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Bicycle Frame Welding and Heat Treating

 

Grades of Aluminum Alloy

7005 is the grade of aluminum alloy most used for my frames. Most manufacturers of alloy high-end frames 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 hydro-forming 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.

TIG Welding

Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the welding ard. The weld area is protected from atmospheric contamination by a shielding gas (usually an inert gas such as argon), and a filler metal is added during the process. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.

TIG (GTAW) is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques.

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.

 T4 heat processing at 480C is required to return the material to its original uniform condition. It can then be aligned to within the accepted degree of tolerance--≤ 3mm.

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 90C for 5 hours. In the second stage, it is 140C 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. When a frame is TIG welded, the area around the weld is subjected to high temperatures. If the frame is left to cool by it's own, the aluminum will return to its previous state. In this case it's either T-6 or T-4. This is called normalizing.

On some metals like steel or Chromalloy the material outside of the weld creates an area of magnetite. Which is a none uniform grain structure. To eliminate the the repair area is preheated to 300 degrees welded and allowed to cool slowly. Thus eliminate the condition.

 


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Last modified: 02/05/17 01:36:12 PM