Flat metal products are often specified according to a set of mechanical properties. The ultimate tensile strength, the 0.2% yield limit, the percentage of elongation, and hardness are applied representations of how the components of a certain material behave in response to an applied force. Tensile strength, yield limit, and elongation are effective metrics for managing the limits of raw material; to what extent it will bend before breaking. This is especially pertinent for stampers. However, there are huge potential benefits to examining a deeper layer of the grain structure that governs its mechanical behavior.
What are metal grains?
A metal is made up of a set of microscopic crystals called grains, randomly oriented throughout the material. The blocks that make up an individual grain are the atoms of the constituent elements of an alloy, such as carbon, iron, nickel, chromium... etc., mixed in a solid solution. The grains of an alloy are produced through a repeated arrangement of atoms, called a crystalline structure, influenced by the chemical composition of the alloy.
A homogeneous section of metal consisting of a repetitive crystalline structure that forms one or more grains can be termed a phase. The mechanical properties of an alloy are a function of the existing crystalline structures in the alloy and the size and arrangement of the grains of each phase.
How are grains formed in an alloy?
The grains of an alloy are formed during its solidification from the liquid state to the solid. Unless great care is taken to facilitate the precipitation and growth of an individual grain when a metal solidifies from its liquid form, solid grains of the thermodynamically preferred phase will essentially precipitate anywhere that the pressure, temperature, and chemical composition of the material allow.
This is because individual grains will nucleate wherever they can and grow until they meet another grain. Due to the different orientation of their crystalline structures, a "grain boundary" is formed at the intersection of the unequal lattices. In the end, the entire metal will be made up of these seemingly randomly oriented grains.
Every time a metal grain is formed, there is a possibility that there will be one or more line defects or missing pieces of a crystalline structure, known as dislocation. These imperfections, the dislocations in a crystalline structure, and their subsequent movement along a grain and across its boundaries are the basis of the ductility of the metal. When all the atoms are where they are supposed to be in a crystalline structure, there is no room for movement beyond the atomic bonds that stretch and vibrations throughout the structure. When an atom is removed, an opportunity is created for another atom to slide into that place, effectively displacing the dislocation. When a force acts on the raw alloy, the added movement of the dislocations in a microstructure allows for plastic deformation without fracture.
How do grains influence mechanical properties?
When a force, such as the rollers of a rolling mill, acts on the alloy, work is done to it, which means that energy is added to the system. If enough energy is added to the plastic deformation, the crystalline networks are strained and new dislocations are formed. It may seem that this should increase ductility because there are more free spaces and more potential for dislocation movement. However, when a dislocation encounters another dislocation, they can lock each other. As the number and concentration of dislocations increase, more and more dislocations lock each other, reducing ductility. In the long run, there will be so many dislocations that no more can form due to cold work, as the existing locked dislocations can no longer move; the atomic bonds of the network stretch and stretch until they break, causing a fracture. This is why alloys are cold worked and limit the amount of plastic deformation that a raw alloy can withstand before breaking.
Grains also play an important role in annealing. Annealing of a sufficiently hardened material essentially resets the microstructure to recover ductility. During annealing, the grains transform in 3 steps:
1. Recovery: Deformed grains fix their crystalline structure by eliminating or rearranging defects
2. Recrystallization: New defect-free grains nucleate and consume the original grains
3. Growth: The new defect-free grains grow and consume each other.
It is essential to understand that there is a minimum level of deformation required to trigger recrystallization. If the material does not have enough stored deformation energy before being heated, recrystallization will not occur, and the grains will continue to grow beyond their original size.
Metal manufacturers can adjust mechanical properties by controlling grain growth. Grain boundaries are essentially a wall of dislocations and also hinder the movement of dislocations. If grain growth is limited, there will be a greater number of smaller grains, which can be considered more "fine" in terms of grain structure. A greater number of grain boundaries implies less dislocation movement and greater strength. If greater grain growth is allowed, the grain structure becomes more "coarse", with larger grains, fewer boundaries, and less strength.
The grain size usually refers to a unitless number, often between 5 and 15 approximately. This is a relative scale related to the average grain diameter; the higher the number, the finer the grain size. The methodology for measuring and classifying grain size is described in ASTM E112 standard and consists of counting the number of grains in a given area. This is usually achieved by cutting a cross-section of the raw material, grinding and polishing, and acid etching to reveal the grains. Counting the metal grains is done under a microscope with a magnification that allows adequate sampling of the grains and can be automated. Assigning an ASTM grain size number suggests a reasonable level of homogeneity in the grain shape and diameter. It may even be advantageous to limit the variation of grain sizes, to 2 or 3 points, to ensure consistency of properties throughout the piece.
