The main objective of the project was to develop a new aluminium wrought alloy that can work at temperatures in the range of 200-250ºC for aeronautical applications such as regulator valves or actuator bodies for air treatment systems. Several heat resistant aluminium based materials have already been developed in the last years that might be applied for such high temperature applications. Notwithstanding, these are either too expensive or present additional technical drawbacks such as insufficient ductility, segregation of components, low machinability or recycling problems.
The present project aimed at developing new alloys through a methodology based on the identification of the effect of up to 12 alloying elements and their combinations in the heat resistance of wrought alloys through the Taguchi methodology. This was based on the selection of critical parameters through design of experiment techniques that minimise the time and resources to be employed in the process. Two orthogonal matrices were to be used, a L16 matrix and a modified L8 matrix. The former employed two levels that were related to the maximum and minimum amounts of the alloying element. The modified matrix incorporates intermediate values. In a first stage gravity casting samples with 24 different formulations were cast by gravity casting and preliminarily tested in order to check whether any of these combinations may reach the established specifications Subsequently, the most promising 2 alloys were further developed.
Researchers belonging to the non ferrous metallic materials and characterisation departments of Tecnalia led the project. Their work was complemented by the researchers at Fundación Inatec, a research institution that is part of the Otua industrial group related to recycling and recovery of metals and Refial, a company devoted to the production of aluminium alloys. They were responsible of the cost analysis issues, environmental aspects and industrialisation aspects.
The main objective of the project was to develop a new aluminium wrought alloy that can work at temperatures up to 200-250ºC. The European aeronautical industry envisages that for near future applications the service temperature of some aluminium components that are part of the air conditioning system of the airplanes will increase to up to 250ºC. Both casting and wrought alloys and thermal treatments were developed for those applications. Furthermore and in order to maintain the competitiveness of the European industry a special attention was required in terms of cost and sustainability of these developments.
In order to carry out this project a consortium formed by three participants was established. Fundación Tecnalia, Fundación Inatec and Refial S.L., all of them based in the Basque Country region in Spain. The coordinator was Fundación Tecnalia.
Two formulations based on Al-Cu alloys were selected at the end of the first year of the project as the most promising alloys to be used at high temperatures. In this second year, the heat treatment parameters were optimised for each of these formulations and they were mechanically and metallurgically characterized. The ageing behaviour of both alloys was also studied, and also the castability, conformability and recyclability of them. Finally, a technical, environmental and cost analysis was performed in order to check the suitability of any or both of the developed alloys to be used in applications requiring long-time stay at high temperatures.
Following the main results achieved in the project were listed:
- Casting of samples with 24 different compositions following the design of experiments approach planned.
- Positive results in the preliminary analysis with tensile properties of extruded specimens at room temperature and 250ºC.
- Selection of the optimum ageing parameters for the heat treatment of the two formulations selected at the end of the first period of the project.
- Results of the formability study of both alloys performed with the use of the Gleeble 3800 thermomechanical analyser.
- Results of the recyclability study concluding that developed alloys have similar recyclability to other Al-Cu wrought alloys.
- Tensile testing results of the extruded samples after applying the optimum heat treatment confirming the increase in strength after the heat treatment due to the precipitation of Al2CuMg particulates.
- Creep testing results showing better creep behavior at all studied stresses for the high copper content alloy.
- Ageing behavior of the alloys showed that strength is reduced between 45 and 55% depending on the composition due to the coarsening of Al2CuMg to form a continuous phase in the grain boundaries.
- Tensile results after the 1000-hour ageing process showed that strength of the developed alloys is slightly higher at 250ºC to other alloys except wrought alloy 2219 and casting alloy AU5NKZr.
- The production cost of the high copper content alloy is slightly higher (0.35 €/Kg) than for the low copper content alloy and both of them are more expensive than the reference 2618 alloy (40 and 50% more expensive respectively).