Originally Posted by
nfmisso
I would not suggest Magnesium. I am a Mechanical Design Engineer, with a strong back ground in materials.
Mg = Magnesium; Al = Aluminum; St = steel; including Cro-Mo; Ti = Titanium
density (mass per unit volume): Mg 1.8; Al 2.7; Ti 4.5; St 7.8
stiffness (force for a given deflection): Mg 44 GPa; Al 69 GPa; Ti 100 GPa; St 205 GPa
yield strength: Mg 70 MPa; Al 6061 T6 276 MPa; Ti-3Al-2.5V 500 MPa; St 4130 435 MPa
reference:
http://www.matweb.com
In practical terms; you can easily permanently bend magnesium with your fingers; it needs to be protected from high pressure loading. For example, if you made handle bars the same size and thickness has standard aluminum handle bars; you could easily leave permanent and deep impressions just by squeezing it with your hands; and the first heavy braking would result in severely bent if not broken bars. As noted, magnesium has some corrosion issues; but they are not significantly different from aluminum.
Magnesium is used for some notebook computers, where its ability to be cast into very intricate shapes provides enough of an advantage to make sense.
For fabricate a traditional style bicycle (joined tubing); magnesium does not make sense. If were to cast or mold the frame in one piece, magnesium MIGHT be worth another look.
I'm sorry, but I
am materials engineer and a large volume of that is nonsense.
If you can manufacture a frame and chassis for a notebook computer in 'magnesium' - which, BTW is as thin as 1mm in some places, and regular handling and impacts do not damage either the chassis or its contents, then how would you be able to bend a large-diameter tube with walls much thicker than that by heavy handling?
No-one uses pure magnesium for structural engineering, in much the same way no-on euses pure aluminium. The lowest strength aluminium alloys used in bicycle manufacture, the AA 6xxx series are of the order of 5 times stronger than pure aluminum. In much the same way, the lowest strength magnesium alloys used in structural engineering are of the order of 7 times stronger than pure magnesium. To put this into perspective, AISI 4130 is at least 3 times stronger than pure iron, so taking the mechanical properties of a pure metal as your starting point is pretty broken. You could have picked one of many Mg- rare-earth or Mg-Mn- rare-earth alloy systems. You'd have found they're very similar in strength to the intermediate aluminium alloys.
For reference, magnesium is not actually very easy to cast at all. Its surface tension is awful and it explosively reacts with water vapour when molten. Old foundry casting techniques for magnesium alloys required someone to dust the stream of metal with sulphur powder to reduce the surrounding oxygen levels, preventing too much burning of the metal. Magnesium sulphide readily disoociates in the presence of air anyways, so there is no risk of it being entrainied in the casting.
I have never felt comfortable with magnesium for bicycle manufacture, that's no secret, but not for any of the reasosn you've mentioned.
It's a film-forming alloy, very susceptible ot reaction with oxygen and forms stable, thin, ceramic oxide films on it surface, all the more so when liquid. As with aluminium, titanium and stainless steels, as a result, no welds in it are sound and no castings with a feed rate above the critical velocity are integral. It is a hexagonal metal, with the concomitant limited slip systems for plastic deformation and it demonstrates low fracture toughness in all three modes.
Titanium I can forgive for its issue with oxide films, because above 600 degrees most titanium alloys dissolve their own oxide and it is the only commercially available metal that actually has an alloy system with oxygen. So while it forms films, its welds are, by definition, oxygen alloyed and safe. Stainless steels have enormous fracture toughnesses no matter their crystallography because they're iron alloys and in almost all cases are blessed by iron's huge ability to absorb plastic work, so a 'defective' weld is still extremely forgiving.
Neither aluminium or magnesium have either of these strengths. At least aluminium is cubic.
Galvanics now, that's whole other argument.