The Savvy Aviator #24: Making Metal Behave

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Metal aircraft parts must satisfy a mind-boggling variety of physical properties: weight, strength, hardness, toughness, springiness, temperature and corrosion resistance, just to name a few. Here's a glimpse at how it's done.

The Savvy Aviator

Metal possesses lots of properties that we find very useful in building airplanes. It's strong, hard, and tough. It's easy to form, work and machine. It's fireproof and can stand up to high temperatures. All in all, metal is neat stuff.

But metal isn't unique in having these properties. Metal is strong, but ordinary cotton fiber actually has higher tensile strength than many of the aluminum alloys we use to build airplanes, and Kevlar is stronger than even high-tensile steel (which is why it's used in bulletproof vests, and now airline cockpit doors). Metal is hard, but diamond and carbides (which are artificial diamonds) are harder. Metal can withstand high heat, but ceramics can withstand much higher heat that would cause metal to melt. Metal is easy to shape and machine, but so is wood -- and aviation-grade spruce is nearly as strong as aluminum alloy, at least if the stress is applied with the grain

So why build airplanes of metal? What makes metal so useful is that it combines all of these properties? You might say that metal is the Cessna 182 of aircraft construction materials: It may not be the best at any particular thing, but it sure does a lot of things well.

Stress/Strain, Elastic/Plastic

What mainly sets metal apart from other materials is how it reacts to stress. In this context, "stress" is defined as an external force applied to an object in such a way as to deform the object. Stress may be applied in tension, compression, torsion, shear, or a complex combination of these. The amount that an object deforms when subjected to a given stress is known as strain. Once again, stress is the force applied to an object (often measured in pounds per square inch -- psi), and strain is how much the object deforms under that stress (usually expressed as a percentage of the object's original dimension).

Take a look at a typical stress-strain diagram for metal:

(27 Kb)
(Click for larger version.)

If a modest amount of stress is applied to a metal object, it deforms elastically -- meaning that when the stress is removed, the object returns to its original shape and size. This is obvious in a spring steel landing gear leg or the aluminum wing spar of an airliner, but it's also true of metal parts like bolts and crankshafts where the deformation is not so obvious.

Hooke's Law (the law of springs) states that strain is proportional to stress during elastic deformation -- in other words, the stress-strain curve is actually a straight line. You can see that this is indeed the case in the curve shown above ... until stress reaches the "proportional limit," at which point the crystalline structure of the metal starts to break apart, and decidedly non-linear things start to happen.

If a metal object is subjected to sufficiently high stress (beyond what's known as its elastic limit), it deforms plastically -- meaning that the deformation is permanent, and remains even when the stress is removed. This might sound bad, but it's actually good. During manufacture, metal's plastic properties when subjected to high stress are what allow us to bend, stamp, forge, extrude, roll, and otherwise coerce metal into complex shapes. In service, those same plastic properties are what permit a heavily loaded metal part to bend rather than break, and therefore to resist fatigue. This important property is often referred to as toughness.

Metal's ability to be elastic under moderate stress and plastic under high stress is largely responsible for making it so useful. Other materials tend to have very different stress-strain curves. Brittle materials like brick, concrete and glass have very limited plastic properties; when stressed to their yield point, they suddenly shatter without warning. Tough materials like rubber, leather and softwood, tend to be elastic all the way to rupture, so they can't be cold-worked into a desired shape the way metal can.

Soft, Hard, And Tough

The simplest metals like iron, aluminum and copper are relatively soft and ductile. In other words, their elastic limit is relatively low so it doesn't take much force to deform them permanently. Where higher strength is required, the elemental metal is usually alloyed by adding a relatively small amount of one or more additional elements that strengthen the metal.

It doesn't take much. Adding just a few percent (by weight) of carbon turns soft iron into high-carbon steel with ten times the tensile strength. Adding about 4% copper to commercially pure aluminum creates 2024 aluminum alloy (the most common one used in aircraft) that's more than four times as strong. These alloying elements work their magic by invading the crystalline structure of the metal in a way that makes the resulting lattice of atoms much denser and harder to dislodge.

Take a look at what happens to the stress-strain curve of iron when carbon is added to form steel:

(30 Kb)
(Click for larger version.)

Notice how the linear portion of the curve gets much steeper, indicating that steel is much harder than iron. It can handle much greater stress without permanent deformation, and a given amount of stress causes far less strain.

But also notice that as more carbon is added to create high-carbon steel, the well-defined "knee" at the yield point that is seen in iron and mild steel tends to disappear. What this means is that high-carbon steel is more prone to sudden failure without warning. It is brittle and doesn't bend before it breaks.

In some applications -- such as cylinder barrels, crankshaft journals, cam lobes and tappets -- hardness is all-important for maximum service life. In other cases -- wing spars and landing gear legs, for example -- toughness (the ability to bend without breaking and to resist fatigue) is crucial. Metallurgists spend a lot of their time coming up with the optimum tradeoff between hardness and toughness for each metal airplane part.

