Titanium Alloy Dimensional Stability

[meekg]

I'm looking to choose a Ti alloy for a reference structure for a physics experiment.
The structure is about 15"x6"x6" and is relatively heavily gutted by machining.
The reference surfaces should not move more than 0.5um during 1 year.
Temperature will be stable.
We will heat treat as much as possible during machining, but I'm wondering if there's a Ti alloy that is particularly good in this respect.

 

[NickE]

I personally dont think that you really want to use a Ti alloy, they are not all that stiff and .5um is a really tight tolerance, esp for machining. In fact I'm not all that sure that you can machine (using normal mechanical means) anything to hold .0000196". Even the tooling shop that does our dies has a hard time getting better than .0001".

nick

 

[meekg]

The fabrication tolerance is much looser than that.
After fabrication, the piece is measured, and other calibration is performed.
However, material creep in excess of 0.5um will take the instrument out of alignment.

Ti was chosen for other reasons, among them strength-to-weight and non-magnetism.

 

[NickE]

Ahhh... I see said the blind man to the deaf mute....

Hmmm.. I think that you are going to have to use the highest strength Ti alloy you can get. Will this be subject to temperature changes? External forces?

nick

 

[meekg]

Storage is -20C to 80C - nothing crasy.
Forces are light. I'm worried about internal stresses.
We'll be gping through several heat treatment cycles and other tricks.
Since internal stresses are a function of (also) microstructure, choice of alloy can be important.

 

[sreid]

Any chance you could consider Zerodur glass from Schott Glass. Complex structures can be machined with abrasive water jet. Usually this is thought of as a low TC material but I'm certain it's creep with time is also low.

 

[meekg]

is it amorphous?
(out of curiosity... we're not changing the material to glass right now... machining too complex, too late..)

 

[sreid]

I think it is amorphus. Here's the starting link.

http://www.schott.com/optics_devices/english/products/zerodur/

 

[sreid]

meekg,

This is an uninformed, uneducated SWAG. I would think that either commercially pure or Alpha Phase Titanium (Ti-5Al-2.5Sn) would be the propper choice. With a single phase alloy one would think that dimentional changes due to slow phase changes (Beta to Alpha) would be minimized or eliminated.

 

[meekg]

yes - that's my thinking too. (Mine was a WAG - what does the S stand for?)

 

[sreid]

Scientific Wild Assed Guess

 

[meekg]

ah.

 

[yates]

I guess it sounds too late to change the material from a Ti-alloy. My worry would be most titanium alloy's natural propensity to remember previous stress cycles. The position of the machining operations in the total manufacturing cycle is vital. Most Ti-alloys are difficult to stress-relieve, and spring-back can often occur. I also hope that the 'heat-treat cycles' you refer to do not include any water quench + temper operations (6Al4V style) as this creates considerable deformation and must precede all machining (even rough machining) operations.

 

[bklauba]

To perhaps risk putting one's foot in some SWAG related material: Have you considered vibratory stress relieving this component? There would be no metallurgical benefit (or detriment) of course, but, if dimensional stability is THE goal, that is indeed why the machine tool industry uses it, often both before AND after rough machining.

Your part is a little small, so its resonances will be a bit high, but reachable with the right gear.

BK

 

[meekg]

We didn't think of that.
If we can't hit the main modes, we can always weigh it down.
Thanks.