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Titanium is more durable and stronger than steel. I checked the price of titanium and it is not too high. Despite that, I could find only one car made of titanium costing, and it cost 2 million.

Does it have anything to do with aerodynamics?

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    $\begingroup$ Why in the world would you think material type has anything to do with aerodynamics? $\endgroup$ Apr 21 '17 at 14:07
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    $\begingroup$ The premise is incorrect. Titanium alloys are nowhere near as strong as the steels used in frame and body components. Most frame members are formed from steels with UTS close to 2 GPa, and elongation at failure of greater than 25 percent. Ti6Al4V are just below 1 GPa. The steels used can absorb a more impact energy than titanium alloys as well. Titanium alloys have higher strength to weight ratio, however. $\endgroup$
    – wwarriner
    Apr 21 '17 at 16:41
  • $\begingroup$ Apologies, I made a mistake and was thinking of the wrong steel class. UTS is about half or comparable to titanium depending on heat-treat. The elongation is still significantly greater (20-60%) which gives greater energy absorption in impacts, which is what really matters in automobiles to minimize risk of injury and death in collisions. Titanium alloys just can't compare for that purpose. $\endgroup$
    – wwarriner
    Apr 22 '17 at 22:48
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The bulk price of titanium doesn't tell the whole story.

A big chunk of the cost of titanium parts is down to the fact that it is significantly more costly to make things out of titanium than steel. Even before you start manufacturing parts there are costs associated with processing stock to the correct dimensions and specifications (in this case thin sheet).

Mass produced car bodies are generally made by cold pressing processes and then welded together and it is the cost of tooling rather than the bulk material price which is the biggest contributor to the manufacturing cost per vehicle. Titanium alloys are significantly more difficult to form and weld than steel. Especially as car design has tended towards deeper forms, compound curved surfaces and tighter radii in pressings.

Welds in titanium are also much more prone to failure due to contamination and thus require much more tightly controlled welding processes and inspection.

The other thing is that stiffness is at least as important as tensile strength in a car body and the stiffness to weight ratio of titanium isn't much different to steel so if you are designing for stiffness you may not even save any weight.

At the other end of the spectrum carbon and Kevlar composites are generally superior to titanium for body shells in almost all respects for low-volume high performance applications and a few carbon parts are starting to filter through to the higher end of production cars. So there isn't even any filter down from high performance applications which tends to be one of the drivers for new technology in production cars.

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    $\begingroup$ Well put. I'll just add that automobile manufacturing, other than in the > \$1million range, is brutally price-sensitive. GM or Ford would change body material if it saved them $10 per car. $\endgroup$ Apr 21 '17 at 14:08
  • $\begingroup$ Machining titanium isn't that easy either... $\endgroup$ Apr 21 '17 at 21:57
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Aside from the manufacturing cost issues in another answer, titanium is much less ductile than steel. In a crash situation, it would not absorb energy permanently (by plastic deformation) but simply "rebound," or possibly vibrate with a large amplitude (several inches peak-to-peak) for a relatively long time span.

(The above is based on practical experience, trying to replace steel by titanium in a containment system to reduce weight - the idea just didn't work).

Also, titanium is quite a reactive metal - titanium fires can be started by friction, and can be as spectacular as burning magnesium ribbon.

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