6061-t6 cylinder head

[celal]

is 6061-t6 alloy suitable meterial for to produce porsche heads?

[HawgRyder]

I would think 6061 is a bit soft....perhaps 7075 would be better.
I have used 7075 - t651 for several projects and found it took a better surface than the 6061.
It polishes up better and is quite a bit harder.
Bob

[Flieger]

What is the equivalent for the RR-51 (?) alloy for the Turbo heads? The low thermal expansion Rolls-Royce aircraft alloy.

[Flieger]

Nevermind. I found it. RR350 = AMS225B

Post #8 here: SC heads on a 930

[celal]

I think 2618 is the best but I am looking for a cheaper options especially for na engine

[chris_seven]

Max,

Do you know the composition of RR 51? I thought that this was a 1930's casting material and would be relatively high in Silicon.

I am not too sure about either 6061 or 7075 for a cylinder head.

6061 is a good general purpose alloy but it has quite high expansion and I can imagine that it will start to lose some of its properties at cylinder head operating temperatures.

Its age hardening temperature is only around 180 degC and continuous exposure to these temperatures may make it overage and lose both strength and ductility.

7075-T651 is an extremely strong alloy. It achieves its properties by a combination of age hardening and then by mechanical stretching T6 defines its heat treatment and 51 defines the mechanical treatment.

I would agree that 7075-T651 machines more readily than 6061 but it does have a couple of disadvantages.

The first is that at the operating temperatures this alloy will almost certainly overage as its basic ageing temperature is only 120 degC.

7075 is also not as corrosion resistant as 6061 particulalry in the presence of chlorides due the the presence of copper rich precipitates within the matrix of the metal.

The corrosion reistance deteriorates even further when it overages.

It will also expand about the same amount as 6061.

Chosing an alloy to make heads is quite intersting and needs a bit of thought.

I would initially want an alloy with around 6-7% Silicon to reduce expansion, I would then look at corrosion resistance and strength.

My first inclination would be to look at some of the 5000 Series alloys as they have excellent corrosion reistance and are solid-solution strengthened rather than precipitation hardened, so their properties are likely to be stable with time at temperature.

I m not sure about their strength when used with moderately high silicon content but I will try to find a suitable data sheet.

I believe that Perfect Bore in UK used to billet machine heads from a 4032-T6 Billet which had been 'squeezed' probably resulting in a T651 type of designation. 4032-T6 is certainly available in sufficiently large blocks. 4032-T6 may not be a bad choice as it has fairly good mechanical strength and is quite stable at temperature.

There was also a post about Yttrium additions to some of the early 911 heads to increase temperature capability and I am not sure if this material is commercially available, I think this was a material available in ingot form so possibly only good for castings.

[chris_seven]

Originally Posted by Flieger

Nevermind. I found it. RR350 = AMS225B

Post #8 here: SC heads on a 930

Very interesting

AMS 225 5% Copper, 1.5% Nickel, 0.3% Manganese

Heat treatment - Solution Treated and aged

Typical Tensile Strength - UTS 270MPa, Yield Strength 200Mpa, elongation 2.5%

Said to have good stability up to 600 degF and reistance to Stress Corrosion Cracking

AMS 4220 4% Copper, 2.0% Nickel, 1.5% Magnesium, 0.2% Cr

Heat Treatment solution treated and overaged.

Typical Tensile Strength - UTS 207MPa, Yield Strength 179Mpa, elongation 1.5%

Both of these are casting alloys and developed for military aircraft in the late thirties early forties.

The Copper content isn't the greatest of news from a corrosion point of view and I must say that neither would be my instinctive choice.

It does show, however, that strength isn't a major factor and that temperature stability ismore important.

[celal]

there are only two company that produce billet cylinder heads. the cmw uses 6061-t6 and 9m uses 2618a

[jcge]

Also known as Hiduminium RR-350

Technical Report AFML-TR-69-100
"Elevated temperature mechanical properties of two cast aluminium alloys"
A.W. Gunderson May 1969
Air Force Material Laboratory
AF Systems Command
Wrigh Patterson AFB Ohio

[304065]

Xtreme Cylinder Heads for Porsche

[chris_seven]

Originally Posted by celal

there are only two company that produce billet cylinder heads. the cmw uses 6061-t6 and 9m uses 2618a

I don't really see the logic in 6061-T6 other than it is agood general purpose alloy with quite good strength, good corrosion resistance and machines quite well but I think it will lose strength quickly at temperature.

I would guess that the 9m heads use 2618 because it is a common piston material and would retain it properties for a longer time at temperature.

The AMS 225B and AMS4220 would also have quite high expansion so maybe this isn't quite so important as I had initially thought.

RR350 is another alloy that uses overaging and has a relativley high copper content and also silver to help with high temperature stress corrosion cracking.

It seems that Continental and Lycoming both use alloys with the designation 355 T-6 and has some Nickel added to improve high temperature performance. Again these are not low expansion alloys.

There are two other alloys that may be good and their designation is 201 and 206

All of these alloys are used for casting and are not available in wrought form.

I would imagine that it would be possible to have this material cast into 'blocks' large enough to machine a head without any sophisticated pattern costs.

To produce a really good casting it would be possible to Hot Isostatically press the block before final heat treatment and this would give very good properties.

Powdered metal parts, P/M parts, densified investment castings - PTI

Would be a great product .

If you have limited your choice to either 6061 or 2618 I would chose the 2618 for this application as it high temp performance is the better of these two.

