It was once a metal more precious than gold. Napoleon the 3rd of France is said to have held a banquet where cutlery made of it was given to his most respected guests... while others made do with golden utensils. It is also what the top of the Washington Monument is made of, symbolizing the nation's industrial prowess at the time when the structure was erected.
The material in question is aluminum. Light, durable, and abundant in nature, it is one of the most widely used metals today, with applications ranging from making soda cans to constructing cars and airplanes. In one form or another, it is also found in the phone you're probably holding right now.
Stainless steel is another metal with widespread use, but not when it comes to phones. In fact, there are very few handsets utilizing stainless steel in their construction – premium models like the iPhone X and iPhone XS, for instance – even though the metal is superior to aluminum in a number of ways. Why is that?
The key strengths of stainless steel
Stainless steel is an awesome material. It is highly resistant to corrosion and it looks lovely when polished, but most importantly, it is very, very strong. In fact, the mechanical strength of stainless steel greatly exceeds that of most aluminum alloys, which is why it is the preferred material for products that must meet a higher durability standard – luxury watches, automotive parts, and in our case, premium smartphones. Being stronger than aluminum allows stainless steel to resist dents, scratches, and bending much better, and that's a good thing if you want your product to last.
The stainless steel frame of the iPhone XS (top) is stronger than the aluminum frame of the iPhone XR (bottom)
Where stainless steel falls short
Aluminum is here to stay, that's for sure. In part, that's due to its versatility – the metal is found in anything from food and beverage packaging to power cables and airplane components. The iPhone XR, the Galaxy S9, the Pixel 3, the LG V40 – all of these phones use aluminum in their construction. Aluminum may not be stronger than stainless steel, but it is strong enough for a device of this kind while giving it a much-desired premium feel.
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The reason why aluminum is preferred over stainless steel by smartphone manufacturers is easy to guess: it is simply cheaper to make a phone's frame and chassis out of it. Aluminum is usually more expensive than steel per unit of weight, but since aluminum is about 2.5 times less dense, a ton of aluminum can theoretically yield about 2.5 times more components than a ton of steel.
On top of that, aluminum is softer, thus easier to machine and turn into actual phone parts – another reason why aluminum is cheaper to use. Below is a video by Apple showing how an iPhone 7 body is made using a CNC machine. Had Apple used stainless steel, the process would have been slower, more difficult, and more expensive.
This isn't the only advantage aluminum has over stainless steel. It is much lighter. It is easier to apply color to. It conducts heat more easily, and phones that run cooler do run more efficiently. But ultimately, the lower cost of using aluminum is the top reason why stainless steel is – and is likely to remain – a metal found only in premium mobile products.
Aluminum series – what are those?
It is worth clarifying that aluminum is rarely used in its pure form. It is simply too soft, which limits its use. In practice, aluminum is often mixed with other metals to create stronger alloys. So when a phone or any other piece of electronics is marketed as made of aluminum, it is in fact made of a blend of metals, with aluminum comprising no less than 85% of any of its alloys.
Currently, there are over 500 aluminum alloys, each with its own properties and metallic content. You will most often see these alloys designated by an individual 4-digit number: the first digit in the alloy number indicates the series that it belongs to, and each series has its properties and composition characteristics. For example, the thin and light aluminum foil you'd find in a kitchen can be made of alloy 1050 or 1100 – an alloy that comes close to pure aluminum. The walls of soda cans are stronger, made of alloy 3004 which is comprised of aluminum and manganese. Mixing aluminum with silicon and magnesium produces the even stronger 6000-series aluminum: Apple is said to have used alloy 6003 for the iPhone 6. And the even stronger 7000-series is made of aluminum mixed with zinc.
While both 6000- and 7000-series aluminum are suitable for making a smartphone's outer components, primarily the latter is found in premium models as it is stronger. Samsung is said to be using 7003 aluminum for the Galaxy S9. Apple uses a custom 7000-series alloy for its phones and watches. And while there also exists an 8000-series, these aluminum alloys are not stronger, contrary to what the larger number may lead us to expect.
For those curious to learn more, the Aluminum Alloys 101 page over at Aluminum.org could prove a valuable resource.
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