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thetunersgroup
02-07-08, 02:08 PM
This is the first of a series of articles we are publishing on the forum about different manufacturing techniques used in the manufacture of automotive parts.

Our goal in publishing these articles is to help people under these processes better, in a light and easy to understand article style format.

Enjoy ...


CASTING VS FORGING VS MACHINING IN METALS

CASTING METALS

To quote from Wikipedia,

"Casting is a manufacturing process by which a liquid material is (usually) poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solid casting is then ejected or broken out to complete the process."

http://www.gmaccessorydealer.com/wheels/forgedversuscast.asp
reads ...

"The quality of cast wheels varies dramatically, depending on process, and sometimes on variables beyond the control of the manufacturer, such as ambient temperature or even humidity ...

Castings tend to be porous -- some carburetors actually leaked fuel through the metal, with no crack or visible flaw present. Porosity is bad, because it means there are places where the metal isn't in direct contact with more metal on all sides.

Voids, which tend to form in the spaces between crystals (a chicken and egg situation), are where cracks begin.

Larger, chunky grains may beget larger voids, and cracks along crystal boundaries will have farther to travel. All these points mean that cast wheels must contain more metal to achieve an acceptable strength, and are thus heavier."

Here is a photo of air bubbles within a cast part (in this case a cast golf club head) ...

http://golf.mizunoeurope.com/images/news/nagai-interview-bubbles.jpg


FORGING METALS

To quote from Wikipedia,

"In modern times, industrial forging is done either with presses or with hammers powered by compressed air, electricity, hydraulics or steam. These hammers are large, having reciprocating weights in the thousands of pounds ...

Forging results in metal that is stronger than cast or machined metal parts. This stems from the grain flow caused through forging. As the metal is pounded the grains deform to follow the shape of the part, thus the grains are unbroken throughout the part. Some modern parts take advantage of this for a high strength-to-weight ratio."

Here is a diagram of that process ...

http://www.scotforge.com/images/photos/graingrowth3.gif

Here are some diagrams of a metal part (shown in red) being made using open die forging ...

http://www.scotforge.com/images/photos/rprocess1.gif
http://www.scotforge.com/images/photos/rprocess2.gif
http://www.scotforge.com/images/photos/rprocess3.gif

http://www.scotforge.com/sf_facts_forgingadv.htm
has some diagrams showing the differences in grain flow between forged, machined and cast parts.

As it says on that page ...

--------
"Forged Bar:
Directional alignment through the forging process has been deliberately oriented in a direction requiring maximum strength. This also yields ductility and resistance to impact and fatigue.
http://www.scotforge.com/images/photos/grain1.gif

Machined Bar:
Unidirectional grain flow has been cut when changing contour, exposing grain ends. This renders the material more liable to fatigue and more sensitive to stress corrosion cracking.
http://www.scotforge.com/images/photos/grain2.gif

Cast Bar:
No grain flow or directional strength is achieved through the casting process.
http://www.scotforge.com/images/photos/grain3.gif

--------

http://www.scotforge.com/sf_facts_companalysis.htm
reads ...

--------
"When Compared to Machined Bar, Open Die and Rolled Ring Forged Metal Parts Deliver:

- Contoured grain flow yielding greater impact and directional strength
- Cost savings in material and reduction of waste
- Less machining and longer tool life
- Broader material options and size ranges

When Compared to Castings, Open Die and Rolled Ring Forged Metal Parts Deliver:
- Directional grain flow and superior final part strength
- Structural integrity and product reliability
- Reduced process control and inspection requirements
- More predictable response to heat treating"
--------

To understand forging's effect on metals in visual terms, take a look at this very high resolution photo of a scan of a sectioned, forged conrod that has been etched to show grain flow.

To take this photo, they cut a forged connecting rod in half, then etched it to highlight the grain flow ...

