摘要:
本文建立了金属熔体近壁面流动剪切模型，并分析了流动剪切对金属凝固的影响。针对A356合金计算结果表明：层流流动的熔体内部剪应力随垂直斜板表面距离的增大而减小，随着流动长度的增加先急剧下降之后趋于稳定；紊流流动的熔体所受的剪应力随着垂直倾斜板表面距离的增大先急剧下降之后趋于稳定，随着流动长度的增加而不断增大；斜板倾角越大，斜板上相同位置的熔体层受到的剪应力越大；熔体垂直斜板表面距离越小，柱状晶所承受的弯曲应力越大；斜角越大，斜板上相同位置的柱状晶的弯曲应力越大；随着熔体在倾斜板表面流动长度的增加，在层流阶段，倾斜板表面柱状晶根部所受的弯曲应力先急剧下降之后趋于平稳，而在紊流阶段，弯曲应力是缓慢增加的；理论分析表明柱状晶在熔体近壁面流动过程受到的最大弯曲应力低于α-Al晶粒的屈服强度，所以斜板上熔体流动产生的弯曲力不能将柱状晶折断，只能将晶粒冲刷游离到熔体中使晶粒增殖，与实验结果相符合。所以本模型可以很好地解释熔体近壁面流动过程中的剪切本构关系以及剪应力对凝固组织的影响。
关键词:
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凝固
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近壁面流动
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剪切
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柱状晶
Abstract:
In this paper, the model of metalic melt shearing flow near the surface is established, and the effect of shearing flow on solidification microstructure of the metal is also analyzed. Calculated results based on A356 alloy melt show that in the laminar flowing melt, the shear stress decreases with increasing length along the vertical direction of the surface of the slope, and the shear stress first decreases rapidly and then stabilizes with increasing length along the flowing direction of the surface of the slope; while in the turbulent flowing melt, the shear stress firstly decreases rapidly and then stabilizes with increasing length along the vertical direction of the surface of the slope, and increases with increasing length along the flowing direction of the surface of the slope. The shear stress at the same position in the melt on the surface of the slope increases with increasing angle of the slope; the shear stress acting on the columnar crystal in the melt on the surface of the slope increases with decreasing length along the vertical direction of the surface of the slope. The shear stress acting on the columnar crystal at the same position in the melt on the surface of the slope increases with increasing angle of the slope;with the increase of the length along the flowing direction, the shear stress acting on the columnar crystal rapidly decreases first and then stabilizes in the laminar flowing melt on the surface of the slope, while the shear stress increases in the turbulently flowing melt on the surface of the slope. Based on the theoretical calculation, the maximum shear stress acting on the columnar crystal in the melt during the shearing flow near the surface of the metalic melt is lower than the yield strength of α-Al grain, so the shear stress induced by shearing flow cannot break the columnar crystal, and only by sweeping the grain into the melt to induce the multiplication of grain, which agrees with the experimental results. So, the proposed model can explain the constitutive relations of the metalic melt shearing flow near the surface and the effect of shear stress on the solidification microstructure.
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