Fig. 7. These micrographs show grain morphology in (a)Al-5% Cu; (b) Al-9.6% Cu; (c) Al-16.2% Cu; and (d) Al-25% Cu.
blade the width of the sidewalk which easily cut through the snow, pushing it to the sides of the walkway. Tis is shown sche- matically in Fig. 6. Dendrites act much like this “V”-shaped plow. In other words, growing alumi- num crystals adapt a dendritic shape as a response to the alloy composition. Growing solid crystals adapt a planar or a non-planar (den- dritic) shape depending on the interaction of two factors. • The growth rate of the crystal. This is usually defined as the velocity of motion of the solid/liquid interface, in microns per second (R), and is controlled by the thermal gradient in front of the crystal (G).
• The rate at which the “piled up” solute elements can be removed, by diffusion, from the solidifying front. Te shape of the solidifying aluminum depends on the amount and type of solute dissolved in the alloy. Te grain size also is influenced by the presence of growth-restricting solutes, like Si and Cu. Tis may be seen by comparing the grains of different Al-Cu alloys in Fig. 7. Tese alloys were solidified at an average cooling rate of 1.8F (1C) per sec- ond. All four figures are shown at the same magnification. Compare this to the crystals in Fig. 4, which are new and just forming. Te arms on the branches of the dendrites are fine, much like needle-shaped leaves on a Christmas tree. Also, the dendrites are growing freely into liquid metal. Tey are still largely unimpeded by neighboring grains. At some point, however, the “trunks” of the dendrites come
in contact with neighboring grains. (Tis type of contact is called “dendrite coherency”.) After this time, any further solidi- fication (and growth of dendrites) can occur only by thicken- ing of the leaves and branches on the dendrite. As a result, the dendrites in the final casting are thicker. Te spacing between arms also becomes larger. It has long been known that the spacing of arms of the dendrite in the casting depends on the solidification time. One of the first detailed studies was published in 1963 by Alcoa researchers who related dendrite cell size to the solidification time.
Fig. 8. Measuring SDAS by linear intercepts is shown.
图7：晶粒形态的显微照 片（Al-5% Cu、Al-9.6% Cu、Al-16.2% Cu、 Al-25% Cu）
道的两侧。这点在示意图图 6中可见。树枝晶很像这个 V型犁。也就是说，铝晶体 以树枝状生长作为对合金成 分的反应。
生长的固体晶体是以平 面化生长还是以树枝晶形态 生长，依赖以下两个相互作 用的因素：
• 晶体的生长率。其常定义为固液界面的移动速度， 以每秒多少微米计算，它受晶体前沿的温度梯度 控制。
• 从凝固前沿开始，“堆积”的溶质元素通过扩散移 除的比率。
铝的凝固形态随溶解在合金中的溶质的量和种类而 定。晶粒大小也受约束生长的溶质的存在的影响，比 如Si和Cu。这点通过比较不同的图7中的Al-Cu合金 的晶粒可见。这些合金凝固时的冷却速度是每秒1℃ 。图示所有图形的放大倍数一样。比较图4的晶体结 构，是新近成形的。树枝晶的枝臂分叉形态很好，很 像圣诞树上的针状树叶。同时，树枝晶也自由生长进 入液态金属。它们仍然基本不受临近的晶粒阻碍。 然而在某些点上，树枝晶的“枝干”与临近的晶粒 接触（这个典型特性叫做“枝晶一致性”）。在这之 后，任何之后的凝固（以及树枝晶的生长）仅能够通 过增厚树枝晶的分支和分叉而发生。结果，最终铸件 中的树枝晶更厚，树枝晶臂的间距也变得更大。 业界长期公认铸件内树枝晶臂的间距由凝固时间决 定。早期详细的研究之一 是1963年由Alcoa的研究 者发布的，将树枝晶晶核 大小与凝固时间相联系。 许多早期论文报道了在
图8：通过线性截断的方法 测试SDAS
62 |
FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION June 2014
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81