Iron can also be alloyed with chromium and/or nickel to form various flavors of stainless steel. These are generally not nearly as strong as carbon steel, but they're much more resistant to heat and corrosion. These chromium and nickel varieties are often used in exhaust systems, heat shields and firewalls.

As I mentioned above, Aluminum can also be alloyed with various elements to improve its structural properties. Copper is added to create the 2000-series alloys most commonly found in aircraft structures, providing excellent strength-to-weight ratio and good fatigue resistance. The biggest downside of this alloy is that it is far more vulnerable to corrosion than pure aluminum. Consequently, it is generally protected by an anodized finish or with a thin layer of pure aluminum ("Alclad"). Other aluminum alloys used in aircraft include the 6000-series (containing magnesium and silicon) and the 7000-series (containing zinc) -- the former is corrosion-resistant, while the latter is the strongest of all aluminum alloys.

Heat-Treating

The strength, hardness and toughness of certain alloyed metals -- including carbon steels and copper-based aluminum alloys -- can also be profoundly affected by heat-treating. Heat-treating is particularly valuable because it permits the properties of a metal part to be changed in the course of manufacture or repair.

Anyone who has built model airplanes is probably already familiar with heat-treating. Suppose you want to make a spring-steel landing gear for your model out of a length of high-carbon steel "music wire" (also known as "piano wire" or "spring wire") available at any hobby shop.

As it comes from the store, music wire is very springy and very tough -- approximately 45 on the Rockwell C hardness scale (RC45) and perfect for a landing gear -- but it's also extremely difficult to bend and form without breaking. The solution many modelers use is to heat the wire with a torch until it becomes a bright cherry red -- about 1400 F (known as critical temperature) -- and then let it cool slowly in still air. This process is called annealing and transforms the steel wire into a soft, non-springy form (about RC25) that can easily be bent and formed to the desired shape. Of course, in this annealed state, the wire is way too soft to be suitable landing gear material -- but that can be easily rectified.

Once the gear is bent to shape, the next step is to heat-treat the wire by heating it back up to critical temperature (bright cherry red, about 1400 F) and then plunging it rapidly into a bucket of water (quenching). This transforms the wire into a very hard condition (RC60+). In fact, the heat-treated wire is now so hard that it is quite brittle and would probably snap off in a hard landing.

The final step is to temper the wire back to the desired RC45 hardness to provide the desired toughness and springiness. This is done by heating the wire back up to a medium blue color -- about 750 F -- and allowing it to cool slowly in still air. The final hardness is a function of the tempering temperature: The hotter the metal is heated during the tempering process, the softer, tougher and less brittle will be the end result.

In case you were wondering, this very same process -- annealing, bending, heat-treating, and tempering -- is precisely the way the flat or tubular spring-steel landing gear legs (like the ones used on single-engine Cessnas) are made.

How Heat-Treating Works

Ferrite Iron:
Body-Centered Cubic Structure
Austenite Iron:
Face-Centered Cubic Structure

The physics and chemistry behind heat-treating of carbon steel is extremely complex, but the basic principle is fairly simple. At room temperature, the atoms of metallic iron normally organize themselves into a crystalline structure known as "body-centered cubic" (BCC) -- basically an array of overlapping cubes with an iron atom at each corner and one more in the center of each cube. In this state, the iron is known as "ferrite" or "alpha-iron."

But when the metal is heated above its critical temperature of about 1400 F, something amazing happens: The iron transforms into a "face-centered cubic" (FCC) structure in which the central atoms migrate to the faces of the cubes. In this state, the iron becomes non-magnetic; in fact, a magnet is sometimes used to determine when critical temperature has been reached.

Steel is basically iron with a few percent of carbon mixed in. When steel is heated to the critical temperature and the iron transforms its crystalline structure from BCC to FCC, the carbon atoms will migrate into the central position of the cubes formerly occupied by iron atoms. This form of red-hot steel is known as "austentite" and is non-magnetic.

If you allow this austentite to cool slowly, iron atoms migrate back into the center of the cubes and force the carbon atoms back out. The result is basically a mechanical mixture of pure iron (Fe -- known as ferrite or alpha-iron) and iron carbide (Fe3C -- known as cementite). The result is soft steel known as pearlite, and the process that creates it is called annealing.

On the other hand, if you cool the austentite quickly by quenching it in water or oil, the carbon atoms are trapped inside the structure, which becomes a body-centered tetragonal form called martensite. This process is called heat-treating, and the result is very hard, very brittle steel -- too brittle for most uses.

So the third step is to heat the steel back up to a temperature well below critical -- typically 200 F to 800 F depending on the final hardness desired. This tempering process allows some of the trapped carbon atoms out of their tetragonal jail cells, and relieves some of the hardness and brittleness. The amount of tempering desired depends on how the metal is to be used; it's strictly a tradeoff between hardness and toughness. A steel cutting tool needs to be very hard, while a steel landing gear leg needs to be tough and springy.

