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

开式锻造用铸锭生产的铸锭在凝固过程中由于体积收缩而产生气孔和缩孔，使后期构件在载荷作用下形成裂纹，从而导致临界失效。因此，开模锻造的目的是关闭这些孔隙，使孔隙表面接触，然后将它们焊接在一起形成材料粘合。已经有大量描述孔隙闭合的标准，但与数值和实验研究相比，大多数标准在预测方面仍然存在显着偏差。在任何已知的标准中都没有考虑到焊接，特别是由于普遍成形条件而可能重新打开的孔隙。因此，第一个资助期的目的是选择一个标准，该标准可以描述孔隙闭合作为不同孔隙几何形状和取向相对于负载方向的函数。“Cicaporo”准则满足恒定载荷方向的这些条件，因此首先选择并参数化。此外，它已经扩展到这样一种方式，它可以描述孔隙关闭，即使在改变载荷方向，通常情况下，在开放模锻由于组件的旋转。在有限元模拟中实现了该准则，并通过具有代表性的体积单元模拟和锻压试验对准则进行了验证。此外，还进行了描述键合形成的模型实验，在不同的温度、变形和表面条件下，推导了描述气孔闭合后焊接的准则。基于这些发现，第二个资助期的目标是尽量减少在第一个资助期制定的标准预测中的进一步不准确性。因此，该准则必须加以扩展，以考虑到孔隙闭合过程中发生的剪切的影响，因为剪切在材料中产生，除其他外，在每个冲程的模具边缘。在第一期资助期模型试验的基础上，推导出一种评价气孔闭合后焊接性能的准则。这可以是一个下限陈述，例如“安全或不安全焊接”或更复杂的连接描述，例如通过指定接触孔表面之间的粘合强度。然后在有限元模拟和实验验证中结合执行孔隙闭合和焊接准则。一旦这已经完成，应用的可能性是扩展到其他材料类典型的开式模锻。

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.