Study On The Technologies Of Morphologies Modification Of Primary Silicon In Al-20%Si Allos

There is a growing interest in hypereutectic aluminum-silicon alloys instead ofconventional materials due to low density, high wear resistance, favorable castability andlow thermal-expansion coefficient. However, the casting of hypereutectic Al-Si alloysalways present relatively coarser and brittle primary silicon particles with shape anglewhich degrade significantly both mechanical properties and mechinablity, and limit thefurther applications of these alloys. At present, several processing methods have beenapplied in fining and modifying the morphologies of primary silicon, such as addition ofmodifiers, semi-solid metal processing (SSM), rapid solidification, and pulse-electricdischarging processing, which is either not as efficient as expected, nor cannot be realizedin industry production due to sophisticated technology & facility, and high expense. Thisresearch has studied systematically on the influences of casting processing, modificationprocessing, diffusion heat treatment and reheating treatment on the size and morphology ofprimary silicon particles based on the prior work, and also investigated the mechanicalproperties, thermal expansivity and wear resistance of these processed materials.Several phenomenons can be observed in this research. Primary silicon particles canbe fined greatly with the increase addition of modifiers; nonetheless, such treatments havelimited influences on the morphologies change of silicon particles, whose sharp angleincrease the probability of both the crack of particle itself and the abruption betweensilicon and Al matrix. When processed by diffusion treatment, eutectic silicon particles canbe granulated and spheroidized obviously, while the sharp angle presenting in the primarysilicon will disappear with the increase of holding time & diffusion temperature. However,too longer holding time will lead to coursing of primary silicon particles, which will bedetrimental to the morphology of silicon particles; ohterwise, the effect of increasingholding time is not as obvious as increasing the diffusion temperature. The effect ofreheating treatment is dependent largely on the origin morphologies of particles, while thereheating temperature should be chosen just beyond the eutectic temperature. The sharpangle can be obtusen in the materials unprocessed by diffusion treatment, but the effect isnot very distinct, while the primary silicon within the materials processed by diffusiontreatment is greatly improved by reheating treatment.The properties test shows that, the plasticity and strength of hypereutectic Al-Si alloysare improved obviously with the refinement and spheroidization of primary silicon, whilethermal expansivity of materials has limited relationship with the morphology of primary silicon particles. Wear resistance test shows that the mass loss of all materials testedincreases with the increase of load while mass loss rate decreases with the increase of load;the wear resistance can be improved by the refinement and spheroidization of Si particles.In this work, several simple, but practical and easy-to-use technologies are utilizedand integrated to fine and spheroidize the primary silicon particles in Al-20%Si alloy, andthe processing effect of this route is very splendid. When utilizing this method in industryproduction, there are not any requirements on equipment modifications, and the rise in theexpense is very limited. As a result, this processing method is proved to be an effective andeconomic solution in modification of morphologies of primary silicon.