Aluminum is one of the most important structural metals employed
in engineering design. By itself in a pure form, aluminum is quite
malleable and rather weak. However, aluminum is most often used
as an alloy, where the addition of alloying elements creates a
material that is both lightweight and can have incredible strength.
Sometimes you don’t even have to add a lot of alloying elements,
just a few percent of different elements. In fact, most aluminum
alloys are at least 90% aluminum. The development of aluminum alloys
was, and continues to be, driven by the aerospace and defense industries.
In particular, aluminum saw a huge surge in alloy development around
WWII, as aerospace engineers designed the most cutting edge military
airplanes requiring lightweight and strong materials. In fact,
the strength of some aerospace grade aluminum alloys such as the
6XXX series approaches that of some steel alloys, while only a
quarter of the weight. Even today when you step on board any aircraft,
the bulk of the structural components will be made from high strength
aerospace aluminum alloys.
We suggest sing a high strength aerospace grade aluminum alloy (AA 6061) to create aluminum frames. This alloy is an industry standard for creating high strength tubing, and comes from a family of aluminum alloys created primarily after WWII to replace the 2XXX series alloys in high strength airframe applications. In fact, some of the most important grades of 6XXX series alloys were developed in the 1970’s for advanced fighter aircraft such as the F-14 Tomcat, which required new high strength/lightweight materials to sustain the stresses common to jet fighter aircraft. The 6XXX series aluminums, including the 6061 alloy, are precipitation-hardened alloys. This means that 6061 is a heat treatable alloy, where the metal is heated to approximately 150oC for about 24hrs. This is where the alloying elements come in. They form nano-scale strengthening particles of the form Mg2Si during the heat treatment. These particles are extremely small and hard, but they help prevent plastic deformation, thereby increasing the strength of the material. This produces a material that is lightweight, stiff and strong.
creating the tubes out of high strength 6061
alloy is not enough, you also have to know how to properly join
these tubes together to produce a strong and durable bike. Understanding the intricacies of the metallurgy behind
welding aluminum tubes together, particularly when working with
a high strength, precipitation hardened 6061 aluminum alloy.
Before fabricating the frame, the tubes are aged to a T4 condition,
which starts the development of the strengthening particles in
the alloy. The tubes are then welded together using a TIG (Tungsten
Inert Gas) welding technique to ensure that the weld zone is
free of contaminants that could embrittle the material and lead
to a lower life expectancy for the whole frame by failure of
the weld. The welding process also produces a heat-affected zone
(HAZ) in the metal of the weld and the surrounding area on the
joined tubes. This HAZ is often associated with poor mechanical
properties due to the diffusion of solute from the weld zone,
which consequently produces less strengthening particle in the
welded material, and by the growth of grains due to the heat
from the welding process. This is exactly why we use 6061
tubing, as it contains the alloying element of Manganese which
is used to control recrystallization and excessive grain growth
when the material is welded and heat treated. After the welding
is complete, the entire bicycle frame is aged to an overaged
state of T6, which develops the strengthening particles in the
material, relieves stresses associated with the welding process,
and minimizes the heat affected zone by allowing the material
in the weld zone to strengthen as well. With the proper heat
treatment and welding technique, our frames can be as strong
in the weld zone as the tubes themselves.
Designing frames out of aluminum can be tricky, and aluminum frames are normally associated with a stiff but harsh ride. We’ve incorporated advanced geometry in all of our frames to create the perfect ride from an aluminum frame. The design allows us to tailor the ride qualities of a frame made from an extremely stiff material such as aluminum. The top tubes and down tubes shapes provide an aerodynamic profile while at the same time designed for maximum efficiency for the torsional loads the frame experiences during riding. The result is an unparalleled ride in the world of aluminum frames, where metallurgy meets design meets aerodynamics.