machining titanium under inert gas shield?

[dsergison]

anyone done this?

since titanium air hardens rather than work hardens, would you be any ahead to use shielding gas when machining it?

and would you need an inert gas, or would co2 work? does flood coolant effectively stop the reaction with oxygen or have any effect that way?

would a mist coolant system running with a shielding gas instead of compressed air be of value?

inquiring minds want to know [img]/ubb/images/graemlins/smile.gif"%20alt="[/img] opiniions?

[Doug_C]

Not sure where you found that information.

Titanium certainly will work harden and sheilding gases are for preventing surface oxidation not hardening. [img]/ubb/images/graemlins/blush.gif"%20alt="[/img]

Air hardening properties are part of the natural molecular change in structure at the critical temperature. Reaching high temps on Titanium can leave the material very brittle with structure damage. In some uses, this must be removed after laser, plasma or torch cutting is used to rough out a blank.

Placing titanium in a controlled(vacuum furnace) environment during HT keeps scale and pitting from starting. This is done mostly for annealing and stress relieving purposes.

Machining Titanium

Welding and HT Titanium

DC

[sergison]

well, from my phd brother in law who works for the army research lab. He runs a electron microscopy lab studying super alloys.

I am led to believe that the hardening from machining is due mostly if not all from it's reaction with oxygen, especially at the elevated temperatures present during machining. And that unlike many other metals titanium doesn't get It's work hardened property as a result of deformation, but as a resuly of it's ability to better soak up oxygen while being heated and deformed.

then I got to wondering if any manufacturers have found this to be so. If so, there seems to be a lack of knowledge about it in the home shop theater [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]

if it is indeed actually so.

[Doug_C]

I can't dispute the accuracy of that statement.

In all the titanium I have cut, I'm just really sceptical there is time for oxygen to do appreciable change immediately upon or during cutting. I guess I would equate that to tarnish on silver plate and not much if anything below the surface. Very possibly detectible under an electron microscope, but insignificant to a cutter that is truely deforming the surface structure by cold working it?

The duration of cutting to rubbing is very short. Let a cutting tool dwell a bit too long and it will become a lot more difficult to cut through it conventionally.

I do know there are age hardening properties that do exist. But that takes a very long time or can be simulated in a controlled atmosphere.

I have read some information on the skins of the SR 71 Blackbird being harder now, than the day it was formed or installed. That was eluded to the heat friction induced by speed, not exposure per say. Part of the history of the material for that plane was a bit humorous. In that there are very few known Titanium mines in the world. We purchased that material from the USSR to make spy planes to spy on them?

DC

[gmacoffline]

im not sure where that info on air hardening came from but there's a little more to it than that, and the usefulness of the info it depends somewhat on the context. the term "air hardening" is used in a number of different contexts, and they have different meanings. in steel, the term 'air hardening' is generally relative to the time and rate of heat transfer from the material to the surrounding material that absorbs the heat - thus the terminilogy here denoting that the chemistry of the steel is intentionally created so that this type of hardening method is successful ( A-1, etc. grades of steel) This chemistry is specific to the heat transfer rate of the boundary between the steel and typical air, and it has little or nothing to do with chemical interaction between the steel and the air. thus, oil hardening steel ( O-1, O-2, etc.) is made with its chemistry specific to the heat transfer rate between the steel and a (typical) quenching oil. the chemistry of the steel is not intentionally affected by the hardening medium, other than a molecule or so at the surface, which is not what we are discussing. the internal chemistry of the steel as far as composition of carbides, etc. changes as a deliberate result of the chemical formulation of the steel and the methodical application and removal of heat. there is little or no intentional change by way of incorporation of other materials from the air or oil into the steel. many steels can easily be work hardened by the heat generated at a cutting point, by a number of tools such as saws, lathes, drills, mills, etc. so this kind of work hardening occurs quite often inadvertently. other work hardening mechanisms include those that deform the metal but do not necessarily heat it to a point where any of the chemistry changes due to heat - these are cold-working properties that can also result in hard spots or surfaces. many metals such as copper and aluminum work harden quite easily by this process and have to be annealed to soften them so that the forming operations can continue with the correct behavior of the metal.

on TI and a number of other materials, including some steels, when the surface of the metal reaches a certain temperature the atoms of the material become much more chemically reactive and can readily form compounds or interact with the elements in the surrounding medium, where it would not do this when cold. this change in chemistry may result in a hardening effect notable when the material re-cools. this chemistry compound change can be said to be a work-hardening if the increase in temperature were due to friction with a tool, such as on a mill or lathe. the difference here is that the hardening occurs by virtue of the metal ( say - steel for example) actually combining with some element of the surrounding medium, to make a new ( non-steel) compound.

hardening is a big topic and there are a number of mechanisms of physics and chemistry involved, it's just not a question with one answer that fits all circumstances.

