Titanium Weirdness

[larry cottrill]

In anticipation of using some of the Titanium tubing (kindly donated by Mark) to build my U-control Test Flyer described elsewhere on this forum, I decided I'd better find out whether I can in fact crush and bend the tubing in the manner illustrated in my drawings. Some of you will recall that Ben and I experimented with heating a length of this tubing in the tailpipe of my Dynajet away back in 2003, as shown in the photos below. The tube tested ended up discolored and warped, as you can see in the second photo, but seemed otherwise unaffected. The key word here is "seemed", since there was no attempt to really test the final characteristics of the sample.

Anyway, I decided to use this sample piece for my crush and bend testing for the model, rather than possibly screw up a fresh piece (since my supply is limited). I chucked the most heavily discolored end of the tube in the vise and started to compress it. To my astonishment, it yielded far easier than a 1/4-inch steel brake line! There was also a sort of very soft "crunching" sound, which I took to be oxide being shelled off. My initial appraisal was that the material had been severely softened.

Silly Me.

Progressing further with tightening the vise, I still encountered practically no resistance, but with an increase in the little "crunching" sound. This was soon followed by shards of metal dropping out of the bottom of the vise. The Titanium was literally crumbling like a dry biscuit in the vise - except that the pieces were razor-sharp slivers of various lengths! The Ti tube was disintegrating between the vise jaws, almost as if I were trying to grip a glass Christmas tree ornament!

So, the material was not softened as I at first imagined, but rather, it had been made astonishingly brittle. The result was something like you might have gotten by squeezing glass lab tubing! Really quite astonishing. I tried the vise at various places along the length of the tube, and found the brittleness less and less pronounced as I went along - meaning that the effect was greatest where the tube had been deepest in the Dynajet tailpipe and therefore, hottest. (I estimate that the very end of the tube penetrated the Dynajet tailpipe to just a little forward of the halfway point of the straight pipe length.) Up at the curved end of the sample (the part gripped in the vise during the heating experiment), the tubing seemed to behave as one would expect fairly hard metal to behave, with plenty of crushing resistance in the vise throughout deformation.

Anyway, I just thought this would be an interesting thing to report. It turns out that even Ti alloy (at least THIS alloy) would be pretty poor as a structural element deep inside a pulsejet. Note that this effect was the result of seconds of exposure to the engine gasses, not minutes or hours.

L Cottrill

[Zippiot]

In a frame does titanium act like carbon steel where if it takes a shot and bends the whole frame needs to be replaced, or is it like chrome-molly where only the bent section needs replacement and the rest of the frame is still solid?

[Jonny69]

Yes, titanium oxidises horrifically which makes it near useless for most high-temperature applications like pulsejet valves. I didn't do the testing because I got the information from the material specialist when I was at university.

Zip, to answer your question I think you would need to know how much it work hardens and fatigues when it is bent out of shape. It is very stiff like spring steel and usually has a thick oxide layer on the outside (the white dusty look)

[Stuart]

Anyway, I just thought this would be an interesting thing to report. It turns out that even Ti alloy (at least THIS alloy) would be pretty poor as a structural element deep inside a pulsejet. Note that this effect was the result of seconds of exposure to the engine gasses, not minutes or hours.

L Cottrill

Thanks for the report Larry. At one point I had contact with a factory in Belarus for possible fabrication of titanium connecting rods for small block Chevy's. Sometimes you can try and get too fancy, so this will keep the titanium bug from buzzing in my brain for temp sensitive stuff.

[Jonny69]

Stuart, I think the temperature is low enough for it to be ok for conrods and pushrods. It seems a common thing to do is nitride the surface which increases its hardness and prevents the oxidation. Billet titanium rods are quite common though.

[Eric]

There are tons of different types of titanium suited for a wide range of applications.

It is notoriously difficult to form and weld, forget about doing any complicated machining with it unless you have lots of time and lots of cutting bits, but you guys are talking about totally different applications and acting like because you exposed it to 3000 degree temperatures that it wont hold up for anything else.

Is there really any material that would make a good structural member deep inside a pulsejet? Not without active cooling.

There are various titanium alloys that you could take that size tubing, bend it into a 4" coil, and have it spring back to normal with extreme speed. Thicker pieces may show deformation and you come back an hour later and its straightened back out. There are others that will survive better at high temperatures, and there are some that are super hard but brittle.

