WEIGHT-SAVING ALTERNATIVE FOR USE IN HYDROGEN-CONTAINING COMPONENTS
EMMINGEN-LIPTINGEN (dj/aj) | The LEIBER Group is not only known as a specialist in forging and machining aluminium. LEIBER also focuses on advanced materials that are precisely tailored to the specific requirements of lightweight construction applications.
For example, LEIBER is working on further developing an alloy within the EN AW-6061A standard alloys that is specifically designed to meet the requirements of hydrogen-containing components. It is marketed under the name AluResist. Alongside high-alloy steels (1.4404 & 1.4435), the base alloy EN AW-6061A is one of the few metallic materials approved for use in hydrogen-containing components in accordance with SAE J 2579 (Standard for Fuel Systems in Fuel Cell and Other Hydrogen Vehicles). The weight advantage of the aluminium alloy compared to the alternatively approved high-alloy steels is obvious here; for the component presented here, it is up to 76%.
Alloy optimisation
The base alloy EN AW-6061A is a heat-treatable aluminium alloy characterised by high mechanical properties, good formability and weldability, and high corrosion resistance. The chemical composition according to DIN EN 573-3 is shown in Table 1. This established alloy is used in shipbuilding, rail vehicle construction, aerospace and the automotive industry. In the automotive sector, it is mainly used in North America.
Table 1: Chemical composition of EN AW-6061A alloy according to DIN EN 573-3:
During the development of AluResist, the chemical composition was critically analysed in accordance with EN 573-3. The strength-determining alloying elements silicon and magnesium were limited in such a way that high component strengths are achieved on the one hand, but the silicon excess is limited on the other. This limitation serves to minimise susceptibility to stress corrosion cracking (SCC) and humid gas stress corrosion cracking (HG-SCC).
The targeted addition of the elements manganese and chromium also serves to limit the excess silicon. These additives also increase microstructural stability, minimising undesirable recrystallised coarse-grained edge zones in the forged components.
Testing / validation
AluResist is optimised for use in hydrogen-containing components. Hydrogen tanks are a common application example. Type IV tanks are now widely used for these tank systems, the structure of which is shown schematically in Figure 1. These tanks have an inner lining made of composite materials and are surrounded by an outer shell made of carbon fibres and other interwoven thermoplastic polymers. The connection area, known as the bosses, and the on-tank valves (OTVs) used in them are examples of applications for AluResist.
Image 1: Type IV hydrogen tank for automotive applications with aluminium bosses and MOTV
During the development of AluResist, the chemical composition was critically analysed in accordance with EN 573-3. The strength-determining alloying elements silicon and magnesium were limited in such a way that high component strengths are achieved on the one hand, but the silicon excess is limited on the other. This limitation serves to minimise susceptibility to stress corrosion cracking (SCC) and humid gas stress corrosion cracking (HG-SCC).
The targeted addition of the elements manganese and chromium also serves to limit the excess silicon. These additives also increase microstructural stability, minimising undesirable recrystallised coarse-grained edge zones in the forged components.
Image 2: Manual on-tank valve: left: original geometry, right: the 76% lighter component in weight-optimised forged geometry made of AluResist
Based on this geometry, prototypes were manufactured from AluResist (see Figure 3), which were then subjected to T6 heat treatment. The appropriate heat treatment parameters were selected on the basis of a series of tests carried out in advance in LEIBER's in-house laboratory.
Figure 3: left: forged MOTV, right: forged and mechanically machined MOTV made of AluResist
The mechanical properties were characterised using tensile tests. Since no mechanical requirements are defined for the EN AW-6061A alloy in DIN EN 586-2, the requirements of DIN EN 755-2 for extruded rods were used as a reference. In the T6 condition, minimum requirements for the yield strength Rp0.2 of 240 MPa and for the tensile strength Rm of 260 MPa with a minimum elongation at break of 8% are defined. The component optimisation requirements used as a basis for the load simulation were set even higher in anticipation of higher characteristic values with Rp0.2 min. 300 MPa, Rm min 340 MPa at 10% elongation at break.
The mean values of the characteristic values achieved across several components are summarised in Table 2. Tensile bar position 1 was taken from the shaft lengthwise to the grain fibre direction of the extruded semi-finished product used. The requirements were significantly exceeded in this tensile bar position. The average yield strength is 362 MPa and the tensile strength is 401 MPa at elongation at break of 12.9%.
The second tensile bar sample in the head of the valve was taken transversely to the grain fibre direction. As expected, this shows slightly lower characteristic values than those determined longitudinally to the grain fibre direction. All characteristic values are above the requirements, both on average and in each individual sample. The requirements of the standard can be exceeded by at least 30% in terms of yield strength and by at least 40% in terms of tensile strength.
Table 2: Mechanical characteristics of AluResist from testing
In addition to the mechanical properties, the microstructure of the manufactured components was examined in detail. For this purpose, macro and micro sections were taken at different positions and examined under a light microscope. Figure 4 shows the macro structure along the length of the forged component. In the shaft area, the elongated fibre structure originating from the extruded semi-finished product is still clearly visible. This is responsible for the anisotropy of the mechanical properties. In the head area of the MOTV, this preferred orientation is deflected more strongly by the forging operation.
The coarse recrystallised surface layer typical of extruded and forged components was successfully reduced to a minimum by adjustments made in the alloy composition: in all relevant areas, the coarse recrystallised surface layer is less than 2.0 mm deep. Only at the end of the shaft, in an area that is removed during subsequent mechanical processing, is the maximum 2.4 mm above this, meaning that the macro structure fully meets the requirements.
Figure 4: Macro structure of the forged and heat-treated MOTV
Acknowledgements
The product design for the demonstrator was carried out as part of the AluScaL project in collaboration with the German Aerospace Centre, Argo-Anleg GmbH, Gränges Powder Metallurgy GmbH, Rosswag GmbH, Gühring KG and the Helmholtz Centre Berlin for Materials and Energy GmbH. The project was funded by the Federal Ministry for Economic Affairs and Climate Protection (BMWI 03LB3032E).