Forging of wheels or other parts

[randywebb]

How much stronger &/or lighter are various ways of making a part?

Say you wanted to make a... wheel.

Forging is the 'best' way to make a metal part like a wheel. The Fuchs are forged, of course. How much stronger is it than a pressure cast wheel.

And how close is gravity casting to the strength of pressure casting?

Where the Mahle gas burner Mg wheels gravity cast?

Later, the 917 wheels were Forged Mg -- but not cheap - $4,000 each, IIRC.


Or what about beams?

Porsche went from the steel front suspension cross-member to a lighter, cheaper, and stiffer, maybe not stronger(?) cast Al version, and then a forged Al version.


Here is part of a website that extols the glories of the forged front suspension cross-member:
http://www.pbase.com/slidevalve911rsr/image/73584707

[DARISC]

[QUOTE]Originally posted by randywebb
[B]And how close is gravity casting to the strength of pressure casting?

Ahhh yes! A burning question that I would like to know the answer to.

Are the cast wheels of Porsches, Ferraris (the ones that come on the cars, not aftermarket wheels), et al, gravity cast or pressure cast?

What well known brands of aftermarket wheels for Porsches are gravity cast and what brands are pressure cast?

By how many percent heavier are gravity cast wheels that are beefed up to be the equivalent strength of similar design pressure cast wheels?

Are gravity castings iintrinsically more brittle than pressure castings? Even when beefed up with more material (thicker sections/).

Do any auto manufacturers offer gravity cast wheels on their cars, as opposed to pressure cast wheels?

How much more expensive than gravity castings are pressure castings?

Are gravity cast wheels essentially a cost cutting measure that results in a wheel that is not the equivalent quality of similar design pressure cast wheels?

Are a predominance of aftermarket wheels gravity cast or pressure cast?

Anyone know if the upcoming Rota wheels (the 17" ones modeled after the Fuchs) are gravity cast or pressure cast?

Are forged wheels of the complexity and quality of Fuchs prohibitively expensive to manufacture today?

[Jeff Alton]

Randy, what about Billet? Throw that into the mix as well.

It would be really hard to give a good answer about how much better, lighter, stronger etc a part is based on the techinque used to make it.

How much extra weight (%) would you consider acceptable to make a similar strength product from say a casting as compared to a forging?

What if the cast part was 95% as strong and reliable as the forged part, but it turns out the forged part is overengineered?

There is so many variables that one would need to build dimensionally identical parts via the chosen methods and them test them to failure to see. Not only that, then one would need to beef up the ones that failed to make them last as long as the best one, and then decide if the added weight was a good trade off. (considering cost and tooling etc).

FEA will only tell you so much, the parts would need to be tested to see.

Mg versus Aluminum is another good question. What are the strength to wieght comparisons?

Lets face it, I think the general agreement is that a forged product offers the best strength to weight ratio. But it is costly to do.

We can also agree that OEM would using a casting process for economy reasons, not for a superior product. BUT, OEM also tests to make sure the product is safe. In the case of wheels, this is usually to a load of 695Kg, although some will test to 1.2- 1.4 the actual vehicle corner weight.

It is safe to say that a company making aftermarket wheels (one piece) casts them as well for cost reasons. They meet their testing requirements that I am sure they do, so why spend money on a forging?

Companies that build 2 and 3 piece wheels use a forging for the center section (or center + outer in the case of a two piece). But, they machine a forged "blank" to the style they want to produce. The "blank" is really quite inexpensive to purchase in bulk. Part of the reason to build a three piece wheel as forged part is because no one would buy it if it was cast because all of the competition builds forged ones.... Also, the typical customer is after a light weight product as well.

Hope I did not ramble too much, but it is a difficult question to answer. Some processes fit some needs better than others.

Cheers

[WolfeMacleod]

I don't kno what the difference would be between forged vs cast wheels, but I can tell you there is an ENORMOUS diference between a forged and a cast sword

I'm assumign the differences would be similar.

[Jeff Alton]

I agree that there is a large differnce, in stength for a given wieght when it comes to wheels.

Tell us about the swords, it will help the discussion.

Alf, you out there, help us out with the swords too!

Cheers

[DARISC]

[QUOTE]Originally posted by Jeff Alton
what about Billet?

A billet is a casting (an ingot, or cut from a ingot). The forging process (hammer, hammer, hammer) work hardens the metal as it is forged (beaten with mighty blows) into shape (like a sword).

