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

采用非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。将平均直径约为20μm的碳化硅悬浮颗粒施加到铝熔体上进行颗粒分离，发现悬浮碳化硅颗粒在几十秒内就转移到了炉壁附近。在0.6~1.5 mm厚度和0.05~0.22粒子密度范围内，粒子积累层厚度(PAR)和粒子密度可随频率(30~200 kHz)和电流(42~163A)而变化。通过对铝液湍流流动的数值模拟，发现铝柱中部的PAR厚度较大，而自由面和底部附近的PAR厚度很薄，预示着强液流的存在。…此外，为了获得刹车材料所需的更厚的PAR，例如4 mm，使用了3 kHz的发电机，由于其较大的蒙皮深度，4.7 mm，预计可以实现更厚的PAR。然而，强烈的电磁搅拌禁止了颗粒分离。数值模拟表明，在壁面附近的速度梯度中产生的升力阻止了颗粒的聚集。通过该模拟，较好地再现了不同频率下PAR厚度的垂直分布，并预测了在3 kHz情况下，如果将电磁力的垂直分布改造为均匀，PAR的形成。基于这样的预测，钼环被放置在自由面和底部附近，以降低那里的流动强度。然而，尽管观察到部分标准杆较低，但改善还不够。为了在不改变电磁分离力的情况下降低流体强度，进行了脉冲施加磁场的试验。结果表明，虽然厚度约为1 mm，但得到了均匀的PAR。需要注意的是，PAR中的颗粒密度超过50%，与其他结果相比，这是非同寻常的，不到20%。最后，对复合材料的变形性和耐磨强度进行了测试，证实了该颗粒增强复合材料的适用性。较少

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