Criteria for the closure and pressure welding of pores in open die forging

• Applicant: Professor Dr.-Ing. Gerhard Hirt
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: from 2016 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 283878720

Ingots produced from ingot casting for open-die forging have pores and shrinkage cavities due to volume shrinkage during solidification, which can cause cracks to form in the later component under load and thus lead to critical failure. The aim of open-die forging is therefore to close these pores so that the pore surfaces come into contact and then weld them together to form a material bond. There are already a large number of criteria for describing pore closure, but for most criteria significant deviations in the prediction remain compared to numerical and experimental investigations. Welding and especially the possible re-opening of pores due to the prevailing forming conditions is not considered in any known criterion.Therefore, it was the aim of the first funding period to select a criterion which can describe the pore closure as a function of different pore geometries and orientations with respect to the load direction. The "Cicaporo" criterion fulfils these conditions for a constant load direction, so this was first selected and parameterised. Furthermore, it has been extended in such a way that it can describe pore closure even under changing load directions, as is usually the case in open die forging due to the rotation of the component. The criterion was implemented in the finite element (FE) simulation and validated experimentally by simulations with representative volume elements and forging tests. Furthermore, model experiments for the description of the bond formation were carried out, in which for different temperatures, deformations and surface conditions the derivation of a criterion for the description of the welding of pores after closing was made possible.Based on these findings, the aim of the second funding period is to minimise further inaccuracies in the prediction of the criterion developed in the first funding period. Therefore, the criterion has to be extended to take into account the influence of occurring shear during pore closure, since shear is generated in the material, among other things, at the die edges of each stroke. Furthermore, a criterion based on the model experiments of the first funding period has to be derived, which can assess the welding of the pores after closure. This can be a lower-bound statement, such as "safely or not safely welded" or a more complex description of the connection, e.g. by specifying a bond strength between the contacting pore surfaces. The criteria for pore closure and welding should then be implemented in combination in the FE simulation and validated experimentally. Once this has been done, the application possibilities are to be extended to other material classes typical for open-die forging.

• DFG Programme: Research Grants