in a mold to get the shape desired. When the metal cools it leaves air bubbles in the inside due to the shrinking of the molten material. The metal is also in no particular pattern so its not as strong as forged.
Heat treatable metals are much better than both of the above when there is an application where they can be applied. They are cut into the shape needed (CNC) then the particles are aligned in the heat treating process to give it the perfect hardness and strength. When heat treated correctly these parts CAN have superior strength over cast or forged parts but only if care is taken to ensure the heat treat process is performed correctly to match the application.

inactiveuser92616

forged is only lighter than cast of equivalent strength. take a cubic inch of forged material and one of cast material, and the forged will be heavier. This is because the cast items have inclusions and voids, making them less dense but also less strong.

Casting is a much cheaper mfg. process than forging.

The technology that really interests me is metal matrix composite molding. They start with pellets of various metals suspended in plastics and waxes, mix the pellets to form the desired alloy, melt to a bondo like goo, and then injection mold them like plastics. The end result is very strong and has tight tolerances. I went on an engineering field trip to one of these places last year, it was really neat stuff.

Porsche-O-Phile

I believe that forging increases the relative hardness of the material due to induction of stresses in it. In the same way that Japanese katana swords are extremely hard (legendarily so) due to how many times they're "folded" in their forging process (in some cases, it was THOUSANDS of times), forged materials are bent / lapped back on themselves and then hammered down again and again, progressively inducing more and more stresses in the material. . . Look at a hardness versus stress-strain curve for steel (and I believe most other metals). It will show that hardness and ultimate tensile / compressive strengths increase with hardness, however ductility decreases. Thus the more you bend or stress a material the stronger and harder it gets, but the more inflexible it gets up to the point it experiences "permanent plastic deformation" and ultimately failure.

When you break a paper clip by bending it back and forth until it fails / breaks, you're actually making the material stronger and stronger (and harder and harder) up to the failure point. In the same way, this is done with forged engine components although it's not traditional "forging and hammering" you might think of that blacksmiths used to do, but it's similar. I beleve the material gains some of its strength from the manner of heating / cooling (e.g. "hot rolling" versus "cold rolling") but also from compressing and some sort of impact, hammer-like treatment to induce the stresses in the microstructure of the material in order to increase hardness. I'd have to go back to my strength of materials texts, but IIRC this is the idea. . .

Porsche-O-Phile

"Rapid quenching" a metal also induces / traps stresses in it as well. This is why smiths will quickly remove a completed item from the forge after final treatment and dunk it into water immediately in order to get the material to fully solidify with the stresses they've induced. Much stronger. I belive this is also done in the production of so-called "forged" components.

Rich Sandor

Kevin, a forged wheel starts off as a large hockey puck shaped chunk of metal, and slowly gets worked into the shape of the wheel.

As stated above, forged wheels are about 25% lighter than a cast wheel of the same size.

Also, as Jeff stated, forging strengthens the metal, but usually when you strengthen a metal, you are also making it more brittle. This is why Fuchs are more prone to cracking whereas cast cup replicas bend easily.

Epic2112

Interesting, thanks everyone

Ben Z.

Travis-
It's not an allignment of particles that increases strength.

Jeff-
You get the award for being the closest!

The way that metals deform is through movement of imperfections in the crystal lattice called dislocations. Here is a picture:



There are various types of dislocations: screw, edge, mixed, but they all do the same thing of moving one plane of atoms at a time. What happens when you work (forge, bend, etc) a piece of metal, more and more of these dislocations form and move. Eventually they begin to interfere with eachother and/or pile up at the boundaries between particles (called grains) in the metal. When this happens the metal begins to get harder to bend (and less ductile), thus stronger.

The ellimination of voids in the metal is also a major benefit of forging. Since all metal was cast at one point, it has had the cast structure at one time (generally dendritic, like snowflakes). Depending on the method of forging, you can actually change the size and shape of the grains. For example, if you work a metal until a bunch of dislocations have formed (these dislocations have high-energy), then you heat it up, the material will try to go to a lower energy state (like most things in nature), and it does this by getting rid of all the defects and ends up forming brand new grains. This is called recrystallization.

Sorry if this is too much info, just trying to clear a few things up.

Epic2112

I hope there is not going to be a quiz.

Ben Z.

Don't worry, it's a pretty tough concept even for the people in the field (I'm not one of them). Some of the stuff is still contested (the piling up at grain boundaries, iirc). It is pretty cool to see the high mag TEM images where you can actually see the dislocations. They look like a pile of tangled spaghetti.

Basically, forging allows you to customize and maximize the metal's properties, but costs more. Casting is cheaper, but you are limited to the original properties of your metal. Although you can heat treat cast metal, but that is a different story.

Epic2112

Isn't heat treating basically, just heating it up until it glows red and then dropping it in water, basically what Porsche-O-Phile was talking about?