Titanium Head Studs

[chris_seven]

it seems that ARP have introduced Head studs manufactured from Titanium which seems like an interetsing idea.

TITANIUM: ARP now offers special order fasteners made of an alloy (Ti6Al-4V) that is specially heat-treated (a process developed by ARP's own Russ Sherman) and provides superior strength to other titanium alloys employed in racing and aerospace. The material has a nominal tensile strength of 180,000 psi, and is very corrosion resistant. The main advantage of titanium, of course, is its weight – which is about 40% lighter than a comparable fastener made of steel. Head studs and accessory bolts are ideal applications for this lightweight material.

Obviously the modulus is a bit lower and there could be some bi-metallic corrosion issues if the studs weren't coated with a PVD layer such as carbide or nitride but has anyone tried or were they used on the 917 or other factory engines?

[HawgRyder]

There would be several considerations.
Price...availability ...and ease of install.
Titanium can be very good when used properly...but in my experience you need to inspect more often.
It goes "soft" at some point...and soon after fails.
We used it for almost every fastener we could in a drag car...and "lost" over 100lbs of wieght (but all the bolts were free from the military supply).
If you are going to disassemble on a regular basis...I see no problem....but for a road car not so good.
Bob

[chris_seven]

Bob,

Very Interesting

Do you know why it goes 'soft' and do you know which Grade you are using?

I did some fatigue work on 6AL4V many years ago but things haev moved on a great deal.

On benefit could be that the 'pull out' forces caused by expansion would be lower and a great number of Racing Motor Cycle engines are now using Ti Studs.

[Flieger]

I believe the 917 used Titanium through-bolts, but not sure about studs. 908/3 or 909 would be the place to look I suppose. But that would probably yield mostly fasteners everywhere else being Ti, maybe ot headstuds.

[Bill Verburg]

Originally Posted by Flieger

I believe the 917 used Titanium through-bolts, but not sure about studs. 908/3 or 909 would be the place to look I suppose. But that would probably yield mostly fasteners everywhere else being Ti, maybe ot headstuds.

neither head studs nor through bolts of the type 912 12 cyl engine used in the 917 was Ti, they were Dilivar

Ti was used for rods and rod bolts

eventually they used Ti for the output shaft for 1 race(Sebring'70) but went back to steel subsequently

Na filled Ti intake valves were eventually and Ti was used for various brackets and even tried on timing gears

160lb of Mg were used in these 542# engines, mostly in the crankcase

[HawgRyder]

Chris...I'm not into metalurgy so I can't give you the answer to your question.
However...I was in the military...and do know from my roomates that worked on both transport and fighter aircraft, that they keep track of the number of hours, landings, etc for all Titanium parts.
The parts get changed out and destroyed (to prevent re-install) and they were very strict about it.
It must be like "working" a piece of steel...eventually the metal goes "soft" and breaks.
If it were not for the "free" aspect of the bolts and nuts...we never would have used them.
Fortunately almost all hardware in the tool crib was class "C" unaccountable...LOL.
Bob

[chris_seven]

Bob,

The fatigue behaviour of Titanium is interesting and for a number of years Titanium was always considered not to have an 'Endurance Limit' which is the stress at below which it will never fail.

Virtually all steels have the limit and can usually considered to have an infinite life when used within this limit.

In recent years 6AL4V which is a commonly used fastener alloy has much good quality published data providing Endurance Limits and I would be comfortable using this data as it can be found in many NASA and other 'Unclassified' Military Publications.

Certainly airframe fasteners used on wing structures can be problematic due to the effects of galling, particulalry when initially specified with slight interference and this would limit re-use but these type of issues will not be a problem for a head stud as this type of damage won't occur.

I am not sure what you mean by working a piece of steel but if you mean bending and reverse bending with significant plastic deformation until failure then the mechanisms are very, very different when we consider the elastic behaviour of a metal.

Severe plastic deformation leads to the formation of voids within the material and these voids grow and coalesce with increasing deformation and this reduces the bulk modulus of the material which is effectively softening.

Elastic processes don't form voids but they can form surface cracks which grow longer with time but in practice the components stiffness doesn't change until a crack initiates and then failure normally occurs very quickly.

There is another process that can occur and this is known as 'cyclic work softening' but this is also associated with plastic deformation and is more commonly classified as Low Cycle Fatigue.

I did some work in this area many years ago using a combined tension/torsion test machine and some 7000 Series Aluminium Alloys but I am not sure if I still have any of the data..... but this is way off topic.

The sad truth is that Ti has some great properties that would make very good head studs but it has other issues such as galling that make it difficult to consider.

I have asked ARP some questions via their Technical Helpline but I am not confident they will reply.

I am trying to find solutions for the problems which don't impact on the benefits. The levels of stress in the head studs even at the highest temperatures that are likely aren't high enough to worry about fatigue failure and I am confident that this isn't an issue.

If I can be confident that we can eliminate galling so that they can be tightened reliably and that they will not result in too much Bi-Metallic Corrosion then I would be happy.

The Bi-Metallic corrosion is unlikely to be much more of an issue than the Stainless Steel and/or Inconel Studs that are being used so the main issue is reliable torque to preload correlation and galling.

[Cupcar]

I saw an original early 906 crankcase which reputedly had Ti studs that were covered with fiberglass insulators.

[Bill Verburg]

Originally Posted by Cupcar

I saw an original early 906 crankcase which reputedly had Ti studs that were covered with fiberglass insulators.

The 906 and later prototypes through 910 were certainly testbeds for the 917, they went to great lengths to make ithem light
the windshield wiper and washer switch was lightened from 53g to 45g by machining the the nut holding it to the dash

14g was saved in the same way from the ignition switch

12g saved by deleting the fuse box cover

there was a 9kg(or more depending on alternator) difference in the electrical parts between the hill climb and long distance cars

brake disks made from chrome plated Beryllium(4.56kg) rather than iron(18.5kg)

Ti stub axles@1kg ea saving 1.55kg over the steel equivalents

Ti conrods, early cars had Ti rod nuts but later ones steel

Ti pad backing and Ti caliper pistons certainly



keep in mind that Ti does not conduct heat very well, they found that the Ti stub axles though lighter were running @400*C compared to 180*C on the steel versions, resulting in a change to ET 300 Molykote grease and using steel on the long distance cars

Ti rear wheel hubs caused similar heat related issues but here it was found to be a non issue

but there is no mention of Ti case bolts or cylinder head bolts, these were always Dilivar w/ the cylinder head bolts f/g coated to keep them warmer

[HawgRyder]

In the years since these engines were assembled and tested...there has to be some advancement in technology.
I wonder if some of those advances could be applied to our cherished machinery?
Keeping an open mind seems to be the order of business.
Unfortunately...we need someone with deep pockets to test all of the proposed improvements...any volunteers? ....LOL
In the mean time...discussion is the best open forum for ideas...simply because it makes us all think.
Bob