Aluminum problems

Discussion in 'Materials' started by Diego San, Oct 21, 2014.

  1. Diego San
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    Diego San Junior Member

    Hello everyone,

    As I told you within my introduction thread, I recently joined a boat manufacturer company. I have learned a lot in these 3 months or so, but I still have a lot of things to learn.

    To make the story short, I am dealing with a fatigue problem in some structures made of Aluminum AA 6063 - T5 used on T-tops mainly, and I just want to know, Which Aluminum alloys do you use most for naval applications? I have read some papers which report a fatigue strength reduction for these alloys with the corrosion. I also have noticed some problems with the welded joints (where we have the greatest problems) , do you use a coat paint or layer on the welded joints?

    The change to a stainless steel alloy is not the best scenario bearing in mind the weight issue; However, have anyone seen the use of schedule 5s pipes for this purposes?

    Thank you in advance
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Firstly 6063 is not ideal alloy. It has half the strength of standard 6082 alloy which is the default standard for shipbuilding.

    For welding you need to ensure high quality welds by an experienced and qualified aluminium welder, not a steel welder retrained. There have been several articles over the past 2years in Proboat Magazine on fabrication/fatigue with aluminium that may be of interest for you. It explains how to avoid fatigue issues from fabrication.
     
  3. Diego San
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    Diego San Junior Member

    Thank you Ad Hoc, I will go through those articles and I will recommend to change the aluminum alloy.

    Regards.
     
  4. PAR
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    PAR Yacht Designer/Builder

    6063 does seem unreasonable on a T top, but 6061, which is also about twice a strong as 6063 might be a cost effective alternative.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Even though 6061 is widely available and used in the states, it is not the alloy of choice elsewhere. It has 3 times as much copper as 6082 and has been prone to corrosion problems. Which is why some Class societies don't allow it, or if you want to use it, demonstrate an existing equivalence using the alloy with a good in-service record.

    I have only ever spec'd 6061 once in the past 20years...and that was for a US build that couldn't get 6082!
     
  6. SukiSolo
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    SukiSolo Senior Member

    One of the better alternatives to 6082 is the 6005 alloy which is probably the closest in terms of tensile strength. The main reason this alloy (6005) has become more popular is that it is much easier to clean from the extrusion die. I've specified it a few times but not for marine use. You would need to research the resistance to corrosion and fatigue characteristics a bit more, but for non marine use it is a useful alloy.

    In the UK 6061 is not as widely available as a stock material at least I've not seen it commonly in stock holders catalogues or ever had an extrusion done in it.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    SS

    Some Class, like LR, wont accept 6005 any more either. Some corrosion issues (also 3 times the Cu content than 6082) and also it seems to be more prone to cracking when extruding complex and thicker sections as its ductility is much less.
     
  8. SukiSolo
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    SukiSolo Senior Member

    Thanks, Ad Hoc. One of the reasons I've used 6005 is at the behest of the extruders themselves when I have asked for 6082!. Not had any problems with moderately complex extrusions ie multiple screwports and other features. For really thin wall ie mast type thicknesses - 1mm to 1.5mm generally stuck with 6082. Not done any serious post forming ie tricky bending with the 6005 either so thanks for the guidance. Probably a lot more 'real' experience of this alloy now too.

    Maybe the problems of ductility relate to very variable wall section thicknessess where 'dragging' through the die may give a problem. Not usually a preferred option with extrusion design, although you can (and I have) get 7:1 ratio in soft alloys for use as heat sinks etc.
     
  9. Diego San
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    Diego San Junior Member

    Thank you all, this has been very formative.

    We are exploring the aluminum market in my country in order to find the alloys you recommend... I am also doing some research among published articles to get more used with this topic.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Being local is not essential, since just about everyone imports their aluminium. Contact the mills direct and if you order a enough, the costs will not be prohibitive.

    You can try Professional Boatbuilder Magazine No.s: 137, 147 and 151 for those articles on aluminium.

    This is also an excellence ref document too:
    http://www.alucluster.com/sites/default/files/Aluminium and the sea.pdf
     
  11. Diego San
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    Diego San Junior Member

    You are quite right; However, I believe there are some local policies regarding those imports. That seems to be a job for the purchasing department, I just would go through them to clarify this matter.

    Thank you for the readings, I will check if we have those numbers.
     
  12. Steve W
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    Steve W Senior Member

    6061 T6 alloy is probably the most commonly used alloy for extrusions in the US and does just fine for things like masts, T tops etc. Nobody is going to build a hull out of it. For many parts, the strongest or the most corrosion resistant is just not necessary whereas availability and cost is.

    Steve.
     
  13. Diego San
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    Diego San Junior Member

    The thing is we are building boats which are having extreme use. thus, we are having fatigue failures from 4 to 6 months.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Which is why you need to use the correct alloys (6082 - T6 , if extrusion and 5083 - 0 if plate), welders that are qualified and experienced in aluminium (not steel trained) the correct filler wires (5356 or 5183) a good QA system and attention to detail in the design.

    If you read those 3 ProBoat articles it outlines nicely the do's and don'ts as well as the nice summary in the link provided above.
     

  15. SukiSolo
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    SukiSolo Senior Member

    You should also find that custom extrusions with the possible exception of hollow sections are not that expensive if you have enough material. Usually done by weight of alloy through the die - the more, the better cost wise. Certainly if you could use 250Kg + on one section it would be worthwhile looking into. It is good practice to run a die once with a large ammount rather than multiple small ammounts as the die gets damaged when cleaning it and tolerances suffer.

    I've never personally had a problem with any custom extrusion not meeting the material designation or the temper condition, but most (not all) have been produced in the UK. Just make sure the extruder has done 6082 T6 before and is comfortable with it.
     
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