OPEN DIE FORGING AND ROLLING
Open-die forging represents a large category of forging processes carried out at elevated temperatures, which can produce forgings of many sizes, shapes and materials including ferrous, non-ferrous, or high-temperature alloys. The initial ingot size is the only limitation to the size and weight of the forged part, with the heaviest ingot known to have been forged to-date being 300 metric tons. For this reason open die forging is most often the process of choice for the production of large metal components, single workpieces or parts with a custom short-run production, that require an optimum structural performance.
The Open-die Forging and Ring Rolling Process
The name "open-die" derives from the fact that the metal is never restrained or confined in the dies, but is worked into its final form by a series of forging operations, usually involving flat-faced dies.
Processes that belong to the open-die forging category include: upsetting, piercing, edging and fullering, cogging, becking, blooming, cut-drop off, among others.
The sequence of operations shared by these processes is usually similar:
- ingot casting
- ingot preheating
- open-die forging process
- heat treatment
Rolling, on the other hand, is the process of reducing the thickness or changing the cross-section of a work-piece by compressive forces applied through a set of rolls. It can be carried out both at cold or hot temperature, and it accounts for about 90% of all metals produced by metalworking processes. Processes belonging to the “rolling” category include: ring rolling, structural shape rolling, rolling of flat plates and sheets, thread and gear rolling among others.
Among these, the ring rolling process is a bulk forming process for axial-symmetric, hollow, seamless parts that can be carried out at cold or hot temperature. Thanks to its favorable grain boundary orientation, optimal for metal fatigue resistance, the process is widely used in the production of rings for the automotive and aerospace industries.
The Numerical Simulation of Open Die Forging and Ring Rolling
Each operation mentioned above has its own unique peculiarities that must be taken into account when using numerical simulation tools.
During ingot casting, it is important to accurately predict any defects that can develop in the casting itself. This includes the presence of porosities due to trapped gases, shrinkage cavities, discontinuities such as cracks, cold shuts and tearings; inclusions (generally non-metallic), and surface defects such as laps and scars. A reliable and comprehensive finite element analysis must be able to capture these defects and transfer these micro-structural properties to the next stage of the process, effectively creating a unified design chain solution.
During ingot preheating, attention shifts to the uniform heating of the ingot up and above its recrystallization temperature, Preheating the Ingottaking into account any possible micro-structural phase change in the material. To avoid the creation of high temperature gradients, sudden temperature changes during heating are generally not recommended. The use of optimization software in simulating the preheating process of an ingot will often result in the discovery of a more efficient heating curve for the ingot. This of course translates to energy savings when implemented on the real process given the large size of the ingot and the time needed for preheating.
The open-die forging process simulation itself can be challenging when setting up the workpiece/hammer relative orientation and positioning, and requires the correct representation of the press kinematics, usually a power hammer or a hydraulic press. However, once the model setup is ready, simulation results can highlight important physical phenomena, for example it can predict whether or not porosities in the cast ingot have been eliminated by forging, or whether the work-piece temperature has lowered enough so that re-heating is needed.
A remarkable software feature that greatly helps the software analyst is the use of sensors to back-trace defects, starting from the end of the forging stage.
After the forging phase, heat treatment is employed to improve the overall mechanical behavior of the work-piece. Here, the accurate prediction of the final microstructure is essential, as different phases have different mechanical properties, and phase transformations during cooling or heating cause volumetric changes in the work-piece. By comparing the experimental time-temperature transformation diagram of the material with the cooling rate, the final volume fraction of the different phases can be calculated, as well as the average grain size for each of them. This information is used to predict the local hardness of the forging.
Machining, such as milling, grooving, boring, drilling, or broaching usually represents the last step in the manufacturing process, and can be easily simulated through the use of numerical simulation code. Accurate modeling the machining process allows manufacturers to improve part quality, predict chip shape, increase material removal rates, extend tool life, and reduce trial and error testing in general.
Its unsteadiness, lack of constraints, contact and material non-linearity combined with the large number of ring rotations required to reach the ring’s final dimensions, make ring rolling one of the most challenging rolling methodologies to simulate via numerical methods. An adaptive mesh in the contact area is often required to fully understand the influence of the tools feed-rate on the most common ring rolling defects, such as profile under-fillings and fish-tailing.
Why numerical simulation is indispensable in Open Die Forging and Ring Rolling
Most open-die forgings and large rolled rings are unique pieces just because of their sheer size. This also means that forging errors in the production of one of these pieces are extremely expensive, and their elimination is the prime motivating factor in the creation of an accurate simulation model of the entire forging process. Today accurate virtual simulation is an essential tool for the designers and foundries producing these pieces. With numerical simulation, engineers are able to predict:
- part defects during the forging process
- process parameters such as temperature, deformation and stresses at each stage of the forging process
- tool wear assessment, including the analysis of the tool’s internal stresses and deformation
- the maximum press load and deflection
At EnginSoft we understand the exceptional value derived from the simulation of the entire process chain (ingot casting, ingot preheating, rolling forging, heat treatment and machining), because experience shows that it can help avoid numerous production errors and delays in obtaining the finished workpiece. This is why where applicable, we advise our clients to work on creating a single virtual model for the design chain within an optimization framework, which is able to take the output from the manufacturing simulation (metallic phases, presence of inclusions, residual stresses, type and location of defects, local mechanical properties…) and feed it to the subsequent structural / computational fluid dynamics analysis.
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