Was RE: [Leica] DSLR choice - magnesium/metallurgy

[douglas.sharp at gmx.de (Douglas Sharp)]

Thanks Frank,
so I assume the future will be carbon fibre sandwich materials, resins 
,glass derivatives or industrial ceramics (TiO2) - or something totally 
new in the way of plastics.
I read recently that one  company is even already making tripods on the 
basis of basalt- was it Manfrotto?.
Interesting to read that titanium also "protects" itself with an 
oxidation surface layer, I thought the stuff was resistant to just about 
anything nature could throw at it.
cheers
Douglas

Frank Dernie wrote:

> Hi Douglas,
> It is true that very few metals are the used as the pure element,  
> AFAIK, and certainly the metals I use, are all alloys. Ally wheels  
> for cars are mainly aluminium, though there is a small percentage of  
> magnesium and other alloying elements. Ferrari use magnesium wheels  
> on their road cars but they have to be immediately surface treated  
> and repainted to avoid corrosion if scratched.  "Ally" wheels corrode  
> readily if used on salt de-iced roads, unless painted, but not as  
> fast or badly as magnesium.
>
> Here is an excerpt from a materials handbook, on the engineering use  
> of magnesium alloys.
>
> "Magnesium, with a density of ?1.7gcm-3 is significantly lighter than  
> most other structural metals. The mechanical properties of the pure  
> metal are however very poor such that only alloys of magnesium are of  
> practical value in engineering applications. Under ?ordinary?  
> atmospheric conditions magnesium and its alloys have a good  
> resistance to corrosion due to the formation of a passive oxide layer  
> as happens with aluminium and titanium. Unfortunately this layer is  
> not completely impervious particularly in the presence of damp air  
> containing salts. Corrosion resistance can thus be quite poor under  
> adverse conditions and care must be taken when selecting magnesium  
> based alloys for certain applications.  The low density of the base  
> metal ensures that the alloys are also extremely light. Aluminium  
> alloys are significantly stronger than magnesium alloys but the low  
> density of the latter affords higher specific strength and stiffness.  
> Magnesium alloys also possess good machinability and can be readily  
> welded using TIG. Having said that they are extremely soft and a  
> ?bulk? penalty must be paid as a large volume of material is required  
> to achieve equivalent properties to other structural metals.  
> Furthermore they are expensive and their resistance to corrosion is  
> not as good as that of aluminium alloys.
>
> There are three primary groupings of magnesium alloys in common use:  
> casting alloys, wrought alloys and those which can be used in both  
> ways. For example;
>
> 1. Magnesium-aluminium-zinc alloys may be precipitation hardened and  
> are used in casting, extrusions and forging.
>
> 2. Magnesium-manganese alloys are readily weldable and mainly used in  
> sheet fabrication processes.
>
> 3. Magnesium-zirconium-rare earth alloys can be used in both cast and  
> wrought forms. They have reasonably high proof stresses, resistance  
> to impact and corrosion and may be easily fabricated."
>
>  The alloys used on cameras will be type 1, the others would be too  
> expensive.
>
> Many metals which corrode form a thin passive layer of oxide on their  
> surface almost immediately, this prevents further obvious corrosion.  
> On aluminium and titanium the oxide layer is thin enough to be  
> "invisible". On brass it tarnishes then stabilises, which makes it  
> good for engineering but tiresome for shiny ornaments!
>
> Basically magnesium is "the new titanium" in the camera marketing  
> game. It is nowhere near as good as titanium but massively cheaper  
> and easier to manufacture from.
>
> It is an unfortunate fashion which will hopefully not last long.
>
> Frank
>
>