Re-welding of 6061-T6 tube

[chief52]

I have an existing equipment rack frame made from square 6061-T6 tube. We need to modify the frame geometry, so want to lengthen one tube. We intend to cut the tube between existing welded corner joints and splice in a longer tube. The rework will provide instruction to grind off all existing weld mnaterial at the joint. My concern is at the existng corner joint we will remove existing weld from the tubes and weld the new longer tube back onto the existng corner tubes.

What are people's thoughts related to expected strength of rewelded corner, decrease in expected tensile strength of material at the corner, etc. We do not plan on re-heat treating after welding.

 

[bf109g]

That's a little iffy. If the original part was heat treated after welding, you can't show that your modification results in the same joint strength as the original.

If you have an 8 oz box on the rack, the reduction in strength may be acceptable, but somebody will have to analyze that.

To get an idea of strength of 6060 T6 after temp exposure take a look at either MIL-HDBK-5 or MMPDS. Granted, the graphs don't necessarily reflect what acutally happens during welding, but you'll get the idea.

 

[chief52]

Thanks, bf109g. That's what I was thinking, diffucult to quantify the loss of strength.

The original frame assy was not heat treated after welding. Various references show strength at the weld filler (5356) approaching the original strength of 6061-T6 without post weld heat treatment (38 ksi filler vice 42 ksi 6061-T6). (Ref Mandal, Aluminum Welding, 2nd Ed, Table 1.3)

On the other hand, I have nothing to indicate strength after re-welding using 5356 filler material (NASA Materials Data Handbook for 6061, Table 12.2 shows auto-TIG strength after rewelding, but for 4043 filler, and we are using Manual TIG).

 

[madvlad]

You guys are way off!

If you don't heat treat after welding, you don't have a 6061-T6 frame, you have a 6061-??? frame. It is NOT going to be as strong as -T6, regardless of what filler you use. It'll probably lose 50% of it's strength, or more.

For certification of aircraft racks we use 8 ksi yield, 18 ksi ult, as for 6061-O...the assumption being that the heat-treat condition near the weld is worst-case. You can't claim the aluminum is any stronger than that, because you don't know for sure.

I would agree that the racks are stronger than 8 ksi, but the FAA/TC won't.

---unless you do a proof test...which can be the easiest solution sometimes...ie one-offs. For example: if you have a 10lb LRU, just bolt in 90lb of steel (ie 9G crash or whatever your load case is), hang the rack in all orientations, examine it for damage, write a 1 page report, and you're good to go.

Chris

 

[MNLiaison]

From Lockheed Stress Memo 129a, welding 6061-T6 aluminum with no re-heat treat will get you 60% of the -T6 base metal strength. With a re-heat treat after welding it's a 85% -T6 base metal strength.

Regards

 

[GregLocock]

I call shenanigans on the 60% number.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MNLiaison]

I must admit I haven't proved it personally. I assume that Lockheed doesn't just throw anything in their stress memos...but who knows. This might be a good little project for some welders and the Instron to get some ball park numbers.

 

[GregLocock]

Do they specify a welding method? we were using a small mig so things might have got rather hotter an usual and - to be fair we didn't measure any stresses afterwards, but the thing kept bending.

There is an awfully large difference between T6 and T0, 60% implies T4 or thereabouts.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[RPstress]

There's some help in this thread:

thread330-225831

I found some data in the ASMH (Aerospace Structural Metals Handbook) and posted it there.

 

[Miner89r]

Many of the Materials and Process engineers at Lockheed don't believe the 60% strength after welding either. When I tried to use it I was told it was not an acceptable reference.