Joining with aluminum foam
In mechanical engineering aluminium foam is starting to be a well known material. It is already used wherever lightweight construction and structural damping are demanded. Particularly for sandwich composites with steel cover sheets the foam shows its strength in rigidity against impulse. The common way of aluminium foam production is using a foamable semi-finished product. Manufactured by a powder-metallurgical procedure this semi-finished product can be produced with nearly any shape of cross section. The increase of volume during the foaming is immense. Even large cavities and gaps can be filled by the involving foam. In addition, aluminium foam is heat resistant up to approximately 500 °C. During the foam process the aluminium foam is able to establish an adhesive bond to other metals. Also it shows a significant decrease in volume during the solidification. Transferring these facts to known applications of other materials, the idea of using aluminium foam as joining material becomes obvious. The verification of this approach has been manifested in a research cooperation between the Osram GmbH and the Fraunhofer Institute. The industrial partner Osram GmbH manufactures high pressure discharge lamps primarily for medical, scientific or projection applications and for decorative illumination. The so called discharge lamp already heats up at the lower range of performance to a few hundred degrees Celsius - comparable to a usual electric bulb. Therefore the discharge lamp is made of fused quartz. For the electric contacting the fused quartz-shaft of the discharge lamp is connected to a metallic cap shell. Due to the high temperatures the bonding is realized with mineral based adhesives. Disadvantages of this method are a limited pot life, the drawn-out process of hardening and a disappointing long-time stability. Also most interfering is the difficulty of implementing this joining process into an automated production process. As one result of the research cooperation the potential of aluminium foam has been used here. By inserting a foamable semi-product between the fused quartz-shaft and the metallic cap shell during the assembling process and a following heat treatment, a durable hard composite between all components will emerge. The bonding maintains its stability due to the shrinkage of the aluminium foam onto the glass bulb and a metallic bond between the foam and the metallic shell. The capping with aluminium foam allows short process intervals and a simple process management. Therefore aluminium foam includes a remarkable potential of rationalisation. The lamps achieve the durability of conventional lamps without any problems.