By Graham K. Rogers

When any product is released by Apple these days, the immediate rush to produce articles has the inevitable selection that insist Apple has lost its edge and claim that nothing has changed. I sat through the video of the Apple Event last week (several times) and read much about the products and what lies beneath. There is far more than meets the eye.

The iPhone 6s and iPhone 6s were of particular interest to me. I saw Steve Jobs make the initial iPhone introduction and had one in my hands the next day. I knew immediately this was a game-changer and was not disappointed. While the early cameras were reasonable, I was looking forward to the latest update as this now has a 12 MP camera and many related software changes.

I was intrigued by an aside from Phil Schiller's during his presentation that the body of the iPhone 6s was made of 7000 aluminum (I will use the US term), a metal used in aerospace applications. The glass was also of a new type. While the body has the same shape, the iPhone 6s is no basic makeover.

iPhone 6s - Image Courtesy of Apple

As far as I am able to discover there are 10 alloys in the 7000 series. The main constituent is Aluminium with various other materials in the mix which may include Chromium, Copper, Iron, Magnesium, Manganese, Silicon, Titanium, Zinc, Zirconium and other substances, depending on the specifications and the intended use of the finished product. The quantities of these materials within each alloy will also differ depending on the specific alloy.

Dante D'Orazio on The Verge, claims that this particular aluminum alloy "is primarily alloyed with zinc, compared to magnesium and silicon in 6000 series aluminum" (Aluminum 6061-T6) used in the iPhone 6. A comment on MacRumors by VinegarTasters back in November 2012, seems accurately to predict the use of the 7000-series alloy (specifically citing 7075).

Apple Watch Sports, Apple Watch Edition, Apple Watch - Image Courtesy of Apple

The first mention by Apple of the 7000-series aluminium alloys is with reference to the Apple Watch. On the main page for the Sport version of the wrist-worn device, Apple's information tells us that this is . . .

stronger than standard alloys. Yet it's very light, making it perfect for the most active lifestyles. It's also exceptionally pure, with a beautifully consistent appearance that's difficult to achieve with traditional aluminum alloys.

Within that short description are several key factors that indicate why such a material would be developed and used by Apple: strength, lightness, purity, consistent appearance. The Watch also makes use of the Ion-X glass: aluminosilicate glass - the same material used in the windows of space shuttles and high-speed trains.

Remember also that Apple created its own specific 18 karat gold for the top of the range Apple Watch Edition: a new alloy that's up to twice as hard as standard gold.

One of the reasons some suspect that the new alloy is being used is to ensure that there is no repeat of what was called, Bendgate. Alex Hernandez on TechAeris, with a hit-seeking headline, seems to speak for Apple in explaining the choice of the 7000 series as the new iPhone material: "7000 series aluminum is much more dense and rigid making it more difficult to machine." I have an iPhone 6 and iPhone 6s (I deliberately put this in my back pocket) and, for the life of me, cannot make these bend in normal usage.

In an item from mid-August, Eric Slivka on MacRumors (with a less-sensational headline) also mentions the bending as a potential reason for the choice of 7000-series aluminum. Included in the article there is some valuable information on the composition, contained in a video the article links to. The alloy appears to include Aluminum, Magnesium, Iron (higher levels), Nickel and Zinc. There are also some interesting macro images of the shell and the anodised surface.

As I work at a faculty of engineering, I went down to the Industrial Engineering Department and sought out one of our experts on metallurgy. He told me that all aluminium commonly has the best strength-to-weight ratio; and specific stiffness, which is

He actually thought that the use of the 7000-series alloy was overkill as it has approximately the same performance as the 6000-series aluminum it supersedes, he told me. With the zinc, this will provide higher gloss compared to other alloys and a very smooth surface that is comparable to stainless steel. With its higher hardness levels it is particularly scratch-resistant: a property always welcomed by owners of such products.

Corrosion of aluminum in a normal environment is relatively high, so it has to be anodised (matching the information in the MacRumors article). Overall, he thought it was a good gimmick and that the 6000-series was good enough.

Locally, the 7000-series aluminium would cost in the region of 500 baht/Kg ($13.88); while pure aluminium is less than that: 200 baht/Kg ($5.55). Stainless steel is less, at 300 baht/Kg ($8.33). Manufacturing obviously puts the cost up - materials plus manufacturing costs - but it would also cost more to polish stainless steel.

The method of producing the iPhone shell could possibly be thin-wall casting, or die-casting, but he speculated that the most probable way would be forging and machining. The comments in the MacRumors article suggested a combined casting and milling (machining) process.

As well as providing me with the link to the VinegarTasters comment, a useful article from DailyTech (Jason Mick), dated 18 June this year, speculates on the relative advantages of 6000- and 7000-series aluminium. Although we know the answer now, the specific type of 7000 aluminium being used is still not confirmed (not yet) - and some of the conclusions by Jason Mick are now known not to be right - the technical background within the article is of good value.

In a more recent posting he speculates further that the alloy is 7003 and its use by Apple is probably wise (see also: "Aluminum 7003, Used in Roofing, May be Tapped by Apple for iPhone 6S/6S+").

iPhone 6s - Image Courtesy of Apple

Neither the online resources I cite, nor my own metallurgy expert particularly think that the use of the 7000-series alloy was needed. Economics is unlikely to figure as Apple tends to use specific materials: right for the application.

However, the concern that Apple has with customer perceptions may have pushed Cupertino into choosing a slightly different material to allay anti-publicity about continued bending problems - by the laws of physics, if something is long enough and enough force is applied it will bend - along with the extra cachet of being able to claim that the latest aerospace materials are selected for its products.

See also:

Cassandra: Apple Event (2) - Crossing the Rubicon with iPads and Accessories

Cassandra: Apple Event (3) - Apple TV: Good in Certain Areas Perhaps

Cassandra: Apple Event (4) - iPhone 6s and iPhone 6s Plus: Desirable

Graham K. Rogers teaches at the Faculty of Engineering, Mahidol University in Thailand. He wrote in the Bangkok Post, Database supplement on IT subjects. For the last seven years of Database he wrote a column on Apple and Macs. He is now continuing that in the Bangkok Post supplement, Life.