| Al-Si alloys are used widedly, while modification of them is the most important methodto improve their mechanical properties. However, there are some problems in the Na-Saltmodification of hypoeutectic Al-Si alloy and eutectic Al-Si alloy, such as the short effectivetime, the loss of efficacy after re-melt and over modification. For P-Salt modifiedhypereutectic Al-Si alloy, the primary Si tends to concentrate at the edge of the sample.This dissertation has studied the effects of high magnetic fields on the solidificationstructure and modification of Al-Si alloys. The phenomena and mechanism of the impacts ofstable high magnetic field(high magnetic field hereafter)on the Na-modified hypoeutectic Al-Sialloy and eutectic Al-Si alloy are studied. Experimental studies on the impacts of high magneticfiled on the structure of the hypereutectic Al-Si alloy and on P-Salt modification are implemented.Moreover, impacts of compulsory floating on P-Salt modification of hypereutectic Al-Si alloy arealso studied. The following conclusions can be drawn from the studies.1) For Al-6w_t％Si hypoeutectic alloy, eutectic Si in solidification structure became thicker underthe condition with high magnetic field. If the Na-Salt modified sample was re-melted and keptat 740℃for 20 minutes under the condition without high magnetic field, its solidificationstructure returned to its origin, i.e. the loss of efficacy after re-melt, while under the conditionwith high magnetic field the effect of modification was remained. It can be said that highmagnetic field could solve the problem of the loss of efficacy after re-melt.2) After Na-Salt modification of Al-6w_t％Si, quickly cooling the samples from 740℃(liquidphase), 590℃(solid-liquid phase) and 470℃(solid phase) under the conditions with andwithout magnetic filed respectively, then the different configurations of eutectic Si couldbe observed. When B=0T, the eutectic Si in solidified structure of the sample quicklycooled from 740℃is short-layer-like. The eutectic Si in solidified structure of the samplequickly cooled from 590℃is long-layer-like. The eutectic Si in solidified structure of thesample quickly cooled from 470℃is sharped-acicular-like. When B=10T, the eutectic Siin solidified structure of the sample quickly cooled from 740℃was thin and distributedevenly. The eutectic Si in solidified structure of the sample quickly cooled from 590℃iscluster-like. The eutectic Si in the solidified structure of the sample quickly cooled from 470℃was ball-particle-like and distributed evenly. It could be said that magnetic filedcould improve the quality of the modification. After diffracting the sample with X-Ray, itwas found that when B=10T, the peak of Si(200) crystal lattice face appared in the samplequickly cooled from 470℃. While B=0T, the peak of Si(200) crystal lattice face did notappear. It means that high magnetic field impelled the growth of the Si in the style of thetwins. The results of the X-Ray diffraction also shows that the Al(200) crystal lattice facetends to arrange in the direction perpendicular to the magnetic strength.3) High magnetic filed enforced the eutectic Si in solidification structure of the Al-12.6w_t％Si eutectic alloy, with high magnetic filed the eutectic Si increased and the spacebetween eutectic Si became smaller. If the sample modified by Na-Salt was kept warmunder the condition with high magnetic filed, the modification is obviously better than thatwithout high magnetic field. When the sample was kept warm for 40 minutes, the effectsof modification without high magnetic filed declined seriously, while the decline of themodification with high magnetic field was less. It means that high magnetic field couldpostpone the effective time of the modification. The observed results from the electron-probe shown that without high magnetic filed Na got together and concentrated on theeutectic Si, while with high magnetic filed Na distributed evenly and the overmodification was avoid.4) During the solidification of hypereutectic Al-18w_t％Si alloy under the condition with highmagnetic filed, it is found that the shape of primary Si changed to plat-like from sheet-like,and concentrated at the edge of the sample and distribute at the whole cross section of thesample evenly. However, this time the eutectic Si was appreciably thicker than thatwithout high magnetic filed. If the sample was modified by P-Salt under the conditionwithout high magnetic field, the plate-like primary Si still concentrated at the edge of thesample, and the center is concentrated with the eutectic Si. While with the magnetic fieldthe plate-like primary Si distributed evenly at the whole cross section of the sample andthe eutectic Si became fine.5) Changing the direction of magnetic filed from the direction paralleled to the axis of thesample to the vertical direction, it found that the solidified structure of the hypereutecticAl-18w_t％Si alloy and the P-Salt modified sample tended to be finer.6) Posing rotary magnetic filed on the P-Salt modified hypereutectic Al-18w_t％Si alloy, theprimary Si would be thick and block-like and concentrated at the edge of the sample. If therotary magnetic filed was posed only during the warming after P-Salt modification, thenprimary Si would be fine and plate-like. It means that compulsory floating is not good to the solidified structure during the solidification after the P-Salt modification, and restrainfloating is good to solidified structure.The studies provided a new methodology to improve the modification effect of Al-Sialloy. It provided a theoretical base for the new method that modifies the hypereutectic Al-Sialloy with a brand new and green method base on high magnetic filed.
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