Titanium and Oxidation

Date 10-12-2008 16:05

From a general point of view the oxidation permeates the cross section of the component. Unlike steel or other non-reactive metals where the oxidation is limited to a few microns in depth, the oxygen reacts with the reactive base metals and affects the mechanical properties of the full cross section (or at least to a considerable depth). Wire brushing the surface will return the color to a bright shiny silver leading to the false impression no oxidation or damage has occurred. There is no means of repairing the damage caused by the reaction with air other than to remove all of the affected metal, i.e., weld deposit and adjacent base metal.

You have to completely shield the weld puddle, weld deposit, and heated base metal until it has cooled below the temperature at which the oxidation reaction takes place. Many specification state the gas shielding has to be maintained until the temperature drops below 450 degrees F. Some literature lists 800 degrees F as the lower limit for shielding gas coverage.

In any event, the welding specification should list the maximum amount of discoloration permitted, light straw, dark straw, blue, etc. permitted. My experience has been anything other than light straw was rejected. Care must be exercised when determining the color of the weld surface because any reflected light from fixtures or surrounding materials can lead to misinterpretations of the color of the weld. The last time I was involved with inspecting welds on reactive metals, we would place the part in "box" that was white and looked at the welds in sunlight, not an incandescent light because the light spectrum was not white. I'm sure there are lights available that do provide a true white light spectrum, but in my case, considering the number of parts being inspected and the time element involved, it was easier to simply perform the inspections during daylight hours.

Best regards - Al

By Lawrence (*****)

Date 10-13-2008 15:19

Plasma Head

Al said it perfectly.

There are some other threads on the forum that break it down even further... Try the search engine and type in "Titanium" and "alpha case"

Titanium is unique in it's sensitivity to reactive gasses and contaminants that enter the molten pool or are left in the heat affected zone when the work is above transformation temperature. When Just a little bit of bad stuff gets sucked in it will pass from from top to bottom of the weld nugget and also has a tendency to follow the molten pool until the weld is terminated.

The contaminants change the Weld and HAZ ductility properties drastically, and the affected area becomes exceedingly brittle. The amount of contamination (O2, nitrogen, surface junk) it takes to foul Ti a weld is in some instances less than the amount of contamination it takes to put an oxide color on the weld face. It may even pass a visual inspection or RT exam and still be totally ruined. This is why hardness testing is becomming more and more prevalant in inspection and expert process control and strict adherance to code requirements and proven standard practices is critical in any Titanium joining operations.

A job like your will require special trailing shields and possibly a purge plate style fixture. If a shield bubble box is available that is another way to go with small Ti parts that drink up alot of heat.

By MDG Custom Weld (***)

Date 10-13-2008 18:54

Usually on really small stuff you have to use a purge bubble or chamber to weld the parts.
I have welded Ti from time to time, but not on parts as small as you have noted.

I did have someone ask me a few days ago if a part that has been welded with improper gas coverage is heated above the critical temp while shielded can it still be used? I could not answer since we always considered it scrap. I don't want to hijack your post, but maybe the answers will help you too.

By Lawrence (*****)

Date 10-13-2008 19:44 Edited 10-13-2008 19:57

MDG

Your question is a good one.... And I think relates to the thread.

It is a very simple thing to strike a 5 amp tig arc and run it across the oxidized face of a titanium weld to remove the surface oxide color........ This is a bogus procedure and can do nothing to improve quality and likely will go a long way to making something that might very easily be dealt with into scrap.

It does however, force us to look at the two distinct types of oxidation related problems associated with Titanium.

The first (for this discussion) is direct wet weld pool contamination or exposure of metal to contaminants in the upper transformation tepms... This is when contamination (atmosphere or surface) is taken into the molten pool or HAZ... This contamination may or may not cause a color indication... It will change hardness and ductility, with very small percentages of oxygen or nitrogen making trouble (on the order of 0.1%) These are the defects while not visible necessarily, must be removed completely until only good metal is left.

