Research On The Unidirectional Solidification Microstructure And Property Of Al-4.5%Cu Alloy

There is plenty of investigation on the effect factors of unidirectional solidification in home and abroad. However, they are basically qualitative analysis. At present there isn't investigation on quantitative analysis between microstructure and controllable parameters. On the groundwork of former researches, we have investigated the development of microstructures, especially dentritic spacing; put emphasis on the studies of quantitative relationship between primary arm spacing of Al-4.5%Cu Alloy and withdrawal rate during unidirectional solidification, which make a theoretical model founded under the condition of local equilibrium of interface and furtherly researched the fracture mechanism and mechanical properties of experimental alloy.A series of Al-4.5%Cu Alloy samples are prepared by the use of drawing crucible unidirectional solidification device, when the withdrawal rate is in the range of 20～220μm/s. The development of microstructures, especially dentntic spacing, is investigated. The results show that the microstructure of unidirectional solidification has obvious directivty; the crystal grows along the normal direction; the grains that depart axial direction fall into disuse.The microstructure changes from cellular crystal to cellular-dentritic crystal and then dentritic crystal. The increase of withdrawal rate leads to microstructure refinement. The primary arm spacing decreases sharply with a withdrawal rate below 100μm/s, and decreases more gently when beyond 100μm/s.We have paid great attention on the investigation of relationship between primary arm spacing of Al-4.5%Cu Alloy and withdrawal rate during unidirectional solidification. Based on the theories of Hunt and Bower, a theoretical model is founded to pridict primary dentritic arm spacing, under the condition of local equilibrium of interface. The experimental data are well inosculating to calculation result by the withdrawal rate of 100～220μm/s, and the error is less than 6.5%. It is hoped that the function can be useful for microstructure analysis and to be helpful to obtain a good match of parameters during unidirectional solidification.We have also furtherly studied the fracture mechanism and mechanical properties of experimental alloy, through SEM in-situ study and tests of tensile strength, extensibility and hardness. The results show that compared with Specimen A (withdrawal rate is 60μm/s), tensile strength of Specimen B (withdrawal rate is 200μm/s) increase 16.7% and simultaneously extensibility improve 85.9% and hardness 67.5%. With the increase of withdrawal rate, fracture mechanism of Al-4.5%Cu changes from quasi-cleavage fracture to ductile fracture, which indicates that there is great effect of withdrawal rate on the fracture mechanism and mechanical properties of unidirectional solidification Al-4.5%Cu.