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I was just reading this article on how Curiosity's aluminum wheels are getting pretty dinged up. Usually titanium alloy is considered an upgrade from aluminum alloys in most applications, from aerospace to bikes to sporks. The "usual" downsides are that it is expensive and difficult to machine. However, those seem like less important factors when you're NASA and the plan is to send an object to another planet and drive over sharp rocks.

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This answer from Reddit indicates there were two drivers, minimizing mass of the wheels for part of the landing sequence and expense. Expense is a tricky one because they wanted a very strong starting material. That limited them to block or extrusions. They could find aluminum pipe extrusion stock, but not titanium, so ultimately that pushed them to aluminum. Since they were machining the wheels down from pipe stock, that placed a minimum thickness for the wheel material due to machining tolerance, so even if they had a titanium source that was cheap enough, they couldn't make the wheel any thinner to lower the weight to what titanium would allow, so that was the other reason they ended up choosing aluminum. Credit to my coworker Tiffany for the Reddit find, and my coworker Matt for also pointing out that the low temperature on Mars would have also exacerbated the brittleness concerns for titanium.

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Aluminum is less dense than titanium. For the same mass, the 0.75mm thick aluminum would have to be replaced with 0.45mm thick titanium. Although the titanium sheet would be stronger in tension, it would probably be more susceptible to tearing from a point load like driving over a sharp rock.

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    $\begingroup$ Titanium is brittle. I suggest adding relevant data from MatWeb to your answer. $\endgroup$ – Deer Hunter Feb 5 '14 at 4:03
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    $\begingroup$ Ti alloy is 1.57 more dense than Al, but has 1.75 greater yield strength and 1.66 times stronger in shear (tikore.com/titanium_facts.htm). So wouldn't that mean it would be less susceptible to tearing even considering reduced thickness from its higher density? I don't have numbers to back it up, but I'm suspecting it might be something about brittleness as @DeerHunter suggests -- JPL probably would rather have Al wheels full of holes than a fractured Ti wheel. Plus the cost and inconvenience in working with Ti. $\endgroup$ – joseph_morris Feb 6 '14 at 20:26
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    $\begingroup$ @joseph_morris: puncturing a membrane gets much easier as the membrane gets thinner. I suspect the brittleness may be involved as well. There are many factors that go into material selection. $\endgroup$ – Ross Millikan Feb 6 '14 at 20:41
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    $\begingroup$ Alloys of both metals can be much stronger (or much better wrt. to some other desired property) than the 100% pure material (and still be 99% - thus no concern mass-wise). Which alloys, if any, are we talking about here? In particular, which alloy of aluminium was used for Curiosity's aluminium wheels? 6061-T6? $\endgroup$ – Peter Mortensen Aug 27 '16 at 20:38
  • $\begingroup$ The alloy is called: AA7075- T7351 (ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20190027256.pdf) $\endgroup$ – Steve Linton May 29 at 11:33
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Aluminium is cheaper, and lighter. In space, every ounce counts. While titanium is ideally strong, it's not necessary.T6 series aluminum is as strong as steel too. Besides, Titanium is more expensive, more difficult to machine and and more difficult to weld.

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    $\begingroup$ -1 until you demonstrate that steel wheels on Curiosity of the same mass would be just as subject to breakage and puncture on Mars as the aluminum ones have been. $\endgroup$ – uhoh May 29 at 11:10
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    $\begingroup$ Materials costs are pretty much irrelevant on a deep space probe. If you're sending a rover to Mars and the choice for a part is three pounds of cast iron or two pounds of gold you build it out of gold because it's cheaper once you consider the cost of getting it to Mars. $\endgroup$ – Loren Pechtel May 29 at 14:50

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