HAZ reducing shear capacity. 3

[wadavis]

I'm reviewing the design of a stout aluminum box beam that has a history of shear failure in the HAZ at the web to flange connection. This is an exercise in optimization, we wish to change the welded detail to increase the shear capacity without overhauling the rest of the beam.

How do you, as the designer, reduce the degradation of the HAZ?

I'm looking at a full penetration bevel with a backer (only access from one side), this increases the bonded surface from the old detail, but adds four times the filler metal. Does that much more filler metal, and therefore heat, have a strong effect on the HAZ?

wadavis
E.I.T.

 

[gtaw]

What the alloy?
What is the welding process?
What is your heat input?
Details, details, details.

Best regards - Al

 

[wadavis]

6061-T6 Aluminum
GMAW
Heat input fails outside my experience.

The box beam has 2 - 1.5" x 7" flanges with 2 - 5/8" x 9" webs with a 4" wide void inside the box beam. Partial drawing attached. The critical load is shearing load pairs at 12" - 24" apart at both ends, with the transverse shear flow along the HAZ zone failing.

The web to flange connection is semi-accessible on both sides for half the connections, but not for the other half. The current detail is a 1/4" bevel, 3/8" fillet.

wadavis
E.I.T.

 

[dhengr]

Wadavis:
You say... “an exercise in optimization,” but may actual mean ‘an exercise in futility.’ When working with aluminum and welding and allowable stresses, more weld usually doesn’t work you out of the problem. The heat from the welding process eliminates the temper in the aluminum for an inch or so, in all directions from the weld, and consequently seriously reduced the allowable stresses in those regions. And, in most cases, more weld doesn’t change that or reduce the HAZ. You have to do something with the details, to move the welding zones to regions of much lower stress, not just add weld to reduce the weld stress. In the regions you are welding, you have both high bending normal stress and relatively high shear stress. The short answer to your question is don’t use welding in those regions.

 

[gtaw]

A "good" weld in 6061-T6 is only 24 ksi. That's down from the as wrought condition of 42 ksi.

The unit stress on the base metal must be considered. As in the case of structural steel, the weld/base metal interface must be considered to prevent the weld from "pulling out" of the base metal. When designing a connection, the yield strength of the HAZ must be considered to ensure the loads are limited to a fraction of the as welded yield strength and within the elastic range.

Reduced tensile test specimens typical break in the HAZ.

The weld is as strong as the base metal in the "as welded" condition if the proper filler metal is selected and the WPS takes into consideration precautions that are somewhat unique to aluminum and its alloys.

When welding with the GMAW welding process, make sure the parameters ensure the spray transfer mode is being used. Do not use pulse or short circuiting transfer when welding aluminum. Keep the interpass temperature low as practical. That means the use of mechanically enhanced cooling, i.e., blowers, directed to the joint between weld passes. Do not use preheat.

Best regards - Al

 

[dhengr]

Wadavis:
How many of these do you make? What are they used for? I’m not sure I understand the load conditions, show them on your sketch. Do listen to Gtaw’s advice, as always, he knows what he is talking about when it comes to welding. If it’s important enough, why not save the money for all the additional welding and consumables, etc. maybe even less costly base plate materials, using normal weld sizes and detailing and temper or retemper the whole beam after welding.

 

[wadavis]

dhengr; ha thanks for the exercise in futility comment, I'll have to view the projects the E.I.T. gets with suspicion. This redesign is in response to shifts in the industry requiring heavier lifts from equipment with the same clearances, we are constrained in our design by the outside box dimensions and a light enough to be handleable. The loading condition is shown in the attached file. These are a piece of specially rigging equipment, there will only be about 12 - 24 of them made and in batches of 4, so I'll have to investigate the retemper process and find out where it sits on the cost/gain. This redesign is in response to shifts in the industry requiring heavier lifts from equipment with the same clearances, we are constrained in our design by the outside box dimensions and a light enough to be handleable. Again, thanks for the input, I'll be investigating the retemper process.

gtaw; It looks like my ignorance of GMAW and preheat process is hindering my progress (The existing design includes pre-heat). Can you recommend reading material regarding spray transfer, circuiting transfer, and the effects of interpass temperature? Can you recommend a welding design guide / textbook?

 

[gtaw]

AWS D1.2 Structural Welding Code/Aluminum is a good start.

Best regards - Al

 

[EdStainless]

Moving the welds may be the best option, and even then you may need to look at heat treatment of the final assemblies.
What if the vertical parts looked like this ] [ instead of flat pieces?
You would need to make the top and bottom plates thinner by 5/8" in order to keep the total flange thickness constant.
It might be worth some modeling to see.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[MikeHalloran]

Right at this point, I'd be looking at hogging two sorta I-beams from 3.5" x 12" flats, and bolting them together at the flanges. ... with no welding.

