Application of Electromagnetic Force to Fabrication of Aluminum-matrix Composites Locally Reinforced with Particles

采用不导电颗粒与液态金属的电磁分离技术，制备了具有高耐磨性和良好变形性能的陶瓷颗粒部分增强铝基复合材料。为了分离颗粒，对铝熔体施加高频磁场，使其悬浮在平均直径约为20μm的碳化硅（SiC）颗粒上。发现悬浮的碳化硅颗粒在几十秒内转移到坩埚壁附近。频率（30 ~ 200 kHz）和电流（42 ~ 163 A）可以改变颗粒堆积层厚度（PAR）和颗粒密度（0.6 ~ 1.5mm），颗粒密度（0.05 ~ 0.22）。通过对铝液湍流流动的数值模拟，预测铝筒中部PAR厚度较大，而自由表面和底部PAR厚度较薄，在自由表面和底部存在强液体流动。为了获得制动材料所需的更厚的PAR，例如4mm，应用了3khz发生器，由于其大的蒙皮深度（4.7mm），预计可以实现更厚的PAR。然而，强烈的电磁搅拌阻碍了颗粒的分离。数值模拟表明，在靠近壁面的速度梯度处产生的升力阻止了颗粒的积聚。通过模拟，可以很好地再现了不同频率下PAR厚度的垂直分布，并对3kHz情况下的PAR形成进行了预测。在此基础上，在自由表面和底部附近放置钼环以降低流动强度。然而，尽管部分观察到薄PAR，但改善还不够。在不改变电分离力的情况下，以降低流动强度为目标，进行了脉冲磁场施加试验。结果表明，在厚度为1mm左右的情况下，得到了均匀的PAR。值得注意的是，PAR中的颗粒密度大于50%，与其他小于20%的结果相比，这是非同寻常的。最后，对其变形性能和耐磨性进行了测试，验证了所制备的颗粒增强MMC的适用性。少

Electromagnetic separation technique of nonconductive particles from liquid metal was applied to fabricate a partially reinforced MMC of aluminum by ceramic particles, which has a high wear resistance and a good deformability. To separate particles, a high frequency magnetic field was imposed on an aluminum melt suspending silicon carbide (SiC) particles of which mean diameter was about 20μm. Suspended SiC particles were found to be transferred to the vicinity of the crucible wall in several ten seconds. The thickness of the particle accumulated layer (PAR) and the particle density could be changed by frequency (30-200 kHz) and current (42-163 A) in the ranges from 0.6 to 1.5mm in thickness and from 0.05 to 0.22 in particle density. The thickness of PAR was large in the middle of the aluminum cylinder, however it was very thin near the free surface and the bottom where the existence of strong liquid flows were predicted by the numerical simulation of turbulent flows in liquid aluminum. … More In order to obtain thicker PAR needed for brake materials, e.g.4mm, a 3-kHz generator was applied, which was expected to achieve thicker PAR because of its large skin depth, 4.7mm. However, intensive electromagnetic stirring prohibited particle separation. Numerical simulation revealed that the lift force generated in a velocity gradient near the wall prevented particle accumulation. By this simulation, the vertical distributions of the PAR thickness at different frequencies were well reproduced and the PAR formation was predicted for the case of 3kHz if the vertical profile of electromagnetic force was reformed to be uniform. Based on such prediction, molybdenum rings were placed near the free surface and the bottom to reduce flow strength there. However, improvement was not enough, though thin PAR was observed partly. A pulse imposition of magnetic field was tested aiming the reduction of flow strength without change in electromagnetic separation force. As a result, homogeneous PAR was obtained though the thickness was about 1mm. It should be noted that the particle density in PAR was more than 50 %, which was extraordinary comparing with the other results, less than 20%. Finally, the deformability and the strength of wear resistance were measured and the applicability of the present particle-reinforced MMC was confirmed. Less