| Popis: |
High temperature Ni-base superalloys are largely used in components of aircraft engines which are subjected to high thermal (above 700ºC) and structural loads. Investment casting has been traditionally implemented for the manufacturing of these components with different levels of complexity and size. However, investment casting of large and complex parts can entail some difficulties which lead to higher casting defect ratios and the need of repairing. Therefore, it is essential to develop easy to repair Ni superalloys castings, i.e., castings with an enhanced weldability or reduced weld cracking susceptibility. Improvement of weldability will also enable the combination of small size castings with forged and rolled components in welded structures reducing overall manufacturing costs and foundry capacity requirements. Cracking susceptibility of Ni superalloys castings is much greater in comparison with equivalent wrought components due to coarser and non-homogeneous microstructures, grain boundary segregations and particular chemistry which promote several cracking mechanisms during welding. A methodology based on Varestraint test (hot cracking test) is introduced to evaluate cracking susceptibility of Ni superalloys with a reduced number of samples. This reduction is particularly important when considering castings since practical conclusions can be drawn with minimum material sampling. Influence of Varestraint testing parameters, machine performance and total crack length (TCL) measurement procedure on scattering of results are assessed. Conditions to reduce scattering and improve repeatability and reproducibility of results are defined, which are essential to compare weldability performance between different alloys and microstructures. |