In the case of work hardening, strength and ductility have an inverse relationship. It is not so clear with grain size. The relationship between ASTM grain size and strength is usually positive and strong. In general, the percentage of elongation and ASTM grain size have an inverse relationship, but excessive grain growth can result in a "dead and soft" material that can no longer be effectively work hardened.
How is grain size controlled?
The grain size of a recrystallized material will vary with time at temperature and cooling rate. The temperature at which recrystallization occurs is determined by the chemical composition and is usually between 30 and 50% of the melting point. While at temperature, recovery and recrystallization processes will compete with each other until recrystallized grains consume all deformed grains. Once recrystallization is complete, grain growth takes over. If the material is not held at temperature for long enough, the resulting structure may be a combination of old and new grains. If uniform properties throughout the metal are desired, the annealing process should be aimed at achieving a uniform and equiaxed grain structure. Uniform means that all grains are more or less the same size, and equiaxed means that they all have approximately the same shape.
To achieve a uniform and equidistant microstructure, each piece must receive the same amount of heat for the same time and cool at the same rate. This critical step requires great precision, and precision rolling partners excel at it. With batch annealing, this is not always easy or possible, so it is essential to wait at least until the entire piece is at temperature before counting the soak time. A longer soak time and/or higher temperature will result in a coarser grain structure / softer material and vice versa.
How does grain structure affect forming?
If grain size and strength are related, and strength is already known, why bother counting the grains?
All destructive tests have variability. Tensile tests, especially at lower thicknesses, are highly dependent on sample preparation. Premature fractures can lead to tensile strength results that are not representative of the actual properties of the material. If the properties are not uniform throughout the piece, taking a tensile piece from an edge might not tell the whole story. Sample preparation and testing can also be time-consuming. How many tests and in how many directions is it possible to perform for a given product? Evaluating the grain structure is additional insurance against surprises.
Beyond strength, isotropy/anisotropy can be better understood through grain structure. Anisotropy refers to the directionality of mechanical properties. A uniform and equiaxed grain structure should be isotropic, that is, have the same properties in all directions. Isotropy is vital in deep drawing processes, where concentricity is key. When a raw piece is drawn into a die, anisotropic material will not flow uniformly, resulting in a defect called earing, where the top section of the cup develops a wavy profile. Inspection of the grain structure can reveal where the lack of uniformity in the piece is and help diagnose the root cause.
Proper annealing is essential to achieve isotropy, but it is also important to understand the level of deformation before annealing. As the material is plastically deformed, the grains will begin to distort. In the case of cold rolling, where thickness becomes length, the grains will elongate in the rolling direction. As the aspect ratio of the grains changes, so will the isotropy and mechanical properties of the material. In the case of a very deformed piece, some of the directionality may be retained even after annealing, resulting in anisotropy. In the case of deep-drawing materials, it is sometimes necessary to limit the amount of deformation before the final annealing to avoid anisotropy.
Earing is not the only grain-related drawing defect. Orange peel can occur when raw material with too coarse grains is drawn. Each grain deforms independently and depending on its crystallographic orientation. The differences in deformation between neighboring grains result in a textured appearance similar to orange peel. The texture is the grain structure that is revealed on the surface of the cup wall. Like pixels on a television screen, the differences of each individual grain will be less apparent with a finer grain structure, effectively increasing the resolution. When it comes to avoiding orange peel, specifying only mechanical properties may not be sufficient to guarantee a sufficiently fine grain size.
To explain the orange peel effect, when the change in dimensions of a piece is less than ten times the grain diameter, the properties of individual grains will determine the forming behavior. As demonstrated by the visual effect of orange peel on the wall of a drawn cup. For an ASTM grain size of 8, the average grain diameter is 885 µin, meaning that any thickness reduction of 0.00885" or less may be influenced by this "microforming effect". This is also one of the reasons why the results of tensile strength and yield limit tests of thinner specimens decrease as thickness decreases and grain size increases.
Although coarse grains can cause problems in deep drawing, they are sometimes recommended for coining. Coining is a deformation process in which a raw piece is compressed to impart the desired surface top
