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مکانیک::
دانه ستونی
Columnar grains grow continually from the outer wall to the bore to give an unacceptable layer in the inner narrow zone, which is sub- sequently machined away.
With the conventional sand casting method, cooling proceeds from two faces, accompanied by the formation of columnar grains meeting toward the cen- ter of the wall of the casting.
This practice may also be followed to delete the formation of columnar grains at the edges.
However, the initial columnar grain structure subsequently disappears because of poor adherence of the latter layers to the substrate.
At low temperature, when both Y2O3 and ZrO2 are deposited, the columnar grains are generally shorter and uniform in thickness, have a smooth surface and are pure white in color.
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