The second type is the Alpha Case... Which is a very thin layer of oxidation (visible via color hues, different colors for different thicknesses of alpha case) in the first few thou of the mateiral which becomes hardened. This layer comes to existence at the lower transformation stage of alpha/beta Titanium alloys and does not extent hardness throught the weld nugget entire. Some trials by TWI and EWI have proven and some governing bodies have incorperated into their practices, that mechanical removal of this Alpha Case can bring the metal back to servicable condition... The hardened alpha case is an area more likely for crack propigation especially under cyclic loading if left untreated.

So striking that low power arc to remove the oxide only creates an opportunity to take the metal back up to a temperature that makes makes it soluable (WHILE STILL SOLID) and allows those contaminants (the pretty colors) to be intruduced deeper into the weld and HAZ, causing brittleness and making the area unusable.

Mechanical removal of alpha case may only be done where approved procedures are in place. In most codes and standards removal of oxide on titanium welds prior to inspection is forbidden. This important note should never be overlooked and strong process engineering evidence backed by thorough mechanical testing must be in place before Titanium welds with colored surface oxides are accepted or treated.

By TimGary (****)

Date 10-13-2008 20:32

Hi PlasmaHead2,

You have an interesting question.
I, along with the other posters here ALWAYS preach complete shielding gas, front and back sides for Ti welding.
Why? That's because to not do so makes contaminated welds, which suffer from embrittlement.
This is not only code requirements but practical as well, as nobody wants their weldment to fall apart.
Now, in this case, you said you are not using any code, except maybe the farm code, which leaves the door wide open.
I don't know what your end use is, but it doesn't sound like it's to hold anything up.
Maybe you don't need shedding for what you're doing. I don't think the material will "oxidize away". I think it will remain corrosion resistant after you clean off the oxide discoloration layer, but I'm just guessing.
I don't think your weldment will have its pre-welded strength, and I'm wondering if it will crack.
My answer would be - If you can afford the material to waste with experimentation, try it, but don't trust it to hold anything that could hurt somebody.

Tim

By PlasmaHead2 (***)

Date 10-13-2008 21:25

Tim, about the only function I'm expecting my pieces to perform is sit on someone's desk/shelf/where ever and look shiny, maybe with a few pretty blue and gold hues. I'm not expecting them to hold up or support anything critical, except maybe a pen or 2. I am well aware of the need to shield Titanium completely while welding and if I was welding to a code I would be following the best practices to the letter, but I'm just a hobbyist playing around in my garage. If I was to try and welding anything that would be of any level of criticality, id go rent another bottle and figure out how to make a purge bubble, but for playing around it's not worth the time/money.

One more question while you're all here...
Would an aluminum sheet be an acceptable working surface for welding titanium? Ideally I know id need a matching grade of titanium for a work surface, but given my circumstances that's out of the question. I know if I tried to weld it on a steel surface that it would more than likely pull the steel into the chips and that they would be contaminated as a result. My train of thought is that aluminum and titanium form an alloy, so at worst id have a slightly alloyed side of the chip. That and I'm thinking the aluminum wouldn't "rust" the same way steel contamination would. Am I on the right track?
Thank you all for your very in-depth replies
-Clif

By TimGary (****)

Date 10-14-2008 01:05

Another great question.
Our TI weldments were fabbed on SS tables and jigs.
I don't know about the aluminum, but I wouldn't trust it as a backing surface meant to use as a heat sink or vapor shield.

I saw a Ti weld once, on 1/2" pipefittings that was made while the welder was running out of gas. The weld surface was a dull gray in color with black color bands at the weld toes. The weld face looked grainy. I don't think it would look good on a desktop.
I do think however, that you could make these welds no problem with only normal GTAW shielding. If your desktop creation were made in a way that the unshielded back side would be concealed, or abrasively cleaned, I don't see a problem. You could get the nice blue color if you wanted.
As for would it corrode, I'm not sure, but I doubt it would detrimentally affect this project.
Please tell me how it works out!

