Crystalline Structure, Grain Direction and its Importance when Bending Polycrystalline Materials


Most metals are commonly found in the polycrystalline form. Polycrystalline materials are solids that are comprised of many differing crystallites, which are also commonly referred to as the metal’s grain. These grains vary in size and in orientation but are usually microscopic and directed from processing conditions. The crystalline lattice structures form during the cooling of the metal from its molten state and are held together by thin layers of amorphous solid. As mentioned above, due to the processing of the metals (e.g., rolling of slabs and blooms down to transitional shapes such as plate, sheet, strip, coil, billets, bars and rods) some alignment of the crystalline structure occurs, which results in texture and can be thought of as the percentage of crystals having the preferred orientation. Texture or grain direction must be taken into account for accurate predictions of material behavior and characteristics when forming/bending.

A metal’s strength is directly related to its grain size; reducing grain size is a common way to improve material strength which often has little to no effect on the material’s toughness. This is extremely important when considering bending of materials to a rather tight radius, like as would be done through the forming of plate from a press brake. Materials with large grains are often susceptible to failure at the material’s grain boundaries which is technically a dislocation at the metal’s slip plane, and often these materials are limited to how tight of a radius they could be bent to before failure occurs. Another consideration to take into account when bending polycrystalline materials through such methods as would be done by a press brake, is the direction that the grain runs through the material. Bending metal against the grain or perpendicular to the grain is a common way to prevent material failure or cracking. Bending with the grain or parallel to the grain direction can be a contributor to material failure, as bending along the grain allows for separation of the grain boundaries. This can drastically limit the length of parts being formed as most materials grain runs in the long direction, but as stated previously reducing the material’s grain size may be a solution to this problem, as grain boundaries disrupt the motion of dislocation through a material.

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