... with really generous radii at the flange to web intersection, of course, and maybe even hourglass tapered webs. Pretty much anything is possible with a big-ass ball end cutter in a 3-axis CNC milling machine. All you gotta do is shovel out the chips.





Mike Halloran
Pembroke Pines, FL, USA

 

[wadavis]

AWS D1.2 looks to be more comprehensive than CSA W59.2 Welded Aluminum Construction which I'm familiar with, I'll be looking into it.

Any comments on the value of Welding Aluminum: Theory and Practice, Fourth Edition?

 

[MikeHalloran]

In days of yore, Alcoa gave away nice books on aluminum- related subjects, including one that just dealt with welding.

Maybe you can find one on Fleabay.
The title is "Welding Alcoa Aluminum", naturally.


Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

6061 is a heat treatable alloy; post-weld heat treat is typical. On it's own post weld it should rebound from -O (annealed) to a -T2 to -T4, given enough time. With heat treat it's back to T6. Do some pull tests on samples to make sure you are using the right filler metal in any case.

There will be some distortion and post-treat straightening required. If that's not acceptable, then the suggestion to move the welds or avoid welding is a good one.

If there's no welding at the end of re-design, 7xxx series aluminum has even higher strengths, allowing smaller sections for less material. 7075, for example.

 

[gtaw]

While aluminum alloy 7075 is known for its high strength, it isn't generally considered to be weldability using one of the common fusion welding processes. Stir friction welding has been performed with success, but it is a solid state weld and limited to relatively thin materials. The zinc, used as an alloying constituent, with its low melting point and low boiling temperature, makes it a poor candidate for fusion welding.

Best regards - Al

 

[3DDave]

Which is why the 7xxx suggestion is prefaced - If there's no welding... - but it doesn't suffer from repetition. 2xxx series are also not particularly weld-worthy. 5xxx are considered very weldable and don't require post-weld heat-treat, but the properties aren't good to begin with. I saw either 2xxx or 7xxx welded one time - it looked like a Pillsbury flaky biscuit with significant delamination.

I finally took a look at the drawing. It's a good candidate for extrusion if the qty is high enough. The cost of the die and the minimum run balance against the cost of cutting out the pieces and setting up to weld with welding's subsequent problems of a HAZ, porosity, and other weld-related failure modes. Still have to add the round cross-rod, but that's it.

 

[MikeHalloran]

The last time I looked at extrusions, 30+ years ago, the biggest extrusion press in the world was in Arkansas, and could do anything you could fit in a 13" diameter circle, so the extant beam would need a chamfer 1" wide x .52" high on each external corner to fit in that extruder, but that wouldn't affect the section properties a lot.

If you moved the webs inboard a bit, you could extrude the flanges with a curve on the distal part, and press them flat before heat treating. This is commonly done for large heat sinks; they are extruded with a circular section and flattened while still soft.

Also, there surely must be a bigger press somewhere by now, so extrusion as per your drawing should be possible, and is worth exploring.

The good news is that the minimum order quantity for that big press was only 5000 lbs., which is not a huge length of such a heavy section, and extrusion dies are surprisingly inexpensive.

The closed box section adds some complexity to the die, so if your application could use a single thick web, that would save you some bucks.



Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

I didn't look first sooo.... it could have been out of reach, but

http://www.shapesllc.com/cap_extrusion.cfm

claim a 24 inch circle and up to 40 pounds per foot.

http://www.bonlalum.com/large-aluminum-extrusions.shtml

are at 16 inches wide, but wimp at 20 pounds per foot.

and Holy Heck!

http://www.aluhorizon.com/custom_shape.html

scroll down the page a bit. Thin walled, but still.

When I started out if a section wasn't in the shapes book, forget it. Even then it better be on a rack.

 

[MikeHalloran]

Well, there you go; easy-peasy.
You can even get competitive bids on the part.
... I suggest that you do.
... after adding some nice fillets to the web/flange interfaces.



Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

MH,

I do think getting a weldment heat treated will be on the easier side in the short term than getting time on a giant extruder, but it's still impressive, the chair especially.

What I want to see someday is variable section extruders. Dang - someone beat me to it: Patent US 5989466. Wonder if one's been built yet.

 

[MikeHalloran]

I wouldn't assume that getting time on a big extruder is a problem. Someone who owns a machine that big doesn't want it to be idle, and should be happy to work with you.

Let me put it in perspective: The 5000# minimum order for that 13" extruder corresponds to exactly _one_ stroke. Building the die will be the biggest delay, and it probably won't be as bad as what you're imagining.

As for stepped sections, you could thin the web of your part with a 1/2" router, or of course a common CNC mill.



Mike Halloran
Pembroke Pines, FL, USA