Eutectic Al alloys with tailored solidification path to probe fundamental aspects of solidification in laser-based AM II

• Applicants: Dr. Markus Apel; Professor Dr.-Ing. Michael Schmidt
• Subject Area: Joining and Separation Technology ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 409726740

Our project goal is to evaluate the Al-Ni-Ce system around its zone of coupled eutectic growth for its suitability for PBF-LB/M. In particular, ultra-fine eutectic structures with superior mechanical properties are originally targeted. In phase 1, we have already verified two of our research hypotheses, namely that in Al-Ni the material microstructures, in particular an ultra-fine binary eutectic, can be designed by selecting hypereutectic alloy compositions and specific processing conditions. Indeed, while the spacing has proven efficient to apprehend the solidification velocity in the case of a eutectic coupled growth, a broad range of other microstructures have been evidenced, both experimentally and numerically (phase-field method), illustrating the strong asymmetric coupled zone of eutectic growth. Moreover, the process and material parameters also influence the melt pool dynamics, and a link between its dimensions and the porosity formation via keyholing has been established. In phase 2 of SPP2122, this link will be investigated in more detail. Especially, the influence of the hatch distance and the laser spot size will receive our attention in order to better understand the keyholing onset dependent on alloy composition and thereby broaden the process window, particularly at low laser scanning speed. Melt pool simulations, as well as experiments with the ternary Al-Ni-Ce alloy, will be performed. Reducing porosity goes hand in hand with generating appropriate microstructures. In this respect, the variety of possible microstructures is expected to become even broader in the ternary alloy system than for binary alloys. A particular focus will be put on the three-phase eutectic coupled growth in order to produce ultra-fine ternary eutectics as well as on a two-phase nanocrystalline structure, evidenced during phase 1 for large scanning speeds, with grain sizes in the range of 50 nm for both phases. This nanocrystalline structure emerges intrinsically, shows remarkably large hardness values and no solidification cracks, however, it seems to be prone to cold cracks.We will systematically perform mechanical testing (hardness, tensile tests) on the produced samples when porosity is low and the microstructure is promising. Our project should then provide, at the end of phase 2, a procedure to build dense specimens with, possibly even locally, optimized properties.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2122: Materials for Additive Manufacturing