Aluminum is one of the most useful and widely used industrial metals. It is light and resistant to corrosion. It is relatively soft, ductile, and malleable and can be formed by machining, casting, drawing, extrusion, or stamping. And it can be alloyed in many ways to produce a wide range of characteristics.

 

Firstcut machines two very different aluminum alloys. 7075-T651, which has zinc as a primary alloy component, is among the strongest aluminum alloys; however, it can be challenging to work with and is quite expensive. At the other end of the spectrum, 6061-T651, which has magnesium and silicon as primary alloy components, is softer, tolerates greater elongation, is less brittle, is easier to form and join, and costs significantly less than 7075.

Firstcut machines aluminum parts in either of these materials for both short run production and prototyping. For production parts, the choice of alloy depends entirely on the application. Tough, strong 7075-T651 is typically the better choice for highly stressed components. For general-purpose applications needing high corrosion resistance 6061-T651 may be better. The respective characteristics of the two alloys include:

 

7075 is used for highly stressed structural parts. It is found in transport applications including aerospace, automotive, and marine. It is also used where strength is critical in applications including worm gears, meter gears, regulating valves, and keys, and where the ratio of strength to weight justifies its cost, as in rock climbing equipment and high-end bicycle components.

6061 is commonly used for structural components in static applications such as frames, brackets, jigs, fixtures, base plates, machine parts, and hydraulic valve bodies. It is also used in applications such as fasteners, truck and marine components, marine fittings and hardware, hinge pins, and appliance fittings in which stresses are lower or parts can be larger to make up for the material’s lower strength.

When parts are machined as prototypes, the method that will be used for production becomes an additional variable. If production parts will be machined, then the prototype can fully reflect performance of the production part. If, on the other hand, production parts will be produced by a non-machining method such as die casting or die stamping then the prototype may only partially duplicate production parts. In that case, choice of alloy should be dictated by the production method. For example, die stamping relies on elongation of the material, so more ductile 6061 may be a better choice for some parts than 7075, which can crack if stretched too far, and this will dictate the choice of material for the machined prototype. Similarly, die-cast parts will not exhibit the same characteristics as heat-treated, machined alloy parts. (In some cases, even machined parts will lose some of their strength and have to be re-treated in a separate post-machining operation to recover specified characteristics.)

Overall, if production is by a method other than machining, the machined prototype will have some, but not necessarily all of the characteristics of production parts. Still, the ability to quickly produce prototypes of similar material can help move the development forward and reduce the likelihood of having to “go back to the drawing board” in later phases of development.

In general, of the two available alloys, 6061 may be the “default” choice for its lower cost and ease of forming. 7075 will be reserved for those applications in which its greater strength and toughness justify its higher cost, brittleness under extreme stress, and production challenges.

Proto Labs Inc.
www.protolabs.com