Nanoparticles’ appeal is that they can give specific proper- ties to certain materials. The nanoparticles in this illustra- tion are tuned to optimize their magnetic properties. Their discovery could lead to a low- cost technology for cleaning arsenic from drinking water.
Weiss, increases in strength between 100 and 120% have been accomplished using 2% volume fraction of nanoparticles in metal matrix composites. “Obvious applications
are going to be in the transportation market, like military, aerospace and cars,” Weiss said. “With the mechanical improvements, you can design parts signifi- cantly lighter.” Li compares the strength- ening of a casting alloy with nanoparticles to strengthen- ing via heat treatment. “Traditionally, alumi- num and magnesium need heat treatment to improve strength, which is nano-scale precipitation from the age hard- ening process,” he said. “Nanoparticles strengthen in the same way. But heat treatment takes a long time and high tempera- ture. Solving that [with nanoparticles] would be significant.” Te general rule is nanoparticles can give aluminum and magnesium the strength of gray iron, along with slightly higher elongation. At the same time, ductility and toughness are unaffected. “Nanotechnology gives us what I call a free lunch,” said
Robert Hathaway, vice president of materials and processe- ses for Oshkosh Corp., Oshkosh Wis. “We can increase our mechanical properties without reducing the ductility or toughness of the material. Te particles are so small, machin- ability should still be good.”
Means to an End One of the earliest challenges of using nanotechnol-
ogy in metalcasting was finding a way to uniformly dis- perse the minute pieces of material throughout the metal. The tiny particles are drawn together by van der Waals force (an attractive or repulsive force among molecules) and tend to stick together in clumps. The attractive force is so high, keeping the particles apart in order to allow the aluminum or magnesium to bond with them takes a lot of energy. Te nano research team’s eureka moment came when it found that using ultrasonic waves was effective in keep-
纳米颗粒的优势在于能够给 特定的材料特殊的性质。在 本插图中的纳米颗粒能优化 它们的磁特性。这个发现能 够清理饮用水中的砷，成本 很低。
然，这些将应用在类似军 事、空间站和汽车等运输 行业，”Weiss说“当机 械强度提高后，就可以设 计更轻的零件”。 李教授比较了热处理 后包含纳米颗粒的铸造合 金的强度。“在传统意义 上，镁铝合金需要热处理 来增加强度，在淬火时形 成纳米级的沉淀物，” 他说，“纳米颗粒也能强 化，相对于热处理来说， 它不需要经历长时间和高 温过程，这个优点十分重 要。”在一般意义上，纳 米颗粒能够给予镁铝合金
灰铁的强度，并能稍微提高延伸率，同时不影响韧性和 强度。
“纳米技术告诉我们什么叫免费的午餐，”威斯康 辛州奥什科什城的奥什科什公司材料加工中心的副部长 Robert Hathaway说，“我们能够增加材料的机械性能 而不降低韧性。纳米颗粒如此之小，切削性能应该也很 好。”
终极手段
早期在金属铸件中使用纳米技术的难点在于如何使金 属中微粒能够保持均匀分散。微小的颗粒由范德华力连 接（分子中相互吸引或排斥的力）并成簇状集合。这种 吸引力很大，需要大量的能量才能使微粒分离以便使其 和镁铝合金结合。
纳米研究团队有了重要发现，在金属凝固时使用超 声波能够有效地分离纳米颗粒并使它们均匀分散。“ 在过去的六年中，我们都致力于研究如何使纳米颗粒在 金属中分散”李教授说，“超声波技术是一种便宜的手 段。”
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FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION Fall 2011
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