Aluminum alloys for bicycle frames
several aluminum alloys used for manufacturing bicycle frames. Most often they are made of
- aluminum alloy 6061 (analogue of our AD33) and
- aluminum alloy 7005 (analogue of our 1915),
- alloys 6066, 6069 and 6013, and
- high, but difficult to weld aluminum alloys 2014 and 7075 - domestic analogues AK8 and V95.
There is also a Chinese advertisement on the Web: "Bicycle pipes made of 6063-T5 aluminum alloy".
Aluminium alloy 6061
Aluminium alloy 6061 - a complete analogue of the good old AD33 domestic alloy according to GOST 4784, as, however, and abortion 7005 - an analog of our 1915. Alloy 6061 – a standard structural alloys, one of the most popular alloys 6xxx series, after the alloy 6060 and 6063, of course. Its value for a fairly high strength properties, high viscous characteristics and good weldability.
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;
- cutting of pipes "on the mustache";
- argon arc welding of pipes into the frame;
- thermal treatment of the frame on the state T6.
Compression of the alloy pipes 6061
The original tubes for the frame are made by pressing (extrusion). Extruded tubes may be seamless and "seaming". Since none of the manufacturers does not indicate, which is the tube, the, probably, they just "suture", cheaper.
Seamless tubes are manufactured from hollow preforms or when pressing solid preform stitch needle-mandrel. These pipes are truly "seamless", because they do not have any hidden longitudinal seams.
"Seaming" pipe made of solid workpiece to special matrixes, most often of the "porthole" type. In this matrix the metal stream is divided into an inner mandrel into four streams, which are then output from the matrix before joined to form four longitudinal welds. Although they are called welds, no conventional welding there, of course, not. This - the so-called compression welding, which occurs under high temperature and pressure with a special metal welding chambers matrix. Correctly and efficiently performed welds absolutely not visible and the strength properties practically do not differ from the base material.
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.
The oval shape is usually obtained by hydroforming technology: by placing the pipe in a special matrix and pumping it inside under high pressure of water or oil.
Butting (from the English "butting") is a thickening of the pipe wall in critical sections and thinning - in lightly loaded. Produced by cold drawing – knurling inside pipe special mandrels.
Trimming aluminum tubes "miter"
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.
Welding of aluminum tubes
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 %, domestic analogue is a welding alloy SvAK5 GOST 4784.
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.
alloy frame Heat Treatment 6061
After that, as a welded frame, subjected to heat treatment for restoration of mechanical properties of the alloy is thermally hardened 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.