Decision – At the very start of the idea to build bicycle frames it was necessary to choose a proper material. Milan Duchek’s pivotal decision which was right, even the best possible one – aluminium alloy AlZn4,5Mg internationally referred to as 7020T6. Let’s only remark that when Duratec company started in 1997, composite materials were not common in cycling and the aluminium alloy was a complete news in the field of building bicycles.
Characteristics of 7020T6 – alloy used for the building of all aluminium models of Duratec brand. High firmness as well as high yield strength - a limit at which the frame is deformed permanently, are characteristic for this material. This is of course unacceptable from the construction point of view. But the utility properties of the frame are not only dependent on the material used, but first of all on its consequent processing. To increase the strength rates, heat treating is performed. To optimise the profile in a certain tension spot, forming processing is used. Conus forming and ovalisation and variable thickness of the wall are mainly used. These parameters make it possible to assess the quality of a frame.
General point of view – At present, two kinds of aluminium alloys are used to make aluminium frames for bicycles. These are 6061T6 alloy and 7020T6 alloy made in Europe or its Taiwan analogue whose chemical composition is slightly different –7005 T6 alloy. Debates which alloy is better for the building of bicycles and why can often be found on discussion servers.
1. Tensile strength and yield strength comparison: European standards in force are crucial for the producers of alloys when comparing materials. Material has to show required chemical composition and tensile figures guaranteed by the producer. These are tensile strength Rm (MPa) and yield strength Rp0,2 (MPa). It is obvious from the enclosed table that 7020 T6 alloy reaches much higher tensile figures.
Tensile strength Rm: is the maximum amount of tensile stress that a material can take before failure.Once the tensile strength is passed, destruction of material occurs.
Yield strength Rp0,2: is the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.
2. Heat treating comparison. It is necessary that the tubes made by drawing from 7020T6 alloy are heat processed. Solution annealing is used for relieving the inner tension and natural ageing (double stage annealing). Natural ageing is a process of curing 7020T6 alloy which is of high importance for building bicycles and it above all takes place at room temperature. It is the reversion of the original mechanical properties guaranteed by the standards after heat intervention (welding). The basic material – tube is thermally heated during welding and after that the material gains its original mechanical properties in the process of artificial ageing. The material gains 90% of strength within the first seven days after welding, the remaining 10% are gained within 2-3 days after welding at ambient temperature. This is an advantage not only during frame production but also at post-warranty service in case of a repair.
Alloy 6061T6 gains its original mechanical properties after welding in the process of artificial ageing. It is necessary to reach the process of returning to original mechanical properties by heating. It is necessary to let the weldment age artificially at prescribed temperature and time (e.g.at 195°C for 3 hours) to reach back the required tensile strength Rm and yield strength Rp0,2. The process of artificial ageing is energy and time-consuming and it is necessary to respect the prescribed rates and times. This property of alloy 6061T6 consequently limits the repairs of frame because it is impossible to attain the process of artificial ageing.