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

用于开式锻造的铸锭在凝固过程中由于体积收缩而产生气孔和缩孔，在载荷作用下会在后一构件上产生裂纹，从而导致临界失效。因此，开式锻造的目的是关闭这些气孔，使气孔表面接触，然后将它们焊接在一起，形成材料粘结。描述孔隙闭合的标准已经有很多，但对于大多数标准，与数值和实验研究相比，预测仍然存在显著偏差。焊接，特别是由于普遍的形成条件可能导致的气孔重新打开，在任何已知的标准中都没有考虑。因此，第一个资助期的目标是选择一个标准来描述气孔闭合作为不同的气孔几何形状和相对于载荷方向的方向的函数。“Cicaporo”准则满足恒定加载方向的这些条件，因此首先选择该准则并将其参数化。此外，它已经被扩展到这样一种方式，即使在改变载荷方向的情况下，它也可以描述气孔闭合，这是由于部件旋转而在开式模锻件中通常出现的情况。该准则在有限元模拟中得到了实现，并通过典型体积元模拟和锻造试验进行了实验验证。此外，还进行了描述粘结形成的模型实验，在不同的温度、变形和表面条件下，可以推导出描述闭合后气孔焊接的准则。基于这些发现，第二个资助期的目的是将第一个资助期制定的准则的预测误差降至最低。因此，必须扩展该标准以考虑在气孔闭合过程中发生剪切的影响，因为剪切是在材料中产生的，尤其是在每一冲程的模具边缘。此外，还必须根据第一个资助期的模型实验得出一个标准，该标准可以评估关闭后气孔的焊接情况。这可以是一个下限陈述，例如“安全地或不安全地焊接”，或者是对连接的更复杂的描述，例如通过指定接触的孔表面之间的结合强度。然后，应在有限元模拟中结合实施气孔关闭和焊接的准则，并进行实验验证。一旦做到这一点，应用的可能性将扩展到开式模锻件的典型材料类别。

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.