TECHNOLOGY STORIES BIKE INFO, GEOMETRY ... - Vintage Trek

esult, it is not strong enough to make a good bicycle frame.
By adding various alloying agents to the aluminum,
different characteristics can be obtained. These alloys of
aluminum have a number which describe the alloying elements.
6061 aluminum has small amounts of magnesium,
silicon, copper, and chromium added to the pure aluminum.
This alloy obtains its strength from microscopic precipitates
(magnesium silicide crystals) that mechanically stop the slip
planes in the aluminum crystals from sliding when force is
applied. As an analogy, they work like putting sand in a
sliding bearing.
Aluminum alloys can also be strengthened by mechanical
working. Cold-drawing the tubing is an example of
mechanical working. This causes microscopic defects and
strains in the aluminum crystal, which make it more difficult
for the slip planes to move.
Welding aluminum
When welding 6061, and aluminum alloys in general,
several undesirable things happen.
With changes in temperature, aluminum changes
dimension more than steel. When a weld puddle cools down,
it shrinks and pulls on the adjacent material. With aluminum
alloys this means a weld distorts the material more
and leaves the material under high residual stress after the
weld is complete. This residual stress adversely affects yield
strength and fatigue life.
If the tube had any strengthening due to mechanical
working, this cold-work induced strength would be lost
near the weld where the material was heated to high temperatures.
Welding removes the strengthening effects of the
T6 heat treatment.
The optimum distribution and size of magnesium silicide
crystals are created by the T6 process, which involves a high
temperature solution-quench followed by lower temperature
artificial age. Exposing the material to the high temperatures
of welding dissolves some of these fine crystals and make
others grow large, weakening the material near the weld.
Heat treatment of aluminum
6061 loses so much strength after welding that we
decided there was no alternative but to heat treat the entire
frame after welding in order to obtain a high strength,
long life, lightweight frame. By heat treating the entire
frame to a T6 condition, the material is brought back to
full strength throughout the frame structure. At 1000
degrees in the oven, part of the solution quench process,
the aluminum is close to its melting temperature. All of the
precipitates present at room temperature dissolve into the
aluminum. This makes it so soft that all of the residual weld
stresses are relieved.
Of course we are not the only manufacturers to solution
quench and artificially age the complete frame. Several
other manufacturers of premium frames also typically do
this on frames made of 6061 or other 6000 alloys.
Often the frames made from 7000 alloys are not heat
treated after welding at all. In other cases they are only
artificial aged after welding, which strengthens the material
which was hot enough for long enough to dissolve
the alloying elements, but does nothing for the rest of the
frame material.
Trek 2002 Tech Manual
In these cases the alloy just got hot enough to partially
dissolve the alloying elements, or just grow the strengthening
crystals to a large size which weakens the material
substantially. This is called over-aging. It is similar to
what happens if you leave the material in the ageing oven
for too long a time. Some of the crystals grow larger in
size, while others shrink or disappear. The net result is
that the weld is strengthened, but the tubing adjacent to
the weld is weakened. So even though 7000 alloys claim
a higher strength than 6061, it is probably less after welding.
Grain growth
In my opinion, the limiting factor for designing aluminum
frames is the fatigue life. If we design a frame in
6061 T6 for the same fatigue strength as Chrome-Moly, the
6061 frame will have a much higher yield strength than
the steel.
I wanted to make our frames even lighter, so in the
early 80’s I started looking for an aluminum alloy with
a higher fatigue strength. There were a few alloys in the
6000 series that had slightly better test numbers.
The problem with the higher strength alloys is that the
presence of the hardening elements causes the microscopic
aluminum crystals (the grains) to grow when the alloy is
at high temperatures or when it is under stress. Larger
grains result in poor strength properties.
In making a Klein frame, we have multiple steps where
we anneal the material with a high temperature oven cycle,
in order to make it soft so we can perform some type of
butting, swaging, forming or bending operation on it, after
which we have to either solution quench and artificially
age it to bring the strength back prior to the next operation,
or we anneal it again to remove the work hardening
effects of the last operation so we can perform further work
to it.
I took a trip to the Alcoa Research center and talked
to several of their material experts. They told me that
I could not use the higher strength 6000 series alloys I
was interested in because we would see uncontrolled grain
growth in our process. 6061 uses a small amount of
Chromium to help slow down this grain growth. That is
what has made it work well for our early frames. So I did
not find a good replacement for 6061 on the first try.
Developing a recipe for a better aluminum alloy
I am not a metallurgist, so I have worked with several
metallurgists during development , who have helped a
great deal. However, I knew our processes and I knew
what was needed to make a better bike. So I knew what I
was looking for and researched other alloys and their use.
Around 1990, I started looking at some Lithium
Aluminum alloys. These are different than typical aluminum
alloys in that they have significantly lower density,
and increased modulus (that means higher stiffness). They
are not perfect, and have some unique problems to overcome.
The aircraft industry spent millions on their development,
but these alloys have not seen a lot of use to
date.
One of the interesting features of the particular lithium
aluminum alloy I was working with was that it utilized
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