Here is some info on metals and modeling


 

 

Metals for Modelers
by Roy Vaillancourt

About the author:
Roy Vaillancourt is the president of Vailly Aviation, a part time venture, catering to giant scale enthusiasts. Roy and his wife, Nancy, have operated Vailly from their home since 1986. Roy, whose specialty is giant scale warbirds, is an active scale contestant who has competed regularly at the Top Gun and Scale Masters Championships for the past ten years. He is a frequent contributor -- on a variety of topics -- to the major scale modeling magazines and to our own Articles and Tips department. A mechanical engineer with two advanced degrees and over thirty years experience in design and manufacturing, Roy works full time as a senior design specialist for Lockheed Martin.


This article discusses the classification and properties of the metals most often used by sailplane modelers.

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ALUMINUM
 

One of the most abundant metallic elements, aluminum is also one of the most versatile engineering and construction materials available today. Its wide range of alloys can satisfy many diverse requirements, including high strength, light weight, good corrosion and tarnish resistance, and good electrical and thermal conductivity. And aluminum's attractive silvery luster makes it easy on the eye.

Although pure aluminum has a relatively high melting point -- 1220 degrees Fahrenheit -- most of its alloys can be easily fabricated, machined and joined. Aluminum is available cast, forged or wrought, such as in plates, bars or rods. It is non-toxic and can be attractively finished by many common methods including painting and anodization -- a chemical etching process. The three alloys most commonly used for modeling are 2024-T4, 5052-H32 and 6061-T6.

2024-T4

T4 is a well known aluminum-copper alloy that has been heat-treated in solution and then naturally aged to a stable condition. It is available in sheet, strip, plate, block, rod and tube form. A very rigid metal, it is easily machined. Its mechanical properties include high strength and excellent resistance to fatigue under tensile or compressive loads. However, because it doesn't bend easily under load, it is not easily formed by bending, as in a brake. In fact, T4 fractures readily at the bend line and is likely to crack at stress concentration points under vibration. Although it is not good for engine mounts it is commonly used to make pistons and connecting rods in engines, pivot blocks, scissors or air cylinders on retracts. It should not be used to make bending brackets of any type.

5052-H32

A strain-hardened and stabilized aluminum-manganese alloy, H32 has moderate to high strength properties but is not heat-treatable. However, it exhibits good welding and brazing characteristics with a high resistance to corrosion. Most commonly supplied in sheet metal form, it is generally not available in thickness over 3/16 of an inch. H32 is excellent for bending into brackets and similar hardware. When bending, keep the internal radius of the bend equal to 1.5 times the thickness of the material or greater.

6061-T6

T6 is an aluminum-silicon-magnesium alloy that has been heat-treated in solution and artificially aged. Of all the aluminum alloys, T6 machines the best and has excellent brazing and welding qualities. It demonstrates very high resistance to cracking under stress and is easily formed by bending or pressure. A medium strength alloy with high corrosion resistance, it is used in many heavy-duty structures such as engine mounts and landing gear components.

For machining aluminum, the best cutting agent and lubricant is kerosene. Next best is a light machine oil like Marvel Mystery Oil, but even candle wax can be used without difficulty.

One thing to remember when finishing aluminum is that for the common man getting paint to stick to it is very tough. Not because we can't paint right, It's because our finishing method does not agree with what aluminum wants to see. aluminum naturally forms a thin surface layer of oxidation. (This is that black stuff you get on your hands when handling untreated aluminum.) This layer forms rapidly on fresh cut material and prevents the rest of the material underneath from oxidizing or corroding. Even if the part looks shinny, this layer may have already formed. This is why aluminum is considered very corrosion resistant. However, this very same layer is what makes it very hard for paint to stick. Well some of you may say, "I'll out smart this material and use some epoxy paint". Truth is, no matter what paint you use if the surface isn't prepared right the paint will peal right off.

