Titanium vs other metals

Discussion in 'Boat Design' started by Stumble, Jun 5, 2012.

  1. Stumble
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    Stumble Senior Member

    Recently I have been playing with the idea of switching to titanium parts for some applications. Acceptingmetalstorm is more expensive, I am curious how much more expensive it really should be.

    For instance lets say I want to replace an aluminium bowsprit that is a 3"x.25"x60" tube with a titanium tube. As I understand it, I could directly replace that tube size with a titanium one of the same dimensions, and be fine. But the it would be much more expensive than it needed to be because the titanium is so much stronger than the aluminium it replaced.

    What I am trying to figure out is how to determine the size tube of titanium I would need to use to get similar strength as the aluminium it replaces. And can therefore price a tube with similar strength as opposed to the size.

    Accepting I wouldn't actually do this without a designer signing off on it, I am curious how it would be done.
     
  2. philSweet
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    philSweet Senior Member

    Stumble, I have this image stuck in my head of you sailing along screaming STARBOARD at some port tacker and him rapping the coaming and yelling back STEEL. Then you smile back, point at your sprit and shout TITANIUM.

    Am I close:?:
     
  3. Stumble
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    Stumble Senior Member

    Lol, I had not considered that. I am just curious about the true "cost" of titanium. A lot of people I know have discounted it as being too expensive. But they base that on direct replacement of aluminium or steel with an equal sized piece of titanium.

    So swapping a #10 316 bolt with a #10 grade 5 Titanium bolt. this may be perfectly reasonable I don't know. But if The grade 5 has twice the tensile strength then is it really necessary to use the same size for the same bit (ignoring issues like fastener holes being predrilled, and other production issues).

    So if it were possible to replace a #10 316 bolt with a #8 titanium bolt, the relative price for the fasteners would change.

    Just for example, these are the retail prices I have for the following:

    #10-24 X 1/2 Hex Head Bolt, Grade 5 - $2.27 each
    #8-32 X 1/2 Hex Head Bolt, Grade 5 - $2.13 each

    #10-24 X 1/2 hex head bolt, 316 - $1.75

    So if you could switch down to the smaller titanium bolt it is still more expensive, but the cost for titanium goes from 130% the cost of 316 to 120%. Not a huge difference, but when speccing out a boat full of stuff it might be worth it.

    At least when combined with the advantages of titanium over stainless.
     
  4. JRD
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    JRD Senior Member

    Gidday Stumble,

    I have in pride of place on my workbench, a set of titanium plated drill bits, guaranteed by the guy on tv to effortlessly glide through.... well aluminium blocks actually (funny how they dont show you a drill going through 316 stainless steel, turning blue with smoke in all directions). Anyway they have a gold coating and seem to last as long as the standard HSS drills they replaced :)

    I started to attepmpt an answer to your question, when after three paragraphs of typing it was obvious I was nowhere close to a useful conclusion.... I decided to replace with the answer with this. Theres actually rather alot of engineering calculation and understanding of both sets of material properties and characteristics required to make a comparison with any level of confidence. As with all the answers here, it depends on the complexity of the structure you have in mind, sorry :(

    If I wanted to make a quick cost comparison, I think I'd get a price for the old and new material based on the section size I already had. If you dont like the price of the titanium at the same section, its unlikely the cost will be much less at the re-engineered section size by the time you account for freight, cutting and handling costs.

    In my own observations carbon fibre seems to have replaced titanium as a more cost effective replacement for light weight parts, and manufacturing techniques are continuously being refined to allow new items to be laminated with high levels of quality assurance. I understand that the capital cost of building titanium manufacturing facilities is high and way out of proportion those required for other metals, which probably explains why it has remained a metal for specialty uses only.

    Edit - I was thinking more about extruded sections and the complex stress combinations they have, with buckling always being a factor when you reduce wall thickness. Comparison of fastenings based on higher strength vs smaller section would be straight forward as you suggest.
     
    Last edited: Jun 5, 2012
  5. Petros
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    Petros Senior Member

    That is a complex question, it depends on the loading of the item you are replacing. Typically you are designing to a combined criteria of bending stress, sheer stress, and deflection, and with tubing also a buckling criteria. Aluminum, steel and titanium have different properties in all of these areas, so you have to design to the critical loading condition. The fatigue strength can be an issue with parts that are cyclically loaded since aluminum has very low fatigue life compared to steel or Ti. that is not likely an issue in this application.

    For example, you can have a column or a truss designed in steel and in aluminum. the steel one might be sized for buckling, because that is the critical load condition, while the aluminum one might be sized for deflection limit (and is stronger in the other areas than it needs to be). Therefore in complex loading conditions or complicated structures (like a truss), the question is not easy to answer.

    for example, a typical Ti alloy has the following properties:

    wt. 0.16 lb/in3
    tensil 128,000 psi
    fatigue 34,800 psi
    shear 79,800 psi
    modulus 16,500,000 psi

    6061 t6 aluminum:

    wt. 0.097 lb/in3
    tensil 40,000 psi
    fatigue 14,000 psi
    shear 30,000 psi
    modulus 10,000,000 psi

    So to get the same strength from say a length of tubing, you need more material with the AL, but how much more depends on the length of the tube, and the dia. So that is not an easy anwer, you need to have an engineer analyze the various loading condition, optimize the design for the two different materials, and than compare total manufacturing costs.

    Usually the Ti comes at a high price overall, and that is why you do not see it used much unless it is in a really weight critical application (such as aerospace).
     