Various alloys of aluminum (including 2000-series copper alloys that are commonly used in aircraft structures) may also be hardened through heat-treating and softened by annealing. The critical temperature of aluminum is a lot lower (about 800 F), but the principle is exactly the same. The main difference is that aluminum is not normally tempered after heat-treating (because it never gets brittle enough to require it), and in fact is usually "aged" or strain-hardened after heat-treating to make it even harder.

Strain-Hardening

Another method of increasing the hardness of metal involves not heat but pressure. When soft metal is compressed by the application of pressure (in excess of its elastic limit), it becomes harder simply because it becomes denser: The atoms in its crystalline structure are closer together than before. (Think of a block of swiss cheese, full of voids. If you compress the cheese and squish out the voids, it becomes denser, harder, but more brittle.) This is known as strain-hardening or work-hardening.

In the case of sheet metal, the most common method of strain-hardening is to pass it between pressure rollers. This can be done when the metal is cold ("cold-rolling") or when it's heated.

For example, the most common aluminum sheet metal used in aircraft construction is known as 2024-T3 Alclad. The "2024" refers to the aluminum alloy composition, which is approximately 4% copper by weight, plus small amounts of tin and zinc. The "T" means that the metal is heat-treated -- and "T3" specifically means it's heat-treated to increase its hardness, and then cold-rolled to make it still harder. The "Alclad" means it's then plated with a thin layer of commercially pure aluminum to improve corrosion-resistance. (And you wondered why aircraft parts are so expensive!)

Another example of strain-hardening is the humble rivet. The rivets most commonly used in aircraft construction are known as "AD" rivets, and are made of relatively soft 2017 aluminum alloy. When the rivet is compressed with a rivet gun and bucking bar (or a rivet squeezer), the alloy is strain-hardened to form a strong joint.

For large metal parts (such as pistons, connecting rods, crankshafts and camshafts), strain-hardening is often accomplished by forging in which a hot metal ingot is pressed into shape in a powerful hydraulic press. Forged parts are usually substantially stronger than cast parts because the forging process strain-hardens the metal as it is being shaped.

Case-Hardening

Alloying, heat-treating and strain-hardening are all methods of hardening a metal part in its entirety -- so-called "through-hardening." Often, however, it's desirable to harden only the surfaces (or perhaps just one surface) of a metal part, while leaving the interior tougher and less brittle. Examples include cylinder walls, crankshaft journals, cam lobes and tappets. The process of hardening only the surface of a part is generically known as case-hardening. There are two methods of accomplishing it: mechanical and chemical.

Mechanical case-hardening is commonly accomplished by cold-rolling or shot-peening. Both methods harden the surface of a metal part by compressing and strain-hardening it.

Steel parts can also be case-hardened chemically, by heating them in an oven and exposing the surfaces to be hardened to a chemical atmosphere. The two most common methods are called carburizing and nitriding. Carburizing involves baking the part in an atmosphere of carbon monoxide (CO), which causes additional carbon to be absorbed into the surface -- thus converting a thin outer layer from mild steel to harder and stronger high-carbon steel. In nitriding, the part is baked in an atmosphere of ammonia gas (NH3), causing the surface to absorb nitrogen atoms that make their way into the interstitial regions of the iron lattice much as carbon does. Both methods result in extremely hard and durable wear surfaces without embrittling the interior of the part.

Think of a case-hardened part (like a crankshaft) as being a lot like a hard-boiled egg -- a hard, brittle exterior shell (or case) surrounding a softer, springier, tougher interior. Although it's hard on the outside and tough on the inside, you need to be very careful not to drop it, or otherwise expose it to stresses that might crack the hard, brittle case.

Maintenance Considerations

Although we've been talking about heat-treating, strain-hardening and case-hardening in the context of making new airplane parts, these processes can be very important in aircraft maintenance as well.

For example, suppose a metal aircraft part gets bent -- say, a pushrod in a landing-gear retraction system. It may be tempting to simply straighten the part and put it back in service. Before doing this, however, it's important to consider that the bending and straightening may well have strain-hardened the part, making it more brittle and prone to cracking and fracture. If the part is highly stressed, it probably needs to be annealed and heat treated again -- or replaced with a new part.

Likewise, if a heat-treated part is weld-repaired, the heat of welding will almost certainly destroy the heat-treatment in the vicinity of the weld. Therefore, after the weld-repair is finished, the part may need to be annealed and heat-treated again.

In a similar vein, an exhaust-system leak will often expose nearby parts -- such as engine mounts -- to high heat. Even if those parts don't appear to be damaged, there's a good chance that their heat-treatment has been destroyed and the strength of the part compromised.

See you next month.


Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.