How does the grain structure of metals in coiled strip affect forming of a part?
Flat metal products are typically specified to a set of mechanical properties. Ultimate tensile strength, 0.2% offset yield strength, percent elongation, and hardness are applied representations of how the building blocks of a given material behave in response to an applied load. Tensile, yield, and elongation are effective metrics for managing raw material limits; how much it will bend before it breaks. This is especially pertinent for stampers. However, there are potentially immense benefits to peering one layer deeper into the grain structure that governs its mechanical behavior.
What are metal grains?
A metal consists of an array of microscopic crystals called grains, randomly oriented throughout the material. The building blocks that make up an individual grain are the atoms of an alloy's constituent elements, such as carbon, iron, nickel, chromium…etc., mixed into a solid solution. An alloy's grains occur through a repeating arrangement of atoms, called a crystal structure, influenced by the alloy's chemical composition.
A homogenous section of metal consisting of one repeating crystal structure forming one or more grains can be called a phase. An alloy's mechanical properties are a function of crystal structures existing within the alloy and each phase's grains' size and arrangement.
How do grains form in an alloy?
The grains of an alloy are formed during its solidification from a liquid to a solid-state. Unless tremendous care is taken to facilitate the precipitation and growth of a single grain when a metal solidifies from its liquid form, solid grains of the thermodynamically preferred phase will precipitate essentially anywhere the pressure, temperature, and the material's chemical composition allow them to.
This is because individual grains will nucleate wherever they can and grow until they encounter another grain. Due to their differently orientated crystal structures, a "grain boundary" is formed at the mismatched lattices' intersection. Eventually, the entire metal will consist of these seemingly randomly oriented grains.
Anytime a metal grain is formed, there is a chance for one or more line defects or missing pieces of a crystal structure, know as a dislocation, to exist. These imperfections, the dislocations in a crystal structure, and their subsequent movement throughout a grain and across grain boundaries are the basis of metal ductility. When all the atoms are where they are supposed to be in a crystal structure, there is no room for movement beyond the atomic bonds stretching and vibrations throughout the structure. When you remove an atom, you create an opportunity for another atom to slide into that spot, effectively moving the dislocation. When a force acts on the bulk alloy, the dislocations' aggregate movement in a microstructure allows for plastic deformation without fracture.
How do grains factor into mechanical properties?
When a force, such as the rolls in a rolling mill, act upon the alloy, work is done to it, meaning energy is added to the system. If enough energy is added to plastic deformation, the crystal lattices are strained, and new dislocations form. This may seem like it should increase ductility because there are more free spaces and more potential for dislocation movement. However, when a dislocation runs into another dislocation, they can pin each other in place. As the number and concentration of dislocations increase, more and more dislocations get pinned together, reducing ductility. Eventually, there will be so many dislocations that no more will be able to form due to cold work since the existing pinned dislocations can no longer move; the atomic bonds in the lattice stretch and stretch until they break, causing a fracture. This is why alloys work hard and limit the amount of plastic deformation a bulk alloy can take before breaking.
Grains also play a significant part in annealing. Annealing a sufficiently work-hardened material essentially resets the microstructure to recover ductility. During annealing, grains transform in 3 steps:
- Recovery: Deformed grains fix their crystal structure by removing or rearranging defects
- Recrystallization: New, defect-free grains nucleate and consume the original grains
- Growth: New, defect-free grains grow and consume each other.
It is essential to understand that there is a minimum level of deformation necessary 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.
Mechanical properties can be tuned by metal manufacturers controlling grain growth. Grain boundaries are essentially a wall of dislocations and also hinder dislocation movement. If grain growth is limited, there will be a higher number of smaller grains, which can be considered "finer" in terms of the grain structure. More grain boundaries mean less dislocation movement and higher strength. If the grains are allowed more growth, the grain structure becomes "coarser," with larger grains, fewer boundaries, and lower strength.
Grain size is often referenced as a unitless number, often between about 5 and 15. 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 rating grain size is outlined in ASTM E112 and involves counting the number of grains in a given area. This is often accomplished by cut a cross-section of the raw material, grinding and polishing, and etching with acid to reveal the grains. The count of metal grains is performed on a microscope under a magnification that allows for adequate sampling of grains and can be automated. Assigning an ASTM grain size number suggests a reasonable level of homogeneousness in grain shape and diameter. It may even be advantageous to limit the variation in grain size, to 2 or 3 points, in order to ensure consistent properties throughout the bulk.
In the case of work hardening, strength and ductility have an inverse relationship. It is not as clear-cut with grain size. The relationship between ASTM grain size and strength is often positive and strong. In general, percent elongation and ASTM grain size have an inverse relationship, but excessive grain growth can result in "dead soft" material that can no longer work-harden effectively.
How is grain size controlled?
The grain size of an annealed material will vary with time at temperature and cooling rate. The temperature at which recrystallization occurs is determined by the chemical composition and often falls between 30 – 50% of the melting point. While at temperature, the recovery and recrystallization processes will compete with each other until the recrystallized grains consume all the 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 the grains are roughly the same size, and equiaxed means that they are all approximately the same shape.
