ความเห็นลำดับที่ 57
A material designed for bicycle frames
Advertising Claims
Many of the advertised properties for different frame materials are the properties of the material in the highest temper state, not the strength that the frame tubing is in after it has been welded or brazed into a bicycle frame. The material may chemically be the same, but the advertised strength is gone.
In addition, the advertised strength is a bulk material property and does not reflect the engineering design of the bike, such as the diameters and wall thickness of the tubing used. These have a huge influence on the overall strength of the finished frame, and at least as much influence on the way the bike rides. Please do not equate advertised material properties with frame durability or low weight. If you want to compare the strength of one frame to another, you probably need to test them both. And if you want to compare the ride, instead of looking at charts you'll need to ride it!
Why Aluminum?
In the early 70's, when I lined up on my first starting line, the bikes around me weighed an average of about 22 pounds. My bike was at least average in quality, yet the frame represented the heaviest part of the bicycle. Even so, I found that it was not stiff enough to keep the drive train in alignment during sprinting efforts.
At the time I was a student at MIT in Boston, Massachusetts. A professor, myself, and some other students started to look at what would make a better material for bicycle frames. The standard high-end bicycle frame was made of double-butted chrome molybdenum steel alloy tubing. Steel is easy to work with, but it is very dense, making even the thin tubes of my high-end steel racing bike into a heavy structure.
Our goal was to to make the frame lighter, stronger and stiffer. To meet thaose goals, our first criteria was a material less dense than steel. As low density alternatives, we looked at Aluminum, Magnesium, Titanium, and Carbon fiber. While each of these looked like they might provide some benefits, we were also looking for an easy way to make a few bikes. We were hoping to find a material that we could obtain easily, and assemble into a strong and light frame.
Only one of those materials met our goals. Carbon fiber needs special molds for each size and geometry of frame to be produced. This would take time and cost a lot of money.
Titanium was very expensive and the welding was difficult. The entire area being heated needed to be shielded from air. Even ignoring the cost, it was difficult to obtain in the tubing sizes we needed for bikes. Most available tubing was CP (Commercially Pure) titanium which did not provide much of a strength benefit.
Magnesium has the lowest density of the metals. Initially this looked good, but it does not have the ductility of aluminum, and does not weld as easily. Also the tubing sizes we needed were not readily available. Another problem was this was in the Boston area, where the streets are salted in the wintertime. We had seen what the salt does to a steel frame, and we knew that magnesium has an even lower resistance to corrosion.
After a lot of research, we decided on aluminum as the material of choice. As we wanted the highest performance frame possible, we started looking at the highest strength aluminum alloys. Unfortunately, they were difficult to weld, had corrosion problems, etc.
Finally we stettled on 6061 aluminum. 6061 was the workhorse of the structural aluminum alloys, and it had most everything we desired. It is easily welded, machines easily, is formable at room temperature, and resists corrosion pretty well (it is used extensively for marine applications). As a real plus, 6061 was used extensively in aircraft, so thin wall tubing was readily available in various diameters.
Aluminum
Pure aluminum is very soft. The molecules align and interconnect such that in pure aluminum, molecular slippage easily occurs in three planes. As a result, 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 assigned by ASTM (American Society of Testing and Materials) which describe the alloying elements. 6061 aluminum has small amounts of magnesium, silica, 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.
Aluminum, to a much greater degree than steel, changes dimension with changes in temperature. When a weld puddle cools down, it shrinks and pulls on the adjacent material. With aluminum alloys this means a weld distorts the material much more and leaves the material under high residual stress after the weld is complete.
If the tube had any strengthening due to mechanical working, this cold-work induced strength would be lost near the weld when the material was heated to high temperatures.
In its optimum condition for bicycle tubing, 6061 is said to be T6, which is an indication of a specific heat treatment. Welding removes both the heat treatment, as well as any benefits from mechanical working.
The optimum distribution and size of magnesium silicide crystals are created by the T6 process, which involves a solution-quench heat treat and artificial age. Exposing the material to the high temperatures of welding dissolves some of these 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. 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 also typically do this on frames made of 6061 or other 6XXX alloys.
Often the frames made from 7XXX 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 material that just got partially hot.
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 7xxx alloys claim a higher strength than 6061, it is probably less after welding.
Grain growth
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, stronger than is needed and thus heavier than is optimum.
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 have to 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 6xxx alloys I was interested in because in our process, we would see uncontrolled grain growth.
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 talked with several metallurgists along the way who have helped a great deal with the development of this alloy. 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 had done considerable research into 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. They are not perfect, and have some unique problems to overcome. The aircraft industry spent millions on their development, but they 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 Zirconium as the ingredient for grain control. From our testing, zirconium seemed like it was particularly effective. So when I decided to attempt to create an alloy specifically for making a bike frame, I decided to get rid of the Chromium used in 6061, and use Zirconium instead.
Since we use multiple heat treat cycles when we manufacture a frame, we needed a high response to the heat treatment. So I added more of the precipitation hardening ingredients Silicon and Magnesium.
I also increased the amount of copper, as it has a strong strengthening effect, and the copper based aluminum alloys show excellent fatigue properties. So I thought more Copper might help increase the fatigue strength of the alloy.
Another requirement we have is the ability to form the material substantially at room temperature when it is in the soft condition. The auto industry uses a couple of 6000 series alloys specifically designed for forming into complex auto body surfaces. These are 6009 and 6010, sheet forming alloys. The notable difference between these and other 6000 alloys is a significant Manganese addition. So I added a little manganese to the alloy to improve the forming ability.
May I have a bit of alloy, please?
The barrier to testing a new "mix" is that you need a good foundry to make a batch for you. A single furnace load of material is 40,000 lbs, or 20 tons of aluminum. If the alloy does not work out well that could be a lot of scrap. So I made my best guess at what the percentages should be, and had the first batch poured.
Great results
ZR 9000 has worked out extremely well. It machines cleaner and with less tearing than 6061. In the annealed condition, it forms very well. It welds very nicely. It has an excellent response to heat treatment. So compared to 6061, it allows us to make the frame without any additional trouble.
In a completed structure, ZR 9000 tests out very well. In tensile tests of complete identical frames, the yield strength is about 1/3rd higher than 6061. On our fatigue testing machines, the ZR 9000 frames endure 5 times the number of stress cycles before failure.
These results are as good as I could have hoped for. We have been able to use the higher properties of the new material to remove weight in places where it is beneficial and increase the fatigue life and dent resistance of the frame tubing.
This is the first material that I am aware of that has been designed expressly for the process by which we make a high performance bicycle frame and thus to optimize the frame's performance.
โดย : นายน่องโต -
[ 11 ก.พ. 2545 - 22:22:06 น.]
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