Aluminum, Stainless Steel, Copper, Brass & Bronze

Aluminum

Aluminum is a great material for many non-wear details. It is about 2.5 times lighter than steel so it can really cut down on inertia and mass in the moving parts of your designs and it is naturally corrosion resistant. From a mechanical perspective, the primary bar stock grade of aluminum that you will use (see below) has almost the same yield strength (where permanent deformation occurs) as mild steel (35kpsi for 6061-T6 aluminum and 36kpsi for 1018CRS), so it sounds like substituting it is a no brainer. But (there's always a but), where aluminum falls down significantly is in stiffness (modulus of elasticity): aluminum has 1/3 the stiffness of mild steel, making it deflect considerably more for the same cross section, so you need to account for that in your designs. There are two types of aluminum stock you will use regularly: bar and cast plate.

Aluminum Bar Stock

Just like ferrous materials, this comes in rectangular, round, and hex in a variety of sizes. There are a ton of different alloys, so your company may have different preferences, but for general detail work, 6061 is a grade that machines well and can be welded. It has a yield strength of 40kpsi, which is strong enough for most details, but, if a higher strength is needed, you should go to 7075 which has a yield strength of 73kpsi. But, 7075 is quite a bit more expensive and it's harder to machine, which is the tradeoff. For 7075 and 6061, you should also specify the tempering method which is typically T6 in our industry (i.e. 6061T6). Price runs around $3.50-4/lb plus a $3-5 cut charge if getting cut to length.

Aluminum Cast Plate

A really nice product for several applications is aluminum cast plate with popular brands being MIC 6, ATP-5 and ALCA5. It is very flat and well toleranced (+/-.005" [.13mm] and has a high quality surface finish. You can get it in thicknesses from 1/4" to 2" and sizes up to 72.5" x 144.5." Because it is cast, it is very stable and very consistent. Just realize that because it is cast, it has a considerably lower yield strength than 6061 or 7075, coming in at just 18kpsi. This won't be a problem in most applications where you are looking for a plate, but just something to keep in mind. Cost is around $4-5/lb, but to be honest, I don't know how it's charged if you buy it at less than a full sized plate, so a quote might be beneficial when trying to decide between a large piece of bar stock or a smaller piece of cast plate.

Stainless Steel

The two most common grades of stainless for detail work are 304 and 316. 304 is less expensive, but 316 is more corrosion resistant and is required in some pharma applications. Your company may have a preference, so check before choosing. If you are designing weldments with stainless, you should add the "L" designation to the material (304L or 316L). Cost is between $3-5/lb with the same $3-5 cut charge for cut to length.

As covered in the tool steel post, there are heat treatable stainless steels, but they are magnetic and will corrode over time, so they should be avoided except for wear details. 416 and 441 are the most popular heat treatable alloys. 416 machines more easily (a VERY relative term, since all stainless takes about 2x the time to machine as carbon steels), but corrodes more. 441 has better mechanical properties and resists corroding, but is more expensive and very difficult to machine.

Copper, Brass, Bronze and Ampco18

Many of you may remember the differences between copper, brass and bronze from your materials class, but I'll restate it here just in case it was an 8am class. Copper is an elemental metal, whereas brass is a copper/zinc alloy and bronze is typically a copper/tin alloy. Copper can be used in our industry for custom electrical connections and for thermal transfer details, but it's extremely soft and shouldn't be used for anything structural or wear related. Brass is a little tougher, but not enough to make it acceptable as a structural/wear item and it really shouldn't be used all that much except for maybe thermal transfer details. The most useful of the three is bronze, which has very good wear properties and low coefficient of friction. 954 bronze is a high strength bronze that should be used when you are seeing significant tensile or shear loads. The other bronze of note is 932 bearing bronze, which has better anti-friction properties than 954. Use bronze for wear plates, bushings (you can buy these off the shelf), etc,

The final material in this section worth mentioning is Ampco 18. It costs a little more than bearing bronze, but it's tougher, harder, has lower coefficient of friction, and holds tolerance better. It's great for high load, high wear sliding surfaces and bushings. You can buy it in round and bar form and make your own details, but there are also a ton of standard bushings, wear plates, gibs, etc made of it. I'm a big fan.