It would be interesting to see just how 2618 loses strength with time at temperature - I have a feeling that piston temperatures are lower than cylinder heads but I don't know by how much.

[Flieger]

I know MAHLE uses a higher silicon alloy than JE for pistons (and cylinders). Have there been overaging problems with them or are they designed with enough margin that overaging does not lower the yield stress to within the operational stress.

Lucky that precipitation hardening and overaging is a function of log10 time, so it takes a while to overage.

[chris_seven]

I have asked several question about overaging of both 4032-T6 and 2618 with regard to pistons but if there is any data it is being kept very quiet.

4032-T6 certainly isn't as strong as 2618 at 300 degC and this is to be expected as at 12.7% Si the metling point would be 600 degC which is around 60 degC lower than 2618.

There is little or no published data concerning aging behaviour of 2618 but it is very similar to many other alloys where you can find information.

I can easily imagine that 2618 could lose half of its tensile strength with sufficient time at temperature but this dosn't seem to be fatal.

If anyne has a very old 2618 piston that is scrap we could estimate is tensile strength from a hardness (Vickers Type) test and see just waht is going on.

I did ask Omega in the UK for some old pistons but without success.

I have an old Mahle and will chack it out next week.

[celal]

Dear Crish,
here is the imformation for 2618 t61
MatWeb - The Online Materials Information Resource

[chris_seven]

Originally Posted by celal

Dear Crish,
here is the imformation for 2618 t61
MatWeb - The Online Materials Information Resource

Thanks,

I have also been digging to see if I could find data on how much strength 2618-T61 loses with time at temperature due to overaging and I finally a found a good paper with some results.



Its room temperature properties when correctly heat treated seem to be around 440 MPa, which is quite reasonable.

At 100degC its properties are basically unaffected even after 1000 hours.

At 150degC it loses about 10% of its strength after 100 hours and 20% of its strength after 1000hrs

At 200 degC it loses about 35% of its strength after 100 hours and 45% of its strength after 1000 hours.

This is broadly in line with what I would expect - 50% overall reduction.

It would be interesting to know what the temperature the head runs at but I can't imagine the bulk temperature would be higher than 200 degC.

Even after suffering a 45% strength reduction this alloy is still about the same strength as the typical cast materials being used in Aero engines.

It dosen't have huge amounts of copper so its corrosion is probably OK and as it won't expand much more than the cast materials it doesn't look too bad.

I think it is a better bet than 6061-T6

[Flieger]

So, assuming the material is not stressed to 60% of its yield strength, everything is good. I hope the designer thought of this.

Although, a crack increases local stress, even circular has a stress concentration factor of 2, right? So if there was a safety factor of 2 for that in there, then a reduction in strength of 40%, there is only a 1.2 factor of safety left for micro-cracks after overaging.

[chris_seven]

Originally Posted by Flieger

Although, a crack increases local stress, even circular has a stress concentration factor of 2, right? So if there was a safety factor of 2 for that in there, then a reduction in strength of 40%, there is only a 1.2 factor of safety left for micro-cracks after overaging.

Max,

This is a very dry subject and I am happy to debate this but would apologise for sending everyone else to sleep.

Safety Factors and the propogation of cracks is a very contentious issue.

If we look at basic theory the sharper a crack the higher the stress concentration and the lower the safety factor.

The limit condition must be an atomically sharp crack - such as a welding crack where tip radius tends to zero. Ths means stress concentration tends to infinity.

This sounds quite good but if this were true then a body with a crack could not withstand any stress whatsoever. merley picking up a cracked part would cause it to break.

This is clealry not true and this realisation led to the development of a field known as Fracture Mechanics.

What really happens is that there is a 'stress intensity' ahead of a crack tip (back to the good old singularity) which causes the material to yield locally.

This yielding causes the crack tip to blunt and limits the stress concentration.

The amount of blunting relates to the size of the plastic zone ahead of the crack tip, which is a material property.

To predict failure really needs a detailed knowkedge of the local stresses in the vicinity of the crack. This has become much easier with the development of FEA models and advanced material testing.

The old 'strength of materials' approach to design using Safety Factors is very limited and at best a guess when stress/strain conditions have not been well established.

I used to give a lecture on the development and use of Fracture Mechanics around the world and I used to propose that we replaced the expression 'Factor of Safety' with the more appropriate term 'Factor of Ignorance'

To be serious, if a crack develops in a piston, it is pretty much the end of its life.

The supporters of 2168 pistons who claim that the material is more likely to support a crack than 4032-T6 are, of course, theoretically correct.

4032-T6 has a lower fracture toughness than 2618 so this is obvious but once a crack develops component life will be very short.

The stress needed to initiate a defect will almost certainly be large enough to propogate the defect so failure is eventually assured.

Sorry to have drifted from the topic.

[Flieger]

Thanks, Chris.

[celal]

dear chris,
I am trying to find a 2618-t61 but just found 2618-t8511 what are te difference between t61-t8511

[chris_seven]

The T designations fo these alloys define the Temper which has been applied.

Temper affects strength, corroison resistance, weldability and dimensional stability.

T61 is a typical temper for a forging which may have significant changes in section.

It means that the component will be solution treated and artificially aged but because of the section changes it will have been quenched into boiling water so the internal stresses will not be excessive.

T8511 is basically a T8 temper. This means that the material will be solution treated and then stress relieved by stretching before artificial aging. The T_511 part of the designation defines the amount of stretch.

It should be OK for your application.