[Doug_C]

the term 'air hardening' is generally relative to the time and rate of heat transfer from the material to the surrounding material that absorbs the heat - thus the terminilogy here denoting that the chemistry of the steel is intentionally created so that this type of hardening method is successful ( A-1, etc. grades of steel) This chemistry is specific to the heat transfer rate of the boundary between the steel and typical air, and it has little or nothing to do with chemical interaction between the steel and the air.

The properties of hardening are in effect a molucular change in structure or phase changes in the material.

As you noted for A1, the air has little to do with hardening. The chemical makeup of the material is mixed to react above a specific temperature and not return to its previous condition. Heat has already made the phase change exist on the way up. Letting it cool naturally just prevents distortion.

O1 or quenched materials are a bit different in their mix, because the oil or quenching process purpose is to freeze it in the critical phase change desired. If let air cool they tend to return to the previous phase. I have wondered also that as the oil strikes the surface, if there is any case hardening effect from momentary carbon production. Not so with water of course. I think that is what you ment by mentioning the
molecule or so at the surface?


Huh?(giggles)

there is little or no intentional change by way of incorporation of other materials from the air or oil into the steel.

But then you restated that later which had me a bit perplexed? [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

on TI and a number of other materials, including some steels, when the surface of the metal reaches a certain temperature the atoms of the material become much more chemically reactive and can readily form compounds or interact with the elements in the surrounding medium, where it would not do this when cold

There are quite a few heat treating processes that intentionally place the metals in a medium(acids, salt, gases, bone, wood) for imparting chemicals into the surface for specific attributes as desired.

Definitely magic and more to learn than might ever be used or remembered when the times comes to use it. Explaining it so someone else can understand it the way each individual does, is a whole story by itself. [img]/ubb/images/graemlins/confused.gif"%20alt="[/img]

TI being exotic and claimed to be found near UFO crash sites, who knows what secrets it holds. LOL!

It cuts just fine the old fashion way. One chip at a time!

DC

[dsergison]

er, um. I got lost. [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]

yeah I know that stuff about steel, it's boilerplate sort of. I hope it wasn't to much trouble to dig up cause I didn't wade through it all real carefully.

titanium case hardens considerably by combining with oxygen at temperatures around 400 degrees.

it's really easy to see much much more than 400 degrees while machining because Ti doesn't transfer heat away like most materials. I have HAND TAPPED a 6-32 hole in the end of a 1/4" ti6al4v rod and watched it smoke the oil and get too hot to touch and discolor the ti. I've cut it on a lathe and watched it smoke. it's really a pain to work.

i'm talking about the use of shielding gass to deny it the ability to surface harden.

[Doug_C]

I found this data sheet that does give an indication of TiN(titanium nitride) as part of a reaction with air. I thought that to be interesting. The Ti02(titanium dioxide) is what I figured would be present, but that takes some time to develop and is used primarily for pigments in paint and plastics. The possibility of Nitrides as in TiN has merit, but no indication of how long it takes to develop or how deep. Nitrides can be harder than carbide.

Titanium

Just as I stated before in heat cutting TI. The area effected is normally removed by conventional cutting methods to prevent fatigue propagation. If you experience higher temperatures in turning or milling causing a problem. I would think the SFM would be too high. Coolant should dissipate any heat buildup to prevent structure damage.

As to the heat buildup while hand tapping a 6-32 hole. That sounds like a chemical reaction I have seen between dissimilar metals and some cutting fluids as they react together.

In Aircraft work, there are rules to working on TI assemblies for specific materials the tools are made from can leave trace materials embedded into the TI and cause corrosion. Much of the finished TI parts go through a passivation process to remove minute traces of any other metals in the surface.

Making an assumption on my part....any time I get into a material that will harden by cold working it, gets categorized as work hardening. Where in reality there may be a lot more principles in physics going on than I can afford to be concerned with. At least until proof of the benefits are common to the field and relatively economical to use.

That leaves the door open for discovery. Possibly fame and fortune? [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]


DC

[dsergison]

That leaves the door open for discovery. Possibly fame and fortune?

yeah I'm all for that! remember, you heard it here first! where do I get my patent? [img]/ubb/images/graemlins/laugh.gif"%20alt="[/img]

hmm, yes it may be that it's reacting with the nitrogen in the air more than anything.

neither my old worn out enco vertical mill nor my 9x20 toy lathe have what it takes to "get under the skin" and make a nice looking cut, so it's hard to test it out [img]/ubb/images/graemlins/frown.gif"%20alt="[/img]

I can't wait to upgrade to something better of a mill.

[GeorgeGaskill]

all my authoritites say to cut as deep as possible to minimize work hardening. Don't take small cuts!

[sergi]

I was hoping that by shielding it you could "sneak up on it" rather than have to make every pass a serious pass.

[Bill D.]

Just to throw in the idea USSR subs had welded titanium hulls. they filledthe interior with shielding gas and put the welders in deepsea duivers suits with breathing hoses. IF gas would work on the lathe I would buil;d a box around the lathe and fuill it with the gas then you need only a small makeup flow in.
Bill D.
PS this assumes the shielding gas is heavier then air..