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FP6

AEROMAG Résumé de rapport

Project ID: 516152
Financé au titre de: FP6-AEROSPACE
Pays: Germany

Final Report Summary - AEROMAG (Aeronautical application of wrought magnesium)

The density of magnesium is only 65% of the density of commonly used aluminium alloys in the aerospace industry and therefore can be a breakthrough technology if used for low weight airframe structures. However to use this low weight material several mechanical properties have to be increased and the technological behaviour improved.

The technical focus of the 'Aeronautical application of wrought magnesium' (AEROMAG) project was the modification of existing and the development of new magnesium wrought products (sheets and extrusions), that provide significantly improved static and fatigue strength properties for lightweight fuselage applications. The specific strength properties of these innovative materials are required to be higher than AA2024 for structural applications (secondary structure) and higher than AA5083 for non-structural applications.

Within this project the overall objective was to demonstrate that magnesium is a suitable engineering material which can be applied for weight savings up to 35 % compared to aluminium. To reach this goal magnesium has to deliver significantly higher weight specific mechanical properties compared to Aluminium. The targets for replacement of aluminium can be divided into two different steps in respect of time scale and risk:
- replacement of medium strength 5xxx aluminium alloys for cockpit and cabin applications
- replacement of medium to high strength 2xxx aluminium alloys for secondary structure or non-pressurised fuselage applications

Therefore new alloys were developed and existing alloys were tested. Appropriate manufacturing processes (rolling, extrusion) were adjusted between material suppliers and universities. Forming and joining technologies require development, simulation and validation for the innovative material and technologies commonly used within aeronautic industry.

Summary of the scientific and technical objectives:
- Static and fatigue strength properties comparable to properties of 5xxx and later 2xxx aluminium alloys
- Damage tolerance behaviour comparable to aluminium alloys
- Weight reduction of single components up to 35 % compared to aluminium
- Cost efficient processes for manufacturing of magnesium semi-finished products (sheets, extrusions)
- Flammability behaviour of magnesium wrought components approved concerning fire worthiness requirements (FAR/JAR)
- Corrosion behaviour of environmentally friendly surface protected components comparable to aluminium
- Investigation of process technologies (forming, welding, bonding, fastening) commonly used in aeronautic industry especially to magnesium
- Development of material models for plastic deformation and failure criteria
- Basic preparation of the qualification of magnesium products for aerospace applications
- Material adapted design and design rules for structural elements made from magnesium

The most promising alloy systems which were selected due to corrosion behaviour, environmental friendliness and mechanical performance for further investigation as wrought products in the project, were Mg-Al-Zn, Mg-Zn-Zr-Re and Mg-Y-Re. Magnesium Elektron joined the consortium to support the material processing step by feedstock supply and knowledge transfer of rolling these specialty alloys. The rolling trials of sheets were successful for most of the selected alloys, such as AZ61 and WE43. Especially the compression curves of several alloys does not show the weakness under compression loads any more, which is typical for most magnesium alloys. For some systems the behaviour under tension and compression is the same, similar to aluminium alloys. Fatigue limits were measured in the range between 150 and 200 MPa for constant amplitude fatigue under tension loading and with smooth specimens.

INPG has done the full characterisation of deformation behaviour at room and elevated temperatures by microstructural analysis and mechanical testing. The objective was to define the forming windows for different alloys and processes. University of Savoie in Annecy performed some forming tests at room temperature with specific devices to study strain localisation effect by image correlation techniques. To determine the forming windows, besides the microstructural characterisation the investigation of the mechanical behaviour of the reference material AZ31B 2.0 mm sheet material at room and elevated temperatures was performed. Efforts were devoted to interpret the data obtained by the image correlation techniques in order to have a criterion for strain localisation and the possibility to measure locally Lankford coefficients. For elevated temperatures microstructure stability, strain rate jump tests and tests at constant strain rate were performed.

Existing and commercially available, environmentally friendly surface treatment technologies were investigated and tested in accordance with aircraft and aerospace standards. The specifications have been defined by the end-users. The main focus of work comprises the definition of pre-cleaning technology, the development of sol-gel technology (TU Vienna), comparison of commercial surface treatment technologies, testing of bare corrosion protection and testing of multilayer coating. Conversion coatings, anodising treatments and hard and wear resistant coatings suitable for Mg substrates were tested, evaluated and rated. AMT&S Alonim, Eurocopter and external companies provided the surface protection systems.

The compatibility of the different pre-treatments to the sol gel process was investigated. The corrosion rate of the untreated substrate decreases by a factor of five, after sol gel coating. Acid pickled samples have a corrosion rate that is five to tentimes lower than that of the untreated substrate. When acid pickling and sol gel treatment are combined, the factors of the two procedures roughly multiply, and the corrosion resistance is enhanced by a factor of up to 60. Because of the good performance and the lower weight-loss during pickling hydrofluoric acid treatment is the recommended method for sol gel coatings.

The additional influence of potential corrosion inhibitors was also investigated. The amount of the additives was calculated to be 5 % of the total mass of the non-volatile compounds in the sol. The films were deposited on samples pretreated with hydrofluoric acid. The anticorrosive effect of manganese and cerium is out weighted by the negative influence on the barrier properties of the sol gel coating in this system. Zinc acetate showed only a minor effect. Triethylphosphate and 1,2,4- triazole proved to be efficient inhibitors, decreasing the corrosion rate by a factor of 1.7 and 3.3 respectively.

Airbus and Technion have carried out some first measurements to compare the general flammability behaviour of aluminium and magnesium sheets and real magnesium components. First it has to be stated that all tests which were carried out passed without any problem the JAR/FAR, Part 25, § 25.853(a) requirements. The reason for that is that the specification was made for non-metallic materials which are often used in the cabin of an aircraft. Airbus found that the time to melt for AZ31B, 2 mm sheet was about half as for comparable AA2024. Without external heat supply the Mg sheet did not continue burning. But to learn more about the flammability of magnesium, the Technion performed additional tests close to a catastrophic scenario. Different magnesium components were directly placed within a burning jet fuel with a temperature of 600 to 800 degrees Celsius. It needed 240 seconds until the components were melted and other 10-20 seconds until they ignited. Coated components could increase the time to only partial ignition up to 10 min. After 1 min when the fuel was fading out, the coated components self extinguished. In comparison to that the Al case melted 350 seconds after the full ignition of the fuel. No ignition of the Al case was observed in this test. But it was important for all cases that there was no ignition observed till melting of the magnesium material.

Different joining techniques were applied to magnesium wrought semi-finished products, in order to promote their introduction on aeronautical structures. The joining processes which are to be investigated are welding (LBW, FSW, TIG), mechanical fastening (riveting) and adhesive bonding. EADS CRC-F has done some first LBW and FSW trials with AZ31 together with mechanical characterisation (static and fatigue). The static behaviour was nearly constant after LBW welding, just a drop in elongation was found. The fatigue behaviour shows some reduction, typical for welds, compared to the parent material. After FSW welding the reduction in fatigue properties was less, but some higher drop in static properties compared to LBW. Some further optimisation of welding parameters is planned. EADS CRC-G concentrated on the detailed investigation of FSW of AZ31, but found similar results. ENSAM did the microscopic investigation (optical, TEM, SEM), hardness measurement, texture measurement and residual stresses measurements of their LBW specimens. Up now it was found that AZ31 shows in general good weldability. Similar investigations are planned for FSW.

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Elke HOMBERGSMEIER, (Manager)
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