Aluminum

Factual Questions
21

Al isn’t that strong. If you use young’s modulous as an indicator of “strength”, which isn’t a scientific concept, you find: (in units of 10**11 Pa)

Al = .7
Brass = .91
Iron = 1.9
Steel = 2.0
Tunsten = 3.6

So steel is almost 3 times “stronger”

If you compare densities in g/Cm**3
Al = 2.7
Brass = 8.6
Iron/steel = 7.8

So the real advantage to Al as a structural materal is weight savings.
Other posters have pointed out its other benefits.

22

Hmmm. According to the link posted by funneefarmer, aluminum metal wasn’t available at all until 1855, at which time it cost $500 per pound (twice the then-cost of gold) – and by 1859, Napoleon had financed projects that reduced the cost of aluminum to $17 per pound (the same as silver). The modern Héroult/Hall process was devised in 1886, but the 6-pound aluminum cap to the Washington monument, despite being made a year later in 1887, still cost over $200.

So, really, there was only about a 4-year span in there during which aluminum was a super-precious metal in league with gold and platinum.

23

Aluminium is also a pain in the arse to spot-weld, which is the current method of joining steel automobiles together. Making aluminium cars involves redesigning your entire automated production line, a big investment.

24

If you have to use three times as much for the same strength, where’s the weight savings? Are the aluminum figures for pure aluminum? Does it get lots stronger in alloy, for a small increase in weight?

It is too clear, and so it is hard to see.

25

Maybe AL bikes are expensive not because the metal is expensive or hard to work with, but because people will pay that much, so that’s what they charge.

Arjuna34

26

Aluminum may not ‘rust’ but it sure as hell corrodes to the point where repair is necessary. Ask any aluminum aircraft or boat owner.

27

ZenBeam said
If you have to use three times as much for the same strength, where’s the weight savings? Are the aluminum figures for pure aluminum? Does it get lots stronger in alloy, for a small increase in weight?

The figures are for pure Al. Yes, there are stronger alloys with similar weights. The biggest advantages occur where a shape is needed but the strength of steel isn’t, such as some forgings. Al is also easier to work in some applications, you can mill it much easier than steel for example.

28

As said, aluminum is light weight. By itself it is not as strong, but can be alloyed with zinc or manganese, which greatly increases the strength.

Aluminum is great for use in aircraft, where weight to strength ratio is important. You can make a slightly beefier part that is much lighter to do the job. Titanium is stronger than aluminum, practically as strong as steel, and lighter than steel, but heavier than aluminum and also more expensive than aluminum, primarily because of scarcity.

Aluminum is easily recycled, and cheaper to recycle than refine new ore. Recycleable aluminum mostly consists of cans, foil, etc, that are basically “pure” aluminum. (Even aluminum in its “pure” state is a very trashy material. At the microscopic level, it is full of other stuff.) However, recycling places don’t seem to like the high grade alloys as much. A guy at work once tried to take a chunk in to sell to a can recycler, but the guy wouldn’t take it. Though I’m sure industrial places can recycle it.

Why is aluminum used in cans? Because it is more ductile than steel, and can be shaped more easily. Compare the extruded shapes of soda cans vs. the rolled shapes of steel food cans. Also, soda cans don’t have to be that strong. There has been much work in design of soda cans to use as little material as possible, to make them cheaper.

But steel is cheaper and stronger, and has certain advantages (such as mentioned above about spot welding). Where weight is not an issue, steel is often preferred.

29

Napoleon III had three sets of flatware. Silver for ordinary guests, gold for special guests, and aluminum for the absolute top drawer.

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams

30
      • As far as flatware goes, gold and silver would tarnish but aluminum wouldn’t.
      • As far as bicycle frames go, the labor involved both in the refining of the material, and in the working of it is involved. Fiberglass frames were seen as a waste of time, I think there’s a 1950’s era bike called the Moonrider that had a swoopy fiberglass frame that was made by joining two molded halves. The catch for modern bikes is that the labor involved in making a fiberglass frame is about the same as for any other composite, and there are other better (stronger, stiffer, lighter) fibers available now that don’t cost much more than glass.
  • When the USSR collapsed, at the time it was predicted (hoped for) in the bicycling community that Ti parts would soon be cheap. That never really happened; a US or Euro built bike frame might cost $2000, where a Russki one might cost $1500. The (early) Russian stuff just didn’t have the quality (i.e., the assembly labor quality) that the US & Euro items did. As time went on, it turned out that there was quite a bit of engineering involved; after that was factored into the products, the prices weren’t that far apart- the difference in price of the raw material became fairly minor. - It’s been a while, but the most expensive frames I heard of were made of Boron or Berryllium, and also out of Russia. I don’t know of anything offhand regularly made out of berryllium, except for - me thinks? - nuclear bomb casings. - MC
31

Mechanical engineers usually consider Young’s Modulus as a measure of stiffness and yield strength (roughly, the stress at which permanent deformation starts to occur) as a measure of strength. Typically, you don’t care much if the airplane wing flexes (as long as you know how much), but permanent deformation or breaking off is undesirable. The weight and Young’s Modulus don’t depend much on the alloying elements, but the yield strength sure does (and it depends on processing, too). Some typical numbers, in megaPascals:

1020 steel 350
1040 steel 490
304 stainless steel 240
440C stainless steel 1,900
6061-T6 aluminum 275
7075-T6 aluminum 500

So you can see that the yield-strength-to-weight ratio of aluminum is typically much higher than steels, except for some difficult-to-work steels.

