Grade 5 Titanium Studs

[FennLane]

I have read, with interest, that ARP Fasteners will now manufacture 6AL4V Cylinder Head Studs.

I am intrigued by this idea.

ARP also state that they have developed a heat treatment schedule that will allow strenghts of up to 170000psi to be developed.

I can imagine that galling could be an issue during tightening or would Titanium Nitride or Carbide Coatings reduce this problem or are they too brittle?

I am sure that bi-metallic corrosion in Aluminium engine could be an issue but again the ocating should reduce this problem.

Does the low modulus of Titanium help to reduce the increase in preload that can damage magnesium castings due to expansion or I am just being too optimistic?

 

[CoryPad]

ARP actually state 180 000 psi ultimate tensile strength for titanium alloy Ti-6Al-4V fasteners.

Galling is definitely a potential problem, especially when mated with an internal thread in an aluminium component.

Titanium nitride and/or carbide coatings are not really suited for this application based on cost and performance.

Typical lubricants include engine oil, extreme pressure lubricants like axle oil, and ARL's own Ultra-Torque lubricant.

Galvanic corrosion with aluminium could be a problem if parts are exposed to a corrosive environment, but that is a lower probability with automobile racing applications. Since the lubricants are not really coatings, they are not much help with corrosion protection.

Low elastic modulus can help with preload variation when clamping magnesium, but the low thermal expansion coefficient of titanium makes things worse. Also, the galvanic corrosion problem is worse, and lastly, the high bolt strength results in problems with allowable contact stress and required thread engagment. It is better to use aluminium screws in magnesium components.

 

[FennLane]

I don't think Aluminium is a good idea for Cylinder Heads Studs which was the basis of my question.

It is quite common to use 17-4PH for head studs and the difference in expansion between Titanium and 17-4PH in an H925 temper is quite small and I am not sure if it is too significant in this application as the modulus difference will probably reduce the pull out force.

It is quite common to anodise Titanium fasteners used in sub sea applications and I am not sure if this is for galling or corrosion.

I would like to understand why nitriding or carbide coatings don't perform in this application.

 

[EdStainless]

6-4 vs 17-4, the 6-4 will stretch twice as much under the same load (given the same cross section).
If a little less rigid clamping is desirable (do 17-4 studs cause deformation?) then Ti is a good option.
For anti-galling thread coatings you need to minimize surface energy while avoiding the risk of cracking or flaking.
PVD and CVD coating do help this, TiN coating is the one that I have seen used.

Of course the standard cautions apply:
use coarse threads,
use rolled threads
select mismatched alloy for the fastener
lubricate threads (lubricant needs to stand up to service conditions, synthetic?)
(this also rules out the use of torque, you need to measure actual stretch)



= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[CoryPad]

If you are using magnesium, then the properties of aluminium screws are much better suited. You may need to use more fastener locations, but the entire system will be smaller and lighter than if you used screws made with materials that have higher strength, elastic modulus, mass density, etc., not to mention the ability to load the magnesium more evenly thus preventing creep.

You are correct that the lower elastic modulus of titanium compared with 17-4PH steel mitigates some of the problem, but the thermal expansion coefficient is still 1/3 that of Mg and Al.

I am not aware of anodized titanium fasteners for subsea applications. Documents from NORSOK, DNV, and recent journal articles do not mention anodizing as a requirement for corrosion protection. If fasteners are anodized, likely it is to reduce galling.

Titanium nitride and/or carbide coatings have high friction coefficient, which is not desirable for fasteners. Also, they are applied by a high-cost vacuum chamber process that is not well suited to batch production of small parts. It is possible to do this, just not widely used.

 

[FennLane]

Unfortunatey I can't re-design the engine casings to use different numbers of fasteners as the engines are early Porsche 911 units. I think this must rule out Aluminium.