Here's a low resolution preview photo:
http://upload.wikimedia.org/wikipedia/commons/thumb/5/5c/ForgedConrodShowingEtchedSection-s.jpg/180px-ForgedConrodShowingEtchedSection-s.jpg

To view the grain structure in high resolution, click on this link ...
http://upload.wikimedia.org/wikipedia/commons/5/5c/ForgedConrodShowingEtchedSection-s.jpg

Looking at the high resolution photo, notice how the grains in the metal are very tightly packed, and they are aligned with the shape and curves of the connecting rod.

This tight density of the grains, and the alignment of the grains, is what gives forged parts their strength. They are simply denser and stronger than a machined or cast part can ever be.

http://www.gmaccessorydealer.com/wheels/forgedversuscast.asp

reads ...

--------
"Forged wheels take advantage of what happens when metal is cold worked. Cold working doesn't necessarily mean you'd want to touch the materials while they're in process, it means the procedures are done at a temperature below the point where the metal starts to melt and regrow a new crystal structure ...

Forging also changes the shape and alignment of the crystal structure. When molten metal solidifies, its grain structure is non-directional, amorphous, grains in the sense of "grains" of sand.

As metal is forged, these grains are stretched in the direction of deformation, making them more like the "grain" of wood. The metal is formed so the grain goes in the directions where strength is needed most.

Think of particle board versus real wood. One is cheap, heavy, and easily formed into a variety of shapes. The other is strong and light.

The forging process, because of the vast pressures involved, also compacts the metal, eliminating porosity and the voids that can be a source of cracks or corrosion. The result is that less metal is required to achieve a given strength, meaning lighter, stronger wheels can be made."
--------

If you'd like to learn more about forging, visit this page ...

http://www.scotforge.com/sf_facts_solution.htm


FORGING EXPLAINED VIDEO

Forging is also used widely in the manufacture of high end golf clubs. The process is equally relevant in terms of car parts.

There is a fantastic online video about how Mizuno forges golf clubs in Japan hosted by Masao Nagai, Mizuno's director of global golf Reseach and Development, which reveals the secrets of Mizuno's forging process.

The video has some excellent info about casting vs forging.

You can view it at

http://golf.mizunoeurope.com/videos/index.php?article=2007_05_29_artofforging

Quite simply forging increases:

1. Directional strength
2. Structural Strength
3. Impact Strength

If you only take one thing away from this article, remember this ...

As it says at
http://www.scotforge.com/sf_facts_solution.htm

"No other metalworking process can equal forging in its ability to develop the optimum combination of properties."

- Adam

Frogger
02-07-08, 03:52 PM
Thanks for sharing. Looking forward to more tech write-ups!

thetunersgroup
02-07-08, 03:54 PM
Thanks for sharing. Looking forward to more tech write-ups!

Hi Frogger,

No worries - I'm glad you enjoyed the article.

- Adam

Quacker
02-07-08, 11:05 PM
reminds me of my materials science classes for engineering science in high school as well as in uni for mech eng :D

well done Adam

DR-JEKL
03-07-08, 09:28 AM
Nice work mate, this helps explain to the young guys why you pay extra for high quality BBS/Rays rims which are forged compared to cheap cast rims from Tempe/Ozzy etc.

thetunersgroup
03-07-08, 11:27 AM
Nice work mate, this helps explain to the young guys why you pay extra for high quality BBS/Rays rims which are forged compared to cheap cast rims

Exactly right DR-JEKL :)

Most of the guys on here who have been around for a while already know this stuff, but the Mizuno forging video I mentioned is of interest even if people already know about forging vs casting. And the section about machining non-forged metal is interesting too.

And yes, this thread should be useful next time one of the new guys claims that cast knock offs of BBS / Rays wheels are as good as the real thing or claims that BBS / Rays wheels are overpriced etc.

A quick link to this thread in reply will save everyone a lot of time in explaining over and over why genuine rims are the go :)

I'll also add a section to this article about Rays own safety standards and manufacturing techniques.

- Adam