Quality assurance of additively manufactured alloys for aerospace industry by non-destructive testing and numerical modeling

摘要

Development of the layer-wise build-up of metallic materials offered significant technical and economic advantages for modern structural components. Airliners are now losing much weight which is due to efficient designs which were made possible by additive manufacturing technologies. In addition to topology optimization, selective laser melting (SLM) induces enhanced micro structural features and mechanical properties. Highly dynamic melt pools result in refined microstructures which increase the strength of the conventionally manufactured titanium and aluminum alloys. At the scanning electron microscope (SEM), it was revealed that SLM produces a very fine cellular dendritic microstructure in aluminum alloys of the Al-Si system. In the recently developed Scalmalloy, the ultra-fine ceramic precipitation of A1-Sc_3 enables the tensile strength to reach 490 MPa and improves fracture strain by 100%. Along with improved microstructures due to high cooling rates, unstable melt pools induce spherical porosity as the release of gases from the powder bed is entrapped by the high solidification rate. Platform heating (PH) which slows down cooling and enhances energy density is proved to stabilize melt pools and improve the degassing mechanism. Al-Si alloys are well-known cast alloys in the automotive industry; however, with the application of SLM technology, the tensile strength of these alloys is now 200% of the cast counterparts. The interest of aerospace industry in a high-strength aluminum alloy encouraged the development of Scalmalloy which has a similar tensile strength to the wrought EN AW 7075. The latter is sensitive to hot cracking during SLM. For assurance of quality of additively manufactured alloys for the aerospace industry, the non-destructive testing (NDT) technology of X-ray computed tomography (CT) is applied in this study to develop finite element (FE) models which will be used in a statistical simulation to deduce a quality-controlled fatigue strength in high- and very high-cycle fatigue (HCF to VHCF) regimes. Modeling of fatigue strength will be based on the quasistatic and cyclic deformation behavior of SLM aluminum alloys.