The physical characteristics of a billet's metal are not changed by being sculpted into shape on a multi-axis CNC milling machine - the density of the billet/ingot is the density of any gravity casting (ingots are just pours of molten metal into a form, no pressure involved).

CNC cutting a wheel out of a billet is good for one off designs and prototype shapes, but are essentially gravity cast.

However, if the billet is initially formed into its shape by forging it, then the metal's properties are changed and I'd guess that a wheel CNCd from a forged bullet would be suprior to one made from an unforged billet.

Any metallurgists looking in?

I've always been knocked out by this image, wnich has been posted before by Jack Olsen and others -

[DARISC]

[QUOTE][i]Originally posted by WolfeMacleod
there is an ENORMOUS diference between a forged and a cast sword

Brittleness being one of MAJOR the differences, no?

[randywebb]

I agree and think we can pretty much rank a raw material in a simple shape (wheel?, beam?) by the way it was formed.

It may be that modern pressure casting can produce a part as strong, or stronger than forging did at one time - but I'll bet that time was not the 1960's.

Pressure casting must be able to produce acceptably safe wheel for high performance street cars or P Ag would not use them on the 997. The C-GT uses Mg wheels - dunno if those are forged or not. I do know a lot of problems with Mg were actually problems with the alloying technique or something.

[Jeff Alton]

I think a wheel can be made plenty strong by any techinque. I am sure a new Porsche wheel is a quality wheel.

But, going back to your original question..... Could it be stronger/lighter if it was forged? Who knows?

From a manufacturing standpoint, the wheel only needs to last the design life of the car. So if the car is designed for a 5 year life span (like some US made cars) then why build a 30 year wheel? Anyone know the design life span of a current Porsche?



Cheers

[DARISC]

[QUOTE]Originally posted by randywebb

Pressure casting must be able to produce acceptably safe wheel for high performance street cars...

Yes, and that is why I have a burning desire to know if gravity cast wheels can meet those acceptably safe standards.

[Jeff Alton]

[QUOTE]Originally posted by DARISC

Originally posted by randywebb

Pressure casting must be able to produce acceptably safe wheel for high performance street cars...

Yes, and that is why I have a burning desire to know if gravity cast wheels can meet those acceptably safe standards.

Don't you also want to know what the standard is??

Cheers

[DARISC]

HAHAHAHA!
Thank you thank you Randy, for starting this thread!

Cheers as well

[WolfeMacleod]

[QUOTE]Originally posted by DARISC

[i]Originally posted by WolfeMacleod
there is an ENORMOUS diference between a forged and a cast sword

Brittleness being one of MAJOR the differences, no?

Correct

[chrisp]

Although not answering your question, it's worth noting that even in forged wheels there is a higher and lower standard. Although forging is usually judged as the best way to maximize the design, some wheel companies will machine from a forged piece of billet and some will shape the wheel during the forging process. When a shape is forged it develops a "grain" which is the flow of the material during the forging process. If the shape is a square block and is then machined, many of the cuts will cross the grain whereas others will run along the grain.

The really expensive forged wheels (like BBS racing wheels) will have a tool shaped like the final wheel and will forge the final ("net") shape. It's expensive, more involved, and requires a unique tool for each final part.

The normal forged wheels (Fikse, CCW, etc) will machine from a piece of forged billet. Still very strong but the final shape requires cuts across the grain which requires them to slightly overbuild the wheel to ensure strength. Although slightly heavier it saves from having to create a unique tool for each wheel center. This is one reason why the machined companies have so many styles whereas net shaped forged wheels (like BBS) are usually only available in one or two patterns.

[island911]

In engineering school the POV is about how close can you get a metal to it's theoretical strength. If you have a perfect microstructure, you've got a winner. Most all of our mfg processes compromise the microstructure. (casting being about the worst) Even forging, is compromised. However, forging allows for the compromises to be controlled better than say casting.

If you choose not to control the microstucture (casting) then the approach is make a bunch and toss (remelt) the bad ones. It's a statistical thing. Porsche saw how cheap it is to tool and create cast wheels . .. AND they reallized that they could x-ray each one, toss the internally cracked ones, and still make money.

Of course, there is also the sales approach of letting the customer buy the cheap Chinese set of shine cast wrenchs for $9.95 (Lifetime warrenty) ... they sell and just replace the cracked ones . .if anyone is willing to step up and admit their expectations of cheapness.

[DARISC]

Originally posted by island911
Most all of our mfg processes compromise the microstructure. (casting being about the worst)

If you choose not to control the microstucture (casting)......

