Aerogels as Nano-Additves for foundry applications

Abstract

The casting of metals and alloys is very often performed into moulds made of sands bonded by organic and inorganic binder-systems. Experience during the industrial tests of the AeroSands showed that there are still many open issues to reduce casting defects associated with the conventional sand-binder system. This knowledge inspired the new developments of inorganic and organic aerogels in a granular form. Simply a small part of any sand used in foundries is replaced by granular aerogels and commonly mixed with the normally used binder-system to prepare cores manually or using a core shooting machine. It turns out that adding granular aerogels (nano-additives) to sand-binder systems are beneficial for sand moulds and cores and lead to improved cast parts. To prove the benefit of the nano-additives scientifically systematically tests are performed. Therefore bending bars having a dimension of 20mm x 20mm x 100mm are used. The matrix includes: type and amount of aerogel, sand, binder-system, metals or alloys, casting temperature, and others. The characterisation and classification of the results is based on several measurement methods for the sand cores like: bending strength, abrasion resistance, gas permeability, thermal decomposition with thermogravimetry combined with FTIR as well as fracture path and bonding bridges by SEM. And further on for the casting and the cast parts: tests to analyze amount and time of gas emission, quality of the surface finishing, the amount of baking due to binder oxidation and pyrolysis, metal penetration, or gas bubbles under the metals surface and ease of core removal. All results of the tests analysis are established in a database. Theoretically one of the major effects to evaluate the best working aerogel granulate is given by the application temperature. But surprisingly for organic aerogels it is shown that they still work during casting processes with temperatures above 350°C, their decomposition temperature. To understand this effect thermal conductivity and thermal decomposition (TGA-FTIR) is measured, evaluated and the results are discussed. Within the presentation it will be shown how perfect the properties of the used aerogels fit the necessities of casting defects. In addition an insight to the industrial applications will be provided.