Those new to the art of metal fabrication are often surprised to learn that like a piece of wood, a piece of metal is also arranged in grains. And, as in woodworking, the grain structure in metals play a critical role in determining their properties and, consequently, their behavior while being shaped.
Metal grains are small, randomly oriented crystals that make up the polycrystalline structure of most metals. Understanding how grain structure affects metal fabrication is essential for everyone from senior engineers and materials scientists down to beginner press brake operators to be able to optimize fabrication processes and produce components with the right properties.
Grain Orientation and Texture
Grain orientation, or texture, plays a vital role in metal fabrication. During plastic deformation processes, grains tend to align in specific directions, creating a “preferred orientation.” This texture can significantly influence the mechanical properties of the metal depending on the direction it is being bent or otherwise fabricated.
For example, in rolled sheets, grains are often elongated in the rolling direction, leading to higher strength along that direction but reduced strength perpendicular to it. Understanding and controlling texture is crucial in applications where directional properties are critical, such as in rolled aluminum sheets used in aircraft fuselages.
Grain Size and Metal Properties
Grain size significantly influences the mechanical properties of metals. Fine-grained metals generally exhibit higher strength and hardness due to the grain boundary strengthening mechanism. Grain boundaries restrict “dislocation movement,” the process by which atoms in a crystalline solid slide past each other, allowing for plastic deformation in materials like metal. Smaller grains mean more grain boundaries, which hinder dislocation movement and enhance strength. Conversely, coarse-grained metals tend to have lower strength but higher ductility, as dislocations can move more freely.
Heat Treatment and Grain Refinement
Heat treatment processes, such as annealing, quenching, and tempering, are employed to modify the grain structure of metals. Annealing, for example, involves heating the metal to a specific temperature and then slowly cooling it, allowing for grain growth and reducing internal stresses. Quenching involves rapid cooling, which can produce a fine-grained martensitic structure in steels, enhancing hardness and strength. Tempering follows quenching and involves reheating the metal to a lower temperature, relieving stresses and slightly reducing hardness to improve toughness.
Grain refinement techniques, such as alloying and thermo-mechanical treatments, are used to produce fine-grained metals. Alloying elements like titanium and niobium can form precipitates at grain boundaries, inhibiting grain growth during processing. Thermo-mechanical treatments combine mechanical deformation and controlled heat treatment to produce a refined grain structure, enhancing the metal’s mechanical properties.
The Influence of Fabrication Processes on Grain Structure
While machining processes like milling can affect the grain structure near the machined surface of a metal, the very nature of fabrication processes which bend or otherwise reshape metal will affect the grain structure of large areas of the workpiece, especially when heat is involved:
- Forming and Forging. In metal forming processes such as rolling, forging, and extrusion, the metal is plastically deformed, which alters its grain structure. These processes often involve working at elevated temperatures (hot working), where recrystallization can occur. Recrystallization leads to the formation of new grains, refining the grain structure and enhancing the metal’s strength. The degree of deformation and the working temperature should be carefully controlled to optimize the grain structure and the resulting mechanical properties.
- Casting and Solidification. During casting, the cooling rate affects the grain size. Rapid cooling promotes the formation of fine grains, leading to a stronger but potentially more brittle metal. Slow cooling allows larger grains to form, resulting in a metal that is more ductile but less strong. Controlling the cooling rate and solidification process is vital for achieving the desired grain structure and, thus, the required properties in the final product.
- Welding. Welding involves the melting and solidification of metals, which significantly impacts the grain structure in the weld zone. Rapid cooling in welding can lead to the formation of fine grains, which can enhance the strength of the weld but may also increase susceptibility to cracking. Preheating and post-weld heat treatments are often employed to control the grain structure and minimize residual stresses, ensuring a sound weld with the necessary mechanical properties.
Grain-Awareness During Fabrication
As mentioned, grains are often stretched out in the direction that a metal workpiece was rolled during production, especially if it was cold rolled. The piece of metal will have greater strength along that axis due to the elongation and alignment of the grains, but it will be weaker perpendicular to it. This is of particular concern to those fabricating thinner workpieces like plate metal or sheet metal, especially if they are bending or rolling it.
Just as a woodworker knows to cooperate with the grain in his workpiece (a cut along the grain of a board is easier and smoother; aligning the wood grain in the direction of stress provides greater strength), so also should the metalworker be aware of the grain in his workpiece and adjust it for optimal fabrication.
Metal will more easily fail along the direction that the grain runs. While that makes it a bit simpler to cut in that direction, it also makes it easier to bend—too easy, that is. Bending with the grain will make it much more likely that the workpiece will experience cracking or a fracture. Bending against the grain takes greater force—and leads to more springback, requiring more overbending to compensate—but it will create a stronger and more robust bend.
Determining Grain Direction
Where possible, a fabrication shop should find out from their metal supplier what the direction and other properties of the grains are in the blanks they are purchasing. Since sheet metal is rolled out and cut down into smaller sections, it is possible that the long dimension of a large blank will be the grain direction.
Looking at the surface of a piece of metal, any subtle irregularities or lines should indicate the direction of the grain, though a magnifying glass or even a microscope may be needed. The best in-shop method would involve cross-sectioning and polishing a sample of the metal and then etching it with acid and examining it under a microscope. Etching with a solution like nital (nitric acid combined with either methanol or ethanol) is very effective in showing the microstructure in steel and iron alloys.
The acid attacks the boundaries of the metal grains, making them appear darker. (If the project is important enough, a shop may need to take a sample to a lab with a scanning electron microscope and ask them for help.)
A shop can also perform a bend test with two sets of samples, gently bending the pieces in each set perpendicular to the ones in the other set. The direction that has the least resistance is likely the direction that the grain is running. The direction that has more resistance—and can be bent further before it cracks—is probably the direction in which bending or rolling should be done.
Going Against the Grain
Understanding and working with grain structure is essential for producing metal components with optimal performance, ensuring their reliability and longevity in various applications. “Going against the grain” may mean being radical in other walks of life, but for experienced metal fabricators, it is a motto to live by.
