product news/產品新聞
machined surfaces often are requested. The application of a vacuum and squeeze pins can improve the quality of the castings, but those measures complicate the process and increase maintenance requirements. However, an alternative diecasting process—semi solid rheocasting (SSR), developed by the Idra Group SRL, Travagliato, Italy, and based on a Massachusetts Institute of Technol- ogy, Cambridge, Mass., U.S., patent—has been effective in improving metallurgic casting quality without increas- ing costs.
Te SSR process uses rapid cooling and applied convec-
tion (stirring) to create non-dendritic semi-solid slurries and can be used for a wide selection of alloys, including aluminum 380, aluminum 390, aluminum 365 and AlMg-
. Te semi-solid alloy, with 10-20% solid fractions, is injected into the mold cavity. Te partially solidified alloys maintain high fluidity and greater flow and cavity filling capacities than alloys in the liquid state. Te cavity can be filled at low speed in laminar condition, and the entrap- ment of air caused by turbulent metal flow due to high injection speeds is reduced. During the filling phase, the alloy already is in solid
5Si2
condition, reducing shrinkage porosity caused by variation of volume due to the passage from liquid to solid state. Compared to traditional diecasting, SSR:
• allows T6 heat treatment to the parts and makes them weldable;
• reduces porosity, which eliminates secondary operations like impregnation and machining;
• reduces casting cycle times by 25%, depending on part thickness;
• leads to longer die life due to lower metal temperature and injection velocity;
• reduces draft angles to 0.2 degrees and achieves near-net-shape;
• lowers consumption of release agent; • achieves wall thicknesses from 2.5 to 12 mm.
Casting trials on two applications have demonstrated
many of these advantages. Parts were selected for casting tri- als based on their need for reduced porosity and overall cast- ing costs, as well as their projected compatibility with the
SSR process. Criteria for selecting a casting for SSR include: • casting porosity can be resolved through slow filling and intensified pressure;
• casting can be filled with a single gate at a speed of 3 to 5 m/second and fill time of 300 to 500 milliseconds;
• multiple metal flow fronts developed during filling must be directed into overflows to avoid flow lines within the casting. All castings in the trial used slow shot velocity calculation
to predict injection velocity before the cold chamber is full of metal to avoid wave formation. Te first casting selected for testing, a pump filter hous-
ing, was a pressure sensitive casting that required extensive machining, impregnation and long cycle times. Te casting was originally converted from gravity to diecasting; therefore,
和挤压的应用可以提高铸件的质量，但这些改进工艺过 于复杂且难以维护。然而，一种新的压铸方法——半固 态压铸（SSR）在不增加成本的前提下，提高了铸件质 量。SSR是基于麻省理工学院的技术专利，由意大利特 拉瓦利亚托的Idra Group SRL研发成功的。 SSR技术通过快速冷却和强制对流（搅拌）来产生非 枝晶半固态浆料。目前可用于多种合金材料的铸造，例 如铝合金380、390、365和AlMg5
Si2 。注入到型腔之
中的半固态合金只有10-20%为固态成分。这种部分凝 固的合金不但保持了良好的流动性，而且比液态合金具 有更好的充型能力。充型发生在低速层流条件下，这样 就大量减少了由于快速充型产生湍流而随着带来的气泡 的数量。
通常条件下由于液态转变为固态过程中，金属液发生 体积变化会产生缩孔。而在SSR充型阶段中，合金已经 处于固态，因而减少了缩孔缺陷。 和传统的压铸相比，SSR具有以下特点: 采用T6热处理技术处理零件，提高其焊接性能 ；
• allows the application of T5 and T6 heat treatment to the parts and makes them weldable;
• 降低孔隙率，可省去浸渗和机加工等二次工序； • 可以采用T5和T6热处理技术处理零件，提高其焊接 性能；
• 根据铸件的厚度，可以缩短25%以上的铸造周期； • 由于较低的浇注温度和速率，可以延长模具的寿命； • 拔模斜度减小到0.2°，达到了近净成形； • 减少脱模剂的消耗; • 可生产壁厚为2.5-12mm的铸件。
两次SSR实验已经证明了这种方法优点很多。这两次 实验采用可用于SSR技术的铸件，并试图减少铸件的孔 隙率和整体成本。采用SSR技术的铸件标准包括： • 通过低速充型和加压，可以降低铸件孔隙率； • 每个浇口的浇注速度为3-5米/秒，在300-500毫秒内 完成浇注;
• 浇注过程中必须浇满，防止多向金属流动面形成流 线。
测试中的所有铸件经过低速射压计算，结果显示，在 低温型腔充填完毕之前，这样的射流速度可以避免金属 液产生波动。
第一件测试的铸件是过滤泵的壳。这是一个对压力 敏感的铸件，需要经过大量的加工和浸渍，加工周期很 长。一开始把铸件的浇注方式由重力铸造改为压铸，但 由于壳壁很厚，很难制造。在没有经过浸渗工艺前，原
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FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION Fall 2011
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