Weld two sections of aluminum mast?

Discussion in 'Boat Design' started by abosely, Oct 16, 2020.

  1. abosely
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    abosely Senior Member

    Using two sections of 6061-T3 6”x ⅛” tubes, one 20’ & one 12’ to make 32’ mast.

    Have read where some have had the joint welded.
    Would prefer to have joint welded if that’s possible rather than using an inner sleeve and riveting.

    Would putting a sleeve inside to assist with alignment, plug weld the mast sections to sleeve and then weld the joint work?

    Would have certified marine welding shop do the welding.

    Being welded would then grind welds flush & polish up.

    Cheers, Allen
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Too many unknowns to answer fully.
    However, are you sure you have quoted the temper correctly..6061 - T3?
     
  3. abosely
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    abosely Senior Member

    Actually I mistyped the temper. :-/
    Meant to be T6.

    Cheers, Allen
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Allen

    That's what I suspected, hence the clarification.
    Well, are you aware of the drop in strength from unwelded to as-welded mechanical properties?
     
  5. abosely
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    abosely Senior Member

    Somewhat, I read mention about having aluminum masts welded, then asked the question.

    I continued reading and trying to learn some about this and saw comments about welded having reduced strength.

    I haven’t pursued this much yet as just starting to learn about the possibility of welding mast.

    Did see a wide range of strength loss stated.

    Now that I know that there is some loss of strength when welding, would like to find out what percentage of strength looking at in this situation.

    I don’t know if welding method makes a difference, or if something can be done to offset the loss of strength in the weld areas.
    Does the size of heat affected area vary with different welding methods?

    Would having a inner sleeve be able to offset the strength loss?

    I guess it would make a deference depending on how the joint was made also.

    Curious what is done when components are welded to a mast and when aluminum boats are built.
    I know there are different alloys used for aluminum boats, but from what I understand 6061 T6 is used in number of applications along with 7000 series.

    Cheers, Allen
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    It goes from 240MPa:
    upload_2020-10-17_13-32-12.png

    down to 115MPa
    upload_2020-10-17_13-32-46.png

    So a significant drop.

    Yes...don't weld it!
     
  7. abosely
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    abosely Senior Member

    Well that pretty much answers that question. :) Thanks for the direct information, now I know this isn't a wise method to use.
    Cheers, Allen
     
  8. Barry
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    Barry Senior Member

    Is there a mechanical method of joining the two sections that would keep the strength of the mast at the "unwelded" specification?

    With fasteners I would expect excessive stress concentrations
    Just curious
     
  9. baeckmo
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    baeckmo Hydrodynamics

    Glue. Many years ago I saw a Russian technology (then the Leningrad Polytechnic, I think), used for load carrying components in rivergoing hydrofoils. The surfaces were prepared and glue (don't know what kind) applied, but for a number of spots. These spots were spot welded to keep things together, and when the glue had hardened, the center of weld spots were drilled away to relieve stress. The structure was impressively light and strong, and they reported a high resistance to fatigue.
     
  10. clmanges
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    clmanges Senior Member

    Glue, yes. You'll still need an insert at the joint. And I have a suspicion that this might be something you'll want to have done by a specialist.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    There are several ways in which this can be done... but this is applicable to whether the joint/sleeve is inside or outside.
    The key being it must be a close fit, not sloppy/loose.

    So, let's consider 2 mast/structural members to be joined....like so:

    upload_2020-10-18_9-27-38.png

    So, we offer up the 2 sections...and the use another member as the joint..or sleeve.

    The question becomes who much of an overlap do you need? Since if, referring to the image, L is very similar to Lm...there should not be any issues. But, if Lm is significantly longer than L, the joint wont allow the member to act as "one single" member, it will still behave as a composite.

    So, under an applied load the member, when one single piece would take up a classic bend shape with a radius R.
    noting the classical bending equation: M/I = (sigma)/y = E/R

    We obviously wish to ensure that when joined, this remains the case. So the joint/sleeve, must allow this to occur, otherwise you will introduce different load paths and behaviour/response, thus more detailed analysis is required.

    When the member bends, if, for the sake of simplicity, it is an RHS, the upper and lower regions will go into tension/compression. Thus there is a force that needs to be transmitted from the original mast via the sleeve.

    So looking in XSA, the number of rivets/bolts etc, must have enough total shear area to transmit the load from the original member through the bolt/rivet into the sleeve and then back the other side.

    The key being the bearing stress at the rivet/bolt location in the beams flange. This is the source of the SCF. Eliminate the high bearing stress, reduces the localise stress.
    The the same procedure on the flange region of the sleeve. And of course not to be placed too close together.
    A simple rule of thumb, the diameter of the rivet/bolt should not be any greater than the thickness of the material is it attached to....so this then determines the number of fasteners you need and thus, can it be done, given the geometric shape of the mast and sleeve.

    And for good measure, apply plenty of glue ...

    Alternative, instead of fasteners, you can, as Baeckmo notes, use just glue. In this case the shear strength of the glue is the means of load transfer.
    All you would do, is add a few pop rivets to prevent possible slip/joint alignment during assembly.

    That's the basic principal.
     
  12. messabout
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    messabout Senior Member

    If Abosely was on the mainland he could arrange to have the entire mast extruded in one piece. There are a whole gaggle of aluminum extruders stretched across the U.S. I have worked with several of them in the past. Most of the time they are willing to do a special part for a little bit more money.
     
  13. fredrosse
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    fredrosse USACE Steam

    While the weld reduces Aluminum yield stress, a tight fitting inner sleeve, say 3/16 wall thickness, with properly arranged plug welds, can be made very nearly as strong as the unwelded continuous mast. Alternately, a tight fitting internal sleeve, having an easily calculated wall thickness, can be selected such that bending moment resistance throughout the mast will meet or exceed the capability of the heat treated 6081 single tube, all with just a single butt weld where the two pieces of 6 inch material meet.

    One issue that has not been addressed here is the loads on the mast vs. position along the mast. There are positions along the mast where bending moment is far lower than the position of maximum bending moment, and if the position of the weld in the mast can be located at a point with bending moment < (165 / 240 ) of the maximum bending moment within the entire mast, then a good full penetration butt weld would do the job even without a sleeve.

    The bending moment as a function of mast length coordinate depends on the mast mounting method, stays, sail positions, etc. and should be available for analysis by one educated in stress analysis.
     
  14. Rumars
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    Rumars Senior Member

    May I ask why riveting is not desired? It is after all the industry standard in splicing masts, plenty of production boats have them. With a round tube there is not even a problem with finding the proper sleeve, you just buy a tube of the same diameter with a thicker wall and have it turned down on a lathe. Use monel rivets, add some epoxy for safety and it's bombproof.
     

  15. abosely
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    abosely Senior Member

    I have these shipped anyway, the longest they have is 20' hence needing the joint.
    What is recommended is 5.5"/140mm x .154"/4mm x 32'/10m. I was going to ask about going with 6"x ⅛” or 5" x 3/16" because hadn't found 5.5" x .154" yet. But sizing possibilities was going to be another question. :)
    If I can get a 5.5'/140mm x .154/4mm x 32'/10m (know 10m is actually 32ft 9.7in) but either is fine for it, as might need trimmed a bit.
    What are some places that might have or make this size?

    Cheers, Allen
     
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