Exploitable results
The project concerned greater utilization of ceramic extrusion dies, in order to take advantage of their potential for long maintenance free life and production of good surface finish. Electro-conductive silicon nitride and zirconia were developed facilitating die manufacture using electrodischarge machining (EDM) methods. Experimental trials demonstrated that electro-machinable ceramic dies with non-circular apertures can be used for aluminium extrusion. Dies with relatively short bearing lengths have been used to extrude 20 billets without sign of deterioration. Methods of housing inserts to maximize their exposure to tensile stress have been developed and these findings may have wider application with benefit to other insert materials such as cemented carbides metal-matrix composites and other materials with limited toughness. More detailed results are summarized below:
silicon nitride and zirconia base ceramics having electro-discharge machining properties similar to steel were prepared;
finite element simulations showed that stress concentration at corners of apertures can be compensated by applying pressure to the outside of the insert;
such external pressure can be applied by shrink-fitting, and can be increased by the use of a three component assembly including a shrink-ring;
the loss of external pressure at the operating temperature of the die due to differential expansion can be overcome by the use of tungsten carbide as a shrink-ring material.
Peripheral cracking of inserts was found to be due to an axial tensile stress generated in the process of shrink-fitting. This problem can be ameliorated by lubrication of the insert housing interface prior to shrink-fitting, the use of thinner inserts and tungsten carbide shrink-rings. One of the partners, CTD, currently has a die developed during this project. This has to be adapted to each individual need, hence the next stage is to develop a universal product.
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