Why Friction-Stir Welding?

I've been wondering about Apple's manufacturing decision to use friction-stir welding on the new aluminum enclosures of the iMac. Apple's used an aluminum enclosure for many of its products; the iPhone, iPad, MacBook, and iMac lines all have or had an aluminum enclosure. None of these products have used stir-welded enclosures—instead they are milled out of a single aluminum blank or use more traditional welding techniques. The stir-welding question wouldn't be as interesting if the iMacs were rolling off the lines by the millions and were being shipped to customers today. Instead, this is the only time I can remember Apple replacing an existing product without making the next generation available for purchase or pre-order in the near future. Apple is effectively killing the iMac market for months while the new machines are produced. That time line has stretched even longer, with rumors that the new iMacs may not be ready until 2013. (It's my best guess that those machines are the 27" iMacs only, as they weren't projected to ship until December when announced.) On Apple's most recent earnings call Tim Cook suggested that the initial stock of iMacs would be severely constrained. The new iMacs are clearly pushing the limits of desktop manufacturing, and these newly adapted technologies are slowing things down.

Why would Apple move to a whole new process that has killed their iMac income for months and will continue to be depressed for the quarter after they start shipping? Elon Musk's quote in Wired's November issue pointed me in the right direction. Musk spoke to SpaceX's use of stir welding for its rockets:

Unlike traditional welding, which melts and potentially compromises some metals, this process works well with high-strength aluminum alloys. You wind up with a stiffer, lighter structure than was possible before. And your material loss is maybe 10%, just for trimming the edges. Instead of a ratio of purchased to flown material … of 10 to 20, you have a ratio of 1.1, 1.2 tops.
— Elon Musk

SpaceX uses stir welding to minimize material waste and still provide a strong structure. Apple is likely using this process for the same reason. With the new iMac's extreme thinness, Apple needed a way to get a stronger structure out of less material. Milling the enclosure was too wasteful—imagine the material waste from milling out the enclosure of a 27" iMac. The time, energy, and material costs of that process would be huge. Instead, Apple's adopted a more efficient, more effective, higher risk technique that should yield better results. As more and more iMacs are made the number of machines compromised due to flaws in the welding process should come down and supply will no longer be constrained.

Apple has adopted a new process to keep costs down and deliver a better iMac. As the friction-stir implementation improves supply and demand will return to balance, and Apple will have fluency in a new manufacturing technique that may replace their existing milling techniques across their product lines, bringing down costs and improving margins on a much larger scale.