  6. Stumble
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    Stumble Senior Member

    I appreciate the thoughts, and certainly understand the difficulty. Heck I spent four hours today trying to figure it our before deciding that my math and engineering skills were woefully inadequate to the task.
     
  7. upchurchmr
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    upchurchmr Senior Member

    Stumble,

    For the last 5 years I have been working to reduce the cost of a military aircraft. With about 100 approved or in work concepts, I have formed some simple rules. The first priority is to change Titanium or composite to aluminum. We just reviewed a design change an the $/# for the same finished part showed Titanium 8x more expensive than made out of Aluminum. Graphite Composite was just about the same (this is not "wet laid" fabric which is much cheaper). Now this is aircraft which is inherently more costly due to the "special" design effort used.

    Depends upon the Ti alloy used, if you Ti 6-4 Mill annealed material it will be cheaper that the most expensive longer lasting Ti 6-4 ELI. I know that is gibberish unless you are an engineer and I just give you the simple facts in hopes you will be happy to not pay 4-8 times the cost of a well designed Aluminum part. Non aerospace might only be 2X.

    You may well find a specific part that is not that expensive. Ti is a great material, stronger/ longer lasting if you use the part for a long time, very non corrosive, easy to weld, and very tough. But not always lighter weight even if you are an experienced engineer who can design both parts (Aluminum and the equivalent Titanium part).

    Fasteners are a different situation, the price depending more upon how many have been made. So pick a common bolt size. Just remember that if you are really trying to get lighter weight, what you are bolting together has to be able to take the load of a smaller fastener.

    Sorry for the long winded, self important post.
    Hope this cures your interest.
    BUT, you might find just the right place where it is worthwhile using it, someday. One of the best reasons would be for corrosion resistance - probably with composites.

    I wish I had buckets of money to use Ti.

    Marc

     
  8. Stumble
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    Stumble Senior Member

    Marc,

    That was pretty much what I have determined up till now. But thanks for the confirmation.
     
  9. DCockey
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    DCockey Senior Member

    Professional Boatbuilder No 132 has a survey article on titanium and boatbuilding.
     
  10. FranklinRatliff

    FranklinRatliff Previous Member

    Bloodhound SSC

    http://www.bloodhoundssc.com/news/l...=customWidgets.contentItem_show_1&cit_id=5378

    Despite having admitted they already have an aft c.g. problem, the Bloodhound SSC land speed team is trying to make their rear bodywork out of titanium (!) and keeps cracking panels. Their back-up choices are steel or aluminum. When I suggested to Andy Green they consider magnesium apparently nobody on the team had even tried to look at what would be involved in doing it with magnesium.

    They will be running on a claypan in South Africa instead of a salt flat so corrosion doesn't even start to be an issue.
     
  11. DCockey
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    DCockey Senior Member

    Sounds like there may be a non-technical reason (sponsorship, prejudices of a team member or ???) for using titanium over aluminum for the bodywork. Usually for bodywork panels bending stiffness and buckling load are major considerations. Looking at the density and modulus above, an aluminum panel can be thicker with higher bending stiffness and buckling load but weigh less than a titanium panel.

    Thin magnesium is generally not used in thin panels due to the fire hazard. This FAA report discusses the flamability of magnesium: http://www.fire.tc.faa.gov/pdf/ads-14.pdf
     
  12. upchurchmr
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    upchurchmr Senior Member

    Franklin,

    I have examined some fittings made of Magnesium on older aircraft. One way to retard the corrosion and flamibility is to coat it extensively in paint. One part I saw had ~.15 thick coatings, but was in good shape after a complete service life - this might be too much paint to retain the weight benefits.

    There is a company who makes inconell braised sandwich panels which are very light. No telling if they would have enough strength at the extreme thinnesses in the sample I had, but the thinner panels can be formed to shape just like sheet metal without damage. Don't know the cost. I can get the contact information from work if this is of interest.

    Stumble - I just saw you are a salesman for a Ti company. What kind of products do you represent? Having an inside ability to get material might make all the difference in the cost of parts, but the cost to machine is about 6x higher than aluminum for my parts, and limited to fewer shops for a good job.

    Marc
     
  13. CDK
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    CDK retired engineer

    Very nasty material, titanium. An old friend of mine gave me a sample plate, 6x10 inches, saying that it had been in his desk drawer for over 20 years. He thought that maybe I could use it.
    I can't weld it, braze it and my metal saw lost its teeth on it.

    There are application though, turbine vanes in jet engines and the heater in etching machines for printed circuits.
    And of course the doors of my fridge, but just like most other household titanium items, that is just paint vaguely resembling the ugly titanium color.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Back in the 80's the story was that titanium prices were being pushed up by alleged hoarding by the Russians to build super-deep diving subs. I wonder if they built any such vessels, or it was just a tale.
     

  15. upchurchmr
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    upchurchmr Senior Member

    Titanium requires very special methods of working.
    Machining has to be at a very slow speed with lots of power and you can't take a very small "bite".
    Even a metal saw will typically be way too fast of a cutting speed and it is very tough - a virtue unless you want to cut it.
    I don't know of anyone who brazes it and I have asked.
    Welding apparently is easy if you have the right equipment. I watched a Ti casting house weld to repair surface defects and they said they require almost no training for their people.

    There was another story about an aircraft made of Titanium, but when one was obtained it was mostly steel according to reports.
     
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