Grain Flow in Forgings III – Effects of Forging on Grain Flow
In the first two parts of this series, we examined the basic definition of grain flow and described the anisotropic properties in metals because of it. The mechanical properties that require the sample to fracture or break will be enhanced when the crack needs to propagate perpendicular to grain flow. The crack is microstructurally diverted when this happens, requiring more energy to propagate, thus enhancing the properties. The second article showed that the raw material that arrives into a forge shop already possesses a grain flow from the suppliers’ operations. We also examined how forging imparts additional changes to grain flow. In this part of our series, we will examine in more detail the effect of forging on grain flow and look at the parameters that affect it.
During the forging process, the metals (and the underlying grains) will plastically deform in the path of least resistance. The metal will flow in the direction and manner that requires the least amount of work. This principle is fundamental. During this deformation, the forging process will impart some grain flow into the metal and the grains. The primary forging parameters that affect grain flow are:
• Shape of the forging
• Forging process
• Design of the preform
• Size of the billet
• Behavior of the metal
• Process conditions
Figure 1 shows an example of grain flow in a forging. In this example, there is a rib region in the upper part of the picture. During forging, the deformation dynamics are such that the metal flows up to fill this region. Note how the grain flow changes within the rib. The grain flow will generally match the contours of the forging die, since the metal will flow around corners and radii. Please note that it is the metal flow during the forging process that produces the resulting grain flow, not the die features. The same die can create different grain flow, depending on the starting shape of the billet and the plastic flow that occurs during forging.
Many forged parts can be produced by more than one type of forging process. In Figures 2 through 4, an illustration of how different processes produce forgings with a significantly different grain flow is shown. This variation is possible in spite of an identical geometry in a round disk forged using different processes. Grain flow can be simulated as shown in these figures using a commercial process model, in this case DEFORM. Various grain-flow tracking capabilities exist to predict grain flow prior to running shop trials. Optimum grain flow would match the maximum principal stress component in service, resulting in the best fatigue life.
The billet that arrives in the forge shop has an initial grain flow that is generally longitudinal. Preform operations are used to change the geometry, microstructure and grain flow. In each preform operation, material flows in the path of least resistance with no influence from future operations. The grain flow changes as a result of this metal flow.
As an example, consider a billet that is upset into a pancake shape during preforming. This upset will change the initial grain flow, which is in the longitudinal/axial direction, into some grain flow in the radial direction (Figure 4). The preform will now have some initial grain flow going into the preform and finish die cavities. The grain flow going into the subsequent forging steps will be different from the grain flow in the initial billet due to the accumulated deformation.
A second example is the use of cross-wedge rolling as a preforming operation, which is shown in Figure 5. There is additional material gathered in some regions, with elongation in others. During this preforming operation there is a reduction (i.e., rolling deformation) in some regions of the bar and gathering in others. The small-diameter regions are generally elongated, with the longitudinal grain flow becoming more pronounced. The larger-diameter regions, where additional metal is being gathered, generally have a radial component to the grain flow prior to final forging operations. Depending on the geometry of the initial bar, the preform and final forged shape, the amount of grain-flow change in these end regions can vary from very little to a significant amount.
Initial Billet Size
To illustrate the influence of initial billet size, we can consider an axisymmetric forging of a spur gear blank, with the gear teeth machined onto the outer diameter. For these teeth to be as fatigue resistant as possible, we would like to produce a forging with as much radial grain flow as possible. Thus, if a fracture started at the root of a tooth, the crack would be propagating in the hoop or circumferential direction, which is perpendicular to the radial direction. Therefore, radial grain flow provides an enhancement of the fatigue resistance of this gear.
The key to increasing the amount of radial grain flow is to upset the initial billet as much as possible. So, if the initial billet has a small height/diameter ratio (by choosing the initial billet with a large diameter), then the amount of radial grain flow imparted during the initial upset is going to be somewhat limited. Note the change in grain-flow direction as the upset increases in Figure 4 to illustrate the point. There are practical limits to which the upset ratio (height/diameter) can be pushed. If the ratio goes much beyond 2, the potential for buckling of the material is increased in the initial upset. Nevertheless, the size of the initial billet – like the shape of the preform – will affect the final grain flow that is obtained in the forging.
Behavior of the Material
Grain flow results from the metal deformation or metal flow. If the material does not flow evenly, then the grain flow may not be in the direction that we may have anticipated. Geometrical and material defects, such as laps or adiabatic shearing, will cause the metal to flow in an undesirable fashion. The grain flow that results will come from the actual flow that occurs, not the flow we might want or might anticipate. Some of the undesirable flow can be caused by inhomogeneities in the material. Inhomogeneities, such as non-uniform temperature or microstructures and non-uniform lubrication, can alter the flow behavior from the design intended by the forging designer. The grain flow in the final forging is produced by the actual metal flow rather than an ideal flow anticipated during the forging design process.
Obviously, the process conditions can have an effect on the material-flow behavior and hence on the grain flow imparted during the deformation. There are some other process conditions that should be considered. For example, if an automotive spindle is being forged, it would be desirable to have the cracking resistance increased across the shaft area where the wheel would be mounted. In order to have this enhancement of cracking (either impact or fatigue), the grain flow should be in the longitudinal direction of the shaft. If the forging process is designed to make an initial upsetting of the billet, we will be changing the initial grain flow from longitudinal to radial and further deformation might enhance the grain flow in this direction. Instead, we might consider not upsetting the initial billet in the normal sense but side striking the billet so that there is a further reduction of the billet diameter. In this scenario the initial longitudinal grain flow that is present in the billet is increased and the shaft will be the beneficiary of an even greater amount of longitudinal grain flow.
The careful consideration of the forging process, with a clear understanding of the final use of the component, is essential when trying to use grain flow to optimize mechanical properties. This understanding requires clear communication between the forger and the customer and a careful consideration of what can be done in designing the overall forging process. This type of dialogue can result in a significant improvement in the final properties of the component. In many cases, there is no increase in production cost to achieve the optimum grain flow.
In this part of our series on grain flow, we have reviewed several aspects of the forging process and how they will affect grain flow. These include the forging shape, the forging process, the preform design, the initial billet size, the material behavior and the process conditions. It needs to be remembered that the grain flow that is imparted is due to the flow or deformation of the material, not just the shape of the component or the dies. If we wish to use grain flow to our advantage, we need to be fully aware of the actual flow that the metal undergoes during the forging operation. The next part in this series will compare grain flow in various processes and provide an example of how machining can influence the region of critical grain flow in a component.
Content for this paper was primarily developed by Scientific Forming Technologies Corporation in partnership with SCRA Applied R&D and FIA. The material was initially developed as a Forging Design Seminar under the FAST program, a multi-year, industry cost-shared program sponsored by the Defense Supply Center Philadelphia and Defense Logistics Agency – Research and Development.
Co-author Dr. Chet Van Tyne is FIERF Professor, Department of Metallurgical Engineering, Colorado School of Mines, Golden, Colo. He may be reached at 303-273-3793 or firstname.lastname@example.org. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at 614-451-8330 or email@example.com.