Tim

By Lawrence (*****)

Date 10-14-2008 11:23

If this is only for desktop ornaments our discussion is way too deep.

You can go for it with aluminum, copper, or brass as a backing plate and It won't make a difference as long as your making pen holders.

The oxide colors will last forever on a desktop and will not corrode further unless they are exposed to chlorine.

In my pony tail artist years I used to make earrings and sculptures from Ti, using argon and atmosphere in different combinations of heat and time in order to produce colors... (green is the hardest to get)...

There are some extra big tig cups you can get for a reasonable price, or even make your own custom cups that will give you much more coverage and flexability when it comes to gas shielding and color.. That is the best way for you to continue your artistic fun with an artists budget.

By PlasmaHead2 (***)

Date 10-14-2008 14:13

Tim, if I had the stainless id use that as a work surface, but I happen to have a few big sheets of aluminum lying around. I haven't seen any black gunk or grey colors. Mostly the weld side of the plate turns dark blue to purple. The back side usually fairs worse and I've had it end up yellow and white more than once.

Lawrence, thank you! that is exactly what I needed to know.
What temperature range do the oxides start forming at? I have an old rod oven that I've been thinking about using for coloring titanium, but it only goes up to 550 F.

Thank you all again for the in-depth answers, I enjoy reading them even if they go over my head.
-Clif

By Lawrence (*****)

Date 10-14-2008 17:59

I don't have the information for color vs temp at hand..

Henry SSBN has way more research on Ti in his basket than I do... Hopefully he will pick up on this thread and let us know what he thinks.

I just used a gas backing plate (copper with lots of holes drilled and about 10cfh) and a big cup.. heating the TI with a light arc and then removing the gas shield from the top at various points to achieve color on artwork. Totally a hit and miss strategy, but it was fun and interesting... The backing plate keeps everything shiny on the bottom.

By ssbn727 (*****)

Date 10-17-2008 21:10 Edited 10-17-2008 21:18

Hi Larry, Hi Al,Tim, MDG, and Allen whom I know is always a sponge to this sort of stuff!!! Weldcome Plasmahead2!!! Good to see everyone is well :) ;)

Been real busy as you well know being a fellow educator and all... Anywho, if you don't want to have a spare cylinder of argon, then get yourself one of those plastic turkey basting bags, and weld the Ti in there!!! It'll do the trick in a pinch and for what you're doing, the excess oxidation will not be too critical. :) :) :)

One thing you need to understand is that the excessive oxidation due to improper shielding of the root side of the weld will increase the amount of interstitials (look it up if you want) but, it simply means metallurgically speaking of course - that the nonmetallic particles on an atomic scale, tends to overcrowd the crystal lattice arrangement (okay, it's a little bit more complicated than that but, I'm trying my best to keep it simple so, bear with me :)) in such a way that the grain boundries start to separate because of the increased "slip planes" due to the overcrowding of the interstitials - otherwise referred to sometimes as "excess garbage' within the crystal lattices, and are not as strong as the base metal was before the introduction of heat, and hypothetically speaking of course - since only a shielding gas was used that only protected the molten metal pool from 'oxidation" or, better described as "atmospheric contamination" at the face of the weld pool as opposed to having up to 3 layers of protection such as the first being an inert atmosphere or if not possible, both a shielding, and trailing gas shield along with a backing gas shield or once again, a totally inert atmosphere, then the addition of a slightly different grade of Ti would be added to coalesce the two members together in order to fuse with added strength, or if you're just autogenously welding the two members (components that make up the assembly - otherwise known as parts) together without the addition of filler metal, then it really doesn't matter whether or not you use a backing gas or inert atmosphere (I suggest a turkey basting bag if you're going to do this on the "Cheap!!!") because, you'll be just weakening the adjacent members of Ti anyway!!!

