Solidification Control Of Al And Al Alloy Under Pulsed Magnetic Field

The mechanical properties of ingot is greatly connected with solidification structure. It is well known that properties of castings are connected with its as-cast structure; fine and equiaxed grain instead of coarse dendritic grain is good for the improvement of its mechanical properties. Therefore, grain refinement is always one of the most important topic in the materials research. Pulsed magnetic field(PMF) is a new physical fields in the refinement of metals, which is attracting more and more attention. However, the mechanism of grain refinement of PMF is still not clear. In this paper, the mechanism of grain refinement of PMF on pure Al is investigated. Meanwhile, PMF is tried to realized during semi-continuous casting of 6063 Al alloy.PMF with different parameters is applied to the pure Al during solidification. The results indicate that the solidification structure is refined when PMF is applied until solidification. The solidification structure is coarse dendrite without PMF, however, the solidification structure is turned to fine equiaxed when PMF is applied. In order to investigated the effects of PMF on solidification process, two K-type thermalcouples was inserted into the melt(one is in the middle of the ingot and the other is on the surface of the ingot). The mesured cooling curves of pure Al shows that the temperature gradient is decreased when the PMF is applied, and the temperature profile tends to be even. The mesured Joule heating curves shows the local input from Joule heating reaches a maximum value of～0.17 W·cm-3 according to the measured results. Estimations with respect to the heat production by the phase change from liquid to solid deliver much larger values of 8 W ·cm-3. Hence, for the magnitude of pulsed currents considered in this study, the Joule heating effect does not play a prominent role at all. Any direct impact on the growing dendrites from a PMF induced re-melting of the dendritic skeleton can be excluded. In order to get the further refinement mechanism, PMF was applied to Al with a certain thickness solidification shell. The results show that when the shell is thin, which can be fractured by PMF. However, PMF has no grain refinement of pure Al when the thickness of shell increase to a certain value.PMF is realized during the semi-continuous casting of 6063 Al alloy by self-made mould with the exciting coil. The experimental results indicate that when PMF is applied to the semi-continuous casting of 6063 Al alloy, the solidification structure is refined greatly. The solidificaiton structure is coarse equiaxed without PMF. However, the fined equixaed grain structure is achieved when PMF is applied. Meanwhile, the solidification structure is more and more uniform with the increasing voltage of PMF. When the voltage of PMF is increased to 700V, the average grain size can be decreased from 2000μm to ～200 μm. The mechanical properties were tested after the heat treatment of the ingot. The results show as the voltage of PMF increasing from 0V to 300V,500V,700V, corresponding Brinell hardness of 6063 Al alloy are about 77.2HB,100.3HB,102.4HB and 104.7HB, respectively. Meanwhile, the corresponding ultimate tensile strength of 6063 Al alloy are about 174.7MPa,220MPa, 230.3MPa and 249MPa, respectively.