Recently, there is an increasing need to produce large forged components for aerospace, naval, energy, and other applications. Open die forging of large cast ingots is the primary process used to produce high quality large wrought components. Cogging or upsetting processes are used in the primary stages during most open die forging.
Large cast ingots often contain defects or undesirable micro-structural features, such as cavities and zones related to casting. Some of these features can remain after hot open die forging, which is an important process for converting large cast ingots into wrought components. During the initial cogging and deformation steps prior to the detailed open die forging operations, any internal cavities should be eliminated. The present work focuses on the closure of internal cavities during open die forging so as to produce a sound component. Hot compression tests were conducted to obtain the flow strength of the cast microstructure at different temperatures and strain rates. The measured flow stress data together with other appropriate material properties were used to simulate the forging steps for a large cast ingot. The numerical simulations for the forging deformation and for the internal cavity behavior were performed using commercial finite element analysis program. Actual defects were measured in commercial ingots with an X-ray scanner. The simulation results for the cavity deformation behavior are compared with cavities measured before and after forging. Through the comparison of experimental results and numerical simulation, a criterion for cavity closure is proposed.
In this upsetting process, changes of hydrostatic stress and effective strain were investigated to determine whether the cavity closed or not. From these results, we know that the transition points of hydrostatic stress related to cavity closing and that effective strain of 0.6 or greater provides an adequate condition for the closure of internal cavities during forging.
Large forged parts in the industry are mostly manufacturing by over 3S forging ratio. However, this standard is based on experience, there is no theoretical basis. To prove the theory, both the experiment and FE-Analysis of cogging process was conducted using cavity unclosed defect part. Forged part after cogging were measured internal defect through UT(Ultrasonic Test). From the result, no defect was found. For the analysis of accurate cavity closure behavior, the same condition was applied for the FE-Analysis. According to the results, the internal cavity was closed through the forging ration of over 2.9S. In addition, the threshold effective strain was measured to be 2.9 in the area deformed by forging ratio of 2.9S.
Therefore, as a results of FE-Analysis, the threshold effective strain and forging ratio for cavity closure were measured to 2.9 and 2.9S respectively.
Also In order to obtain the threshold effective strain value applicable to industrial, FE-Analysis model has no internal cavities, the numerical simulation was preformed under the same condition with cavity model.
According to the measurement result, the threshold effective strain was confirmed to be 2.1 in the area deformed by forging ratio of 2.9S.
Thus, a minimum threshold effective strain for application to the industry has like to proposal to over 2.1.