patch repair in large 6013-T6 Curved ALUM Sheet

[edmeister]

Situation:
Have a 7 Foot length of curved Sheet .063" THK - 6 Inch radius made from 6013-T6. (Aerodynamic leading edge) Require to remove a damaged section (ie: 5" Diameter)from anywhere on this surface. Propose to cut out & weld in flush filler of same material.

Problem:
-What will be the Post weld Temper in the local area?
-Any suggested techniques to restore original -T6 temper on such a large item without distorting other areas?
-Who would have facilities for such a procedure?

 

[MikeHalloran]

Post weld temper will be -0 ,dead soft.



Mike Halloran
Pembroke Pines, FL, USA

 

[TVP]

Aren't these type of parts peen formed? Likely the alloy sheet is already in the T6 condition and then it is peened into the desired shape. There is no way to weld a small section and expect it to have the appropriate shape AND strength. I suggest you talk to the appropriate people with the manufacturer, etc. before attempting a repair.

 

[edmeister]

The skin is originally machine - milled with .040" THK Pockets & then Press-formed to shape. There is no other operation done to these panels. We are working with the MFR to discover an alternate means of repair. These Skins are in close proximity to a Heated de-ice tube which does not permit standard sheet-metal repairs- filler & internal doubler (Head-conduction issues).
- A procedure to weld repair a small segment & heat treat uniformly the entire length without distortion is the goal.

 

[MikeHalloran]

I'm aware of a technique called 'hammer welding', wherein a full penetration butt weld is reheated with a gas torch and the bead is hammered flush with at least one sheet face, using a dolly and a body hammer ... on steel. I've never heard of anyone doing it on aluminum, but I won't say it's not possible.

The -T6 condition involves solution heat treat and artificial aging, and I think it's normally done on an unformed sheet that may even be under tension.

You can probably find a heat treating oven that will handle a 7 foot article, but I'm pretty sure distortion would be a huge problem.

Some car bodies are now made from laser- welded blanks, wherein e.g. high strength steel, bake-hardenable steel, and ordinary mild steel cut sheets are butt- welded with essentially no bead and subsequently formed in normal press tooling. There's less scrap, and the more expensive materials are used only where they're needed. Again, that's done in steel.

There exist 5-axis laser machines that can cut arbitrary profiles in all faces of rectangular (steel) tubing. Maybe they could also cut odd shaped holes around damaged areas in the leading edge, then cut a blank that could be formed to fit the hole exactly. I think laser cutters can also produce a very fine weld, again with essentially no bead. I don't know if they can do it in aluminum. I assume that they could be induced to deal with aluminum, given sufficient money.

Where I'm going with that thought is maybe (this would clearly be done at a well tooled depot) the patches and holes could be cut with edges like the finger joints you find in modern wooden molding, laser welded, and left in the -0 condition.

I'm assuming that the finger joints would distribute the stress along a narrow weld line whose developed length greatly exceeds the girth of the patch, reducing the stress at the weld line proper, and making postweld heat treatment unnecessary.

That may or may not work, depending on how the leading edge is actually stressed. Maybe the factory has some FEA wizards available to help you evaluate that idea.





Mike Halloran
Pembroke Pines, FL, USA

 

[RPstress]

6013 is usually formed in the T4 condition and optionally aged to T6.

Since T6 is a 375 F/190 C heat treat, the anti-icing air must be a bit less than this. You might get away with a BMI adhesive bonded scarf repair, but it's pushing the limits of practicality, to say the least.

To get T6 properties back post-weld, ASMH recommends solution treatment at 1000 F/540 C + water quench + age. You'd think that would warp it up a treat.

The as-welded properties aren't too bad, surprisingly. According to ASMH a GTA sheet weld with 4043 filler rod had Ftu 36 ksi and Fty 26 ksi. Re-aging upped these to 38 ksi and 35 ksi, but for such a small increase in Ftu it's probably not worth the risk. However, these properties are unqualified, so some sort of test programme would probably be necessary.

The main reason for going to the expense of aging a leading edge is birdstrike. Penetration speed is roughly proportional to Ftu, so a knockdown of 0.7 (36 ksi vs. 52 ksi for T6) is quite large. It depends on how much of the bird resistance is from the LE and how much from structure behind it (usually a spar web).

If birdstrike isn't a worry, then clearing it stresswise with a big hole (i.e., ignore the welded material - a conservative approach) should be possible, though it does beg the question as to why it was made T6 to start with. On large aircraft the leading edge is often regarded as essentially secondary, with all of the wing bending and torsional strength coming from the main box aft of the leading edge.

 

[Compositepro]

Perhaps these issues is why this type of repair usually is done with riveted patches. This question may be better posted in the Aircraft Engineering forum.