The previous articles in this series looked at the ways grain flow can be observed in forgings, how forging can change grain flow and how the grain flow in forgings compares to grain flow produced by other processes. In order to achieve the benefits that grain flow can impart to a forged component, there is a need to understand the end service of the component to produce a forging with the grain flow of the required orientation in the critical region(s) of the component.
Grain flow is the orientation of grains and inclusions/particles within the metal caused by plastic deformation. Remember that grain flow can have a very positive influence on the mechanical properties that involve a fracture or a crack, such as impact resistance, fatigue life and ductility. The influence of grain flow on strength or hardness is not as significant, however, since these properties are primarily controlled by chemistry and heat treatment.
This article will review the grain flow on a set of collar forgings on the M224 60-mm mortar system. The mortar collar is used to attach the mortar tube to the bipod or tripod assembly. This article will also illustrate how simulation software can be used to predict grain flow in a forging prior to sinking of a die and forging trials. As was discussed in prior articles, grain flow enhances fatigue and fracture properties when the grain elongation is aligned with the maximum principal stress component in service. These examples will illustrate the grain elongation in the area that wraps around the barrel. To optimize any forging for the intended service, it is imperative to interact with the customer and understand the final application of the component. This ensures that the grain flow is designed to benefit the end user.
Modification of Grain Flow
Generally, the initial billet that arrives in the forge shop already possesses some grain flow from the deformation that it received from the metal supplier. During forging, the grain flow can be modified by changing the billet size, forging design, preform design or forging process. We covered these modifications in previous articles. The examples presented in this article illustrate the development of grain flow through the process and how computer simulations can effectively provide details of how the grain flow evolves during the forging process.
Using Simulation to Predict Grain Flow
The commercial FEM software, DEFORM™, from Scientific Forming Technologies Corp. is used to illustrate grain development in the case studies in this article. A utility named FLOWNET was developed in the early 1990s to track patterns through the forging process. By maintaining pattern connectivity, highly deformed regions, such as flash, show a highly elongated and aligned pattern that matches grain flow observed in production. In cases where the grain flow in the initial billet is highly elongated, patterns with representative ellipses can accurately predict changes with deformation.
Pattern tracking has been used for decades with excellent results, including tracking some elusive defects such as piping or flow localization. The method can be applied to 2-D models or sliced sections of a 3-D model. In complex 3-D forgings, a subsurface representation of the billet or 3-D geometry can be tracked and analyzed during post-processing. When experimental measurements of grain flow have been performed, the measured grain orientations have been within a few degrees of the simulation model prediction. These experimental validations provide credence to the simulation results.
Mortar Collar Forgings
An aluminum mortar collar must be strong, tough, fatigue resistant and durable. The primary grain flow shown in Figures 4-7 are representative of the grain flow tracking the surface shape and contours. In both components, the clamping force around the barrel would typically represent the highest stress in service. The grain flow shown in the region that wraps around the barrel (Figures 4 and 6) illustrates the development of an ideal grain flow for this type of application.
The busting, blocking and final forging operations are shown for both the top and bottom mortar collars. The interior metal movement could be observed in the actual simulation. We can only see snapshots of the metal movement in these static pictures. Nevertheless, it is clear that there is significant (directional) grain flow resulting from these forging operations.
Figure 4 shows the forging, the forging after trimming, the grain flow in the forged and trimmed component and, most importantly, the grain flow in the component after final machining to the required dimensions. Of greatest note is the grain flow in the machined component and the orientation of this grain flow with respect to the loading in final use. In this example, grain flow in the thin web region is in the horizontal direction and parallel to both the upper and lower surfaces of the web. This orientation of the grain flow provides extra resistance to any cracking or fracturing that would occur in this web region, assuming the crack starts at one surface and propagates perpendicularly from that surface to the other surface.
The process of examining grain flow so that it can be optimized in a forging operation can be done with the sectioning, grinding, polishing and etching of the forging as described in Part 1 of this series (October 2014). The use of simulation software can also be a fast, efficient and economical method to monitor the grain flow, especially the grain flow in the interior regions that will become the surfaces of the end-use component after final machining. In a simulation or a physical sectioning, it is possible to superimpose the machined shape on the area being evaluated. Both of these methods provide the opportunity to look for deleterious end grains in high-stress regions of the finished part.
Grain flow in a forging should always be considered in three dimensions. It’s easy to study a cut-up section or simulation output in the form of a 2-D slice. Visualization of grain flow in 3-D in a simulation is possible, whereas physically, this requires two cut faces being studied. In addition to press loads, potential defects and metal flow, present-day simulation software can track the movement of metal and allow the forge shop to see “into” the part nondestructively with the grain flow that has developed due to the plastic deformation imparted to the metal during the various forging steps.
In order for the forging to produce a component that provides the benefits of grain-flow enhancement of properties, a clear understanding of the final part geometry and loading conditions is needed. This understanding requires clear communication between the forger and the customer. In the examples presented in this article, the use of simulation software is illustrated and shows how it can be used to track the changing grain flow that is imparted during each step in the forging process. Our next article will explore grain flow in open-die and large forgings.
Content for this paper was primarily developed by Scientific Forming Technologies Corporation in partnership with SCRA Applied R&D and the Forging Industry Association – Department of Defense Manufacturing Consortium (FDMC). 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. As part of this program, the U.S. Army Benét Laboratories, Watervliet, N.Y., graciously shared content regarding the mortar collars. These components were forged by Consolidated Industries of Cheshire, Conn.