It comes down to this simply put... If you introduce heat to any reactive metal, whether it be CP (Commercially Pure) grade Ti or Zirconium for that matter, and you do not properly shield it in one way or another, and add the matching filler metal for that metal - there's an excellent bet that the fused metal will be weaker than the base or parent metal was originally, because all of those extra, overcrowding oxygen, and nitrogen atoms that are part of the interstitial family of metallic elements or compounds will weaken the crystal lattices, and therefore weaken the grain boundries or grains themselves!!! In other words too much oxygen, nitrogen, carbon, and other "interstitials" equal unecessary GARBAGE that weakens the weld - PERIOD!!! On the other hand, the proper amount of interstitials used in a metallurgical matrix will potentially increase some of the desired mechanical properties of certain metals however, strict controls must be adhered to in order to achieve this :) :) ;) Gotta go!!! Heading down to the "Burgh" from Wellsville!!!

Btw Larry, Al, and Allen, I'm the new Senior Year Welding Instructor at Alfred State Technical College which is part of the State of New York University system, otherwise known as SUNY, and I've got to get the last of my stuff back up here to finish my relocation to Olean, NY so, forgive me for not participating here as much as I normally did in the past!!! Once I get settled in, I'll be frequenting my "Second" home here again very soon!!! I also played around with some Ti to see what colors I could get, and ended up making a "Peacock" out of CP grade 2 Ti which when I find the pics, I'll post it up here. :) ;)

Respectfully,
Henry

By ssbn727 (*****)

Date 10-17-2008 21:22

Oh yeah - Btw Plasmahead2, there's some good info you can access in "Google books" on Titanium but, you'll have to do some "mining" if you want to find out what temps to use in order to achieve the colors you're seeking so, happy hunting!!! :) :) ;)

Respectfully,
Henry

By PlasmaHead2 (***)

Date 10-18-2008 16:22

Awesome Henry, thank you very much! :) :)

Another quick question for you, would temp sticks work ok for titanium or would they react with it and do horrible things to the metal?

Thanks again,
Clif

By ssbn727 (*****)

Date 10-22-2008 01:37 Edited 10-24-2008 04:34

Tempsticks + Titanium = Bad, Horrible :) :) ;) Infared Pyrometer = better, although not always so accurate under certain conditions so, periodic calibration is necessary for accuracy! ;) ;) ;)

Respectfully,
Henry

By PlasmaHead2 (***)

Date 10-23-2008 20:31

Thank you once again, and good luck with teaching the next generation of welders. :) :) :)
-Clif

By MDG Custom Weld (***)

Date 10-20-2008 13:41

It's good to here from you Henery. I'm really glad to here you're doing so good.
Thanks for the great information, as always :)

By ssbn727 (*****)

Date 10-22-2008 01:47

Man you Guys are making me blush!!! :) :) ;)

Coming from you gents whom I have the utmost respect for means so much to me!!!
Thanks again for the kind words. :)

Respectfully,
Henry

By DaveBoyer (*****)

Date 10-22-2008 02:52

Henry, good luck with the new job.

By aevald

Date 10-17-2008 21:50

Hi Henry, AWESOME. Great to hear of your newest endeavor and also thanks for simplifying these items so that I can get a handle on the materials that you've covered here. Certainly sounds as if you have been very busy, not anything new I'm sure, however all the same I can tell that you haven't let any grass grow under your feet. Definitely give us all a rundown on your situation once you get better settled. Good to hear from you and don't work yourself into the ground. Best regards and congratulations on your newest chapter in life. Allan

By Lawrence (*****)

Date 10-17-2008 23:41

Henry!!!!!!!!!!!!!!!!!!!

I couldn't be happier for you.

The right man in the right spot!

By 803056

Date 10-18-2008 20:34

Henry, I'm happy to hear your absence is because of a "Happy Event". Congratulations!

Good to hear from you and I love your dissertation.

Best regards - Al

By jrw159

Date 10-23-2008 20:48

Hank,
Congratulations to you, AND the lucky students who will benifit from your knowledge. :-)

Best of luck,
John