As opposed to gravity casting (cheap aftermarket wheels), does pressure casting (O.E.M. and high end aftermarket wheels), to any degree, "control the microstructure" of the metal?

Or is the fact that pressure cast metal is denser (and less prone to having voids and bubbles) than gravity cast metal the primary reason that it yields a stronger result?

Is the difference in cost between gravity casting and pressure casting as great as the diference in cost between fully forged wheels (Fuchs, BBS) and forged billet/machined wheels (Fikse, CCW)?

Are any of the "cheap Chinese sets of shine cast" category aftermarket wheels on the market today pressure cast?

Or is pressure casting technology used primarily by automakers and high end aftermarket wheel makers, who are willing to spend more money for wheels of higher quality to help protect themselves from lawsuits in the event of wheel failure, wherein claims of inferior gravity casting technology (as opposed to pressure casting technology could be made - y'all know how pesky them derned persecutin' attorneys kin be!)?

[rfn026]

Keep in mind that for wheels sold in the United States aftermarket there are no standards. SEMA keeps hinting that they might develop some sort of standard but the wheel companies are fighting this.

Most of the rice burner stuff is gravity cast in China. This is the lowest end of the food chain.

btw - This is a subject no wheel company, except BBS, wants to really talk about.

Richard

[ljibis]

Originally posted by DARISC
As opposed to gravity casting (cheap aftermarket wheels), does pressure casting (O.E.M. and high end aftermarket wheels), to any degree, "control the microstructure" of the metal?

No, a casting is a casting. The crystalline structure of the metal in a casting is determined by the type of metal being cast, the dimensions of the finished piece, and it's rate of cooling.

Cast pieces cool from the outside in. The outside layers of metal have a different crystal structure than the inner layers because the outside cools faster. The crystalline structure grows from the outside in. This results in a selective orientation of the slip planes.

Near the center of the piece the cystallization takes place slowly, so the crystals tend to be larger and have a random orientation. Thus the outside of a cast piece tends to be stronger than the inside. This is not necessarily a bad thing, as surface wear resistance can be a good quality for many items. But your inner structure will be more ductile, and will flex with impact.

As has been stated previously, forging mechanically stretches and orients all the crystals in one direction which provides superior strength. Your material is consistently strong all the way through. You also have a weight advantage, since you can get more strength out of less metal in a forging.

Originally posted by DARISC
Or is the fact that pressure cast metal is denser (and less prone to having voids and bubbles) than gravity cast metal the primary reason that it yields a stronger result?

You got it. Using pressure to fill your casting gives you better control over your process. You can make more complicated parts using pressure casting than you could with a gravity cast.

Pressure casting shouldn't necessarily be seen as a sign of increased quality or strength. It does note, though, that they relied on more than gravity to fill the molds. Better process control usually results in better quality, and I would argue that your average pressure cast wheel will always be better than your average gravity cast wheel.

[DARISC]

Originally posted by ljibis
Better process control usually results in better quality, and I would argue that your average pressure cast wheel will always be better than your average gravity cast wheel.

Given that, in terms of safety and performance, is it foolhardy to choose, based on aesthetic and cost saving reasons, a gravity cast wheel over a pressure cast wheel?

Especially for use on a high performance car?

Or, should it be of no concern because pressure cast wheels are not "that much" better?

If the latter is the case, then why does "no wheel company, except BBS, want to really talk about it"? (I realize that BBS are forged billet/machined wheels but the question still seems relevant)

[island911]

Originally posted by randywebb
...
Forging is the 'best' way to make a metal part like a wheel. The Fuchs are forged, of course. How much stronger is it than a pressure cast wheel?
....

That's a tough question.
First, castings vary in quality, due to process controls and even operators.
Second, the knock off (cast) would need to be designed for casting. ...That is, the geometry plays a big role in how the structure performs. (both in the casting/flow/cooling and the net shape when loaded/load path (compression vs tension)).
Third, there is the consideration of life cycles. The initial "test strgth" may be close (w/ a heavier structure), but the cast has a much lower life expectancy. (given the same stress levels)
Fourth, the type of aluminum you will need for casting pour, vs what you can use for forging. (this is a biggie) ... a 2014-T6 kicks a 295-T6 butt (70 vs 36ksi UTS) ...guess which is for forgings.

Yet, there are those who will maintain (hope?) that "new technology" will bring / has brought casting up to competitive performance. (shudder)

Of interest is Alcoa and their ubiquitous forged Aluminum truck wheels. The common placeness of forged wheels in even trucks says quite a bit about the cost/performance of forged vs cast wheels.