The objectives are to develop zirconia and silicon nitride ceramic materials suitable for dies for hot extrusion of metals.
Experimental trials have shown that electro-machinable ceramics dies with non-circular apertures can be used for aluminium extrusion. Although the dies tested in trials reported here had limited life, more recent trials ondies having much shorter bearing lengths have been more promising and these have been used to extrude 20 billets without sign of deterioration so far. Methods of housing inserts to minimize their exposure to tensile stress have been developed and these findings could 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 those of steel have been prepared.
- Finite element simulations show 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 is 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. The problem has not occurred in recent trials where these measures have been adopted.
The ceramics are to be made electrically conductive so that they can be cut by conventional EDM machining techniques. The aim is also to improve the engineering design of the dies so that more complex shapes can be extruded without damage to the die.
OX11 0RA Didcot
ST10 2HF Stoke-on-trent
W14 0QL London