Grain Flow in Forgings IV – Various Processes and Influence of Machining
The focus in the first three parts of this series was on the basics of grain flow and their effects on mechanical properties. How the forging process changes the initial grain flow from the incoming billet and the effect of various forging parameters on the grain flow in the final product have been summarized. It is noteworthy that the grain flow comes from the movement of the metal during deformation throughout the forging process, not just the final forging operation. In this fourth installment, the typical grain flow for various manufacturing processes will be reviewed. The advantage of near-net-shape forging and the interaction of machining with grain flow will also be discussed.
Grain flow, as we have seen, can be one of the major advantages of forging. This advantage can be illustrated by examining a single component manufactured using three different processes: forging, machining and casting. Figure 1 shows an illustration of the grain flow in a simple U-shaped component produced by the three processes.
Grain Flow in a Component Produced by Different Methods
The forging process deforms metal in a die cavity with large amounts of plastic deformation. This typical contoured grain flow generally follows the die contour as material bends around the various radii in the forging tools. The flow is based on the workpiece deforming in the path of least resistance as the dies are closed.
In contrast, the component made from the machining of bar stock or plate does not have any metal movement during the fabrication process. Without metal deformation, the grain flow is the same as the initial bar or plate. The grain flow in this machined component has not changed from the direction and orientation of the bar or plate that was received from the metal supplier.
The component can also be made by casting. In the casting process, the liquid metal solidifies into the shape of the mold. In a cast component, there is no plastic deformation given to the metal and, hence, no grain flow. Any grain directionality is related to the solidification process, which may be perpendicular to the surface as opposed to parallel in a forging. The cast component will also have some porosity, voids and chemical inhomogeneity due to the solidification process. These features cannot be avoided and are inherent to a cast product.
This example provides a typical illustration of the same component made by three different processes. The components produced by forging and machining have a clear grain flow resulting from prior plastic deformation. The cast component does not have grain flow. For any critical service component, a service-life comparison would be the most important measurement. It is a safe assumption that a typical U-shaped part would be subjected to forces that cause this U to bend and open up further (i.e., increased separation of the two ends at the top of the U). Impact or cyclical loading could cause a crack to develop at high stress levels and cause one of the vertical legs of the U to fracture. Then we would expect the grain flow to play a major role in enhancing the impact or fatigue resistance if it is oriented properly.
Examination of a cast component would typically reveal porosity and a lack of grain flow. While the casting may possess tensile strength, a crack could easily propagate from one side of the leg through the component to the other, moving from one pore or void to the next. In fact, the casting will fracture at much lower energy levels, and fatigue failure will occur at much lower stress levels relative to a wrought structure. Remember that a wrought structure is one that has been mechanically worked to close the pores and voids and to reduce the chemical inhomogeneity.
The component made by machining from bar stock or plate has grain flow defined from the original material. The grain flow is not as large as in the forging because the amount of deformation is less. More importantly, the orientation of the grain flow needs to be examined with respect to the path that the fracture or crack would take as it propagated from one side of the component to the other.
In the case illustrated in Figure 1, we find that the crack opening is parallel to the grain flow that the metal received from the prior deformation by the supplier. When the crack moves across a component parallel to the grain flow, the crack can more easily propagate along the interface between the metal and any inclusions and along the grain boundaries that run to longer lengths in the grain-flow direction. This component will have a higher impact and fatigue resistance relative to the casting, but the orientation of the grain flow relative to a potential crack path is not optimum. Thus, it’s likely that the mechanical property levels are less than those found in a completely homogeneous wrought structure having no grain flow at all.
Finally, an examination of the forged component reveals a grain flow that was modified during the forging process. The elongated grains typically follow the contours of the die in which it was forged. Any crack that started at one side of the vertical leg would need to propagate perpendicular to the grain flow to cross the leg of the component. Under these conditions, one would expect to find the impact and fatigue-resistance properties that depend on cracking or fracture to be enhanced. This forged component would be more resistant to impact or fatigue relative to the machined component and should be significantly higher than the cast component.
Note that a key aspect to obtaining this forged grain-flow increase in properties is to ensure that the grain flow is perpendicular to the expected fracture path during service. Therefore, it is important to know how the component is going to be used and something about the loading conditions. Clear communication between a forge shop and the final customer is necessary to optimize a forging design and process that will yield an optimum part for its intended service.
Changing Grain Flow
Figure 2 shows two different forgings that illustrate another important point. In the example presented previously, the assumed grain flow was illustrated to be fairly smooth and continuous. In reality, the forging process can impart a fairly complex grain-flow pattern that varies as you scan from one location in the component to another. In Figure 2, the imparted grain flow will enhance the properties in different directions at different points in the component since the grain flow will vary. Knowing the critical locations for loading of the component in service is required to ensure that the grain flow in those locations is in the optimum direction.
Influence of Machining on Grain Flow
Figure 3 shows a spindle that has been forged. It appears to have an excellent grain flow along the shaft where the loading would be expected to occur. The fatigue and impact resistance along this shaft region will be enhanced for cracks or fractures that would propagate from one side of the shaft region to the other.
There is a subtlety here that needs to be brought to light. Prior to putting this spindle into service, it is machined to the proper dimensions for a bearing race. Machining can influence the effect of grain flow by removing a surface with grain flow in one direction and exposing a surface with grain flow in a different one. The machining itself does not cause significant plastic deformation. Though there is no change in the grain-flow direction, the metal removal yields a different location in forging the new surface. The grain flow on the finished part is the important one to consider. Figure 4 shows the machined profile on the forging and the component after it has been machined.
Figure 5 illustrates an issue that can arise. This figure shows a region of the component where a set of ball bearings rotates in a raceway under load. As these balls rotate around the component, a cyclic stress occurs. Because of the cyclic loading, fatigue or surface spalling in this region of the spindle must be considered.
On closer examination (Figure 5 inset), we find that the grain flow in that region is no longer parallel, but it is perpendicular to the surface. This orientation at the surface occurs because some of the original forging was machined off. This surface was originally located in the interior of the forging. Nevertheless, a grain flow perpendicular to the surface of a component where cyclic loading is expected may result in an unexpected or premature failure. A crack could develop, and it would then run from the surface into the component parallel to the grain flow. This situation is not desirable. In fact, it is the opposite of what is generally an optimum grain orientation to maximize the resistance to fatigue and cracking.
This example illustrates quite well that a clear understanding of the “final” component –
its geometry, its use and its loading – is needed to properly provide the grain-flow improvements that forging is able to impart to a component. The forging process is ideal for metal components requiring strength, toughness and fatigue resistance. That said, the forging design and process should be developed with an eye on the loading in service to maximize performance and service life.
The comparison of grain flow (or lack thereof) for components made by three different processes was examined. The casting would generally perform the worst. The machined component, although having grain flow, should be better than a cast component, but it would not necessarily have the impact and fatigue resistance that can be found in a forging. We also saw that the grain flow in a forging can vary from location to location and that machining the final component can expose unfavorable grain flow to a critical surface location in the component.
The message is that forging grain flow can indeed enhance component properties. However, it is incumbent upon the forger to know the details about the end use of the component being produced. Our next installment will look at this need to understand the final component, with details of two example forgings and the use of simulation software to track the grain flow during forging.
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.