The best way to prepare aluminum is to dip the part in a chromic acid etching bath just prior to priming. This bath removes all oxidation and applies a thin protective top layer that adheres to the aluminum and accepts any paints very well. Painting should begin as soon as possible after the part is dried off but it is possible to wait up to 24 hrs. with out too much harm. Once the primer is on all other painting can proceed at a normal rate. Well most of us don't have a chromic acid bath at home so how do we handle this problem.? The best way is to sand the entire part with 320 or 400 wet - dry sandpaper used wet. Dry the part off by use of a heat gun or forced air. Wash right away with Acetone or thinners compatible with the paint you will be using. Again dry off and commence applying primer. Once the primer is on you can relax. If you had to stop anywhere in the process before you got primer on the part you'll have to start all over with the sand paper etc. The point here being that you have to get primer on an oxidation free surface.

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LEAD

Probably the second most used metal used by modelers. When was the last time you built a nose heavy plane? Lead is most commonly used by modelers as ballast. This bluish-gray metallic element is very dense with a specific gravity of 11.35. This means it is very heavy for a given volume. Lead is a soft, malleable and ductile material. Its melting point is quite low at 625 degrees F. Lead is also useful for generating electric current in electrochemical applications such as batteries. It can be readily and inexpensively fabricated into many forms. It is used as an additive to some metals to make them easier to machine and it is also a major alloy in most solders. Lead is sometimes added to "plain" bearings to aid in lubrication and ease of fabrication.

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COPPER

Probably the first metal to be smelted from its ore, Copper is a very useful material that has a number of desirable properties. It resists corrosion, provides outstanding electrical and thermal conductivity, and has good ductility. While its strength-to-weight ratio is relatively low, Copper is considered a heavy metal. Pure Copper melts at 1981 degrees F. It can be polished to a high luster. It is non-magnetic and combines well with other metals to form a wide variety of useful alloys. It is easy to fabricate and can be joined mechanically by soldering or brazing very easily. Copper and its alloys tend to work harden and can be either hot or cold worked to increase its strength. What this means is that as you bend Copper, if you were to bend the same area back and forth repeatedly the material actually gets stronger at the bend. The down side is that as the strength and stiffness goes up so does its stress cracking probability. So the trick here is to know just when to stop working it before you start to fatigue it. For most of our uses today there are two basic forms of Copper.

C110

This is 99.9 percent pure Copper. It can be bent, riveted, drilled, milled, filed, soldered, brazed and welded to most any configuration. Most common use is electrical connections and ground straps etc.

C112

A harder version of the C110 that can also be easily brazed and soldered. It is harder then the C110 so it may require annealing prior to bending and /or shaping. Annealing is a softening process that is accomplished by heating the part to a burgundy red color and letting it cool naturally. The annealing process can be applied to an entire part or it may be done locally (only to certain areas).

Like aluminum, Copper also likes Kerosene as a cutting agent when machining it. Also like aluminum; Copper forms a thin oxidation layer that helps it become very corrosion resistant. This oxidation layer, however, does not form as fast as aluminum's so painting Copper is not as big a chore. The do-it-at-home modeler should follow similar techniques as used on aluminum when it comes to painting Copper.

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BRASS

Brass is really an alloy of 70 percent Copper and 30 percent Zinc. Brass is an excellent metal for cold working and shares many of the same properties of Copper but Brass is stronger. Increasing the Zinc content increases strength and ductility. Brass can also be annealed the same way Copper is. Brass is considered a "self lubricating" metal and very rarely requires a lubricant in either machining or in use. Brass sometimes has Lead added to aid in machining and forming. There are many special alloys available but the three basic forms of Brass in use today are:

C260

Known as "cartridge brass" C260 has a high Zinc content that gives it optimum strength and ductility yet still retains the high formability of Copper. It has excellent cold workability and is used extensively in the automotive field. It is also the most common form of Brass used for plumbing goods, builder's hardware, and ammunition components.

