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Why is it, that machined metals, milled out of a primitive shape, tend to bend upon impact rather than crack or shatter? Why do they behave differently to stresses when they are cast into the final shape?

I find this rather confusing. Don't they have the same lattice? Are there more defects introduced when casting it in complex shapes?

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    $\begingroup$ This is a very broad question, covering a lot of metallurgy. But, consider the stresses in a cooling object and how that might impact elastic and plastic behavior. (Of course, there is a reason for annealing or other heat treatments for near-net shape castings, as well as them not being net shape). $\endgroup$ – Jon Custer Sep 6 '18 at 12:46
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Jon Custer is right- this is a very broad question, about which whole chapters in materials science textbooks have been written. I'll furnish a broad perspective which I hope will get you pointed in the right direction.

First of all, all common metals begin their useful (i.e., engineering) lives as raw solids that were cast from the molten state. But in the case of (for example) a piece of steel bar stock that is being milled to shape in a milling machine, that bar underwent significant post-casting treatment to transform it from a rough billet of as-cast steel into a square chunk of stock: the billet got mechanically deformed while red-hot through a series of incremental squeezing and rolling processes- all of which had the effect of significantly altering the microstructure (grain size, grain orientation, etc.) of the metal and thereby altering its physical characteristics which include its ability to be bent without fracture or to withstand impact loads without cracking.

Second, those "secondary processes" also furnish the opportunity to cut out and discard things like slag inclusions and voids which occur commonly during the casting process, so that the finished piece of bar stock is usually a lot more defect-free than a cast part and hence more robust.

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