If you are not careful while welding it, you can actually burn the titanium. It typically needs a complicated preheating session, welding, and then annealing cool down period.

You likely have a pieces of 4 al 2 v that isnt well suited for high temperatures, which you then exposed to the inside of a pulsejet which is one of the hottest operating environments around, that no metal will truely survive well in.

Eric

[Mark]

http://www.key-to-metals.com/Article122.htm
http://www.titanium.com/titanium/aerospac.cfm

[Zippiot]

How do you weld titanium (I assume you will braze it)?
The only way that I know of is to space the two pieces apart, just a little, and run a special laser between them...

[hagent]

Hi Larry,

What did you really want to use a high temp material for anyways?

Thanks

[Stuart]

Stuart, I think the temperature is low enough for it to be ok for conrods and pushrods. It seems a common thing to do is nitride the surface which increases its hardness and prevents the oxidation. Billet titanium rods are quite common though.

Oh, I'm sure con rods are great in titanium, it was right after the Berlin wall fell and I had a connection to a plant in Belarus that would have done them cheap too.

I am just now aware of the high temp limitations of titanium.

[Dang911]

Ok as far as this whole controversy about TI, maybe I can contribute. First of all, Titanium welds normal using TIG. I have herd of it being welded with MIG only once, and that was using special TI wire, (hard to find). TI is easier to weld than aluminum, in my opinion.

As far as its heat properties, off hand I do not know what grades/alloys are good for high temp, but some TI is far superior to steel for working strength at high temperature. I work at an FAA repair station, and all of the turbine duct work is 2-5" TI tubing, I almost built my lockwood out of TI, BUT being my first lockwood engine I wanted to make sure it was functional. Maybe some time down the road I will go all TI.

The duct work is used in the turbines thrust reversers, and takes the thrash of high temp, high pressure turbine exhaust. See pics

[larry cottrill]

Hi Larry,

What did you really want to use a high temp material for anyways?

Thanks

Hagen -

I don't really care about the high temp properties in the case of this tubing. I will use it by simply bonding it together with epoxy (i.e. J-B Weld). That will be plenty good enough for the truss in the model fuselage and the fuel cylinder cage.

The reason Ben and I ran it was simply that it seemed like an interesting material, since I knew that my dad had welded Titanium tailcones for turbojets back in his days at Solar Aircraft. I assumed (wrongly, it turns out) that the Ti alloy tubing would have a good chance of working well in the pulsejet interior where other materials were getting badly torn up. I had just gotten several pieces from one of Mark's eBay forays, and had no idea what to do with any of it, so it seemed like the Dynajet test was a logical thing to try.

This thread was NOT an attempt to show that Titanium is "just no good" - my dad produced afterburner tailcones from it, for Pete's sake. I was just reporting how the results of a particular experiment ended up. It would have been an incomplete picture of reality just to leave it as I thought it was at the time of my jetZILLA article, after I had seen the actual effects while trying to perform ordinary metalwork on the affected piece.

This 1/4-inch OD tubing is the most amazing metal I have ever had in my hand. It hefts like a paper drinking straw, but when you try to flex it between your hands, it's like you're trying to bend a solid steel rod. Unbelievable, really, if you're not accustomed to handling it. It is bendable around a pipe held in the bench vise, but kinks badly if you try to get a very tight bend. I need to do more experimentation with a fresh piece to know what I can get away with in working it by hand. It cuts surprisingly easily with the jeweler's saw, but then it is extremely thin wall material, of course - only about half a mm, maybe less.

L Cottrill

[Mike Everman]

I love Ti. It's a joy to lathe turn, and has a beautiful color. I just spot welded some .020" and it took it well. Even that thin, it's so very stiff, it's like Larry says, it kind of goes against common sense when you hold or try to bend it.

[hagent]

Hi Larry,

I was just wondering if you had some cool idea that you were trying to impliment and thought that Ti might be acceptable.

I'm not sure if totaly imersing it up the tail pipe would be a true test of how well it would perform as a CC for example. It was just a cool test to see what would happen. Has anyone used Ti for a CC ? I always thought that it might just oxidize like Mg would once it got hot enough.

If you ever wanted to put something in a CC and have it survive you could try SiC, I think you can buy some from CREE. They come up to 3" diameter waffers. Of course making it into something usable might be tough. It has properties like diamond. Might be fun though.

Might make a nice Aerospike nozzle.

Take care,