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

由用于开式模锻的铸锭生产的铸锭由于在凝固期间的体积收缩而具有孔隙和缩孔，这可能导致在载荷下在后面的部件中形成裂纹，从而导致临界失效。因此，开式模锻的目的是封闭这些孔隙，使得孔隙表面接触，然后将它们焊接在一起以形成材料结合。已经有大量的标准来描述孔隙闭合，但对于大多数标准，与数值和实验研究相比，预测中仍然存在显着偏差。在任何已知的标准中都没有考虑焊接，特别是由于主要的成形条件而可能重新打开的孔隙。因此，第一个资助期的目标是选择一个标准，该标准可以描述孔隙闭合，作为不同孔隙几何形状和相对于载荷方向的取向的函数。“Cicaporo”标准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，它已被扩展到这样一种方式，它可以描述孔隙闭合，即使在变化的负载方向，通常是在开式模锻的情况下，由于组件的旋转。该准则在有限元（FE）模拟中实现，并通过具有代表性的体积单元和锻造试验的模拟进行了实验验证。此外，还进行了描述粘结形成的模型实验，其中针对不同的温度、变形和表面条件，推导出了描述闭合后气孔焊接的标准。基于这些发现，第二个供资期的目标是尽量减少第一个供资期制定的标准预测的进一步不准确性。因此，该标准必须加以扩展，以考虑在孔隙闭合过程中发生的剪切的影响，因为剪切是在材料中产生的，除其他外，在每个冲程的模具边缘。此外，必须根据第一个供资期的模型实验得出一个标准，该标准可以评估封闭后气孔的焊接情况。这可以是下限声明，例如“安全或不安全焊接”，或者是更复杂的连接描述，例如通过指定接触孔表面之间的结合强度。孔隙闭合和焊接的标准应在有限元模拟中结合实施，并通过实验进行验证。一旦完成了这一点，应用的可能性将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.