Best Conrod Materials? 11

[jp83]

Hello, I'm new to this site. I hope someone may be able to help me.

I'm designing conrods for an 320hp, 4 cylinder, 2.0 litre engine. I'm basically looking for a material to use. Money is no real object, I just dont really have a direction to go with in the investigation. I have been told that EN24V steel alloy could be good, but I really want the lightest possible material that can get. I thought about titanium alloys?

If anyone has any suggestions for me, they would be much appreciated.

Thanks

 

[patprimmer]

Gary

All materials have some unique but limiting qualities, otherwise we would use one material for everything from drinking water to rocket fuel.

Have you tried a google search on titanium, titanium alloys, titanium properties, titanium connecting rods etc.

Magazine editors are not always qualified nor experienced engineers. They often have arts degrees. They may have business degrees. If they are unscrupulous their opinions may be swayed by their advising customers.

If F1 use titanium in rods, but not cranks, I guess they do it for a reason, that reason being overall balance of properties vs overall balance of properties required, like bearing qualities of the journal.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Metalguy]

Gary,
Ti can be examined very well via dye penetrant, esp. fluorescent (Zyglo). While they are not as sensitive as mag., a good examiner will find cracks if they are present.

I'd have to look it up, but I'm not sure that Ti alloys such as the popular 6/4 alloy are more sensitive to notches than 300M. Another factor in fatigue cracking is corrosion, which is a factor in nearly all fatigue. Ti alloys are FAR more resistant to CF than steels, depending on the actual envirnment, of course.

 

[GaryFB]

Dear PatPrimmer; Yes I have done many Google searches on titanium and have researched the subject for years. The Acura NSX uses titanium connecting rods, the new Corvette street car is using titanium connecting rods, the new Porsche GT3 street car is using titanium connecting rods; but that does not tell me anything. Reducing the recipricating mass will reduce the loads on the rod bearings, this is good. Reduced recripricating mass can provide better endurance and possibling an increase in the engines rpm.
I believe this thread started by JB83 asking about what material would be best used for connecting rods. I think we could discuss H, I, and A beam designs too. We could discuss the advantages and disadvantages of many different materials aluminum, titanium, and steels. So far I have only read about opinions and not facts. I believe alot more goes into manufacturing a connecting rod then just the choice of material. I believe the heat treat processes and the shot peening have a very important role.
If anyone knows more about the titanium alloys and the different manufacturing processes for titanium I think it would be a valuable contribution to this forum.
I would like to thank everyone in advance for contributing, I find this to very a very interesting subject. Thank you.

 

[Metalguy]

One "fact" is that the strong Ti alloys have a fatigue curve similar to steel and unlike Al alloys, in that they have an infinite life if the stresses, etc. are below some value.

 

[patprimmer]

Gary

What exactly are you trying to do.

On the one hand you seem to have a considerable but incomplete collection of data, both anecdotal and real, on the other you seem confused as to what to do with it.

Are you trying to decide whether or not to use titanium rods in a particular engine, or on a range of engines, or are you contemplating making them.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NickE]

"I believe alot more goes into manufacturing a connecting rod then just the choice of material. I believe the heat treat processes and the shot peening have a very important role."-Gary

Gary-

Here's how I would go about a material/process selection in view of your statements.

0)Determine goals. IE A connecting rod that will NEVER fail is possible but wont reach the goal of high RPM operation. One that can handle 25,000rpm is also possible, this one could be cost porhibitive though.

(The above is a complex way of saying: Cheaper, Stronger, Longer lasting, lighter. Pick 2)

1)Decide on the material. (the reason I put this first is that almost all the other details hinge on this.)

2)Design the part. This would take into account specific stiffness, specific strength, geometry, stress concentration, etc...

3)Determine manufacturing method. This must take into account the goals in 0 the material in 1 and the part design in 2.


A good example is a part I helped do significant development work on. Many materials are compatible with the design and production method. We had to decide on an optimum though. After much effort 17-7CH900 was chosen. Yes its more expensive than 301, lower strength than top quality carbon steel, less conductive than Alloy360, lower fatigue performance than 7C27Mo2; however the compromises made in the material selection have given us a part that works in ninety odd percent of the applications I've seen. Yes there is always one thing better and we could say that Elgiloy would make a phenomenal connecting rod. It costs >$160/lb though. Let alone forging it, processing it, heattreating it.

nick

 

[patprimmer]

Great input Nick.

I hope that is what he is looking for.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GaryFB]

Hi Everyone; The original forum post from JP83 asked a very general question about which material could he have connecting rods made and what did people in this forum know about the titanium alloys. Personally I think it was an excellent question. There are many materials, many processes, and many heat and surface treatments used to produce connecting rods. There are now some new steel alloys that have exceptional strength to weight values, it would be invaluable if a few material engineers could tell us about these alloys.
My point about titanium is that there is more to titanium then most people know. Some connecting rod manufactures forge their titanium, which is the very best way to manufacture a titanium connecting rod. Some manufacturers are making titanium connecting rods from extruded or billet plate. Titanium in its best alloy and best forging, still only has 70% of the tensile strength of a high strength steel. Titanium in its best alloy and best forging has half the modulus of eleasity of high strength steel. Just from the tensile strength and modulus it should be obvious that titanium in it absolute best alloy and forging is NOT equivalent to high strength steel.
In a race engine, where the titanium connecting rod can be replaced often, titanium could reduce the weight of the connecting rod by 25%. If the titanium was the same dimensions of the steel rod, the weight savings could be as much as 40%.
Are you going to accept a machined, billet titanium connecting rod, or will you want one of the better forged alloys? How many companies have forging dies, and how many can forge titanium? The real question becomes, do you want to pay $10,000.00 for a forging die, plus the cost of machining, heat treating, and surface finishing?
You can purchase a high strength steel rod for $300.00 each, the cost of a titanium specialty rod could easily cost over $1000.00 each. In the end, the ti rod may be lighter, but you will have to replace it more often.
One of the better titanium alloys is called Ti-6246 (Ti-6Al-2Sn-4Zr-6Mo) originally developed by Pratt & Whitney over 35 years ago, and is available from Timet. There is a newer Ti alloy called Ti-200, maybe someone can obtain some info on this alloy?

 

[Metalguy]

Why do you think Ti rods would have to be replaced? I already told you that there is a stress level below which Ti doesn't develop fatigue cracks. A good Ti rod can be both lighter and stronger than a good steel rod, the only exception *might* be if the steel were something like the 350 alloy used for *centrifuges*. The Ti rod would be larger in cross section than the steel, so you can stop being concerned with E.

 

[GaryFB]

Metalguy, thank you for your responses, I am beginning to understand the subject much better. I think we would both agree that the weight savings would not be 40%, titanium vs steel? In order to keep the big bore round and within tolerance, the mass would have to be larger. I think you would agree that having a titanium connecting rod made by an "aftermarket" supplier is no guarantee the rod was designed or manufactured "below stress levels"?
Based on my research, I think a titanium connecting rod, manufactured and forged from the better alloys could have a weight savings of 25%, but it would not have the tensile strength of a similiar 4340 steel rod.
My only point is that purchasing titanium rods that are lighter, is no guarantee that you are receiving a connecting rod that is equal in strength to steel. If the weight of the ti rod is 40% less then the steel, then the steel rod was over engineered, or the ti rod was under engineered.

 

[unclesyd]

Here are several good papers on the design and mechanics of conrod design.
If you register you can get all the data you will vere need for a lot of conrod materials in Fatigue Database, just below these articles.

http://www.steel.org/autosteel/bar_rod/index.htm