To achieve a uniform and equiaxed microstructure, every work-piece should see the same amount of heat for the same amount of time and cool at the same rate. This critical step requires a great amount of precision, and where precision reroll partners excel. With batch annealing, this is not always easy or possible, so it is essential to at least wait until the entire workpiece is at temperature before counting the soak time. A longer soak time and/or a higher temperature will result in a coarser grain structure / softer material and vice versa.
How does the grain structure affect forming?
If grain size and strength are related, and the strength is already known, why bother counting grains, right?
All destructive tests have variability. Tensile testing, especially at lower thicknesses, is heavily dependent on sample preparation. Premature fractures can result in tensile strength results that are not representative of the actual material properties. If the properties are not uniform throughout the workpiece, taking a tensile coupon from one edge might not tell the whole story. Sample preparation and testing can also be time-consuming. How many tests in how many directions is it feasible to perform for a given product? Evaluating the grain structure is extra insurance against surprises.
Beyond strength, isotropy/anisotropy can be better understood via the grain structure. Anisotropy refers to the directionality of mechanical properties. A uniform and equiaxed grain structure should be isotropic, meaning it has the same properties in every direction. Isotropy is vital in deep drawing processes where concentricity is critical. When a blank is drawn into a die, anisotropic material will not flow uniformly, resulting in a defect called earing, where the upper section of the cup develops a wavy profile. Inspecting the grain structure can reveal where the non-uniformities are in the workpiece and help diagnose the root cause.
Proper annealing is essential in achieving isotropy, but it is also important to understand the deformation level before annealing. As the material is plastically deformed, the grains will begin to distort. In the case of cold rolling, where thickness is converted to length, the grains will elongate in the rolling direction. As the aspect ratio of the grains change, so will the isotropy and bulk mechanical properties. In the case of a severely deformed workpiece, some of the directionally may be retained even after annealing, resulting in anisotropy. For deep-drawn materials, it is sometimes necessary to limit the amount of deformation prior to final annealing in order to avoid earing.
Earing is also not the only grain-related deep drawing defect. Orange peel can occur when drawing raw material with grains that are too coarse. Each grain deforms independently and as a function of its crystallographic orientation. Differences in deformation between neighboring grains result in a textured appearance that someone thought looked like an orange peel. The texture is the grain structure revealing itself on the surface of the cup wall. Like pixels in a television screen, each individual grain's differences will be less apparent with a finer grain structure, effectively increasing the resolution. When it comes to avoiding orange peel, specifying mechanical properties alone may not be enough to ensure a fine enough grain size.
To expound upon the orange peel effect, when the change in dimensions of a workpiece is less than ten times the grain diameter, the properties of individual grains will drive forming behavior. As evidenced by the visual orange peel effect on the wall of a drawn cup, instead of deformation being averaged over many grains, it will reflect each grain's specific size and orientation. For an ASTM grain size of 8, the average grain diameter is 885 µin, meaning any reduction in thickness of .00885" or less may be influenced by this "micro-forming effect." This is also one of the reasons why the tensile and yield strength results of thinner tensile specimens decrease with decreasing thickness and increasing grain size.
While coarse grains can cause problems for deep drawing, they are sometimes recommended for coining. Coining is a deformation process where a blank is compressed to impart the desired surface topography, such as George Washington's profile on a quarter. Unlike drawing, coining does not usually involve much bulk material flow and requires a great deal of force, which may only plastically deform the surface of the blank. For this reason, minimizing the flow stress at the surface by using a coarser grain structure can help mitigate the force needed for proper die filling. This is especially applicable in open die coining, where dislocations on surface grains are allowed to flow freely instead of accumulating at grain boundaries.
The importance of engineer-to-engineer communication between metal formers and material suppliers
Deep draw manufacturers and precision metal stampers who produce metal parts would be well served to partner with metallurgists at a precision reroller with high level technical capabilities. Reroll partners with these capabilities can help producers of stamped parts optimize their material down to the grain level.
A strip or foil manufacturer with technical capabilities has the materials knowledge and expertise required to measure and control a wide range of subtle variables that can positively impact a stamped parts’ production and ultimate quality.
Critical to manufacturing success of precision parts is two-way, engineer-to-engineer collaboration, especially when applied to new product development. When the metallurgy and engineering experts on both sides of the supplier-manufacturer relationship integrate into a single team, it can have transformative effects on finished parts. Ulbrich’s team of engineers and metallurgists are deeply experienced in producing metal for stampers to some of the tightest tolerances and material characteristics in the industry. Working together, especially when it comes to grain, we can help your business overcome some of your most difficult metal challenges.
The trends discussed above are generalizations that may not apply to any specific part formed from flat sheet or strip metal. However, they do highlight the benefits of measuring and standardizing raw material grain size while designing a new part to avoid common pitfalls and optimize forming parameters. Reach out to an engineer today to discuss the requirements and specifications you’re working with on your next project.