C330

Most widely used for the fabrication of tubing. A low Lead content of .5 percent gives this alloy good machinability and excellent cold working properties. It can be fabricated by forming, bending, machining, piercing and punching. It can also be brazed, soldered and welded similar to Copper. Of all the Brass alloys this is the one that is used most widely for brazing steels and dissimilar metals together.

C360

Considered a "Leaded" Brass, this alloy also has a high Zinc content of up to 37 percent. The inclusion of lead gives this high strength alloy a "free-cutting" quality making it easier to machine. Often called "Leaded Brass" or "Free machining Brass" it finishes well and is the most easily plated, soldered and brazed Brass alloy.

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STEELS

For our purposes there are basically two types of steels that could be found in the average shop, Carbon Steel and Stainless Steel.

Carbon Steels

Carbon Steels come in a variety of alloys. Too many to list here. The predominant elements in Carbon Steels are Iron and Carbon. The Carbon content can range from a few hundredths to just over 1 percent. Doesn't sound like big numbers , does it?

( Low Carbon Steels 0 - .30 %, Medium Carbon Steel .31-.70 %, High Carbon Steel .71-1.3 % ). Carbon Steel in its various forms represents more than three-quarters of the steels in production today. Carbon Steel is generally fine grained, and has little to no alloying agents. Most Carbon Steels are classified as hot rolled, cold drawn or cold rolled and are available in bar, sheet, wire, tubing, and structural shapes. They can also  be forged and casted. Carbon Steels are heat treatable to a degree. The carbon content is what gives these steels their heat treatable strength properties. For example; the higher the carbon content, the stronger the material can be heat treated to. The music wire we use for landing gear is one of the medium Carbon Steels heat-treated to a tough condition. One draw back to these steels is that they contain high amounts of Iron. This means they rust easily. They should not be left bare, as they will form an oxidation layer of rust. Unlike aluminum, this oxidation layer keeps on going until it has taken over the whole part. Eventually the part will deteriorate and disappear. Leaving you with a pile of rust....... The best cutting agent for carbon steels is plain old motor oil. 30W works the best. Straight out of the can or bottle. Finishing steel is very easy. Clean off all oils and sand off all rust followed by a wipe down with thinner. Apply a coat of primer as soon as possible and finish off with the color of your choice.

Stainless Steel

Stainless Steels are high-alloy steels well known for their outstanding corrosion resistance. Valued for tough mechanical properties such as high strength and extreme thermal capacities they provide low maintenance and long service life. Typically Stainless Steels are iron-nickel-chromium alloys with a generally high percentage of nickel. There are two classes of Stainless Steels, Non-Ferric (300 series) and Ferric (400 series). The Ferric class (400 series) contains a higher percentage of iron and approximately 12 % chrome and even though these steels are classified as "stainless" they do rust. The 400 series is magnetic and is heat treatable while the 300 series is not magnetic (generally) and is not heat-treatable. The 300 series contains a higher percentage of nickel and approximately 17 % chrome. It is the higher contents of nickel and chrome that give the 300 series their corrosion resistance. Most fasteners such as nuts, bolts and washers are of the 300 series. In cases where extreme high strength is required nuts and bolts would be made from heat treated 400 series and then coated to prevent corrosion. The best cutting agent for most Stainless is USED motor oil thinned with a little Kerosene. The older the motor oil the better. You know, the stuff you drain out of your car after 70000 miles. Don't mix it with rocks or sand, just add a little Kerosene and your good to go.

Finishing any Stainless is just like finishing any Carbon Steel with one exception. The non-ferric series does not rust and therefore does not require any finishing at all if you don't need it painted. It can be left bare and will hold its luster for a very long time. Much longer than most of our models survive. The ferric series does rust so it should be given the prep and prime and paint treatment.

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I hope that this has helped you gain a little more knowledge about the various materials we use and why we use them. Should you have any specific questions relative to any of these materials, Please drop me a line and I'll try to answer your questions as best I can.

Good luck on your metal working.