Casting-ProcessesI.ppt-NDLRDspace.pptVIP

* This slide introduces the third factor: Solidification effects * The solidification effects are different for different materials. Plastics solidify into an amorphous material with no microstructure. Semiconductor casting or crystal growth which is a form of casting results in a single crystal. The casting parameters can effect the defect structure which can affect the electronic properties. Silicate glass casting results in solid objects which are also amorphous. The casting parameters have very little effect on the properties. * Solidification of metals depends very strongly on the metal, whether it is pure or alloyed , and strongly on the casting parameters. We consider pure metals and alloys separately because the crystal growth characteristics are different. * Because the coldest spot in the mould/metal system is the mould wall, crystals form there first. Heat is extracted out of the mould through the mould walls, so a temperature gradient is driven into the melt and a solidification front advances from the walls. Since pure metals solidify at a single temperature, the grains tend to grow on equal axis and to propagate outwards from the wall. Crystals need a nucleus to form in the first place and if there are no nucleating agents present, they will initiate randomly. Seeding the melt with nucleation agents provides many sites where crystals can start which results in more uniform, evenly distributed grains * Alloy solidification depends on the phase diagram. Eutectic compositions have a single melting temperature and behave like pure metals. Non-eutectic compositions tend to have a melting range and we need to look at the phase diagram to determine what happens. Slide 33 provides an example of a phase diagram. * This diagram illustrates what happens with a typical alloy. See pages 119-122 for a description of what happens during solidification in this system * This is an amplification of the previous slide * This figure from the book shows the dif