In some cases, permanent deformation is allowable but fracture is not. For those cases, you look at the tensile strength (or other similar measures). Aluminum enjoys a similar advantage per kilogram there, too.

jrf

32

Irishman wrote:

But zinc and manganese are both silghtly denser than iron. If it takes a lot of Zn or Mn to make the Al decently strong, wouldn’t that defeat the purpose?

33

Actually, Young’s Modulus is a measure of the linear elasticity (i.e. amount of “stretch” under a given load before yielding) of the material, not strength. It’s also known as the “Elastic Modulus.” “Yield strength” is the amount of stress that a material can withstand before permanent deformation occurs. “Ultimate strength” is the amount of stress (under 1 tmie load) that a material can withstand before it breaks.

The choice of metals for aircraft (I’m discussing aircraft here because it’s my area of expertise) is based on strength, resistance to high temperatures, weight, and cost. For example:

  • Aluminum is used in the airframe (e.g. structure making up the fuselage, wings, empenage, etc.) because of the relatively low load and temperatures it is exposed to. The low weight is prefered since it makes up a majority of the aircraft structure.

  • Steel is used in higher pressure and temperature components such as environmental control valves. Steel is relatively cheap and generally doesn’t create enough of a weight penalty to justify the use of more expensive titanium.

  • Titanium is generally used in the engine where the temperatures are too high for most steels. The temperatures and pressures are way too high for aluminum. Also, the engines make up enough of the overall structure such that steel would be too heavy.

34

This (see Figure 5) gives a graphical representation of Young’s Modulus, yield strength, and ultimate strength. Young’s Modulus is merely the slope of the “linear region” of the stress-strain curve in Figure 5.

35

You may also want to scroll down to Figure 6 of that link. OK, I’ll shut up now.

36

We can’t just compare stifnesses and densities; geometry counts too. For example, the deflection when bending a simple beam of rectangular cross-section is inversely proportional to Young’s modulus, the width of the beam, and the cube of the height of the beam. So, if you made beams of equivalent stiffness from steel and aluminum, and the cross-sections were both squares, the width and height dimensions of the aluminum beam would be about 30% greater than the steel beam (the ratio of Young’s moduli is about 3, so you need to make that up by increasing w*h^3 by a factor of 3; 3^0.25 = 1.31). But the volume is linearly proportional to the linear dimensions, so the aluminum beam would be about 75% more volume than the steel beam (1.31^2 = 1.73). Since aluminum is about 2.9 times less dense that steel, the aluminum beam would weigh about 60% of the steel beam’s weight (1.73/2.9 = 0.60).

That amount of weight reduction is significant in many cases.

The amount of weight reduction depends on the details of the geometry and loading, but there are lots of other common cases which yield similar results.

Finally, in some obscure corners of the design world, the ability to anodize or hard-anodize aluminum and obtain an essentially corrosion-free surface is important. I design equipment for use in semiconductor fabs (industry-speak for factories), and you just can’t use carbon steel in a modern clean room unless it’s coated with a petroleum product or painted, and those introduce their own problems. Stainless is a PITA, especially when it’s mirror finish. So there’s lots of aluminum in clean rooms.

jrf

37

Yup. And from that, you might infer that strength is increased greatly by the addition of a pinch or two of alloying elements. Your inference would be correct. By weight:

6061:

Silicon 0.40 - 0.80%
Iron 0 - 0.35%
Copper 0 - 0.10%
Molybdenum 0 - 0.15%
Magnesium 0.80 - 1.2%
Chromium 0.04 - 0.35%
Zinc 0 - 0.25%
Titanium 0 - 0.15%

7075:

Silicon 0 - 0.40%
Iron 0 - 0.50%
Copper 1.2 - 2.0%
Molybdenum 0 - 0.30%
Magnesium 2.1 - 2.9%
Chromium 0.18 - 0.28%
Zinc 5.1 - 6.1%
Titanium 0 - 0.20%

jrf

38

In addition to the spot-weld problem mentioned previously, sheet aluminum springs back a different amount after being formed. I think Ford was going to try an aluminum Probe, and one of the problems mentioned was making a whole new set of (expensive) dies for the presses. It ends up being cheaper to make an aluminum car from scratch rather than modifying existing models.

39

Beryllium is highly toxic.

John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams

40

So’s Arsenic. What’s your point?