Forging Versus Casting

I found this in a comment on Tam's blog. I think it gives a pretty good explanation of the relative merits of making metal parts via casting versus forging. In this case we are talking about making parts for guns, but it applies equally well to any kind of metal parts.

Roberta X: Your preference for forgings is not superstition. Unless the casting process is exquisitely well-controlled, dissolved gases will undissolve as the metal cools, forming bubbles ("voids"). Forging crushes the voids and pressure welds them so they will stay closed. It also smears out other inclusions so they are less likely to cause failures, and finally it work hardens the metal.

The reason for using castings is always cost. Forging and then milling to final shape is expensive. Casting costs considerably less, and with alloys like Zamak that don't contract when they freeze, there will be considerably less final milling. But there's good reason to be leery of castings where strength is critical.

One example: A motorcycle company once decided to save a buck or so on it's brake levers by changing from forged aluminum to cast aluminum. They also contracted the castings to the low bidder. The result was voids in the castings and a series of usually fatal accidents where the levers broke just when the rider was braking harder than ever before.

Now, selecting a casting wasn't the whole problem. There are foundries that could cast that aluminum alloy with no voids, but purchasing selected a foundry with experience mainly in decorative castings. Secondly, nowadays it doesn't cost that much to x-ray every casting. You don't have to have someone standing there studying the picture, either, automated inspection computer programs can do the job.

And then there's the second issue: with castings, good performance in casting becomes a more important criteria in selecting the alloy than the final mechanical properties. A steel slide with the same design defects might fail at the same spot, but with the right alloy the slide would deform and jam so you could not fire the gun again before it cracked all the way through and sent pieces flying. With firearms in particular, the failure mode matters!

I was going to say that the arguments about cost are predicated on mass production, but even if you are just making one, casting can be quite a bit quicker. You do have to make the mold, which can be just as difficult as making the actual part, but you can use softer materials like wood or aluminum. Forging is another process entirely, and you wouldn't use it if you were casting a part. There are ways to make any of these processes more efficient for mass production, but in all cases they require expensive machines.