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Does 6Al-4V Titanium stress relief require a vacuum/inert gas oven?

Does 6Al-4V Titanium stress relief require a vacuum/inert gas oven?

(OP)
MIL-H-81200 for the stress relief of Titanium indicates 900° to 1200° F for 60 to 240 minutes. It also talks about various furnace atmospheres including vacuum, inert gas and others. It does not give specific requirements for the stress relief atmosphere. I've seen references to scaling occurring above 1000°F. Can I do a 900°F stress relief in an air atmosphere, or does this need a protective atmosphere? This is a welded structure from .080 thick sheet metal for a low pressure duct on a commercial aircraft.

Thanks,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Does 6Al-4V Titanium stress relief require a vacuum/inert gas oven?

KW... be advised that each Titanium CP/alloy type requires different SR time/temperatures.

An old welder friend of mine had similar problems with many 'one-off' Ti BAD parts were repairing.

He cleverly got around this by building a 321-A CRES box [thin sheet] large enough to accommodate the part. He welded shut the sides and one-end; then made a cover with tight flanges but open corners, which he screwed into position [CRES screws/nutplates]. He then welded a fitting to the opposite end of the box [fitting welded in-place over a hole!].

He then attached an inert gas-tube [or convoluted CRES HOSE] to the weld-fitting and passed the hose/tube thru the oven wall... which had a port in it that he had made specifically for this purpose.

He then flowed inert gas [IG, argon] into the box at a significant rate as the oven heated-up... which effectively purged the air from the box, replacing the local atmosphere with argon. As the oven went past ~700F to +800F he then reduced the inert gas flow to a low-steady rate for the entire SR bake process. Obviously the argon flowed from the weld-fitting on one-end of the box to/thru the 'leaky' cover on the opposite of the box. Since this process was essentially 'atmospheric pressure' there was no strain on the box from the gas or the heat. we used this for years!

NOTE. A friend's daughter did a science project on Ti-6Al-4V: she discretely heated coupons to various temperatures 'in-air' for a minimum of 15-minutes [oven stabilized at temperatures before insertion]. They went thru all of the classic color changes from straw to purple to chocolate brown indicating changes with alloy/grain/oxidation changes [alpha, alpha-beta, beta, case etc]. She Did very well at the science fair; and Her dad and I got coupon sets for mishap investigations involving over-heat/fire on Ti 6Al-4V structures. The coupons were hand-cleaned to represent real-world of mishaps only... not pristinely cleaned as in a lab.. which fully suited our purposes and hers!

http://files.engineering.com/getfile.aspx?folder=f...

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Does 6Al-4V Titanium stress relief require a vacuum/inert gas oven?

(OP)
Thanks Wil!

I think we're going to send this out this time, but building a box like that might be a good solution if we're running into this a lot. I've seen titanium weld repairs that have a color check to verify weld acceptability. I can't remember for sure which side blue was on, but the white at 1100° was unacceptable. I'll keep a copy of that for future reference.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Does 6Al-4V Titanium stress relief require a vacuum/inert gas oven?

Large pieces of Ti are SRed in air,and then pickled to remove the skin.
Thin stuff should be done in vac or Ar.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Does 6Al-4V Titanium stress relief require a vacuum/inert gas oven?

(OP)
We did a test on a sample of 6Al-4V at 900°F for one hour in air and it came out as a gold color, which for welds would be acceptable. Would this be enough of a justification that we can SR at 900°F for one hour in air and as long as the color of the duct does not turn medium blue, white or worse that it could be accepted as is?

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

RE: Does 6Al-4V Titanium stress relief require a vacuum/inert gas oven?

Parts can be wrapped in stainless steel foil (sold specifically for this purpose) so that a regular oven can be used for heat treatment.

RE: Does 6Al-4V Titanium stress relief require a vacuum/inert gas oven?

TIG welding of titanium components like this duct is often performed using a "glove box", which is purged using an inert gas like argon. AMS 2801 sec. 3.1.2.2 provides guidance on furnace atmospheres used for stress relief. It notes that "parts with net dimensions shall not be heated above 1000degF in air or non-inert atmosphere furnaces unless coated with a protective coating." Any alpha case present on the part surface after stress relief/heat treat can be removed as described in ASTM B600.

RE: Does 6Al-4V Titanium stress relief require a vacuum/inert gas oven?

Note... just to add some clarity to my original post...

My 'welder friend' always welded with inert gas completely surrounding the weld zones.

For most 'routines weld-jobs' this would include carefully placed gas-nozzles flowing toward the weld from every necessary angle/side for total gas immersion [before, during, after welding].

However... he 'loved' to weld in the inert atmosphere under a 'blown' acrylic bubble that was fully purged of air and filled with slow-flowing inert gas. My fuzzy memory sorta remembers that he had [2] bubbles: (a) a~24 x 24 x ~12-high and (b) a 36 X 36 X ~18-high and a special table. There were at least 2-pairs of integral pressure-tight gloves that we worked with and port(s) in the supporting table for the TIG cables and grounds... and sometimes a turntable he used to rotate the work-piece... plus multiple inert-gas ports. I think he also made 'sheet-CRES boxes' that allowed him to boost the height of each bubble for added working space/clearance. I believe he also had other 'homemade improvements' in the set-up that facilitated the work he did on aircraft engine parts.

NOTE.
My friend was one of the most talented, disciplined, skilled and knowledgeable welders that I've ever met. Unfortunately he had a 'few personality quirks'... the most egregious of which was the knack for disappearing ~an hour or two in the middle of the workday whenever he had nothing 'pressing-to-do' [to run errands, take a late lunch, etc], which just drove his rather narrow-minded manager nuts. The day they 'fired him', I gave him a two-thumbs-up verbal recommendation to another company who hired him on the spot with a raise and better benefits. DANG.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Does 6Al-4V Titanium stress relief require a vacuum/inert gas oven?

About 11 or 12 years back I did a contract job for a company that NG had offloaded some design work to involving weld fixtures for titanium HP pneumatic ducts on the F-35. It was amazing how involved the design process of these weld fixtures was. I worked closely with some great manufacturing specialists at NG, and they wanted things done in a very specific way. The duct pieces had to be constrained in the fixture to minimize residual stress after welding. It was preferred that the weld sequence could be completed without removing the duct from the glove box, since the glove box purge procedure was a bit of a hassle. So things like the size/shape of the fixture base plate, how the duct pieces were constrained, how the duct shape was oriented relative to the base plate, and access around each joint for making the weld and in-process visual inspection, were all carefully considered in the fixture design. In fact one significant part of designing each fixture was 3D modeling the work space required by the welder around each joint, and the line of sight provided across the glove box wall.

The finished weld fixtures were beautiful. But like most things on the F-35 project, I'm sure they cost a fortune.

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