Porsche used to use conventional steel studs but issues with magnesium cases (introduced in 1969), the higher temperature used for emissions control and a change of cylinder material from Cast Iron to Aluminium led to significant problems. Porsche introduced a controlled expansion alloy stud (Dilavar) to reduce the pull out forces caused by expansion but6 these studs commonly suffer from brittle fracture - I suspect to to SCC and chlorides as the studs are exposed to road dirt on the lower side of the engine. I believe Dilavar is a precipitation hardening Austenitic Stainless Steel with a Cof E of 18ppm/degK. They have a dreadful reputation in Porsche circles although the lastest variation has a resin coating and a larger waisted diameter seem to survive but they are vastly expensive.

17-4 PH studs are commonly used as a replacement and don't seem to cause issues with Aluminium engine cases (Re-introduced in (1974/75). Many magnesium cases have already had to repaired with inserts. 17-4PH is readily available in the Uk and is cost effective but its selection is not entirely compelling. I would prefer 15-5PH but again this is difficult to source in the UK.

I spoke today with a specialist manufacturer of Ti studs and bolts and tell me that they normally clear anodise for subsea and they tell me this is to reduce galling but I am not entirely convinced. I don't believe this is a requirement.

I am aware that nitride and carbide treatment are PVD processes but cost is not too much of an issue but I do agree that High Friction is a non-starter.

In Japan Ti Head Studs are being offered with a nano-ceramic coating and reported tensile strentgth of 230ksi but I cannot find a trace of this technology in the UK.


I would also consider using A 286 Stainless Steel but am concerned about SCC in the presence of chlorides.

 

[FennLane]

Just found this old article which seems to suggest Silver Plating may hold some answers.

http://www.dtic.mil/dtic/tr/fulltext/u2/297320.pdf

 

[Screwman1]

I am not aware of a Ti alloy that will reach 230ksi.
Part of the issue with these studs is the need for them to have good elasticity in the joint. The idea of increasing the cross sectional area actually increases the spring rate of the fastener and causes more problems with embedment and pullout, making the situation worse. The 17-4 is a good choice if you keep the geometry the same, it resists SCC and doesn't have too many problems with notch sensitivity.
I don't see Ti as being a good choice for this application; the CTE and modulus are against you and you will have a hard time getting the yield strength that you need for these bolts. Galling will be a problem with repeated assemblies.
One of the problems with this application is thread failure in the magnesium cases and pulling more clamp load will only make that worse. I would look at doing a joint analysis to see if you can adjust the fastener design or the nut to get more elasticity without creating additional clamp load. The thread diameter in the cases might have to be increased to get better pullout resistance (keeping the shank the same or reducing it), but you are getting close to the barrels already. It's a famously nasty joint design; they had a marginal design and then kept trying to get more performance out of it without changing the envelope and at some point you reach a limit of can be done unless you decide to change some of the laws of physics.

 

[FennLane]

I agree about the Ti alloy and the information is here:

http://performancepart.wordpress.com/2012/02/19/jasma-titanium-racing-head-studs/.

The studs are not tightened to give huge preloads - only around 24 lbsft on an M10 x 1.5 stud. The increase in preload dus to expansion is significant but still not huge and I calculated around 44ksi total at a typical operating temperature. I don't think yield strength is too much of an issue that these levels stress.

I would accept that use of high strength exotic materials is a bit of overkill in this application but the Motorsport market always like the biggest number.

Porsche have also adopted a policy where they use the high Cof E material on the exhaust side studs and a conventional steel on the upper side and this must result in uneven clamping and perhaps uneven distortion of the cylinder.

Measuring stretch in a stud is costly as the only Ultrasonic devices that I have seen coast around $3k and I am not sure how well they calibrate.

I have just bought a load cell so we can gauge up a fastener and start to determine an accurate picture of the clamping forces.

 

[CoryPad]

Is there a magnesium component in the joint? If so and if this is for a racing engine application, then the titanium parts should be fine as long as there is limited exposure to a corrosive environment and as long as the clamping force developed is not so high to cause creep. You will get a benefit from the low elastic modulus and low mass density compared with iron, nickel and/or cobalt-based alloys.