In the aluminium foundry, using a heating source directly immersed into a liquid metal provides numerous advantages compared to conventional gas or electrical furnaces using radiant heating, as far as cost saving and product quality are concerned. However, the development of immersion heaters comes up against difficulties: mainly thermal fatigue and corrosion as aluminium is one of the most aggressive metal melts due to its high affinity to oxygen and its ability to dissolve a great number of materials. During this project, the mechanism and kinetic of molten aluminium attack on MZ composites have been clarified and the great ability of the ceramic to resist aluminium corrosion demonstrated.
The long term use of the claddings is a major source of concern. The thermo-mechanical behaviour of the elaborated RSMZ material has been studied and it should provide the required durability. Small prototypes have been manufactured and successfully tested in conditions close to the solicitation on service: diving, maintaining and pulling out phases in molten aluminium. This proves the high potential of mullite-zirconia for immersed heater cladding for holding aluminium melts. However, the industrial slip casting of real-size claddings needs to be made more reliable.
The firing of porcelain etc. requires kiln furniture with good mechanical properties up to 1400 C. Silicon carbide ceramic proved the best element so far, despite an expensive cost. The elaboration of an over-stoechiometric mullite-zirconia with an higher alumina content has shown that this material has the required properties to be used as a kiln furniture, for a reduced cost. The good creep and oxidation resistance of the material at high temperature have been established. However, improvements on the industrial die pressing of the plates are needed to confirm the high potential of the ceramic towards this application.
High temperature applications in industry require ceramic materials with high resistance to creep,thermal shock and corrosion.The outstanding properties of reaction sintered mullite-zirconia (RSMZ) ceramics developed in a previous EURAM project let us think these materials are suitable candidate for many high temperature applications.In order to show the large potentialities of the RSMZ ceramics,we have selected two different applications:kiln furniture used to hold pieces like porcelain or refractory tiles during the firing process and immersed heater cladding used to protect the heating element in molten aluminium.
The first step of the project will consist in elaborating the RSMZ ceramic. The elaboration process will be optimized according to the required final microstructure. Then,a maximum of two different compositions will be selected after preliminary corrosion, creep and mechanical tests. In a second step,the following studies will be carried out in the selected compositions:
1-corrosion mechanisms and kinetic studies;
2-thermal fatigue behaviour studied in connection with subcritical crack growth propagation;
3-creep resistance in oxydizing environment.
A finite element modelling will be done to determine the stress concentration locations during in-service loadings and to optimize the shape and size of the thermal heater.
Finally prototypes will be manufactured and tested in industrial service conditions.For immersed heater cladding,the aim is to attain six months without failure.Kiln furniture (plates,beams and rollers)made of RSMZ must have comparable performances as other available ceramics used for the same applications.After the tests,the in-service degradation will be characterized to ensure that long term extrapolations are reasonable.
Funding SchemeCSC - Cost-sharing contracts
28500 Arganda Rey - Madrid
36780 La Guardia