Bicycle Frame Making with Aluminum Alloy – Key Processes: T4 vs T6 Heat Treatment, AL 6061 vs AL 7005

by Sabinna on August 10, 2010

Since I have new aluminum alloy model on the drawing board, here is a brief coverage of key considerations in aluminum alloy frame construction behind the production of these frames.

Grades of Aluminum Alloy

7005 is the grade of aluminum alloy I use for my frames. Most manufacturers of alloy high-end frames in Taiwan use this, whereas China-based manufacturers will mainly employ 6061. Neither grade of alloy is “better” than the other in an absolute sense. It simply depends on what your purpose is.

One property of 6061 is that it is more malleable than 7005–it can be more easily “drawn”, opening the way to creating a variety of tubing designs through hydroforming processes for example. It is also more resistant to corrosion and fatiguing.

The properties of 7005, however, make it more suitable for creating butted tubing. Although it is more susceptible to fatigue than 6061, this will only take place over many millions of cycles (High Cycle Fatigue), and so is not something to realistically be concerned about. Key differences become clearer when looking at the different heat treatment processes applied to each type.

T4 and T6 Heat Treatment

A frame is created first through welding the tubes of the front “triangle”, which is not quite the right word because of the head tube. Except in those frames when the top tube and the down tube actually form an angle, rather than a curve, you are talking about an irregular quadrilateral. Details aside, after this is completed, the  chain stays and seat stays are then welded onto the completed structure.

Aluminum alloy is a relatively soft material. But the welding messes this up with the area around the weld becoming harder as well as producing a raw frame that needs alignment.  T4 heat processing at 480°C is required to return the material to its original uniform condition. It can then be aligned to within the accepted degree of tolerance–≤ 3mm. Heat treatments are outsourced to a specialist company.

The next heat treatment process is T6 which has the aim of hardening and thus strengthening the aluminum alloy. It’s different depending on whether 7005 is used or 6061.

T6 treatment for 7005 takes place in two stages. In the first, the frame is subject to a temperature of 90°C for 5 hours. In the second stage, it is 140°C for 16 hours. T6 treatment for 6061 is 180° for one 8 hour period. Basically, with 7005 you get a rigid frame considerably lighter than one made from 6061.

Another aspect to all this is the cooling process. The standard procedure for cooling 6061 after heat treatment is liquid chemical. Rapid cooling in this way causes distortion in the tubes, and increases the weight of a frame cooled in this way. 7005 is more susceptible to distortion, so frames made from this are air cooled.

Another significant problem is the disposal of the used chemical. Once a factory would expel this sort of effluent directly into the environment. But with strict controls in place, alongside of a more environmentally aware population, this is no longer an option in Taiwan.

The final process is careful QC control in which each frame in a production undergoes systematic checks. I will document this process in as much detail as possible in the next few months. The upshot of all this that utilizing 7005 produces a cost effective, light and extremely durable frame that can be used to build a road bike that is lighter than most of the low-quality carbon offerings that are increasingly appearing in the market these days.

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