Originally Posted by aegisdesign
I'm not sure where you got those figures from but they look wrong to me coming from a bicycle materials background. There's not much difference between 6061-T6 and 7005-T6 for instance, the two most commonly used aluminium alloys used in bicycles which both have around 40-42 kpsi. The choice usually comes down to manufacturing processes and geography rather than a materials choice. USA = 6061, Taiwan = 7005 since 6061 is more labour intensive and requires more post welding work.
There's also some nice 6000 series alloys. For instance 6069 has a 55-71 kpsi yield after T6 and still offers 20-25% elongation as opposed to 10-13% for other aluminiums. It's almost approaching Titanium levels but without the weight penalty.
That'd probably be more apt for Apple here as the clip would have bent and sprung back into shape instead of permanently deforming.
The other nice alloy is 2014-T6 which beats most of the 6 and 7 series alloys but is a bitch to work with. Typically, it's only used for handlebars as making a bike out of it is brutal.
And then there's the Scandium alloys. Mmmm.
Yup. There's much more to it than just material strength and the choice here is obviously manufacturing. There's some some aluminium sausage machine in China squeezing out n shuffles a second out of it's die.
China is almost entirely 7005. I wonder though if Apple heat treats the clip though as there's quite a difference between unheat treated and treated and with some alloys you've also got to age the alloy post processing. ie. it gets stronger if you leave it a month.
A nice steel spring clip perhaps then they could have done away with the hinge entirely though it may not have looked so pretty.
Or people could just be more careful, or is that not so obvious?
I was just throwing out some numbers off the top of my head, based on some work I've done recently, comparing high strength aluminum/titanium/stainless steel/FRP. You can go to the Alcan, Alcoa, or Corus websites for datasheets on aluminum, there are many other databases, MatWeb, military databases for metals/FRP/materials, etcetera. When I stated those numbers I was thinking 6061-T6 for 6000 series, and 7075-T6 for 7000 series, and Kaiser 7068 aluminum currently tops the list at 99 ksi yield, obviously with STA and HT alloy metals there are literally hundreds/thousands of choices for design. BTW, I had to use some 7075-T6 in a recent design.
The friction stir process (or friction stir welding) allows the high strength 7000 series to be used quite effectively in situations where it is normally rivited (e. g. airplanes), since all other welding techniques weaken the material in the weld area and high strength 7000 series can't be welded via the other welding methods due to it's poor weld characteristics via these conventional weld methods, whereas FSW has a softening factor ~1 across the weld (in fatigue design), FSW is also very dimensionally stable (e. g. doesn't warp the material).
BTW, you're elongation numbers seem a bit high? SOP is to take the yield strength divided by the modulus (both in same units such as ksi), to get allowable elongation (in percent), metals are typically ~1% or less (roughly).