Applications of Aluminium

Aluminum bicycle frame 6061

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Aluminum alloys for bicycle frames

Several aluminum alloys are used to make bicycle frames.. The most common alloys are:

as well as alloys:

  • 6066, 6069 и 6013
  • 2014 и 7075.

There is also such an advertisement on the Web: “bicycle pipes made of aluminum alloy 6063-T5”.

Aluminium alloy 6061

Aluminium alloy 6061 is a standard structural alloy, one of the most popular alloys 6xxx series, after the alloy 6060 и 6063, of course. Its value for a fairly high strength properties, high viscous characteristics and good weldability.

Table 1 – The chemical composition of 6061 aluminium alloy (EN 573-3)

Table 2 – The mechanical property limits of 6061 aluminium alloy (EN 755-2)

How to make bicycle frames

Let us consider the technology for production of bicycle frames made of aluminum alloy 6061. It includes five main technological stages:

  • pressing and cutting to size of billet pipes;
  • molding;
  • cutting of pipes "on the mustache";
  • argon arc welding of pipes into the frame;
  • thermal treatment of the frame on the state T6.

Tube extrusion: seamless and porthole

Tubes for bicycle frames are made by extrusion. Extruded tubes can be seamless and porthol. Since none of the manufacturers indicate the type of their tubes, then they, of course, namely "porthole".

Seamless tubes are made on special extrusion presses.. These tubes are truly "seamless", since they do not have any hidden longitudinal extrusion seams (Fig. 1).

Porthole tubes are made from a solid billet on special porthole dies. In this matrix the metal stream is divided into an inner mandrel into four streams, which are then joined before exiting the die to form four longitudinal extrusion welds (Fig.. 2).


Fig. 1 – Principle of seamless tube extrusion [3]


Fig. 2 – Schematic of welding chamber (porthole) die hollow extrusion.
(a) Cross section showing metal flow into port streams and around the mandrel. |
(b) Billet entrance face of the die set [3]

Forming aluminum tubes

After the molding operation, the initially round pipes receive a more complex and more "strong" geometric shape: the ovality of the section, variable wall thickness of pipes and the like.

Hydroforming

The oval shape is usually obtained by hydroforming technology: placing the pipe in a special matrix and pumping water or oil into it under high pressure (Fig.. 3).

 


Fig. 3 – Schematic representations of tube hydroforming processes [4]

Butting

  • Usually tubes have the same thickness, which provides the same strength characteristics along its length.
  • Tubes with single, double and triple butted have different thicknesses, which allows the frame to withstand high stresses at the ends of the tubes.
  • Single butted tubes will be thicker at one end, where strength is required only in a certain place.
  • Double butted tubes are thicker at both ends.
  • Triple butted pipes serve the same purpose., as double butted pipes, but further reduce the weight of the middle of the pipe (Fig. 4).


Fig. 4 – Tube butting [5]

Tube mittering

In order to easily and reliably weld the frame tubes need to be "tricky" and precisely trim. After such trimming the two pipes are joined with each other completely without a gap. It is on special machines, which provide a clean and precise cut.


Fig. 5 – Byke tube mittering [2]

Welding of aluminum tubes

Welding technology

To connect pipes clipped into a single structure most commonly used welding, for pipes made of aluminum alloy 6061 - argon-arc. When the argon-arc welding the necessary heat is obtained between the non-consumable tungsten electrode and the work piece. To prevent oxidation of the welding zone an inert gas is used, argon. Argon arc welding is well suited for welding thin-walled tubes and, Moreover, it gives a neat welds. For welding alloy 6061 most commonly used welding wire from the alloy 4043 (in international designation) with a silicon content of about 5 %.

Welding heat-affected zone

The main consequence of a change in welding the physical properties of the base metal and the weld due to heating in the weld zone. In this small zone near the weld, which is called the heat affected zone, It occurs the greatest degradation in mechanical properties, in particular, fatigue strength.

Strong heating causes a change in the aluminum alloy structure - growth of secondary phase precipitates. This coarsening of precipitates reduces the strength characteristics of the material, such as tensile strength, on 30-35 %. for instance, If the tensile strength of the alloy 6061 before welding was about 310 MPa, after welding it decreases to 185 MPa. See also here.


Fig. 6 – The zones of a TIG weld showing parent material, HAZ, and filler material [2]

Heat treatment

After that, how rama cooked, it is subjected to heat treatment to restore the mechanical properties of the heat-hardened alloy 6061, that have been lost during welding. Typically, this heat treatment on the condition of T6. It consists of three stages:

  • heating for hardening: heating up to 530 ° C with an exposure of about 1 hours to obtain the solid solution alloying elements of aluminum, in the case of aluminum alloy 6061 – magnesium and silicon;
  • hardening: rapid cooling to room temperature to delay alloying elements in solid solution;
  • artificial aging: heating up to 175 ° C with holding for 8 hours to extract fine precipitates dispersed strengthening phase.

After such heat treatment state T6 receives alloy and aluminum alloy full designation 6061 takes the form: 6061-T6.

Sources:

  1. What aluminium alloys are best for bicycle frames? – shapesbyhydro.com/
  2. Material and Design Optimization for an Aluminum Bike Frame /F. Dwyer, A. Shaw, R. Tombarelli – WORCESTER POLYTECHNIC INSTITUTE – 2012
  3. Aluminum Extrusion Technology / P. Saha
  4. A state of the art review of hydroforming technology / C. Bell & J. Corney1 & N. Zuelli & D. Savings – Int. J. of Material Forming (2020) 13:789–828
  5. https://www.bikeexchange.com.au/blog/bike-frame-materials-explained