Austenitic Sandwich Materials

Prof. Heinz Palkowski, Günther Lange
Institut für Metallurgie, TU Clausthal
Robert-Koch Strasse 42, D-38678, Clausthal-Zellerfeld, Germany

The future developments in mechanical engineering, vehicle and energy system engineering must concentrate on solutions for processes, machines and materials which carefully treat resources and energy and at the same time keep the technical lead with new and innovative products. Lightweight construction concepts are able to be maintained and operated costs efficiently, reduce production costs, increase the product life for economic reasons reliability of use or optimize the freight of payloads.

Steel has become less favorable in previously dominated areas, e.g. in the automobile industry since lightweight materials such as aluminum and magnesium based alloys as well as synthetic materials and composite materials have gained a broad range of acceptance.

Steels with a higher strength and a higher young modulus than conventional steel cannot quite compensate the advantage of these materials for lightweight construction, despite the advantage of a lower price, a better forming behavior, a higher strength and the possibility of recycling without problems. A trend-setting solution for a higher demand of steel use seems possible by the development of high-strength, austenitic steels with a large manganese content. These steels show comparable mechanical qualities, and at the same time are more economical and in addition permit lightweight construction.

Sandwich systems represent an interdisciplinary concept by combining the areas of material choice, production engineering, design and function integration for the fulfillment of the high demands on modern materials. The sandwich material connects the advantages of miscellaneous materials (e.g. low density, high bend resistance, sound and vibration insulation, energy absorption, high load-capacity at a low weight, need adapted qualities) with each other.

Applications

Nowadays these new materials and designs are appreciated as key technologies for innovative research and development. The further development of the materials, the optimization of material applications and the necessary manufacturing method with reduced costs and time are permanent research objectives.

These new compound systems open new, future-oriented applications. The weight reduction is considerable for this task. A combination of steel/synthetic material/steel has the advantage of a higher strength opposite to corresponding steel and, depending on the choice of the steel grade, a high corrosion resistance. These sandwich materials find their way in more and more industrial applications such as automotive-, building-, transport-, chemical-, aerospace- and airplane industry.

The first essays and theoretical based works from to the "sandwich" topic are from 1935-1945. Applications are found not only in aircraft construction but also in the automobile manufacturing industry, in architecture, in shipbuilding engineering as well as in the sports and leisure industry. Some examples are described in the following.

Sandwich sheet metals increasingly find their way into the automobile industry. They are used for car bodies both for of lightweight reasons and for sound reduction. Sandwich materials are used with a homogeneous or inhomogeneous core of foams and other hard materials.

Examples of components of sandwich constructions are cowl application, gear box covers, hoods, car boot covers, oil pumps and chassis frame components. A well known example for the use of sandwich sheet metals in the automobile industry is the lightweight construction bodywork (Ultra Light Steel Auto Body).

Some of the components, such as spare wheel hollow and cowl application were manufactured of steel sandwich sheets. These components can be executed up to 50% lighter with the same properties concerning geometry and function than with normal deep drawing steel. The material consists of two thin steel sheets which are bonded with a thin polypropylene material layer as core material.

Material manufacturing

In one investigation sandwich materials with high-grade steel sheets were researched. They combine a good corrosion behavior and acid resistance with good damping behavior and noise reduction. For the production of the sandwich materials a 0.5 mm thick polyolefin foil was used.

The first manufacturing method to be tested was a press-joining process. This was performed discontinuously by an 8" and 10" rolling stand. The high-grade steel sheet metals [X2CrNiMo17 12-2 (1.4404) and X6CrNiMoTi17 12-2 (1.4571)] with a thickness of 0.5 mm were first cleaned and degreased. The steel was than coated with a defined layer of adhesive. The used adhesive agent is a conventional product based on epoxy with resin. After activating the adhesive the upper sheet metal was joined with a 0.5 mm thick PP/PE-foil in a rolling process. During the next step the produced upper sandwich was bonded with the lower sheet metal, also by rolling. For durable and reproducible adhesive bonding an activation temperature of the adhesive of 254°C +/-2°C was needed. The necessary dwell time of the coated sheet metals was 30 seconds in a stationary convection oven.

The other way to produce sandwich material is the discontinuous method. This manufacturing method was carried out with a cooling and heating system deduced in a laboratory press system. For the sandwich production a sufficient set of the granular material was mixed with the adhesive agent. This mixture was inserted as a packed bed between the cleaned and degreased sheet metals. At temperatures between 260 to 300°C the sandwich materials were then pressed for about 60s. To reach an even core layer thickness, the sandwich material was pressurized at 445 MPa. After the press process the sandwich material annealed to room temperature with a cooling rate of 10°C/min. For adjusting the core layer thickness and the thickness of the complete sandwich material a metal frame was used as a spacer.

These sandwich materials were examined in different tests for the bond strength of the individual layers and for their forming behavior. Deep drawing behavior is for example examined in the Erichsen Test. The height of the cup is a reference value to compare different sheet materials.

The difficulties in the deep drawing process of sandwich systems dwells from the different behavior of the used materials, e.g. polymer and high-grade steel.

The wrinkling of the metal can be counteracted with blank holders for mono materials. The material is forced into the desired flow direction. With sandwich systems, e.g. metal/PP/PE/metal the metal layer can flow despite a blank holder in the polymers, so that it can come to wrinkling in the metal layer. The higher the resistance of the polymer is brought into line with that of the metal, the bigger the resistance is against wrinkling.

For the deep drawing process of sandwich materials the knowledge about the blank holder pressure and force was necessary. Too little blank holder pressure increases the risk of wrinkling.

Conclusion

If some years ago sandwich systems were used only in individual sections, then they will find the way to more and more industry areas today. By the combination of construction and material they offer the substitution of classic mono materials, because next to lightweight construction they offer quantities like anti-corrosion protection and damping.

The development of new materials and technologies still stands at the beginning. New adapted material systems like natural fiber composites, hybrid structures of metals, polymers and ceramics increasingly gain meaning in future. The development for adapted composites, the processing of a material construction matrix for composite materials as well as the improvement on the adhesion and cohesion qualities by shift transitions graduated are future design objectives. Furthermore at the beginning of the material design process the aspects of the environmental protection and recycling have to consider.

Tool concepts and procedures should also be reconsidered for the component production from sandwich materials or be developed newly or adapted to the materials.

Aspects of the environmental protection and recycling are getting more important in these considerations from the beginning of the development. The use of natural fibers can serve as reinforcements in a matrix material between two metal sheets.

The interest in research and development in the area of these new materials has increased strongly during the last few years. In combination with other fields of research, like the nanotechnology, the biotronics, the mechatronics and the material development, the sandwich materials offer a large and important spectrum for the future.