Development of high strength aluminum alloys for laser powder bed fusion

Despite the widespread use of Laser Powder Bed Fusion (LPBF), only few high-strength Al alloys are suitable for this process due to the occurrence of hot cracks during solidification. In this thesis, strategies to design Al alloys processable by Laser Powder Bed Fusion were successfully developed and applied to 2xxx and 6xxx series alloys. Novel Al alloys were designed to stimulate proper solidification behavior and to achieve improved mechanical properties by means of tailored heat treatments. CALPHAD-based simulations revealed a useful tool for the selection of alloying elements able to suppress hot cracks. Processability and performance of novel Al alloys were assessed by an extensive experimental work. The work was carried out within the framework of SAMOA (Sustainable Aluminum additive Manufacturing fOr high performance Applications) EIT Raw materials upscaling project, and the alloy design was carried out in compliance with the European Commission guidelines for critical raw materials. Specifically, the content of Sc and Si, which are defined as “Critical Raw Materials”, was minimized or avoided. Two different strategies were identified to suppress hot cracks: (i) the addition of grain refiners (Ti, B and Zr) and (ii) the formation of an abundant eutectic phase mixture at the end of solidification. The former route promotes the formation of a fine equiaxed microstructure capable to accommodate thermal stresses and avoid solidification cracking. Al-Cu-Mg-Ni-Fe-Ti-B and Al-Mg-Si-Zr alloys were designed and developed following this route. The second approach was applied to the Al-Cu-Mg system. By increasing the Cu content, an abundant eutectic phase mixture is stimulated and revealed capable to backfill and “heal” incipient cracks that nucleate at grain boundaries during last stages of solidification. The strategies were applied to four main case studies, which are summarized in the following: a) An Al-Cu-Mg-Ni-Fe alloy (2618 grade) was modified with different amounts of Ti and B to improve the processability and high temperature resistance of the alloy. Samples with no hot cracks and relative densities higher than 99.7 % were produced by LPBF. Microstructural features, aging response and mechanical properties of the alloys were studied and compared with those of the Al-Cu-Ti-B-Mg-Ag (A20X), a commercial alloy for high temperature applications specifically developed for LPBF. b) An Al-Mg-Si alloy (6182 grade) was doped with Zr following two different procedures. The first relies on mechanical mixing of Al powder with ZrH2 particles, whereas the second one is based on the alloying of the molten Al with Zr during the gas atomization process. The pre-alloyed Al-Mg-Si-Zr alloy showed relative density higher than 99.6 %, no hot cracks within its microstructure and mechanical properties comparable with those of the extruded 6182 counterpart. On the contrary, the use of ZrH2 particles led to high fraction of gas pores, which are ascribed to the entrapment of molecular hydrogen in the melt pool during solidification. c) The effect of Cu on the processability of a 2024 Al alloy was investigated. Cu is responsible for the eutectic Al-Al2Cu reaction at the end of solidification. An abundant eutectic phase mixture was revealed capable to enhance the solidification cracking resistance of the material. A refined microstructure was found in samples with more than 10 wt.% of Cu, featuring no hot cracks and relative densities higher than 99.5 %. Similarities and differences with respect to Al-Cu alloys produced by conventional welding techniques were highlighted and discussed. d) An Al-Mg-Zr-Sc alloy with reduced content of Sc was processed by LPBF. Its microstructural features, aging response, crystallographic texture and mechanical properties were investigated. Simulations of Al3(Sc,Zr) precipitation were performed to predict the yield strength of the aged alloy. Tensile properties of the alloy were compared to those of Scalmalloy, a commercial Al-Mg-Zr-Sc alloy featuring a higher amount of Sc.